Abstract
Tunnel construction by mining method in water-rich sandy cobble strata faces challenges including surrounding rock instability, water and sand inrush, and settlement control. The conventional shallow buried mining method suffers from poor grouting effectiveness and low support efficiency. The sandy cobble layer, characterized by high porosity and large particle size, leads to difficulties in hole formation for advance support, non-uniform grout diffusion, and susceptibility to face collapse, necessitating process optimization to enhance safety and efficiency. The mined tunnel in the station rear distribution line section of Xi'an Metro Line 5 and Line 21 at Xi'an East Station measures 178.672 m in length with a cross-section of 13.4 m × 11.493 m. The arch crown traverses a water-rich sandy cobble layer (cobble particle size 20–200 mm), with the phreatic water level at a depth of 12 m. The construction requires undercrossing highways and shallow-foundation residential buildings, imposing stringent settlement control requirements. The original design employed 3 m small pipes and single-fluid grouting; however, difficulties in hole formation and insufficient grouting during construction resulted in schedule delays and safety risks. Based on the characteristics of the sandy cobble stratum, this study proposes the following optimization scheme: optimization of advance support parameters—small pipe length reduced to 2 m, circumferential spacing decreased to 300 mm, one pipe per longitudinal frame, and adoption of cement-sodium silicate double-fluid grout (grouting pressure 0.1–0.3 MPa) to improve grout diffusion and consolidation efficiency; improvement of construction techniques—adhering to the principle of 'short thin pipes, light disturbance, rapid solidification, and stable arch foot', adding a 3–5 cm shotcrete cover to stabilize the working face, and combining the bench method with the double-side wall drift method for stepwise support to reduce surrounding rock exposure. Following optimization, construction efficiency increased to 2 m per day, with monthly progress reaching 60–70 m and the construction period shortened by nearly half. Surface settlement, convergence, and building settlement all remained below warning thresholds, validating the technical reliability. This study has developed a rapid construction technology for shallow buried mining in water-rich sandy cobble strata, resolving the challenges of grouting and support efficiency while ensuring safe and efficient project implementation. The findings provide key technical references for subway construction in similar geological conditions, advance tunneling technology in complex strata, and deliver significant economic and social benefits.
Full Text
Construction Technology of Shallow Buried Tunneling through Water-Rich Sandy Gravel Strata
Wang Yizhuo
Guangzhou Metro Engineering Consulting Co., Ltd., Guangzhou 510000, China
Abstract
Tunnel construction in water-rich sandy gravel strata faces significant challenges, including surrounding rock instability, water and sand inrush, and settlement control. Conventional shallow tunneling methods suffer from poor grouting effectiveness and low support efficiency. The high porosity and large particle size of sandy gravel layers make advance support hole formation difficult and prevent uniform grout diffusion, often leading to face collapse. These issues necessitate urgent process optimization to enhance both safety and construction efficiency.
The underground excavation tunnel for the connecting line behind Xi'an East Station of Xi'an Metro Lines 5 and 21, measuring 178.672m in length with a cross-section of 13.4m×11.493m, traverses water-rich sandy gravel strata (cobble particle size 20–200mm) at the crown, with the phreatic water table located 12m below the surface. The construction required undercrossing highways and shallow-foundation residential buildings, imposing stringent settlement control requirements. The original design employed 3m small pipes with single-fluid grouting; however, difficulties in hole formation and insufficient grouting resulted in schedule delays and safety risks.
To address the characteristics of sandy gravel strata, this study proposes an optimized scheme comprising two key improvements. First, advance support parameters were optimized: small pipe length was reduced to 2m, circumferential spacing decreased to 300mm, with one ring installed per support frame, and cement-sodium silicate double-fluid grout was adopted (grouting pressure 0.1–0.3MPa) to improve grout diffusion and consolidation efficiency. Second, construction techniques were improved following the principle of "thin-short pipes, light disturbance, rapid solidification, and stable arch foot," which involved adding a 3–5cm shotcrete cover to stabilize the working face, combined with bench cut method and double-side wall drift method for step-by-step support to minimize surrounding rock exposure. Following optimization, construction efficiency increased to 2m per day, achieving monthly progress of 60–70m and reducing the construction period by nearly half. Surface settlement, convergence, and building settlement all remained below warning thresholds, validating the reliability of the technology.
This study has developed a rapid construction technology for shallow tunneling in water-rich sandy gravel strata, resolving the challenges of grouting and support efficiency while ensuring safe and efficient project implementation. The findings provide key technical references for subway construction in similar geological conditions, advance tunnel construction technology in complex strata, and deliver significant economic and social benefits.
Keywords: Metro tunnel; Sandy gravel layer; Stratum grouting reinforcement; Shallow tunneling method construction technology