Undrained strength
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Undrained Strength. CIVE 640. 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). CU – Weald clay. Board – Figure 28.6. 1. undrained = no vol change

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Undrained Strength

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Undrained strength

Undrained Strength

CIVE 640


So far

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)


Cu weald clay

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 = (+)


Cu weald clay eff stress paths

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


Undrained strength

ESP

The ESP

= fn (OCR and p’ - e conditions before shear, i.e., p’o and eo )

 fn (loading type)


Drained vs undrained

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


Undrained shear strength

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)


Undrained shear strength1

Undrained Shear Strength

Board – Table 28.1


Undrained strength

“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.


F 0 continued

“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)


Cssm undrained

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


Cavitation

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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