بررسي اثر ناهمگوني مقاومت برشي زهكشي‌نشده در پايداري شيب هاي طبيعي

EFFECT OF THE HETEROGENEITY OF UNDRAINED SHEAR STRENGTH ON THE STABILITY OF NATURAL SLOPES

در اين نوشتار، مقايسه ‌يي بين روش هاي تحليلي عددي مشخصه و احتمالاتي انجام، و اثر ناهمگوني تصادفي ويژگي‌هاي خاك بر روي ضريب اطمينان پايداري شيرواني هاي رسي بررسي شده است. ضريب اطمينان پايداري شيب رسي در شرايط زهكشي‌نشده با لحاظ تغييرپذيري فضايي ويژگي هاي مقاومتي محاسبه و قابليت اعتماد به محاسبات ضريب اطمينان با تحليل هاي اعتباري بررسي شده است. با استفاده از تيوري ميدان تصادفي در تركيب با روش تفاضل محدود و در چارچوب شبيه سازي هاي مونت كارلو، توزيع آماري ضريب اطمينان پايداري شيرواني محاسبه و مشاهده شد كه ميانگين ضريب اطمينان تحليل هاي تصادفي در مقايسه با تحليل هاي مشخصه، كاهش قابل ملاحظه ‌يي دارد. نتايج به‌دست‌آمده از اين مطالعه، اهميت تحليل هاي اعتباري را مشخص ساخته است و لزوم محاسبات ضريب اطمينان پايداري شيرواني ها را در يك چارچوب احتمالاتي نشان مي دهد.

Slope stability analysis is a geotechnical engineering problem characterized by many sources of uncertainty. Some of the uncertainties are related to the variability of soil properties involved in the analysis. The influence of spatial variation on the mean of the safety factor is discussed in both deterministic and stochastic numerical analysis contexts. A numerical procedure for a probabilistic slope stability analysis, based on Monte Carlo simulation, which considers the spatial variability of soil properties, is presented to assess the influence of randomly distributed undrained shear strength. In the proposed method, the commercially available finite difference numerical code, FLAC 5.0, is merged with random field theory generated using the Cholesky decomposition technique. For each realization, the random field of cu is generated by Cholesky decomposition at the center of the element level, and the in-situ stresses in the slope are generated by applying gravity loading. This method considers the spatial variability of soil properties, and applies a strength reduction method to estimate the safety factor of a slope. Considering the case of a 5.0 m high cohesive soil slope of 54.5° (a range of coefficients of variation (COV) from 10% to 50% in cohesion values), parametric studies are performed to study the effect of stochastic soil cohesion on the statistics of the safety factor, in comparison to the deterministic solution available for the uniformly constant cohesion property. Effects of the statistical range of variability of cu on the mean of the safety factor are also presented and give an important insight into their effect on reliability analyses. The results obtained in this study are useful to understand the role of cohesion variations in slope stability analysis under different slope conditions and material properties. The coefficient of variation of undrained shear strength was proven to have a significant effect on the reliability of safety factor calculations. The observations made from this study help to explain the requirement for slope stability calculations in a probabilistic framework. It is further demonstrated that the variability of soil properties translates into a substantial reduction in the safety factor (in an average sense), compared to the corresponding deterministic (homogeneous soil) case.