Best Internal Drainage System/Filter for Retaining Wall Application

[LOTE]

I have perused several posts on different filter mediums for french drains. I am curious what people think for the best retaining wall drainage and filter system for varying soil types. For this discussion, I am thinking more for a mechanically stabilized earth wall, but let me know if you have thoughts on other wall types too. Let me know your comments on each of these scenarios:

Scenario 1 - clean crushed stone backfill for the reinforced fill, sandy retained fill. I don't see much need for a drain pipe or filter, other than a between the back of the reinforced zone and the retained backfill. Since the retained fill is sandy, just a geotextile fabric between the two zones seems appropriate. The wall face will need sufficient weep holes or unobstructed panel joints for water to seep out.

Scenario 2 - clean crushed stone backfill for the reinforced fill, clay retained fill. I have read that it is not a good idea to place geotextile fabric against clay as the fabric will clog. Are there alternative products? I have heard a sand filter is good to filter out clay, but if the transition between the retained and reinforced zone is vertical, a 12" vertical column of sand is not very practical to construct.

Scenario 3 - sand in the reinforced zone, clay retained fill. Here, I would think it would make sense to wrap the drain pipe in 12" of clean crushed stone and wrap that with geotextile fabric. The sand in the reinforced zone will filter out the clay before getting to the fabric and the fabric keeps any fine sand particles out of the pipe.

Scenario 4 - high fines in the reinforced zone (35% max fines per NCMA). Here is where I really get stuck. You don't want fines clogging the drain pipe, so a geotextile fabric somewhere in the system makes sense. Typically a vertical column of clean crushed rock will be placed directly behind the wall face with a drain pipe at the bottom. Would you put fabric between the clean crushed stone and fine reinforced fill? Should there be a sand filter between the fabric and fine backfill? Is a vertical column of sand really feasible to construct?

 

[MTNClimber]

Scenario 1 - I would include a drain pipe wrapped in filter fabric just for the appearance of a properly designed wall. If the wall moves due to something out of your control, the first thing someone will say is that there's no drain. The drain may not collect any water, but it's cheap enough and will defend you against that type of argument.

Scenario 2 - We would install filter fabric in that situation, but we don't work with retained saturated clays. All retained clayey soils are typically dry on our projects. If you were worried about it, a designed filter would make sense, but installing a 12" wide strip sand filter is nothing short of a pain for the contractor.

Scenario 3 - You would have to design the retained fill as a high fines reinforced fill if you genuinely thought the clay would migrate into your retained fill.

Scenario 4 - You probably want a toe drain and heel drain to keep the high fines reinforced fill dry. (see Chapter 4 of NCMA's best practices)