Detection of an abandoned mining channel under a residential complex by ground penetrating radar

Ground Penetrating Radar (GPR) investigations were conducted in a 2-storeyed newly built residential complex. The complex had 124 flats in 31 blocks spread over 1.2 hectare. Though conventional geotechnical investigations had revealed the SPTvalues <N>ranging 11-72and refusal in boring met at 0.5 - 4.0 m depth, no indication of any underground cavity or channel was observed. The flats were ready for occupation when in Dec. 1990, a small portion near the stair case between two flats caved-in after moderate rains in the region. Assuming it an isolated void, the filling and compaction was done. Around Aug.1991, the region experienced intermittent rains and a bigger portion between two blocks caved-in to about 2.5 m depth. It caused severe damage to the adjoining flat; its kitchen portion suffered subsidence and wide spread cracks developed in load bearing walls. The event called for non-destructive high resolution geophysical investigations. Since mining of Kaolin and Mica was carried out in the surrounding region several decades ago, it was considered that some abandoned mining channel might be buried under the damaged blocks. Thus GPR investigations were preferred in the first instance.