Failure of a Retaining Structure in a Metro Station Excavation in Nanchang City, China

This paper presents the failure of a retaining structure in special geologic conditions featuring a high level of groundwater and very thick sand layers in a metro station excavation in Nanchang, China. The retaining structures were composed of cast-in-place piles, three-axis mixing piles, and bislurry grout. Although much effort was expended in designing piles that would perform well in the specific geologic conditions, failure, including sand boils and ground collapse, still occurred. Remedial measures were taken to prevent further large failures and to maintain the stability of the strut systems. The reasons for the failure were investigated afterward by analyzing the monitoring data and conducting a series of field and laboratory tests on core samples of the three-axis mixing piles and bislurry locations. The bislurry grouting was unsuccessful in preventing seepage of ground water due to the low permeability of upper clay and silty sand, which became the main reason for the failure. Instead, high-pressure rotating-jet grouting proved to be an effective way to form a waterproof wall during excavation in these site conditions. This case provides a wealth of experience and guidance for future similar excavations with regard to mitigating this type of failure.