Study on stability of exit slope of Chenjiapo tunnel under extreme rainstorm conditions

The exit slope of the Chenjiapo tunnel is located in Xuanen County, Hubei Province, China, and rainfall is one of the main factors inducing landslides. During the tunnel excavation, the left side of the front edge of the slope slid downward and caused a 6.27 × 104 m3-large landslide. Moreover, a 8.69 × 105 m3-large unstable slope was formed under the combined action of rainfall and the excavation. Because the front edge of the slope has been sliding, further tunnel excavation and extreme rainfall may induce massive landslides. This not only threatens the safe construction of the tunnel but also directly risks the operation safety of the expressway at a subsequent stage. To reveal the failure process of the Chenjiapo tunnel exit slope under extreme rainfall conditions, the slope stability is studied under five rainfall types and three rainfall intensities by conducting numerical simulations using the GeoStudio software. The results show that under the condition of front-peak rainfall, the safety factor of the slope first decreases and subsequently increases with increasing rainfall time. The slope is damaged at the 18th hour of the rainfall, and the plastic zone completely penetrates the upper soil layer at the end of the first day of the rainfall. In addition, the maximum horizontal displacement of the slope, which is up to 0.233 m, is the maximum among those under the five rainfall types. Under the conditions of equal-intensity, stepped, medium-peak, and back-peak rainfall, the safety factor of the slope decreases with increasing rainfall time. The slope begins to be destroyed between the second and the third days of the rainfall, and the plastic zone begins to be fully penetrated. Therefore, for the same rainfall time and total rainfall amount, the front-peak rainfall is the most harmful to the slope stability. Under 50, 70, and 90 mm/day rainfall intensities, the safety factor of the slope decreases with increasing rainfall time. Compared with the other two rain intensities, the slope is damaged first at the 27th hour under the 90 mm/day-rain intensity. At the end of the rainfall, the safety factor of the slope is the smallest under the rainfall intensity of 90 mm/day, which is 0.887. This indicates that a high rainfall intensity is associated with easy damage to the slope. After the rainfall, the safety factor of the slope immediately recovers, the horizontal displacement gradually rebounds, the distribution range of the plastic zone begins to decrease, and the slope returns to a stable state after 12 days of the rainfall ending. The exit slope of the Chenjiapo tunnel may fail under extreme rainfall conditions; therefore, it is urgent to adopt reinforcement measures, such as an anti-slide pile as the main support and drainage and sealing slope cracks as the auxiliary ones.