Computing the stability and strength of rock foundations under concrete gravity dams

Abstract

1. Analysis of the studies and computations suggests that loss of stability due to shearing is characteristic for concrete gravity dams constructed only on extremely weak rock and semirock foundations. On sound rock and rock of medium soundness, concrete dams form a strong contact with the foundation and lose their stability if there is compressive failure of the foundation with rotation about a pivot center located beneath the foundation surface. Rock foundations for which limiting rotation of the structure is the basic type of failure are characterized by a critical shear coefficient tan ψcr≥1.0#x2013;1.1. 2. It is recommended that gravity dams and their foundations be designed on the basis of two limiting states: on the condition of limiting rotation, which corresponds to the dam's loss of stability and is referred to as the first group of limiting states, and on the condition of incipient rotation for which the local strength of the rock foundation is exceeded; the latter is referred to as the second group of limiting states. 3. The initial stage of rotation (incipient rotation) is accompanied by crack formation in the rock foundation in the region of the upstream face after attaining the ultimate, tensile strength (Rt) with the smallest principal stresses. The final stage of rotation (limiting rotation) is accompanied by failure of the rock mass beneath the downstream face of the structure with the smallest principal stresses that are attained for the ultimate compressive strength (Rc) of the rock mass. It is recommended that the stability and strength of the rock foundations of concrete dams be computed from the conditions of incipient and limiting rotation, using these characteristics of the tensile Rt and compressive Rc strengths of the rock mass, which have a clear-cut physical meaning. 4. Stability computations performed for gravity dams on the basis of the condition of limiting rotation yield safety factors for stability, which correspond, to actual fracture kinematics and are therefore more reliable. 5. Conversion to dam-stability computations based on the conditions of limiting and incipient, rotation make it possible to: a) utilize the additional bearing capacity of sound and medium-sound foundation rocks; b) estblish grounds for economical gravity dams of reduced size, using engineering measures that ensure their adaptability to normal operations as cracks form in the foundation; c) reduce the, depth of the cut formed in rock foundations for dams, assuming that rock characterized by the parameter tan ω≥1.0–1.1 can remain, in the foundation, since in this case loss of stability occurs not as a result of sheart, but due to rotation of the structure, which is characterized by high stability safety factors; d) use engineering methods correctly and efficiently, to increase the stability of dams, such as, for example, their anchorage in the rock foundation, which creates a significant restraining moment that is not considered in computing the shear resistance of the dams. 6. The method proposed to compute the stability and strength of rock foundations of gravity dams can also be expanded to buttress dams and retaining walls.