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Sunday, 27 April 2014

All eyes on the slopes

Monitoring to prevent slope failure need not be done by the experts alone. Layman residents can and should also do it.

ACCORDING to a report in The Star last month (March 28), in Kuala Lumpur alone there are more than 800 slopes that have been identified as high-risk for failure.

These slopes are in areas such as Cheras, Bukit Tunku, Bukit Dinding and Sungai Penchala.

In the report, Ikram chief operating officer Mohd Taufik Haron was quoted saying that a technical team was sent out to each slope to document any signs of distress.

“These signs include soil erosion, clogged drains or land movement. In this way, we can focus on the slopes which need maintenance before any slope failure occurs,” he said.

“Water is one of the main contributors to slope failures in a country like ours which has a high level of rainfall. Under such conditions, water control on slopes is paramount,” says Eriko Motoyama, programme director of Slopewatch, a community-based NGO that helps residents monitor their slopes for signs of landslides.

This involves looking out for bare slopes, damaged drains, cracks, leaking water pipes, vegetation growing through cracks, or loose rocks or debris on and near the bottom of the slope.

With a bit of knowledge, monitoring to prevent slope failure can be done by layman residents, Motoyama says, and one of the first things to do is to get to know the dynamics of a slope.

Ir. Shaik Abdul Wahed in his book Slopes Made Simple explains that control of water simply means directing water on the surface of the slope, usually stormwater, through a system of drains so that water flows down and away from the slope.

When drains are affected or construction of the slope was badly done, water can saturate the soil inside the slope. The combined weight of soil and water can increase dramatically and lead to failure.

Man-made slopes are typically built with a system of drains that control water (see graphic). When you stand in front of a slope, you will notice that the surface is sectioned into parts, which are called “berms”. Each section has a “berm drain”, which catches the water coming down from the slope face immediately above and from the berm itself.

At the top of the slope, there is a “cutoff drain”. The drain at the very top of a slope intercepts and safely carries away water coming from areas above.

At the bottom, there is a “toe drain” that accepts water from the slope face directly above, from the area below the slope such as roads and all water discharged by the cascade drains. Toe drains will channel all this water safely away to the public drain or suitable water course.

An integral part of the drainage system are the “cascade drains”, which are vertical drains that run down the length of the slope. Cascade drains have a special function. They are used when water has to be channelled from high ground to lower ground.

They not only carry water coming from the cutoff drain at the top and the berm drains along the slope, but they also break up the high energy of water coming down from drains at the upper levels so that water from the top and along the slope does not damage the drainage structures below.

This is done by providing a series of steps or obstructions along the drains so that the water, instead of rushing down like one tall waterfall, comes down in a number of small waterfalls. Cascade drains are designed so that the energy of the water is dissipated considerably before it reaches the bottom of the slope.

One final feature of the drainage system are “sumps”, which are junctions that allow water from more than one drain to be collected. The water is then channelled in a specific direction, such as to a cascade drain or external drain.

The sumps provide a means for water coming from various directions to safely meet head-on, which causes a lot of mixing and considerable loss of energy. It is important that both the inlets and outlets to the sump as well as the sump itself are kept in good condition, free of obstructions, and are able to provide free flow of water into the sumps at all times.

Also, remember that all the drains and structures function as a system and that any weak areas such as cracks, holes and defects due to design flaws or normal wear and tear can compromise the entire slope.

Thus, when inspecting a slope, it is necessary to look at the entire drainage system as a whole.

Be vigilant for slope safety

Tags / Keywords: Property , slope management , safety


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