Mud Mat

[SL26]

I have an issue with a mud mat that I can use some advice on. I have a large mass concrete slab (50'-0" x 50'-0" x 6'-0") on a project with heavy reinforcement and a very large embed frame. we placed a 6" mud mat about 5'-0" below grade and place some 6" concrete walls on top to form a containment area that would allow us to work conveniently and to seal the subgrade. The subgrade was brought to compaction and tested prior to placing the mud mat. A shrinkage crack formed in the mud mat after all the rebar and embeds were in place (about two weeks of work). During this time we had several rain days and the containment area would fill about 18" with water. Is there a concern with water damage to the subgrade under the mud mat?

 

[oldestguy]

No way to comment on this without soil data. Your question s best answered by you local geotech engineer. Any comments you get here, without us knowing the FULLsite conditions and actual loads, likely could be off target.

 

[SL26]

The soil is clay/sand (51% sand and 49% clay). Blow counts at subgrade are 15 but 6'-0" down they are only 6. Only compaction at subgrade was called for in the report.

My thoughts were if the containment pit held the water, then there might be a chance of some localized seepage at the crack but the chances of it saturating under the full mud mat would not occur.

 

[oldestguy]

With those soil conditions and the loads, expect plenty of settlement as well as some possibility of soil rupture failure. This deserves careful attention by a geotech engineer and a examine the possibility of structure distress due to non uniform settlement. The details due to the water containment are incidental and probably not nearly as important as the major stability question for this structure. Without full details don't expect from folks like me or others to be fully correct in our statements.

 

[SlideRuleEra]

We placed a 6" mud mat about 5'-0" below grade...

Doubt the mud mat was designed or intended to be a structural slab. If there was "leakage", anywhere, would the mud mat bridge over the weak spot? Take oldestguy's advice, investigate. There is too much at stake to "loose" a sizeable concrete placement because of an overly optimistic assumption that everything is ok.

BTW, a good Contractor working on a major project below grade does not "sit back" and wait for rain to stop. One or more employees should be detailed (24/7) to keep the "hole" pumped out until below grade work is complete.

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