Abstract:

Underground obstacles, such as concrete piles, soil retaining structures, pipes and trenches and underground structures, are often encountered during shield boring in the construction of urban rail transit works. These pile foundations are normally underpinned, dismantled or manually cut. However, these conventional treatment methods have such disadvantages as high cost, long construction period and severe influence on the surrounding traffics. A 400 mm model shield is used to cut blind concrete, glass fiber concrete and reinforced concrete, so as to study the feasibility of pile foundation cutting directly by shields, to make analysis on the modification of cutter heads and to obtain shield boring control parameters. Furthermore, the experience of the pile foundation cutting directly by shields in the construction of No. 7 line and No. 10 line of Shanghai rail transit works is described. Conclusions drawn are as follows: 1) It is feasible to cut reinforced concrete directly by shields; 2) Before shield launching, the cutter head should be modified, and a number of advance cutting tools and shell tools should be installed on the cutter head; 3) During the cutting of the pile foundations directly by shield, the shield should advance slowly (with less than 10 mm/min advance speed), and the earth pressure, thrust and cutter head torque set should be stable; 4) During the cutting of the pile foundations directly by shield, lubricating and frictionreducing materials should be added into the excavation chamber so as to prevent the screw conveyor from being jammed by concrete blocks. After the shield passes the obstacle point, secondary grouting should be made to fill the annulus space behind the segment lining on basis of the monitoring results so as to control the settlement of the ground surface and the nearby buildings; 5) Cutting pile foundations directly by shield has advantages,including low cost, high safety and small environmental influence.

Key words: shield, pile foundation cutting, reinforced concrete, underground obstacle