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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.
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*
|
WALL TYPE |
MAXIMUM SAFE HEIGHT |
WIDTH: HEIGHT RATIO |
ADVANTAGES/LIMITATIONS |
|
Dry masonry |
4 metres |
1:1 to 0.6:1 |
Well drained; flexible; relatively low cost; low strength threshold |
|
Composite masonry |
8 metres |
0.75:1 to 0.5:1 |
Better drained than mortared masonry, but with reduced strength |
|
Mortared masonry |
10 metres |
0.75:1 to 0.5:1 |
Relatively easy to construct on steep terrain; cannot tolerate settlement; poor drainage |
|
Gabion |
10 metres |
Width = ½h + 0.5 |
Flexible without rupturing; tolerates poor foundations; well drained; relatively low cost for strength |
|
Reinforced earth |
8 metres |
Depends on design |
Reinforcing expensive or difficult to obtain; difficult to achieve tension |
|
Reinforced concrete |
10 metres |
Depends on design |
Relatively costly; requires advanced technical skills to build; poor drainage |
* 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.
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
|
SLOPE |
WALL HEIGHT |
BASE WIDTH |
TOP WIDTH |
|
30 - 35° (58 - 70%) |
1.5 - 2.0 m |
1.25 - 1.5m |
0.75 m |
|
35 - 40° (70 - 84%) |
2.0 - 2.5 m |
1.5 - 2.0m |
0.75 - 1.0 m |
|
40 - 45º (84 - 100%) V |
2.5 - 3.0 m |
2.0 - 2.3 m |
1.0 m |
|
SPECIAL FEATURES OF DRY MASONRY RETAINING WALLS · Careful design of dry masonry retaining walls can make them highly effective. · Lay the foundations back into the slope at 1v:3h. · Dress all stone (if it is rounded) into rectangular blocks. · Lay stones so that they are tied into the slope, so that only the small ends, not long sides, are at the face of the wall. · Overlap all joints. · Use stones as large as possible. If mainly small stones are available, use large ones at least every one metre to improve the tying. · Keep the angle of the foundations (1v:3h) with each layer of stone. The outer face of the wall will automatically come to 1h:3v if this is done. · Use flatter stones for the top layer. Cover the top of the retaining wall with soil or build a bound masonry band along the top to stop it unravelling. · Dry walls co-exist with, and are strengthened by plant roots. Encourage or plant vegetation. · Use the dimensions in Figure 2.2 for dry stone retaining walls, depending on slope angle. |
|
SPECIAL FEATURES OF GABION CONSTRUCTION Gabions have many possible functions, including their use in toe walls, revetments and retaining walls. They have special properties of strength, flexibility and free drainage. Practical features · The normal width to height ratio is: width = ½ height + 0.5 · Ensure drainage is provided from the lowest point of the foundations · Use heavily galvanised high-grade steel wire complying with the latest Nepal Standard. · Mesh should be, either a heavily galvanised mild steel or a triple-twist hexagonal mesh (i.e. 1.5 complete turns), nominally of 100 mm width and 120 mm length. · Panel frames should be made using 8 SWG wire, and mesh should be of 10 SWG wire. · Special attention must be paid to binding the boxes together along the seams (selvedging). · Wire all gabion boxes together using 12 SWG wire, allowing an additional 5% of wire for binding and tying. · During construction, add four or five cross-trusses (of 10 SWG wire) per square metre in each horizontal direction. · Ensure that the minimum dimension of all stones is larger than the wire mesh size. · Stones should be tabular and angular. · All stones should be carefully and densely packed, not just the facings. · Wire the lids down with additional wire of 12 SWG. · Backfill behind the gabion structure with a filter blanket to improve drainage. |
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.
