The Tokyo Electric Power Company, Inc. has been constructing a large pumped storage Power station with an output of 1,280 MW by two dams on. the Takase River, a branch stream of the Shinano River.
Nanakura Dam, as the lower storage is a earth-rock dam with a height of 125 m and a volume of 7.4 million m3. Construction was completed in 1976 and by 1979 reservoir had filled.(Figure 1.)
The bedrock of the dam site is composed mostly of granite and in the vicinity of the right-bank of the dam crest and in the directly contact zone of the core at Streambed exist fractured zones upstream and downstream through the axis of the dam. This report will describe the process for treating the fractured zone in the contact to the core zone at the streambed, and the grouting technique which used of the allowable grouting pressure determined from effective confining pressure at each Stress level based on the laboratory model test, in order to prevent the occurrence of hydraulic fracturing of core at the Contact zone by fault grouting from the gallery in the foundation rock.
The geological features of the dam site are characterized by a preponderance of granite, penetrated by a barely perceptible dike of porphyrite and lamprophyre approximately 1 to 5 m wide.
This granitic bedrock is a coarse, ash-colored biotitic granite with sufficient bearing power for the foundation of a earth-rock dam, but in the streambed zone there are three fractured zones of decomposed granite and partially argillized granite that are in contact with the core zone of the dam and run from upstream to downstream through the axis of the dam.(Figure 2.)
These fractured zones have an influence on the bearing power and permeability of the dam.
Table 1 gives values obtained from surveys and tests performed mainly on the fractured zone for the bearing power and permeability of the foundation. They show clearly that the shearing strength of the fractured zone is somewhat greater than that of the core material of the dam. But as it is highly permeable, it was feared that leakage from the reservoir through the fault and piping of fault material would occur.
The fractured zone, which is in contact with the core zone, must not be allowed to produce shear failure, due to the load of the dam, or destructive deformation of the core zone, due to differential settlement.
Furthermore, it must have the impermeability against seepage from the reservoir through the foundation so as to prevent seepage from causing piping of fault material.
With these imperatives in mind, the stability of the fractured zone was investigated and calculations were made using FEM.
