Exploration, Design And Excavation Of Clinch River Breeder Reactor Foundations

1. The successful completion of large excavations in rock requires implementation of a design that is comprehensive yet flexible enough to produce a safe, economical and technically acceptable excavation. This paper summarizes the planning, exploration, design and construction associated with the excavation program for the Clinch River Breeder Reactor Plant in Oak Ridge, TN. 2. In preparation for construction of the safety related structures associated with the Clinch River Breeder Reactor Plant (CRSRP) in Oak Ridge , Tennessee, extensive excavation and foundation preparation work was required to meet static and seismic loading conditions imposed by these structures. The excavation for these structure measured 4?5 feet by 300 feet in plan view and extended to depth of approximately l00 feet below finished grade. The excavation required the removal of 500,000 cubic yards of residual soils and 1.1 million cubic yards of rock primarily consisting of laminated siltstone and limeatone. To minimize rock excavation and provide crane access to the bottom of the excavation, 75-foot high , nearly vertical rock slopes in the north, south and east portions of the excavation were required. These rock slopes required an extensive rock bolting program to assure the stability of the excavation during the expected seven-year construction period for the project. 3 An extensive subsurface investigation and laboratory testing program was performed to delineate-subsurface conditions and establish engineering 9 properties of the foundation materials. The field investigation consisted of soil and rock borings, mapping of exposed rock outcrops and in-situ testing. Soil samples and rock core obtained from the borings were tested to determine index properties and strength parameters of these materials. Laboratory testing of these samples included unconfined compression tests, Poisons ratio and unit weight determinations. In-situ testing included continuous down-hole geophyaical logging, determination of seismic compression and shear wave velocities using crosa-hole and up-hole methods, and performance of Goodman Jack tests to determine the in-situ modulus of elasticity. These tests were performed in different orientations with loading directions corresponding to strike, dip and intermediate orientation. The results of the field and laboratory testing are summarized in Figure 1. These values do not necessariIy reflect overall in-situ rock mass properties because of the influence of geoIogic discontinuities. As the size of the Ioaded area increases more rock discontinuities are affected. Therefore, the impact of the discontinuities on the overall properties of the rock mass had to be carefully evaluated to develop realistic design parameters for the rock mass. 3.1 Site Geology The CRBRP site lies near the western border of the Appalachian geosyncline, which was active during most of the Paleozoic Era. The site is underIain by limestones, siltstones and dolomites. Weathering and erosion are the dominant geologic processes since Paieozoic time. The rock is generally covered by residual soils except at some Iocations near the CIinch River where ancient terrace and recent alluvial soils have been deposited. Figure 2 iIIustrates a typical profile of the site.