ABSTRACT
Factors such as rock cover, rock type and rock quality can exert a dominate influence on the behavior of a concrete tunnel lining. This paper will present and discuss the investigation, evaluation and use of such factors at the Rocky Mountain Pumped Storage Project in the design of the 10.67m (35 foot) diameter concrete tunnel lining. A plane strain, finite element analysis was performed on the concrete/rock system. The finite element model consisted of a concrete lining zone and three (3) rock zones; a highly destressed rock zone, a transitional rock zone and an intact rock zone. Zones were differentiated from one another by varying the modulus for each zone. Results of this study yielded a concrete lining strain from radial expansion which was used in the crack control calculations for the concrete tunnel lining. After the tunnel excavation was performed, plate jacking tests and additional seismic surveys were run on the tunnel walls. This testing served to confirm the rock modulus values used in the preliminary analysis. However, results were observed which indicated that a two zone rock model would more accurately represent in-situ rock conditions than the three-zone rock model used in the preliminary analysis. This revision to the concrete/rock model was made and a final concrete tunnel lining analysis was performed.
INTRODUCTION
High hydrostatic heads conveyed through large diameter tunnels in present day pumped storage hydroelectric projects have stretched pressure tunnel lining rule-of-thumb design methods to the limit. At a good site in which the potential for a catastrophic tunnel failure is remote, rule-of-thumb design may be adequate. However, if one is not so fortunate as to have an ideal site, historical rule-of-thumb design methods can be found lacking. Alas, such information can not be known without adequate site geotechnical and geophysical testing. During the past few years, several failures of pressure tunnels during first filling have amply highlighted a need to re-evaluate the use of traditional rule-of-thumb pressure tunnel design methods. Factors such as rock cover, rock type and rock quality can exert a dominate influence on the behavior of a pressure tunnel lining. Each of these parameters must be adequately identified and evaluated for inclusion in the pressure tunnel lining analysis, design and subsequent construction. The specific characteristics of each site must be determined while the use of vague generalities and rules-of-thumb for detail design must be eliminated. The initial decision which must be made is a determination of the type of lining which is required for the project. If a watertight lining is needed, normally a steel lining must be provided. If some water loss is acceptable, a concrete lining or perhaps even no lining at all may be appropriate. Any type of concrete lining will be permeable to some degree. The acceptability of water loss through a concrete lining can be evaluated based on these questions:
Is the pressurized tunnel inside a potential hydraulic fracturing or hydraulic jacking area?
Is the rock erodible?
Is landslide potential critical?
Is water loss critical to the project operation?
Is there a low ground water level?
