Why do landslides occur and will there be more in future?

09 Sep 2022

Straits Times, 9 Sep 2022, Why do landslides occur and will there be more in future?
The factors that contributed to a landslide at the Housing Board's Clementi NorthArc Build-To-Order site last Friday are still being investigated.

Engineering experts tell The Straits Times that such incidents in Singapore are commonly caused by heavy rain. They say that with more frequent bouts of intense rainfall projected, more landslides may occur.

Here, they explain how landslides happen and whether parts of Singapore are more prone to these incidents.

Q What causes landslides in Singapore?
A Landslides occur when the sliding force of a slope, which comes from the weight of its soil and load imposed on the slope, exceeds the internal strength of the soil in the slope that resists these forces.

Professor of civil and environmental engineering Harianto Rahardjo from Nanyang Technological University said various factors can contribute to landslides.

One example is additional weights placed at the top of a slope or heavy rain infiltrating the soil, said Prof Rahardjo, who helms the university's research on unsaturated soil mechanics.

As Singapore is located next to the equator, where rainfall is abundant, frequent and intense downpours are the most common causes of landslides here, said Associate Professor Darren Chian from the National University of Singapore's department of civil and environmental engineering.

With Singapore's relatively flat topography, he said, landslides are smaller in scale, less damaging and generally less common here than in other countries in the region.

Q Are parts of Singapore more prone to landslides?
A No specific part of Singapore is more prone to landslides, said Prof Chian and Prof Rahardjo.

In addition to the variable nature of ground conditions across Singapore, there is the practice of topping up land in an area with soil from elsewhere.

This adds to the diversity of the type of soil found near the ground surface, said Prof Chian.

Landslides occasionally occur at slopes that have been altered for the construction of buildings when heavy rain causes water to accumulate beyond the slope's drainage system capacity, said Dr Teo Tee Hui.

Dr Teo, who is from the Singapore University of Technology and Design, has done work on predicting landslides in other countries.

But this is rare in Singapore as there are typically precautions in place, he added, noting that the Clementi landslide was unexpected.

"Slopes altered for construction or natural slopes are usually checked and reinforced to prevent landslides in most cases," said Dr Teo.

Q Will the frequency of landslides increase in future?
A "With greater intensity and frequent rainfall, occurrences of landslides are expected to rise," said Prof Chian.

This is because water has more time to gather and seep into the ground, leading to rapid reduction in shear strength of the soil that is supporting the slope against sliding, he added.

A slope made of moist soil is kept stable by capillary suction between the soil particles, which contributes significantly to the shear strength of the soil, said Prof Rahardjo.

When too much water infiltrates a slope, this decreases capillary suction, decreasing the shear strength of the soil, he added.

This, in turn, increases chances of the slope giving way.

The number of landslides across the island spiked last year as a result of intense rainfall.

The National Parks Board told The Straits Times in June this year that this could have been due to the abnormally high rainfall last year, which was nearly 70 per cent more than in the previous year.