CIRIA Publication C516 Modular gravity retaining walls: design guidance, London 2000;
CIRIA Publication C637 Soil nailing – best practice guidance;
CIRIA Publication C708 Use of vegetation in civil engineering; and
TRL Report 537 Soil nailing for slopes.

Compliance references

UK National Annex to Eurocode 7: Geotechnical design (NA to BS EN 1997-1:2004);
BS 6031:2009 Code of practice for earthworks;
BS 8002: 1994 Code of practice for earth retaining structures;
BS 8006-1:2010 Code of practice for strengthened / reinforced soils and other fills;
BS EN 1997-1: 2004 Eurocode 7: Geotechnical design;
BS EN 14475:2006 Execution of special geotechnical works – reinforced fill;
DMRB Volume 2 Section 1 Part 3 (BD 68/97) and Part 4 (BA 68/97) Crib Retaining Walls;
DMRB Volume 2 Section 1 Part 5 (BD 70/03)Strengthened/Reinforced Soils and other Fills for Retaining Walls and Bridge Abutments. Use of BS 8006: 1995; and
DMRB Volume 10 Section 1 Part 4 (HA 58/92) The Road Corridor.

Technical references

CIRIA Publication SP123 Soil reinforcement with geosynthetics, London 1996;
CIRIA Publication C637 Soil nailing – best practice guidance, London 2005;
CIRIA Publication C516 Modular gravity retaining walls: design guidance, London 2000;
CIRIA Publication C580 Embedded retaining walls – Guidance for economic design, London 2003;
CIRIA Publication C591 Infrastructure cuttings - condition appraisal and remedial treatment, London 2003;
CIRIA Publication C592 Infrastructure embankments - condition appraisal and remedial treatment, London 2003;
CIRIA Publication C641 EC7 - Implications for UK practice: Eurocode 7 Geotechnical design, London 2008;
CIRIA Publication C708 Use of vegetation in civil engineering;
TRL Report 131 A literature and design review of crib wall systems, Crowthorne, 1995;
TRL Report 537 Soil nailing for slopes, Crowthorne, 2002; and
TRL Report 515 Vegetation for slope stability, Crowthorne, 2001.

Typical criteria

Are the slopes exposed to erosion from wind and water run off?
Are there site constraints dictating the slope angle and/or the landtake?
Does the design require the slopes or walls to be vegetated or in other ways integrated with the surrounding landscape?
Is there risk of instability or failure of slopes?

Technical considerations

Slope surfaces might require stabilisation against either erosion, shallow or deep-seated failure. Depending on the steepness of the slope (strengthened slopes with angles >70^o are classified as retaining walls), different geosystems or combinations of geosystems can be used to improve overall stability.

Normally, vegetation provides a natural erosion control system, however it can be difficult to establish on steep slopes in exposed locations particularly if they are north facing. The resistance of the vegetation, particularly during the establishment phase, can be improved by the use of geotextiles to retain the top soil and moisture while being easily penetrated by the plant root systems. For more extreme growing conditions, some types of geosynthetics are available which incorporate a factory-prepared layer of pre-seeded top soil containing slow release fertilisers and water retaining polymers to speed up germination and support growth. Often these systems include a facing geogrid, which will provide a permanent anchor to the vegetation and erosion protection to the soil, and a biodegradable fabric, which will eventually degrade to enable the roots to penetrate the substrate.

Geosystem facings to steepened slopes and retaining walls are commonly connected to geosynthetic/steel reinforcement (or soil nails/anchors) within the body of the slope or the retained ground which acts to improve its overall stability. Horizontal reinforcement such as geotextiles and geogrids can provide a wrap around facing when extra length of the material is wrapped around the slope surface of the reinforced layer and then anchored back into the fill. Alternatively, steel mesh facing can be anchored to the horizontal reinforcement, typically for steeper slopes (up to 70°). Prefabricated steel mesh units, which provide a facing element and a stiff panel that can be pinned in the soil to provide reinforcement and anchorage to the facing element, can also be used, particularly for slope failure repairs.

For walls, i.e. slopes steeper than 70°, structural facing systems and gravity retaining walls are used. These are typically of modular construction and have a significant mass, e.g. precast concrete blocks, concrete or timber interlocking boxes (forming crib walls), gabions filled with durable coarse fill, concrete walls. Anchorage points for horizontal soil reinforcement might be provided on the internal face of the hard facing structure. Specific finishes can be provided to ensure integration in the landscape and “softening” of the surface, e.g. texturing/colouring of the exposed face of concrete blocks, use of local stones for the facing layers of the fill within gabions, crib walls partially filled with top soil to favour growth of vegetation.

Economic considerations

Geosystems work with the soil available on site by increasing its stability and limiting its erosion. Use of geosystems results in significant savings in terms of construction time, material cost and waste disposal charges. These are some examples of the savings that can be realised:

Protection of the face of slopes and walls contribute to the establishment of vegetation, a natural system to control soil washing out and water run off, which could otherwise cause for example clogging of drains, flooding, slip failures disrupting adjoining structures.
By enhancing the action of reinforcing elements such as nails, ground anchors, geotextiles, geocells etc., facing systems contribute to strengthen the existing soil. This means that only minimal materials (i.e. the reinforcing geo-components) need to be procured and transported to site, hence realising important cost savings.
The costs of excavating, transporting and disposing of unsuitable soils and their replacement by imported higher quality fill are avoided.

Environmental considerations

The highlighted savings are matched by environmental benefits. Geosystems achieve resource efficiency through reduction of the need for virgin/imported materials and waste minimisation.

This results in savings of natural resources and landfill space. Furthermore, the considerable reduction in transport requirements (for haulage of large quantities of new materials and waste) results in significant avoidance of greenhouse gas emissions.

In many cases the construction period is shortened with environmental benefit in terms of reduced construction noise and less traffic disruption in the adjoining area.

The systems can also provide enhanced conditions for the establishment of vegetation, thus contributing to erosion control and carbon absorption, minimisation of water runoff, increased biodiversity.

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