A method for detecting and pretreating boulders during shield tunneling in granite strata

Boulders are spheroidal and weathered rock fragments that exist in softly weathered granite strata. Boulders can cause severe damage to the cutter heads of shield machines and can even lead to the failure of shield tunneling during the excavation of metro tunnels in granite strata. However, there is no effective method to safely and rapidly cope with this problem on a large scale. This study proposes a method to detect and to pretreat boulders during shield tunneling within granite strata. The detection method is a combination of microtremor surveying and supplementary drilling exploration to ensure both rapid, large-scale detection and high detection accuracy. Based on the microtremor survey results, principles of interpretation are developed to predict the presence of boulders and evaluate the safety status of shield tunneling. Furthermore, the predicted boulders are precisely confirmed by supplementary drilling exploration. After the boulder locations are detected, a combined surface drilling and preblasting technique are adopted to pretreat the boulders. This method was successfully applied in the shield tunneling of Fuzhou Metro Line 5 in China. The application shows that this method is effective for the prediction and pretreatment of boulders during shield tunneling. The prediction of boulders has high accuracy. The preblasting of boulders improves the tunneling efficiency, which significantly reduces the total thrust and torque on the cutter head, and markedly increases the tunneling speed.