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

DESCRIPTION	DEPTH	MECHANISM*	FUNCTION REQUIRED
Rills and gullies form in weak, unprotected surfaces. Erosion should also be expected on bare or freshly prepared slopes.	Usually in the top 0.5 metre, but can become deeper if not controlled.	Erosion on the surface.	Armour, Reinforce, Catch.
Mass slope failure on a shallow slip plane parallel to the surface. This is the most common type of landslide, slip or debris fall. The plane of failure is usually visible but may not be straight, depending on site conditions. It may occur on any scale.	Frequently 0.5 metres or less below surface (or along a local discontinuity).	Planar sliding (translational landslide or debris slide).	Reinforce, Anchor, Catch, Drain.
Mass slope failure on a deep, curved slip plane. Many small, deep landslides are the result of this process. Large areas of subsidence may also be due to these.	Usually > 1.5 metres deep.	Shear failure (rotational landslide).	Anchor, Support, Drain.
Slumping or flow where material is poorly drained or has low cohesion between particles and liquefaction is reached. These sometimes look similar to planar slides, but are due to flow rather than sliding. The resulting debris normally has a rounded profile.	Frequently 0.5 metres or less below surface.	Slumping or flow of material when very wet.	Drain, Reinforce.
Collapse due to failure of the supporting material. This usually takes the form of a rock fall where a weaker band of material has eroded to undermine a harder band above. These are very common in mixed Churia strata.	0.5 to 2 metres in road cuts; deeper in natural cliffs.	Debris fall or collapse.	Reinforce, Support.

* 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

FUNCTION*	CIVIL ENGINEERING TECHNIQUE	BIO-ENGINEERING TECHNIQUE	COMBINATION OF BOTH
Catch	Catch walls	Contour grass lines or brush layers	Catch wall with densely planted shrubs
	Catch fences	Shrubs and large bamboo clumps	Catch wall with bamboo clumps planted above
Armour	Revetments	Mixed plant storeys giving complete cover	Vegetated stone pitching
	Surface rendering	Grass carpet	jute netting with planted grass
Reinforce	Reinforced earth	Densely rooting grasses, shrubs and trees	Wire bolster cylinders and planted shrubs or trees
	Soil nailing	Most vegetation structures	Jute netting with planted grass
Anchor	Rock anchors	Deeply rooting trees	Combination of soil anchors and deeply
	Soil anchors		rooting trees
Support	Retaining walls	Large trees and large bamboo clumps	Retaining wall with a line of large bamboo clumps
	Prop walls		planted above
Drain	Masonry surface drains	Downslope and diagonal vegetation lines	Herringbone-pattern wire bolster cylinders and angled grass lines
	Gabion and french drains	Angled fascines or brush layers	French drains and angled grass lines

* 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)

EXPECTED CONSEQUENCE IF THE SITE IS NOT TREATED	PRIORITY RATING
Slope movement threatens houses	Priority 1 (i.e. very high priority)
Slope movement threatens complete loss of road	Priority 2
Slope movement threatens partial loss of road	Priority 3
Slope movement threatens complete road blockage	Priority 3
Debris may fall on top of pedestrians or vehicles and cause injury	Priority 3
Slope movement threatens loss of productive farmland	Priority 4
Slope movement threatens blockage of drains	Priority 4

