Engineer: Excavation and drop in water table caused sink holes

The Star 3/5/2004 By LEONG SHEN-LI

Gue ... 'sinkholes and cracks in houses could not have been caused by vibration from blasting'

SINKHOLES and soil settlement, which has been appearing near the Stormwater Management and Road Tunnel (SMART) project along Jalan Cheras and Jalan Tun Razak, should no longer occur once the initial open-cut excavation works are completed. 

Geotechnical engineer Dr Gue See-Sew said once initial excavation and blasting work was finished, the water-table, which had dropped because of excavation work, would return to its original level, removing the main cause of sinkholes and soil settlement. 

“I cannot say that there will be no more sinkholes or soil settlement, but their incidence is highly unlikely,” said Gue, who has been engaged by SMART to look into the appearance of sinkholes and cracks in the walls of buildings near the project sites. 

He said excavation works would have already triggered most of the sinkholes which could have occurred in the area. 

Explaining the phenomenon, Gue said about a third of Kuala Lumpur was built on limestone bedrock. Sinkholes tended to occur with such a geological formation. 

“Limestone surfaces have a lot of cavities which are either hollow or filled with water. When the water-table is lowered because of excavation work, the layer of soil or alluvium above the cavity will increase in weight because the support from the water-table disappears,” he said. 

The increase in weight may cause the underside of the soil originally covering the cavity to slowly collapse, creating a sinkhole when all the soil falls through. 

“This situation is aggravated by rain, which causes the soil above the cavity to get saturated with water. This increases the weight of the soil significantly, causing it to collapse into the cavity,” he said. 

Gue said initial soil investigation work had helped identify most of the cavities along the tunnel alignment and treatment work by filling such cavities with grout was then carried out.  

Grout consists of liquid cement or chemicals which, after being injected into the cavity, would solidify and then plug the cavity. 

“However, it is impossible to identify each and every cavity, and to know exactly how big a cavity is,” he said. 

Gue added that sinkholes and cracks in houses could not have been caused by vibration from blasting. 

“The sinkholes, soil settlement and cracks which occurred in the houses and buildings were located far away from the excavation sites,” he said. 

Meanwhile, the project's tunnelling expert Gusztav Klados said tunnelling work should not create any sinkholes or soil movement because of the special tunnel boring machine (TBM) which had been chosen. 

He said the TBM was a slurry shield type machine, which could create a pressurised shield at the front end of the machine by using pressurised air and bentonite, a type of liquid clay. 

“Again, we work with the same principle of maintaining the water-table level. The two tunnel boring machines (TBM) are specially built to ensure that the water-pressure in front of the TBM does not fall due to water flowing through the machine while boring work is being carried out,” he said. 

Klados said soil investigation was carried out along the tunnel alignment and treatment work was being carried out to ensure that nothing unexpected would occur. 

“When boring works start, we will have teams who will constantly carry out soil settlement measurement, water-table measurement and even visual monitoring,” he said, adding that dilapidation surveys had been carried out on hundreds of buildings to establish their structural condition before work began. 

He said special attention would be given to places where the tunnel alignment ran under highly populated areas, major roads and highways and railway lines.