PETALING JAYA: Internal erosion of soil may have caused the rockfall at the New Klang Valley Expressway (NKVE), said a soil erosion consultant.
It could have been due to large amounts of water seeping into the back of the rock layer, said Valentino Tew Kia Hui of VT Soil Erosion and Research Consultancy.
The slope, which Tew thinks is made up of soil at the back and granite rocks in the front, could have been subjected to intense rainfall.
If any area receives more than 34mm of rainfall in a day, I give it a value of 1 erosion risk frequency. This means there is a possibility of an erosion event occurring in a particular area, he said.
According to the Meteorological Services Department, 125mm of rain fell on Monday, 2mm on Tuesday and finally 11mm on Wednesday, the day of the rockfall.
He recalled that there was about 2.5 erosion risk frequency around Sungai Buloh in November 1999, which meant that there could have been a possibility of two erosion events occurring then.
Once the water seeps into the soil, a process called internal erosion could have occurred in the soil structure, therefore washing away soil particles.
Internal erosion is a process which cannot be seen on the surface but happens within the soil itself, he said.
Tew hypothesised that cavitations had formed in the soil, which held the granite in place and eventually slid away, causing the massive rockslide.
However, he said there might not have been anything that could have been done to prevent the mishap.
Tew, who wrote a book in 1999 with Dr Roslan Zainal Abidin of Universiti Teknologi Mara identifying areas along the North-South Expressway that were at risk to erosion, ranked that portion of the NKVE as being moderate to high risk.
Preventing an occurrence of landslides is a process which requires long-term initiatives such as constant monitoring and continual maintenance. The costs for such monitoring and maintenance are expensive but necessary to prevent the loss of life, he said.
Institution of Engineers president Prof Abang Abdullah Abang Ali said most landslides were preventable and its occurrence could be avoided if slopes and hillsides were designed and constructed properly.
The problem most engineers face in the design and construction is the cost. Most clients are reluctant to pay. So the problem starts when insufficient research is done, contractors are not advised properly and these structures cannot survive forever, he said.
Constant monitoring is required, Prof Abang Abdullah said.
However, he refused to speculate the cause of the rockfall until further studies are conducted.
Although no showers were recorded yesterday, water was continuous flowing from the hill slope into drains where the rockfall occurred.
A worker involved in the clearing of the debris said that since the incident, water from the hills had been flowing down onto the road and drains.
The flow downhill is also quite heavy and fast, he said.
He added that the water would eventually flow into the valley about 200m from the site.
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