All natural slopes we see along the countryside, hills and mountains have been there for millions of years.
Over time, these slopes have undergone changes, failed, collapsed, developed, their own stabilizing systems and are now stable or just beginning to be stable.
These stabilizing systems include slope faces covered with natural vegetation such as grasses, shrubs, trees and formation of natural water channels that have developed over the years.
The moment we make any changes to the existing natural system for any reason, such as cutting trees and excavating soil, we are disturbing and upsetting the natural system. What this means is that if the given slope was marginally stable in its natural state, we now make it less stable.
Or if it was stable to begin with, we now introduce changes that can make it unstable. Either way, when dealing with natural slopes we have to be very careful. Such slopes require careful evaluation.
To ensure that the changes we make to the natural system will give us stable slopes, we have to assess what will make it stable and very carefully design substitutions or changes we intend to make to suit us. We do this by studying factors such as the soil inside the slope, the slope height, its inclination, its existing vegetation, how its surroundings may influence the slope, and the movement of water in and around the slope. We also have to consider where our structures are going to be in relation to the slope.
All of the above have to be taken into consideration in the slope design.
Almost all slopes at various development sites are designed by engineers. Slopes are designed taking the above factors into consideration and based on strength characteristics of the soil in the slope. Strength characteristics of soil in the slopes are obtained by carefully obtaining and testing the samples taken from the soil inside the slope.
To ensure that these slopes are safe, engineers would also have to consider the effects of factors being introduced such as new heights of slope separating two ground levels, the steepness or angle of inclination of the slope, the proximity of structures to the slopes, the surroundings, and the control of rain water on, in and around the slope.
Of these considerations, the control of water is the most important aspect of the final design and construction of the slope. Because almost all slope failures are caused or aided by water in one way or another, the control of water plays a very important role in the design and maintenance of the slope.
Among the problems caused by lack of water control include, but are not limited to:
Erosion of slope faces and loss of protective vegetation cover
Sinkholes and piping holes caused by loss of fine material from the soil
Softening of the soil below the slope surface
Softening of the toe at the bottom of the slope
Weakening of the slope by water entering the slope
Only after a slope design is completed can the proposed development work begin. But care for the slopes does not end there. To ensure the continuous safety of slopes against failures, these slopes have to be looked after or maintained. This means that once the construction is completed, it falls on the enduser to make sure the slope remains safe through maintenance of the slope, a subject we will be addressing later in this book.
Among the 213 development sites inspected for a study, it was found that although the slopes seemed reasonably well designed and proper drainage was provided, the slopes were still undergoing distresses because effective maintenance was lacking at these places.
In some cases the lack of maintenance was found to be so bad that the slopes required costly redesign and reconstruction. In other cases, slopes had already failed while some others were about to fail and would result in costly damage.
Apart from ensuring that all slope-related issues are addressed within the development, it is also essential that careful consideration is given to the effect of our development on surroundings and the effects of the surroundings on our development. No slope-related development can be designed as an isolated development, be it a simple home, institutional development or large housing/commercial development. All our cuttings, fillings, and drainage must consider the adjacent areas.
SLOPE DESIGN AND SURROUNDINGS
The relationship between slope design and the surroundings is best explained in the following example.
Two structures built along the slope are dependent on each other for their safety. Should the slope below the yellow structure fail or become unstable due to failed drain or seepage, it will affect the blue structure below. If the slope above or below the blue structure is in distress due to bad drainage and the slope fails, it will destabilize the yellow structure above.
This means that when living in hillside areas, the adjacent or the neighbouring areas become very important. This is because when maintenance and slope safety is not fully understood by everybody and when neighbours ignore each other, issues leading to instabilities can go unnoticed. This in fact is the best stage for disaster.
Hillside dwellings in Cameron Highlands and Kundasan