Height Of Slab Foundation 1

[Inspector807]

I have a foundation that is 48-64"s tall 8x8x16 block, then filled with sand clay(would it matter if it is pure sand?) that is literally just dumped in the foundation and smoothed out with small bob cat tractor(no machine tamping done) and a 4" slab poured on top, is it possible that no additional reinforcements of the walls or slab area would be required other than the moisture barrier and reinforcement wire and grade beams, this is in residential construction, my concern is the compaction and wall reinforcement, if the soil is not compacted then what good are the grade beams under the loadbearing walls, it seems that in the future the piping under the slab could also sag if they are not supported properly, this is a situation i deal with almost daily any input or ideas would be greatly appreciated

 

[Focht3]

While not recommended, this may be acceptable practice - depending on the fill used and the underlying soil conditions. The "sand clay" material is a definite "no-no." Granular fill may be acceptable. Then again, it may result in a disaster.

Where are you located?

 

[themoor]

Inspector807,

Grade beams are useful because they transfer sustained loads to the foundation (from the house to the CMU foundation walls in your case). Assuming that the grade beams are properly tied into the foundation walls & constructed, and their is no seismic influence; the absence of soil inside the foundation would not affect the performance of the grade beams. It's the soil below the grade beams that affects their performance. And you are right, the potential for sag increases with the less a fill material is compacted. In my opinion, don't worry about pipes sagging (their "loads" are neglible). Moor importantly, the slab itself will sag, causing the problems associated with cracked and uneven slabs. And just as Focht3 mentioned; an 8" wide foundation can "slice" through the soil or settle or otherwise fail, OR it can perform as intended, all depending on the underlying soil conditions.
As for wall reinforcement- as long as the 48"-64" walls act as single structures, they will undergo bending. The rigidity of the CMU may not be enough to withstand the bending, therefore vertical reinforcement should be installed. Likewise, vertical bars will help tie the house into the foundation.
Notice that my comments are from a geotechnical point of view. And even though residential construction generates relatively "light" permanent loads, there are structural concerns inherent to your scenario that I am not qualified to address.

To All:

Is it necessary to include a disclaimer in our forums???-
My comments are my opinions based on my personal experiences and interpretation. They are not recommendations or professional advice.

 

[boo1]

Some additional info is required:
Are the cells filled with mortar and reinforced?
Are you in a seismic area or high wind zone?
What code body is your area controlled by?
Is the fill moisture controled?
I assume the block wall is installed on a reinforced footer.

Ungrouted 8" wall height is limited to 48".
We use sand clay (red sand) as fill with out problems, but the sand must moist and be installed in 8-12" lifts with vibrator or tamper run in between them.
With improper compaction the grade beams (usually 12"x8" thk could become un supported) .
I have seen settling lead to cracking of the floor, plining problems and doors sticking.

With this much fill, consider a raised wood floor system it may be cheaper!

 

[boo1]

Some additional info is required:
Are the cells filled with mortar and reinforced?
Are you in a seismic area or high wind zone?
What code body is your area controlled by?
Is the fill moisture controled?
I assume the block wall is installed on a reinforced footer.

Ungrouted 8" wall height is limited to 48".
We use sand clay (red sand) as fill with out problems, but the sand must moist and be installed in 8-12" lifts with vibrator or tamper run in between them.
With improper compaction the grade beams (usually 12"x8" thk could become un supported) .
I have seen settling lead to cracking of the floor, plumbing problems and doors sticking.

With this much fill, consider a raised wood floor system it may be cheaper!

 

[Inspector807]

Thanks for all of your input, i agree with what is written, as far as a raised floor it seems builders in this area refuse to do so. as far as the earlier reply saying he wouldn't be that concerned with the piping in the slab, some runs are 20' long and if they aren't supported by compacted fill i would think the piping would sag under normal use and cause future problems with the drainage of the sanitary system, i am a building inspector and some of you may have read my posts in the past dealing with similar problems in residential construction, i have since left that jurisdiction after struggling for more than a year to resolve issues of poor construction with the building official without making any head way, i have not given up i am still looking for ways to improve conditions in the construction industry, any ideas or input is always appreciated.

 

[boo1]

In my area fills of the depth require PE design stamp and 95% or 90% modified proctor compaction tests.

 

[Focht3]

Inspector807:

I feel you pain. Your former boss needs to either be fired or retire; he is costing the public money by allowing contractors to cut those kinds of corners that have a high risk of damaging the residence in the future. Shame on him.

I am also worried about the plumbing lines. Poor fill compaction can have long term impacts if the pipe is not properly supported. You have a right to be concerned.

Where are you located? (state or province)

themoor:
I consider all my comments and advice as without warranty of any kind since I have not been retained to consult on any project discussed on this forum. I am provided with only that information that the requester deems important, and I always reserve the right to revise my remarks if I am presented with information that is new to me. Think of this forum as a conversation at a professional society meeting - I frequently tell my friends to "hire me" if they want my opinions to count...

I also frequently advise the requester to find a local geotechnical engineer (or structural, mechanical, etc.) to provide them with qualified professional advice. If they only choose to follow some of my advice, then they will have a hard time suing me!

 

[Inspector807]

was working in ga at the time but as i said found out i was fighting a losing battle from that side of the fence. i live in sc, thanks for your input

 

[boo1]

So SBC should be your governing code body.
Check foundation chapter:
Ungrouted 8" wall height is limited to 48".

Maybe a geotech firm can do an inservice to train the entire office on proper fill techniques and the reasons why!

 

[Inspector807]

I'm ahead of you there i did try to arrange just that but was told not to make WAVES, and was refused the request. I believe its was an attempt to keep the prep requirements to the same standards as the had always had to keep confrontations between himself and the builders he was trying to keep happy to a minimum,i can find no other reason for not wanting to get more education in that area beings that he knew the magnitude of problems we were having dealing with the preparations of the raised slab foundations.All it boils down to is the future homeowners are being sold substandard homes and are being left with the bill to correct these defects.

 

[RG88]

I agree with boo1 on the code reference. I have seen improper backfill in this situation result in long term problems, slab cracking, settlement, and plumbing disasters.

As a former inspector, and having been the head of an inspections division, I understand your difficulty. This will sound harsh, but my experiences in inspections and that of my peers is to leave the common sense and understanding at the door when you show up to work, inspect to the code, pure black and white, if it doesn't fall within the code, cite it for the builder to have an engineer evaluate it. I don't particularly think that this is right everytime, but it is what keeps the inspectors and builders on a level field with the liability of this 'grey' decision where it should be...on a professional's judgement. Field engineering costs are passed to the homeowner, but as unfortunate as that seems, I am certain that most will agree it is the best way to protect the public, mitigate liability, and remain politically neutral with the contracting public. (Inspecting to the code and nothing more is the hardest thing to accept as an inspector, especially when you there because you want to serve the public.)

As far as learning more, I must commend you. Keep looking for that better jurisdiction, one that will reward you for continuing education...my experience is that this is typically the jurisdiction with the reputation for being strict! Also, one where the head of inspections is a professional engineer, not just an old salt that government cannot terminate due to tenure.

One other thought, the engineer who does the evaluation will probably be willing to explain to his/her findings to you. Free training!, and who knows, such a contact may prove useful in the future!