Research On Determination Method Of Overburden Thickness Of Rongjiang Forth Road Underwater Tunnel

Underwater tunnel is an important means to cross the water barrier,a technological invention of human beings to overcome the obstacles of nature and get along with nature harmoniously,and it is also the embodiment of human modern civilization.The advantages of an underwater tunnel,unlike a bridge,include a small footprint,protection from harsh environments,no damage to the natural environment,and little impact on the navigation of waterways.The construction of underwater tunnel needs to overcome the important technical problems such as the determination of the minimum overburden thickness.In the design process,we have to face a variety of geological conditions,so how to properly consider the water depth,surrounding rock characteristics,tunnel size and other factors.How to give consideration to the cost and safety of underwater tunnel reasonably;How to effectively guarantee the safety requirements of underwater tunnel construction and operation is the key problem to be solved urgently for designing a minimum overburden thickness in line with the reality.For tunnel engineering project,it will be beneficial to shorten its period and reduce the cost of investment,and bring more economic benefits.Therefore,based on the researches of many scholars,this paper takes the Rongjiang No.4 Road Crossriver Tunnel in Ganzhou as the engineering background,studies and analyzes the overburden thickness of the tunnel from three aspects:engineering experience,vault thickness criterion,and numerical simulation.The main research contents and results of this paper are as follows:(1)The empirical engineering methods for determining overburden thickness--Norwegian empirical method,minimum water inflow method and top water mining method are summarized.The influence of various factors on overburden thickness is analyzed from the aspects of surrounding rock grade,rock and soil properties,bedrock buried depth,water depth and tunnel section size.The results show that the minimum overburden thickness of Rongjiang No.4 underwater tunnel is 11.8m under various empirical methods.Finally,the applicability of the empirical methods is evaluated and the necessity of using the weight function method to determine the minimum overburden thickness is proposed.(2)The self-supporting arch thickness of tunnel surrounding rock is calculated by mechanical method and the discrimination method of deep and shallow buried boundary depth of mountain tunnel.The self-supporting arch thickness of surrounding rock arch is taken as the criterion,and the minimum vault thickness when it can provide the self-stabilizing and selfsupporting ability of tunnel surrounding rock is taken as the minimum overburden thickness of underwater tunnel.The feasibility of this criterion was verified by taking 31 underwater tunnels built at home and abroad as verification conditions.It is concluded that the minimum overburden thickness calculated based on the self-supporting arch thickness is generally small compared with the actual minimum overburden thickness of the tunnel.Finally,this criterion method is applied to the fourth underwater tunnel in Rongjiang River,and the minimum overburden thickness is obtained to be 15.75 m.(3)The FLAC3 D finite difference calculation program is used to simulate the excavation of tunnel with different overlying thickness.At the same time,the control variables of surrounding rock grade conditions and water depth are carried out.A total of 150 calculation conditions are obtained.Based on the calculation results,the deformation criterion of surrounding rock stability is adopted,that is,the characteristic dislocations of the vault,the shoulder,the waist and the bottom of the tunnel surrounding rock under various working conditions are quantitatively extracted.The results show that the displacement increment of the tunnel vault will decrease first and then increase with the increase of the overburden thickness.150 working conditions are applied to Rongjiang No.4 Road underwater tunnel by analogy,and the minimum displacement increment of tunnel vault appears when the overburden thickness is 14 m.