Joints play an important role in the behavior of dam under normal and shear loading conditions. Existence of discontinuities such as joints causes reduction of strength and increase of deformability in dam structures. Thus the safe management of dam operation requires a precise evaluation of the dam stability in terms of the shear strength of the concrete lift joint or a discontinuity in the rock mass. The shear strength of these two types of joints is analyzed in this paper. Then the applicability of acoustic emission (AE) in monitoring the shear behavior of joints is discussed. In order to identify shear mechanism of joints the AE source location is evaluated.
One of the most important cases of instability of concrete dams generally results from pre-existing geological features in the foundation such as faults, joints, soft rocks and solution channels, the contact joint between dam body and foundation and joints in dam body. Therefore, stability analyses and assessment in term of shear strength of these weaknesses are essential and form a critical part of the safety assessment. Shearing process of the discontinuities can be divided into two types: constant normal load (CNL) condition, where the normal load acting on the discontinuity remains constant during the shearing process, and constant normal stiffness (CNS) condition, where the normal load varies according to the normal dilation. For studying the joint behavior in dam body and its foundations, it can be considered that, depending on the orientations of the joint sets and their depth, each joint is free to dilate and it is subjected to normal load. Hence, to study these conditions, the most appropriate laboratory experimental shear test set-up is constant normal load (CNL). Besides conventional monitoring techniques using extensometers, strain gages and joint meters, the acoustic emission (AE) monitoring has been experimented in several civil engineering structures. The acoustic emission monitoring can be used, as a supplementary tool, for the assessment of damage and instability of structures during construction and operation. Acoustic Emission is a transient elastic wave that is generated by the rapid release of energy within a material. The current status of structural assessment via the acoustic emission method is that stability is associated by no AE activity, while instability is indicated by varying degrees of AE activity. Obviously, the greater the AE activity, the greater is the degree of instability. In most cases, failure is associated by a large burst of AE activity . When joints (construction joints in concrete or rock joints in foundation) are under loading, once the critical state of shear stress is reached, a certain part of the joint is deformed. The identification of this predisposed stage before total failure is important to reduce the danger of sudden release of deformation energy. This prevention process can be done by AE monitoring.