Roadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.)

Roadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.) Section One - Stabilising slopes with civil and bio-engineering (introduction...) 1.1 Problems of slopes and their solutions 1.2 Steps for the stabilisation of slopes (introduction...) Step 1: Make an initial plan Step 2: Prioritise the works Step 3: Divide the site or slope into segments Step 4: Assess the site Step 5: Determine civil engineering works Step 6: choose the right bio-engineering techniques Step 7: Design the civil and bio engineering works Step 8: Select the species to use Step 9: Calculate the required quantities and rates Step 10: Finalise priority against available budget Step 11: Plan plant needs Step 12: Arrange implementation and prepare documents Step 13: Prepare for plant propagation Step 14: Make the necessary site arrangements Step 15: Prepare the site Step 16: Implement the civil engineering works Step 17: Implement the bio-engineering works Step 18: Monitor the works Step 19: Maintain the works

Roadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.)

Section One - Stabilising slopes with civil and bio-engineering

Figure

This section provides:

· a brief introduction to the common problems of slope instability, and ways of solving them;
· a straightforward procedure to identify and implement the appropriate treatments for each site, following a series of logical steps summarised in Figure 1.3, on page 14.

1.1 Problems of slopes and their solutions

Ideally, the causes of slope instability would be well understood and appropriate solutions would be easy to select. However, this is rarely the case and engineers must make assumptions about the causes of slope instability, based on their knowledge and experience of the terrain. This is particularly true in Nepal, where slopes tend to be long and steep, and the climatic variables are as yet poorly understood. Attaining a desired factor of safety may not be feasible.

With such a variety of materials and sites, choosing stabilisation techniques is a complicated process. There are many variables, most of which cannot practicably be measured in the field. Therefore, it is not possible to set quantitative limits on many of the parameters. This section outlines instead a practical analysis to help the engineer to decide on the best course of action.

Bio-engineering is not a substitute for civil engineering. It offers engineers a set of tools to complement those already available in solving a range of shallow slope problems.

Bio-engineering serves two distinct roles: providing additional techniques for stabilising shallow failures and controlling erosion; and enhancing civil engineering structures by protecting them and maximising their effectiveness. In both roles, bio-engineering techniques must be carefully integrated with civil engineering structures.

Every slope has a different variety of erosion and failure processes at work on it; often, there will be more than one process affecting each part of a slope. Freshly prepared slopes (i.e. those just cut or filled) are usually subject to erosion, and so all slopes need to be stabilised. These erosion and failure processes must be identified before remedial work can be started. Examples of the most common problems are given in Figure 1.1¹.

¹ For a more comprehensive list of the common forms of failure and erosion, refer to pages 12 and 13 of TRL Overseas Road Note 16, Principles of low cost engineering in mountainous regions.

Shoulder erosion threatening the edge of the road pavement. Even the smallest slope problems must be tackled

A deep planar slide on the Arniko Highway has removed a portion of the road

Figure 1.1: Common types of erosion and slope failure

* The mechanisms of failure or erosion are covered in detail in the Reference Manual.

Figure 1.2: The main engineering functions of structures, with examples of techniques

* The six main engineering functions are defined in Section 1.2 below and are elaborated in the Reference Manual.

Figure 1.3: Flow chart to show the progression of the steps for slope stabilisation

Figure 1.2 shows examples of civil and bio-engineering techniques that have been devised to overcome these common problems.

1.2 Steps for the stabilisation of slopes

In the steps outlined below and shown schematically by the flow chart in Figure 1.3 on page 14, the engineer follows a logical procedure to plan and implement the works.

These steps initially use as their basis the six main functions of both civil engineering and bio-engineering techniques of slope stabilisation, given in Figure 1.2. In more detail, engineering structures serve to:

Catch eroded material moving down the slope. Movement may occur as a result of gravity alone, or with the aid of water as well. Material is caught by a physical barrier such as a wall or the stems of vegetation;

Armour the slope against erosion from runoff and rain splash. This is most effectively done using a continuous cover of low vegetation (inert coverings such as stone pitching tend to be expensive over large areas). Partial armouring is often provided, for example by using lines of grasses, brush layers or wire bolsters;

Reinforce the soil by physically stiffening it to increase its resistance to shear. Plant roots are effective at reinforcing soil;

Anchor surface material to deeper layers by soil pinning. This helps to reduce mass movements at depths greater than provided by general reinforcement. The roots of large plants emulate soil anchors or rock bolts;

Support a soil mass by buttressing. On a large scale, a retaining wall or the roots of large plants (and their stems once they have caught some debris) such as big bamboo clumps can buttress a soil mass. On a micro scale individual large stones or smaller vegetation perform this function.

Drain excess water from the slope. Drier materials tend to be more stable than wetter ones -many failures occur when the material reaches a point of liquefaction. Standard civil engineering drains can be provided; vegetation planted in lines angled down the slope help to drain the surface layers.

Sites are assessed using a standard procedure. The choice of stabilisation techniques (both standard civil and bio-engineering) depends on an identification of the functions needed to stabilise and protect the slope. These steps lead through the process to give a logical application of the techniques available.

To implement slope stabilisation works including both civil and bio-engineering, follow these steps.

Figure 1.4: Summary calendar of civil and bio-engineering works

Step 1: Make an initial plan

Early in the fiscal year, you must prepare for the process of checking all slopes along the road and planning the year's work. The calendar in Figure 1.4 shows how the various steps involved, as described above, will be scheduled through the year.

Note that there are three critical points on this calendar; all are governed by seasons. These are:

· seed collection, which starts in Mangsir for many species;
· civil engineering works, which must be complete before the rains start in Jestha; and
· site planting, which usually starts in late Jestha or Ashad, as soon as the rains are reliable.

All of the other activities must be completed on schedule for these to be carried out in the right season. This accounts for the heavy planning and design load early in the fiscal year.

Step 2: Prioritise the works

Inspect the road and make a list of the sites that require treatment. Prioritise them according to the importance of stabilisation.

Weighing the seriousness of the existing or potential failure against the damage it could cause helps to prioritise and schedule works. Figure 1.5 shows the priority to be given to different slope movement problems. The scale goes from a distinct threat to human life (e.g. houses might be lost or the entire road could be carried away) to the situation where a slope problem can cause only limited damage (e.g. occasional blocking of drains), and where treatment is more of a preventative measure (see Figure 1.5).

In many cases, budget constraints allow only the higher priority sites to be addressed. Sites up to priority 3 should always be treated if at all possible. However, the aim should be to move towards treatment of all sites under a regular maintenance programme.

Figure 1.5: Prioritisation of repair work (from the perspective of the Department of Roads)

Figure 1.6: Typical slope segments, showing patterns of material movement

Step 3: Divide the site or slope into segments

A slope segment can be defined as a length of slope with a uniform angle and homogeneous material that is likely to erode or fail in a uniform manner.

The mechanical, hydrological and biological processes at work in a slope are many and complex. Nevertheless, before any remedial work can begin, it is necessary to identify the major factors contributing to instability in order to decide on appropriate action. The assessment and treatment of each site is based on the use of one or more techniques for each segment. So it is necessary to divide the slope into its component segments. Some common examples of slopes are given in Figure 1.6.

While carrying out this step on site, it is useful to sketch the site on the pro forma given in Annex A.

Before proceeding to Step 4, you should have a good knowledge of the site. You should know how many segments make up the slope, and have an idea of how the main processes at work within each segment contribute to the overall instability of the site.

From now on, each segment of slope must be considered a separate entity for treatment: both civil engineering and bio-engineering techniques should be planned for each segment rather than for an entire site.

Careful site inspection Is essential for successful stabilisation

Figure 1.7: Common types of erosion and slope failure

* The mechanisms of failure or erosion are covered in detail in the Reference Manual.

Step 4: Assess the site

This is the most important step, and the one on which you must spend the most time. You must make a site visit, and you will need:

· either some copies of the pro forma in Annex A or a notebook;

· a 30-metre tape measure;

· a clinometer or an Abney level (if you do not have one of these, take this handbook with you and use the angled slope lines in Annex A to estimate slope angle in profile); and

· an altimeter, or maps or site drawings of the roadline that show the altitude.

Carefully assessing a site, through an investigation on the ground, is the key to applying good engineering practice. Without proper investigation and assessment, both civil and bio-engineering techniques are likely to fail. This section gives only a very brief guide; more details on site assessment are provided in Section 2 of the Reference Manual.

The objective of Step 4 is to arrive at a more detailed appreciation of the factors contributing to instability within each slope segment. In order to achieve this, you will need to look carefully at each segment of the site and note down the following facts (more details are given in the paragraphs below).

Figure 1.8: The main physical factors affecting slopes

Erosion and failure processes

Each site has a different variety of erosion processes at work, which must be identified before remedial work can be started; often, there will be more than one process affecting each slope segment. A list of the erosion and failure problems is given in Figure 1.7. But remember that most sites, however small, are the result of a combination of these processes. It is assumed that freshly prepared slopes (i.e. those just cut or filled) are subject to erosion, and so all slopes need to be treated¹.

¹ For a more comprehensive list of the common forms of failure and erosion, refer to pages 12 and 13 of TRL Overseas Road Note 16, Principles of low cost engineering in mountainous regions.

List the erosion and failure processes at work in each segment of slope and mentally crosscheck it against the functions required of the stabilisation measures. Sketch these on paper for your later reference and to help with the design of structures. If you need more details on any aspect of site assessment, refer to Section 2 of the Reference Manual.

Other factors

You must identify and note down all of the physical factors affecting a site. Those additional to the basic features of slope segments are listed in Figure 1.8. Some are internal (e.g. springs) while others are external (e.g. river undercutting).

