Principles of Geotechnical Engineering (MindTap Course List)
Principles of Geotechnical Engineering (MindTap Course List)
9th Edition
ISBN: 9781305970939
Author: Braja M. Das, Khaled Sobhan
Publisher: Cengage Learning
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Chapter 4, Problem 4.1CTP

The properties of seven different clayey soils are shown below (Skempton and Northey, 1952). Investigate the relationship between the strength and plasticity characteristics by performing the following tasks:

a. Estimate the plasticity index for each soil using Skempton’s definition of activity [Eq. (4.28)].

b. Estimate the probable mineral composition of the clay soils based on PI and A (use Table 4.3)

c. Sensitivity (St) refers to the loss of strength when the soil is remolded or disturbed. It is defined as the ratio of the undisturbed strength (τf-undisturbed) to the remolded strength (τf-remolded)) at the same moisture content [Eq. (12.49)]. From the given data, estimate τf-remolded for the clay soils.

d. Plot the variations of undisturbed and remolded shear strengths with the activity, A, and explain the observed behavior.

Chapter 4, Problem 4.1CTP, The properties of seven different clayey soils are shown below (Skempton and Northey, 1952).

(a)

Expert Solution
Check Mark
To determine

Find the plasticity index for each soil using Skempton’s definition of activity.

Explanation of Solution

Determine the plasticity index of the Beauharnais soil using the relation.

A=PI%ofclay-sizefraction, byweight

Here, A is an activity.

Substitute 0.52 for A and 79 for % of clay–size fraction, by weight.

0.52=PI790.52×79=PIPI=41.08

Similarly, calculate the plasticity index for the remaining soils.

Determine the remolded shear strength of the Beauharnais soil using the relation.

τfremolded=τfundisturbedSt

Here, τfundisturbed is the undisturbed shear strength and St is the sensitivity.

Substitute 18kN/m2 for τfundisturbed and 14 for St.

τfremolded=1814=1.3kN/m2

Similarly, calculate the remolded shear strength of the remaining soils.

Summarize the calculated values of plasticity index and the remolded shear strength as in Table 1.

Soil% Clay fraction (<2μm)Activity APlasticity Index
Beauharnois790.5241.08
Detroit I360.3612.96
Horten400.4216.80
Gosport550.8948.95
Mexico City904.5405
Shell haven411.3354.53
St.Thuribe360.3311.88

(b)

Expert Solution
Check Mark
To determine

Find the probable mineral composition of the clay soils based on PI and A.

Explanation of Solution

Determine the probable mineral composition of the clay soils based on PI and A.

Refer Table 4.3, “Typical values of liquid limit, plastic limit, and activity of some clay minerals” in the textbook.

Take the mineral composition of the Beauharnois soil as Illite for the activity value of 0.52 and the plasticity index value of 41.08.

Therefore, the mineral composition of Beauharnois soil is Illite.

Similarly, calculate the probable mineral composition of the remaining soils.

Summarize the calculated values of mineral composition as in Table (2).

SoilActivity APlasticity IndexMineral composition
Beauharnois0.5241.08Illite
Detroit I0.3612.96Kaolinite
Horten0.4216.80Kaolinite
Gosport0.8948.95Illite
Mexico City4.5405Montmorillonite
Shell haven1.3354.53Montmorillonite
St.Thuribe0.3311.88Kaolinite

(c)

Expert Solution
Check Mark
To determine

Find the remoulded shear strength of the clay soil.

Explanation of Solution

Determine the remoulded shear strength of the Beauharnais soil using the relation.

τfremolded=τfundisturbedSt

Here, τfundisturbed is the undisturbed shear strength and St is the sensitivity.

Substitute 18kN/m2 for τfundisturbed and 14 for St.

τfremolded=1814=1.3kN/m2

Similarly, calculate the remolded shear strength of the remaining soils.

Summarize the calculated values of remolded shear strength as in Table 3.

SoilActivity AUndisturbed Shear strength (τfundisturbed)(kN/m2)Sensitivity (St)

Remoulded Shear strength

(τfremolded)(kN/m2)

Beauharnois0.5218141.3
Detroit I0.36172.56.8
Horten0.4241172.4
Gosport0.89292.213.2
Mexico City4.5465.38.7
Shell haven1.33367.64.7
St.Thuribe0.33381500.3

(d)

Expert Solution
Check Mark
To determine

Plot the variations of undisturbed and remolded shear strengths with the activity A.

Explanation of Solution

Refer Table 3.

Plot the graph between the undisturbed, remolded shear strengths with the activity A as in Figure 1.

Principles of Geotechnical Engineering (MindTap Course List), Chapter 4, Problem 4.1CTP

The shear strength of the clay obtains from two components, one is cohesion, which is the cementing force between particles, and second one is frictional resistance, which is mainly due to the particle movement of one particle over another. The cohesion contribution is greater to the shear strength, when the clay activity is greater. Although no reliable correlation can be developed from Figure 1, both the undisturbed and remolded shear strengths certainly show increasing trends as the activity increases.

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