THE INVESTIGATION OF DISSOLUTION SUBSIDENCE INCORPORATING MICROGRAVITY GEOPHYSICS AT RIPON, YORKSHIRE

Dissolution subsidence affords some of the most difficult ground condi tions with which engineering geologists have to deal. Within the UK, a reas underlain by gypsiferous Permo-Triassic strata, most notably arou nd Ripon in Yorkshire, are prone to dissolution structures and resulta nt building failures are well documented. Conventional drilling of suc h unstable sites is often a 'hit and miss' affair and most geophysical techniques do not provide sufficient resolution to offer adequate con fidence in the results. Proposals for the redevelopment of a site with in the urban area at Ripon could not rely on such frequently inconclus ive methods and it was necessary to implement a phased approach to sit e investigation. Following a desk study, high-resolution microgravity geophysics was carried out both inside and outside the existing buildi ng. This indicated a major negative anomaly of peak amplitude - 74 muG al. Subsequent static core probing, rotary drilling and trial trenchin g confirmed the existence of a potentially unstable breccia pipe which could therefore be taken into account in the engineering design.