A comparison of in-situ and laboratory shear wave velocity measurements in compacted backfill
27 June 2012
The construction of a new nuclear power plant requires an excavation more than 90 feet (27.4 meters) deep. The excavation is backfilled with compacted sandy soils. The power plant is embedded about 40 feet (12.2 meters) below grade, thus bearing on about 50 feet (15.2 meters) 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 crosshole seismic testing methods, were conducted on a 20 foot (6.1-meter) 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 20 to 25 feet (6.1 to 7.6 meters) apart. The fourth and final level of testing was conducted nominally at plant grade. Crosshole 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 (25 and 50 feet [about 7.6 to 15.2 meters] of backfill placed) are evaluated. Results from the final two test levels of backfill, along with results of crosshole 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.