## 3 Types and Mechanism of Tunnel Bottom Drum
### Single-Factor Analysis
The deformations at the left and right arch feet vary, with greater vertical displacement on the left side, leading to an increased sliding tendency between rock layers on both sides. This uneven settlement causes rock dislocation and tensile failure in the primary support structure. 3. When other conditions are held constant, increasing the rock dip angle from 5\({}^{\circ}\) to 25\({}^{\circ}\) results in tunnel floor heave ranging from a minimum of 22.99 mm at a 5\({}^{\circ}\) dip angle to a maximum of 29.58 mm at a 10\({}^{\circ}\) dip angle. The maximum value is 1.3 times greater than the minimum. Compared to the variation in tunnel floor heave under different lateral pressure coefficients, the influence of the rock dip angle is relatively minor. 3. **Characteristics of tunnel floor heave under different hard rock thicknesses** From Figure 11 and the numerical simulation results, it can be concluded that 1. The tunnel floor heave decreases with the increasing thickness of the hard rock layer, and the rate of decrease accelerates. When the thickness of the hard rock layer increases from 20 cm to 50 cm, transforming the layered rock mass from thin to medium-thick, the floor heave reduces from 28.28 mm to 26.92 mm, a decrease of 4.8%. As the thickness increases further from 50 cm to 90 cm, transitioning the rock layer from medium-thick to thick, the tunnel floor heave continues to diminish from 26.92 mm to 24.56 mm, a reduction of 8.7%. The thicker the layer becomes, the more the layered rock approximates a homogeneous rock mass, resulting in a smaller degree of floor heave. 2. When the dip angle of the rock stratum is 0\({}^{\circ}\), the layered rock mass can be considered as a flexural member under uniform load. Under high ground stress, the effect of the single-layer thickness of the layered rock on tunnel floor heave follows this pattern: the thin layer > medium-thick layer > thick layer. As the single-layer thickness of the rock mass increases, the integrity of the surrounding rock improves, its deformation resistance strengthens, and consequently, the tunnel floor heave decreases. 3. When other conditions are constant, when the thickness of hard rock increases from 10 cm to 90 cm, the minimum value of tunnel floor heave that occurs in the thickness of 90 cm is 24.56 mm, and the maximum value that occurs in the thickness of 10 cm is 28.28 mm, and the latter is 1.1 times the former. Compared with the lateral pressure coefficient and rock dip angle, the influence is small, and the influence order is the lateral pressure coefficient > rock dip angle > hard rock thickness. Figure 11: Fitting curve of tunnel bottom displacement with different thicknesses of hard rock strata. Black line: the connecting line between numerical simulation results and field testing results; red line: the fitted curve based on the formula. * **Tunnel Bottom Drum Characteristics under Different Thin Layer Thickness** From Figure 12 and numerical simulation results, it can be drawn that 1. The tunnel floor heave increases with the increase in the thickness of the thin layer. When the thickness of the thin layer increases from 1 cm to 9 cm, the tunnel floor heave gradually increases from 15.40 mm to 30.48 mm, increasing by 198%. 2. Compared with the lateral pressure coefficient, rock dip angle, and thickness of hard rock strata, the order of influence is the lateral pressure coefficient > thin layer thickness > rock dip angle > hard rock thickness. 5. **Characteristics of tunnel floor heave under different elastic modulus of surrounding rock** From Figure 13 and numerical simulation results, it can be drawn that 1. The tunnel floor heave decreases as the elastic modulus of the surrounding rock increases, with the rate of decrease accelerating. When the elastic modulus rises from 1.5 GPa to 3.65 GPa, the floor heave reduces from 30.43 mm to 29.17 mm, a 4.2% decrease. As the elastic modulus further increases from 3.65 GPa to 13 GPa, the heave drops from 29.17 mm to 15.94 mm, a reduction of 45.4%. 2.