STUMPS UNDER-NEAR SLAB

[LA21640]

POURING A MONOLITHIC SLAB NEAR(10') AN EXISTING HUGE PECAN TREE RECENTLY CUT DOWN--WILL ROTTING ROOTS CAUSE VOIDS UNDER SLAB--DAMAGE SLAB OR WILL SLAB BE STRONG ENOUGH TO IGNORE

 

[oldestguy]

It all depends on what the slab is used for. It is unlikely to have a noticeable affect.

I've seen some pretty crummy building sites containing a lot of logs, stumps, etc. I suspect the rotting leaves some "loose soil", but arching around these weak places seems to occur and the building owner is not affected. I've never found a settled building or similar problem caused by buried logs or stumps. A sawdust pile is another matter.

Personnaly I built a garage over some large stumps and figure the slab has enough strength to bridge these weaker areas.

Doing a grubbing job to take out the stump and roots is more likely to leave large loose zones than if you didn't try it.

 

[cvg]

taking the opposite stance than oldestguy, it is much easier to clear and grub, fill and recompact now than it is to tear out the finished structure later and reconstruct the slab. That's why most engineers specify no roots or other organic material in the subgrade or in the fill. Do you like to gamble?...

 

[oldestguy]

Let me add one thing. I assume are using a stiff concrete mix that is "strong", say at least 3,000 psi. Also, the slabs I work with are reinforced with steel re-bar, not the "chicken wire" roll out stuff that usually ends up on the bottom of the slab.

I once found an owner (Sears) would not allow compacted fill to be placed on a natural ground situation where there were scattered roots from grass. After stripping topsoil, it took quite a bit of convincing the owner's engineer that he had nothing to fear with these scattered fine roots. So, you can take this to the extreme if cost means nothing. Of course no problem for that store done some 40 years ago.

On another job a house corner settled. The contractor had removed a boulder from that area, leaving things loose. Of course the right thing would be to excavate all that and fix it up, but not every contractor is that concientious.

I find that generally owners will take the more economical approach when there is no clear cut reason for spending money.

 

[bothunter]

Vegetation in the soil is to be avoided. Have you contacted a nursery and asked about pecan trees, especially about the size of the root ball?

 

[bb29510]

a problem with roots sytem is, that it will wick water up from a deeper elevation.