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hollow sounds under slab

cutawooda's picture

Just finished my house and it isnt even 7 months old. I was walking around and noticed several resonant spots that seem to have no filler dirt under them. ( I have stained concrete floors so every step is heard).  I even noticed that at and exterior wall by a toilet, it resonates. Why would this be? It has not rained in a while but i am careful to wet my perimeter with a soaker hose. Should I do anything about it....CAN you do anything about it. If there is a problem what is the solution?  I dont like this clay soil out here. I am unfamiliar with it and it terrifies me. I just moved up here from houston and the soil there was so much more,....predictable. I hear that this clay swells alot and falls alot.

(post #58860, reply #1 of 10)

Sounds like poor compaction prior to concrete placement possibly. You could drill holes and have a skilled slabjacking (mudjacking) company fill the voids, I've done that quite often, mostly with the purpose of raising sunken concrete, but It can also be done as a preventative measure.


What type of foundation do you have, is this a slab on grade? Is there proper drainage around the perimiter?


(post #58860, reply #2 of 10)

Where are you?  I lived in the Dallas area for 14 yrs, and the soil was very expansive, high in clay content.  Foundationn repiar was a big business.


You might ask the archjitect that did the plans, and see if the slab was built to be unsupported.  Could be that there were voids left under the slab to allow for expoansion.


Do it right, or do it twice.

I'm sorry, I thought you wanted it done the right way.

(post #58860, reply #3 of 10)

If you financed your house, the builder must have had compaction tests run on the dirt previous to puring any concrete, to satisfy the lender. Here you need a minimum of 90 to pass but builders prefer 95% on compaction, taken at 6" and 8".


They also test the soils to determine class, moisture and dry density. All that will tell you what you may have there.


Look at it in your files or ask the builder to get you a copy. If the tests were done and passed, you may only have some bad spots.

(post #58860, reply #4 of 10)

Since one bad spot specifically reported so far is near the toilet, I would start by having a plumber certify that there are no leaks flushing soil away.

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #58860, reply #5 of 10)

In one of my branch banks (in a former life) we started noticing doors that swung by them selves, wouldn't latch, etc.  Did some checking and discovered an irrigation line under the slab that had sprung a leak.  Washed out part of the fill under the safe deposit vault, causing the door to go badly out of alignment.  It wa the landlord's problem, and they fixed it by pumping grout under the slab to re-level it.


 


Do it right, or do it twice.

I'm sorry, I thought you wanted it done the right way.

(post #58860, reply #7 of 10)

I acted as GC. I was there for the pour and asked my concrete guy to beef up the slab with 24 inch footing and #5 rebar in the footings.Everything was done correctly. I must admit though,..I would have liked to see the fill dirt packed down with a compacter. Hind sight is 20/20.

(post #58860, reply #8 of 10)

I hate to say it, but if the fill wasn't clean granular fill AND compacted with a compacter, then IMO it wasn't done correctly. I think you've found your problem :( As mentioned previously a good slabjacking company should be able to fill the voids under your slab with minimal damage to the surface. Luckily it sounds like the re-bar you placed has saved your slab from cracking so far.


I am paranoid about compaction, with the freeze-thaw cycles we get here proper ground preparation is imperative. I'm currently preparing a slab on grade for a 30x30 garage. The first gravel fill was all compacted with a diesel roller compactor and then any topping to get the grade exact was compacted again with a plate compactor, it's cheap insurance.


(post #58860, reply #9 of 10)

"I would have liked to see the fill dirt packed down with a compacter."

I agree with sled, my eyebrows raised high at that comment!

.

Excellence is its own reward!

 

 

Oh Well,

We did the best we could...

(post #58860, reply #10 of 10)

I would have liked to see the fill dirt packed down with a compacter.  So how was the fill placed?  Just dump it out of the truck and spread it around?

Do it right, or do it twice.

I'm sorry, I thought you wanted it done the right way.

(post #58860, reply #6 of 10)

The compaction for the slab proper may have been up to par but the back fill for the waste line (from the toilet) may not have been so thoroughly compacted thus allowing more localized settlement.

Do you know what kind, if any, reinforcement was placed in the slab? Was a structural engineer involved in the design of the slab? With adequate rebar (not mesh) the slab may be able to bridge small amounts of such localized settlement, but you would, of course, be left with the hollow sounds from foot traffic.

I'm no soils engineer, but I would think that if the backfill was properly compacted and kept at a constant moisture content, there wouldn't be that much concern for expansion. But I could be wrong.

If the hollow sound from foot traffic is more than you care to endure, like others have mentioned, you can have it pressure grouted. If there wasn't adequate reinforcing in the slab, I'd suspect that is something you should definitely consider to avoid all kinds of headaches down the road.


...........

Dennis in Bellevue WA

woodnut@anatechsys.com

........... From Beautiful Skagit Co. Wa. Dennis