Slope angle(s)

Record the slope angles and assign each segment to one of three classes: <30°, 30 - 45°, or > 45° Slopes of less than 30° will need only mild treatment;) those falling in the other two classes will require more substantial stabilisation.

Figure 1.9: Common characteristics of well-drained and poorly drained soils

Slope length

Record the length of each segment of the site as < 15 metres or > 15 metres. A slope length of 15 metres represents a practical dividing line between 'big' and 'small' site segments. Slope segments longer than 15 metres are prone to greater risks, for example of gullying. Also, cost constraints may lead to a compromise over the desired intensity of work. Segments with very long slopes (greater than 30 metres) are singled out for special consideration in step 5 (see Figure 1.11).

Material drainage

This relates to the internal porosity of soils and the likelihood of their reaching saturation, losing cohesion and starting to flow. Materials with poor internal drainage tend to have more clay than sand. They are prone to slumping at a shallow depth (e.g. < 500 mm) if they accumulate too much moisture. In such a case, stabilisation requires some kind of drainage in addition to other functions.

For convenience, materials need to be classed only into 'good' or 'poor' drainage. Figure 1.9 provides a guide.

Segment moisture

The moisture regime of the entire site must be considered although, in the field, this can only be estimated. In assessing sites, it is necessary to determine into which of four categories each segment falls.

Figure 1.10 summarises the main factors and how they can be identified.

Altitude

Altitude is the main determinant of temperature in Nepal and therefore regulates the local climate to a large extent. It is necessary to know the altitude to a reasonable degree of accuracy (ideally +100 metres) when the actual species are selected for bio-engineering works.

Figure 1.10: Environmental factors indicating site moisture characteristics

* Loam is the name given to a soil with moderate amounts of sand, silt and clay, and which is therefore intermediate in texture and best for plant growth.

Figure 1.11: Assessing the requirements for civil engineering treatments

Step 5: Determine civil engineering works

At this stage, standard civil engineering structures (e.g. gabion and other types of retaining structures, breast walls, prop walls and revetments; check dams; masonry drainage systems) should be considered. In later stages, small-scale civil engineering structures used only for surface protection (i.e. stone pitching and jute netting) are considered as options where appropriate.

Some sites will not require the building of structures, but will instead be stabilised using only bio-engineering techniques. In most cases, however, bio-engineering techniques will also be employed to enhance the effectiveness of civil engineering structures. The series of questions in Figure 1.11 helps to simplify the process of assessing the requirements for major civil engineering treatments. These must be integrated with bio-engineering measures, but normally need to be implemented first.

If civil engineering structures are to be used, they must be designed and constructed according to normal practice. Apart from the key design details referred to in Section 2, these are beyond the scope of this manual. A useful reference work is TRL Overseas Road Note 16, Principles of low cost road engineering in mountainous terrain.

The next step concentrates on shallow (< 500 mm depth) stabilisation and surface protection using bio-engineering techniques, and on areas around civil engineering structures.

Step 6: choose the right bio-engineering techniques

Having completed step 5, any deeper-seated problems will have been addressed by conventional civil engineering measures, such as retaining walls and drainage systems. This step gives details of bio-engineering and other related techniques for protecting the surface, stabilising the upper 500 mm, and improving surface drainage; and for enhancing and protecting large civil engineering structures. These are required as part of the whole stabilisation package; bio-engineering must be fully integrated with any civil engineering structures.

The flowchart in Figure 1.12 suggests appropriate techniques for different slope segments. It is assumed that these are combined with appropriate civil engineering structures where necessary to enhance slope stability. Many factors determine the optimum technique or combination of techniques, but only the most important have been included here.

The seven columns in Figure 1.12, (a) to (g), are summarised below.

(a) Slope angle(s)
3 classes: <30º, 30 - 45°, or >45° (measured in step 4).

(b) Slope length
2 classes: <15 metres or >15 metres (measured in step 4).

(c) Material drainage
2 classes: good or poor (estimated in step 4).

(d) Site moisture
2 classes: wet/moist or dry/very dry (combined from the four estimated in step 4)¹.

¹ The four classes determined in step 4 (as well as the altitude of the site) are required to establish the actual species to be used for bio-engineering, in step 8.

(e) Potential problems
The potential problems to be encountered on each slope segment have been identified in step 4.

(f) Function required
Once you have assessed the most likely potential problems on a slope segment you can select the most appropriate engineering functions required (i.e. catch, armour, reinforce, anchor, support or drain) for each segment. In bio-engineering, the functions required by the treatment determine the plant types used and the way they are propagated. This is given in detail in step 8.

Figure 1.12: Choosing a bio-engineering technique

Special conditions

* Possible overlap with parameters described in the rows above. † May be required in combination with other techniques listed on the rows above. ‡ Only the common potential problems listed in Figure 1.7 are given here. 'Any rocky material' is defined as material into which rooted plants cannot be planted, but seeds can be inserted in holes made with a steel bar. 'Any loose sand' is defined as any slope in a weak, unconsolidated sandy material. Such materials are normally river deposits of recent geological origin. 'Any rato mato' is defined as a red soil with a high clay content. It is normally of clay loam texture, and formed from prolonged weathering. It can be considered semi-lateritic. Techniques in bold type are preferred. Chevron pattern: <<<<< (like a sergeant's stripes). Herringbone pattern: ¬¬¬¬¬ (like the bones of a fish).

(g) Techniques
One or more techniques that are known to be successful on sites for each category are given. However, the general picture may not cover every case and so this flowchart cannot be considered to be fully comprehensive: some local variation may be needed and this, of course, is the reason for having an engineer on site.

Once this step has been completed, it is possible to move on to the detailed design of the works for each site.

Step 7: Design the civil and bio engineering works

Design the civil and bio-engineering works using normal procedures. It is more cost effective to design the works so that they are carried out in a fully integrated way. As usual, you should bear in mind the resources and budget available for the work. Make the designs as detailed as you can at this stage.

Practical design considerations for the most common civil engineering structures are given in Section 2.

Details of the design of bio-engineering works are given in Section 3.

Padang bans is a valuable small bamboo for bio-engineering works on high-altitude roads

Step 8: Select the species to use

It is important to select the right species for use in each bio-engineering technique. To do this there are three factors to consider: function/technique, propagation and site suitability.

Function/technique

Having worked through the previous steps, you will have determined the functions required for each slope segment and will now have identified the techniques you need to use. The most appropriate class of plant to use depends on the techniques. These are summarised in Figure 1.13, but are also shown in the table listing the main bio-engineering species (Figure 1.14).

Propagation

There are various methods of propagation appropriate for the main plant classes, but individual species can be propagated only by certain of these methods. The method of propagation to be used is often determined by the function required and the bio-engineering technique being used. For example, if grass lines are to be planted, the species chosen must be capable of propagation from slip cuttings; if brush layering, palisades or fascines are to be used, the shrubs or small trees must be capable of growing from hardwood cuttings.

Site suitability

Whatever the function and propagation method required of the plants by the bio-engineering technique, the plants selected must be able to grow in the site being treated. The suitability of each species to their growing sites is complex, but there are some straightforward rules that simplify the matter. The three main aspects of the environment affecting plant growth are as follows.

· Temperature. This is very closely related to altitude for most of Nepal. In choosing the species, therefore, the site altitude measured in step 4 is used.

· Moisture. This is very difficult to quantify. It was assessed in step 4 for the site and classed as one of wet, moist, dry or very dry. This is now used to choose the species.

· Nutrients. The main species used in bio-engineering are all tolerant of very poor soils. Therefore the nutrition factor can be ignored at this stage.

The final choice of species according the technique for which it is to be used, and the site characteristics of altitude and moisture, is made by reference to Figure 1.14.

Figure 1.13: Bio-engineering techniques and appropriate plant classes

* A shrub is a woody plant with multiple stems growing up from the ground; a tree has usually one stem growing up from the ground. For bio-engineering purposes, shrubs and small stature trees have the same functions, since the rooting patterns tend to be similar.

Figure 1.14: Selection of species for bio-engineering by groups of techniques

Planted grass lines (all configurations) and vegetated stone pitching gully beds
These grasses are grown from slip or rhizome cuttings

This table gives the main species used for bio-engineering in Nepal. A range of plants is available for each particular location. A list of all tested bio-engineering species is given in Annex B. Full details of the main bio-engineering species are given in the Reference Manual.

Figure 1.14: Selection of species for bio-engineering by groups of techniques

Species for grass seeding and turfing

This table gives the main species used for bio-engineering in Nepal. A range of plants is available for each particular location. A list of all tested bio-engineering species is given in Annex B. Full details of the main bio-engineering species are given in the Reference Manual.

Step 9: Calculate the required quantities and rates

Calculate the quantities and rates required for the works. This is a standard procedure and, for work by the Department of Roads, must follow the schedules established by the government for this purpose.

Rate analysis norms for bio-engineering are given in the Reference Manual.

Step 10: Finalise priority against available budget

You can now finalise the work to be undertaken in the year's programme. This entails determining the right balance between the resources available and the seriousness of the failures on the sites that need to be stabilised.

The prioritisation made in step 2 showed how important it is to stabilise each site. This should be re-examined to check that the higher priority sites can all be covered. In certain cases it may be necessary to return to steps 7 and 8, to reconsider the design of the civil and bio-engineering works with a view to reducing costs and covering more sites.

Figure 1.14: Selection of species for bio-engineering by groups of techniques

Species for brush layers, palisades, live check dams, fascines and vegetated stone pitching walls
Shrubs/small trees grown from hardwood cuttings

* Required for live check dams only

This table gives the main species used for bio-engineering in Nepal. A range of plants is available for each particular location. A list of all tested bio-engineering species is given in Annex B. Full details of the main bio-engineering species are given in the Reference Manual.

