Comparison of In Situ and Laboratory Shear Wave Velocity Measurements in Compacted Backfill

The construction of a new nuclear power plant requires an excavation over 27.4 m (90 ft) deep. The excavation is backfilled with compacted sandy soils. The power plant is embedded about 12.2 m (40 ft) below grade; thus bearing on about 15.2 m (50 ft) of compacted backfill. During licensing and design of the plant, the shear wave velocity profile of the proposed backfill was estimated using results from laboratory testing [combined resonant column and torsional shear (RCTS)] and in situ shear wave velocity measurements. RCTS tests were conducted on representative samples of potential borrow sources. In situ velocity measurements, which included spectral-analysis-of-surface waves (SASW) and cross-hole seismic testing methods, were conducted on a 6.1-m- (20-ft-) thick embedded test pad of the potential borrow. During construction of the production backfill, in situ shear wave velocity measurements, using SASW methods, were taken at four levels during backfill placement, spaced nominally 6.1 to 7.6 m (20 to 25 ft) apart. The fourth and final level of testing was conducted nominally at plant grade. Cross-hole seismic testing through the backfill was also conducted at this level. This paper compares the results of estimated values made during the licensing and design phases of the project, based primarily on laboratory test results, with in situ shear wave velocity measurements made during the actual construction. In particular, SASW results from the first two levels of backfill tested [about 7.6 and 15.2 m (25 and 50 ft) of backfill] are evaluated. Results from the final two test levels of backfill, along with results of cross-hole testing, were not available at this writing. The comparison of laboratory results and in situ testing provides good agreement The assumptions made to develop the estimated values during the licensing phase are compared to the as-built measurements of shear wave velocity accounting for actual backfill materials and construction practices reflected in the in situ values.