The official report into the 16 December 2022 Batang Kali landslide

The author of The Landslide Blog is Dave Petley, a renowned expert in the field of landslide research and control.

After the devastating landslide on December 16, 2022 in Batang Kali, Malaysia that claimed 31 lives, the government launched an investigation. This was not a public inquiry, but rather a forensic analysis to determine the reasons behind the landslide and gather insights. The official report was recently released in Kuala Lumpur and has sparked significant attention within Malaysia.

The report, written in English, can be accessed on the internet. It was headed by CKC, the Slope Engineering Division of the Public Works Department, with contributions from numerous other organizations. Its primary findings are:

• The landslide shifted from a rotational failure to a debris flow.

• The reason for the landslide was excessive rain that resulted in elevated levels of water in the soil, partly due to seepage.

• There is insufficient proof to indicate that human actions played a role in the occurrence of the landslide.

It’s difficult to dispute any of these points, and I believe the analysis has been executed effectively. CKC is a robust organization, although I concur with others that it could benefit from some strengthening (refer to the following).

However, there are several interesting observations in the report. The research indicates that a unique series of occurrences took place, involving two instances of slope failures. This is depicted in the diagram provided in the report.

During the initial failure, a significant landslide took place on the upper slope, resulting in a debris flow that reached the base of the slope. This landslide also caused a temporary blockage of debris, which collapsed after 20 minutes and formed the final shape of the area.

Although it is common for slopes to experience multiple phases of failure, it is unusual that the second failure did not result in any retrogression or an increase in the distance traveled. While it is not impossible, it is unexpected.

Two out of the three witnesses provide accounts of two stages of malfunction, but they do not provide much information on the specifics of these failures (which is expected since it occurred at night). The project team has recreated the initial slope (discussed further below) in a simulation that takes into account seepage. The simulation shows the occurrence of a rotational failure that reached the road, caused by elevated pore water pressures. This appears to be a plausible explanation.

The team determined that following the initial failure, the debris created a dam on the slope. This dam then moved again, causing a second landslide due to the water trapped from the scarp region. However, please note that according to the diagram shown above, the first failure also reached the farthest point on the slope, as I had previously pointed out.

From my perspective, there is no provided explanation for how this interpretation was determined, and there doesn’t seem to be any acknowledgement or exclusion of other potential models. For instance, it is possible that a smaller failure occurred lower on the slope with a shorter distance traveled, followed by a secondary retrogression that resulted in the complete runout. Alternatively, the initial failure may have only affected a portion of the total landslide width, with the second failure involving the remaining material.

There is insufficient evidence to prove or disprove either of these hypotheses. However, I cannot dismiss them solely based on the information provided. The interpretation appears to be influenced by a 2D slope stability model, but caution must be exercised when interpreting such models. The specific layout of the landslide is likely highly dependent on estimated input parameters.

Another interesting feature is the embankment located on the top portion of the incline, towards the lower side of the road. This can be observed in the illustration provided in the report as well:

The structure made of material fills is situated in a highly crucial location on the incline. It is worth noting that the back edge of the ultimate collapse aligns with the road’s edge, which is either directly on or very close to the edge of the fill. This can be clearly seen in the image on the cover of the report.

According to the report, the fill embankment was first observed in satellite imagery in 1992. The shape of the embankment, as depicted in the cross-section, is quite uncommon. I am interested in understanding the reasoning behind constructing an embankment in this manner. However, the report does not provide sufficient information about the purpose of the embankment. It appears to be a crucial aspect that warrants further discussion.

• It seems that there is an increase in weight on the top of the final landslide.

• The substance can have the ability to let liquids pass through and also retain water, resulting in increased water pressure within the pores.

• The first failure could potentially be caused by the embankment collapsing.

This is important because the report’s conclusion states:

While I do not disagree with the conclusion, it appears to me that it has adopted a specific perspective on the definition of human activities.

The Malaysian media has received a varied response to the report. However, the most significant long-term reaction comes from Kua Kia Soong, a previous member of Parliament for Petaling Jaya, who penned a letter expressing his thoughts.

We need to completely revamp the regulations for developing hillslopes. This includes strict environmental impact assessment requirements, regular upkeep, and open communication with all members of the community. It is crucial that engineering projects prioritize professionalism and a strong dedication to stability and user safety. Only by tackling these deeply ingrained problems can we avoid future devastating disasters.

Absolutely! A great beginning would involve additional funding for CKC.

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