Step 11: Plan plant needs

Calculate the exact need for plants for the bio-engineering works. This can be done with reference to Section 3, which gives the plant spacings for each of the bio-engineering techniques. This will allow you to list the precise plant requirements for the programme. In turn, this is what must be produced by your nurseries, provided by the contractors or obtained from elsewhere.

It is standard practice when planning the growing of plants in a nursery to allow for losses during production. This is covered in Section 4. Therefore at this stage you should calculate the exact site needs, and you do not need to add an allowance for losses before the plants reach site. It may, however, be useful to add a contingency quantity of plants in case site conditions vary from those expected, and more plants are required.

Figure 1.14: Selection of species for bio-engineering by groups of techniques

Species for shrub/small tree planting and large tree planting
Shrubs/small trees grown from seeds/polypots

This table gives the main species used for bio-engineering in Nepal. A range of plants is available for each particular location. A list of all tested bio-engineering species is given in Annex B. Full details of the main bio-engineering species are given in the Reference Manual.

Step 12: Arrange implementation and prepare documents

In this step, the question as to whether the works are to be carried out by contract or through a direct labour force is considered. Both have advantages in different situations. Small-scale works are normally best done through daily-rated labour. Both the government regulations and the private sector in Nepal provide considerable flexibility for either system.

Whichever method of implementation is chosen, it is necessary at this stage to prepare the appropriate documentation for the works to be undertaken. For contracting, standard specifications for bio-engineering are given in the Reference Manual. Standard specifications for civil engineering structures are also available from the Department of Roads.

Figure 1.14: Selection of species for bio-engineering by groups of techniques

Species for seeding (on site) with shrubs/small trees or large trees
Robust plants grown from seeds

* Utis and khanyu should be seeded by broadcasting (broadcasting is where seed is thrown over the surface in as even a way as possible, but forming a totally random, loose cover) only. The other species have larger seeds and can be direct seeded (direct seeding is where seeds are sown carefully by hand into specific locations in a slope, such as in gaps between fragmented rock).

This table gives the main species used for bio-engineering in Nepal. A range of plants is available for each particular location. A list of all tested bio-engineering species is given in Annex B. Full details of the main bio-engineering species are given in the Reference Manual.

Step 13: Prepare for plant propagation

There are three main considerations in producing plants.

· Seeds must be collected for the grasses, shrubs and trees that are needed for the programme (step 11). Seed collection times for bio-engineering plants are given in Annex B. The timing of this operation is critical (for obvious biological reasons) and orders must be placed in time. The calculation of the required seed quantities is given in Section 4.

· Fill grass slip beds with stock to give sufficient of the right species of plants. Section 4 gives all the details of grass bed preparation and production in nurseries.

· Nurseries must be checked to make sure that they have all the resources necessary for the production season. Below about 1200 metres, nurseries should be prepared in Mangsir and Poush (mid November to mid January) for growth and production to start in Falgun (February). Section 4 provides the details for nursery preparation and production.

Higher altitude nurseries need a longer phase of production. Above about 1500 metres, and certainly above 1800 metres, most plants need to grow for a year in the nursery. Plants raised from seeds sown in Shrawan (July-August) of one year will be planted out on site in Ashad (June-July) of the following year. Nurseries between 1200 and 1800 metres must be planned on an individual basis depending on the particular local micro-climate. Some nurseries in this zone take six months and some take a year to produce usable plants.

Figure 1.14: Selection of species for bio-engineering by groups of techniques

Species for large bamboo planting

This table gives the main species used for bio-engineering in Nepal. A range of plants is available for each particular location. A list of all tested bio-engineering species is given in Annex B. Full details of the main bio-engineering species are given in the Reference Manual.

Step 14: Make the necessary site arrangements

Kanda phul can be grown from hardwood cuttings and prefers dry sites between 500 m and 1500 m altitude

The final part of the design phase involves issuing the necessary instructions for site responsibilities and quality control. Check that all staff understand the designs for every part of the site works, and that they know the implementation programme and where their responsibilities lie.

If contractors are being employed, it is also necessary to finalise the liability for defect repair if this has not already been established in the contract. It may be necessary to make special arrangements if different civil engineering works and bio-engineering contractors are being used on the same sites.

At this stage it is also necessary to check that safety measures are understood and will be complied with. See page 10 for a Safety Code of Practice for Working on Slopes.

Step 15: Prepare the site

Before civil engineering structures can be put in place and bio-engineering treatments applied, the site must be properly prepared. The surface should be clean and firm, with no loose debris. It must be trimmed to a smooth profile, with no vertical or overhanging areas. The object of trimming is to create a semi-stable slope with an even surface, as a suitable foundation for subsequent works.

Trim slopes to a straight profile, with a slope angle of between 30° and 60°. (In certain cases the angle will be steeper, but review this carefully in each case.) Never produce a pronounced convex or concave profile; these are prone to failure starting at a steep point. Trim off steep sections of slope, whether at the top or bottom. In particular, avoid convex profiles with an over-steep lower section, since a small failure at the toe can destabilise the whole slope above. Remove all small protrusions and unstable large rocks. Eradicate indentations that make the surrounding material unstable by trimming back the whole slope around them. If removing indentations would cause an unacceptably large amount of work, excavate them carefully and build a prop wall.

In plan, a trimmed slope does not need to be straight. An irregular plan view is acceptable and, in most cases, reduces costs because protrusions do not need to be removed.

Remove all debris and loose material from the slope surface and toe to an approved tipping site. If there is no toe wall, the entire finished slope must consist of undisturbed material.

Where toe walls form the lower extreme of the slopes to be trimmed, you can use the debris for backfilling. Where backfilling is practised, compact the material in layers, 100 to 150 mm thick and sloping back at about 5°, by ramming it thoroughly with tamping irons. This must be done while the material is moist.

Dispose of excess spoil carefully, in an approved tipping site. Just throwing it over the nearest valley side wall is not good enough. Much slope instability and erosion is caused in this way. Always include adequate provision in your estimates for haulage to an approved safe tipping area.

A carefully prepared site has more chance of reaching stability

Trimming slopes

To trim slopes effectively, follow these steps.

1 Check that all prior construction work has been completed and that the site is clear of equipment.

2 Define the type of site. Possibilities are as follows: minor trimming required only on part of site; keeping rill or gully pattern in plan section; trimming to a new designed plan section; new retaining wall to be backfilled.

3 Make a site visit and explain to the site staff and workers exactly what the finished site should look like. Draw sketches to ensure their understanding.

4 Ensure that there is safe access to the site. On very steep slopes, make new paths if necessary. Make sure that ropes or ladders are provided. When trimming a site, always work from the top of the site, moving down the slope. Check that all safety requirements have been fulfilled (refer to the section on safety in the Introduction, pages 10 and 11).

5 Carry out a trimming survey. Put in pegs and lines as necessary.

6 Cut notches through the mass to be trimmed to give the final cut lines.

7 Trim in steps from the top, using the steps as ledges for the labourers to stand on during trimming.

8 If backfilling is required behind a retaining structure below, compact the trimmed material as you go. This will require halting the trimming, redistributing and compacting the debris as backfill. Compact in level layers approximately 100 - 150 mm thick, laid back into the slope at about 5°. If possible, add water while compacting the material.

9 Complete the main trim. Then go back to the top of the slope and work down again, carrying out the final trim. This should give a clean, smooth surface, good enough for vegetation to be planted on.

10 Check the final trim line. If protrusions or indentations remain, go back and re-do those parts; if the profile is satisfactory, clean all debris off the slope finally and tidy up.

11 Dispose of excess spoil safely (see below).

Trimming should be done logically, to remove as much or as little as necessary. On this slope, the weaker material has been trimmed ready for grass planting, while a hard rock outcrop has been left intact

Spoil disposal

However much care is taken to minimise quantities of spoil, it cannot be eliminated altogether. Controlling the disposal of spoil is very important, because it can give rise to a variety of problems, including:

· erosion of the spoil tip itself;

· the smothering or removal of natural vegetation. Once stripped of plant and soil cover, slopes usually take three to five years to re-vegetate, and as many as 10 years on steeper and more sterile slopes;

· instability within the spoil material itself, especially when infiltrated by water;

· overloading and resultant failure of the slope;

· disruption of existing runoff patterns and siltation of water courses and drainage channels;

· disruption to agricultural practices.

Even on valley alignments, spoil tipping must be carefully controlled. This debris should have been pushed down to the river; left like this, it surcharged the slope and contributed to a slide

You can minimise spoil problems by taking two steps. The first is to identify those operations that will generate spoil, the places where it will be generated and the quantities involved, no matter how small. The second is to plan for its disposal by designating safe tipping sites.

You are responsible for designating suitable sites, and your criteria for their selection should aim to avoid the problems listed above. When construction is being undertaken through a conventional construction contract, you should ensure that both the contractor and the construction workforce are aware of the restrictions on the disposal of spoil, the location of approved spoil disposal sites and specific requirements for the management of these sites. Strictly enforce contract specifications regarding spoil disposal.

You may choose either to discard spoil, or to turn it into landfill. Observe the following guidelines:

· when you are creating a landfill site, make maximum use of terraces, level ground and spurs;

· if spoil tipping has to be done on steep slopes, select areas formed in resistant bedrock. Tipping should result in no more than the removal of vegetation and shallow soil, with negligible slope incision thereafter. Bitumen drum disposal chutes can be used to convey the spoil down a short slope to a safe site below;

· build many small spoil benches rather than a few large ones, to avoid slope overloading;

· provide a drainage blanket beneath a spoil bench where there is any indication of a spring seepage at or near the spoil site;

· compact spoil benches during construction. While benches cannot be compacted in the formal sense, you can construct them in definite lifts normally not more than 0.5 m thick, with the top surface of each lift approximately horizontal. This will allow machines involved in spreading the spoil to track the surface and provide some degree of compaction;

· where spoil benches are constructed on agricultural land, form the tip into a benched profile so that it can eventually be returned to agricultural production. In the meantime, the risers between levels must be protected against erosion by applying vegetation or constructing dry stone walls;

· where the top surface of the bench is large, reduce runoff by providing regular shallow interceptor drains. The slope of these drains should be constant as far as is practicable and should not be so steep as to induce erosion;

· on completion, leave spoil benches in their required shape and plant them with grasses, shrubs and trees to encourage maximum stability and resistance to erosion.

