Investigating the mechanical properties of sandy cobble strata is essential for optimizing the design and construction of urban tunnels, thereby controlling ground deformation and ensuring tunnel stability. This paper aims to comprehensively investigate the mechanical properties and energy characteristics of heterogeneous sandy cobble strata. Numerical simulations are employed to examine the stress-strain behavior and energy evolution mechanism in scenarios with and without interfaces between the soil matrix and blocks. Subsequent analysis focuses on elucidating the effects of the internal stochastic structures, which characterize heterogeneity, on the overall strength and energy characteristics. The results indicate that the presence of interfaces significantly compromises the overall strength, while exacerbating the occurrence of a tortuous plastic zone around blocks. The volumetric block proportion (VBP), which represents the volumetric content of cobbles, has a significant impact on the overall mechanical behaviour. In the context of high VBP, block sizes, counts and orientations play substantial roles. Finally, the discussion reveals that when blocks are modelled using the elastic model, the overall strength is significantly overestimated compared to the strain-softening and Mohr-Counlomb models, especially in scenarios with high VBP and in-situ stress. It provides an unsafe evaluation (i.e., overestimation) of tunnel stability.

Sudden leaks often occur when constructing shield tunnels within saturated sandy cobble strata. Therefore, it is important to examine the reasons for water seepage and understand the mechanisms behind such problems. This paper presents a study that combines laser scanning technology with the Python programming language to create software for monitoring tunnel deformation. The software was employed in a practical subway tunnel scenario, successfully acquiring deformation data pertaining to the tunnel's structural segment through the analysis of point cloud data from the tunnel lining. Furthermore, the seepage-stress coupling theory was employed to establish a three-dimensional model of shield tunnel excavation, interlinking groundwater and stratigraphic factors with the sequence of shield tunnel excavation. The origins and mechanisms of water damage resulting from seepage and leakage are explicated through an examination of the seepage field, displacement field, and deformation of the tunnel structure pre- and post-excavation. Additionally, on-site monitoring data is considered. The mechanism of tunnel leakage is outlined as follows: Tunnel excavation completion induces alterations in the seepage field, leading to an accelerated inflow of groundwater into the soil beneath the tube sheet during shield excavation. The tube sheet of the shield tunnel, composed of sand and gravel layers, experiences vertical elliptical deformation that exacerbates shifts in the displacement field due to tunnel deformation's inception and progression. Excessive tube sheet deformation triggers fracture cracks, ultimately engendering the creation of seepage channels. These channels, in turn, foster seepage and water damage. The results of this paper provide a reference for preventing and remedying water infiltration and leakage in shield tunnels constructed of sandy cobble strata.