As explained before, all manmade slopes are designed taking into account the properties of soils that go into the construction of the slope. Such designs also take into account what will be at the top or bottom of the slopes. Almost all these designed slopes provide for stable slope angle and proper drainage. In every case, the greatest attention is always paid to the control of water.
Design and construction of slopes vary with soil types, heights of slopes, and their importance to surroundings. Designs can range from very complicated to very simple.
Properly designed and constructed slopes very rarely have problems, and because they do not have too many problems, we take them for granted and tend to forget them. However, these well-constructed slopes will also give trouble if they are not regularly maintained.
Therefore, the construction of slopes plays a large role in their behaviour during service. Badly constructed slopes, for which very little attention has been paid to good practices, will require a lot more maintenance since they are easily affected by action of water that leads to endless problems of erosion, seepages, and slips.
Almost all development in hilly areas consists of some cutting and filling. In every case, how this is to be done is specified or designed by engineers. Slopes should be trouble-free when care has been taken to observe the requirements of the engineer during construction.
This means that all cut and exposed areas should be protected during and after construction against water and all fill areas should be well-packed and similarly protected.
Filling operations require particular care as these are most susceptible to ingress of water. Loosely filled ground can soak up water and also erode at a rapid rate, causing instabilities and ugly erosion scars.
All filling operations should involve proper preparation and filling in small thickness of spread soil layers, with each layer well-compacted using rollers or tampers. Well-compacted ground has high strength, does not readily soak up water and does not erode easily.
Doing things properly is good practice. Good practice is nothing more than the intelligent use of instructions and guidelines provided by the engineer in his specifications or the use of relevant Codes of Practice, plus a bit of common sense.
The process of cut, fill, spread and compact, layer by layer, is carried out until the desired slope height is reached. Particular attention is paid to the material used for filling which exludes rocks and construction waste. This way, the whole fill is built up of a number of well-compacted, stable soil layers.
Dumping of soil or simply pushing soil from one area to another is a bad practice whereby the soil in the fill remains loose and weak. Such practices also pay very little attention to unsuitable materials such as parts of trees, rocks, and even small boulders, all of which get buried into such dumped fills. Parts of vegetation (trees, stumps, or branches) or rocks should never be used in fillings as this makes the soil very difficult to compact.
Once construction is finished, slopes are protected against rainfall by providing suitable vegetation to cover all exposed slope faces. Once such cover uses grass and is known as turfing, in cut as well as in filled slopes. Vegetation cover varies from ordinary grass to specially developed grass having deep roots to ferns.
Considerable attention must also be paid to the installation of drain elements, from the preparation of the base of the drain to the top edge of the drain. Well-constructed slopes with good vegetation cover do not easily allow water to enter the slope face and water can run off to the drains provided.
In the case of bad construction, the soil inside the slope is loose, and regardless of vegetation cover and drainage provided, water will still get into the slope. This loosens the soil inside the slope, washing away fine particles, and causing piping, rapid erosion and in time, slips. For slopes constructed with difficult soils such as those containing mainly clay (more than 35 percent), getting proper compaction of soil is very difficult to begin with. In addition, where such work is badly supervised, the packing of soil is rarely done, and simple dumping is adopted, we have a slope with numerous potential problems due to loose fill in the slope. In these cases, even trimming of the slope to its required shape is difficult. Such slopes are simply covered over with grass and passed on to the end user, who now has to put up with endless problems.
Anything built on badly filled areas will experience all sorts of annoying problems such as cracks, erosions, sinkholes, settlements, and collapses. The enduser of the slope-related property has no control over construction, but he ends up with the mess which actually is the responsibility of the engineer.
Although all slopes are provided with required drainage structures to control water, they do not function on badly constructed slopes. The drains cannot be properly constructed on bad fills and get further damaged as slopes start getting into distress such as localized settlements and small slips. Such small distresses, particularly those affecting water control stuctures such as drains, will set the stage for bigger slope failures or landslide.
Moreover, because of the loss of soil around and below the drains, some of the drains actually trap water and provide protected areas (against fogging) for mosquitoes to breed.
Constructing slopes with minimal maintenance in service is easy. It requires some care and training in the rudiments of construction. It also requires adequate supervision from the design engineers to ensure works are properly carried out. At sites where any form of earthwork is involved, the compaction of soil is essential. Compaction is not difficult. Depending upon the size of the project, compaction can be carried out manually using anything suitable.
To compact the soil in place, a variety of methods and tools are available, from hand tampers, mechanical tampers such as vibrating plates, to huge rollers.
A well-compacted earthwork site usually appears neat, even during construction. Rain does not cause too much runoff damage since the soil is well-compacted.
GOOD AND BAD CONSTRUCTION PRACTICES
ABOUT GOOD CONSTRUCTION PRACTICES
Good construction practices consist of following guidelines laid out in the Codes of Practice for earthworks and contract specifications, or Terms of Reference (T.O.R.). Every construction contract includes these documents. Bad practice occurs when, deliberately or out of ignorance, these documents are ignored.
Methods described in the guidelines are mandatory as they are required under the T.O.R. for such works and also under the Code of Practice for Earthworks (MS1754:2004). or similar local Code of Practice. For construction works, no matter how small, it is essential that the contractor observes requirements of the T.O.R. It is also essential that the owner employs someone capable of ensuring that the contractor does in fact follow the requirements of the T.O.R.
Construction of slopes, which merely pretend but do not incorporate fully the requirements of the T.O.R and the Code of Practice, will inevitably encounter some form of trouble during construction and in service. Such slopes will require much maintenance and will show distresses more often, leading to repairs.
Slopes built up by simply dumping soil remain loose throughout the depth of the filling. The soil in such slopes cannot be packed and remain loose. Such loose soil allows water to enter easily and is washed away by rain, causing rapid erosion. Rain water
entering the soil inside the slope can cause leaching (removal) of fine soil particles. This can lead to piping (small water channels) inside the slope, which in time can get bigger and bigger and lead to slumps or slips. Such slopes can be easily recognized by tell-tale erosion gullies, slips, ugly loss of vegetation, sink holes, and badly damaged drains.
At the end of the day this is unfortunate for the enduser who has no control over bad construction practices, but has to suffer and even pay for correcting bad construction.
Badly filled slope with structure supported on piles. Following rain, a part of the filled slope below the structure has collapsed, exposing piles and pile caps. Repairs now are not easy and even if repaired, there will always be a chance that, sooner or later, more slips will occur.
Structures constructed on bad fills have settlement problems and show cracks and similar distresses even before construction is completed. Again, repairs cannot give long-term confidence or comfort to occupiers. Plastering over the cracks is not a solution. This is an easy way out. This simply passes the problem to the enduser who has to put up with misery for the rest of the time he occupies such a structure.