Back-Analysis of a Debris Slope through Numerical Methods and Field Observations of Slope Displacements: Fid-Bau Portal

Back-Analysis of a Debris Slope through Numerical Methods and Field Observations of Slope Displacements

Pandit, Koushik / Singh, Mahendra / Sharma, Swati / Sandhu, Har Amrit Singh / Sahoo, Jagdish Prasad

Back-analysis of a landslide-prone site is often felt required to estimate the physical and geo-mechanical parameters of geo-materials present in it. For this, a combination of numerical techniques like Limit Equilibrium Method (LEM) and Finite Element Method (FEM) are utilized to inversely compute the engineering properties by validating the observed field data which is often the slope displacements recorded over a period of time. This approach is suitable for realistic stability assessment, reliable prediction and competent engineering design of landslide control measures. The present case study area is located near the Tehri Dam in Tehri Garhwal District of the Uttarakhand State, India. The landslide, known as the 12.55 km slide, is situated on the right bank of Bhagirathi River, approximately 13 km away from Tehri Dam in the river downstream. The stability of this slide is important since it often blocks the connecting road between Tehri Dam and Koteshwar Dam. This road is vital for the dam operation as well as for the locality. Hence, stability assessment and long-term monitoring of this slope is significantly important. For this, at first, a typical cross section parallel to the main sliding direction is selected and profiled for the stability assessment. This prevailing slope is then analysed using probabilistic limit equilibrium method. The shear strength properties of the slope debris materials were adopted as the random variables following normal distribution for determining its probabilistic factor of safety. After it, the mobilized shear strength parameters are back-analysed for a target safety factor of 1.0 (i.e. the critically stable slope condition). Since the landslide is reported to be moving slowly along the controlling shearing plane, the shear strength of the slip surface has either come close to or reached its long-term strength. It is then followed by a finite element stress analysis of the slope based on Mohr–Coulomb yield criterion for the debris and rock mass materials. From this analysis, slope displacements at the road level are documented and the elastic parameters (i.e. elastic modulus and Poisson’s ratio) are back-calculated to validate the observed slope displacements at road level from remote sensing interpretation over the years. In this way, a parametric analysis has been performed by varying both the elastic parameters and validating the road level displacements for recent past years. Once, the elastic parameters are validated based on actual slope displacements, the corresponding mobilized shear strength parameters are also revised for the study duration. Finally, these new set of shear strength parameters were utilized to obtain the global safety level of the slope as well as the year wise road displacements. The present study prepares a base work for a calibrated long-term monitoring of the slope displacements and will be useful for slope strengthening measure designs.