# Model Test on Excavation Face Stability of Shallow-Buried Rectangular Pipe Jacking in Sand Layer
## 1 Introduction  
Rectangular pipe jacking demonstrates 20-35% higher space utilization efficiency compared to circular alternatives by eliminating ground leveling requirements, making it particularly suitable for constructing shallow-buried underground infrastructures in sandy soil strata, such as utility tunnels, metro stations, and pedestrian passages [1, 2, 3]. However, the excavation process inevitably disrupts the original geostatic stress field. Given the low cohesion characteristics and weak soil arching effects inherent in sandy soils, shallow burial conditions significantly increase the risk of excavation face instability, potentially triggering substantial ground subsidence. Consequently, a comprehensive understanding of failure mechanisms, including support pressure variation patterns, soil failure modes, and soil arch evolution dynamics, during rectangular pipe jacking operations becomes crucial for both engineering safety assurance and environmental impact mitigation. [PERSON] et al. [4] investigated excavation face instability in sandy gravel strata during rectangular pipe jacking through numerical simulations, revealing soil plastic deformation characteristics and developing a wedge-shaped support pressure calculation model. [PERSON] et al. [5] subsequently performed scaled model tests on pipe curtain box culverts in sandy layers, demonstrating the structural reinforcement effects on rectangular excavation faces. [PERSON] et al. [6] systematically analyzed face stability under various reinforcement configurations through case studies of actual projects. Building on [PERSON]'s displacement solution and stochastic medium theory, [PERSON] et al. [7] established an analytical framework for stratum disturbance prediction during parallel rectangular pipe jacking operations. [PERSON] et al. [8] further refined the wedge-shaped model by incorporating advanced numerical simulations of soil failure mechanisms ahead of excavation faces. Addressing operational safety concerns, [PERSON] et al. [9;10] developed an optimized limit support pressure model through comprehensive analysis of pressure distribution patterns in earth pressure balance chambers during muck removal processes. Notably, [PERSON] et al. [11] formulated a three-dimensional instability model using spatial discretization techniques and derived a support pressure calculation method based on limit analysis upper bound theory, although its computational complexity necessitates MATLAB-based numerical solutions. [PERSON] et al. [12] extended the investigation to saturated sand conditions, conducting coupled hydro-mechanical analyses of support pressure evolution, pore water pressure distribution, and failure mode development in large-section rectangular pipe jacking projects. While research on rectangular excavation face stability has gained momentum, current methodologies predominantly rely on theoretical assumptions and numerical simulations, with limited experimental validation through physical modeling. Furthermore, existing instability models and support pressure formulations frequently extrapolate from circular face research outcomes, inadequately addressing shape-induced mechanical differences. This methodological gap underscores the need for dedicated experimental investigations capturing the unique failure characteristics of rectangular geometries. This study conducts systematic model tests investigating rectangular excavation face stability under varying overburden depths and soil compaction states. The instability process is physically simulated through controlled retraction of support plates, while employing non-contact photogrammetry and full-field deformation monitoring systems to capture. This is conducive to reducing the interference of monitoring equipment on soil deformation and force in previous studies: (1) real-time support pressure fluctuations, (2) deep soil displacement fields, and (3) ground settlement patterns.