3 January 2013

Greater clarity, and some controversial media statements, about the Bukit Setiawangsa landslide in Malaysia

Posted by dr-dave

Over the last few days it has become much clearer as to what happened at Bukit Setiawangsa in Malaysia on 28th/29th December.  In particular, better media images suggest that this was indeed a large-scale, probably rotational, failure behind and through the engineered slope.  Probably the best image gallery can be found here, which includes this shot :

http://fz.com/content/cliffhanger-setiawangsa

This aerial view is also very helpful:

http://mynewshub.my/2012/12/31/benteng-60m-di-bukit-setiawangsa-tertinggi/

Great care is needed in over-analysing these images to come up with definitive statement on the cause of this failure as this appears to be a landslide that is more complex than it might initially appear.  For example, on the left side of the slope (as it appears on the image above) the boundary of the failure appears to lie at the edge of or even outside of the reinforced slope (it is a strangely linear lateral scarp) for most of its length.  In the lower portion though the scarp cuts back through the concrete.  Note the additional deformation (buckling of the concrete) in this lower portion of the slope as well.  Indeed, it appears that there is  more movement on the left side of the landslide than on the right (note the tilt in the displaced structures at the top of the dispilaced mass).

Meanwhile, there is a distinctly curious press report about the landslide in the online version of The Star newspaper, quoting Prof Dr Tajul Anuar Jamaluddin, an Associate Professor at Universiti Kebangsaan Malays.  The article says:

‘Shotcrete’ caused collapse

The Bukit Setiawangsa landslip was likely caused by the unsuitable slope protection method used for the hill, according to experts.  Universiti Kebangsaan Malaysia Assoc Prof Dr Tajul Anuar Jamaluddin said the wall used to protect the slope was known as a “shotcrete” wall which prevents water from entering the soil. “However, water can still seep into the slope from areas not covered by the wall but the wall also prevents water from flowing out. In other words, water can flow in but not out. When this happens, the groundwater pressure builds up and breaks through the wall, causing the landslip,” he said when contacted

Dr Tajul, who teaches geology, said shotcrete walls were also not meant to be built on highly metamorphic rocks at Bukit Setiawangsa, which are not strong enough.  “The proper slope protection for such rocks should be something permeable like a high tensile strength wire mesh wall. This method allows water to seep in and out and vegetation to grow to prevent erosion,” he said. Dr Tajul said that improper use of the shotcrete wall was common in Malaysia. “Shotcrete walls should only be applied for granite slopes which are solid.  However, they are commonly used because they are three to four times cheaper than the permeable walls,” Dr Tajul said. He added that the Bukit Setiawangsa slope was cut too high and steep at 60° while houses were built too close to the edge. Sensitive land development specialist Dr Tew Kia Hui said shotcrete walls were designed to prevent erosion but should not be constructed in areas with unstable earth. “Such walls cannot withstand a heavy load above the soil. This includes houses, apartments and boulders,” he said.

Of course I have no idea as to whether these are correct quotes, or if they have been taken out of context, but they seem to be to be strangely misleading.  These comments have already been addressed in a letter from the Institution of Engineers Malaysia, but let me add some thoughts.

First, whilst shotcrete might have been used to provide the final finish, this is not really what one would consider to be a shotcrete wall.  In this slope the primary strength was probably provided by the prestressed ground anchors (which are the pairs of studs that can be seen across the surface).  Frequently in failures of this type the ground anchors either failed to penetrate the slip circle or they were not sufficiently strong to provide the necessary additional strength to the slope (which may be because the anchors were too weak or the strength of the soil was incorrectly evaluated).  I have no way of knowing whether this was the case here, and it is not sensible to speculate at this stage.

Second, drainage may or may not be a problem at Bukit Setiawangsa, but there is no reason why an engineered slope of this type should lead to drainage problems if properly designed.  The key issues are to ensure that adequate drainage holes are provided and that these are maintained correctly.  I am sure that the forensic investigation will look at this carefully.

Third, when engineered slopes like this fail they do not do so because the water pressure breaks through the shotcrete.  They do so because the rock / soil mass displaces.

Fourth, engineered slopes such as this are in common use around the world, with great success.  If properly designed there is no reason why they should be avoided in this sort of setting or lithology.  I remind regular readers of the absolutely amazing ship lock wall at the Three Gorges Dam site, which I featured last year.  This slope is 102 m high:

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Fifth, the last thing that anyone would ever use in this situation is a wire mesh wall.  The images above show that this slope is in weathered residual soil; a mesh could not retain such a material (which would flow through the gaps) and would not provide sufficient strength to avoid a rotational failure.  Wire mesh walls have their uses; this is not one of them.

Sixth, I  am surprised at the comment at the end by Dr Tew Kia Hu, which seems to imply that engineered slopes cannot have structures at the crest.  I wonder if he was aware that this is a slope with ground anchors rather than just shotcrete (which would explain the comment).   Actually the buildingst Bukit Setiwangsa are very light weight in comparison with the mass of the slope, and are unlikely to be the cause of the problem.  Elsewhere, really big structures are built at the crest of engineered slopes, but of course the design does need to be appropriate.

Of course, Malaysia recently completed a National Slope Masterplan to reduce the occurrence of these sorts of events.  The correct approach here is of course first to ensure that the slope at Bukit Setiwangsa is safe; second to learn lessons from it; and third to implement the Masterplan in full. As has been shown elsewhere, this is a proven way to manage slopes properly.