Do not permit the following:

· tipping of spoil into stream channels other than major rivers, as the increased sediment load will lead to scour and siltation downstream;

· tipping of spoil on to slopes where road alignments, housing areas or farmland downslope might be affected;

· use of areas of past or active instability and erosion as tip sites, unless they are at least 50 metres from the road;

· the discharge of runoff over the loose front edge of a tip bench during or after construction;

· tipping of spoil in front of road retaining walls, where impeded drainage could soften the wall foundation.

Careless spoil disposal on long, steep slopes can cause very extensive damage

Figure 1.15: Checklist to assess the quality of bio-engineering site works

Step 16: Implement the civil engineering works

Dressing stone during the construction of dry stone dentition

Civil engineering works must be completed before the start of the rainy season. This usually disrupts work seriously from Jestha (May-June) onwards. Hence the calendar in Figure 1.4 (step 15) suggests carrying out site preparation works in Magh (January-February), and implementing the civil engineering works between Falgun and Baisakh (mid February to mid May).

All works must be carried out to a high standard. For this it is necessary to ensure that adequate site supervision and monitoring are provided.

Step 17: Implement the bio-engineering works

The actual implementation of bio-engineering site works normally begins in Ashad (late June). However, this depends on the onset of reliable monsoon rains. In the east of Nepal it may be slightly earlier, perhaps even in Jestha; and in the far west a little later, perhaps not until the second half of Ashad. The start should usually coincide with the time when farmers start to plant rice on non-irrigated khet land in the local area.

As usual, it is necessary to provide adequate site supervision to ensure that the works are carried out to the highest possible standard. Section 3 gives the construction steps for all bio-engineering techniques.

Step 18: Monitor the works

Check that the works have been completed to a high standard on the site. Figure 1.15 gives a simple checklist for assessing the quality of bio-engineering works. It is not fully comprehensive, but gives the main indicators to look for.

If the plants are being attacked by animals, or are likely to be, provide protection. Move on to step 19 to plan the maintenance inputs required by each site.

Grass planting on an eroded landslide scar. The supervisor is monitoring the works closely

Step 19: Maintain the works

The maintenance of bio-engineering sites is part of roadside support maintenance. This is split between routine and preventative maintenance activities.

The maintenance of roadside vegetation should be planned to ensure that maximum benefit is attained from the existing infrastructure. Most maintenance activities have to be carried out at a specific time of year. You should consider each site independently because maintenance interventions are site-specific for each slope in each roadside area.

In order to plan roadside support maintenance carefully, it is necessary to follow these steps.

(a) Devise a schedule of checks for all roadside support maintenance activities (i.e. list the maintenance tasks and the intervention times).

(b) Devise a schedule of sites for each check.

(c) Carry out the checks punctually at the allotted times for every selected area.

(d) Monitor the programme to ensure that the maintenance takes place as required.

The calendar in Figure 1.16 summarises the timing for the recommended bio-engineering maintenance operations. (Full details of bio-engineering maintenance are given in Section 5.)

WHERE TO FIND MORE INFORMATION

In this site handbook, more information is given on each step. However, because of the large amount of information, the handbook is split up into a series of technical sections from this point on. The main sections are as follows.

Section 2 Civil engineering techniques: design features of the main civil engineering works and construction details of the smaller scale techniques not covered by other manuals.

Section 3 Bio-engineering techniques: construction details of all the bio-engineering techniques used by the Department of Roads.

Section 4 Plant production and nurseries: full practical information about the propagation of plants for bio-engineering and the management of nurseries.

Section 5 Maintenance of bio-engineering sites: practical guidelines on every maintenance task under routine and preventative off-road (or roadside support) maintenance related to vegetation.

In addition, the Reference Manual contains a great deal of supporting information.

Figure 1.16: Calendar of bio-engineering maintenance operations

Roadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.) Section Two - Civil engineering techniques (introduction...) 2.1 Retaining walls 2.2 Revetment walls 2.3 Prop walls/Dentition 2.4 Check dams 2.5 Surface and sub-surface drains 2.6 Stone pitching 2.7 Wire Bolster Cylinders 2.8 Other civil engineering techniques

Roadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.)

Section Two - Civil engineering techniques

Figure

This section outlines the main civil engineering structures used for slope stabilisation and erosion control in conjunction with bio-engineering:

· retaining walls (Section 2.1);
· revetments (Section 2.2);
· prop walls/dentition (Section 2.3);
· check dams (Section 2.4);
· drains (Section 2.5);
· stone pitching (Section 2.6);
· wire bolster cylinders (Section 2.7);
· other civil engineering techniques: notes on their use (Section 2.8).

This section describes the main features of civil engineering structures and the ways in which they may be integrated with bio-engineering techniques. It does not give full design details of structures such as retaining walls and check dams but provides references to sources of further information.

Techniques that have been tried extensively in the Nepal road sector but rejected by the Department of Roads (e.g. waterproof slope covers and non-living wattle fences) are not included in this section. The reasons for their rejection are given in the Reference Manual.

2.1 Retaining walls

Functions

Retaining walls help to support mountainside slopes, or support the road or slope segments from the valley side. They are designed to stop an active earth pressure. Toe walls are normally considered to be a type of retaining wall found at the base of a slope or segment of slope.

Sites

Any slope where there is a problem of deep-seated (> 500 mm) instability, or where the steepness of the slope makes benching impractical.

Practical features

· Use dry masonry in every case where it is applicable (see special features of dry masonry walls below). Only use other types of wall when you are certain you need greater strength and can justify the additional cost.

· Careful design and supervision of foundations are of paramount importance.

· While excavating foundations, remove debris to a safe location. Do not allow it to be thrown down the slope.

· In most locations, solving the drainage problem is a major difficulty. Therefore consideration should always be given to using the best-drained of structures.

· In bound masonry and reinforced concrete walls, weep holes of a minimum width of 75 mm, sloping downwards, should be given every one metre along and up the wall. There should be a line of weep holes along the wall at the lowest level at which it can be drained.

· Backfilling is critical: many walls are not backfilled and so retain nothing but air! Always ensure that retaining walls are properly backfilled and compacted in layers. Place a drainage blanket of aggregate with a porous membrane of filter fabric (geotextile if possible; but otherwise hessian) over weep holes or drainage areas.

· Once construction is complete, ensure that the slopes around the structure are tidied up and treated using appropriate bio-engineering measures. All surplus debris must be removed, or it will encourage the development of erosion.

A dry masonry retaining wall with a scupper culvert

Figure 2.1: Comparison of retaining wall types*

* Despite these general criteria, design must always be site specific rather than based on a 'typical' design.

Figure 2.1 compares the main types of retaining wall.

Integration with bio-engineering

Bio-engineering techniques should be used in conjunction with retaining walls, according to site characteristics, as follows:

· Protection of backfill.
· Protection from scour and undercutting of the foundations and sides.
· Flexible extension to the wall by planting large bamboos, shrubs or trees above the wall, increasing the catch function (refer to Sections 3.9 and 3.7 for details of these techniques).

Further information

There is an entire chapter on road retaining walls in TRL Overseas Road Note 16, Principles of low cost road engineering in mountainous terrain (chapter 11, Road retaining walls, pages 116 to 126). The design of retaining walls is also covered in most geotechnical engineering text books.

2.2 Revetment walls

Function

Revetment walls are constructed to protect the base of a slope from undermining or other damage, such as grazing by animals. They give only protection, not support, and are not used on large, unstable slopes, where substantial retaining structures may be required. Breast walls are normally considered to be types of revetment.

Sites

Along the base of inherently stable cut slopes where seepage erosion can destabilise the base of large slopes; along the foot of abandoned spoil tips which have reached their angle of repose; along the foot of large fill sites.

Practical features

· Excavate a foundation at the foot of the slope until you find a sound layer to build on.

· Construct walls of freely drained materials wherever possible, such as dry stone masonry or gabions.

· If using cement-bound masonry, include weep holes to drain water from behind the wall and reduce hydrostatic pressure. Weep holes should have a minimum width of 75 mm, slope downwards, and be constructed every one metre along and up the wall. There should be a line of weep holes along the wall at the lowest point at which it can be drained.

· The back face should be vertical; the front face should slope back at the rate of 330 mm horizontally per metre of height (a gradient of 3:1);

· The ends of the wall should turn in to meet the slope, and should be raised about 250 mm: water falling on them will then run down over the wall and not scour the ends.

· If there is a risk of people or animals damaging the top of a dry stone wall, provide a capping beam of cement-bound masonry.

· Once the wall is complete, finish backfilling behind it and compact the fill thoroughly at a steep angle (at least 30°) to rejoin the original slope as high up as possible; plant into the fill as soon as you can.

Figure 2.2: A guide to the dimensions of dry masonry retaining walls on different slopes

Integration with bio-engineering

A revetment toe wall is protected by grasses planted on the slope above

Bio-engineering techniques should be used in connection with revetment and toe walls as follows:

· Protection of backfill.
· Protection from scour and undercutting of the foundations and sides.
· Flexible extension above the wall by the use of large bamboo or shrub and tree planting, increasing the catch function: refer to Sections 3.9 and 3.7 for details of these techniques.

