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Abstract:
Rocky strata is the most desirable part of earth crust for site selection in most of the civil engineering projects like highways, railways, tunneling, hydropower etc. Although it provides relatively stronger, stable, safe and economically workable site during construction and implementations, they exist in uneven orientation and geological attitudes forming kinematic instability potential. On the other hand during construction it requires side cutting, drilling and blasting on the stable rock slope resulting in generation of the new cracks and mobilization of the existing discontinuity planes. This causes failure of rock slopes in different modes like plane, wedge, toppling etc. A typical case of stability analysis for three different kinematically instable plane failure areas has been analyzed here by kinematic, pseudo static and numerical modeling approaches. Techniques for restoring the natural state of equilibrium in such rocky zone that has been the headache for engineers since decades is recommended. At adverse condition when these slopes are subjected to surcharge and seismic loadings solution to rock slope stability becomes challenge for engineers. That is common problem to be faced in a mountainous country like Nepal.
Slope failures are recurrent phenomena in the Nepalese Himalayas. The study area comprises the hill slopes along road stretches at different locations of country. Three different slopes in this study area with potential planar rock slide have been undertaken for investigation and modeling. To investigate the failure process in the rock slides, the slopes are modeled as a continuum using 2D finite element plain strain approach. Shear strength reduction analysis is performed to determine the critical strength reduction factor. The computed deformations and the stress distributions, along the failure surface, have been compared with the field observations and analytical solutions and found to be in good agreement. The analysis results indicated rock slide slopes to be stable at two locations and unstable at one. The rock slide modeling could depict the exact pattern of failure involving 3 sets of discontinuities simultaneously as observed in real-field scenario which is a major limitation in case of limit equilibrium analysis. The field-observed stable slope comes to be stable through FEM analysis also.
The stability analysis of rock slopes has been a challenging task for engineers because the rock mass constituting the slope often has discontinuities in various forms, resulting in different types of slope failures. There are many literatures on this topic but they are seldom considering several parameters including surcharge and seismic loads that govern the stability of the rock slope against slope failure in field projects which cannot be omitted in design of slope stabilization measures in a rocky terrain of highly seismic vulnerable zone like Nepal. Therefore, in the present work, the analytical expression for the factor of safety of rock slope considering all possible parameters is derived, along with a discussion of its special cases in view of different practical situations.
Road networks are considered the back bones of development in our country. Most of roads alignments in hilly area are chosen to avoid rocky fracture zone causing in a longer route due to lack of confidence. However rock slope can be stabilized by different techniques like scaling, anchoring, bolting, grouting, netting etc. Rock anchoring is among best solutions but the question is whether they do maintain stability in adverse situation of earthquake. A typical case can be studied along Arniko Rajmarg at Khadichour, in Sindhipalchowk district where the upper face of the road having planar rock slide stabilized by anchoring is studied in this work. In other roads where road widening has become a problem this study will provide similar alternative solution for analysis and stabilization. This work is proposed for the case when the alignment or site is mandatory to be stabilized. The basic concept is to determine the factor of safety for typical rock slope under different situation of load like seismic, surcharge, water pressure giving due consideration to existing crack and discontinuity to decide the required anchor bolt and also for deciding the realistic situation of pre-anchored rock slope and suggest alternate protection measures. Achievement of objectives from series of modeling field verification and laboratory experiment shall be fruitful for all road hydropower, tunneling for stabilization of planar rock slope along the country. Hence it will be a spotty option when there is no other option.
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