Study of the Engineering Geological Problems of the Havasan Dam,with Emphasis on Clay-Filled Joints in the Right Abutment

Havasan dam site is located in northwest of Iran. The planned concrete dam is to be built on Cretaceous limestone. Faulted and fractured limestone is exposed at the dam abutments and in the reservoir area. Rock mass properties including the deformation modulus and uniaxial compressive strength were calculated using different rock mass classification systems (RMR, Q, GSI and DMR). Laboratory tests indicate that joint filling materials contain clay with low to high plasticity (CL to CH) and low to medium potential swelling pressures. X-ray diffraction analysis confirms that the reason for potential swelling of joint fillings is the existence of clay minerals (such as illite and montmorillonite). The study results about the shear strength of clay-filled joints show that under JRC–JCS condition (laboratory scale), JRC _n –JCS _n (large scale) and normal stress equal to 0.25–4 MPa, the range of shear strength of clay-filled joints will be equal to 0.2–2.17 and 0.14–1.72 MPa. In some areas dissolution along the joints results in high permeability, especially in the right abutment. Three dominant joint sets occur in the exploration galleries which have been excavated in the right abutment. The maximum aperture of these joints varies from 7 to 9 cm, and the joints are typically filled with clay. Preliminary analysis shows that the presence of open joints which will cause seepage of water, combined with the impact of the clay-filled joints and forces acting on the slopes, could lead to slope failures and rock falls. In addition, the assessment of slope stability results in abutments using limited equilibrium method and Swedge software under dynamic and static conditions shows that two wedges formed on the slopes of the abutment by the natural joints are potentially unstable. The rock wedge on the left abutment is smaller but presents higher sliding potential. In addition, there is no probability of planar failure due to the geological condition of the dam abutments. This paper summarizes the site investigation and subsequent analysis, which resulted in a recommendation not to construct this site. We offer some potential mitigation plans to consider if a dam were to be built at this site.