Chapter 12, Problem 12.23P

### Principles of Geotechnical Enginee...

9th Edition
Braja M. Das + 1 other
ISBN: 9781305970939

Chapter
Section

### Principles of Geotechnical Enginee...

9th Edition
Braja M. Das + 1 other
ISBN: 9781305970939
Textbook Problem

# A 15-m thick normally consolidated clay layer is shown in Figure 12.55. The liquid limit and plastic limit of the soil are 39 and 20, respectively. a. Using Eq. (12.46) given by Skempton (1957), estimate the undrained cohesion at a depth of 11 m below the ground surface as would be obtained by conducting a vane shear test. b. Using Bjerrum’s (1974) correction factor [Eqs. (12.61) and (12.62)], estimate the design value of the undrained shear strength determined in Part a. c. If the clay has the potential of becoming overconsolidated up to OCR = 3.0 due to future ground improvement activities, what would be the new undrained cohesion? Use Eq. (12.47) given by Ladd et al. (1977).

(a)

To determine

Find the undrained cohesion at a depth of 11m below the ground surface.

Explanation

Given information:

The liquid limit (LL) of the soil is 39.

The plastic limit (PL) of the soil is 20.

The saturated (γsat) unit weight of clay is 20.5kN/m2

The depth (H1) of the sand above the water table is 3m.

The unit weight (γ) of dry sand is 16kN/m2.

Calculation:

Find the plasticity index (PI) using the relation.

PI=LLPL (1)

Substitute 39 for (LL) and 20 for (PL) in Equation (1).

PI=3920=19

Show the empirical relationship between undrained cohesion (cu) and effective overburden pressure (σo) as follows:

cu(VST)σo=0.11+0.0037(PI) (2)

Here, cu(VST) is undrained shear strength from vane shear test and (σo) is overburden pressure.

Substitute 19 for PI in Equation (2).

cu(VST)σo=0.11+0.0037(19)cu(VST)σo=0.1803 (3)

Calculate the overburden pressure (σo) using the relation.

σo=H1(γ)+H2(γsatγw) (4)

Here, H2 is depth of the clay

(b)

To determine

Find the design value of undrained shear strength by using Bjerrum’s correction factor.

(c)

To determine

Find the new undrained cohesion.

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