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

MECHANISM*	DESCRIPTION	DEPTH
Erosion on the surface.	Rills and gullies form in weak, unprotected surfaces. Erosion should also be expected on bare or freshly prepared slopes.	Usually in the top 0.1 metre, but can become deeper if not controlled.
Gully erosion	Gullies that are established in the slope continue to develop and grow bigger. Large gullies often have small landslides along the sides.	Usually in the top 0.5 metre, but can become deeper if not controlled.
Planar sliding (translational landslide or debris slide).	Mass slope failure on a shallow slip plane parallel to the surface. This is the most common type of landslide, slip or debris fall. The plane of failure is usually visible but may not be straight, depending on site conditions. It may occur on any scale.	Frequently 0.5 metre or less below surface (or along a local discontinuity).
Shear failure (rotational landslide).	Mass slope failure on a deep, curved slip plane. Many small, deep landslides are the result of this process. Large areas of subsidence may also be due to these.	Usually > 1.5 metres deep.
Slumping or flow of material when very wet.	Slumping or flow where material is poorly drained or has low cohesion between particles and liquefaction is reached. These sometimes appear afterwards like planar slides, but are due to flow rather than sliding. The resulting debris normally has a rounded profile.	Frequently 0.5 metre or less below surface.
Debris fall or collapse.	Collapse due to failure of the supporting material. This normally takes the form of a rock fall where a weaker band of material has eroded to undermine a harder band above. These are very common in mixed Churia strata.	0.5 to 2 metres in road cuts; deeper in natural cliffs.
Debris flow	In gullies and small, steep river channels (bed gradient usually more than 15°), debris flows can occur following intensive rain storms. This takes the form of a rapid but viscous flow of liquefied mud and debris.	The flow depth is usually 1 to 2 metres deep.

* 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).

Erosion and failure processes	List
Other factors	List
Slope angle(s)	3 classes: <30º, 30-45º, or>45º
Slope length	2 classes: 15 metres or>15 metres
Material drainage	2 classes: good or poor
Site moisture	4 classes: wet, moist, dry or very dry
Altitude	Determine: use an altimeter, map site drawing

Figure 1.8: The main physical factors affecting slopes

POTENTIAL FACTOR	DESCRIPTION
Fault lines	Small fault lines may cause differential erosion in parts of the site.
Springs	There may be seasonal springs within the site, which cause localised problems of drainage or slumping.
Slip planes	The main plane of failure may not be the only one. Many sites have secondary, smaller slip planes additional to the main failure mechanism
Large gullies	Large gullies nearby may erode backwards and damage the site. Alternatively, they may discharge, causing deposition on the site.
Landslides	Nearby landslides may extend headwards or sideways, or may supply debris on to the site.
River flooding	A large river below the site may flood badly, damaging the site by either erosion or deposition, or a combination of both.
River cutting	Rivers below the site may move in floods, undercutting the toe of the site.
Catchments	If there is an extended catchment area above the site, it could lead to a large discharge, which causes bad damage by erosion or deposition.
Drain discharge	The discharge of drainage water must be safeguarded to avoid causing erosion or mass failures. Poorly sited or inadequately protected discharge points can cause severe problems.
Khet and kulo	Khet (rice paddy) land or a kulo (irrigation channel) above a site usually means a large volume of water infiltrating into the slope, with a greater potential for failure or large-scale erosion.
Construction activities	Construction activities on or near the site may lead to undermining through excavations, or surcharging through spoil disposal in the wrong places.

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

MATERIAL DRAINAGE CHARACTERISTICS	TENDENCY TOWARDS GOOD DRAINAGE	TENDENCY TOWARDS POOR DRAINAGE
Overall drainage	Freely draining material; dries quickly after rain storms	Slowly draining material; tends to remain wet for long periods after rain; behaves like firm dahi (curd)
Soil particle size	Coarse textures; loams and sandy soils	Fine textures; clays and silts
Porosity	Large inter-connecting pores	Small pores
Material types	Stony colluvial debris; fragmented rock; sandy and gravelly river deposits	Residual soils of fine texture; debris from mud flows, slumps, etc; rato mato (red clay loam soil)
Slope types	Fill slopes; cut slopes in stony debris (colluvium)	Cut slopes in original consolidated ground

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.

Wet:	permanently damp sites (e.g. north-facing gully sites).
Moist:	sites that are reasonably well shaded or moist for some other reason.
Dry:	generally dry sites.
Very dry:	sites that are very dry; these are usually quite hot as well (e.g. south-facing cut slopes at low altitudes).

