Stabilising slopes

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When forming benches across side slopes in locations of restricted width, some form of slope stabilisation may be required to allow steeper side slopes. Steep river banks, which may be susceptible to erosion and collapse may require stabilising and reinforcing before the path is built. There are hard engineering methods (use of stone etc.) to reinforce slopes, or soft engineering methods using ‘natural processes’ including tree roots for binding the surface. Soft engineering methods can take some time to become established but can provide a satisfactory long term solution in many situations. Some hard and soft engineering methods can be combined to take advantage of both durability and aesthetic benefits.

Soil reinforcement involves the use of a variety of geotextiles, both man-made and natural. Slopes as steep as 60 degrees can be formed, but require complex civil engineering techniques beyond the scope of this guidance. Further information should be sought from a civil engineer or specialist manufacturers of available slope stabilisation materials.

Retaining walls can also be complex structures requiring specialist design. A well built wall looks very attractive and can be an additional feature on a path. However, a retaining wall beside and below a path edge may present a falling hazard to path users and some kind of handrail may be required, depending on the height involved and local setting.

Timber walls

These comprise support boards or logs with timber or steel stakes driven into the ground. They are low cost, easy to install and can be reasonably unobtrusive. Timber walls can deteriorate after a relatively short period (5 to 10 years), although they can often be repaired or partially replaced to extend their life.

Gabion walls

gabion wall

These are wire mesh rectangular baskets filled with stone, which are available in various sizes. Stone should be 200mm down to 100mm graded, crushed rock built into the basket very much like a dry stone wall. Gabions should be perfectly rectangular and should not sag or move about. Gabions can be stacked up and even pinned together(or pinned into a large slope) to form large walls. The stonework can be infilled with soil and seeded to help stabilise the gabions, as well as giving them a more natural look. They are fairly cheap and will last a long time (15 to 20 years). However, they are very vulnerable to vandalism – cutting the basket wires will result in an immediate collapse. Gabion baskets are also vulnerable to rust in coastal environments and need to be filled with larger stone if they are to be exposed to wave or current action.

Stone walls

Retain a slope with a solid stone wall laid either dry or mortared. A good source of local stone is essential (purchasing stone should be avoided if possible for budget reasons). Careful choice of suitable sizes and shapes of stones is essential for stability, bearing in mind that a retaining wall has to withstand considerable pressure from soil pushing behind it. Skilled stonemasons/ wallers should be used to undertake construction. Stone walls can last a lifetime if well built and they are very resistant to vandalism.

Rock walls

These are essentially walls constructed with large rocks. If a supply of large rock is available on site they can be used in a variety of situations to stabilise large slopes, support extensive causeways and protect bridge abutments from water scour. A large excavator will be required to place them into position and care should be taken loading them on to large dumpers.

Specification
Specifications for small retaining walls are given in the TCV ‘Footpaths’ handbook
If larger or more complex walls are required, seek specialist advice from a consultant structural engineer. There are a number of techniques available, some of which are simple and low cost

 

Maintenance
Annual inspection should be undertaken for all retaining structures to record visual condition and structural stability
A longer term technical inspection cycle (e.g. every 5 years) by a suitably competent person is recommended

 

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