Soil study may help prevent new landslides
NST 14/06/2006

AFTER the Hulu Klang landslide on May 31, geotechnical experts were interviewed by the Press and, as usual, there was no shortage of theories about the possible causes, including residual granite overlying the parent granite rock; rainfall of 44.6 mm, continuously over three days in an area with high-risk ratings; and the compromising of soil stability by land clearance and river diversions.

Such responses, unsupported by investigation, are unhelpful. They may even be wrong. For instance, the rock underlying this site is not granite but metasediments. They are amply exposed in earthworks in the adjacent site. Even if the rock in this area were indeed granite, that could not by itself explain the disaster.

Granite covers large parts of Peninsular Malaysia and almost inevitably, residual granite (soil) overlies parent granite rock.

Does this mean that slopes in granite areas are inevitably unstable? As a corollary, would land not underlain by granite be any safer, everything else being equal?

It may be true that several rainy days preceded this failure and certainly land had been cleared. But the same two conditions apply equally to many other parts of the Klang Valley.

Yet, it is not explained how these two factors could have interacted to trigger a landslide here and not elsewhere.

Is some other unknown factor involved? Or had this slope been simply cut too steep or the reinforced earth wall inadequately designed?

It is not clear at present if any river diversion, the remaining proffered cause, is involved and how it could have contributed to the slope failure.

This slope disaster is the latest of a series in Hulu Klang serious enough to have attracted wide Press coverage. However, there have also been numerous landslides which have escaped attention because they were either small or without dire consequences.

Many slopes here, scattered over an area of about 2km-3km by 1km, are clearly unstable. They should be studied to determine the underlying causes, especially when large parts of Hulu Klang have already been developed or are being developed.

The study should also attempt to identify specific slopes that may be particularly unstable so that residents in such areas would have time to consider their options, and developers can avoid such areas or otherwise incorporate safety measures into their projects.

The alternative is to allow the residents, present and future, to stay on and suffer the consequences when another slope fails one not-so-very-fine day.

The problem of slope instability is not confined to the Hulu Klang area but is a nationwide problem.

Despite all claims to the contrary, the mechanics of slope stability in residual soils is not well understood and that is why slope instability is so common, and so many different explanations can be given by as many experts to each single failure.

When different and even completely contradictory explanations are given for the same phenomenon, all explanations must be wrong or at most only one can be correct. That is a sure indication that the phenomenon is not well understood.

Unless, we completely stop building on slopes, an option we can ill afford, the only viable long-term solution is to conduct research to increase our understanding of the nature of residual soils.

Hopefully, when we have understood enough, correct design procedures can be developed to replace the present hit-or-miss approach based on unproven assumptions of the nature of residual soils.

N.C.N.Shah Alam