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

SITE MOISTURE FACTOR	TENDENCY TOWARDS DAMP SITES	TENDENCY TOWARDS DRY SITES
Aspect	Facing N, NW, NE and E	Facing S, SW, SE and W
Altitude	Above 1 500 metres; particularly above 1 800 metres	Below 1 500 metres; deep river valleys surrounded by ridges
Topographical location	Gullies; lower slopes; moisture accumulation and seepage areas	Upper slopes; spurs and ridges; steep rocky slopes
Regional rain effects	Eastern Nepal in general; the southern flanks of the Annapurna Himal	Most of Mid Western and Far Western Nepal
Rain shadow effect	Sides of major ridges exposed to the monsoon rain-bearing wind	Deep inner valleys; slopes sheltered from the monsoon by higher ridges to the south
Stoniness and soil moisture holding capacity	Few stones; deep loamy* and silty soils	Materials with a high percentage volume of stones; sandy soils and gravels
Winds	Sites not exposed to winds	Large river valleys and the Terai
Dominant vegetation	e.g. amliso, nigalo, bans, chilaune, katus, lali gurans, utis	e.g. babiyo, khar, dhanyero, imili, kettuke, khayer, salla

* 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

QUESTION	FUNCTIONAL IMPLICATION	ACTION IF THE ANSWER IS "YES"	USE OF BIO- ENGINEERING
Is the slope segment or the whole site subject to a deep-seated (>1 metre depth) shear (rotational) failure?	Major reinforcing, anchoring or physical support required.	If the failure plane can be identified, use conventional civil retaining walls to support the toe. Alternatively, it may be possible to remove weight from higher up on the slope by heavy trimming.	Bio-engineering measures will mainly be used to armour backfill and foundation areas. If trimming is carried out, bio-engineering measures will be needed to armour the new bare surfaces.
Is the slope segment very long (greater than about 30 metres), steep and in danger of a mass failure below the surface?	Reinforcing or physical support is required. Armouring is also required. Bio-engineering measures alone may be adequate, but where a large volume of surface runoff is possible, physical structures are also necessary.	If suitable foundations are available, use retaining walls to break the slope into smaller, more stable lengths. Some other kind of physical scour check should be used, such as wire bolster cylinders.	Bio-engineering measures must be designed to reinforce and armour the slope between the physical structures.
Is the foot of the slope undermined, threatening higher segments or the whole Slope above?	Strong physical support is required. Bio-engineering measures will enhance civil structures.	Investigate the necessity of building revetment, toe or prop walls.	Bio-engineering measures will mainly be used to armour backfill and foundation areas.
Is there a distinct overhang or are there large boulders poorly supported by a soft, eroding band?	Localised physical support or anchoring are required. Support can be given using a civil structure.	Consider prop walls or dentition to support the overhang.	The direct seeding of shrubs on fragmented rocky slopes can provide anchorage.
Does the slope segment have a rough surface; or is it covered in loose debris; or is it a fractured rocky slope; or does it have any very steep or overhanging sections, however small?	Armouring is required, but only after the slope has been altered to stop it shedding loose material.	Trim the slope as far as possible to attain a smooth, clean surface with a straight profile in cross-section.	The trimmed slope will need to be armoured afterwards by the appropriate bio-engineering measure.
Is there water seepage, a spring or groundwater on the site, or a danger of mass slumping after heavy rain?	Deep drainage is required.	Investigate the need for a drainage system involving french or other sub-surface drains, depending on site conditions.	Deep drains can be enhanced by surface bio-engineering systems (e.g. downslope planted grass lines).
Is the slope made up of poorly drained material, with a high clay content?	Techniques used on this sort of material must be designed to drain rather than accumulate moisture.	There is a danger of shallow slumping. Investigate the need for a surface drainage system.	An appropriate bio-engineering system (e.g. downslope planted grass lines) is often adequate on its own.
Is the site a major gully, subject to occasional erosive torrents of water?	Major drainage is already present; heavy armouring is required.	Use masonry check dams to reduce the scouring effect.	Between the check dams, use large bamboo planting, live check dams or vegetated stone pitching.

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.

How to use the flow chart in Figure 1:12 There are two methods: either 1. Use it as a prescriptive system to determine the treatments required, based on the site assessment described in step 4; or 2. If you have already determined a treatment, use it to check that your choice is suitable against normal practice.

