Experiment and statistical assessment on piping failures in soils with different gradations

Abstract

ABSTRACTA series of upward seepage tests on sand specimens with three grain size distributions (i.e., two uniform soils and one gap-graded soil) were conducted to better understand the seepage failure of sea defense systems and coastal hydraulic structures. The objectives were to provide a detailed description of the piping progress and failure mechanism of soils subject to seepage and to investigate the influence of soil gradations on the piping failure modes. In addition, a data set consisting of a wide range of grain size distributions was compiled to assess statistically the accuracy and applicability of critical hydraulic gradient, icr, prediction methods. Experimental results indicated that the piping failure modes of sand specimens depended on soil internal stability, which was related to the grain size distribution. Piping failure of uniform sand, classified as internally stable soil, was associated with an effective stress equal to zero, whereas piping failure of gap-graded sand, classified as internally unstable soil, was caused by the internal erosion of fines. The average icr values of uniform sand are in good agreement with Terzaghi’s theoretical value. However, the average icr value of gap-graded sand is far lower than Terzaghi’s icr value. The statistical results indicated that Terzaghi’s method with Kezdi’s internal stability criterion shows the most accurate results with the smallest variance (model bias mean μ = 1.04 and coefficient of variation (COV) = 8.08%) for predicting the icr of internally stable soils. For internally unstable soils, methods based on the force equilibrium on a soil particle generally produce reasonable μ values (μ = 1.20–1.75), but the COV values are considerably high. The discrepancy between the predicted and measured icr values is discussed. The Results and Discussion in this study provide insightful information for the design, strengthening, and maintenance of coastal waterfront structures against soil piping and erosion.