Further information

Revetments are covered in TRL Overseas Road Note 16, Principles of low cost road engineering in mountainous terrain (page 136), with typical design diagrams given on page 137.

2.3 Prop walls/Dentition

Function

The term 'prop wall' also covers support walls and dentition. On very steep cut slopes, prop walls are used to support blocks of harder rock where they are underlain by softer rock bands. Where differential weathering occurs due to variations in adjacent strata, large segments of slope can become destabilised by a soft rock band eroding away underneath it. This presents two options: either remove all the material above or support it with a prop wall.

Prop walls do not usually offer total support to the full weight of all slope material above. Rather, they stop the erosion of softer bands below harder bands supported on them.

Sites

Only on steep cut slopes. Anywhere that a large slope-trimming job can be avoided by installing a relatively small wall. This technique is particularly useful in bands of alternating hard and soft rocks, such as are common in the Churia ranges.

Practical features

· Excavate a foundation on a band of rock that is as hard as possible: this must be underneath the band that is being replaced by the prop wall and must show evidence of much greater resistance to erosion.

· Using dressed stones and a cement: sand mixture of 1:4 or 1:3, build a cement masonry wall following the line of the slope.

· For sections less than 2 metres high only, use dry stone masonry built with well-dressed stones.

· In cement-bound masonry, weep holes should be at least 75 mm in diameter, sloping downwards and should be installed every 500 mm (horizontally as well as vertically); they must also be included in the lowest level of masonry.

· Normally, the bound masonry wall should be no more than 500 mm thick; if support deeper than this is required, it can be provided by careful dry stone packing behind the masonry wall. There must be no cavities allowing collapse of even very small areas behind the wall.

· When the lower surface of the material to be supported is reached, it is important to pack in the stones and mortar very tightly; the whole wall is useless if the last millimetre is not solidly completed.

Integration with bio-engineering

Prop walls are usually used to support bands of harder strata and so there is usually a limited scope for close integration with bio-engineering techniques. However, where conditions give rise to a need for additional protection, bio-engineering techniques should be used as follows:

· Protection from scour and undercutting of the foundations and sides.

Further information

Prop walls are covered in TRL Overseas Road Note 16, Principles of low cost road engineering in mountainous terrain under retaining structures (chapter 11, pages 116 to 126 and revetments (pages 136 and 137).

Constructing a cement masonry dentition wall to prevent undercutting of the upper part of a steep cut slope in differentially weathered gneiss

2.4 Check dams

Function

Check dams are simple physical constructions to prevent the downcutting of runoff water in gullies. They ease the gradient of the gully bed by providing periodic steps of fully strengthened material. Check dams are designed to accept an active pressure if it applied in the future, while permitting a safe discharge of water (and perhaps debris) via a spillway.

Sites

Any loose or active gully. In any rill that threatens to enlarge. In general, anywhere on a slope where there is a danger of scour from running water.

Practical features

· Choose locations for the check dams so that the maximum effect can be achieved using the minimum possible volume of construction. Refer to the box on the spacing of check dams.

· Excavate a foundation in the gully bed until you find a sound layer to build on. The base of the dam should be at least 660 mm thick if it is one metre high; for every additional metre of height, add a further 330 mm to the width.

· Construct the check dam using the best-drained and most cost-effective materials. If possible, use dry stone masonry or gabions to improve drainage. If this will not work, use concrete-bound mortar.

· If using concrete-bound masonry, include weep holes to drain water from behind the check dam and reduce hydrostatic pressure.

· The ends of the dam should be keyed right into the gully sides and should be raised at least 250 mm to form a central spillway or notch: this ensures that water coming over the dam will then run down the middle and not scour the ends.

· An apron must be provided below the dam to ensure that energy is dissipated and that flow continues in the centre of the gully below the check dam.

· If there is a risk of people or animals damaging the top of the dam, or if it is in a gully likely to take a large flow of water, point the top layer with cement mortar.

· Once the construction of the check dam is completed, backfill behind the wings and sides, and compact the fill thoroughly.

Small check dams at frequent intervals are effective, even in very steep gullies

Integration with bio-engineering

Bio-engineering techniques should be used in connection with check dams as follows:

· Protection of backfill and gully floor above check dam.
· Protection from scour and undercutting of the foundations and sides.
· Construct live check dams between civil check dams, to reduce water velocity in the gully and improve stability (refer to Section 3.12); or line the gully bed with vegetated stone pitching (refer to Section 3.14 for details of this technique).

Further information

Check dams are discussed in TRL Overseas Road Note 16, Principles of low cost road engineering in mountainous terrain (page 108), with typical design diagrams given on pages 110 and 111.

Stone-pitched surface drains on a wet landslide scar

2.5 Surface and sub-surface drains

Function

Surface drains are installed in the surface of a slope to remove surface water quickly and efficiently. Surface-water drains often use a combination of bio-engineering and civil engineering structures.

Cascades are surface drains designed to bring water down steep sections of slope.

Sub-surface drains are installed in the slope to remove ground water quickly and efficiently. In practice they can be installed to a maximum of 1.0 to 1.5 metres (although the design depends on site conditions). Sub-surface drains are usually restricted to civil engineering structures, and do not normally use bio-engineering measures. However, bio-engineering techniques can be used to strengthen the slope around the drain.

Sites

Any site less than 35°. Certain drain types can be used on slopes up to 45° (e.g. drains constructed using gabion wire or concrete-bound masonry). Cascades are normally used on slopes steeper than 45°.

The same site, two years later, following establishment of the protective grass cover between the drains

Figure 2.3: Surface drains and cascades, and sub-surface drains: design and integration with bio-engineering (all drainage systems are assumed to be dendritic)

Figure 2.3: Surface drains and cascades, and sub-surface drains: design and integration with bio-engineering (all drainage systems are assumed to be dendritic) continued

Practical features

· Always design drainage systems to run along natural drainage lines. Choose locations for the drains so that the maximum effect can be achieved using the minimum possible volume of construction.

· Always ensure that drain outfalls are protected against erosion.

· Only use a rigid geometrical pattern of drains on newly formed fill slopes where there are no clear natural drainage lines.

· Excavate a foundation until a sound layer to build on is located. Drains must be well founded like all other civil structures.

· Run main drains straight down the slope. Feed side drains in on a herringbone pattern.

· Never use contour drains: these block very easily and are also highly susceptible to subsidence. A blocked or cracked drain can create terrible damage as a result of concentrated water flow.

· Design and construct the drains in such a way that water can enter them easily on the higher side but not seep out on the lower side. Use weep holes and thick (³ 20 gauge), black polythene membranes carefully to achieve this.

· A flexible design is usually an advantage. Concrete masonry can be easily cracked by the slightest movement in the slope, and then leakage problems result.

· If there is a risk of people or animals damaging the drain, make sure that the construction is strong enough (e.g. use gabion rather than dry stone construction).

· Once the drain is completed, backfill around it and compact the fill thoroughly.

· Apply appropriate bio-engineering measures to enhance the effectiveness of the drain.

· Where the site requires deeper drainage and the machinery is available, drains can be drilled into the slope.

· Figure 2.3 gives comparison details of the main drain and cascade types.

A gabion cascade in heavy rain. This type of structure can transport large volumes of water down steep slopes without damage

Further information

Surface drainage on slopes is covered on pages 136 to 139 of TRL Overseas Road Note 16, Principles of low cost road engineering in mountainous terrain.

Sub-surface drainage on slopes is covered on pages 139 to 140 of TRL Overseas Road Note 16, Principles of low cost road engineering in mountainous terrain.

2.6 Stone pitching

Functions

A slope is armoured with stone pitching. This gives a strong covering. It is freely drained and will withstand considerable water velocities.

Note that in Section 3.14, among the bio-engineering techniques, full details are given of vegetated stone pitching: that is a stronger form of stone pitching, with emphasis given to its strengthening by vegetation.

Sites

Any slope up to 35°. This technique is particularly useful on slopes with a heavy seepage problem, in flood-prone areas or where vegetation is difficult to establish, such as in urban areas. It is also useful on gully floors between check dams and for scour protection by rivers.

Materials

· Boulders;
· Tools for digging and for dressing stones.

The largest available stones should be used which permits pitching to be done effectively on the site. The stones used should have one large flat side, and should be of equal size and angularity.

Spacing

Stone pitching effectively gives a complete surface cover.

Construction steps

1. Prepare a sound slope before constructing the stone pitching; it must be free of loose debris and topsoil, and trimmed to an even surface.

2. Bed the stones down well into the slope surface. Excavate as necessary to ensure an even upper surface to the stone pitching.

3. Build the stone pitching carefully, with the stones fitted together firmly, as if it is a dry masonry wall. Stones should be perpendicular to the slope, with the main point or narrow side down.

4. In drains and gullies, a rough surface can be left to retard water flow.

5. For further strengthening it is best to plant grasses or the hardwood cuttings of shrubs through the stone pitching (see below and Section 3.14).

6. Other options for strengthening are either to use a gabion mattress (of 0.3 to 0.5 metre thickness) instead of dry stone pitching; or to use cement mortar (but this can impede drainage).

Integration with bio-engineering

For the best effects, bio-engineering techniques should be used in connection with stone pitching as follows.

· Strengthening of stone pitching: plant grass slips in the gaps between stones: see Section 3.14.

· Increased strengthening of stone pitching: insert live cuttings of shrubs into the gaps between stones: see Section 3.14.

Maintenance

If stones are displaced, the pitching should be repaired as soon as possible. Otherwise the maintenance depends on the type of bio-engineering used (refer to the relevant part of Section 5 for details).

Main advantages

Stone pitching forms a strong and long-lasting method of reinforcing a slope surface and stopping gully development.

