Consol on very soft silts 2

[jmgray]

What are some methods to determine consolidation values on very soft, fine grained sandy silts/silty sands. The soils I'm dealing with are 0 blows/ft SPT, and even carful transportion is changing their properties.

I a google search came up with Multiple Deployment Model Pile for in-situ testing, but I was unable find enough info to understand if its able to provide in-situ consol data.

Can someone point me to information on either short term consol tests set up in the field or in-situ methods for classifing these easily disturbed samples?

 

[TDAA]

How deep/thick are the deposits? What are you looking at for loads and size of structure? What tollerance for movement do you have?

I have been in that situation before. The reason I ask the questions is that in my experience, it has been better to either go deeper, or provide another improvement vs. trying to test, and build upon the material.

 

[jmgray]

There are compressible deposits down to approx 40'. The site is planned to be developed with light weight res, so deep founds might not make economic sense.

We trying to gather data for a surcharge program. The upper 20' is a clayey silt and seems appropriate for surcharge. The soil from 20-40' is sandy silt/silty sand, we would like some consol data on this soil to determine if sand content is such that surcharge would still be effecive.

 

[BigH]

You could use pvc wick drains during your surcharge - then use extra surcharge to act as the structural loads of the residences. Build out the settlements - even wait a bit longer to be sure. Don't forget that the roads will be settlement prone too - and services.

 

[TDAA]

So, Light weight Residential. We normally assume wall loads of 1-3 KLF, an column loads up to 25K. Also, I am assuming that you had some higher blow counts near the surface, since you didn’t mention floating the rig on a barge.

With that being said, I would look to improve the soils directly below the footings, to improve the capacity, and reduce the footing size and the pressure applied to the underlying poor soils. Also, perhaps the site is more suited to crawlspace only construction (no basement).

How big are the footing, that you anticipate meaningful pressure increases to a depth of 40 feet? Also, how much of a surcharge do you have planned for light res const? Sorry that I am questioning more than answering, but based on my limited knowedge of the site, I see different alternatives. Perhaps you have ruled them out allready.

 

[jmgray]

Nearly all the soil from 0-40' had blow counts around 0-4 per foot, light truck traffic can drive on the crust without issue. This site is located in california, and it is along a levee. A typical foundation for this area and type of soils is a PT slab. A general sheet surcharge for the site is being considered to mitigate for settlement of both structures and improvements. The design grades are expected to remain close to the OG. The amount of surcharge may be something like 6' depending on, of course, the expected settlement with a given load, the time rate, and the underlying soils. We have done surcharge jobs else where in the area. Soil was recovered by a shelby and consol ran in our labs (arriving in a different condition due to the unavoidable influence of transportation).

I believe the above approach is the standard of practice for determining a surcharge program. Is there another way, however, to get consol data? It may be as simple as setting up a temp lab in a trailer at the site, or it may be a more exotic solution. So I guess my question is what cool shit have you guys seen?

 

[dgillette]

How about CPT with stops for pore-pressure dissipation? Not sure how well it works, but Peter Robertson of U of Alberta has published a method for getting coeff of consolidation. That will get you time for consolidation, but not amount of settlement. Not sure, but it may require the Hogentogler-type cone with the piezo element just behind the cone just ahead of the sleeve, rather than a Wissa-type cone with the element at the very tip of the cone.

 

[BigH]

Lest we all forget about the very rudimentary correlations by the wigenots using liquid limit and Cc? - may be enough. Also, there is the correlations of Lambe and Whitman of Compression ratio (Cc/(1+eo) given in their soil mechanics book.

 

[oldestguy]

How's about a test fill?

Many a time I have found this beats all lab testing, since it test the site. You then can make a juedgment as to how high to make it versus what timing you need. It also gives you scoop on primary and secondary effects.

 

[bigyaks]

How about a Skip Test?

Prepare a firm level area and drop a skip full of sand onto it; measure the ensuing settlements with a dumpy level.

