Characterization of faults and shear zones and their impact on temporary and permanent construction of the MTRC Kwun Tong line extension alignment

Abstract

Direction of the nearby maximum principal stresses and the mass weathering of the surrounding geology have been accepted as some of the most significant factors in the design of underground excavations and structures. However, presence and exact location of localized fault/shear zones is extremely difficult to determine by conventional ground investigation techniques. More often than not, the effect of the fault/shear zones is not fully realized until they become exposed by the actual excavation. Thus, this case study was set out to determine whether the orientation of fault/shear zones can be accurately predicted based on hydraulic fracturing tests carried out in the vicinity. Detailed geological information had been gathered from mapping records which were carried out during the excavation and construction of the MTRC Kwun Tong Line Extension (KTE) project. Based on the mapping data, 17 as-encountered and inferred fault/shear zones, and the corresponding temporary support and permanent structures, along the KTE alignment were carefully examined. The degree of temporary support and reinforced concrete lining were, respectively, found inversely proportional to the rock mass quality in terms of Q-value and the presence of completely weathered material in the tunnel crown. The orientations of the inferred fault/shear zones examined in this study agreed with the direction of the maximum principal stresses measured from hydraulic fracturing tests carried out in the vicinity. The findings of this case study suggest that orientations of fault/shear zones can in fact be predicted and mitigation measures may be prepared before the effect of fault/shear zones becomes fully manifested.