Wet, very loose sand 2

[Arizona3106]

I have a home site with rock on one side. On the other side is up to 23 feet of very loose, low plasticity, clayey sand, with water levels at about 4 feet. Just looking for other ideas for support, except for driven piles or conventional concrete piers. There is a little resistance in the upper foot, but N values are 2 at 2 feet depth. Below the water table, the SPT hammer sinks in a few inches prior to the test. There is zero lateral support available here, correct? Maybe a little lateral support for a grade beam?

 

[L0k]

what kind of structure do you want to build? how heavy is the structure?

 

[Arizona3106]

Sorry - By home site I mean just a higher end wood framed house.

 

[Guest262653]

Just some possible solutions off the top of my head (which will probably end up being more expensive than the ones you already ruled out):
- micropiles;
- bored concrete piles;
- jet grouting;
- soil replacement.

Or create a deep indoor swimming pool...

 

[BigH]

How about a sketch . . .?

 

[Arizona3106]

@BigH - Site plan is attached. Adjacent to the large rock outcrops, the wet loose soil is about 5 to 6 feet. The site is covered with native grasses. A couple cottonwood trees along the north side.

@avscorreia: Yea, need to add some kind of cool water feature to this place. Would slender piles be an option, given what i'm assuming is 0 lateral support? Does grout injection work under water?

 

[Guest262653]

Regarding the slender piles (micropiles), it all depends on the steel pipe diameter and thickness. If you really don't consider any lateral stiffness, you need to design it to the full unbraced length. But is it really that bad? Just a small continous lateral stiffness is usually enough to keep buckling problems at large.

As for jet grouting, it's not a standard low-pressure grout injection. It works at high pressures (typically 20 to 40 MPa), so under water works aren't really a problem. Some additional info:

http://www.skanska.co.uk/global/sevices/piling/datasheets/jet-grouting.pdf

http://www.spezialtiefbau.bilfinger.com/fileadmin/con-spezialtiefbau/verfahren/en/Jet_Grouting.pdf

http://www.bauerpileco.com/export/sites/

http://www.bauerpileco.com/documents/brochures/construction/Jet_Grouting_03-13.pdf

Nevertheless, you should check its applicability to your problem and its economic feasibility with a local contractor.

For the problem you have in hands, in my part of the world it would probably be more efficient to use micro-piles or, if the slenderness requirements were too high, regular cast-in-place RC piles (Continous Flight Auger piles may be an option but again, check with a local contractor).

 

[Arizona3106]

I do like the idea of micropiles. However, I am a little lost as to what amount of lateral pressure can be relied upon for design. I understand that if phi is 0, at least the passive resistance resistance should probably be the saturated unit weight, so maybe an equivalent fluid pressure on the order of 30pcf?

 

[Arizona3106]

oops, I meant submerged unit weight, not saturated.....

 

[cvg]

building on wet, very loose sand, 4 feet above the groundwater level - sounds like there is risk of liquifaction.

 

[oldestguy]

I question the low blow counts in saturated sand as meaning anything significant. Could well be caused by the mode of drilling and the type of "casing". Even if it is loose, with the light weight building proposed, a "floating slab" reinforced properly would seem to be one method. Another is a short term surcharge, followed by a light bearing pressure for any footings. Determine that pressure as less than the surcharge pressure, possibly assisted by an undercut and compacted fill at footings to further distribute the pressures. Cost should govern.

 

[Arizona3106]

I guess the big question I have is determining allowable lateral soil pressures in this stuff - I feel like I'm working in soup. Any thoughts?

 

[L0k]

since the ground water is quite high, a better solution is using timber piles.

 

[BigH]

if the whole of the house is on the loose sand - 5 to 6 ft deep, I might well consider taking it out. You could form a basement area - storage beneath the floor slab. if the 23 ft of loose clayey sand is under the house - a different story of course. I'd consider what OG discussed. you could preload the loose soils, let it settle, remove and build. You structure if very light - the one consideration is differential settlements as part of house would be on "hard" area and the other on a deeper "softer" material. But with a dynamic preload concept where you could use the fill for landscaping later, this might be the golden ticket! Piling for a single house would be prohibitive I would believe - if you were building 10 to 20 houses, then maybe.

 

[Arizona3106]

Thanks for the suggestions - I had considered preloading the site, but I don't feel very comfortable that it would work for this case.

I've had sites before with deep loose soils where we've run a large sheeps foot compactor on the surface for several days. Additional borings showed compaction kept getting deeper and deeper until the site was ready for development. We lowered the water table with pumping prior to compaction.

 

[oldestguy]

As an example of preloading, one site had from 6 to 30 feet of dumped concrete slabs, misc. earth and probably some rubbish snuck in. A 8 - 10 ft.high preload berm was moved across the site. That was 30 years ago. Some undercutting was done replaced with compacted fill at footings. Various commercial buildings were built, including a movie theater. All worked successfully. Of the similar filled sites I was involved with not one has had any settlement problems that I know of. In some cases settlement platforms were used, but due to rapid settlement found, they are no longer needed.

 

[Arizona3106]

OG - How did it work on the sites with shallow water?

 

[oldestguy]

Can't recall any all saturated, non organic. Most jobs with water table were granular soil (usually fill) over organic soils, such as sedimentary peat, fibrous peat, etc. where the organic soil controlled the timing. Then I didn't worry about upper the fill since it was the much less compressible and more permeable material than the deeper organics. There may have been a pond at the bottom of the site mentioned, but before I was involved. In your case, just consider it as slowly draining soil and place settlement platforms and evaluate the settlement versus log of time chart data as if it were a clay. My bet it will go darn fast to a point where the overload required time will be a shirt stay for the surcharge (days, not weeks). If in doubt, run the whole plan via a geotech familiar with surcharging so th evaluation makes sense to him/her..