1. Undrained Strength CIVE 640

2. So far… 3 test types: 1. CD 2. CU 3. UU 3 steps for each step 1. soil specimen obtained 2. consolidation phase (und. or dr.) 3. shearing phase (end = failure)

3. CU – Weald clay Board – Figure 28.6 1. undrained = no vol change 2. oc clay is dilative 3. no peak is observed 4. for dilative clay, u e during shear = (-). Thus, the effective stress inc during shear 5. nc clay is contractive (u e during shear = (+)

4. CU – Weald clay – eff stress paths Board – Figure 28.8 a 1. ESP is curved (backwards) for NC clay 2. As OCR approaches 1, backwards curvature increases 3. OCR may be such that u e @ failure = 0 4. A highlights the difference between ESP and TSP

5. ESP The ESP = fn (OCR and p’ - e conditions before shear, i.e., p’ o and e o )  fn (loading type)

6. Drained vs. Undrained Board – Figure 28.8 a 1. q f - p’ f relationship (envelope in MIT space) is independent on drainage conditions 2. Indeed, there is a q f - p’ f - e relationship for a clay with a given OCR, independent on drainage conditions

7. Undrained Shear Strength Undrained Strength q f = fn (effective stress) q f = fn (p’ o, A f,  ’, and c’) and is also a fn of drainage conditions but  fn (loading type) fn (OCR)

8. Undrained Shear Strength Board – Table 28.1

9. “  = 0” Consider a clay with p’ o = 16 psi 3 UU tests (no additional cons. allowed in step 2 ): 1. p o =  3 = 10 psi 2. p o =  3 = 30 psi 3. p o =  3 = 100 psi q f for all = 5 psi Why? Confinement change = u change. Since the effective stress = ct during this step, the strength (= fn eff stress) is constant.

10. “  = 0” continued If q f is plotted vs. p o A family of straight lines appears on the MIT space q f = fn (p’ o ) Board – Table 28.2

11. CSSM - Undrained Contractive (B) u = + during shear  ’ decreases Dilative (A) u = - during shear  ’ increases Undrained No vol change = No e change Santamarina et al 2002

12. Cavitation Occurs when –u < ~15 psi (1 atm) May occur during shear, leading to “drained-type” behavior. Peak strength becomes a function of the total confining stress p o More pronounced in sands