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

START (a) SLOPE ANGLE	®(b) SLOPE LENGTH	® (c) MATERIAL DRAINAGE	® (d) SITE MOISTURE	® (e) PREVIOUS/POTENTIAL PROBLEMS ‡	®(f) FUNCTIONS REQUIRED	® (g) TECHNIQUE(S)
> 45°	> 15 metres	Good	Damp	Erosion slumping	Armour, reinforce drain	Diagonal grass lines
			Dry	Erosion	Armour, reinforce	Contour grass lines
		Poor	Damp	Slumping, erosion	Drain, armour, reinforce	1 Downslope grass lines and vegetated stone pitched rills or 2 Chevron grass lines and vegetated stone pitched rills
			Dry	Erosion, slumping	Armour, reinforce dram	Diagonal grass lines
	<15 metres	Good	Any	Erosion	Armour, reinforce	1 Diagonal grass lines or 2 Jute netting and randomly planted grass
		Poor	Damp	Slumping, erosion	Drain, armour, reinforce	1 Downslope grass lines or 2 Diagonal grass lines
			Dry	Erosion, slumping	Armour reinforce drain	1 Jute netting and randomly planted grass or 2 Contour grass tines or 3 Diagonal grass lines
30° - 45°	>15 metres	Good	Any	Erosion	Armour, reinforce, catch	1 Horizontal bolster cylinders and shrub/tree planting or 2 Downslope grass lines and vegetated stone pitched rills or 3 Site grass seeding, mulch and wide mesh jute netting
		Poor	Any	Slumping, erosion	Dram, armour, reinforce	1 Herringbone bolster cylinders & shrub/tree planting or 2 Another drainage system and shrub/tree planting
	<15 metres	Good	Any	Erosion	Armour, reinforce, catch	1 Brush layers of woody cuttings or 2 Contour grass lines or 3 Contour fascines or 4 Palisades of woody cuttings or 5 Site grass seeding, mulch and wide mesh jute netting
		Poor	Any	Slumping, erosion	Dram, armour, reinforce	1 Diagonal grass lines or 2 Diagonal brush layers or 3 Herringbone fascines and shrub/tree planting or 4 Herringbone bolster cylinders & shrub/tree planting or 5 Another drainage system and shrub/tree planting
< 30°	Any	Good	Any	Erosion	Armour, catch	1 Site seeding of grass and shrub/tree planting or 2 Shrub/tree planting
		Poor	Any	Slumping, erosion	Drain, armour, catch	1 Diagonal lines of grass and shrubs/trees or 2 Shrub/tree planting
	<15 metres	Any		Erosion	Armour catch	Turfing and shrub/tree planting
	Base of any slope			Planar sliding or shear failure	Support, anchor, catch	1 Large bamboo planting or 2 Large tree planting

Special conditions

Any *	Any*	Any*	Any*	Planar sliding, ear failure	Reinforce, anchor	Site seeding of shrubs/small trees †
> 30°	Any	Any rocky material		Debris fall	Reinforce, anchor	Site seeding of shrubs/small trees
Any loose sand		Good	Any	Erosion	Armour	Jute netting and randomly planted grass
Any rato mato		Poor	Any	Erosion, slumping	Armour, drain	Diagonal lines of grass and shrubs/trees
Gullies < 45°	Any gully			Erosion (major)	Armour, reinforce, catch	1 Large bamboo planting or 2 Live check dams or 3 Vegetated stone pitching

* 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

TECHNIQUES	PLANT CLASS TO USE	PACE REFERENCE TO FIGURE 1.14
Planted grass lines (all configurations) and vegetated stone pitching gully beds	Grasses grown from slip/rhizome cuttings	page	28
Brush layers, palisades, live check dams, fascines and vegetated stone pitching walls	Shrubs */ small trees grown from hardwood cuttings	page	30
Large bamboo planting	Large bamboos	page	33
Site seeding with grass	Grasses grown from seed	page	29
Turfing	Small sward grasses	page	29
Site seeding with shrubs/small trees	Robust shrubs/small trees grown from seeds	page	32
Shrub/small tree planting	Shrubs/small trees (grown from seeds/polypots)	page	31
Large tree planting	Large trees (grown from seeds/polypots)	page	31

