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Selecting shear strength data for semi-compacted clays

Selecting shear strength data for semi-compacted clays

Selecting shear strength data for semi-compacted clays

I have a geotechncial investigation report for a berm construction project in which the following is included:

*boring logs w/ blow counts
*index testing
* Unconfined compressive tests on in-situ foundation density
*standard and modified compaction
*hydraulic conductivity tests on in-situ foundation material
*in situ density tests

Note: On-site clay will be used to construct the berms.  Since the berms are low-consequence, and will not be subjected to a hydraulic head, we are looking at reduced compaction efforts at the insitu moisture content.  In-situ water content ranges from 9 - 19%, and the optimum for standard compaction is ~10 - 11%.

Here's my question: I have decent data to establish the shear strength of the foundation, but no triaxials done on the compacted clay.  In-situ clay is described as stiff to med stiff.  Is it a decent assumption to use the same cohesion as the in-situ foundation material?
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RE: Selecting shear strength data for semi-compacted clays

no.  If you have no control on compaction moisture content and you have no long-term strength parameters you can't really perform an engineering assessment.

Just how plastic are these soils?  I'm guessing not too plastic 'cause the Wopt is pretty low for clay.

What berm slopes are you considering?

Can you provide LL, PI and percent sand content?


¡papá gordo ain't no madre flaca!

RE: Selecting shear strength data for semi-compacted clays

The soils are a lean CL, with LL ~34, and PI ~21.  Sand content varies, but is typically 16 - 20%.

Slopes are very flat for most of the project (7H:1V), but there are a few stretches of 2H:1V.  I need to let the designer know if these need to be flattened.  I have seen some references to "semi compacted" dikes in which in-situ moisture content can be used for flatter slopes.  However, I'm not sure on how to determine how flat the slopes have to be before you can ignore water content.


RE: Selecting shear strength data for semi-compacted clays


First, there is no simple answer to what you are asking.  Additional required information includes the following:
how high are the berms?
what is the purpose of the berms?
what is the strength of the foundation soils?

Your question focuses on undrained strength, but drained strength may control.

As for "semi compacted", never heard of it.  You either compact it or you don't.  However, there are many different degrees of compaction.

Figure out what you are really trying to do, then post again.

Mike Lambert

RE: Selecting shear strength data for semi-compacted clays

The berms are a max height of 8', and are being used to contain dry river sediment.

 The semicompacted reference comes from a US Army Corps of Engineers manual for construction of containment facilities for dredged material.  Table 6-9 in Chapter 6 states that semicompacted dikes can be constructed at natural water content, but require flatter slopes.  The semi-compacted refers to a reduced 15-blow test and the allowance of compaction with a crawler tractor or hauling equipment.


Foundation materials are a med stiff to stiff clay, c~1 - 1.25 tsf.

GeopaveTraffic, I don't imagine you would ever employ this method if you are in the field of transportation/ road construction.  However, the only consequences of a slope failure on this site is increasing maintenance costs for the site.     

RE: Selecting shear strength data for semi-compacted clays


The best advice is to get some of the material, compact it to various densities, and run strength tests on it.  if you don't want or can't do that, then you can only use your experience with similar soils to determine how steep the slopes can be constructed.  

Mike Lambert

RE: Selecting shear strength data for semi-compacted clays

w-opt = 10-11%
w = 9-19%
Not a good combination.

If this material was mostly near or dry of optimum, I might have suggested that for an 8' high embankment, at 2:1, with no reservoir behind it, and low consequences if there is a slide or settlement, you ought not to worry too much about testing.

However, this stuff looks like it's way wet of optimum, and could be difficult to compact or even run equipment on unless it is dried out by plowing, blending, sunshine, etc.  If it's wet of optimum, there is a physical limit to how dense it can be made by compaction.

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