Preamble

Technical Research and Engineering Practice of Excavating Existing Structures' Pile Groups in Composite Strata with Super-Large Diameter Shield Tunneling Machines

Zhang Haibin, Lu Yangyi, Zhan Chun, Dai Changkang, Liu Jiazhi, Fang Cheng
(Guangzhou Metro Engineering Consulting Co., Ltd., Guangzhou 510000, China)

Abstract

As shield tunneling technology extends toward super-large diameters and complex geological conditions, safety control during the excavation of existing building pile groups has become a critical engineering challenge. This paper takes the Guangzhou Haizhuwan Tunnel Project as a case study, drawing upon the engineering practice of a 15.07m super-large diameter slurry balance shield machine passing beneath the Zhonghe Business Building and residential complexes, to systematically investigate the construction risk control system for shield excavation of pile groups in composite strata.

The study reveals the deformation mechanism and risk pathways during shield tunneling beneath pile groups: during the interaction between the shield and piles, as the cutterhead engages the piles, significant changes in side friction resistance occur at the pile-soil interface, triggering soil stress redistribution. In composite strata, the argillaceous siltstone contains 35% clay minerals, where low-specific-gravity slurry causes soil collapse and rapid increases in mud density, leading to cutterhead mud caking and accelerated tool wear. Additionally, during load transfer between the underpinning structure and original structure, uneven load transfer at the moment of pile cutting can easily cause structural instability.

Based on this mechanistic understanding, an innovative "soil conditioning-cutter layout-parameter matching-dynamic monitoring" four-in-one intelligent control system was developed, achieving three major technical breakthroughs. First, innovation in parameter coupling mechanisms: a multi-parameter coupling function was established among face pressure (±0.1 bar), cutterhead rotation speed (0.8–1 rpm), and thrust force (8000–11000 T). Analysis of trial excavation data revealed that when face pressure fluctuation exceeded ±0.2 bar, surface settlement variations reached 10–30 mm, whereas when controlled within ±0.1 bar, settlement variations remained stable within ±5 mm. The study also revealed a proportional relationship between mud density (1.15–1.2 g/cm³) and shield torque/thrust, where each 0.05 g/cm³ increase in mud density increased thrust by approximately 1500 T and torque by approximately 800 kN·m, providing quantitative basis for parameter matching. Second, breakthrough in information feedback closed-loop control: a dual-source joint monitoring technology was developed combining "above-ground building vibration-displacement monitoring system + shield cutterhead dynamic response," and a deep learning-based prediction model for superstructure vibration was constructed, enabling real-time "monitoring-analysis-decision-adjustment" closed-loop control that reduced shield parameter adjustment response time to within 15 minutes. Monitoring data showed that when building settlement rate exceeded 0.5 mm/day or tilt change exceeded 0.005°/day, the system automatically triggered parameter optimization, adjusting face pressure by ±0.05–0.1 bar and reducing cutterhead speed by 0.1–0.2 rpm, effectively controlling structural deformation. Third, innovation in shield gap control technology: the K-ni method was applied for shield gap filling, combined with dynamic mud density control and optimized synchronous grouting (grouting volume controlled at 130%–135% of theoretical gap, grouting pressure 0.15 MPa higher than excavation face water-soil pressure), effectively controlling overlying soil settlement during shield passage and reducing surface settlement by 30%–40% after K-ni grouting.

Engineering practice demonstrated that after implementing prestressed concrete underpinning beams to achieve physical isolation between building piles and the shield tunneling zone, the intelligent control system successfully completed continuous grinding of 67 cast-in-place piles and pipe piles. According to field monitoring results, building settlement and tilt were both significantly better than control thresholds.

In terms of numerical verification, comparison between predictions from a three-dimensional dynamic calculation model of "underpinning structure-underpinned structure" and the XGBoost (eXtreme Gradient Boosting) machine learning algorithm with measured data showed that the relative error of key parameter predictions was ≤10%, validating the high-precision predictive capability of the control system. The research findings provide a replicable technical pathway for efficient, low-disturbance tunneling with large-diameter slurry shields in composite strata, offering engineering guidance value for resolving the challenge of pile group avoidance in urban core area underground projects. It should be noted that this technical system is primarily applicable to argillaceous siltstone composite strata with 30%–40% clay mineral content. For strata with non-rounded gravel content exceeding 50%, working conditions with soft silt interlayers thicker than 3 m, or extreme hydrogeological conditions where groundwater level is more than 20 m above tunnel crown, further optimization of soil conditioning, K-ni injection parameters, and mud density control ranges is required. In high-pressure water-bearing strata, shield tail sealing systems and backfill grouting processes should be enhanced to prevent groundwater infiltration and resulting stratum instability. The clarification of these technical boundary conditions provides a scientific applicability assessment framework for similar projects.

Keywords: super-large diameter slurry shield; pile group excavation; building undercrossing; composite strata