All exposed earth in fills, cuts, on flat terrain, or on slopes are subject to the impact of raindrops, which in time can cause erosion, gullies and even landslips. The exposed grounds, particularly those along slopes, have to be protected against erosion.
This protection is provided in a number of different ways, from covering slopes with plastic sheets, spraying with cement/concrete, using a combination of pitched stones in cement mortar, grasses, shrubs, and even beautifully trimmed grass with a sprinkling of flowers. The thing to remember is that all these covers have one primary objective: to protect exposed earth surface against water, mainly rain. Where this requirement is not met, such cover not only has no meaning, but can cause more damage than no cover at all.
Vegetation Cover
Of all the covers we can provide for our slopes, the most aesthetic and easiest to provide is vegetation. Vegetation cover varies from simple grass turfing to ferns. They are not only pleasing to look at, they are almost-although not quite-maintenance-free.
Temporary Cover
Temporary covers are used to protect exposed slope faces against the action of water. These covers are usually provided during construction or soon after a landslide to protect the exposed slope face. Almost all such temporary covers are plastic sheets.
TURFING: CLOSE TURFING AND SPOT TURF
Once a slope is completed, its surface has to be protected against rain. If not covered, within a short time, falling rain can cause considerable erosion and gullies. Covering exposed slope surface with grass is about the easiest method. For this reason, special types of grasses have been developed for slopes, mostly for use on highway slopes. One such grass is called Vertiver.
The most commonly used grass cover provided for slopes and other exposed grounds is turfing. Turfing comes in about 20cm x 20cm square patches of grass and is used to cover the slope face. Each patch is referred to as a turf. When turf patches are placed close together such that the slope face is completely covered with grass, this is referred to as close turfing. Close turfing requires a lot of grass patches and can be expensive.
A cheaper turfing method is to place turf patches at least one square apart. The slope face (or any exposed soil surface) is thus covered with numerous spots consisting of patches of turf. This is referred to as spot turfing. Most of the time, spot turfing does not provide complete cover as expected.
Because of exposed soil around the patches of turf, rain can cause soil around the turf to erode, leaving patches of turf higher than the surrounding area and create gullies or deprive turf of nutrient and eventually kill the grass in the turf.
The grass cover provided on exposed surfaces of slopes is meant to provide protection against the impact of rain and running water. To be effective, this cover has to be provided such that the whole of the exposed surface is covered, which the spot turfing cannot provide.
Advocates of spot turfing anticipate that in time grass in each patch of the turf will grow sideways and the grass patches will merge. This will completely cover the exposed ground surface and will then be as good as close turfing but at nearly half the cost. But this does not always work. This is because before the grass can grow sideways, rain or wind can cause enough erosion to wash away the soil around the turf patch.
The turf can also dry out because of no watering or the exposed surface in between the patches of turf can erode, suspending turf patches above the surrounding ground and forming small gullies caused by rain. Any one of these actions can prevent the spot turf from growing sideways and providing effective cover. In almost all cases, spot turfing on slope causes more damage than no turfing.
SPOT TURFING: WHY IT DOES NOT ALWAYS WORK
CASE OF SPOT TURFING IN PRACTICE
OTHER TYPES OF COVERS
Gunite
Gunite is "mortar conveyed through a hose and pneumatically projected at high velocity onto a surface" or "a cement, aggregate (sand) and water mixture that is sprayed using compressed air".
The gunite is used to cover slopes because it is dense, strong, and waterproof. It is also used to stabilize rock slopes where rocks are likely to dislodge themselves following erosion by rain.
The gunite covers are rigid and cannot tolerate movement. There also cannot be too much loss of ground support under them.
The gunite is not placed in forms or plastered by hand, but is impacted or jetted like water from a hose, onto any type of shape or surface, including vertical or overhead.
Like everything else related to slopes, gunited cover on slopes has to be carefully placed and looked after. Should the rigid concrete cover crack and water penetrate the slope surface between the gunite cover and the soil it is protecting, a lot of damage can occur. The washing action of water can lead to considerable erosion behind the gunite cover, and in time, create a large gap between the gunite cover and the supporting slope face. This erosion can go undetected until too late, by which time the surface cement cover may begin to break up, exposing considerable erosion behind.
Damaged and failing gunite cover. Water has eroded the soil behind the gunite cover, leaving the thinly sprayed gunite sheet unsupported. Eventually, the concrete gunite sheet will collapse under its own weight due to lack of support. Once this happens, the damage will spread, in this case making the whole slope cover useless and very costly to repair or replace.
“Overnight 46 houses have had to be evacuated at Bukit Setiawangsa, a highly desirable Eastern suburb of Kuala Lumpur...”
This slope is some 43 meters high covered with sprayed concrete. “ ...the wall (slope cover) used to protect the slope was known as a “shotcrete” wall which prevents water from entering the soil. However, water can still seep into the slope from areas not covered by the wall but the wall also prevents water from flowing out. In other words, water can flow in but is not able to flow out. When this happens, the water pressure builds up and breaks through “the wall” (slope cover), causing the landslip,”
STONE PITCHING
Stone pitching is another method used to provide protective covering to exposed slope surfaces.
Stone pitching consists of large stones set in cement mortar along the slope face.
This type of cover appears more attractive than the gunite, but it is also rigid and has problems similar to the gunite type of cover.
Stone pitching is easier to install as this method does not require specialised equipment necessary for guniting operations.
Stone pitching is carried out by hand and no machinery is required, but it is dependent on the operator's skills. Stone pitching is useful for only small heights up to three or so meters.
FABRIC COVERS
SHEETS
These are temporary covers used to protect exposed slope surfaces. They are usually applied during construction or soon after a slope failure to protect the exposed slope face against the effects of rain. The primary function of these sheet covers is to stop water from falling on exposed slope surface and entering the slope or washing away materials from the exposed face. Unless the slope is protected, further slope failures can occur. Almost all such temporary covers now used are plastic sheets.
Care must be taken when covering the slopes to ensure that:
Rain water does not fall directly on the exposed surface and cause erosion
AND
Water from other parts of the slope, particularly from above the failed area, does not enter the failed surface
Water from other parts of the slope is usually a lot more concentrated and can cause greater deterioration of the failed surface. What all this means is that any slope area covered with sheets must consist of complete cover such that all sources of water are prevented from entering the exposed slope area.
For this reason, partial covering, covering in bits and pieces or partially neglected covers do more damage as they not only allow rainfall on the exposed areas, but they allow the water falling on plastic covers to enter the slope face in a more concentrated form and cause greater damage.
EXAMPLES OF BADLY PROTECTED SLOPE FACES
Failed area is not protected against the rain and the water from above. Such covers tend to concentrate water at the bottom of each sheet and instead of protecting, aggravates the erosion. Such covers are more damaging than no cover. Such bad examples are just about everywhere.
Failed slope covered with badly placed or badly maintained sheet covers