Development of analog material and model experimental design based on the strength reduction method

The rapid development of rail transit has led to increasingly complex tunnel spatial layouts, substantially elevating construction and operational risks. The strength reduction method has been widely applied in tunnel engineering; however, existing studies have predominantly relied on numerical analysis, whereas field evidence and experimental validation remain limited. To address these issues, we developed a rock–soil analog material in which shear strength parameters can be quantitatively reduced through controlled heating. By integrating laboratory-scale physical model tests with numerical simulations, we investigated the potential failure modes of tunnel groups under surrounding-rock strength degradation as well as the associated displacement–stress evolution. Taking the Hongyancun tunnel group in Chongqing, China, as an engineering background example, we aimed to provide targeted guidance for the design and risk management of tunnel groups.

The analog material was prepared using paraffin wax as the binder and quartz sand together with 400-mesh barite powder as aggregates. Temperature was introduced as an external control variable, enabling controlled reductions in the material’s shear strength parameters through heating. The mechanical properties of the analog material were characterized using a ZJ-type strain-controlled direct shear apparatus and a universal testing machine, based on which the mass ratio of paraffin wax:quartz sand:barite powder was determined as 6:56.4:37.6. To achieve stable and precise control while heating the physical model, an in-house intelligent temperature-control device was developed for accurate heating and temperature regulation. Numerical simulations and laboratory excavation–heating tests on a tunnel group were conducted to analyze the failure patterns and the evolution of displacement and stress fields following quantitative reductions in the surrounding-rock strength. During the tests, displacement transducers and embedded strain blocks were installed to monitor displacement and stress responses throughout excavation and heating.

Based on the strength-reduction concept, an analog material was successfully developed in which cohesion and internal friction angle decrease proportionally with increasing temperature, allowing the mechanical behavior of Grade Ⅲ–Ⅴ surrounding rock to be effectively reproduced. The developed intelligent temperature-control device enabled accurate heating and stable temperature regulation of the model material. The tunnel-group heating tests indicated that the sustained development of damage in the pillar rock (intervening rock mass) was the primary factor triggering collapse of the tunnel group. By contrast, even when localized failure occurred in noncritical regions, it typically did not directly lead to overall instability of the tunnel group.

Herein, we propose and validate a temperature-controlled physical modeling approach for quantitatively understanding reductions in the rock strength surrounding tunnels. The developed analog material and temperature-control system effectively capture potential failure zones and instability modes of tunnel groups under strength degradation during excavation and operation. The findings provide experimental evidence and practical references for optimizing the spatial configuration of complex tunnel groups, identifying critical locations, and managing risks throughout the construction–operation lifecycle.