Abstract
Taking the EPB shield tunneling of Guangzhou Metro Line 18 through full-face sand layers at large burial depth as the engineering background, this paper summarizes the key technologies for soil conditioning during construction and draws the following conclusions: 1) When shield tunneling through water-rich sand layers, the high water content, high groundwater pressure, and strong permeability of the excavated soil make screw conveyor spewing prone to occur. Therefore, anti-spewing devices should be installed at the screw conveyor outlet, and corresponding soil conditioning measures should be implemented according to ground conditions to prevent spewing. 2) Soil conditioning represents both a critical challenge and key focus during shield tunneling in water-rich sand layers. According to empirical experience, the fine particle content in typical strata should exceed 30% to impart adequate fluidity and water-sealing capacity to the excavated soil. However, in this project, the fine particle content of the strata was only 11%–14.5%, yet no spewing occurred and the screw conveyor discharged soil smoothly, demonstrating that the adoption of foam injection alone for soil conditioning according to the specific ground conditions of this section is feasible. Consequently, suitable soil conditioning methods should be determined through testing and other means based on ground conditions during construction. 3) As sand layers are prone to collapse when disturbed, the shield can rapidly traverse the sand layer section by increasing tunneling speed and maintaining stable construction parameters, thereby avoiding excessive disturbance to the strata. Simultaneously, the excavated soil volume should be strictly controlled, and the quality of synchronous grouting and secondary grouting should be ensured to maintain surface settlement within normal limits.
Full Text
Preamble
Study on Muck Conditioning Technology for Deeply Buried Shield Tunneling Through Full-Face Sand Strata
Zhu Siran, Xie Wenda, Chen Minjie
Guangzhou Metro Engineering Consulting Co., Ltd., Guangzhou 510000, China
Abstract
This paper examines the key muck conditioning technologies employed during the construction of Guangzhou Metro Line 18, where an earth pressure balance (EPB) shield tunnel was driven through full-face sand strata at considerable depth. The study yields three primary conclusions. First, when tunneling through water-rich sand layers, the high water content, elevated groundwater pressure, and strong permeability of the muck can readily induce spewing from the screw conveyor. Therefore, anti-spewing devices should be installed at the screw conveyor outlet, and appropriate muck conditioning measures must be implemented based on ground conditions to prevent spewing.
Second, muck conditioning represents a critical challenge during tunneling in water-rich sand. Empirical evidence suggests that fine particle content must exceed 30% to ensure adequate muck fluidity and water retention. However, in this project, despite fine particle content of only 11%–14.5%, no spewing occurred and screw conveyor discharge remained smooth, demonstrating that foam injection alone was a viable conditioning strategy for the specific strata encountered. This underscores the importance of selecting conditioning methods based on site-specific conditions through experimental validation.
Third, because sand strata are prone to collapse when disturbed, the shield should traverse these sections rapidly by advancing quickly while maintaining stable operating parameters, thereby minimizing excessive ground disturbance. Simultaneously, strict control of excavation volume and ensuring the quality of synchronous and secondary grouting are essential to keep surface settlement within acceptable limits.
Keywords: earth pressure balance shield, water-rich sand layer, spewing, muck conditioning