Laying drain tile in stone trench under slab- @bottom or center?
In preparing to lay perforated drain tile (4″ Schedule 20 perforated pipe) in their trenches under a basement slab, there is a difference of opinion between my contractor and the illustrations in Building Science Corporation “Basement Insulation Systems” research report (2006). My contractor suggests that the filter fabric lay directly on the clay substrate, the pipe directly on top of that, and the 3/4″ stone on either side of and above the pipe. Then we’d wrap the fabric 360 degrees around. However the illustrations show the pipe in the center of the trench (in cross section), with some stone below as well as on the sides and above. My contractor reasons that water will always be standing in the first scenario, whereas in the second scenario, water in the trench will enter the pipe and drain out. I agree that his logic makes sense. What do you say?
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You are probably using schedule 36 SDR (sewer & drain) pipe - sch 20 is electrical conduit.
As long as the holes in the bottom third of the pipe are below the bottom of the basement slab, the correct installation requires a bed of washed, 3/4" crushed stone (ideally 4-6 inches), then the pipe with holes down, then 6" of the same stone on the sides and over the pipe, with the heavy duty (road-bed, not landscape) filter fabric wrapped like a burrito around the entire "filling" (which requires that it be set down first).
If the pipe is directly on the filter fabric over slay, then small particles of soil will get washed into the pipe until it clogs. With the pipe sitting up on a bed of stone, any small particles (fines) that get washed through the fabric will settle out at the bottom rather than in the pipe. The pipe's purpose is to evacuate ground water either rising or moving laterally (surface water should not be draining into the perimeter drain, but must be dealt with by gutters, downspouts and leaders and grading) before it rises to the bottom of the slab and wicks into the concrete.
Correction of typo: that's schedule 35 SDR pipe.
Thanks so much for your valued advice - huge help!!!!!!!!! :-) Anita