Research on stabilization of Silvan dam spillway slopes and desing of safe slope

In this study; it is aimed to investigate the stability of the excavation slopes under static and dynamic conditions and to design safer slopes in critical sections and to determine slope support (stability enhancing) options for the construction of the spillway discharge channel of the Silvan Dam Supply Construction in Silvan district of Diyarbakır province within the scope of the GAP project of the General Directorate of State Hydraulic Works (DSİ). For this purpose, a 4-stage study was carried out consecutively as literature review, field studies, laboratory tests and slope stability analyses. In this framework, firstly; morphology, geology, structural geology and hydrogeological data of Silvan region were investigated in previous studies. In the second stage; the units in the field were examined, the soil profile, lithology, engineering geology and rock mass properties of these units and the orientation of the discontinuities were determined from the geotechnical drilling data and measurements made in the field and measurements were taken from the slits observed in the field. As a result of the geotechnical studies carried out in the investigation area, it was determined that the groundwater level not being within the expected slip planes does not constitute any obstacle in terms of slope stability. In addition to these, 6 cross sections were prepared for slope stability analysis by using the geological map created from the measurements made in the field and the thicknesses of different units, track width, track height and slope angle obtained from the existing borings and the location of the groundwater level. In the third stage, unit volume weight and shear strength parameters were determined in the laboratory on rock and soil samples taken from the site. Uniaxial compressive strength, Hoek-Brown failure criterion, Geological Strength Index (GSI) of the Midyat formation, which has the characteristics of a weak rock mass in the field, and slope stability input parameters were determined by RocData v5.0 (Rocscience, 2015) using data obtained from slope rubble and uncontrolled fill material. In the last stage of the study, the slope/slope angle of the discontinuities measured in the field was analysed kinematically using Dips v7.0 (Rocscience, 2019). As a result, it was shown that discontinuity-controlled instabilities such as planar, wedge and overturning are not expected in the section of the slopes to be formed in the Midyat formation, therefore, circular (mass) slip is more important for both this unit and the slope rubble and uncontrolled fill material. In order to evaluate the stability of the slopes under seismic conditions, the maximum horizontal ground acceleration value expected at the site was determined using the Turkey Earthquake Hazard Map Interactive Web Application (AFAD). As a result of the limit equilibrium analyses performed along six different sections representing the excavation slopes considering static and earthquake conditions, values lower than an acceptable factor of safety of 1.3 were obtained for the slope geometries envisaged in the project in all sections of the excavation. This determination necessitated the investigation of a safer slope support design for the projected slope geometries, which yielded safety coefficients of at least 1.3 in the static case and at least 1.1 in the seismic case. Considering the feasibility and economic conditions for slope bracing, soil nail, rock bolt, ibo bolt and pre-stressed cable anchor bracing were studied as alternatives. According to the analyses performed for different slope support options, it was concluded that the design of two supports or three supports together in different sections of the excavation slope area would be important in terms of creating safe slopes. The results of all analyses were evaluated together and the safe slope supports were determined according to the selected safety coefficient values.