Abstract:

When EPB shields bore in the typical fullface dry sand strata and/or composite strata consisting of dry silty clay and sand in Shijiazhuang, problems such as large thrust force, large torque, unsmooth releasing of mucks, serious ground surface settlement, serious wearing of cutterhead and screw conveyor and difficulty in shield attitude control may occur. In the paper, the type selection and boring parameters of the shields for the running tunnel from Heping Hospital Station to Lieshilingyuan Station on Line 1 of Shijiazhuang Rail Transit Project are analyzed. Conclusions drawn are as follows: 1) When EPB shields bore in fullface sand strata, bentonite slurry and foams must be injected so as to condition the ground and reduce the friction; 2) When EPB shields bore in the composite strata consisting of silty clay and mediumcoarse sand, only foam injection is needed; 3) The fundamental target of bentonite slurry injection is not to condition the ground, but to improve the lubrication effect; 4) When EPB shields bore in such strata, attention should be paid not only to the successful boring of the shield, but also to the protection of the components and parts of the shield. Therefore, when EPB shields bore in the composite strata consisting of clay and sand, it is necessary to inject some water or bentonite slurry. 

Key words: Shijiazhuang Metro, EPB shield, dry sand strata, shield type selection, ground conditioning, boring technology