COLLAPSE OF historic chalk tunnels is believed to be the likely cause of dramatic subsidence in a residential street in Reading last month that resulted in the evacuation of 200 people.
The front of two houses collapsed on 4 January when a large crater, 6m to 8m in diameter and over 4m deep, suddenly opened up, bursting a water main and fracturing a gas main. After emergency repairs, most residents were allowed to return to their homes.
Reading Borough Council called in local consultant Peter Brett Associates to advise on making the structures safe and on the investigation needed to determine remedial measures.
Within hours the firm's structural team had designed and implemented temporary support to the damaged houses.
About 140t (90m 3) of crushed concrete was placed in the crater, providing a semistable base for scaffold erection.
Investigations, designed by PBA geotechnical engineers Dr Clive Edmonds and John Talbot, began immediately around the subsided area and moved successively outwards in an attempt to locate competent ground.
So far, the investigation has comprised a closely spaced array of 70 dynamic probes along the street and in the back gardens of the houses. At selected places cable percussion boreholes are also being sunk to 35m for sampling and insitu testing. Further probes and boreholes are planned plus a CCTV survey in one borehole where a void was found. A desk study and research of historic use of the immediate vicinity is also underway.
Site geology consists of Reading Beds overlying the Upper Chalk.
Edmonds said that investigations indicated that collapse of historic chalk tunnels are the cause rather than a solution feature.