4.4 Bank and Slope Stabilisation
Upland slopes are prone to slippage, particularly when vegetation has been lost. Initial loss and erosion may be caused by pressure of use, but fragile vegetation, thin friable and mobile soils, high rainfall, and frequent freeze thaw action all contribute. Slopes will need stabilizing if a path solution is to be effective.
The revetment wall is solidly built to retain loose or unstable ground on steep slopes. The stabilised slope will then provide a better base for revegetation. Revetments are also used to support and consolidate banks along path edges. The most typical situations for its use are:
- on open eroded slopes, or gullies associated with the old path alignment
- where the path traverses a slope, either on one line or zigzagging
- to support a lower path edge from collapsing down the slope
- to retain the bank or slope above from collapsing onto the path, either at the path edge or on the slope above
Bill of Quantities (example)
Using natural weathered stone construct an informal revetment wall to retain the slope above the path. The construction must be solid and stable, with large foundation stones, off-set joins, pinned and backfilled firmly. Pack gaps between the courses with turf, and turf over the top to blend with the upper slope.
Where revegetation over an eroded slope is necessary the revetment may be combined with turf banks and transplants, or geotextile with seed and fertiliser (see Restoring Vegetation).
The revetment is a rough-faced, random coursed, drystone wall. On steep slopes the structure may need to be a formal retaining wall, of approximately 500mm height, or more. Preferably, a less formal approach should be used, with large boulders butted together along the path edge to support the banking. Both should be made to look as natural as possible by incorporating turves into and over the structure.
Revetments are built from the following:
- large boulders for informal revetments
- variable sized, block stone for formal revetment walls
- spoil for back-filling
- turf for landscaping the revetment
These are described in detail in Materials and Use. Stone for revetments should be in its natural form with the outer faces weathered, preferably lichen or moss covered, to blend with the surroundings.
Method of construction
The key to a solid revetment is the foundation. Whether it is the more formally constructed wall or the random boulder edge, a solid base should be excavated and levelled to build on. This should be to at least one third of the depth of the base stone.
- use the largest stones for the wall base stones, progressing with courses reducing in size towards the top; the final course should use stone that is large enough to form a solid top to the wall
- the courses should form a batter, leaning into the slope, to provide more resistance to any slumping of the slope behind
- outer stone faces should not protrude, as these may be used as steps, by people or animals, to climb over the wall, which will ultimately result in weakening of the structure
- lay the stone a course at a time, butting adjoining stones tightly, and with off-set joints, to provide a solid structure
- pin each course from behind with smaller stone wedges, to ensure that no movement occurs, before the next stone is laid
- backfill any space behind the revetment as each course is laid; it is essential that this is packed tightly to minimise movement and settling of the soil which inevitably happens after construction is complete
- fill gaps between courses on the face of the wall with turf off-cuts to help create a natural appearance
- revetments above the path should be topped off with turf, and landscaped into the upper slope
- to keep walkers off the top of revetments below the path edge, spoil and turf should also be used on the path edge
- revetments supporting the lower path edge should have spoil and turf in front of the foundation stones, to help stabilise and blend them with the lower slope
- revetments on open slopes should have turf and spoil above and below to blend into the slope and aid stabilisation
Key points to watch:
- always build on top of securely wedged stone - if the course below is loose then all those above will be unstable
- extend the revetment by one metre past the end of the bank that requires stabilising, to prevent banks collapsing around the ends
The following maintenance task should be carried out regularly:
- re-packing of loose stone work with turf or stone wedges
- re-turfing of any areas where turf has died or been damaged
- revetment walls can be very visible on steep slopes - avoid high formal structures
- use turf with care to help walls blend into the slope
- source turf and stone away from eroded slopes to prevent further erosion
HEALTH AND SAFETY HAZARDS
- if working on a revetment over one metre high, take precautions to ensure the safety of the worker; wear PPE, and take particular care when working on the top courses of the wall
- minimal impact is the over-riding aim of pathwork - a certain amount of erosion on slopes is acceptable in an upland landscape