Bond–Slip Behavior Between Shotcrete and I-Shaped Steel with Studs in Tunnels: Experimental and Numerical Study via Push-Out Tests

The bond–slip behavior between I-shaped steel and shotcrete crucially influences the cooperative deformation and overall bearing capacity of tunnel initial support. To clarify the bond–slip mechanism between the steel section and shotcrete, this study conducted push-out tests on five groups of stud-reinforced specimens, investigating the effects of stud geometry and arrangement. The tests revealed two primary failure modes—tensile and expansion failure—governed by stud quantity and strength. The typical bond–slip curve comprises four stages: non-slip, ascending, descending, and residual. The quantitative results highlight that stud arrangement and diameter significantly impact bearing capacity. The quincuncial layout achieved an ultimate load of 372.4 kN, which is 16.8% and 54.9% higher than that of the double-row (318.9 kN) and single-row (240.5 kN) parallel arrangements, respectively. Increasing the stud diameter from 13 mm to 16 mm boosted the ultimate load by 16.7% (from 240.5 kN to 280.7 kN). The residual load was approximately 62–87% of the ultimate load. Based on these findings, a bond–slip constitutive model was developed, showing high agreement with experimental data. This study provides theoretical support for optimizing steel–shotcrete support systems in tunnels, though generalizability is limited by sample size and the exclusion of long-term load effects.