September 4-7, 2017, Delft, the Netherlands - 25th Meeting of the European Working Group on Internal Erosion.
8
Contribution for assessing filter efficiency in zoned dams
A. Benamar
Normandie Univ, UNIHAVRE, CNRS, LOMC, 76600 Le Havre, France
S. Azirou & A. Tahakourt
LGCA, Faculté de Technologie, Université de Bejaia, Algeria
Keywords: erosion, filter, flow, plasticity, porosity.
Dam filters are mainly designed using filter criteria based on the grain size distribution. The main design criteria against which performance is assessed are a criterion for retention of fine particles. The characteristic size of finer fraction influences the size distribution of the filter pore and hence the retention capacity of flowing particles, and the permeability of the filter itself. This paper reports experimental results obtained on the soil-filter system behaviour subject to different hydraulic and geometrical conditions. The device used for erosion test in vertical flow conditions is quite similar to that described by Sherard et al. (1985) for No Erosion Filter (NEF) test. It is devoted to investigate the filtration of cohesive soils by granular filters with the presence of a crack. Many core soils and filters were used. The objective of this study was to determine the effectiveness of the filter to protect the silt submitted to erosion under controlled water flow. The hole erosion test (without filter) performed on lean clays is devoted to investigate the erodibility of the soil and also to provide the boundary condition at the filter inlet for further test including a downstream filter. The test was carried out using three successive pressures (25, 50 and 75 kPa) and the eroded mass was measured after each pressure step. The results show that overall applied pressures the eroded mass increases. A method based on porosity reduction and particle size distribution of base soil, for estimating the filter efficiency, is presented. The model should help in the design and the quality control during filter construction. Moreover, important results are developed for predicting the filter performance and capacity retention of fine particles. Particles transport and filtration through each granular filter were analysed as regards to filter retention capacity and particles size selection. The plasticity of base soil influences greatly the filtration since slightly plastic soils are more erodible than plastic soils. The analysis of hydraulic conductivity in the filter is of a great concern and leads to understand the filtration process. A comparison of the efficiency of the filters is assessed toward the usual required criteria and the most appropriate for the dam filters. Matching experimental results with filter design criteria reveals that many of them are conservative. The retention capacity is computed for many combinations of filter-base soil at each applied hydraulic load. The results indicate that the retention capacity for different combinations increases with applied pressure. As assessed previously, the eroded mass from base soil broadly increases with increasing pressure. The filter porosity variation was assessed and correlated with clogging particles volume. The evolution of such parameter may be an indicator of likely filter clogging. A new approach of filter clogging was proposed by evaluating a damage index which is affected by various parameters such as the ratio D15/d85 and the size of eroded particles. An approach linking the geometrical parameters (damage index) to the hydraulic conductivity leads to an estimation of the filter performance which provides an interesting and realistic criterion. The analysis of the base soils erodibility (without downstream filter) indicates that plasticity is an important parameter in the consideration of erodibility rate of the base soil. The grading analysis of retained particles provides more quantitative data about the particle size selection. The results show that slightly plastic base soil (CL2) produced the deposition of a large number of particles compared to the very plastic soil (CL1) which is less erodible but dispersive.
A. Benamar
Normandie Univ, UNIHAVRE, CNRS, LOMC, 76600 Le Havre, France
S. Azirou & A. Tahakourt
LGCA, Faculté de Technologie, Université de Bejaia, Algeria
