Investigation of a large ground collapse and countermeasures during mountain tunnelling in Hangzhou: a case study

Abstract

This paper presents a case study of ground collapse (30 m wide and 16 m deep) during tunnelling under a water channel in the city of Hangzhou, China. The ground collapse damaged the constructed tunnel locally, and remedial measures had to be taken to resume the tunnelling works. In this paper, a forensic geotechnical and geological study was undertaken in which the geological condition at the collapsed site was reviewed, the causes for the collapse were investigated, and the development of the soil failure mechanism was evaluated. Remediation measures undertaken at the project site are also discussed. During this forensic investigation study, the strata over the tunnel was found to be of low strength and furthermore erodible to groundwater seepage. Groundwater seepage was detected, which was attributed to be the triggering force that contributed to the collapse. Tunnelling construction using explosives for blasting rocks was attributed to be one of the causes of the failure. A four-step rapid remediation measure was proposed on site, which included filling of the collapsed crater, reinforcement of collapsed debris in the tunnel, grouting and dewatering, and removal of debris and tunnelling. Tunnelling was resumed soon thereafter. The field monitoring data validated effectiveness of the proposed rapid remediation.