Main limitations

Stone pitching is relatively expensive in comparison with bio-engineering measures such as brush layers.

2.7 Wire Bolster Cylinders

Function

Wire bolster cylinders (in cross-section, a gabion tube of 300 mm diameter filled with stone) are laid in shallow trenches across the slope. They prevent surface scour and gullying (by reinforcing and fulfilling an intermittent armouring function), and provide shallow support. Bolsters can be laid in two ways: (1) along the contour; or (2) in a herringbone pattern (¬¬¬¬¬) to double as a surface drainage system.

Sites

On most long, exposed slopes between 35° and 50° where there is a danger of scour or gullying on the surface. Contour bolsters are used on well drained materials; slanted (herringbone pattern) bolsters are used on poorly drained material where there is a risk of slumping.

Constructing a wire bolster (see construction step 5, overleaf)

Completed wire bolster cylinders on a steep colluvial slope

Materials

· Woven gabion panels;

· 16 mm rebar or high yield steel rod cut into 2 m lengths;

· Boulders:

for contour bolsters, angular,
smallest dimension > 100 mm;
for herringbone bolsters, rounded,
smallest dimension > 100 mm;

· Tools for digging trenches and for working with gabion wire;

· Sledge hammers.

· For herringbone bolsters, thick (³ 20 gauge), black polythene sheet to line the trenches.

Gabion bolster panels are normally 5 m × 1 m. Where larger bolsters are required 5 m × 2 m panels can be woven. They are made on a conventional gabion weaving frame but with a smaller mesh than usual: this is normally 70 × 100 mm, triple twist. Heavy coated 10 SWG wire is used for the border and 12 SWG for the mesh.

Spacing

Contour bolsters are normally spaced as follows,

slope < 30°: 2000 mm centres;
slope 30 - 45°: 1500 mm centres.
Herringbone bolsters are placed at 1500 mm centres.

Construction steps (contour bolsters)

1 Trim the area to be treated to an even slope with no small protrusions or depressions which will interfere with the bolsters.

2 Starting about 2 metres from the bottom of the slope, mark out a contour line across the slope with the aid of a spirit level.

3 Dig a trench along the line: the trench should be about 300 mm wide and 300 mm deep (Figure 2.4; a).

4 Lay a gabion bolster panel lengthways along the trench: make sure the edge of the panel on the lower side is flush with the edge of the trench.

5 Fill the bolster with stones larger than the mesh size (Figure 2.4; b, c).

6 Fold the upper edge of panel over the stones and join it to the lower panel edge. Leave a 100 mm flap from the upper edge extending over the lower edge (Figure 2.4; d).

7 Join abutting bolsters: form the bolsters into a continuous line across the slope and close the extreme ends with wire.

8 Backfill the material around the bolsters, compact it and clean away surplus debris.

9 Drive steel bars into the ground at right angles to the slope every 2 metres along the bolsters. Position them immediately below and touching the bolsters, and drive them in far enough so that they cannot be pulled out by hand (Figure 2.4; e).

10 Cover remaining site: repeat steps (2) to (9) up the slope at the spacing required until the area is covered.

11 Starting from the top of the slope, clean away surplus debris and make sure that backfill is complete and firm.

12 Implement bio-engineering works throughout the site.

Figure 2.4: Wire bolster construction

Construction steps (herringbone bolsters)

1 The site to be treated should first be trimmed to an even slope: there should be no protrusions or depressions that will interfere with the bolsters; loose rocks should be removed if possible;

2 Starting about 2 metres from the bottom of the slope, mark out the lines for the bolsters; they should be at 45° to the line of the slope and each slanting piece should normally be 5 metres long (although the design must be flexible to take individual site conditions into account).

3 Dig trenches along the lines, about 300 mm wide and 300 mm deep.

4 Lay a sheet of black polythene along the bottom and lower side, but not the higher side, of the trench.

5 Lay gabion bolster panels lengthways along the trenches. The edge of the panel on the lower side should be flush with the lower edge of the trench.

6 Fill the bolster with stones larger than the mesh size. Stones must not be packed carefully on top of each other as this reduces water flow: instead, they must be poured in from above and packed firmly but at random within the mesh.

7 Fold the upper edge of the panel over the stones and join it to the lower panel edge; at the end of the pattern, where the slanting lines meet, the bolster ends should be closed over with wire but not joined to adjacent patterns: this is so that each stack of V patterns can fail without affecting the pattern next to it.

8 Repeat steps 3 to 7 at 1.5 metre intervals, installing a series of bolsters up the slope.

9 Once the slanting lines are complete, dig a trench straight down the slope and install a 'vertical' bolster in it to collect water from the bottoms of each V, and run it to the base of the slope. Tie the herringbones or ribs to the spine.

10 Backfill the material around the bolsters, compact it and clean away surplus debris as necessary.

11 Drive mild steel bars into the ground at right angles to the slope every 2 metres along the bolsters: they should be positioned immediately below and touching the bolsters, and should be driven in far enough that they cannot be pulled out by hand.

12 Implement bio-engineering works throughout the site.

Integration with bio-engineering

The spaces between the bolsters should be treated with appropriate bio-engineering as soon as the subsequent rains have broken, as follows:

· Between wire bolster cylinders: plant shrub and small tree seedlings at 1000 mm centres throughout the slope treated, according to site characteristics and as determined by the instructions in Section 1.2.

· If a more complete surface protection is required, the surface can be planted or seeded with grass between the wire bolster cylinders, using the techniques described in Sections 3.1 to 3.5, according to the site requirements described in Section 1.2.

Maintenance

Maintain the bio-engineering works according to the needs of the particular treatment used.

If rills develop between the bolsters and threaten to undercut them, small-scale stone dentition should be used to support undermined places and stop scour erosion. In extreme cases, fully stone-lined gullies can be made between bolsters in order to shed large amounts of accumulated runoff without damaging the slope.

Main advantages

Bolsters form the strongest and longest-lasting method of armouring a slope surface and preventing gully development.

Main limitations

Bolsters are relatively expensive in comparison with bio-engineering measures such as brush layers.

2.8 Other civil engineering techniques

Wire netting

Function

Wire netting (usually gabion wire mesh) is spread over the surface of a rocky slope. This can be carried out to reduce the shedding of rock debris and slow the degradation of the surface.

Sites

Slopes composed of hard rock with a degree of fragmentation leading to the occasional shedding of debris particles larger than about 100 mm.

Comments

The main difficulty with this technique is fixing the wire mesh to the face of the slope. This is normally done by hammering steel pegs into rock cracks or by cementing them into depressions. If this is done satisfactorily, then it can be a very robust measure.

If the slope has not been trimmed well in advance, the accumulation of loose boulders behind weakening wire netting could release a bigger and more dangerous load in one go, rather than a gradual shedding of individual boulders. But with a good maintenance regime this technique could be used to advantage.

Gunite (shotcrete)

Function

Gunite is a cement-stabilised aggregate sprayed on to a wire mesh slope covering. It can be used for surface armouring and to bind together the surface of weathered and fractured rock slopes.

Sites

This technique has potential on steep (> 50°) cut slopes less than about 30 metres in height.

Comments

This technique has been used successfully in Hong Kong and Malaysia. The main limitation is the difficulty of ensuring slope drainage through the covering, even when numerous weep holes are provided: it is generally considered to be inappropriate on slopes with high groundwater seepage rates, such as are common in Nepal. The expense is also a limiting factor.

Chunam

Function

Chunam is a lime-based plaster applied as a slurry across the surface of a slope. It waterproofs and supports the immediate surface, armouring against scour of the surface and weathering of the material below. It can be reinforced using wire mesh already attached to the surface.

Sites

This technique has potential on steep (> 50°) cut slopes less than about 30 metres in height.

Comments

This technique has been successful in limited areas only. It is essential to install drainage or weep holes, sloping towards the outside of the material. Even with these, many surfaces have failed when treated with chunam because too much water has percolated behind the surfacing from higher up the slope and it has flaked away from the less weathered material behind. The best successes might be achieved in naturally dry sites, where hard chunam facings can stop surface erosion during heavy rains.

Cement slurry

Function

A watery cement slurry is poured into the ground. It percolates along pores and gaps in the material. When it sets, it binds the material together and increases the cohesive strength. The main engineering function is to reinforce.

Sites

This technique has potential in highly permeable debris materials, such as colluvium with a low proportion of fines.

Comments

There are no records of the widespread use of this technique in Nepal, but it has been used occasionally on colluvial slopes above roads. It is probably only worth using on the very porous materials described above, where there is adequate void space to absorb a critical quantity of slurry. Its best application is to reinforce material immediately upslope from a retaining structure which is considered too weak, such as a gabion wall which has bulged: in this situation, it may avoid the necessity of replacing one threatened structure with a stronger one. It could also be used in a purpose-designed situation, where there is inadequate space to construct a wall of the desired thickness, and where cement stabilisation of the retained debris can increase the factor of safety satisfactorily. In emergencies, it might be used shortly before the start of the monsoon rains, when there is not enough time to build a normal structure.

Reinforced earth

Function

A proprietary or purpose-designed material is laid at intervals into debris, which is built up in layers to form a slope of earth strengthened with the reinforcing material. The result is intermediate in strength between a straightforward fill slope and a retaining structure. The main engineering function is to reinforce.

Sites

This technique is best used on fill slopes, where the design angle is relatively low and the major disturbance involved in construction can be accomplished more easily.

Comments

There are various proprietary systems of reinforced earth, but there are no records of them being tried for slope stabilisation in Nepal. A form of earth reinforcement could be undertaken using a material such as gabion mesh laid into the slope at intervals as it is backfilled. However, reinforced earth systems present complex slope stability calculations, and the standard retaining structures are preferable in most cases.

