Model test of the mechanism underpinning water-and-mud inrush disasters during tunnel excavation in sandstone and slate interbedded Presinian strata

Water-and-mud inrush disasters have become a major challenge in underground engineering for the construction of tunnels in sandstone and slate interbedded Presinian strata. Disaster prediction and prevention rely in part on realistic modeling and observation of the disaster process, as well as the identification and examination of the underlying mechanisms. Based on the geological conditions and the historical records of the Xinping Tunnel on the China—Laos Railway, an engineering geological model of the water-and-mud inrush was established. A physical model test that accurately reproduced water-and-mud inrush during tunnel excavation in sandstone and slate interbedded strata was also carried out. Then, testing was conducted that examined the stress and strain, seepage pressure, and high-leakage flow of the surrounding rock. The results indicated that the water-and-mud inrush proceeded through three stages: seepage stage, high-leakage flow stage, and attenuation stage. In essence, the disaster was a catastrophic process, during which the water-resistant stratum was reduced to a critical safety thickness, a water-inrush channel formed, and the water-resistant stratum gradually failed under the influence of excavation unloading and in situ stress—seepage coupling. Parameters such as the stress and strain, seepage pressure, and flow of the surrounding rock had evident stage-related features during water-and-mud inrush, and their variation indicated the formation, development, and evolution of the disaster. As the tunnel face advanced, the trend of the stress—strain curve of the surrounding rock shifted from sluggish to rapid in its speed of increase. The characteristics of strain energy density revealed the erosion and weakening effect of groundwater on the surrounding rock. The seepage pressure and the thickness of the water-resistant stratum had a positive linear relationship, and the flow and thickness a negative linear relationship. There was a pivotal point at which the seepage pressure changed from high to low and the flow shifted from low to high. The thickness of the water-resistant stratum corresponding to the pivotal point was deemed the critical safety thickness.