Settlement of Cohesive Soils without Consolidation Testings 3

[GeotechVols]

Our company does basic geotechnical explorations for mostly commercial buildings and some small municipal structures. Often clients do not have the budget to afford a consolidation test to obtain consolidation settlement rates so we can estimate settlement of Clays.

Can I get some suggestions on how some of you find settlement in clays?

We have clients request estimated settlements and I'd like to give them a ballpark range. I should note, our geotechnial explorations collect only SPT N-values generally, we also provide moisture contents and atterberg limits for about 25% of the split spoons obtained, or a representative sample. Our clients typically provide foundation loads, if not we assume them based on similar projects we've done in the past. Our geology consists of moderately plastic soils, some being CH and some being CL. We don't have expansive clays. and we have the occasional shale. We work in the Tennessee, Virginia, North Carolina, Kentucky, Georgia and Alabama areas.

 

[GeoPaveTraffic]

Where to start...

First, if clients want settlement values; then they need to pay for a proper investigation. That includes borings in sufficient quantity and quality to support the required analysis. This includes undistrubed, or as near as possible, samples and proper laboratory testing.

Second, you can't provide settlement estimates without the building load and grading plan, period. If you are guessing at the loads you might as well just guess at everything.

Third, you can estimate consolidation parameters based on moisture content, liquid limit, and plastic limit. I've done it, but don't like it.

As long as engineers are willing to stick their neck out by making recommendations and predictions without real information, clients will continue to expect more and to pay less. Only us engineers performing the services are to blame. Sometimes you just have to say, if you want xxxx then we have to do additional work and that additional work costs more money.

Mike Lambert

 

[GeotechVols]

Valid point! Its one I side with all the time, but am usually overruled due to my lack of experience. The use of a consolidation test leads me to another question. If you have one consolidation test done at the approximate foundation bearing elevation, do you assume all your soils to have the same characteristics to the depth of your boring? Or is this a case where if a client had all the money in the world, you would run consols at every interval?

I apologize for the elementary questions, after a couple year out of college I'm beginning to focus in on the soil mechanics and my own intrepratations than whats just been thrown at me.

 

[GeoPaveTraffic]

Generally, more is better, but there are limits as you point out.

Remember almost all of the load is in twice the footing width, so that generally limits the area of interest.

It comes with experience, best recommendation is to think about how much load is being added to the soil at different depths.

Mike Lambert

 

[Ron]

Mike...well said!!

 

[BigH]

Consolidation testing isn't always the "cat's meow" either . . . It needs to be tempered with experience not only of a particular engineer but also overall experience in the general area in which the structures are built. Is there a history of settlement sensitivity? Most, not all, but a vast majority of geotechnical reports for, let's say, "routine" jobs are based more on experience of the engineer than on hard cold calculations. Hardly ever saw a consolidation test run for such "small" jobs - use of rule of thumb, etc.

In clays, if all I had was the SPT values, I would estimate the undrained shear strength - how does this relate to a "normally consolidated" clay with Su approx equal to 0.23pv'? Then I'd have a red-light or green-light on whether consolidation (not reconsolidation) was a factor. If not a factor, then settlements would be small and a "typical" statement would be "with an allowable bearing pressure of xx kPa, total settlement is not expected to exceed 25 mm and differential settlement would be less than 15 mm".

If you would be in the consolidation range (Cc), you would be looking at an anticipated "maximum" settlement - and any time dependency - like over 6 years, say.

Allowable bearing pressures given usually on such small jobs (and perhaps minimum footing sizes) would be such that settlements don't really come into play - unless of course, settlement is caused by fills rather than footings.

I've always remembered something that I was told a long time ago when dealing with clays - - if you actually come to within 30% of the real settlement, you've had a good day . . .

 

[coneboy]

I am not sure if a CPT contractor is available in your area?
What about the use of solid stem auger drilling for bulk sampling?

In our area of the west coast almost all of the consultants conduct solid stem augers sampling for bulk samples inconjunction with CPT testing to determine the equivalent SPT, Su etc values. In our area, typically, only the biggest and most intricate projects use mud rotary drilling and SPT testing. Would your senior staff members be interested in this approach?

The solid stem auger sampling procedure can be done very quickly (30 foot hole in clay = 30 min) and CPT testing is also very quick, compared to SPT testing. Once you become familar with examining CPT data for silts and clays you will see why it is preferred by so many (but not all). Look at the costs and see what you think.

Coneboy

 

[BigH]

coneboy: In Toronto, our drillers used to be able to put down a 20 to 25 ft solid stem auger hole with sampling at 2.5 ft intervals to 15 ft and then 5 ft thereafter in less than 45 minutes. In my view, the spoon sample obtained would be very beneficial for not much time difference.

 

[GeotechVols]

I think CPT would be very beneficial too. Unfortunately, the contractor that do CPT aren't very close and I don't know what the cost of their services is, but I know its much more expensive than a standard CME truck rig. Around Tennessee we can get $9.50 /foot with like a $350 mob. Any idea what CPT costs?

 

[GeotechVols]

BigH, well said. The more I think about it the engineers around our area generally say the same thing. The never have any calculations to back it up but just with certain loads and consistency of soils what its going to settle. I don't have my soils books in front of me, but I'll have to look more in depth to the relationship of Su approximately equal to 0.23pv', cause I actually haven't heard of it before.

 

[coneboy]

BigH
That is some great production!
Good value for your site investigation buck.
I doubt any of the drillers that I work with could match that speed.
Of course, the soils we typically deal with are soft and saturated so the solid stem auger borehole would not stay open all the way to the bottom.


Geotechvols
I don't know what the mob/demob would be, but a CPT truck usually runs at approx $300 to $350 an hour. If you assume a 70 foot CPT test in an hour on a good day then the costs might be in the $5/ft range. Of course, you can't really compare the accuracy of the two testing methods (my biased opinion).

Try getting a free example demo plot of the soil shear strenghts vs depth from a cone contractor and see if it to your liking. Fairly nice correlations exist for determining the shear strength values. Once you get more comfortable you can also model the soil OCR
values. We use the method you mentioned which involves looking at the Su/EOS ratio (.22).

Coneboy

 

[aeoliantexan]

For overconsolidated clays, the ultimate bearing capacity is approximately 6 times the undrained strength (Qu=6Su). A safety factor of 3 makes the allowable bearing pressure about 2Su. The preconsolidation pressure is roughly 4 to 5Su (see Skempton's equation). For modest loads, say 1- to 3-story commercial buildings, the settlement will be small if the bearing pressure is well below the preconsolidation pressure. So the geotech gets the undrained strength from unconfined compression tests or from correlations with N, picks a number in the lower one-third of the set, and recommends an allowable bearing pressure of about 2Su. Elastic settlement will be small, but there has to be some settlement, so the geotech says the settlement will be less than one inch, or maybe one-half inch, depending on what the building will tolerate, what the structural engineer will tolerate, and what construction quality he can expect.

A great deal of geotechnical engineering is done as described above. The trick is to identify the cases where it might not work. Say a site with compressible soils at depth and grading requires a lot of new fill load on the site. Or a site with an old gully full of poor fill. Or architectural details that won't tolerate a half inch of differential settlement.