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RE: Protection of Drain Pipe Under Mat Foundation

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The Styrofoam solution sounds much better than burying 2x12 lumber and plywood under the masonry water feature.
It sounds like you're designing a reflection pool or swimming pool structure with significant loads. The reason to cover the pipe with a compressible material is to keep the weight from bearing on the pipe, right? You say the pipe is 24" below the mat. You'll probably dig most of the dirt off the top of it to remove the compacted soil fill. The trench could be dug square over the top of the pipe. If it's 24" deep then 24" of Styrofoam should absorb any settlement and compaction. The rough trench could be refilled around the square Styrofoam block layers with sand or dirt.
An alternate material that's easy to work with might be loose perlight or vermiculite, which is available in large bags from a garden nursery supply or light-weight aggregate supply source. Styrofoam could be used on top in conjunction with the perlight material to keep the concrete out of the perlight. Or just cover the trench with plastic for the pour.
The wood bridge is a bad idea, in my opinion. The wood will rot and degrade in a few years. That would allow soil redistribution into the trench and possibly uneven settlement. I don't like to leave wood in the ground for several reasons, even in dry Southern California.
Dave Gaines
(626) 794-4117 home
(626) 410-3631 cell
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From: Bill Allen [mailto:t.w.allen(--nospam--at)]
Sent: Thursday, May 29, 2008 9:34 AM
To: Seaint
Subject: Protection of Drain Pipe Under Mat Foundation

Dear Colleagues:


I have a small mat foundation supporting a masonry water feature. The mat is 29 ft. x 22.50 ft. For static load cases, the maximum actual soil bearing pressure is about 700 PSF. For lateral load cases, the maximum soil pressure is approximately 1,300 PSF. The mat is 15” thick and is reinforced with #5 bars at 7” and is 4” below top of finish surface (bottom is at -19 inches). There is no geotechnical engineer on this project. This project is located in Southern California (Orange County) and was permitted under the 2001 CBC.


During construction, it was discovered that there is an existing 10 inch diameter drain line running through the area of the mat foundation. For various reasons, it is desired not to relocate the drain unless absolutely necessary. The drain line is located 24” below grade. I believe this is to the top of the pipe, but I need to verify this.


I’m thinking that I could specify a layer of Styrofoam on top of the pipe to prevent it from crushing. Alternatively, I could specify a small plywood “bridge” over the pipe. This plywood bridge would be supported by a 2x12 installed vertically on either side of the pipe. Maybe I haven’t looked at this problem long enough, but I don’t see that this will adversely affect the stresses in the mat foundation nor significantly affect the soil bearing pressures which would affect the geotechnical performance of the foundation.


Is this really not a big deal or should I do something to address the issue?




T. William (Bill) Allen, S.E.


Consulting Structural Engineers
V (949) 248-8588 F(949) 209-2509