What tests to run on a shelby tube

[hydroponder]

I am working on a site that has approximately 10 feet of fill that is loose to medium dense sand (N ~ 3-10) on top of approximately 10 feet of loose clay (N ~ 1-5) on top of some stiff clay till (N > 10). From historic photos, it appears that the site was originally a wetland that was filled in for a parking lot. We are now looking at putting up a senior living facility and a medical building. We drilled 15 borings and took shelby tubes in the two layers of clay. I am wondering what are the best tests to run on these shelby tubes? I have never collected shelby tubes and do not have a lot of experience in this area.

I spoke with the lab and they said I could run either a UU test ($160) or a CU test ($500). They also said that I would need to tell them the confining pressure for running the CU test?? They also said I could run an Unconfined Compression test ($72).

So, what tests do you recommend and what information will be given to me with the test results. Any other recommendations regarding the foundation for the buildings? Piles, geopiers, excavate??? Thanks.

 

[dgillette]

Oedometers, oedometers, and oedometers, and perhaps both UU and CU.

How many oedometers that actually is depends on the size of the site and how consistent the material is from place to place. You need them in both clay materials. If you put the buildings (presumably pretty light) on shallow foundations on the sand, I think you could probably get adequate bearing capacity from strip or spread footings, but potential for differential settlement is likely to control the design. Even if you put in some sort of deep foundation or excavate down to the stiffer clay, there is a good chance that settlement will govern your design, with bearing being secondary. N>10 is not high enough that I would write off differential settlement without testing. It would have to be N>>10.

If you think you might be excavating, you would want undrained shear strength for the materials exposed, for stability of cut slopes. I prefer UU over unconfined because it helps to reduce the effect of sample disturbance. The oedometer tests can give you a reasonable estimate of undrained shear strength using correlations with material type and preconsolidation stress. (See C.C. Ladd's SHANSEP papers, which your geotech should at least know of.) If you think you might use piles bearing in the lower clay, then you might want CU with pore pressure measurements to get the effective stress parameters for long-term (drained) strength.

Does that help you, hydroponder?

DRG

 

[hydroponder]

Yes it does. Thank you DRG.

 

[hokie66]

Driven piles.

 

[BigH]

hydropounder - clay is not loose, please, it is very soft to soft. hokie66 indicates piles - certainly this is the most obvious and most postive. However, you can look at several other ground treatment techniques - remove 1.5 m or so of the loose sand fill (assuming it is clean and not full of garbage, then recompact it using a heavy roller. You can then put in wick drains and preload the site. You might also consider stone columns, etc. Lots of toys to play with.
The real problematic soil is the 3m (oops, 10 ft) of very soft to soft clay). If you can densify the sand fill to say average N value of 20, you might be able to ensure that the zone of influence of your building foundation (presuming 2 storey or less) pressure distribution wholly in the sand. This would not "load" the clay unless you are also regrading the site and placing additional fill. Lots of variables on a site where you have shallow fill over shallow soft clay. In India, we used wick drains and built a 10 m high retaining wall on 6 m of similar clay. It worked - but we did have 1000mm of settlement!

 

[hokie66]

The problem with preloading a site such as this is that impatience often overpowers reason. The geotech gets less than he asks for in terms of time and settlement plate monitoring, then the structural engineer is expected to adopt a suitable shallow footing system, when there really isn't one. Always a gamble, and the appropriate party should take the risk, but when something happens, all involved are in the firing line. It depends a lot on the type of building; that is, how movement sensitive it is.

 

[BigH]

hokie - true but that's why I suggested wick drains - time is cut tremendously - as we had to surcharge load (in India) and couldn't "preload" as the site was too soft to put on preload - we still had something like 45 days for the 95% consolidation level

 

[oldestguy]

There is another option than piles, preloading, etc. It is use of a reinforced slab (raft) foundation, sometimes looking like a waffle upside down to provide rigidity. Add to that provisions in the building to minimize the differential settlement "effects" that may still occur with control joints at all openings and in masonry exterior walls.

I've had to use this over old dumps, peat filled river beds, etc. The main drawback seems that the utilities are either under the slab or within the "cavities" between the "T" sections of the slabs and future access to them is practically impossible. We've put 5 story apartments on these situations and no problems with depth of "T" on the order of 4 feet.

For the structural engineers we tell them to design the slab so that any area "X" x "X' has no support under it. For single story buildings we usually make "X" = 10 ft. for lack of a better guide.

 

[BigH]

Good point Oldestguy - I, too, have used rigid slabs over soft soils - saving grace here is the 10 ft of sand fill.