Numerical Simulation On Reasonable Freezing Mode For The Combined Pipe-Roof And Freezing Pre-support Technique In Gongbei Tunnel

Artificial freezing method, by means of drilling the freezing pipes into soil,which can transfer heat to reduce the the soil temperature, can translate the aquifer into frozen formation to enhance the soil stability, reduce the deformation and cut off the groundwater, so that the various construction can be carried out within the protection of this special construction techno logy.The main application of artificial freezing method in tunnel can be lied as follow:enhancing the soil of the departure and reception of shield, stabilize the of butt joint of the shield tunnel in the underground or the bottom of the sea, shield tunnel gushing, repair cave-in accident and so on. Artificial freezing method has been widely used because of its main advantages:first, reliable water sealing,helps to dryp-excavation,and ensure construction quality of various types of structures.second, Protecting the Environment,no foreign material invade into the soil,so small disturbance to the surrounding environment. third, strong practicability, can be used in different soil layers under normal circumstances.On the basis of the horizontal freezing method pre-reinforcement construction in the hidden digging section of Hong kong-zhuhai-macao bridge gongbei tunnel, according to the design of freezing method in the "Gongbei tunnel construction drawing design", by using theoretical analysis, numerical simulation and other methods, the study have been made on the law of development of permafrost for freezing combination mode of Circular Freezing pipe+ Strengthen pipe+ Limiting pipe. The main contents of this paper are:(1) To list the main freezing composite modes according to the design of freezing method in the "Gongbei tunnel construction drawing design" and similar project experience;(2)To design experiment projects for a variety of freezing design experiment projects and simulate the development rule of the temperature field for those projects with numerical models by using the large-scale numerical software, considering the influence of the latent heat for Frozen wall extending speed; And simulate the law of development of the ground frost heave displacemen and strain field with sequential coupling analysis method, by introducing the coefficient of thermal expansion to consider the influence of the surface frost heave deformation by artificial freezing method.According to the requirements of freezing time, frozen wall thickness and the frost heave control, three test programs are made to verify:① whether the the frozen wall thickness required can been formed by the mode of only circular freezing pipes, make sure the necessity of opening the Strengthen frozen pipes for auxiliary freezing.② Whether the the frozen wall thickness required can been formed by the mode of ’circular frozen pipe as the main pole, Strengthen frozen pipe as the auxiliary pole", set for the rationality of opening the Strengthen frozen pipes 15 days before the excavation.③To determine the lag open time of the Strengthen frozen pipes in the circular freezing pipes and circular freezing pipes together freezing mode.(4) Making sure the necessity of opening the limiting pipes and giving some suggestions for the timely opening time and Salt water appropriate hot temperature of the limiting pipes.(5) Optimizing the freezing combination mode of "Circular Freezing pipe+Strengthen pipe+Limiting pipe". Finding out the best freezing combination mode to save the cost of freezing and strengthen the freezing schedule management, according to the freeze period, frozen wall thickness requirements and frost heave control indicators.