Relationship between Su, qu anc c
Relationship between Su, qu anc c
(OP)
What is the relationship between he undrained shear strengh(Su) and the unconfined compressive strength(qu) of a soft clay? AASHTO gives simplified pressure distributions with values based on the undrained shear strength for cohesive soils. My boring data gives 0.7 tsf for qu and SPT N values ranging fom 2-4. I have been told that one can assume that Su=0.5qu, but this is also the value for cohesion (c). Any help is appreciated.





RE: Relationship between Su, qu anc c
Su = c
- but not always. You have to look at the soil type, stress history, type of load, load application and duration, soil creep, etc.
Why are you dealing with this very geotechnical engineering issue, and not the project geotechnical engineer?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Relationship between Su, qu anc c
Check on the N values - check the other properties (Atterberg limits and the natural moisture content compared to the LL and PL). Also, discuss this with your geotechnical engineer. Focht3 is right - this is the "job" for the geotechnical engineer. What AASHTO chart/table are you looking at for pressure distribution vs Su?????
RE: Relationship between Su, qu anc c
Worst than that, SPT, although a result of the soil strength is a function of so many parameters that it really shouldn't be correlated with anything except relative density of sands ( not gravels, not silt, not clay).
To answer your first question, the undrained shear strength is, by theory and practice, equal to one half of the unconfined compressive strength under undrained conditions. This relates back to the Mohr-Coulomb failre surface so it is not an emperical correlation.
As far as correlating any strength conditions of clayey soils to the SPT I suggest that you don't take this approach. The vane is a simple test that gives much more reliable information.
I assume that the qu you refer to is from a pocket penetrometer or did you conduct a test on an undisturbed sample?
RE: Relationship between Su, qu anc c
RE: Relationship between Su, qu anc c
While this represents a more desirable circumstance (by cleaning up our nomenclature), it isn't true - yet. We still have many remnants of the use of "c" in design procedures - for example,
fs = α·c
for axial pile capacity analyses. Total stress, not effective stress, concepts still dominate this area of geotechnical design. And ½·qu is still referred to as 'c' in many design offices.
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Relationship between Su, qu anc c
Shear strength, by definition, is the effective cohesion (c') plus the effective stress times the tan of the effective angle of internal friction. (Bishop) Our testing should then be aimed at obtaining c' and the effective angle of internal friction.
The unconfined compression test assumes that the internal friction angle is zero, which is generally not correct and thus our undrained shear strength is 1/2 the deviator stress which is not entirely accurate either.
I agree with jdmm on this one. An accurate laboratory vane, this is not a field vane or hand vane but a stationary laboratory, bench mounted lab vane apparatus that can measure the shear strength on shelby tubes samples will generally give better results, especially when testing clays and clay tills. The beauty of the laboratory vane is one can get several tests throughout the length of one shelby tube sample. For example take one lab vane test, push out a little sample, take another lab vane test and so on. Averaging the results will give a far more reliable result that an unconfined, especially if the till is quite rocky.
RE: Relationship between Su, qu anc c
I'd love to run CU's with pore pressure measurements all the time (ah, the fees!) but it won't happen. CPT and DMT won't work, and SBP is limited to the upper 15 feet (~5 meters) of soil in many areas. (G increases rapidly with depth around here.) Our depth to constant soil moisture is 15 to 20 feet, sometimes more.
The unconfined compression test is here to stay, at least until we have a better-cheaper-faster alternative -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Relationship between Su, qu anc c
RE: Relationship between Su, qu anc c
RE: Relationship between Su, qu anc c
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"
RE: Relationship between Su, qu anc c