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Thursday January 3, 2013

Misleading statements confuse public over slope failure

THE Institution of Engineers, Malaysia (IEM) is concerned about some statements made in The Star regarding the slope failure at Bukit Setiawangsa.

The article referred to is “Shotcrete caused collapse”.

The title is misleading as it prematurely states that the said failure was due to shotcrete even before a proper conclusion was made based on investigations, facts and substantiated evidence.

The statement: “The Bukit Setiawangsa landslip was likely caused by the unsuitable slope protection method used for the hill, according to experts” is overly speculative with no substantiated basis.

There could be many mechanisms or factors contributing to and triggering the slope failure.

Speculation may cause the public to have the wrong idea about the nature of slopes and how it could have failed.

Universiti Kebangsaan Malaysia Assoc Prof Dr Tajul Anuar Jamaluddin said the wall used to protect the slope was known as a shotcrete wall which prevented 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.”

In fact, one of the functions of shotcrete is to prevent direct precipitation or surface runoff from infiltrating the slope body.

To drain out water from elsewhere into the slope, there are methods, likes weep hole drains at regular intervals over the shotcrete and long horizontal subsoil drains from the shotcrete surface into the slope mass.

This relieves the build-up of water behind the shotcrete surface and also deep into the slope. 

For the said failed slope, drain holes were provided on the shotcrete surface to relieve the water build-up, if any.

“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.”

It is presumptuous to assume the role of the shotcrete surface in the original design of the slope.

In fact, after investigation, it may be that the failure was due to other factors such as from the prestressed ground anchors that were the main support of the slope instead of the shotcrete surface.

“The proper slope protection for such rocks should be something permeable like a high tensile strength wire mesh wall.”

This is a very alarming public statement on the solution for slope stabilisation recommended by Dr Tajul.

It is extremely dangerous to suggest using a “high tensile wire mesh wall” for stabilising the slope with such challenging configuration (50m high slope with very steep gradient).

This statement can have a detrimental consequences to public safety if one simply adopts such advice to construct the slope.

Engineering design is the role of the professional engineer registered with the Board of Engineers Malaysia under the Registration of Engineers Act.

Geologists should refrain from giving engineering solutions or professional advice related to designs involving public safety. 

“Shotcrete walls should only be applied for granite slopes which are solid.”

This is a contradictory statement. If the granite is solid, there would be no need to have shotcrete to protect the granite surface from surface erosion. 

Shotcrete is generally weaker than solid granite and so it defies logic to rely on a weaker material (shotcrete) to maintain the stability of a stronger material, likes solid granite.

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.

Dr Tew failed to elaborate on why the shotcrete is not suitable for areas with unstable earth. 

Without considering the slope stabilisation design holistically, it would not be appropriate to conclude the suitability of shotcrete in this case.

IEM, as a professional body of engineers, has among its members, experts in the field of geotechnical engineering.

We will be pleased to assist the Government in the detailed investigation of the failure and to recommend necessary action to prevent recurrence of such a failure.

IR. PROF. DR JEFFREY CHIANG CHOONG LUIN

Honorary Secretary

THE INSTITUTION OF ENGINEERS, MALAYSIA

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