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RE: Protection of Drain Pipe Under Mat Foundation

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I would think that as long as what ever is between the bottom of the slab and the top of the pipe will compress some without attracting too much load (i.e. it is not stiffer material that the adjacent soil), then it should not be an issue.  It would mainly be an issue if you had similar compressible material (i.e. same soil) but with the "hard spot" (i.e. the pipe) under it OR if the slab sat direction on the pipe.  If the slab/mat can settle without compressing some material (say the styrofoam) such that much load transfers through the material to the pipe, then you should be good.  Thus, I could see a solution like styrofoam working.  I am not sure how you are envisioning the plywood bridge working, but that is probably me just missing it.
To me the issue is more about the pipe crushing than affecting the mat performance.  The mat should be able to easily span over a 10" "soft spot" from the pipe line if you go with the compressible material option.
Adrian, MI
-----Original Message-----
From: Bill Allen [mailto:t.w.allen(--nospam--at)]
Sent: Thursday, May 29, 2008 12:34 PM
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