70% to 85% compaction under new home

[AlabamaJohnny]

What is the best way to repair a bad soil compaction already under a structure? It appears the house was constructed without the soil being compacted correctly.

 

[Ron]

There's not a good way. You can do a chemical grouting on close spacing to stabilize the soil, but outside that, not much.

 

[msquared48]

Pin Pile or underpinning under the exterior foundation to lower the bearing stress on the soil. Underpin the interior footings.

Mike McCann
MMC Engineering

 

[AlabamaJohnny]

Does the underpinning need to go down to the bedrock?
What about High Density Polyurethane or Deep Injection (DI) Process that employs high density expanding polymers?

 

[msquared48]

Depending on the soil, underpinning does not have to go to bedrock. A 2" diameter pipe pile droven to refusal with a 90# jackhammer will yield 2 to 4 tons per pile, depending on the soil. Check with a local soils engineer in your area for the exact value to use.

Mike McCann
MMC Engineering

 

[WHEngineer]

What type of foundation do you have? Slab-on-grade or crawlspace? With a slab-on-grade, you can often full-depth pressure grout with a cement grout if you have a relatively sandy soil. Clayey soils do not perform as well with grouting. Pin piles or helical piers can also be used and are especially effective on crawlspace or like foundations with well-defined load bearing members. Many of the firms in the southeast US will use the weight of the structure to jack against with pin piles. Helical piers are screwed into the ground until they reach an adequate bearing strata. Many of the pin pile and helical pier systems have capacities of up to 100 kips, with pin piles being limited by the amount of resistance available. Due to the structural span weakness of typical residential footings, piers or piles will often have to be placed at 5 to 7 foot intervals. Local foundation companies can give you rough costs, although it is better to have a geotechnical engineer perform an exploration of the soils (often hand auger/dcp instead of the more expensive drill rig). Using a geotech can save you on the number of piers required or advise of other alternative repairs.

 

[AlabamaJohnny]

This is a house in the desert in California. It is a Slab-on-grade. The soil is a sandy soil. One cost estimate shows using 22 Push Piers down to 30 feet and Uretek Method of High pressure injection. The house is split level about 3000 sq ft with the garage under part of the house. The cost estimate also talks about taking the push piers down to bedrock. The inspectors say the house should be bulldozed. I'm not so sure this house can't be saved. The question is at what cost. If the house were finished and on a good foundation, it would only be worth about $255K today. The house is only 85% complete and on a bad foundation. The grading around the house is not complete either.

 

[MiketheEngineer]

Toss up..

 

[WHEngineer]

Any idea how deep the bad fill is? That may affect options, especially with grouting. Shallow fill on level or slight slopes may be relatively economical. Deep fills or fills on steep and/or poorly benched slopes may be more of a demo and start over project.

 

[AlabamaJohnny]

The house is on top of a ridge. The grading and compaction plan called for turning the soil down to 6 feet and compaction test every 2 feet. There is a compaction test report that was good for the final grade under the main structure, but there are no reports showing they started at 6 feet deep.

 

[BigH]

A story I could tell on this type of event . . .

 

[cvg]

with the high cost of underpinning or tearing the house down, I think I would get another geotech out to investigate and find out for sure what your foundation conditions are...

 

[WHEngineer]

I agree with cvg on getting a geotech. With shallow fill, there are several options that may be cost effective depending on bearing strength of the underlying residual soils. A local geotech should be able to apprise you of the various options.

 

[aeoliantexan]

Poorly-compacted soil can often be densified with compaction grouting. Holes are cored through the slab; a hole is augered to good bearing material, and a 2-inch pipe dropped in, or the pipe can be driven. Very low-slump grout or concrete is pumped through the pipe, expanding to form a bulb of grout surrounded by densified soil. The pipe is pulled up one or two feet and the process repeated until the slab begins to rise. This can be done on roughly 6 or 8-foot centers under the entire building if necessary.

Pin piles are usually 3-inch diameter steel tubing jacked down to refusal with a hydraulic ram reacting against the weight of the structure. They can be installed along the perimeter of the building and fastened to L-brackets that catch the bottom of the slab. They can be installed in the interior through holes cored through the slab and a small capital formed to support the slab. There are a lot of contractors that do only this type of work in Texas and Oklahoma; I'll bet there are plenty in California. The cost should be a lot less than the value of the house. Your situation is worth some study.

I agree that helical piles and small augered piles are potential alternatives.

First you need a good geotech investigation to define the problem and identify a bearing stratum.

 

[Ron]

Ditto on getting a Geotech out to evaluate.

One other consideration is the configuration of your bearing walls in the house. If only perimeter, then pin piles, helical piles, and other recommended positive support to a bearing stratum are good ideas. If you have interior bearing walls, you might be better with a soil remediation technique to maintain consistent bearing. Several have been mentioned.

How do you know that the near-surface soils are not compacted. I realize you have no test results but that doesn't mean that compaction didn't occur....possibly!

 

[oldestguy]

I have a few questions that will help all.

Has an elevation survey been done? This can be done quickly with a garden hose and clear tubes on each end and two yard sticks.

Plot up a topog drawing to use as a guide for repairs.

What do you see as to problems? Cracked slabs? Wall cracks?

Chances are in sandy soil the settlement is done with already, unless you plan to add fill against the house, further causing settlement.

If slabs are supporting things, I'd look first at compaction grouting, not mini piles. You might also look at fixing the worst parts and then patch cracks. It's surprising how much uneven floors no one will notice later.

 

[AlabamaJohnny]

Thanks for all the advice gentlemen. I looked at the structure first hand this weekend. I could not believe what I saw. The house appears to be a total loss. The slabs are cracked on both levels of the house. The garage slabs have been cut out and re-poured in several different places. The back slab off the garage under the two-story is cracked beyond repair. This appears to be a 30’ x 20’ slab. The south wall of the garage supports the two-stories. It has a concrete footer and cement blocks up to about 4 feet. I wouldn't want to try an underpinning on that wall. There are major cracks in the walls on both levels. This house is a perfect example of what happens when the soil compaction plan is not followed. Without a good foundation, any structure is doom to fail. I’ve never seen so many construction errors on one structure. This would have been a beautiful home.

 

[oldestguy]

All may not be lost. I've seen one house like this placed on an old ash dump. It continued to settle with time. It was brought back by a contractor with patience and did not have to be torn down.

Again, even with cracked concrete, that is not the end of the world. Are the walls tipping in? The question of how uneven it is elevation-wise needs to be found before giving up. Like I said, unless you are concerned that a ball rolls across the tilted floor, many won't notice some of these things, and if they do, maybe the "deal" they got on the place may make up for it.

Are the buried utilities affected? Does the waste water flow in the right direction, etc.?

Block walls can be reinforced from the inside.

Another aspect deals with the construction contract, the "designer's" responsibilities. Who was in charge? There may be some legal recourse also.

 

[DRC1]

It may not be a total loss. Houses are very light structures. It may be possible to cast piers outside the building, put needle beams under the sills, and replace the foundations. If you go down 8 feet below the sill, you should be out of the soils of concern, plus now you have a full basement. If somebody messed this up, are they on the hook for the repairs? You may want to dicuss what to do going forward with your attorney.