Filter testing of broadly graded cohesionless tills

Abstract

Filter criteria have generally been checked experimentally with relatively uniform bases (protected soils). However, these criteria may not be applicable when nonuniform soils are used, especially broadly graded cohesionless tills with nonlinear grain size curves. Such soils were extensively used at the James Bay project as impervious core material, and a testing programme was undertaken to establish the margin of safety associated with the application of current filter criteria: the design was based on the arbitrarily defined portion of the base soil grain size curve. Also, a more general objective of the programme was to assess the role that self-filtration plays in the filtration process of such broadly graded cohesionless soils.This paper presents and discusses the factors that have been found relevant in the development of a representative laboratory simulation of the complex filtration phenomenon; namely, maximum particle size, density, saturation of the samples, direction of flow, intensity of the gradients, and development of hydraulic fracturing. Some preliminary test results are given for a till from the LG-3 dam that showed a marked bend in the grain size curve at [Formula: see text]. It is seen that particles coarser than this size do not significantly influence the self-filtration process. Finally, the test results demonstrate that the filters specified for the LG-3 project are more than adequate. Key words: earth dam, filtration, broadly graded cohesionless soils, internal erosion, laboratory tests, design.