For retaining wall construction, there are varying suggestions online for whether filter fabric should be placed at the back of the drainage stone. This post intends to clarify when (and when not) to use filter fabric behind the drainage stone.
At the top of the drainage stone
First, there should always be filter fabric* at the top of the drainage stone. This is a must. You want to keep organic content, grass roots, and fine soil from the surface from migrating into the drainage stone. The top fabric should extend up the back of the wall several inches and attach to the wall with construction adhesive*. The fabric then extends at least 6 inches past the back of the drainage stone.
But what about the back of the drainage stone?
If you have sand or gravely soil behind the drainage stone, it is a good idea to place filter fabric at the back of the drainage stone to prevent the sand from infiltrating into the drainage stone and drain pipe*.
However, with clay or silts (i.e. fine backfill material), the filter fabric will clog, adding significantly more hydrostatic pressure to your wall. Do not place filter fabric between fine backfill and drainage stone.
Check out this video for a demonstration, and this video for further explanation.
To determine if your site has silty or clayey soil, check out this guide. If the guide gives you GM, GC, SC, SM, ML, MH, CL, or CH then do not use filter fabric behind the drainage stone, unless additional measures are taken as described below. If the guide gives you GP, GW, SP, or SW then use filter fabric behind the drainage stone.
But will this red clay stain the face of my wall?
Yes, it is true that clay can stain the face of a retaining wall.
To prevent staining of the wall face with clay soil, a sand layer is placed between the fine backfill and drainage stone. The sand acts as a filter to keep small clay particles from reaching the drainage stone. When a sand filter is used, filter fabric is placed between the drainage stone and the sand filter. Check out the cross section below.
Additional Thoughts
Never place filter fabric directly on the back of the blocks in a segmental block wall. The drainage stone should fill any gaps between the segmental blocks. This helps lock the blocks in place and add necessary weight to the wall system to prevent movement. The presence of filter fabric directly on the back of the blocks prevents drainage stone from filling these voids.
When filter fabric is necessary, use non-woven 6oz geotextile fabric*.
For additional information on retaining wall drainage, check out this post.
*Amazon Affiliate – If you use these links and make a purchase, I may be compensated.