Page - 1351 - in Book of Full Papers - Symposium Hydro Engineering

2. THEORY OF SHEAR ZONE DEVELOPMENT Measurements of ACE dams [1][4][5][6] have shown that the deformation behaviour of those types of dams differs from the deformation behaviour of other dam structures. Since the measurements provide only fragmentary information about the overall behaviour of the dam, theoretical considerations are necessary to supplement the data in order to obtain a fundamental understanding about the ongoing effects in the dam. For a long time there have also been presumptions about the existence of shear zones in the dam body, however, general knowledge about the development and the effect on the stability was not theoretically explored in detail. The following chapter provides theoretical considerations about typical stress paths in ACED dams and the formation of shear zones. When analysing the stress and deformation behaviour of dams, at least two significant load stages have to be considered: end of construction and first impounding. The loads acting on the core and the stress distribution in the dam body completely change when the dam is impounded which means that the dam material undergoes significant stress paths during the impounding as well as during the reservoir operation under fluctuating reservoir levels. Arising stress differences between upstream and downstream are compensated by deformations. The asphalt concrete core acts as a thin impervious and flexible membrane and transfers the horizontal water load to the supporting downstream dam shoulder. Since the core provides limited resistance to the applied horizontal loads, the AC tries to reach an equilibrium state, which can only be achieved by a horizontal deformation in order to mobilize the resistance of the downstream dam shoulders. The magnitude of horizontal deformation depends on the stiffness of the adjacent zones and on the shear strength of the shoulder material. As a result of the horizontal core deformation towards downstream, the horizontal stress in the upstream dam zone adjacent to the core significantly decreases. For the upstream dam shoulders the stress paths can easily be visualised by a σ – τ diagram as depicted in Figure 1. For the end of construction, the stress state which is depicted by Mohr's circle is far off the Mohr-Coulomb failure line. Due to impounding and the buoyancy of the upstream shoulder material, σ1 and σ3 are reduced, whereby Mohr's circle is shifted, but does not reach the Mohr-Coulomb failure line. The horizontal deformation of the asphalt concrete core to the downstream side reduces the minor principal stress σ3, K’0 = σ3/σ1 decreases and the diameter of the Mohr circle increases until it reaches the Mohr-Coulomb failure line, activating a sliding wedge. The inclination of the sliding plane ψ and therefore the size of the mobilized wedge depends on the shear strength of the dam shoulder material and can be determined from the σ – τ diagram by Equation 1. 𝜓 = 45 + 𝜑 /2 [1] The mobilized wedge closes the rising joint between the downstream dam shoulder and the deformed asphalt concrete core. The mobilization of a shear zone reduces the shear stress in the surrounded area and protects those zones against shear deformations. 1351