Methods for Buttress Dam Stability Assessment Considering Monolith Interaction and Structural Cracks

QC 20261106

Abstract

In stability assessments, concrete buttress dams are typically assumed to act as intact structures, with monoliths considered in isolation. This neglects the impact of effects such as preexisting cracks and interaction between monoliths. Neglecting these mechanisms can lead to either over- or underestimation of the load-bearing capacity and may also affect the predicted failure behavior. The aim of this thesis is therefore to improve the understanding and assessment of the stability and failure behavior of concrete buttress dams, with particular focus on monolith interaction, structural cracks, and the requirements of numerical modeling.

These phenomena were assessed with a combination of physical and numerical models. The physical model tests were performed to establish the failure behavior of isolated and interacting buttress dams with both cracked and intact monoliths. Finite element (FE) simulations were used to reproduce the physical tests, validate the numerical approaches, and extend the analyses beyond the experimentally tested configurations. The numerical models ranged from relatively simple predefined frictional interfaces to distributed concrete cracking and a reinforced rough-crack model incorporating aggregate interlock and reinforcement action.

Monolith interaction was shown to significantly alter both the load-bearing capacity and failure behavior of buttress dams, with the largest increases in resistance observed for short dams with steep cross-valley foundation inclinations. Failure of individual monoliths was found to be unlikely while interaction and contact with adjacent monoliths were maintained. However, considerable displacement may be required to mobilize this interaction. The associated increase in resistance should therefore generally not be credited in the primary stability assessment, but rather be regarded as reserve capacity that may be important for predicting the global failure behavior of the dam.

Structural cracks have been shown to affect both the load-bearing capacity and failure mode of buttress dams. A cracked buttress dam can first be represented as blocks separated by smooth planes with Coulomb-friction interfaces, with aggregate interlock and reinforcement crossing the crack omitted. If this simplified representation demonstrates stability, it is sufficient for assessing the load-bearing capacity. However, these models may misrepresent failure modes and are less suitable for predicting the actual behavior of the dam. If the simplified block model does not demonstrate adequate stability, the effects of aggregate interlock and reinforcement crossing the crack may need to be included. For reinforced rough cracks, the initial crack width was found to strongly influence the load-bearing capacity and, in some cases, the governing failure mode. Therefore, a model that includes reinforcement but represents the crack interface using Coulomb friction cannot generally be assumed to provide a conservative estimate of the load-bearing capacity.

The required numerical model complexity was found to depend on the behavior of interest and the objective of the analysis. Simplified models were adequate for failures dominated by rigid-body motion, whereas three-dimensional interaction, material failure, and reinforced crack behavior required more detailed representations. These findings also have implications for dam monitoring, where stability assessments can help identify relevant local and global response quantities and structurally important cracks. However, the monitoring implications remain qualitative, and further validation under representative field conditions is required to reduce uncertainties associated with real foundation interfaces and aged structural cracks.

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