Load and resistance factor design of slopes and MSE walls
Load and Resistance Factor Design (LRFD) methods for slopes and MSE walls were developed based on probability theory. The challenges of developing LRFD for slopes and MSE walls include the representation of spatial variability of the soil parameters in slopes using Gaussian random field, which is computationally demanding, and the examination of multiple ultimate limit states for both external and internal stability checks for LRFD of MSE walls. ^ For each limit state, a rational framework is developed accounting for different levels of target probability of failure (or target reliability index) based on the consequences of failure of the structure (i.e., attainment of the limit state). The conventional equations for loads and resistance in the current MSE wall design guides are modified so that the equations more closely reproduce the ultimate limit states (ULSs) in the field with as little uncertainty as possible. The uncertainties of the parameters, the transformations and the models for each ULS equation are assessed using data from an extensive literature review. ^ For LRFD of slopes, several slope cases were considered. Each was defined by the mean and variance values of the live load and of the strength parameters and unit weight of each soil layer. Gaussian random field theory was used to generate random realizations of the slope (each realization had values of strength and unit weight that differed from the mean by a random amount). A slope stability analysis was then performed for each slope to find the most critical slip surface and the corresponding driving and resisting moments. The probability of failure was calculated by counting the number of slope realizations for which the factor of safety did not exceed 1 and dividing that number by the total number of realizations. The mean and variance of the soil parameters were adjusted, and this process repeated until the calculated probability of failure was equal to the target probability of failure. Optimum load and resistance factors were obtained using the ultimate limit state values and nominal values of driving and resisting moments. ^ For LRFD of MSE walls, the First-Order Reliability Method was successfully implemented for both external and internal limit states and a reasonable Resistance Factor (RF) value was calculated for each limit state for different levels of target reliability index. ^
Rodrigo Salgado, Purdue University.
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