A detailed understanding of geological conditions is required to define realistic failure mechanisms and derive the required inputs for dam stability assessments. This paper initially outlines the rock material and rock mass properties at five New Zealand dam sites and then summarises approaches to the selection of design input parameters and analytical stability methods. The dams are founded on rock mass types that include closely-jointed greywacke, foliated schist and jointed sintered to moderately welded ignimbrite. Rock mass characterisation and laboratory strength testing provide inputs for classification systems such as the Geological Strength Index and failure criteria such as that of Hoek-Brown, but even where these are well defined, care is needed in the recognition of appropriate failure modes in the foundation geological model. The case histories include examples of the use of the Hoek-Brown failure criterion for estimating global rock mass strength, the Barton empirical equation for discontinuity shear strength and the empirical estimation of rock mass deformability. Analytical methods include both limit equilibrium and finite element methods and use of Strength Reduction Factor for sliding stability. A critical prior requirement for analyses is to assess whether they are for global rock mass strengths or structurally-defined kinematic mechanisms. The discussion section considersinfluences on input parameter values such as the Hoek Brown material constant mi.
Stability assessments are required for both the design of new dams and performance evaluations of existing dams. Apart from small dams with low hazard ratings, their design should be based on comprehensive, site-specific investigations and geotechnical properties, rather than precedent or empirical charts. Most dams have individual or unique characteristics, and there is a real challenge in selecting investigation techniques most suitable for the nature of the site and applying appropriate methodologies in the design process. This paper outlines experiences in obtaining design parameters for the foundations of five existing dams in New Zealand, and the analytical approaches used for stability assessments. Initially there is an overview of foundation intact rock material properties obtained from laboratory testing and rock mass properties obtained from rock mass characterization. Then, derived input parameters are used in limit equilibrium and finite element analyses of dam stability, using either global rock mass strength and deformability or shear strength along structurally controlled defects. The paper finishes with some discussion of the factors that influence the design parameters outlined in the paper.
Figure 1 shows a map of New Zealand with locations of large dams [1], many of which are part of hydroelectric schemes. The dams are founded on a range of lithologies that include Upper Palaeozoic to Mesozoic-age greywacke and schist, Tertiary-age sedimentary rocks and Quaternary-age ignimbrites. The five case history dams, (four gravity and one arch), are founded on greywacke, schist and ignimbrite.
Unconfined testing (unconfined compressive strength - UCS or qu, Brazilian tensile strength - st and Youngs modulus - Ei) has been performed on cores obtained from block samples and/or triple tube drilling at most of the sites.
