Application of geophysical methods to determine bedrock conditions within a tunnel alignment

The Upper Scioto West Interceptor Sewer is a 10-kilometer long bedrock tunnel under construction in Columbus, Ohio. Tunneling progress was halted when the tunnel boring machine (TBM) encountered a clay-filled solution valley measuring approximately 75 meters (240 feet) wide. A long and expensive delay followed while a vertical shaft was excavated ahead of the TBM, and a horizontal tunnel was hand dug through the soft material to reach the TBM. A geophysical survey was contracted to identify additional problem areas in advance of further tunneling. The survey included two conventional geophysical techniques, seismic refraction and microgravity, and a new technique named Forward Looking Seismic. Seismic refraction provided a profile of the bedrock surface along the tunnel alignment. Microgravity also detected bedrock valleys as well as low-density bedrock zones attributed to fracturing, solutioning, etc. The third method, Forward Looking Seismic, was conducted as a preliminary test. The FLS test detected two geological features ahead of the TBM, a low-density bedrock zone and a clay-filled void. The low-density zone corresponded to an area where jet grouting was required to facilitate tunneling. An estimate of the location and orientation of the void was successfully made.