Loose Sands Problem 2

[SocalSoils]

We have done borings and lab testing on the soil for a partial reconstruction of a commercial building. The issue is the loose sands encountered at the site. Blow counts were as low as 7 blows/ft, which is a very low number that we were not expecting. Furthermore, the dry densities confirm these low blow counts by being as low as 95 pcf at 15-20 feet. The fines contents were relatively low. The odd thing is the consolidation tests show very good settlement performance (3% strain at 8x overburden) and nearly zero collapse when saturated at 3x overburden.

I acknowledge the low densities may be a result of disturbance, considering the low cohesion. But disturbance doesn't account for the low blow counts. Is there precedence for these kinds of low blow counts, low densities, but good consolidation and collapse performance? It doesn't make much sense to me.

The foundations may be mat foundations, assuming they are feasible. Also, what are your experiences with various ground improvement options given the loose sands and their relatively low fines contents? The majority of the commercial structure will remain in-place, so care must be taken to the nearby existing foundations.

Thanks

 

[Ron]

You should not be concerned about consolidation. Consolidation tests on clean sands don't tell you much! You do need to be concerned about elastic settlement and "load induced compaction". For loose sands that are that far below the surface, you can do some compactive remediation. Inserting a 24-36 inch diameter pipe attached to a vibratory pile hammer will densify these soils. Do this on a 5-foot grid. Works great!

 

[oldestguy]

I suspect you are not running relative density testing, finding the loosest and densest conditions and then seeing where you are on relative density. I would expect what you are testing is a uniform sand that has very little range between loose and dense conditions. Under that set of conditions and with a uniform material your blow counts will be low, but that does not mean you don't have a condition where settlement would be a problem at typical footing bearing pressures, such as 3,000 or 4,000 psf.

To help you resolve it, run a plate bearing test with depth setting similar to a footing, to force the test into looking at settlement only. Pro-rate the test result up to a footing size.

At times we have to throw out low blow count as meaning much (since it likely is testing shear strengh) but that settlement is what you really want to know. By the say your term "consolidation" in the test you ran applies only to squeezing out water of cohesive soil. That test however is showing you the low range between loose and dense condition. It probably is your best indication of the soil behavior.

As an examples of this condition, I (when I was working) used a 1/2" rod probe to judge the degree of compaction of fill in general before trying in-place density testing. Then, comes a uniform sand in a condition that I thought was very loose due to the rod almost dropping in. Running field density as well as the lab loose and dense tests showed the relative density was well above the usual acceptable 70 percent. One of the buildings I recall setting on that soil with footings at acceptable pressure is the Sears store at East towne, Madison, WI. No compaction needed. Still in great shape 40 years old.

Now and then we gotta examine the reason for what we might otherwise think is a bad condition and use some other criteria in evaluating it.

 

[SocalSoils]

Thanks for the response, Ron. What I meant by consolidation test was using a consolidometer to measure elastic settlement. Any thoughts as to the low blow counts (7), dry density (95) but relatively high Youngs modulus? It just doesn't jive to me from a soil fabric standpoint, but maybe this is too demanding of a question. Appreciate any response.

 

[SocalSoils]

Oldest Guy, you are correct, it is a uniform sand. That is a good idea to run emin and emax tests and find the relative density. That would also explain a lot of what my response to Ron was asking.

 

[BigH]

A couple of questions - what size footings would you envisage? What bearing pressures do you actually need? Are you in a seismic zone? What size footings did the structure use originally and what were the bearing pressures? Has the existing structure shown any signs of settlement problems?

If in a seismic zone, then you may/have to carry out some densification - but this also might impact your existing structure as you said you are doing a partial reconstruction. And then what would the authorities require about upgrading the part of the structure that you are not rehabilitating?

If settlement, up to 25 mm is not a problem, I don't see why you can't get 100 kPa (about 2000 psf) allowable bearing pressure. Would this be enough? There are quite a number of good papers/reports issued on settlement in sands (

http://ece.umass.edu/sites/default/...of-Shallow-Foundations-on-Granular-Soils.pdf)

Don't overthink the issue - do you have a problem? Yes, No, maybe - if Yes, then as the others have pointed out - some study is needed and this might take the line of more investigation or ground improvement techniques - which could include plate load tests - but you need to ensure that for a plate load test, the soils remain at least the same with depth as the zone of influence of a 300 mm dia plate is only 600 mm. If maybe, engineering judgment will be most definitely required - along with the evidence you have collected or need to collect. If no, then use of charts such as that found in the URL given or in such books as Bowles might be considered.

My view - a little conservatism in the soil conditions is good insurance.

 

[Okiryu]

Just wanted to add: what is the depth of water table? If your loose sands are saturated and if you are in a seismic area, liquefaction will need to be checked.

 

[oldestguy]

One more comment. I liken this to what would happen if you had a large box full of marbles and placed a "footing" on it. Ya probably can't do much to the box of marbles to make it any better or worse for support support. However, where you place that "footing" can make a difference.

 

[Rockersimon]

A question: Is there any organic material present?

Also, given the nature of sand (a friction material) one might consider taking advantage of the fact that all settlement will be immediate. How high is the building in question? If, say 80 %, of the load is applied during construction then the expected remaining settlement will be relatively small.

Furthermore, try a stress analysis (e.g. Boussinesq) and determine your 'depth of influence' (e.g. there the change in effective stresses is less than or equal to 20% of the insitu stresses). Depending on the influence from other foundations (existing older buildings 'counts' as insitu stresses) I would expect you'd find a small 'penetration' of the stresses and hence a small portion of underground actualle affected by changes in stress.

I've attacked an example with two footings influencing the underground.

Hope it helped solve your problem!

 

[SocalSoils]

Thank you all for your responses. To answer some of the questions: groundwater will not be an issue and there was no organic material present.

 

[Riggly]

If the driller does not maintain a hydrostatic balance inside the augers and outside the augers, a misleadingly low blow counts within the sand stratum is possible. This is especially true in fine grained sand that does not drain as readily as we assume sand to drain, and is located significantly below the groundwater level. High hydrostatic pressure outside the augers can loosen the sand at the bottom and result in false low blows. Was that case possible at your site?

 

[SocalSoils]

The low blow counts were encountered well above the groundwater table and capillary fringe. If groundwater was present, then your scenario would make a lot of sense. Thanks for the response.