## 2 Statistical Analysis of Tunnel Floor Heave
### Statistical Analysis of Tunnel Floor Heave in Gently Inclined Surrounding Rock  
(1) **Depth**  
As illustrated in Figure 3a, tunnel floor heave in gently inclined surrounding rock is predominantly observed at depths ranging from 200 to 500 m, with occurrences diminishing beyond 500 m. This trend is attributed to two key factors: the scarcity of gently inclined surrounding rock at depths exceeding 500 m and the elevated ground stress encountered at greater depths. Consequently, tunnel sections buried at depths of 200 to 500 m within gently inclined surrounding rock are particularly susceptible to floor heave.  
(2) **Lithology**  
As shown in Figure 3b, the floor heave of the gently inclined surrounding rock tunnel is mainly distributed in mudstone, sandstone, and shale strata, among which mudstone accounts for 36.3%, sandstone accounts for 25.6%, gypsum accounts for 12.9%, and shale accounts for 11.0%, so it can be found that mudstone and sandstone are prone to the risk of tunnel bottom bulge under slowly dipping surrounding rock conditions.  
(3) **Surrounding rock grade**  
As depicted in Figure 3c, grades III and IV of surrounding rock are the most common, with each accounting for 45.9% of the observed cases, while grade V constitutes 14%.  
(4) **Layer thickness**  
As indicated in Figure 3d, both thin and medium-thick layers are affected, representing 21.6% and 78.4% of the occurrences, respectively.  
(5) **Inclination**  
As illustrated in Figure 3e, 52.7% of cases occur within a 10\({}^{\circ}\) inclination, 41.2% between 10\({}^{\circ}\) and 40\({}^{\circ}\), and only 6.1% at inclinations above 30\({}^{\circ}\). Therefore, the risk of tunnel floor heave is more likely to manifest at inclinations below 30\({}^{\circ}\).  
(6) **Ground stress**  
As shown in Figure 3f, tunnel floor heave in gently inclined surrounding rock is primarily concentrated within the 10-20 MPa stress range, with fewer occurrences in the 20-30 MPa range and even less below 10 MPa. This indicates that floor heave generally occurs when ground stress reaches a certain threshold, and the coexistence of high ground stress and gently inclined surrounding rock is observed in some cases, particularly when local stress levels reach 20-30 MPa.  
(7) **Cumulative floor heave**  
The maximum recorded cumulative floor heave is 122 mm, with a weighted average of 31.9 mm. The majority of cumulative heave values are below 50 mm, with 61.5% being less than 25 mm and only 6.2% exceeding 100 mm.  
(8) **Floor heave rate**  
Figure 2: The proportion of influencing factors of tunnel floor heave during construction.  
The maximum recorded heave rate is 2.22 mm per month, with the majority of heave rates falling below 2 mm per month. Heave rates exceeding 2 mm per month account for only 6% of the cases.  
Figure 3: Floor heave statistics of a gently inclined surrounding rock tunnel. **(a)** The proportion of different tunnel depths under the condition of gently dipping surrounding rock. **(b)** Different lithologic ratios of surrounding rocks under gently dipping conditions. **(c)** The proportion of different surrounding rock grades under the condition of gently dipping surrounding rock. **(d)** Different thickness ratios of surrounding rock strata under the condition of gently dipping surrounding rock. **(e)** The proportion of different dip angles of surrounding rocks under slow-dip conditions. **(f)** Different in situ stress ratios under the condition of gently dipping surrounding rock. **(g)** A statistics chart of cumulative floor heave of a tunnel under the condition of gently inclined surrounding rock. **(h)** A statistics chart of the floor heave rate of a tunnel under the condition of gently inclined surrounding rock.