Shear Strenght Tests for unsaturated OC clays

Question

Shear Strenght Tests for unsaturated OC clays

Okiryu (Civil/Environmental)

(OP)

25 Nov 15 03:44

Hi... What is your preference for verifying shear strengths for unsaturated OC clays? I think that it will depend on the type of application, but I am asking this because I have a project where a bunch of tests are required and want to give a good and practical explanation of the tests that I will be selecting. For building shallow foundations, I do unconfined compression tests since the lab reports the elastic modulus so I can use it to calculate elastic settlements. In very few occasions where shear strength controls, I have used Su from this test (Su=c=qu/2).
For slope stability, remolded samples and CU tests may be more appropriate to calculate fully softened or residual strengths.

This is a medium size warehouse. Have not started the field investigation yet, but in my area these OC clays are unsaturated medium stiff clays with SPT N-values up to 10.

Would like to know your thoughts. Thanks.

fattdad (Geotechnical) · Answer 1 · 2015-12-03 18:02:15

generally accepted practice is to use saturated strengths in geotechnical design. Are you familiar with unsaturated soil mechanics? Do you recognize the correlations between soil water characteristics and shear strength? Is there any engineering design that will be in place to limit any possibility of saturation during the design life?

My thoughts are to correlate or test to obtain phi and c, bearing in mind that such correlations are based on saturated testing in the laboratory.

f-d

ípapß gordo ainÆt no madre flaca!

killswitchengage (Geotechnical) · Answer 2 · 2015-12-04 16:00:01

Good question, just yesterday i was reading a book untitled : Mechanics of unsaturated soils , and i can tell you its not that easy. To make it simple if you feel that the local climate is arid or semi arid ( rate of infiltration is higher than precipitation or its just the precipitation that's low around the year ) then use this soil mechanics of unsaturated state. Otherwise, its a bit tricky and i found my self quickly overwhelmed . Lastly , i should warn you about SPT data in clays its not the best approach to do .

Okiryu (Civil/Environmental) · Answer 3 · 2015-12-05 13:55:07

f-d, you have a good point. Although I have had the chance to analyze unsaturated soils for slope stability analysis, it is still a topic which I am not familiar with. Also, I do not considered unsaturated conditions for foundation design. Saturated conditions are more conservative so this approach is generally accepted, I think. The grade of saturation of my clays is about 90%. I cannot say that clays will not be saturated during the design life of the structure. I think that it may be good to check how much increase in water content is needed in order to clays become saturated. For allowable bearing pressure calculations, when I have unsaturated clays I consider elastic settlements. Of course, for saturated conditions, I use consolidation theory (1-D).

Just curious to know what is the approach of other people in this situation. Thanks !!

fattdad (Geotechnical) · Answer 4 · 2015-12-07 14:05:49

also bear in mind that you may have 90 percent saturation, but the soil will act as saturated. There are plenty of clays that are below the water table and still don't have S=100. It's also likely that a soil with 90 percent saturation will not have sufficient air to allow for easy flow of water (i.e., buildup of excess pore pressure will occur).

f-d

ípapß gordo ainÆt no madre flaca!

Okiryu (Civil/Environmental) · Answer 5 · 2015-12-07 14:19:38

f-d, thanks again. What can be the factors to consider that clays may act as saturated even having S=90%? Is any paper that you can recommend about this?

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Shear Strenght Tests for unsaturated OC clays