A thesis submitted in partial fulfillment 'of the requirements for the degree of Master of Engineering, School of Engineering and Technology

Thesis (M.Eng.) - Asian Institute of Technology, 1990

Earth dam engineering had increasingly emphasized the detrimental effects of concentrated leaks developed through the cracked core, which lead to considerable erosion of fine materials, mainly clay and silt. Dispersed clay particles rarely exist due to the presence of calcium and other multivalent cations in the reservoir waters that encourage flocculation. Therefore, economical filter design should attempt to consider the retention of clay flocs rather than individual dispersed particles. In t h is respect , a filter design approach based on filter permeability and retained equivalent particle sizes was investigated . Series of filtration tests were carried out simulating a concentrated leak in the cohesive core. Slurries containing fine materials obtained from a residual lateritic s oil (passing sieve #200) were percolated through sand filters. A anhydrous salt (CaCl2 ) was added to the clay slurries to e n courage flocculation , where t h e corresponding clay floc sizes would represent typical field (reservoir) conditions. The 'equivalent' floe sizes were determined by hydrometer sedimentation tests. The granular filters tested within t he scope of this study included an array of fine to coarse grained sand , both uniform and well graded. The influence of the filter grading was particularly investigated by varying the uniformity coefficient. Another set o f a common tests included several groups of graded filters , each group having specific particle sizes , D5 or D10 or D15 etc. It was found that the permeability of the filter is a specific function of the smallest particle fraction of the filter , namely D5 or D10 . The effectiveness of the filter for a given flocculated clay suspension could be assessed by measuring the flow rat e and the turbidity of the effluent. By representing on a log scale , t h e equivalent floe sizes ( os5) versus the filter permeabilities for t h e complete series of tests, it has been possible to identify a linear boundary which distinctly separates t h e effective filters from the ineffective ones . This relationship established for these fine erodible particles may be used as a reliable design guideline for the protection of dam cores con structed with soils of similar mineralogy.