Further information

Reinforced earth structures are covered in TRL Overseas Road Note 16; Principles of low cost road engineering in mountainous terrain (page 122), with typical design diagrams given on page 120.

Soil nailing

Function

Tensile strengthening is added to the slope in the form of steel bars inserted into the soil (or surface layers). Insertion is possible to a maximum depth of about 5 metres.

Sites

Any slope that is liable to creeping planar mass failures and where access for machinery is feasible. It is not effective against erosion or many shear failures.

Comments

There are two main methods. One uses a procedure of drilling and grouting, and the other hammers the nails in, normally using a mechanical percussion device. Both have advantages in certain situations, but both require machinery to gain access to the slope. The high cost of the technique makes its usefulness in Nepal dubious compared with the established systems of slope stabilisation.

Further information

Information is available from the companies offering proprietary systems of soil nailing, and their agents. No proven examples of this technique are known in Nepal.

Roadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.) Section Three - Bio-engineering techniques (introduction...) 3.1 Planted grass lines: contour/horizontal 3.2 Planted grass lines: downslope/vertical 3.3 Planted grass lines: diagonal 3.4 Planted grasses: random planting 3.5 Grass seeding 3.6 Turfing 3.7 Shrub and tree planting 3.8 Shrub and tree seeding 3.9 Large bamboo planting 3.10 Brush layering 3.11 Palisades 3.12 Live check dams 3.13 Fascines 3.14 Vegetated stone pitching 3.15 Jute netting (standard mesh) 3.16 Jute netting (wide mesh) 3.17 Mulching 3.18 Vegetated gabions 3.19 Live wattle fences 3.20 Hydro-seeding

Roadside Bio-Engineering - Site Handbook (DFID, 1999, 160 p.)

Section Three - Bio-engineering techniques

Figure

This section gives details of the design and construction of the main bio-engineering systems used for stabilising slopes and controlling erosion. These are:

· grass planting, seeding and turfing (Section 3.1 to 3.6);

· shrub and tree planting and seeding (Sections 3.7 and 3.8);

· large bamboo planting (Section 3.9);

· brush layering (Section 3.10);

· palisades (Section 3.11);

· live check dams (Section 3.12);

· fascine constructions (Section 3.1 3);

· vegetated stone pitching (Section 3.14);

· jute netting: detailed information on its use and construction: standard mesh (Section 3.15) and wide mesh (Section 3.16);

· mulching: detailed information on the use of mulch as an aid to bio-engineering (Section 3.1 7);

· vegetated gabions (Section 3.18);

· live wattle fences (Section 3.19);

· hydro-seeding (Section 3.20).

Details of all of the civil engineering measures used in combination with these bio-engineering systems are given in Section 2.

3.1 Planted grass lines: contour/horizontal

Planting grass lines. Use a planting bar to make holes just big enough for the roots

Function

Grass slips (rooted cuttings), rooted stem cuttings or clumps grown from seed are planted in lines across the slope. They protect the slope with their roots and, by providing a surface cover, reduce the speed of runoff and catch debris, thereby armouring the slope. The main engineering functions are to catch, armour and reinforce.

Sites

Almost any slope less than 65°. This technique is mostly used on dry sites, where moisture needs to be conserved. It is most widely used on well-drained materials where increased infiltration is unlikely to cause problems. On cultivated slopes less than 35°, horizontal lines planted at intervals across the field can be used to avoid loss of soil and to help conserve moisture, as a standard soil conservation measure. Planted grass lines at intervals are essential if cultivation has to be carried out on slopes greater than 35°.

Place the grass into the hole, taking care not to tangle the roots or have them curved back to the surface

Materials

· Grass plants raised in a nursery or cuttings obtained elsewhere;
· Short planting bars;
· Line string;
· Spirit level;
· Tape measure (30 metres);
· A means of transporting plants to site;
· Hessian and water to keep the roots moist;
· (Optional) Manure or compost.

Fill the soil in around them, firming it gently with your fingers

SPECIES SUITABLE FOR PLANTED GRASS LINES: CONTOUR/HORIZONTAL

Spacing

Line spacing depends largely on the steepness of the slope.

Within rows: plants at 100 mm centres (except padang and tite nigalo bans, which should be spaced at 500 mm centres)

Row spacings:

slope < 30°: 1000 mm;
slope 30-45°: 500 mm;
slope > 45°: 300 mm.

Where this technique is used on agricultural land, a compromise must be reached between ease of cultivation and reduction of soil and water movement. A vertical interval of 2 metres or more is generally adequate.

Construction steps

1 Prepare the site well in advance of planting. Remove all debris and either remove or fill in surface irregularities so that there is nowhere for erosion to start. If the site is on backfill material, it should be thoroughly compacted, preferably when wet.

2 Always start grass planting at the top of the slope and work downwards.

3 Mark out the lines with string, using a tape measure and spirit level. Make sure the lines run exactly as required by the specification, along the contour.

4 Split the grass plants out to give the maximum planting material. Trim off long roots and cut the shoots off at about 100 mm above ground level. Wrap the plants in damp hessian to keep them moist until they are planted. Remember that you will need two slip cuttings per drill (planting hole) if the grass is a fibrous rooting type (e.g. babiyo, kans, khar, phurke, etc.) but only one if it is rhizomatous (e.g. amliso, padang bans, etc.), and only one rooted stem cutting or seedling.

5 With a planting bar, make a hole just big enough for the roots. Place the grass into the hole, taking care not to tangle the roots or have them curved back to the surface. Fill the soil in around them, firming it gently with your fingers. Take care to avoid leaving an air pocket by the roots.

6 If compost or manure are available, scatter a few handfuls around the grasses. This is especially important on very stony sites, where compost or manure can help to improve early growth. You may have to incorporate it into the surface material to prevent it being washed off.

7 If it looks rather dry and there is no prospect of rain for a day or two, consider watering the plants by hand.

Maintenance

This normally involves:

Protection (check on Kartik 1);
Weeding (check on Shrawan 1, Bhadra 1 and Aswin 1);
Grass cutting (check on Poush 1).

Main functions

Contour grass lines catch material moving downslope. They also armour slopes on highly impermeable materials by retarding runoff and reinforcing slope materials.

Main limitations

Contour grass lines can increase the infiltration rate to the point of liquefaction on poorly drained materials, particularly on steeply sloping, fine-textured debris.

3.2 Planted grass lines: downslope/vertical

Function

Grass slips (rooted cuttings), rooted stem cuttings or seedlings are planted in lines running down the slope. They protect the slope with their roots, provide a surface cover and help to drain surface water. They do not catch debris. The main engineering functions are to armour, reinforce and drain. Using this technique, a slope is allowed to develop a semi-natural drainage system, gullying in a controlled way.

Sites

Almost any slope less than 65°. It is mostly used on damp sites, where moisture needs to be shed. It is also most widely used on poorly drained materials where an increase in infiltration can lead to liquefaction of the soil.

Materials

· Grass plants raised in a nursery or cuttings obtained elsewhere;
· Line string;
· Triangular set square or frame with a plumb line;
· Spirit level;
· Tape measure (30 metres);
· A means of transporting plants to site;
· Hessian and water to keep the roots moist;
· (Optional) Manure or compost.

Vertical grass lines allow a slope to develop a semi-natural drainage system, reducing infiltration and the likelihood of liquefaction of the soil

Spacing

If the site is a newly cut slope, then a simple geometrical pattern can be used. The normal spacing is as follows:

Within rows: plants at 100 mm centres (except padang and tite nigalo bans, which should be spaced at 500 mm centres)

Row spacings: 500 mm.

SPECKS SUITABLE FOR PLANTED GRASS LINES: DOWNSLOPE/VERTICAL

However, if a gully system has already partly developed, then the spacing is defined naturally. Lines of grass should not be more than 500 mm apart if possible and, if ridges are bigger, a series of small lines in a chevron pattern (<<<<<) is required to protect gaps. Careful supervision is required on site to ensure that all planted lines follow the direction of natural fall.

Construction steps

1 Prepare the site well in advance of planting. Remove all debris and either remove or fill in surface irregularities so that there is nowhere for erosion to start.

2 Always start grass planting at the top of the slope and work downwards.

3 Mark out the lines with string, using a tape measure. Use the spirit level, string and set square, or the frame to check the maximum line of fall. Make sure the lines run exactly as required by the specification, down the slope or drainage line.

4 Split the grass plants out to give the maximum planting material. Trim off long roots and cut the shoots off at about 100 mm above ground level. Wrap the plants in damp hessian to keep them moist until they are planted. Remember that you will need two slip cuttings per drill (planting hole) if the grass is a fibrous rooting type (e.g. babiyo, kans, khar, phurke, etc.) but only one if it is rhizomatous (e.g. amliso, padang bans, etc.), and only one rooted stem cutting or seedling.

5 With a planting bar, make a hole just big enough for the roots. Place the grass into the hole, taking care not to tangle the roots or have them curved back to the surface. Fill the soil in around them, firming it gently with your fingers. Take care to avoid leaving an air pocket by the roots. Mound the soil along the grass line, to encourage water to run mid way between the lines rather than close to the plant stems.

6 If compost or manure is available, scatter a few handfuls around the grasses. This is important on very stony sites, where it can help to improve early growth. You may have to incorporate it into the surface material to prevent it being washed off.

7 If it looks rather dry and there is no prospect of rain for a day or two, consider watering the plants by hand.

Maintenance

This normally involves:

Protection (check on Kartik 1);
Weeding (check on Shrawan 1, Bhadra 1 and Aswin 1);
Grass cutting (check on Poush 1).

If gullies develop, they must be controlled. If they become large enough to endanger the site, they must be checked with dry stone pitching and small check dams. If caught early enough, a few stones may be all that is required to create an armoured rill in which the runoff can safely pass. If allowed to grow too big, much more work will be required.

