Page - 1353 - in Book of Full Papers - Symposium Hydro Engineering

shear stress due to the mobilization of the sliding wedge influences the stability of the core. Figure 3 depicts the situation in the core during impounding, when shear stress τ” from the mobilization of the wedge superimposes a vertical stress state σ’1 = σ’v and σ’3 = σ’h, leading to a rotation of the principal stress with the angle of β. σ’’1 and σ’’3 are the state of the principal stresses after rotation with 𝜏 "/𝑧 , the mobilization rate of the shear strength. The diagram clearly shows that even if high shear stresses burden the core, the rotation of the principal stresses only have little impact on the mobilization rate of the shear strength 𝜏 /𝑧 . Fig. 3: Stress state in the core (σ1’, σ3’), superimposed by shear stress τ” from the mobilization of an upstream wedge 3. MEASUREMENTS Different measurements on dams with central sealing membranes [1][4] showed some indications about the existence of shear zones in the upstream dam shoulder. In order to complete the theoretical considerations in Chapter 2, measurements of the 85 m high Feistritzbach Dam and the 155 m high Finstertal Dam (core height 96 m) are presented below. The Feistritzbach Dam is situated in southern Austria and is designed with a 70 - 50 cm thick vertical asphalt concrete core, slightly inclined in the upper part. Particular tests were performed to examine the physical and mechanical properties of the asphalt concrete core material. In accordance with the design of the dam and the rockfill material properties, a softer bitumen and a less stiff mix were chosen. The tests on the asphalt concrete core material showed a soft behaviour for primary loading and a stiff response for unloading and reloading conditions. If the strain exceeded 5 to 6% (depending on the confining pressure), the material was dilative and the volume increased significantly. The failure criteria of the material was determined with φ' = 37°, c = 0 kN/m², ν = 0.25. Due to partly servere weathered rockfill materials the settlements of the dam reached about 1.0% of the total dam height after first impounding. Despite large deformations, the dam has been operated for over 28 years and the asphalt concrete core behaves quite well and no leakage through the core was observed. The dam is well instrumented with horizontal and vertical inclinometers, as well as different settlement gauges and earth pressure cells etc. The measurement 1353