Get the weight of sand + skip by using local weighbridge before and after delivery of the skip. If you are worried about collapse on inundation then bund the skip and flood the area and see what happens.

Buildings Research Establishment (UK BRE) guidance note 427.2 "Low Rise Buildings on Fill" gives more info.

 

[Grouser]

How about Dymnamic Compaction? "Old cranes never die...". Non-plastic silts should be a good candidate for this technique. Slightly plastic silts may also be improved.

Cheers

 

[dgillette]

I wouldn't recommend DC for silt unless it can be pretty well dewatered beforehand. The permeability is just too low. We had a project back around 1990 where it worked quite nicely in the coarser material, but when the fines got over something like 30 or 40%, things went south. This was in placer tailings that ranged from clean gravel and cobbles down to sandy silt, depending on which part of the process the material came from. Also, I've never heard of DC being successful at depths over 10 m in any material, and jmgray's site goes deeper than that.

Remember that jmgray wrote "Nearly all the soil from 0-40' had blow counts around 0-4 per foot, light truck traffic can drive on the crust without issue." With those blowcounts, you would need a thick gravel pad just to keep from losing the weight in the mud on each drop.

Old cranes might not die, but dynamic compaction can sure age them prematurely. The sudden release of the weight causes odd dynamic forces the mast isn't designed for; structural connections and sheaves get hammered. The major DC contractors mostly use dedicated DC machines instead of conventional cranes.

Anyway, this is more likely to work as a surcharge project, or possibly a ground replacement project (stone columns or some such), than as a DC project.

 

[Mccoy]

How about the (expensive) flat dilatometer?
it's considered one of the most reliable (may be THE most) in situ tests to figure out the edometric modulus. Even tough you get the horizontal component only (isotropy must be assumed).
Also, first I would make sure the blade cell is enough sensitive to your weak soils.

Also, putting together 2-3 methods wouldn't be a bad idea (CPTU, limits, flat dilatometer). And see if they converge.

If they, don't, then oldestguy's test fill appears the only way. But it sounds slow, and scale might be a problem (should be large enough to affect deeper layers, just as the real thing would do).

 

[jmgray]

Thanks for the all the replies,

Bigyak what type of equipment do you refer to as a skip?
Contractors around here refer to small clam-shell bucket loaders (around 1yd^3 max)as a skip, I have a feeling the jargon in your location may be different.

DDC was considered during our 1st site visit, but the exploration revealed much less sandy soil than was expected.

A test fill, maybe as the 1st stage of a rolling surcharge seems likely.

We have a CPT contractor in the area that has flat dilatometer- they seldom are requested to use it. I dont think anyone from my company has experience with the results and correlations. From what I gather, the dilatometer is more common in europe than in the US. Will the dilatometer give consolidation rates on low permeable soils? I would think that the value would be rough based on the short consol sampling time. -I find this is a very interesting test and would like to know more if anyone has links to info they have found useful.

 

[BigH]

jmgray - google for Dr. Paul Mayne of Georgia Tech. He has a number of articles on many of the varieties of in situ testing that can be downloaded from his site. If your site soils are "sensitive", rolling surcharge can be enhanced by remoulding your soil in situ. A major causeway in Northern Ontario was done this way by explosives.

 

[Mccoy]

jmgray,
dilatometer is used here in italy almost only in hi-budget jobs, and when some reliable measurement of consolidation is desired.
Professor Marchetti, the inventor, mantains a website with some illustrative plots and material (the following should at least in part answer to your questions, providing you're able to work out the units - which I wasn't):

http://www.marchetti-dmt.it/pageslevel1/m_comparisons.htm

you might even want to get in touch with marchetti or his collaborators:

marchetti@flashnet.it

I think they'll be eager to reply, all guys (and gals) who have been working at the dilatometer's development for 15 years or more.

I'm going to look for a third-party opinion, i'll try and see if I can come up with something.


Dilatometer use has been supported by the experimental results of Schmertmann, and more recently Paul Mayne.
I think main drawbacks are cost and sparse sampling compared to, for example, electrical CPT.