Support Mechanism And Diseases Treatment Technologies For Tunnels With Soft Base

With the increasing of the numbers of tunnels in soft rock, engineering disasters occur frequently due to the inefficient bearing capacity of tunnel bases. It is necessary to pay more attention and to do further researches to those tunnels. By means of theoretical analysis, numerical simulation and field tests, the load calculation method for soft bases under arch feet, supporting mechanism, main treatment measures and scheme to deal with engineering diseases have been studied. A technology system for construction of tunnels with soft bases has been set up. The main contents and conclusions are as follows:(1) In order to check base bearing capacities of tunnels in soft rock, a new analysis method to calculate the loads on soft bases is developed, in which the load-bearing characteristic of tunnel supporting structure is considered. As to shallow buried tunnel in weak and soft rocks, on basis of the theory of load & structure method, formulas to calculate base loads in symmetrical and unsymmetrical tunnel are deduced, with necessary simplifications of the friction from surrounding rock and the resistance of steel bars to the supporting structure. To deep buried tunnel with soft base, the bearing capacity can be directly evaluated through construction numerical simulation according continuous medium theory. The calculation method is verified by comparing the theoretical result with the field test.(2) An analytic model of the foot steel pipe is built, with a view to its characteristic of design and construction, and behaviors of load and deformation. Based on the Winkler's assumption, a model of beam on elastic foundation is put forward to analyze the foot steel pipe. Formulas are deduced and software is written with Visual Basic for the calculation. Graphics of the reasonable length corresponding to size of steel pipe and elastic resistance coefficient of soft rock is brought forward for the design of foot steel pipes.(3) Bearing behavior and failure module of foot pipes are studied through numerical simulation with FEM. The influence to the bearing capacity is discussed, including the strength of surrounding rock and the parameters of steel pipe. Some important results different from engineering experiences are gained by theoretical analysis: 1) to foot steel pipe that can not penetrate the soft base, its reasonable dip angle is very small for low longitudinal bearing capacity; 2) steel pipe with diameter of 42mm is unsuitable for foot steel pipe because its low cross bearing capacity; 3) to a given geological condition and pipe parameter, the reasonable length of steel pipe is certain, extra longer pipes can't obtain larger capacity; 4) surrounding rock near the support structure should be especially reinforced by drilling grouting holes along the whole steel pipe. Based on those results, design and construction methods are presented.(4) Main engineering measures are researched to increase the bearing capacity of soft base and the resistance of structure to settlement. On the basis of analysis of support mechanism, design theory, construction method and applicable condition, a design table is established for measure selection to treat with tunnel on soft base.(5) Combining with engineering instances of No.3 Youshui Tunnel and No.4 Youshui Tunnel, No.1 Chang'gan Tunnel, principles for treatment are raised through the analysis of common diseases in tunnel with soft base. Directed against the different demands of normal construction and diseases treatment, systematic schemes are established including the geological survey, calculation and analysis, engineering measures selection, monitoring and assessment.