* 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

MOISTURE	WET	MOIST	DRY	VERY DRY
ALTITUDE		Grasses
2500 - 2000 m	Padang bans	Padang bans	Tite nigalo bans
	Tite nigalo bans	Phurke
		Tite nigalo bans
2000 - 1500 m	Amliso	Amliso	Amliso	Babiyo
	Kans	Babiyo	Babiyo	Kans
	Katara khar	Kans	Kans	Khar
	Padang bans	Katara khar	Katara khar
	Phurke	Khar	Khar
	Tite nigalo bans	Padang bans	Phurke
		Phurke	Tite nigalo bans
		Tite nigalo bans
1500 - 1000 m	Amliso	Amliso	Amliso	Babiyo
	Kans	Babiyo	Babiyo	Dhonde
	Katara khar	Dhonde	Dhonde	Kans
	Khus	Kans	Kans	Khar
	Phurke	Katara khar	Katara khar	Narkat
	Sito	Khar	Khar
	Tite nigalo bans	Khus	Khus
		Narkat	Narkat
		Phurke	Phurke
		Sito	Sito
		Tite nigalo bans
1000 - 500 m	Amliso	Amliso	Amliso	Babiyo
	Kans	Babiyo	Babiyo	Dhonde
	Katara khar	Dhonde	Dhonde	Kans
	Khus	Kans	Kans	Khar
	Phurke	Katara khar	Katara khar	Narkat
	Sito	Khar	Khar
		Khus	Khus
		Narkat	Narkat
		Phurke	Phurke
		Sito	Sito
500 m - Terai	Amliso	Amliso	Amliso	Babiyo
	Kans	Babiyo	Babiyo	Dhonde
	Katara khar	Dhonde	Dhonde	Kans
	Khus	Kans	Kans	Khar
	Sito	Katara khar	Katara khar	Narkat
		Khar	Khar
		Khus	Khus
		Narkat	Narkat
		Sito	Sito

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

Moisture	Wet		Moist		Dry		Very dry
Altitude	Clump grasses (for seeding)	Small sward grass (for turfing)	Clump grasses (for seeding)	Small sward grass (for turfing)	Clump grasses (for seeding)	Small sward grass (for turfing)	Clump grasses (for seeding)	Small sward grass (for turfing)
2500 - 2000 m
2000 -	Kans	Dubo	Babiyo	Dubo	Babiyo	Dubo	Babiyo
1500 m	Katara khar		Kans		Kans		Kans
	Phurke		Katara khar		Katara khar		Khar
			Khar		Khar
			Phurke		Phurke
1500-	Kans	Dubo	Babiyo	Dubo	Babiyo	Dubo	Babiyo
1000m	Katara khar		Dhonde		Dhonde		Dhonde
	Phurke		Kans		Kans		Kans
	Sito		Katara khar		Katara khar		Khar
			Khar		Khar
			Phurke		Phurke
			Sito		Sito
1000 -	Kans	Dubo	Babiyo	Dubo	Babiyo	Dubo	Babiyo
500 m	Katara khar		Dhonde		Dhonde		Dhonde
	Phurke		Kans		Kans		Kans
	Sito		Katara khar		Katara khar		Khar
			Khar		Khar
			Phurke		Phurke
			Sito		Sito
500m-	Kans	Dubo	Babiyo	Dubo	Babiyo	Dubo	Babiyo
Terai	Katara khar		Dhonde		Dhonde		Dhonde
	Sito		Kans		Kans		Kans
			Katara khar		Katara khar		Khar
			Khar		Khar
			Sito		Sito

