Study On Water Inrush Mechanism And Risk Evaluation Of Subsea Tunnel Crossing Weathered Trough

Subsea tunnel is an important mean of transportation as a cross-sea passage which can connect islands far from the mainland.It can facilitate people's travel and promote regional economic development.Subsea tunnel brings us convenient transportation.However,a series of safety accidents in the procedure of construction also gives us a wake-up call.Subsea faults,weathered troughs,cracks,and other geological structure may be connected with seawater.Once the tunnel excavation makes that form channel,the seawater becomes the source of water inrush,which is prone to collapse and water inrush hazards.How to cross the unfavorable geological section successfully is the key point of subsea tunnel design and construction.This paper is depending on the Haicang tunnel project to carry out the study.Based on an extensive collection of predecessor's research data,topographic and geomorphological characteristics,formation lithology,geological structure,and hydrological characteristics of the study area were investigated in detail.The mechanism of water inrush through the weathered trough of the subsea tunnel was analyzed using theoretical analysis,numerical simulation and model test.The main research results and conclusions are as follows.(1)According to the analysis of geological characteristics of the Haicang subsea tunnel crossing the weathered trough,the physical,chemical and mechanical effects of water on the weathered trough are systematically revealed.The formation conditions of water inrush induced by weathered trough are analyzed from the effects of the water source,water inrush conduits,and external disturbance.The typical influencing factors of water inrush through the weathered trough are selected by the complete investigation on all potential disaster-causing factors in the process of undersea tunnel construction.This paper systematically derived the permeability water pressure at the fissure of weathered groove and the expansion strength factor of composite fracture and obtained the critical criterion of weathered groove crack expansion against hydraulics and fracture mechanics.(2)This paper establishes the numerical models for the excavation of subsea tunnel without support and the excavation with support,respectively.It is found that the characteristics of tunnel displacement field,stress field and seepage field are affected by tunnel excavation disturbance in a range of about double the tunnel diameter when crossing weathered trough.After supporting,we find that shotcrete support and advanced grouting can enhance the stability of surrounding rock,and the stress increases obviously in the grouting reinforcement area.By strengthening the support,the pore water pressure in the weathered trough decreases,the seepage path changes,and the permeability of water into the tunnel decreases.(3)A three-dimensional simulation test system for the catastrophic evolution of the subsea tunnel water inrush was developed,which consists of a model test stand,water supply system and a multi-information acquisition system.The similar materials of tunnel wall rock and weathering trough were developed,and the two similar materials with different properties were used to make a model.It is found that the variation law of parameters in each stage of water inrush is significant agreement with the observed the water inrush phenomenon,the stress field,the displacement field and the characteristics of seepage field in the process of tunnel excavation.The serious influence of seawater on the weathered trough is mainly concentrated in the area of one time the diameter of the cave.The weathered groove will decrease under the combined action of excavation and seepage,and the cracks in the zone will begin to formation,and the influence range will expand.The weakened material of weathered trough is carried out by seawater,and the loss of solid particles will increase the permeability in the weathered trough.As a result,the water inrush conduits linking between the seawater and the weathered trough was finally completely connected and induce form water inrush.(4)This paper constructs the Bayesian network topological relation of water inrush risk combined with graph theory.The prior probability and conditional probability of the influencing factors of water inrush are obtained which combined with the evaluation language of experts on the probability of each bottom event risk based on fuzzy set theory.The risk probabitity of water inrush from the large cross-section subsea tunnel through the weathered trough is 65%by using GeNle software.Finally,the posterior probability of the target node of water inrush risk is calculated by controlling the influencing factor node of water inrush and the degree of influence on its associated nodes.These results predict the probability of water inrush risk.The main influencing factors are analyzed by reverse diagnosis under the condition of node status.