Site characterization for the investigation of tunnel 8 collapse

Abstract

A section of the high speed rail tunnel collapsed before the installation of permanent lining. The section designated as Tunnel 8 was located in Miaoli County in Northern Taiwan. The resulting spoil blocked approximately 50 in of the tunnel. A crater of 28 in in diameter, with a maximum depth of 12.1 in was formed at the ground surface, at approximately 60 in above the crown level of the tunnel. A site characterization program was carried out to define the boundaries of the collapsed zone, provide ground water information and stress and stiffitess parameters for the outer and final concrete lining design. Six 100 in deep, rotary cored holes were drilled vertically from the ground surface, within the collapsed zone and penetrated through the tunnel level. SPT, pressuremeter and suspension P-S logging tests were performed in these boreholes. Seven open boreholes, approximately 60 in deep, were drilled laterally from the concrete barricade at north and south end of the collapse zone within the tunnel at various angles. Geophysical tests which included seismic refraction tomography and resistivity tests were conducted from the ground surface. The data collected from the site characterization coupled with the geological background were analyzed and an image of the possible disturbance zone was created. The paper describes details of the site characterization, their interpretation and results of the analysis.