Foundation performance of Gardiner Dam

Abstract

Gardiner Dam is founded on a highly overconsolidated clay shale that had a profound influence on embankment design and performance. The shale contains preconstruction shear zones whose specific location, extent, and strength characteristics were virtually impossible to determine from test samples and soil logs. Analyses of slumped valley slopes and of shallow excavation slopes that moved during early stages of construction overestimated the strength of dormant shear zones that were activated during subsequent embankment construction. Major design modifications consisting of extensive slope flattening and toe-berm loading were undertaken to compensate for the weakness of the shear zones.About 2 m of foundation settlement and over 2 m of lateral displacement occurred under the central section of the 64 m high embankment during construction, but virtually none happened at the toe. Annual post construction increments of lateral displacement are related mainly to the yearly cyclic reservoir load. Although these deformations have decreased to the order of 2 cm/year, comprehensive monitoring and critical evaluation are maintained in order to ensure long-term safety and functional adequacy of the project.The design and construction of Gardiner Dam proceeded at the forefront of knowledge and understanding of the residual shear strength and behavior of highly overconsolidated clay shales. The construction and postconstruction performance of the foundation, which is described in this paper, indicates that much more information and research are needed to fully understand the long-term behavior of such materials.