Geocomposites are often used as curtain drains in earth structures to dissipate seepage-related pore water pressures as a means of increasing stability. In this application, geocomposites function as both filters as well as in-plane drains. One issue that should be considered in the design of curtain drains constructed with geocomposites is a possible reduction in transmissivity due to penetration of soil particles into the filter. During initial stabilization of flow across a soil-filter interface, a stable filter cake will form in the vicinity of the soil-geocomposite interface. Although this process has been evaluated for soil filters, it has not been studied in detail for geosynthetic filters, which are significantly smaller in thickness.

The depth of penetration of particles into geosynthetic filters, and the corresponding change in lateral transmissivity is evaluated in this study using both theoretical and experimental approaches. The theoretical approach is used to estimate the depth of penetration based on the grain size distribution of the protected soil and pore size distribution of the geotextile filter. The experimental approach involves the use of a new permeameter which can be used to measure both the cross-plane permittivity and the in-plane transmissivity of the soil-geosynthetic interface in a single test. This permeameter includes a gradient ratio setup, which permits measurement of the hydraulic gradient within the soil as well as across the soil-geosynthetic interface. The permeameter was designed as a flexible-wall permeameter, in order to control the effective stress and use back-pressure saturation.

Phosphogypsum was used in this study to evaluate the depth of penetration of soil particles into geosynthetic drainage layers. It was selected as its wide particle size distribution, low plasticity, and high solubility make it prone to filtration failure due to piping. In addition, it may also cause problems in drainage systems due to precipitation of suspended phosphogypsum particles within the geocomposite drainage core.

A theoretical model was developed to estimate the depth of penetration of soil particles during infiltration. Monte Carlo simulation was used to evaluate the depth at which different size soil particles would become lodged within a continuous flow pathway through the geotextile filter. The protected soil grain size and geotextile pore size distributions were used in this analysis. When verified with the results from the experimental component of this study, this model can be used to assess other soil-geosynthetic combinations.

Two experiments were conducted in this study using the combined permittivity-transmissivity test. The first test involves evaluation of the in-plane and cross-plane flow behavior of a geocomposite (consisting of geotextiles bonded to either side of a geonet drainage core). The results from this test are expected to be useful to assess the behavior of geocomposite curtain drains used in phosphogypsum waste stacks. The second test involves evaluation of the in-plane and cross-plane flow behavior of different layers of geotextile. The results from this test are expected to be useful to assess the depth of particle penetration within a geotextile filter.

The theoretical model and experimental testing approach presented in this study were found to be useful tools that can be used in practice to evaluate filtration criteria and clogging of geotextiles used with problematic soils.