In situ sampling, density measurements, and testing of foundation soils at Duncan Dam

As part of the B.C. Hydro Dam Safety Review of Duncan Dam, a comprehensive two-phased program of field and laboratory investigations was carried out between 1988 and 1992 to evaluate the potential for liquefaction of the foundation soils during an earthquake. The initial screening level phase 1 field investigations comprised basic soil sampling, standard penetration tests, and cone penetration tests to determine the spatial distribution of the foundation soils beneath the dam and to characterize the basic engineering properties of the soil units. A screening study using Seed's liquefaction assessment method indicated that soil unit 3c consisting of uniform fine-grained sand would be susceptible to liquefaction. The phase 2 investigations consisted of detailed and careful soil sampling to obtain high-quality in situ samples of the unit 3c sand for laboratory triaxial and simple shear tests. Samples were obtained using a conventional fixed piston sampler and a specially modified Christensen double core barrel sampler. The samples were frozen to minimize sample disturbance during handling and transport. Ground freezing was also conducted using liquid nitrogen, and the frozen soil was sampled using a CRREL core barrel. Borehole density logging was performed to assess the quality and level of disturbance of the soil samples. This paper discusses the procedures and results of the phases 1 and 2 field investigations. Key words : standard penetration test, cone penetration test, geophysical logging, energy calibration, soil sampling, freezing.