Comparative Study on the Mechanical Responses of Tunneling in Composite Strata Using Different CRD Excavation Sequences: A Physical Model Test

Constructing tunnels in composite strata using the Cross-Diaphragm (CRD) method poses significant construction risks and technical challenges, often triggering engineering disasters such as surrounding rock instability, excessive surface settlement, and support structure failure. Based on a mined tunnel project of Guangzhou Metro Line 18, this study conducted laboratory model tests to systematically investigate the effects of different excavation sequences (left-right and upper-lower excavation) on the evolution of surface settlement, lining stress, and surrounding rock pressure under homogeneous and " upper-soft and lower-hard " composite strata. The results indicate the following:(1) In homogeneous hard soil, left-right excavation achieves better settlement control; in composite strata, upper-lower excavation is superior by activating hard-rock arching. (2) In composite strata, soft-hard coupling induces sustained tensile stress at the arch shoulder, which contrasts with the full-section compression in homogeneous soil and thus requires focused monitoring and tensile-resistant design. (3) At the early excavation stage in composite strata, delayed unloading-load transfer causes a paradoxical increase in vault surrounding-rock pressure. (4) Based on a comprehensive comparison, when tunnelling in composite strata using the CRD method, the upper‑lower excavation sequence is strongly recommended.