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Slope Failure at Tanjung Bungah, Penang

posted Sep 8, 2019, 6:48 AM by jeffery jim

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I just got a copy of the Commission of Enquiry Report which highlighted the culprit of the whole incident, the Principal engineer of KAA, Ir. Khoo Koon Tai. He committed gross negligence and could have remedied the problem for more than ten (10) occasions but he prefer to hang the skeleton in his closet.

First, what he did is not value engineering. It is the opposite of value engineering for any cost saving design and exercise. After 4 landslip events, he did nothing. What really surprised me is his inability to call for interim monitoring before committing to any design change and slope rehabilitation. Leave alone the other basic engineering practice which he deliberately ignored.

Dumbfounded by the whole case, I wonder if he have the access to knowledge for monitoring and supervision purposes. I think he lacked of understanding when it comes to engineering and design philosophy. When encountered by extreme case or unknown, it is best to design assisted by series of testing. That is the main rule which is clearly mentioned in Eurocode.

To be frank this site is relatively small or puny in comparison to some of the road construction sites. With four slopes and three berms, it is considered as a minor slope in comparison to some road project slopes which reached up to eight to twelve berms. It is deemed to be an everyday slope for most of us.

After the second landslip and the deviation from the conceptual design, he should have halted all works and review the slope FOS. It is true that what attained during site investigation may varies, it is also a proper practice by discharging his duty and liability with simple geotechnical instrumentation such as piezometer and inclinometer.

These instruments will assist in checking the ground water table and also soil movement which allow one to understand the actual scenario of the hydrostatic pressure and the strata movement. When inclinometer ratio is more than 0.2, it is time to stop work at the affected or adjacent areas.

Another thing which rarely good engineers or clerks-of-work are able to pick up is wet or saturated water condition at the top slope and stress crack lines or fissures.This indicates the matured slip circle formation where cohesion and internal angle of friction have lapse and the stability is now critically dependence on the self-weight and counterbalance at the slope toe. By rule of thumb, if you find stress fissure at the top slope, the FOS is around 1.1 to 1.2 where JKR Guideline for slopes require around 2.0-2.5 for different criteria which forms the global stability requirements.

The top slope fissure usually indicates Acrisols (low base and plasticity clay) soil is saturated with water. That the easiest tale-tell sign that one may need in order to build a conclusive preliminary condition of a slope. It means the load is more than the soil's ability to hold on through the internal angle of friction and cohesion.

In case fissures turned to crack lines which are more than 1 or 2mm wide, the first thing to do is to order for a counterbalance measure to be placed at the slope toe (rock-filled gabions) in order to contain the slip. Second, horizontal drain should be installed with tarpaulin lining for the swale-like earth drain instead of deep surface drains for draining ground water and reduced water flow net which subsequently stabilize the slope by reinstating the internal angle of friction for soil. Lining is helpful against introducing or recharging the ground water due to infiltration and due to surface runoff during storm event.

Once mitigation measure have taken place, it is time to install piezometer and inclinometer at critical areas for monitoring. No earthwork is allowed 10m to 15m away from this slope. After a period of 60 to 90 days, sacrificial soilnail or rock bolt test is allowed.