Soil strength in slope stability analysis

[pileexpert]

Dears,
We are going to design an anchored wall up to 36 m excavation, the soil has the following conditions:
0-10m depth clayey sand
more than 10m sandy clay
there is no water table observed.
from the top to bottom SPT-N>50 blows/30cm
direct shear test on disturbed sample shows phi=30 deg and c=10 kPa

my question is that to choose soil strength parameters in most manual I've got it tells if SPT for sand >50 then very dense condition and phi>40 deg and for clay N-value>32 it is hard condition and Cu>180 kPa.
Now Is it true to choose sand layer parameters as phi=40 deg and c=0 kPa and for clay choose phi=0 and c=180 kPa?
if so we usually consider Cu for undrained condition but since there is no water, is it still correct to choose just Cu for clay layer?
ther question is that we have clayey sand layer is that correct to just consider phi and ignore c in ther layer?
is the labaratory results correct?
Please guide me on this.
Regards,

 

[fattdad]

You should consider both short- and long-term strength conditions. These will be the same for the granular soils and different for the fine-grained soils. You have to decide how many fines draws the line.

So, what's the effective strength of the hard clay?

f-d

¡papá gordo ain’t no madre flaca!

 

[pileexpert]

hanks fattdad for your reply.
the effective strength for clay is almost the same as sand phi=28 deg and c=10 kpa but the sample was disturbed,
dont you think that c value is very small as i found in soil manual for clay with N-value more than 32 the Cu would be more than 180 kPa?
the other question is since there is no water table in the field, we sould still consider short term condition, if so why?
and could you please tell me with the soil I mentioned above what are the typical strengths for effective and total strengths?
thanks.

 

[fattdad]

There is undrained soil behavoir both above and below the water table. I am not going to spend the time to convert metric to english units dive into my references and noodle out design parameters for your project. Sorry.

f-d

¡papá gordo ain’t no madre flaca!

 

[pileexpert]

Fattdad
I am not looking for what parameter I should use in the project I want to know the reason how an undrained condition can occure above water table and the values I put here are just for clarification of my question,
anyway I change the parameter into english units and I would be very grateful if you could help me.
SPT-N value for all layer>50 for 12"
based on disturbed sample on clay and sand phi=30 deg and c=200 pound per sq foot.
as I mentioned before most textbook gives a phi>40 deg for sand with SPT>50 and Cu>3800 pound per sq foot
now could you please clarify me with the first question.
thanks.

 

[GeoPaveTraffic]

First if you an excavation that is 100 feet deep and are asking these questions, you really need someone with more experience working with you.

As far as your specific questions:
1. Undrained shear can and does occur above the water table. Clay does not have to be saturated to behave in an undrained condition. The slope should be checked for undrained behaivior of the clay.
2. There is not enough information to determine if the phi=30 degrees is a good value or not. You indicate that the sample was distrubed, but not how the test was ran. Was the sample recompacted and then sheared or just sheared?
3. There is not enough information to provide "typical" values for your conditions. Blow counts are meaningless for drained strength in clay. You would need Atterburg limits to be able to get a prelminary idea of the drained clay strength.

Good luck, I think you are going to need it.

Mike Lambert

 

[pileexpert]

Thanks GeoPavetraffic for your reply
Of course sb with more expreinced should design this system, I have jist joined a company and they have asked me to involved in this project and of course they would check my results, but I am here to learn more from you guys.
It is intersting for me that you refer to atterburg limits to check clay strength. could you please tell me how. for this project LL is between 30-35 and PI is also between 10 -14.
the lab results from direct shear test (CD) and i am not sure they have recompacted or not but i think they should otherwise the results dont make sense, do they? can we rely on the results for clay because of disturbence. since I think soil cohesion is lost due to disturbence.
my main quesion is (as many textbooks) for sand layer can we say phe is more than 40 deg since blow count is more than 50
and can we just consider phi=40 for the sand layer and put c=0 knowing that the fist layer is clayey sand not pure sand?
thanks

 

[GeoPaveTraffic]

Things are not nearly so simple as you seem to want to make them.

A clayey sand, may or may not behave is a true sand. Depends on the amount of clay. Just because the blow count is high does not always mean a high phi angle.

As for the clay, if you don't know how the direct shear test was ran, then you CANNOT use the results. Results without understanding how they were obtained have NO meaning. As for the Atterburg limits, there area several published relationships between drained phi angle and limits. Look them up. As for long term soil cohesion, while I generally include it in my analysis, unless you have experience with the material (which it appears you do not have) then I would not use long term coheision in my analysis.

Mike Lambert

 

[fattdad]

For normally-consolidated clay, the ratio of undrained shear strength to overburden pressure is often taken as 0.2. Now, do you know enough about the stress history to know whether the clay layer is normally consolidated with respect to its overburden pressure?

"Drained conditions" have no bearing on whether the stratum is above or below the water table (as Mike points out). Rather the bearing is during shear will the pore-water pressures be able to drain at the same rate as shearing. In fine-grained soils this is suspect. In coarse-grained soils it's more likely.

Please bear in mind that many clays below the water table are not even saturated. They will display undrained soil behavor, however.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

I'm just wondering if there is, in fact, a water table. It was indicated that no water table 'was observed.' - this doesn't mean it isn't there - but that in the short time frame the hole was open before backfilling, they didn't observe. I can't believe, for an excavation this deep, that no piezometer or standpipe was installed to determine the water table (if or if not present) and if present, to permit long term monitoring before excavation begins.

 

[274490]

BigH made a good point. Please check the variation of water table levels.

You should use effective shear strength for the retaining wall design.

By the way, there is no undrained shear strength for clay if it is above the water table.

 

[GeoPaveTraffic]

274490,

I disagree with your statement that "... there is no undrained shear strngth for clay if it is above the water table."

If you shear an unsaturated clay very quickly you will get a strength similar to shearing a saturated clay very quickly. Additionally, these strengths will be very different than a strength determined by shearing the same clay very slowly.

Mike Lambert

 

[274490]

For any soil sample, you should have the same effective shear strength no matter you shear the soil quickly or slowly. This has been approved by testing the same clay in CD and CU Triaxial tests.

The undrained shear strength is not a fundamental material property of soil. Undrained shear strength is only the represented shear strength of the soil under the influence of pore water pressure.

For saturated clay, the undrained shear strength is significantly different from drained shear strength because pore water pressure is not dissipated during the quick shearing test. Both effective stress circles of slow and quick tests should follow the same effective strength envelope.

If the clay is totally dry, the effective stress circles for fast and slow tests should be the same without generating any pore water pressure in the soil. In other words, the same shear strength should be obtained.

If the clay is in a partial-statured condition, you may have two shear strengths for fast and slow tests due to the influence of negative pore water pressure.