A Consolidated Undrained test- exam problem

[AdrianMatei]

Dear community,
The following is a short abstract; if you're time does not allow to read it all, then please proceed to reading the question
Abstract : I have recently undergone a geotechnical and foundation engineering exam, at which i performed miserably; i'm a civil engineer, who did manage to pass 2 other examns, which are more in line with my daily activities (reinforced concrete, masonry and wooden structures design/ steel and composite structures design). Thinking that mastering geotechnical engineer makes for a better civil engineer in general, i started studying for this exam aswell. The exam proved to be extremely difficult, for several reasons; where i was able to finish the 2 other exams in time and with a high grade, with this particular geotechnical engineering exam i was not even close to making it; in fact, i'm quite ashamed of the grade i got. Regardless, i'm willing to further study and take it again next year, hopefully with better results. Which brings me to the following question, given at this exam, that i still dont manage to quite understand. It reads as following:
Text of the Problem: (Note that the problem is not in english, so i will translate it as closely as i can; most of the notations in the problem are widely used in geotechnics books, but i will try to offer some context.
"You are given a geotechnical study to verify; it is necessary to provide to the designer values of the total and effective shear strength parameters for a plastic-consistent, normally consolidated sandy clay; the results will be used for the calculations required for a geotechnical design. For this end, it has been solicited to use a triaxial compression test, type CU ( consolidated- undrained), that includes a measuring of pore water pressure, on saturated samples. The following values were found in the geotechnical study, which were determined in the triaxial compression device:

Translation from the picture: proba 1= sample no.1 ; proba 2 = sample no.2; proba 3= sample no. 3;
For sample no.1 , you are given the detailed results from the test (the pore water pressure before applying the chamber pressure is 0):

Translation from picture: "deformatie specifica axiala"= specific axial strain; deviator= deviatoric stress; u = pore water pressure;
And now, here comes the question:
1. In your opinion, the results presented to you are:
a) correct, because the test was realised under the asked conditions;
b) correct, because the resulting shear strength parameters are in accordance with the state/nature of the sample;
c) incorrect, because the test was not realised under the asked conditions;
d) incorrect, because the shear strength parameterrs are not in accordance witht the state of terrain (site conditions)"
Note that any question at this exam can have 1 or 2 correct answers; none of the participants was able to provide a correct answer to this question. (Also, forgot to mention, none of the participants was able to pass this exam).
Thank you for your patience; hope i can get some insight into how to break down this problem.
Respectfully, Adrian Matei

 

[EireChch]

the first thing I would do is to plot your total stress and effective stress mohr circles, see if the data looks reasonable.

I would also check your skempton B pore pressure parameter. The answer option of 'was it under the asked conditions' leads me to think they might be asking, was it consolidate properly at the start?

Sorry I dont have much time to interrogate the data, nor can I easily check if the questions I have asked are reasonable/logical based on the data but its where I would start anyways!

 

[AdrianMatei]

Thank you for taking the time to answer.
Upon computing the total shear strength parameters i did come up with a value of the cohesion of +100 kPa, which would indicate that the value is indeed not in accordance with the expectations for a normally consolidated clay (I would assume?). This would make option d) a solid option;
The correct answers given be the committee were c) and d). So i might be halfway through figuring out what the problem is about. I will look into the value for the B parameter, which should teoretically be 1, since the clay sample was initially saturated? But say the value i will obtain isnt 1, would that mean that test wasnt carried 'under the asked conditions'?. My initial thoughts when reading option C) was that perhaps the CU test isnt a good option for determining both total and effective shear strength parameters, hence the test didnt provide the information that was asked, but i immediately dismissed the posibility since, as far as i knew, CU test, when measuring water pore pressure, does indeed allow for determining both sets of shear strength parameters.

 

[EireChch]

A normally consolidated clay can have a cu of 100kPa if its taken from deep enough. plot your mohr circles to get some better answers from others.