Research On The Criterion To Determine The Rock Cover Of Subsea Tunnel By Numerical Method

Because of its unique advantages, subsea tunnel will become the preferred mode of transport to cross strait or gulf. With the rapid development of economic and social in our country, link between the coastal areas will be more closely. The construction of subsea tunnel will boost regional integration and bring huge economic and social benefits. At present, China is at the beginning stage of subsea tunnel construction. In terms of the investigation, design and construction of subsea tunnel, we need to enrich theoretical research and practical experience to meet the urgent needs of the construction of subsea tunnels in the future.The depth of rock cover is a key factor in the construction of subsea tunnel, which is related to the stability and costs. To determine the depth of rock cover is a complex problem, which involvs many factors, such as the stability of surrounding rocks, the seepage of seawater, costs. To seek the minimum rock cover which can meet the safety need in the course of construction and operation, will shorten the length of the tunnel, save a lot of project cost, and bring huge economic benefits. However, due to the lack of design theory and practical experience in subsea tunnel construction in our country, the depth of rock cover is desigined mainly on basis of foreign engineering analogys. Most of existing engineering analogies focus on analysis to single factor, such as the surrounding rock properties, water depth or water inflow. At the same time, the geological conditions is different from foreign to domestic. So when it is applied in domestic subsea tunnel, either the risk is higher, security to be tested; either too conservative, resulting in high costs. With the rapid development of numerical simulation techniques, numerical methods are applied in the research of subsea tunnel. However, numerical methods to determine the rock cover have no unified decision guidelines to follow. So in this paper, on the basis of previous research and my school’s undertakening research project about subsea tunnels, take Jiaozhou Bay Subsea Tunnel as background, to simulate and compute the excavation process of the subsea tunnel in different rock cover depth, in order to explore the criteria of determining the rock cover with numerical methods by extraction and analysis to calculation results. The main contents of this paper are as follows: (1)Analysised the realated factors of subsea tunnel rock cover and existing engineering analogies. The mechanism of surrounding rock features, water load, construction methods, cross-section shape and size is explained. The core theory of Norwegian Empirical Methods, Japanese Empirical Method of Minimum Discharge, Domestic Empirical Formula for Underwater Coal Mining and their advantages and disadvantages are also analysised. At last, a brief analysis to the necessity of weighted engineering analogy and numerical methods.(2) The project group found that the vault’s displacement of subsea tunnel has a minimum value when the rock cover depth increasing. This criterion is called the minimum displacement method. The subsea tunnel’s excavation process with various rock cover depth is simulated by FLAC3D finite difference calculation program and the key points of surrounding rock’s displacement, principal stress and the volume of plastic state zone is quantitatively extracted. Based on the results, the above characteristics variable’s variation of the key points on vault, spandrel, hance and arch bottom is obtained. Meanwhile, according to the FLAC3D’s single seepage mode calculation, the variation of subsea tunnel water inflow is obtained,too. By analysis of the above characteristic variable’s variation with rock cover depth, the reasonableness of the minimum displacement method is verified.(3) The results is sensitive to the parameters imported in numerical method. So different parameters of surrounding rock elastic modulus, such as Poisson’s ratio, internal friction angle, cohesion and the water depth were selected to simulate the excavation calculations in order to assess the sensitivity of those parameters to the minimum displacement method.(4) The safety factor of subsea tunnel with various rock cover depth is calculated by using the FLAC3D software, with strength reduction and gravity increase method. The surrounding rock’s displacement, stress and the plastic zone volume in the critical failure and normal state is quantitatively extracted to obtain their variation. By comparing the variation of displacement, stress and safety factor of surrounding rock in the critical failure and normal state, a criterion by safety factor to determine subsea tunnel’s rock cover depth was proposed.