DOE OSTI2018
This study evaluated the influence of diameter and grain size on the initiation and continuation phase of piping erosion. Twelve tests were performed in a laboratory setting in which sand was fluidized in a controlled setting and hydraulic gradients within the sand, flow rates through the sand, and expansion of the sand columns were measured as the sand was fluidized. This study was conducted to improve the understanding of the resistance a vertical column of sand offers to piping erosion. From the fluidization tests conducted in two permeameters with different diameters, it cannot be concluded that permeameter diameter has a significant influence on a specimen’s resistance to piping erosion. Although one test conducted in the smaller diameter permeameter did record hydraulic gradients lower than those recorded in tests conducted in the larger diameter permeameter, this was not a consistent observation for all tests run in the smaller diameter permeameter. Further analysis ought to involve more tests run in each permeameter as well as incorporating additional permeameters with both larger and smaller diameters than those used in this study. From the fluidization tests conducted using the well-graded, coarse-, medium-, and fine-grained sands, it appears that resistance to piping is dependent on grain size and grain size distribution. The coarse-grained sand offered the most resistance to piping, the fine-grained sand offered the least, and the medium-grained and well-graded sands offered similar resistances between those of the coarse-grained and fine-grained sands. Gradients at which sand boils were observed, maximum gradients reached before the sand fluidized, and the minimum gradients reached at the conclusion of each test also appear to be influenced by grain size and grain size distribution. Tests conducted on the coarse-grained sand recorded the highest and most closely spaced sand boil, maximum, and minimum gradients, indicating that it offered the highest and most constant resistance to piping, while the fine-grained sand tests recorded the lowest sand boil, maximum, and minimum gradients, indicating that it offered the least resistance to piping and that resistance decreased as the tests progressed. The medium-grained sand tests recorded sand boil, maximum, and minimum gradients between those of the coarse-grained and fine-grained sands. The well-graded sand tests recorded the lowest sand boil gradient, which could be indicative of the occurrence of suffusion erosion occurring in conjunction with piping erosion, whereby the finer particles were washed out of the sand matrix at relatively low gradients, forming sand boils while the relatively coarser matrix remained resistant to piping. All tests recorded maximum gradients that were lower than the theoretical critical gradients calculated using Equation 4. This is likely due to the changes in the internal configuration of the sand as the piping process 33 initiates. In order to better understand the influence of gradation on resistance to piping, more sand gradations should be tested in future work. Measured hydraulic gradients in the lower portion of the fluidized sands were higher than those measured in higher portions. This could be explained if the density of the fluidized sand was not uniform, with more material near the base of the column than near the top, which is contrary to the findings of Sellmeijer. Furthermore, the tests were terminated when the maximum height of the upstream reservoir was reached but did not give any evidence that gradients had leveled off to a uniform “final gradient”. It is likely that gradients would have continued to dissipate had more hydraulic head been applied to the upstream reservoir. In order to better understand this, additional testing should be conducted using an upstream reservoir capable of simulating higher hydraulic heads.