Experimental study on dynamic mechanical characteristics and failure features of surrounding rock in layered composite rock mass tunnels

• Tunnel geometry dictates dynamic response: • Circular tunnels exhibit 17–25% higher peak strength and strain recovery than semicircular arch tunnels due to uniform stress distribution. • Semicircular arch tunnels suffer 20% greater wave attenuation and 32–42% higher fragmentation from stress concentration at sharp corners. • Soft-layer ratio controls fragmentation: • Increasing soft-rock content (0:1 → 1:1) raises fractal dimension by 30–40%, intensifying fragmentation and reducing particle size. • Energy dissipation mechanisms revealed: • Higher soft-rock ratios increase reflected energy and absorbed energy but reduce transmitted energy due to impedance contrasts. • Distinct failure modes uncovered: • Circular tunnels: Radial cracks initiate symmetrically from the hole’s sides. • Semicircular arch tunnels: Simultaneous cracking at vault, arch foot, and sidewalls forms asymmetric mesh-like fractures. • Methodological innovation: • First combined SHPB-DIC-high-speed imaging study quantifying dynamic fracture evolution in layered tunnels, enabling precise crack-path analysis. To study the dynamic mechanical behavior and failure characteristics of layered composite rock masses under complex geological conditions, specimens of circular and straight-wall semi-circular arch tunnels with varying soft and hard rock thickness ratios were prepared using rock-like materials. Vertical tunnel impact tests were conducted employing a high-speed camera, a separated Hopkinson compression rod (SHPB) test system, and a digital image correlation (DIC) system. Dynamic stress-strain curves, fracture morphology, specimen fragment fractal dimension, energy dissipation, crack evolution, and failure modes were then analyzed. The results indicate that the influence of tunnel geometry on the dynamic mechanical properties of layered composite rock masses should not be underestimated. The wave attenuation amplitude of the straight-wall semi-circular arch tunnel specimens exceeds that of the circular tunnel specimens, with its peak strength and peak strain being relatively lower. Additionally, the ratio of soft to hard rock layers significantly affects fracture morphology. As the proportion of soft rock increases, the degree of fracture in the composite specimens under impact load increases substantially. Furthermore, the relationship between energy proportions and the soft rock ratio was analyzed, with an exploration of the failure characteristics of the surrounding rock in the composite formation. The differences in crack germination types for tunnel specimens containing circular and straight-wall semi-circular arch tunnels, under varying soft and hard rock proportions, were systematically analyzed.