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

Moisture	Wet		Moist		Dry		Very dry
Altitude	Shrubs/ small trees	Large trees*	Shrubs/ small trees	Large trees*	Shrubs/ small trees	Large trees*	Shrubs/ small trees	Large trees*
2500 -	Bainsh	Phaledo	Bainsh	Phaledo		Phaledo
2000 m
2000 -	Bainsh	Phaledo	Bainsh	Phaledo	Namdi phul	Phaledo
1500 m	Namdi phul		Namdi phul
1500 -	Bainsh	Dabdabe	Bainsh	Dabdabe	Kanda phul	Phaledo
1000 m	Namdi phul	Phaledo	Kanda phul	Phaledo	Namdi phul
	Saruwa/		Namdi phul		Saruwa/
	bihaya		Saruwa/		bihaya
			bihaya		Simali
			Simali
1000 -	Assuro	Dabdabe	Assuro	Dabdabe	Assuro	Dabdabe	Assuro
500 m	Bainsh		Bainsh		Kanda phul		Kanda phul
	Kanda phul		Kanda phul		Saruwa/
	Saruwa/		Saruwa/		bihaya
	bihaya		bihaya		Simali
	Simali		Simali
500 m -	Assuro	Dabdabe	Assuro	Dabdabe	Assuro	Dabdabe	Kanda phul
Terai	Bainsh		Bainsh		Kanda phul
	Kanda phul		Kanda phul		Saruwa/
	Saruwa/		Saruwa/		bihaya
	bihaya		bihaya		Simali
	Simali		Simali

* 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

Moisture	Wet		Moist		Dry		Very dry
Altitude	Shrubs/ small trees	Large trees	Shrubs/ small trees	Large trees	Shrubs/ small trees	Large trees	Shrubs/ small trees	Large trees
2500 -		Lankuri		Gobre salla		Gobre salla		Gobre salla
2000 m		Painyu		Lankuri		Lankuri
		Rato siris		Rato siris		Rato siris
		Utis		Utis		Utis
2000 -		Chilaune	Keraukose	Bakaino	Keraukose	Bakaino	Keraukose	Bakaino
1500 m		Khanyu		Chilaune		Chilaune		Gobre salla
		Lankuri		Gobre salla		Gobre salla		Khanyu
		Painyu		Khanyu		Khanyu		Rani salla
		Rato siris		Lankuri		Painyu
		Utis		Painyu		Rani salla
				Rani salla		Rato siris
				Rato siris		Utis
				Utis
1500 -	Keraukose	Chilaune	Areri	Bakaino	Areri	Bakaino	Areri	Bakaino
1000 m		Khanyu	Dhanyero	Chilaune	Dhanyero	Chilaune	Dhanyero	Khanyu
		Lankuri	Kanda phul	Khanyu	Kanda phul	Khanyu	Kanda phul	Rani salla
		Painyu	Keraukose	Painyu	Keraukose	Painyu	Keraukose
		Rato siris	Tilka	Rani salla	Tilka	Rani salla	Tilka
		Seto siris		Rato siris		Rato siris
		Utis		Seto siris
				Sisau
				Utis
1000 -	Dhanyero	Khanyu	Areri	Bakaino	Areri	Bakaino	Areri	Bakaino
500 m	Dhusun	Painyu	Dhanyero	Kalo siris	Dhanyero	Kalo siris	Dhanyero	Kalo siris
	Keraukose	Rato siris	Dhusun	Khanyu	Dhusun	Khanyu	Dhusun	Khanyu
	Tilka	Seto siris	Kanda phul	Painyu	Kanda phul	Khayer	Keraukose	Khayer
		Sisau	Keraukose	Rani salla	Keraukose	Painyu	Tilka	Rani salla
		Utis	Tilka	Seto siris	Tilka	Rani salla		Sisau
				Sisau		Sisau
500 m-	Dhanyero	Khanyu	Dhanyero	Bakaino	Dhanyero	Bakaino	Dhanyero	Bakaino
Terai	Dhusun	Seto siris	Dhusun	Kalo siris	Dhusun	Kalo siris	Dhusun	Kalo siris
	Keraukose	Sisau	Kanda phul	Khanyu	Kanda phul	Khanyu	Keraukose	Khanyu
	Tilka		Keraukose	Seto siris	Keraukose	Khayer	Tilka	Khayer
			Tilka	Sisau	Tilka	Sisau