Main functions

Downslope grass lines provide the maximum amount of surface drainage by channeling runoff and minimising infiltration. They still armour against erosion and reinforce the slope.

Main limitations

On impermeable materials, runoff can become damaging. In drier sites, grass plants can suffer from drought due to the increased drainage. On some weak materials, rills can develop down the side of the plant line, damaging the grass slips and reducing their growth.

3.3 Planted grass lines: diagonal

Function

Grass slips (rooted cuttings), rooted stem cuttings or seedlings are planted in lines running diagon-ally across the slope. They armour the slope with their roots and by providing a surface cover. They have limited functions of catching debris and draining surface water. The main engineering functions are to armour and reinforce, with secondary functions to catch and drain. This technique offers the best compromise of the grass line planting systems in many situations.

Sites

Almost any slope less than 65°. It is mostly used on poorly drained materials where an increase in infiltration can lead to liquefaction of the soil. It is also useful on damp sites, where moisture needs to be shed. It should be used whenever there is doubt as to which grass line planting system should be used, as a result of uncertainties over site environmental characteristics or material properties.

Materials

· Grass plants raised in a nursery or cuttings obtained elsewhere;
· Line string;
· Tape measure (30 metres);
· Triangular set square or frame with a plumb line (optional);
· Spirit level (optional);
· A means of transporting plants to site;
· Hessian and water to keep the roots moist;
· (Optional) Manure or compost.

Spacing

If the site is a newly cut slope, then a simple geometrical pattern can be used. The normal spacing is as follows:

Within rows: plants at 100 mm centres (except padang and tite nigalo bans, which should be spaced at 500 mm centres)

Row spacings: 500 mm.

However, if a gully system has already partly developed, then the spacing is defined naturally. Lines of grass should not be more than 500 mm apart if possible and, if ridges are bigger, a series of small lines in a chevron (<<<<<) or herringbone (¬¬¬¬¬) formation is required to protect gaps.

Construction steps

1 Prepare the site well in advance of planting. Remove all debris and either remove or fill in surface irregularities so that there is nowhere for erosion to start.

2 Always start grass planting at the top of the slope and work downwards.

3 Mark out the lines with string using a tape measure. Make sure they run exactly as required by the specification, diagonally across the slope or towards drainage lines. It may help to use a spirit level and set square or frame to check the maximum line of fall.

4 Split the grass plants out to give the maximum planting material. Trim off long roots and cut the shoots off at about 100 mm above ground level. Wrap the plants in damp hessian to keep them moist until they are planted. Remember that you will need two slip cuttings per drill (planting hole) if the grass is a fibrous rooting type (e.g. babiyo, kans, khar, phurke, etc.) but only one if it is rhizomatous (e.g. amliso, padang bans, etc.), and only one rooted stem cutting or seedling.

5 With a planting bar, make a hole just big enough for the roots. Place the grass into the hole, taking care not to tangle the roots or have them curved back to the surface. Fill the soil in around them, firming it gently with your fingers. Take care to avoid leaving an air pocket by the roots.

6 If compost or manure is available, scatter a few handfuls around the grasses. If the site is very stony, this is important for improving early growth. You may have to incorporate it into the surface material to prevent it being washed off.

7 If it looks rather dry and there is no prospect of rain for a day or two, consider watering the plants by hand.

SPECIES SUITABLE FOR PLANTED GRASS LINES: DIAGONAL

Maintenance

This normally involves:

Protection (check on Kartik 1);
Weeding (check on Shrawan 1, Bhadra 1 and Aswin 1);
Grass cutting (check on Poush 1).

Main functions

Diagonal grass lines armour and reinforce slopes effectively, and can also drain and catch material moving down the slope. This system appears to combine the best features of both horizontal and vertical planting in the majority of sites.

Main limitations

Where the specific advantages of contour or downslope planting patterns are critical, diagonal planting should not be used. On certain very weak materials, small rills can develop down the slope.

3.4 Planted grasses: random planting

Function

Grass slips (rooted cuttings), rooted stem cuttings or seedlings are planted at random on a slope, to an approximate specified density. They armour and reinforce the slope with their roots and by providing a surface cover. They also have a limited function of catching debris. This technique is most commonly used in conjunction with standard mesh jute netting, where complete surface protection is needed on very steep, harsh slopes. In most other cases, however, the advantages of one of the grass line planting systems (i.e. contour, downslope or diagonal) offer better protection to the slope.

Sites

Almost any slope less than 60° that allows grass planting. Normally used only on sites where jute netting (standard mesh) has already been applied. This implies slopes steeper than 45° and less than 15 metres in length, where moisture is not a serious problem.

Materials

· Grass plants raised in a nursery or cuttings obtained elsewhere;
· Short planting bars;
· A means of transporting plants to site;
· Hessian and water to keep the roots moist;
· (Optional) Manure or compost.

Spacing

Plants should be at an average of 100 mm centres (i.e. 100 plants per square metre). No gap should exceed 200 mm.

Construction steps

1 Apply the jute netting (standard mesh) well in advance of the monsoon, as described in Section 3.15. Start the grass planting as soon as the rains allow. If the site has not been treated with jute netting, prepare it well in advance of planting: remove all debris and either remove or fill in surface irregularities so that there is nowhere for erosion to start.

2 Always start grass planting at the top of the slope and work downwards. Workers should stand on the pegs holding the netting, not on the netting itself.

3 Split the grass plants out to give the maximum planting material. Trim off long roots and cut the shoots off at about 100 mm above ground level. Wrap the plants in damp hessian to keep them moist until they are planted. Remember that you will need two slip cuttings per drill (planting hole) if the grass is a fibrous rooting type (e.g. babiyo, kans, khar, phurke, etc.) but only one if it is rhizomatous (e.g. amliso, padang bans, etc.), and only one rooted stem cutting or seedling.

4 With a planting bar, make a hole just big enough for the roots. Place the grass into the hole, taking care not to tangle the roots or have them curved back to the surface. Fill the soil in around them, firming it gently with your fingers.

5 Plant grasses at random over the surface, but aim for an average spacing of 100 mm centres (i.e. 100 plants per square metre). No gap should be greater than 200 mm.

6 If compost or manure is available, scatter a few handfuls around the grasses.

7 If it looks rather dry and there is no prospect of rain for a day or two, consider watering the plants by hand.

Maintenance

This normally involves:

Protection (check on Kartik 1);
Weeding (check on Shrawan 1, Bhadra 1 and Aswin 1);
Grass cutting (check on Poush 1).

Main functions

Random grass planting armours and reinforces slopes effectively. This is particularly the case when it is used in conjunction with standard mesh jute netting.

Main functions and limitations

Where the specific advantages of contour, downs-lope or diagonal planting patterns are critical, random planting should not be used.

SPECIES SUITABLE FOR PLANTED GRASSES: RANDOM PLANTING

3.5 Grass seeding

Function

Grass is sown directly on to the site. It allows easy vegetation coverage of large areas. This technique is often used in conjunction with mulching and jute netting to aid establishment. The main engineering functions are to armour and, later, to reinforce.

Sites

Almost any bare site with slopes up to 45°. Grass seeding is mostly used on well-drained materials, where increased infiltration does not give rise to problems.

Materials

· A supply of a carefully chosen grass seed;
· Tools to scarify the surface to be sown;
· Mulch (cut plant material) or hessian sheeting to cover the seed once sown (see Section 3.17).
· On slopes of 30° to 45°, wide mesh jute netting will be required to hold the mulch in place on the slope (see Section 3.16).

SPECIES SUITABLE FOR GRASS SEEDING

In grass seeding, spread the seeds or grass seed heads liberally over the slope. Ideally, the whole surface should be very lightly covered in seed material

Construction steps

1 Well in advance of the date of sowing, prepare the site. Remove all irregularities likely to allow slumps or gullies and clean loose debris away.

2 Immediately before sowing, scarify the surface of the slope. This means scratching the surface or carrying out basic cultivation to give a loose surface into which the germinating grass seeds can send their roots.

3 Start sowing from the top of the slope and work downwards. Spread the seeds or grass seed heads liberally over the slope. Ideally, the whole surface should be very lightly covered in seed material. An application rate of 25 grammes per square metre is normal.

4 Cover the seeds completely with a layer of mulch, made from cut herbs such as ban mara (Eupatorium adenophorum), or with hessian sheeting. A vegetation mulch is preferable. Wide mesh jute netting (150 mm × 500 mm mesh size) should be used to hold mulch on to the surface if the slope is greater than 30°.

Maintenance

This normally involves:

Protection (check on Kartik 1);
Weeding (check on Shrawan 1, Bhadra 1 and Aswin 1);
Grass cutting (check on Poush 1).

Main functions

Grass seeding armours surfaces effectively: it can be used to create an even cover over all surfaces. It reinforces slopes after a few years of growth.

Main limitations

This technique gives none of the structural advantages of grass slip planting. Plants take longer to develop from seeds than from slips. Very heavy rain in the days immediately after sowing can lead to seeds being washed off the slope, or to damage to the very small seedlings.

3.6 Turfing

Function

Turf, consisting of a shallow rooting grass and the soil it is growing in, is placed on the slope. A technique commonly used on gentle embankment slopes. Its only engineering function is to armour.

Sites

This technique can be used on any gently sloping site (less than 30°). It is normally used on well-drained materials, where there is a minimal risk of slumping

Materials

· Flat shovel with a sharp edge to cut the turf;
· Old khukuri to cut the turf to shape;
· Water to keep the turf moist;
· Wooden rammer (mungro);
· If the slope to be turfed is greater than about 25°, wooden pegs about 300 mm long and 30 mm in diameter will be required.

SPECIES SUITABLE FOR TURFING