## 1 Introduction  
Block caving is a highly cost-effective underground mining method for large, low-grade ore bodies, relying on gravity-induced fragmentation and controlled draw [1]. The stability of the bottom structure, which includes critical components such as drawpoints, drifts, and pillars [2], is essential for the economic viability of this method. However, the dynamic nature of caving, particularly the impact of large block falls, poses significant challenges to structural integrity [3]. Recent failure of underground cavers in block caving mines, such as the Chuquicamata mine collapse in 2018 [4, 5], have underscored the need for a better understanding of impact-induced damage mechanisms. Unlike traditional mining methods, block caving subjects the bottom structure to both static stresses (from overburden pressure) and dynamic loads (from falling blocks) [6, 7, 8]. This dual-loading scenario makes conventional stability analyses insufficient, necessitating advanced experimental and numerical approaches [9, 10]. The bottom structure must withstand three types of loads: static stress from the overlying caved material [11; 12], dynamic impacts from large blocks (up to several meters in size) [13], and cyclic degradation due to repeated loading-unloading during production [14]. Field observations and numerical studies suggest that progressive damage accumulation (e.g., pillar spalling, drift convergence) often leads to sudden collapses [15]. For instance, at the Northparkes mine in Australia, unexpected block falls caused severe damage to drawpoints, highlighting the need for predictive models. [PERSON], [PERSON] [16] examines the flow characteristics of caved ore and rock under the influence of multiple coarse particles through physical and numerical tests, highlighting the importance of interparticle interactions, void fractions, and force chains in shaping the isolated movement zone. [PERSON], [PERSON] [17] investigates the influence of pre-existing joints on rockfall fragmentation during impact using the discrete element method, revealing that joint geometry and spatial position significantly affect fragmentation, impact force, and kinetic energy. Despite these risks, most existing studies focus on static stability [18; 19], while dynamic and cyclic loading effects remain understudied [20; 21]. The damage to the bottom structure is usually caused by the impact of large rockfalls on the slope. Large block falls generate high-energy impacts, inducing stress waves that propagate through the bottom structure [22]. Laboratory experiments using split Hopkinson pressure bars (SHPB) have quantified single-impact rock fragmentation [23], but repetitive impacts (simulating actual mining conditions) are rarely studied [24]. Existing research faces several unresolved issues, such as the effect of cyclic loads on the long-term strength of pillars and drifts, and the inability to clearly identify critical stresses for different failure modes [25; 26]. Additionally, the impact of dynamic loads is often overlooked, which may lead to a potential underestimation of risks. True triaxial tests have been widely used in mining applications [27]. Conventional triaxial tests (\(\sigma_{1}>\sigma_{2}=\sigma_{3}\)) cannot replicate the true stress anisotropy (\(\sigma_{1}\
eq\sigma_{2}\
eq\sigma_{3}\)) in block caving environments [28]. True triaxial testing has emerged as a superior method for simulating mining-induced stresses [29], with applications in pillar bursting [30], excavation damage zones [31], and rock fracture propagation [32; 33]. However, no prior study has applied true triaxial testing to investigate cyclic block impacts on bottom structures. This gap limits the accuracy of numerical models (e.g., FEM, DEM) used in mine design. This paper addresses the deficiencies of the aforementioned research. Firstly, a large rockfall impact test on the bottom structure of multiple funnels under true triaxial stress was conducted.