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Re: Protection of Drain Pipe Under Mat Foundation

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Sounds like your soils allowables are close to, if not over, the limit. I just wanted to point out that a 29'x22.5' map with a 700psf soil pressure gives a weight of cmu of 29'x22.5'x700psf=456,000 lbs of masonry. That is a lot of masonry. The 1300psf under lateral seems really high too. Things to check: What is your weight of masonry? What is the vertical center of gravity of cmu? what is your allowable base shear coefficient?

On 5/29/08, Gerard Madden, SE <gmse4603(--nospam--at)> wrote:
In your mat foundation model, you may want to remove the soil supports for a certain distance on either side of the pipe run and see how the forces re-distribute in the mat per Scott's concerns. You may want to add some bars in the mat to bridge the ineffective region of soil.

Your soil pressures are low, but without the soils report, you may get nailed if you get over 1333 psf seismic.


On Thu, May 29, 2008 at 11:28 AM, Steve Gordin <sgordin(--nospam--at)> wrote:
I would suspect that a "10 inch diameter drain line" would have at least some traffic rating.  If so, such rating would easily allow soil pressures about 1,000 PSF. 
Styrofoam above the pipe will only redistribute pressures into the soil and into the mat.  As a result, the lateral pressures on the pipe may increase.  Placing Styrofoam on the sides of the pipe will increase the "span" etc.
It may be worth investigating if the drain line has traffic rating.  If it does not, it may be worth encapsulating the pipe into a continuos concrete "arch", say, 6" min. thick, no or nominal reinforcement  There is nothing to loose.   I doubt that it will require special wood "bridging" (36" thick layer of wet concrete creates pressure of only 450 PSF).  The additional cost will be close to nothing.  The pipe will be more durable, and if it would ever become damaged/clogged, it will have to be abandoned to avoid damage to the "water fixture."
V. Steve Gordin, SE
Irvine CA
----- Original Message -----
From: Bill Allen
To: Seaint
Sent: Thursday, May 29, 2008 09:33
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
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