Propagation of Seismic Settlements at Depth to the Ground Surface-A Case History

摘要

Storage of large-diameter metallic casks containing spent fuel required the construction of concrete pads at an existing nuclear power generating facility. The subsurface investigation which included: piezocone and seismic piezocone penetration testing (CPTu, SCPTu), standard penetration testing (SPT), intact thin-walled tube sampling and laboratory testing, encountered zones of weak/compressible (soft) and/or liquefiable soils at depths greater than about 55 feet (16.8 m). Understanding the site geology, particularly the depositional environment, was a key factor in the overall assessment A 2.3-foot (0.7-meter) SPT rod drop in one borehole indicated a possible void. A seismic event at the site could trigger settlements at depths below 55 feet (16.8 m) that could propagate to the ground surface. Seismic settlements at depth were calculated for two different mechanisms: (a) compression of weak, soft materials, and (b) liquefaction-induced settlements. Settlements at depth, including a postulated collapse of a void, were propagated to the ground surface using an empirical soft-ground tunneling approach and a numerical model using Plaxis 2D. The results showed that both the empirical and numerical models predicted similar settlements at the ground surface, which were significantly smaller than those at depth, and within acceptable limits for the concrete pads.