Improving advance rates of TBMs in adverse ground conditions

Abstract

AbstractA new approach is required to TBM tunneling where adverse ground conditions exist if predictable advance rates are to be achieved. It is well understood that when good ground conditions exist, TBMs vastly outperform drill and blast advance rates. However, with today's available TBMs, these high performance rates can soon be negated if adverse ground conditions are encountered. It is a common misconception that a shielded TBM will be able to cope with all adverse conditions. This is definitely not the case, as projects like Pinglin in Taiwan, Hallandsås in Sweden and San Francisco in Ecuador, will attest to.This paper describes adverse ground conditions as related to TBM tunneling, including squeezing ground, rock bursts, fault zones, and high water pressure. Optimal ground support techniques and potential problems with specific TBM types are discussed. Finally, a new concept TBM that integrates both ground conditioning and ground control techniques is introduced. This new concept TBM endeavors to facilitate predictable advance rates, even in difficult conditions, using novel concepts such as a retractable front shield and improved rock support systems in L2 area. This retractable front shield incorporated into a Double Shield feature allows for immediate ground conditioning behind the cutter head and 360 degree probe drilling, while still maintaining the increased safety and simultaneous tunnel lining characteristic of Double Shield TBMs.Excellent access immediately behind the cutter head is maintained by eliminating the peripheral thrust cylinder utilized in standard shields and using the basic floating gripper design to pass thrust to the cutter head via a main beam.