Study of cohesive soil-granular filter interaction incorporating critical hydraulic gradient and clogging

Granular filters are used in embankment dams to prevent core materials from internal erosion, while draining seepage water to prevent saturation of the downstream embankment. Current filter design criteria based on particle size ratios are not valid for cohesive soils, which provide significant resistance to erosion due to cohesion. Relying on this, filters are designed for many dams using coarse gravels with pore sizes in the range of 5-10 mm. Performances of dams in field condition are reported satisfactory except when very large sizes gravels are used, leading to insufficient compaction of clay with gravel and the likely problem of reduced particle contact. The problem of \u27crack\u27 is identified in the impervious core. Generally this type of cracking has not been a consideration at the design stage and thus subsequent failures are detected. To mitigate this, filter gradations are reduced to pore sizes between 0.3-5 mm. The erosion of soils and performance of filters or dam structure in general all depend on the actual field condition existing in the core: intact or cracked. In the case of intact core, soil erosion is insignificant and filter gradations are much wider, however, those perform successfully. Where a crack develops in the core, the seepage pressure is large, erosion is significant (the eroded particles are in the range of 10 microns), and filter gradations are narrow. The proposed analytical models reveal that the critical hydraulic gradient is a key indicator in designing filters for cohesive dam cores. For a particular soil in an intact core, the required critical hydraulic gradient for destabilization of soil particles is significantly high (around 1000) and thus filter gradations are wider. In a cracked core, however, the seepage condition can be quite severe, the critical hydraulic gradient is comparatively small (around 100), and the required filter gradations are thus narrower. The predicted results are verified by experimental modeling and also by comparing them with the results of previously proposed models. Experimental investigations have been carried out using compacted clay with a central pinhole representing a crack. Both the analytical and experimental results show that the effective cohesion (C) in intact core and the critical shear stress (Tc) in cracked core are the key governing factors respectively in evaluating the critical hydraulic gradient. Filter properties, such as pore sizes, also have a significant influence. In addition, the critical hydraulic gradient varies with effective stresses due to the weight of overlying layers. When filter pores are small, there is potential for the filters to become clogged, since porosity and subsequent permeability are too low. Clogging is a specific characteristic of interface of base and filters where filter pores are blocked by eroded base particles. This leads to improper drainage, which is responsible for high pore pressures in the core thus promoting failure of the dam. Another aspect of this thesis is to investigate clogging in granular filters due to the flow of clay slurry generated from crack walls. According to laboratory tests, clogged filters are identified based on the gradual decrease in flowrate and permeability over 5 hours of testing period, without attaining a constant value. The experimental results indicate that the uniformity coefficient, porosity and subsequent permeability of filters are the main factors in evaluating clogging