Technical Advisory Council for Onsite Wastewater Treatment


Use Of Geo-Fabric As A Soil-Stone Separator In Drainfields


Geosynthetic fabrics are commonly used as the separation material on top of drainfield stone to provide separation between the soil cover and the stone.  The purpose of the fabric is to minimize soil particle (i.e. sediment) movement through the stone to the infiltrative surface.  If sediment moves to the infiltrative surface, it will reduce the infiltrative capacity of the surface beyond the reduction that is caused by the development of a biomat.


It is important for the soil absorption system to “breath” or pass oxygen rich air into the stone and liberate gases that develop.  Therefore any cover material must not become clogged with sediment to the point that air cannot pass freely through.  Lighter weight fabric will be superior to heavier in this regard due to its relative inability to trap and hold sediment. 


The Technical Advisory Council (TAC) has surveyed the available materials and specifications that are in use by other states for cover separation fabric.  A well-documented specification for this purpose was not found.  There are numerous parameters used to describe geofabric products and there is currently no industry standard method of specifying cover fabric.  Based on information received from manufacturers and the experience of fabric users in Michigan and other states, the following specifications are recommended for cover fabric until more is learned (bold specifications are considered most important):


            Use Non-woven fabric

Weight – Not to exceed 2.0 oz. per square yard 

            Trapezoidal tear strength  -- 10 lbs minimum.  (ASTM D 4533-91)

            Puncture Strength  -- 8 lbs minimum  (ASTM D 4833-88)


For additional information, the state of Wisconsin Administrative code Chapter 84, Section 40, page 203  ( ) contains information on fabric for this use also. 


A current list of suppliers of filter fabric material meeting this specification is maintained on the MOWRA web site under the TAC Information section.  Users are encouraged to error on the light side if fabric of the above specifications cannot be located.  Also, users are encouraged to comment on this specification and provide additional information on sources of materials available for this application.  As more information becomes available, this specification will be revised.  To provide information on other products, contact the chair of the TAC.  For contact information, see and click TAC Information.



This Advisory was approved by the Technical Advisory Council August 31, 2005. 


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