Removing an extensive amount of overburden can initiate adjustments in the foundation mass. Rebound adjustments induced by this removal include, in addition to elastic response, elements of visco-elastic and plastic response which have to be taken into account when the foundation is loaded by subsequent construction. The accurate measurement of foundation response can be important in the design and construction of deep foundations and can be essential in the construction of very deep foundations. In 1974, a large foundation excavation was undertaken for the two unit Grand Gulf Nuclear Station. Overburden removal ranged in depth from 65 feet in the turbine area to 110 feet in the containment area. Very long, rod-type Multiple Position Borehole Extensometers measured the rebound. The design of the extensometers and the dimensions of the installed instruments, are discussed. Graphs are included which show the adjustments measured by each extensometer during the deepening of the excavation. The measured rebound for each transducer package of each extensometer is summarized. The data is compared to the predicted values which are based on a mathematical model developed using laboratory test results and empirical methods. The resulting information forms part of the permanent record of construction for the nuclear power station.
Grand Gulf Nuclear Station is located 25 miles south of Vicksburg, Mississippi. The station consists of two units, each 1250 Mw boiling water reactors. Limited site work began in April 1974. In this area the soils consist of loess, underlain by terrace deposits which in turn are underlain by the Catahoula Formation. The Catahoula is the competent bearing stratum for the plant foundations. The top of the loess is at the ground surface and varies from approximately El. 206 to El. 140. The interface of the loess and terrace deposits is approximately El. 130.
