<p>Excavation in deep weak clastic rock engineering frequently results in significant deformation and cracking of the surrounding rock. This study employed a self-developed mechanical excavation test device under true triaxial stress to conduct scaled-down excavation tests on three types of clastic rock, aiming to clarify the cracking mechanisms induced by excavation. The findings indicated that in contact-cemented clastic rock, post-excavation, multiple short cracks formed on the floor, leading to the detachment of several blocks and eventual overall uplift. In addition, small wedge-shaped blocks gradually detached from the crown. During step loading of horizontal boundary stress in porous-cemented and basal-cemented clastic rocks, longer cracks developed on the floor, cutting through the surrounding rock and causing the uplift of large blocks from the floor and the gradual formation of a V-shaped failure at the crown. Significant excavation-induced deformation was observed within three times the tunnel radius from the excavation boundary, decreasing progressively towards the far field for all three types of surrounding rocks. Excavation induced tensile strain within the surrounding rock, with floor deformation increasing over time. The stability of surrounding rock improved, and deformation decreased from contact-cemented to porous-cemented to basal-cemented clastic rocks post-excavation. Floor heave and local crown peeling in contact-cemented clastic rock were attributed to high redistribution stress caused by excavation and low cementation performance of the surrounding rock. This study provides an experimental foundation for understanding the mechanisms and preventing disasters related to squeezing deformation of weakly cemented surrounding rock under high horizontal tectonic stress.</p>