Granular materials exhibit complex mechanical behaviors during unloading, yet the underlying micro- and meso-scale mechanisms remain unclear. This study employs a discrete element method to simulate a series of triaxial tests on sand and pebble specimens with varying initial densities under different unloading stress paths. While dense specimens demonstrate strain softening and dilatancy, loose samples exhibit shear contraction. To quantify the underlying fabric evolution, persistent homology (PH) theory is adopted to analyze the particle contact force networks. The results reveal that the average strength of this network correlates strongly with the macroscopic stress–strain response. For dense samples, network strength rapidly increases to a peak coinciding with the deviatoric stress maximum, then gradually decreases with further shear. Crucially, this evolution is path-dependent: the average contact force network strength increases approximately 20% more during unloading in the minor principal stress direction compared to unloading in the major principal stress direction. This quantitative analysis of force chain degradation provides a mechanistic explanation for the observed strain softening, highlighting the dominant role of the unloading stress path. In contrast, loose specimens, which initially lack an obvious force chain network, show negligible microstructural evolution during unloading.

To study excavation face instability in upward shield tunneling, a 3D numerical model was established using ABAQUS software v2023 under different depth ratios (C/D = 1, 1.5, 2, 3, 4), with reference to the upward shield tunneling project of the Midosuji Utility Tunnel Construction Project in Japan. The model simulated soil as an elastoplastic material governed by the Mohr–Coulomb criterion, with dimensions of 60 m × 60 m × 60 m and boundary constraints applied to soil surfaces. This study explores variations in the limit support pressure and soil failure zone during the instability process. Simulation results were validated through scaled model tests (1:50). The study findings reveal that (1) for varying depth ratios, the support pressure on the excavation face decreases as the depth ratio increases, with diminishing reductions at higher ratios. (2) At shallower depths (C/D < 3), the soil failure zone above the excavation face is nearly conic. At deeper depths (C/D ≥ 3), the failure zone resembles a “bullet head” shape. (3) At shallower depth ratios (C/D < 3), surface displacement shows slow-to-rapid transitions. At deeper depth ratios (C/D ≥ 3), surface displacement remains nearly constant. (4) Post-instability, stress concentration at the horizontal tunnel’s top opening causes segments to deform into an inverted V-shape. (5) Soil stress changes are categorized into three zones—stress release, soil wedging, and stress transfer—with each zone expanding as depth increases.

With the growing scarcity of surface space, underground development has become essential for expanding human living space. Among various tunneling methods, pipe jacking stands out due to its economic advantages and minimal environmental impact. Recently, vertical pipe jacking has been explored as an innovative technique for constructing shafts that connect horizontal tunnels to the ground surface. However, the evolution of jacking force during vertical pipe jacking with increasing jacking distance remains poorly understood. Understanding this evolution is critical for selecting jacking equipment, designing the horizontal tunnel lining against reaction forces, and preventing construction failures. Unlike horizontal pipe jacking where self-weight is negligible, the proposed model reveals that in vertical pipe jacking the self-weight of the pipe and machine above the excavation face increases with jacking distance while the overburden pressure decreases, resulting in a parabolic-like jacking force trend—a novel finding not reported in previous pipe jacking literature. This paper proposes theoretical formulas to quantify the three components constituting the jacking force: face resistance at the cutting head, frictional resistance along the pipe surface, and the dead weight of the machine and pipe above. The influence of jacking distance on each component is systematically analyzed. Parametric studies under standard and varied conditions reveal that under standard conditions, jacking force follows a parabolic trend—rapid initial increase, followed by slower growth, and eventually a slight decrease. The maximum jacking force consistently occurs at L = L₀ − 1 m, identifying the most unfavorable construction stage where special attention to tunnel lining deformation is required. Increasing outer diameter transitions the force curve from quasi-parabolic to “half diamond” shape, while doubling the friction coefficient approximately doubles the jacking force. These findings provide practical guidelines for vertical pipe jacking design and construction, including equipment capacity selection, friction reduction strategies, and monitoring priorities.

The emerging upward shield method (USM) for constructing vertical shafts has been used in various projects, including the Midosuji utility tunnel in Japan. A scaled-down model testing system, featuring a geometric similarity ratio of 1:30, was developed specifically for studying the USM. This system incorporates sand inflow control, propulsion control, data acquisition, and water level control. It facilitates detailed observation and recording of parameters such as vertical displacement of surface soil layers, support force at the excavation face, and earth pressure within the model box. Consequently, it enables investigation into the excavation face instability process, modes, and the formation and evolution of the soil arch zone above the excavation face during upward shield tunneling. Additionally, through the application of particle image velocimetry (PIV) technology and GeoPIV-RG software v1.1, quantitative analysis of soil displacement fields during excavation face instability is conducted, capturing microscopic displacements and deformations of soil planes. This approach more accurately elucidates the accuracy of understanding the dynamic response of soil. Pre-test research using the model testing system explores the variation patterns of excavation face load displacement, vertical earth pressure within the failure zone, surface displacement, and internal soil displacement during the instability process. Analysis reveals that excavation face load variation typically progresses through three stages: rapid growth, slow growth, and descent. Moreover, vertical earth pressure shifts upward in tandem with excavation face displacement, while overall surface displacement initially shows slight settlement followed by accelerated uplift.