Triaxial tests were performed on undisturbed samples from the same depth of organic clay whose pre-consolidation load, determined from consolidation tests, was in the range 90 to 160 kPa. The principal stresses at failure of two CD tests were Data from one CU test on the same clay are shown below. The effective consolidation pressure was 330 kPa and the specimen was loaded in axial compression. Test No. 1: σ3 = 200 kPa, σ1 = 704 kPa, Test No. 2: σ3 = 278 kPa, σ1 = 979 kPa Stress Difference (kPa) Strain (%) Pore Pressure (kPa)

(a) Plot the Mohr circles at failure and determine ∅' from the CD tests for the normally consolidated portion of the failure envelope.

(b) For the CU test, plot curves of principal stress difference and pore pressure versus strain.

(c) Assuming that the single CU test for which data are given is representative for CU tests run at pressures well above the pre-consolidation stress:

(a) What is ∅ in terms of total stresses above the effects of pre-consolidation?

(b) What is ∅' determined by the CU test above the effects of pre-consolidation? (After A. Casagrande.)?

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Stress Difference (kPa) Strain (%) Pore Pressure (kPa) 30 0.06 15 60 0.15 0.30 32 90 49 0.53 73 120 150 0.90 105 180 1.68 144 210 187 238 4.40 240 15.50