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

Moisture	Wet		Moist		Dry		Very dry
Altitude	Shrubs/ small trees	Large trees	Shrubs/ small trees	Large trees	Shrubs/ small trees	Large trees	Shrubs/ small trees	Large trees
2500 -		Utis*		Gobre salla		Gobre salla		Gobre salla
2000 m				Utis*		Utis*
2000 -		Khanyu *	Keraukose	Bakaino	Keraukose	Bakaino	Keraukose	Bakaino
1500 m		Utis*		Gobre salla		Gobre salla		Gobre salla
				Khanyu *		Khanyu *		Khanyu *
				Rani salla		Rani salla		Rani salla
				Utis*		Utis*
1500 -	Bhujetro	Khanyu *	Areri	Bakaino	Areri	Bakaino	Areri	Bakaino
1000 m	Keraukose	Utis *	Bhujetro	Khanyu *	Bhujetro	Khanyu *	Bhujetro	Khanyu *
			Keraukose	Rani salla	Keraukose	Rani salla	Keraukose	Rani salla
				Sisau
				Utis*
1000 -	Bhujetro	Khanyu *	Areri	Bakaino	Areri	Bakaino	Areri	Bakaino
500 m	Keraukose	Sisau	Bhujetro	Kalo siris	Bhujetro	Khanyu *	Bhujetro	Khanyu *
		Utis*	Keraukose	Khanyu *	Keraukose	Khayer	Keraukose	Khayer
				Rani salla		Rani salla		Rani salla
				Sisau		Sisau		Sisau
500 m -	Keraukose	Khanyu *	Keraukose	Bakaino	Keraukose	Bakaino	Keraukose	Bakaino
Terai		Sisau		Khanyu *		Khanyu *		Khanyu *
				Sisau		Khayer		Khayer
						Sisau

* 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

Moisture	Wet	Moist	Dry	Very dry
Altitude		Large bamboos
2500 - 2000 m	Kalo bans	Kalo bans	Kalo bans
2000-1500 m	Choya bans	Choya bans	Kalo bans
	Kalo bans	Kalo bans	Nibha bans
	Nibha bans	Nibha bans
1500 - 1000 m	Choya bans	Choya bans	Mal bans
	Dhanu bans	Dhanu bans	Nibha bans
	Kalo bans	Kalo bans	Tharu bans
	Mal bans	Mal bans
	Nibha bans	Nibha bans
	Tharu bans	Tharu bans
1000 - 500 m	Choya bans	Choya bans	Mal bans
	Dhanu bans	Dhanu bans	Tharu bans
	Mal bans	Mal bans
	Tharu bans	Tharu bans
500 m - Terai	Dhanu bans	Dhanu bans	Mal bans
	Mal bans	Mal bans	Tharu bans
	Tharu bans	Tharu bans

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

TYPE OF WORKS	SIGNS OF GOOD WORKS
Individual plants	A bright, healthy colour.
	Showing no signs of wilting.
	Well proportioned (i.e. not stunted or very tall and thin).
	Crowing fast, with a number of long new shoots.
	Without signs of discoloration on the leaves.
	Without signs of insect attack on the leaves or shoots (e.g. holes eaten in the leaves).
	Without any obvious signs of disease.
	Undamaged.
	Not yellowed, except in the later part of the dry season.
Whole sites	Completely treated, with no gaps or areas missed out.
	Evenly covered.
	Fully tidied up, with no loose debris on the slope.
	Showing no signs of instability.
	Stable enough to survive the early rains while plants get established.
	Generally looking good, complete and healthy throughout.
Grass lines	Complete, with plants at the spacing specified within the rows.
	The right distance between the rows, according to specification.
	Even, with no gaps or poor plants in them.
	Straight, according to specification.
Brush layers and palisades	Complete, with the right number of cuttings per running metre.
	The right distance between the lines, according to specification.
	Even, with no gaps or dead cuttings.
	Straight, according to specification.
Fascines (minor excavations needed to check)	Complete, with the right number of cuttings per running metre.
	The right distance between lines, according to specification.
	Straight, according to specification.

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