Study On Pipe Roof Effect And Face Stability Of Large Section Pipe Roof-Box Jacking Method

With the development of transportation rapidly, more and more large section and shallow buried tunnel will be constructed in city. Pipe roof-box jacking method is a kind of trenchless knowledge which has the advantage of little affect on ground activity and environment, especially for the case passing through road with heavy traffic and important zone. Compared to ordinary pipe roof method used in Japan or other countries, the pipe roof-box jacking method (RBJ method) is no need to improve the soft soil within the pipe roof space, and consequently the project cost and time can be reduced greatly. Base on the Beihong Road tunnel in shanghai city, this paper studies the mechanical effect of steel pipe and the stability during the box jacking by means of analytical equation, numerical simulation, laboratory test and site measurement.Researches of the pipe roof mechanism focus on two aspects which are the unloading or overloading effect of top row of pipe and guiding effect of the bottom row.Underexcavation and overexcavation models have been presented based on the elastic foundation beam theory and the corresponding deflection equations and other controlling equations have been deduced. In addition, the affections of pipe's diameter, deformation magnitude, and coefficient of subgrade reaction on unloading ratio or overloading ratio, settlement range and inner forces of steel pipes have been discussed. When the settlement produced, the pipe roof can transfer some covering load from active sliding zone to stable zone, and the unloading capability of pipe roof is increasing linearly as the pipe's diameter, deformation magnitude, coefficient of subgrade reaction increased, and the settlement range expands large as the settlement magnitude and stiff rigidity increased, but decreased with coefficient of subgrade reaction increased. When the heave produced, the pipe roof can transfer some covering load to passive zone, and the overloading ratio and heave range of pipe roof is increasing as the pipe's diameter, heave magnitude, coefficient of subgrade reaction increasing, while coefficient of subgrade reaction has almost little influence.Laboratory test has been done by use of clay and sandy soil respectively to testify the theory. When box jacking in clay under the circumstance of underexcavation, heave displacement has been produced near grid, and the test value has same regular pattern with calculate one. When box jacking in sandy soil under the circumstance of overexcavation, settlement has been produced near the grid and the test value is coincide with the calculated one. Taking the 4^th and 5^th box as examples which are jacked under the conditions of underexcavation and overexcavation...