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Concrete house on loose sand. potential for settlement issues?

kgalvtxsse (Structural)
31 Jan 11 22:06
I am in the process of designing a Insulated Concrete Form house that will be built on Galveston Island Texas. The house has a ground floor/garage, a 1st floor and a 2nd floor. Total weight with concrete floors  and walls is around 1.5 million pounds over a footprint of around 2,000 ft^2.  The soil profile is a SM from ground surface to a depth of 15ft below ground surface with blow counts from 12 to 25, from 15ft to 24ft below ground surface is a SM with shell, blow counts from 5 to 10, and from 24ft to 30 ft below ground surface is a CH with 1tsf pocket pen at 24ft and 4tsf at 28 ft below ground surface.

The foundation will be slab on grade with wide and deep perimeter and interior beams or a "mat" foundation. I am not worried about a bearing failure of the sand, my concern is settlement of the loose sand/shell layer. The houses that will be built adjacent (to the left and right, possibility within 10ft) will be on piling that will be driven 10 to 15 ft below grade. I am concerned that the vibrations from the pile driving will cause settlement after the structure built.

I have looked into calculating the potential settlements but I honestly don't have much confidence in the numbers I would get given what I see as the uniqueness of this situation.

The lot required about 1000 cubic yards of fill to level up the back yard, so the fill was delivered and stockpiled on the house foundation area to provide a surcharge. This was done in December.

So my questions are..... what do the experts on this site think about this, anyone with similar past experience?  Should I drive some pilings while the surcharge is in place in order to vibrate the sand while it is loaded with something other than the house? Was surcharging the foundation a waste of time?  Should I just install sub pilings to carry the load (25ft 12 in diameter pile capacity is 4.2 tons, over 350 piles assuming no foundation support other than the piles)?

Thanks for your thoughts,
Scott  
oldestguy (Geotechnical)
1 Feb 11 13:12
Surcharge of loose sand is a good idea and may solve most of your concerns about settlement.   I've used surcharge on dumped fills for many years with success.  

In your case, if there is settlement, it likely occurs during construction so that any fixing can be done then.

 The "grid" of beams and the slab combination usually works great where settlement is likely.  Underground plumbing has to be all done first. I have recommended using this on soft river sediments and a few times on old dumps generally for apartment buildings of 3 and 4 stories. In each case no problems.  However, the buildings also were designed with joints in all masonry and conventional walls, including at every door and window.

You should take into account that even though your slab is quite rigid, it still may flex enough to cause walls to crack.  It is amazing what joints can hide as to settlement conditions.

For designing the mat or similar foundation one rule that sometimes applies is to design so that any area 10x10 might not have any support under it.  You may have to back off a little on that size area if it gets too difficult to do.

 
aeoliantexan (Geotechnical)
2 Feb 11 20:03
A rough estimate of the settlement due to compression of the sand is about 3/4 inch to 1-1/2 inch.  This is based on an old correlation of E = (8 to 15)N, with E in tons per square foot.

According to your numbers the average pressure of a mat will be about 750 psf.  If the fill was placed uniformly over the entire building area, it was probably about 8 feet thick and weighed about 800 to 900 psf.  If all the fill will be removed and the foundation built at the original grade, the fill was almost a 100% preload, a little skimpy, but possibly effective if the mat delivers a nearly uniform pressure.  I have successfully preloaded loose silty sand, increasing the modulus by roughly a factor of 5 from virgin compression to recompression.

We are left to worry about compression of the firm clay.  It may have been precompressed by the surcharge if it was in place for several months.  Is everything below 30 feet incompressible?  The house is wide enough that it cause significant compression of a normally consolidated clay well below 30 feet.

Pile driving could cause settlement of the house if it is close by.  I don't think the preloading will reduce that risk significantly.  Can your owner wait until the piles have been driven for the houses next door?

I would be concerned that 25-foot piles would concentrate loads on the soft clay and suffer non-uniform settlements.  If the very stiff clay at 30 feet is quite thick, it may be capable of supporting piles with a reasonable capacity, say 20 tons.

This project needs a good geotechnical investigation and recommendations by an engineer with plently of local experience.  If the owner won't spend the money, you should consider walking away.

 
Ron (Structural)
2 Feb 11 20:16
I agree with oldestguy...surcharge the site and move on.
kgalvtxsse (Structural)
2 Feb 11 21:10
Thanks for the replies.
A geotech report was done for the pile capacity, the borings were taken to a depth of 30 ft below surface. The 4 tsf clay layer extends very deep > 50 ft. The fill is stacked 8 to 10 ft high and around 8 ft of the surcharge fill will be removed. Assuming the fill is in the 110 pcf range, it is a clayey sand, the surcharge should be in the 800 psf range. Also forgot to mention that the water table is about 6 ft below the ground surface and the site could be completely submerged during a hurricane.

Waiting to start the house till the neighbors have built is not a option, but driving two to four pilings along the property line to vibrate the foundation while the site is surcharged is an option. My concern is that the sand layer will not find its equilibrium under the load without some form of agitation, although the Geotech feels it is not an issue, I am looking for other opinions.

Thanks,
Scott
 
aeoliantexan (Geotechnical)
4 Feb 11 16:02
I have only limited experience to apply to your question.  

Some research done a couple of decades ago showed that existing pile foundations in loose sand settled when new piles were driven nearby.  Movement occurred with each blow and continued as long as the driving continued, then stopped.  I observed the same behavior while driving H piles near existing Franki PIFs.

Based on this limited information, I would expect that driving 2 or 4 piles won't do much to reduce the effect of the subsequent pile projects next door, whatever the effect will be. That is a seat-of-the-pants guess.

Distance from the driven pile is important, of course.  I would expect the effect to reduce with the cube of the distance.  If you do drive a few piles, I suggest monitoring settlement at various distances.  You could use the results to estimate the effect of the subsequent construction.

Perhaps someone else can provide more quantitative input.
FixedEarth (Geotechnical)
5 Feb 11 10:44
Have you considered Mat foundation?  The bearing pressure may end being less than the removed surcharge stress and this will result in minor settlement.  

You many need to assume water is at Elv. 0 ft and design the Mat for uplift.
BigH (Geotechnical)
5 Feb 11 10:45
If you really have a "firm" clay - and I would think that the pp would be a bit low, I would be surprised if you don't have at least 800 psf overconsolidation - in other words, based on what you have given, I would not be that worried about the "firm" clay - as you note, 4 ft lower it isn't firm anymore - it is very stiff. With the surcharge on, as you did, I would also doubt that there would be any additional settlement in the loose sand - except for creep effects.  I would be pretty confident on what you have and have done.  As a matter of interest, though, what have the other builders in the area done in these conditions?  Is your design "new" or standard.  If standard, any problems with the other homes?
kgalvtxsse (Structural)
5 Feb 11 11:57
I think I am going to proceed forward with two or four 30' long 8" diameter piles driven into the clay layer to vibrate the sand just to be safe. 99% of the new houses in the area are stick framed built on 12"X12" piling. Most of the older stick frame houses are slab on grade and are doing fine. Any of the concrete houses that have been built in the area that I am aware of have been founded on deep driven concrete piling. This is one of the only concrete houses with a mat or grid foundation that I am aware of.  

Thanks,
Scott

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