# Model Test on Excavation Face Stability of Shallow-Buried Rectangular Pipe Jacking in Sand Layer
## 5 Conclusions  
To investigate the instability evolution mechanism of the shallow-buried rectangular pipe jacking excavation face in the sand layer, model tests were carried out in this study, and the following four conclusions were obtained:  
1. Support pressure evolves through consistent stages irrespective of overburden ratio or density, and it shows nonlinear changes due to the influence of soil arch. And enhanced density intensifies soil arching, reducing support pressure while amplifying its post-failure rebound.
2. Shear bands propagate progressively from excavation base to the ground during instability. The deep soil successively develops into non-deformation, triangular shear zone (slight deformation), arched shear zone (local instability), and vertical shear zone (overall instability). Meanwhile, the failure angle increases with soil density but is depth-independent, approximating twice the internal friction angle (\(\approx\)2\(\varphi\)) for rectangular excavation faces.
3. During face instability, abrupt trapezoidal settlement surfaces emerge. Negligible settlement occurs prior to arched shear band penetration, but post-penetration triggers overall instability, forming these trapezoidal surfaces that exhibit progressively increasing settlement magnitude, rate, and extent. The critical settlement Scr depends on soil density but not overburden ratio. While the settlement rate is invariant to both parameters, larger overburden ratio increases displacement to reach Scr. Critically, the trapezoid geometry is governed by horizontal soil arching, where the equilibrium arch angle \(\beta\), at a range of 41\({}^{\circ}\)-48\({}^{\circ}\), correlates with the excavation aspect ratio and is independent of both soil density and overburden thickness.
4. Mechanistic linkages exist among support pressure, deep soil displacement, and ground settlement. Triangular shear band initiation causes pressure to drop to P1. Subsequent arched shear band development activates arching, minimizing pressure. Arch disruption induces localized instability, initiating pressure ascent while triggering new  
Figure 14: Generalized diagram of instability mechanism for rectangular excavation face.  
peripheral arched bands. These ultimately evolve into vertical surface-penetrating shear bands, coinciding with rapid ground settlement and pressure stabilization.  
The instability mechanism revealed in this study provides a foundation for establishing a three-dimensional support pressure calculation model. Combined with more realistic on-site monitoring, it is of great significance to establish a calculation model of the rectangular pipe jacking face.  
Conceptualization, Y.Z. and P.Z.; Data Curation, Y.Z. and Y.X.; Funding Acquisition, P.Z.; Investigation, Y.Z.; Methodology, Y.Z. and P.Z.; Validation, P.Z.; Visualization, P.Z. and J.M.; Writing--Original Draft, Y.Z., Y.X. and P.Z.; Writing--Review and Editing, Y.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by PowerChina Guiyang Engineering Corporation Limited Postdoctoral Fund Project (YJBSXM240002), the National Science Foundation of China (52008383), and the Open Project Fund of Engineering Research Center of Rock-Soil Drilling & Excavation and Protection, Ministry of Education (202306).  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
Author [PERSON] was employed by the company PowerChina Guiyang Engineering Corporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.