Mud Mat as Structural Element 2

[JAX91]

My company is engineering some foundations for an industrial facility. The geotech report states the upper ten feet of existing grade is loose and gives an allowable bearing capacity of 1,500 psf. Our client hired a third party engineer to make foundation recommendations for the site. This third party engineer is not a geotech. He is just a third party. The third party engineer recommends excavating the entire 1.5 acre site to a depth of 3'-0" (frost depth). He then recommends placing a 6" thick, 1,500 psi mud slab reinforced with 6x6#10 welded wire fabric, and placing all foundations on top of it. He is claiming the mud slab will reduce settlement because it will "effectively spread out the loads by a factor of 3 to 5 compared to standard spread footings." Am I out of line thinking this idea will not work? Once the mud mat is relied upon to spread load, do you not have to then meet the minimum code requirements for a footing/mat foundation? ACI requires a minimum of 6" of concrete be placed above rebar for a foundation element. Coupled with the 3" clear cover required against earth, that means we would need at least 9" of thickness of concrete. Adding in the mesh thickness puts the required thickness closer to 10". Then we would need to provide minimum steel. In addition, does such a thin, weak mat on loose soils really even spread the load much further than the thickness of the mat? Also, would there not be access issues? I don't think you could get a pump truck to pump concrete out to the middle of the 1.5 acre site, which means you would need to drive a concrete truck out there. How do you get a concrete truck down into the pit? Can the weak mud mat handle the axle loads of a concrete truck? Some of these answers I can actual calculate, but others are opinion. I would appreciate anyone's thoughts on the subject.

 

[SlideRuleEra]

JAX91 - I agree with you. A mud mat makes a good all weather working surface, allows accurate placement of the structural foundation rebar, and prevents the structural foundation concrete from being contaminated with soil... and that's all it does (but which is certainly important). IMHO, at best the mud mat can be thought of as "really good backfill". Other than that it is what I like to call "structurally invisible". The third party engineer has unrealistic expectations.

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[Ron]

Agree with you and SRE...the mud mat is a construction convenience and does nothing to mitigate either settlement or bearing capacity. Your 3rd party engineer is not helping.

 

[BigH]

A mud mat (or blinding concrete) is never a structural element - would not count on it for any strength or load distribution. Had a client in a former French Colony in SE Asia who was upset that for a foundation excavation (which the contractor had excavated a bit too small in one corner) that we had placed the mud mat on what was open and then did a second placement for after the contractor enlarged the excavation. He got all excited about how, he, as a structural engineer and putatively a lecturer, said this was all wrong in that the blinding concrete was structural. Our structural engr and I looked at each other, shrugged our shoulders and went on our way. . . .

 

[darthsoilsguy2]

Wow... this 3rd party has a Very Unique Way of Approaching this Situation....

i've been wondering where those professional liability insurance payments have been going.

 

[msquared48]

A mud mat, or slab, is really nothing more than a means to prevent the owner's wife from getting her high-heeled shoes muddy. Nothing more.

Mike McCann, PE, SE (WA)