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Undrained Strength

This text provides a comprehensive overview of undrained shear strength in soils, focusing on three test types: Consolidated Drained (CD), Consolidated Undrained (CU), and Unconsolidated Undrained (UU). It discusses the essential steps of each test, the behavior of different clay types (dilative vs. contractive), and the effective stress paths and relationships involved. Key concepts such as the effect of Overconsolidation Ratio (OCR) and pore water pressure changes during shear are also examined, along with relevant figures and tables that illustrate these critical geotechnical principles.

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Undrained Strength

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  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, ue during shear = (-). Thus, the effective stress inc during shear 5. nc clay is contractive (ue 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 ue @ 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 eo )  fn (loading type)

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

  7. Undrained Shear Strength Undrained Strength qf = fn (effective stress) qf = fn (p’o , Af, 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. “f = 0” Consider a clay with p’o = 16 psi 3 UU tests (no additional cons. allowed in step 2 ): 1. po = s3 = 10 psi 2. po = s3 = 30 psi 3. po = s3 = 100 psi qf 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. “f = 0” continued Board – Table 28.2 If qf is plotted vs. po A family of straight lines appears on the MIT space qf = fn (p’o)

  11. CSSM - Undrained Undrained No vol change = No e change Contractive (B) u = + during shear s’ decreases Dilative (A) u = - during shear s’ increases 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 po • More pronounced in sands

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