Geotechnical Problems Caused by Unknown Cellars

Some months ago there was a collapse in a public park in Budapest, Hungary. On the location called Kőbanya the limestone was covered by 10-30 m of good quality clay. For that reason clay was excavated in general in open-pit mines while limestone was exploited in quarries and in underground tunnels as well. The main reason of the accident was consideret to be the collapse of an unknown tunnel or cellar. The surroundings of the void was investigated by different gophysical metods to find the reason of the event and to map the potencial location of latter collapses. DC geoelectric, GPR, seismic diffraction and tomogrphic methods were applied. The findings were proven by drillings and CPT soundings.