ABSTRACT 1 ABSTRACT
Case histories for small diameter tunnels in Austin, Texas, are analyzed to gain insight into particular problems encountered during tunnel excavation in weak sedimentary rock, and the performance of different TBM excavation systems are compared.
2 INTRODUCTION
Since 1973, more than 75 km of small (2.4 to 3.2 m) diameter tunnels have been excavated or are planned in the vicinity of Austin, Texas. Complete project reports are available. In this paper, information from six tunnel contracts in three rock units is presented, performances are compared, and conclusions are suggested.
3 AUSTIN AREA GEOLOGY
The City of Austin is located at the boundary between two physiographic regions: the Edwards Plateau, the "hill country", to the west; and the prairies of the coastal plain to the east. The hill country is a dissected plateau, incised by numerous streams. The boundary between the "hill country" and the prairie corresponds to the locus of the Balcones Fault zone. The effect of this fault zone on the rock in the area is illustrated in Fig. 1, a west to east geologic cross- section in the vicinity of Austin. The entire fault zone is about 8 km wide. The bedrock geology in the Austin area includes chalk, claystone, marl, limestone, dolostone, dolomitic limestone, and volcanoclastic materials of Cretaceous age. The rock units outcrop in belts with general strike north-northeast and shallow dip (less than one degree) to the southeast. Additional attention in this paper is directed towards excavation in the Glen Rose Formation to the west of the fault zone and in the Austin and Taylor Groups to the east of the fault zone. The particular difficulties of tunneling in the faulted material will not be addressed here.
4 TUNNEL PROJECT DESCRIPTIONS
The Glen Rose, Austin, and Taylor rock units have been tunneled in six contracts between 1973 and 1986. Information on these projects is presented in Table 1. Tunneling for the Austin Crosstown Wastewater Interceptor (ACWI) was completed in 1974. The 5029-1 contract began in the Taylor Group rock at the east, and continued through the Austin Group into the fault zone. The Calweld TBM (Smith International) used was fully shielded and had the capability to thrust off primary rib and lagging support or to react torque and thrust with four x-bracing sidewall grippers which could be extended through the shield. The ACWI 5029-3 contract began in the fault zone and continued west into the Glen Rose Formation. The Robbins TBM used was not fully shielded and thrust reaction was obtained with opposing sidewall springline grippers. The Onion Creek Wastewater Interceptor (OCWI) was completed between 1984 and 1986, with two major tunneling contracts. The Section II contract was excavated in the Taylor and Austin Groups, including local volcanoclastic deposits. A shielded Lovat TBM was used and thrust was generated with tail shield hydraulic cylinders extended against rib and lagging support. The OCWI Section IV contract was excavated entirely in the Taylor Group. Thrust and torque reaction for the two Lovat TBMs was mobilized through an expandable thrust ring or gripper section located immediately behind the shield.