Uplift resistance and mobilization of buried pipelines in unsaturated sands

Pipelines are commonly buried in depth of 1m to 2m in which the soil condition is most often partially saturated. However, the conventional design guidelines and available analytical models for predicting the uplift resistance and mobilization are based on assumption that the soil is either dry or fully saturated. In unsaturated soils, such an assumption often leads to under-estimate of uplift resistance and over-estimation of uplift mobilization. Enhanced strength and stiffness of unsaturated soils due to suction can increase the uplift resistance along with substantially different peak mobilizations as compared to tradition estimations. This can lead to under or over-estimation of remedial measures during feasibility/design stage, thus false bidding/project estimations. This paper examines the effect of partial saturation on the peak uplift resistance and mobilization of pipelines buried in sandy soils using numerical modelling. Two-dimensional finite element (FE) analyses were conducted on the basis of steel pipeline buried in unsaturated finer sand behaviour of which was modelled using modified Mohr-Coulomb model. Firstly, the numerical model was validated against the reported full scale test data, and then the analyses were extended for various pipe sizes, cover heights and degree of saturations of sands. Results showed that the peak-mobilization can be significantly different than that suggested by current guidelines, and strongly depend on the factors considered in the current study.