مهندسی عمران شریف (Feb 2018)
EFFECT OF WATER PRESSURE INSIDE THE CRACKS ON THE BEHAVIOR OF CONCRETE GRAVITY DAMS UNDER FLOOD LOADING
Abstract
Tensile cracks at the upstream face of concrete gravity dams, especially at the dam base as induced by tensile stresses, due to the reservoir hydrostatic load are the most likely type of Damage in this type of structures. Given the possible intrusion of water under the reservoir heading into such tensile cracks, considering an actual distribution of water pressure inside the crack may intensively affect the stress state around the crack tip. Eventually, propagation of the crack, and hence, a change of COD profile mutually modifies the water pressure distribution along the crack. This coupled hydro-mechanical process has played an important role in design and safety assessment of concrete gravity dams. If this interaction is not considered in the stress analysis or safety assessment of concrete gravity dams, results of analyses can be misleading.The aim of this paper is to present a simple 2D model to simulate the process with hydro-mechanical interaction based on nonlinear fracture mechanics (NLFM) and Biot effective stress concept. The proposed model includes: 1) the modified smeared crack model for the mechanical behavior of cracking concrete; 2) the effective stress equation in which both terms of pore pressure and effective porosity are increased linearly with tensile damage index in a cracked element.The proposed model is validated by the wedge-splitting test of a concrete cube specimen under a displacement control loading. The specimen is subjected to a mechanical load as well as to an internal hydrostatic pressure of 0.5 MPa. Next, the Koyna Dam as a well-known example of a concrete gravity dam, was analyzed under flood loading considering water-crack interaction; moreover, cracking status of the dam and the maximum sustainable reservoir level before dam collapsing were studied. Numerical analysis results confirm the significant effect of water pressure inside the cracks on the overall response of the dam. In fact, considering hydrostatic pressure inside the cracks has significant effects on the scenario of dam collapse and the maximum sustainable reservoir level.
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