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
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
|
SPACING OF CHECK DAMS Check dams should normally be placed where: · they protect weak parts of a gully from scour; In most cases, gullies are so irregular that the spacing of check dams will be determined by ground conditions. However, if the gully is sufficiently uniform, the spacing of check dams can be determined using the relationship devised in 1973 by Heede and Mufich. This states that where X = check dam spacing in metres, K = 0.3 when tanS The effective height (HE) has to be estimated by the engineer on site. It is a function of the foundation conditions and the construction material used. The height should normally be maximised to reduce the number of check dams required. |
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
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)
|
DRAIN TYPE |
| | | |
|
STRUCTURE |
BIO-ENGINEERING |
MAIN SITES |
ADVANTAGES |
LIMITATIONS |
|
Unlined natural drainage system (rills and gullies already developed on bare surfaces). |
Grasses in the rills and gullies, and grasses and other plants on the sides. |
Existing landslide scars and debris masses. |
By far the cheapest form of surface drain. Rapid drainage is assured. |
There is a risk of renewed erosion in exceptionally heavy rain in weak materials. |
|
Unlined earth ditch system. |
Grasses and other plants on sides and between feeder arms. |
Slumping debris masses on slopes up to 45°, where the continued loss of material is not a problem (e.g. in debris masses well below a road, draining straight into large rivers). |
By far the cheapest form of surface drain. |
There is a serious erosion hazard, especially on steep main drains, so this type should be used only where further erosion is not a problem. Leakage into the ground may also occur. |
|
Unbound dry stone system of ditches. |
Grasses between stones (as vegetated stone pitching), and grasses and other plants on sides and between feeder arms. |
Almost any site, however unstable, where the ground is firm enough to hold stone pitching and the flow of water is not too excessive for this construction technique. |
A low-cost drain type. Strong and very flexible. These two features make it good on unstable slopes. |
A membrane of thick, black polythene may be required to stop leakage back into the ground. |
|
Bound cement masonry ditch system. |
Grasses and other plants on sides and between feeder arms. |
Only on stable slopes with suitable material for good foundations. |
A strong structure for heavy discharges. |
Relatively high cost. Very inflexible, so there is a high risk of cracking and failure due to subsidence and undermining. |
|
Wire bolster cylinders (herringbone pattern). |
Grasses and other plants on sides and between feeder arms. |
Almost any site, however unstable, without excessive amounts of stone, but where the ground is firm enough to hold the structure. The drainage discharge should not be excessive. |
A medium-cost shallow type of drain. Very strong and flexible, which makes it good for unstable slopes. |
A membrane of thick, black polythene may be required to stop leakage back into the ground. |
|
Open gabion ditch system. |
Grasses and other plants on sides and between feeder arms. |
Almost any site, however unstable, where the ground is firm enough to hold a relatively big structure, and where a large volume of discharge is possible. |
A large and high-cost type of drain. Very strong and flexible, which makes it good for unstable slopes. |
A membrane of thick, black polythene may be required to stop leakage back into the ground. |
|
CASCADES | ||||
|
Dry stone cascade. |
Grasses and other plants along the sides. |
Any slope section steeper than 50° where foundations are adequate and discharge is relatively low. |
A low-cost form of cascade with a degree of flexibility. |
A membrane of thick, black polythene may be required to stop leakage back into the ground. |
|
Mortared masonry cascade. |
Grasses and other plants along the sides. |
Very stable slope sections steeper than 45°, where foundations are very good. |
A strong structure for heavy discharges. |
Relatively high-cost and inflexible cascade type, so there is a high risk of cracking and failure due to subsidence and undermining. |
|
Gabion cascade. |
Grasses and other plants along the sides. |
Any slope section steeper than 45°, where foundations are adequate and discharge is likely to be high. |
Very strong and flexible, which makes it good for unstable slopes. |
A relatively large and high-cost cascade type. A membrane of thick, black polythene is required to stop leakage back into the ground. |
|
Concrete cascade. |
Grasses and other plants along the sides. |
Very stable slope sections steeper than 45°, where foundations are very good. |
A very strong structure for the heaviest discharges. |
Very high-cost and inflexible cascade type. The risk of cracking and failure due to subsidence and undermining is partly offset by the innate strength of the construction. |
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
|
DRAIN TYPE |
| | | |
|
STRUCTURE |
BIO-ENGINEERING |
MAIN SITES |
ADVANTAGES |
LIMITATIONS |
|
SUB-SURFACE DRAINS | | | | |
|
French drain system (perforated pipe of durable, high grade black polythene, 150 mm diameter with approximately 40 holes of 5 mm per metre) in a drainage medium of aggregates). Drain can be made more resistant to disruption by building it in a casing of gabion. |
Grasses and other plants along the sides and between feeder arms. |
Almost any site, however unstable, where the ground is firm enough to hold the structure and the flow of water is not too excessive for this construction technique. |
A relatively low-cost and common sub-surface type of drain. Very flexible, which makes it good for unstable slopes. |
A membrane of permeable geotextile should be used. If the flow is too great, piping may occur underground. The outfall must be monitored to check that the drain is functioning, but the hidden nature of the drain means that this cannot always be fully ascertained. |
|
Site-specific design of drain to pick up seepage water. An open ditch or a drain with a flexible gabion lining is preferred. |
Plant grasses and other species along the sides. |
Any slope with obvious seepage lines. |
Specific drains can be designed for any site, leading to the optimum collection of water. |
Great care is needed to ensure all seepage water is trapped by the drain. Movement in the slope may affect this. |
|
Deep surface drain types (deeper versions of the surface drains described above, designed to catch shallow ground water seepage). |
As for each surface drain type described above. |
As for each surface drain type described above. |
Open drains allow easy cleaning and repair, as well as monitoring of effectiveness. |
The usual practical maximum depth is about 1.5 metres. Special care must be used to allow water to seep into the drains. |
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.
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).
|
WHY YOU SHOULD AVOID USING CUT-OFF DITCHES OR CATCH DRAINS ABOVE CUT SLOPES Cut-off ditches, otherwise known as cut-off drains or catch drains are: · almost certain to become Mocked; A cut-off ditch becoming blocked or cracked is a common cause of a landslide or the severe erosion of a cut slope in Nepal. Damage to a cut slope can be considered the usual outcome of the installation of a cut-off ditch. This warning applies to all surface ditches that are out of sight of the road, and therefore they are best avoided unless there is no alternative. Water should be brought down the slope along its natural course, protected with vegetation and civil structures as required, and if necessary carried into the nearest roadside ditch by a cascade. Localised damage can then be seen and repaired as soon as it occurs. |
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.
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.
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.
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