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Kampung Kopungit Landslide: A Simple Desktop Study

posted Sep 6, 2021, 8:05 AM by jeffery jim
It is a mellow and rainy afternoon. It is pretty much a good time to have a good read but then, I came across an update about landslide at Kampung Kopungit, Kota Kinabalu, Sabah. I did a study similar geotechnical reconnaissance work previously but it is on a large structure belong to Telekom Malaysia.

2. Kopungit hill is a known to many Kota Kinabalu residents as an exciting hill which provide easy tracking where the trail is rather friendly and also challenging. Geologically, this hill a mix of a few types of topography - swamp at the foot of the hill and surrounded by moderate and high hills with slope of more than 25 degree. This village is home to many villagers in a highly concentrated and dense population where houses are build too close to each other regardless of the terrain.

3. The landslide triggered in between two types of soil association which is the Lokan Soil and Dalit Soil at a higher elevation. The main soil unit for both soils is the Acrisols with dispersion of Cambisols at the ridge where water tanks are sitting. The main concern of Acrisols type of soil is its nature which is usually porous by nature until it is stripped down and form hard surface crust. This clayey type of soil forms impermeable surface which allows water runoff on the surface. Nevertheless, consistent rain and the formation of rills on the slope disposition the soil from the top to the lower section part of the slope.

4. Water runoff plays important role in devastating event such as landslide when soil constituents start to form additional overburden on slope areas which have been in equilibrium due to the cohesiveness of the clay and certainly some angle of friction resistance derived from larger and coarser aggregates with alluvial related origin. This disposition leads to disequilibrium and certainly periodic water saturation at lower elevation does not help; instead, adding pore pressure at the whole equation of the geotechnical calculation. Hence, the slope is no longer stable and triggers landslide as a mechanism to shed the overburden until a shallower stratum can remain in equilibrium. Sliding occurred and soil moves; and dwellers and properties will experience the wrath of mother nature.

5. A short desktop investigation shows that this event is not an event which occurred suddenly and compulsively. Landslide is the outcome of progressive loss of bonding in soil matrix. For the past two years, satellite images shows that there are progressing soil erosion surrounding the affected area as per attached photos. There are also clear indicators where temporary remedial works were on place where tarpaulin is used to cover the slope before permanent slope stability restoration works can progress.

6. Tarpaulin is a great remedial but it is momentary and should not be placed on the slope surface without further engineering considerations. In many cases, half covered slope is worse than status quo as water saturation at the toe of the slope will weaken the overall slope stability and introduce the trigger point. This can be avoided if proper drainage and water diversion as well as tension cracks at the ridge is remedied. The tension crack might not be visible in this particular case but it is developed in between weakened soil and stable soil which sits directly under the water tank structure with pile foundation.

7. These geotechnical issues aggravated by site condition where most slopes have close to 90 percent gradient which is almost 1:1 in ratio for horizontal and vertical. The soil erosion possibility increases by hundreds of folds from a flat soil to undulating hill and grievously hazardous at this kind of steep and high slope.

8. The other condition which many may not be clear here during design consideration or site valuation is the overburden introduced by vehicles and residential at these slope. Apart from overburden, what is significant and consequential condition would be the pore pressure buildup at certain areas which is an impermeable layers under the houses and road. Water infiltration rate which is not uniform and therefore leads to difference which is not considered when modelling the geotechnical condition based on drained and/or undrained conditions of a triaxial shear test.

9. One of the method which is not introduced here is the utilization of gabions as counterweights at the toe slope with proper interceptor and horizontal drains as a temporary measure before the construction of proper retaining wall.

10. The use of rubble wall as permanent solution is not advisable since TM building which is nearby gone through such slope failure from the use such engineering feature. Unknown or undetermined finite element modelling may not able to consider all complex stress conditions of this particular slope and hence, design criteria involving overturning and sliding should be increased to 10.0 for factor of safety.