Integrated Investigation for Geological Detection and Grouting Assessment: A Case Study in Qingdao Subway Tunnel, China

Subway tunnel construction plays a significant role in the development of urban underground space. Advance surface grouting treatment of adverse subsurface geological bodies is an effective measure to avoid the accidents during subway construction. This paper describes the successful application of geological prospecting and grouting assessment at the tunnel site of Qingdao subway line No. 1. The study area is located in a coastal bedrock fissure water distribution area. It was decided to conduct surface grouting to reduce the high risk of water inrush. Integrated investigation consisting of electrical resistivity tomography (ERT), seismic ahead prospecting, surface vertical drilling and cross-hole ERT was conducted to ascertain the geological conditions and guide the grouting scheme design. Surface ERT and seismic ahead prospecting were adopted to provide a rough survey of water bearing conditions and fractured zones. Then, detailed imaging was conducted using cross-hole ERT inversion with the inequality constraint based on the information obtained using surface vertical drilling. The shape and location of three water bearing structures were identified. Based on the integrated investigation results, an appropriate grouting scheme was designed. After the application of grouting, cross-hole ERT was carried out again to assess the effect of grouting by comparing the vibration of twice inversion results of cross-hole ERT and resistivity tests of the grout sample. Such information serves as a basis for guiding geotechnical excavations in complex urban environments and avoids the risk of construction accidents.