Experimental Study of Subsurface Erosion in Coarse-Grained Sediments: Fid-Bau Portal

Experimental Study of Subsurface Erosion in Coarse-Grained Sediments

Pachideh, Vahid / Fakher, Ali / OmidNaeeni, Seyyed Taghi

Permanent redirection of groundwater flow in urban areas is one of the issues that can be addressed in drilling operations. In this case, in the worst scenario, water can create rills by displacing and washing away some of the soil particles, weakening the soil it in terms of resistance. This phenomenon is commonly referred to as “subsurface erosion” or “internal erosion”, and soils that are expected to experience this phenomenon are referred to as “internally unstable”. Due to the fact that experimental studies are the best method for understanding internal erosion, a large-scale apparatus was constructed in this study to investigate this phenomenon. The dimensions of the apparatus are such that it can test coarse-grained soils with a maximum grain size of 30 mm. Other specifications of the apparatus include the ability to apply load, adjust the inlet head and flow rate to the sample under investigation. To develop an appropriate model for internal erosion, various trial and error methods were used to develop the testing procedure, which was then evaluated. In the developed method, internal erosion is modeled through a three-stage test with a constant head throughout the test and an increase in discharge at each stage. In this method, the installation of sandbags at different levels on the sample prevents to a large extent the flow passing through the wall areas, which is one of the common errors in internal erosion tests. Using this method, eleven tests were performed on the concave, linearly, uniformly, and gap-graded soils. The tests reveal that the soilsuffusion in the linearly and uniformly-gradedsoils is unlikely to occur in a catastrophic manner, while it is very likely to occur in the gap-graded soils. Additionally, occurrence of erosion in the uniformly-graded soils is localized compared to the gap-graded ones. The results of the tests on the gap-graded soils demonstrate that in addition to the extent of the gap intervalin the particle size distribution curve, the sieve size at the end of the sample in the apparatus is also very effective. Therefore, internal erosion will occur in a test on a gap-graded soil if two criteria are met simultaneously: (a) its gapratio is greater than 5 or 4.75; (b) the ratio of the sieve size at the end of the sample to the beginning of the gap interval is greater than 3 or 2.38. Regardless of the effect of sieve size, tests show that internal erosion in soils with a gap ratio greater than 5 is certain, less than 3 is unlikely, and between 3 and 5 is probable.