MINIMUM PORE SIZE AND PERCENT OPEN AREA- SOME NEGLECTED FACTORS IN GEOTEXTILE FILTER DESIGN

The design of geotextile filters currently tends to focus on defining the maximum value of a representative geotextile pore size, often 090 or 095. This is usually based on the soil grading characteristics which might be expressed in terms of the soils average particle size d50 and coefficient of uniformity Cu. Sometimes another representative particle size such as d90 may be used, as well as the linear coefficient of uniformity. In some cases the soil density may also be taken into account.
To investigate the design criteria for geotextile filters a series of Calhoun Permeameter tests were carried out on woven and non-woven geotextiles. Thick compressible non-wovens were deliberately excluded to eliminate the effect of geotextile compression when buried in the soir.
The results enabled the establishment of suggested design criteria based on upper limit values for the 090/d90 ratio related to the soil’s uniformity coefficient. The behaviour of a few soil-geotextile combinations appeared to conflict with this simplistic approach. Further investigation revealed that two other parameters were also influencing filter formation, namely the percent open area and the minimum acceptable 090 value.
The importance of these factors can be highlighted by considering the use of a’ geomembrane. Although this theoretically meets geotextile filter design criteria based solely on an upper limit 090 it clearly would not function as a filter. Additional constraints on the permeability of the geotextile would prevent such a nonsensical design being accepted. However, the permeability constraints primarily arise from the need to prevent any build-up of pore water pressure at the geotextile interface. A more detailed consideration of the effect of the percent open area and the lower limit for 090 on the formation of soil particle bridges at the filter interface is justified. This paper presents the laboratory results and discusses these two factors. Comparisons are made between the design of geotextile filters and granular filters, and design recommendations are given.