ROADS in Singapore are checked for potholes, cracks or other physical defects regularly, said the Land Transport Authority (LTA) yesterday.

Highways and major roads, for instance, are inspected every two weeks, and smaller roads every eight weeks. Inspections are also carried out

immediately when members of the public call in, added a spokesman. It was explaining its procedures for road maintenance, following queries

from The Straits Times on the sinkholes that have appeared in recent months. Experts say that sinkholes are part and parcel of the rapid

urbanisation that Singapore is undergoing, and existing measures could be tightened to prevent any incidents. There have been four cave-ins

in three areas so far this year. Two of them - one in Woodlands Road last Saturday, and one in Keppel Road in January - were caused by burst

water pipes, the authorities said. A sinkhole also appeared in Clementi earlier this month. Four days after it was filled in, the road gave way again.

The LTA is still investigating this. The lane-wide depression in Woodlands appeared next to a construction site for the MRT's Downtown Line 2.

Experts say sinkholes are caused by several factors, including water pipe leakages, excavation works and rainwater, which may erode the sand

and silt underground. When the soil matter directly beneath the roads trickles to deeper cavities and the surface collapses, a sinkhole forms.

It does not help that Singapore has a complex network of utility lines such as power cables, water pipes and sewage lines running underground

at depths of 3m to 6m.