Civil Engineering Reference
In-Depth Information
Example 11.9:
Effective stress paths
A layer of soft, normally consolidated clay is 9.25 m thick and has an existing effective
overburden pressure at its centre of 85 kPa.
It is proposed to construct a flexible foundation on the surface of the clay, and the
increases in stresses at the centre of the clay, beneath the centre of the foundation, are
estimated to be
Δ
σ
1
=
28.8 kPa and
Δ
σ
3
=
19.2 kPa.
Consolidated undrained triaxial tests carried out on representative undisturbed
samples of the clay gave the following results:
Cell pressure
=
35 kPa
Strain (%) Deviator stress (kPa)
Pore water pressure (kPa)
0
0
0
1
10.4
0.4
2
20.7
4.8
3
29.0
9.7
4
33.2
13.2
5
35.8
16.6
6
37.3
17.9
6.8
37.8
19.3
(failure)
Cell pressure
=
70 kPa
Strain (%) Deviator stress (kPa)
Pore water pressure (kPa)
0
0
0
1
20.7
4.1
2
42.7
12.8
3
54.4
22.1
4
63.4
30.4
5
66.1
34.8
6
71.7
37.9
7
75.8
40.7
(failure)
By considering a point at the centre of the clay and below the centre of the founda-
tion, draw the effective stress paths for undrained shear obtained from the tests and
indicate the effective stress paths for the immediate and consolidation settlements that
the foundation will experience.
Assume that K
0
=
1
−
sin
φ
and determine an approximate value for the immediate
settlement of the foundation.
Solution:
The first step is to plot out the two effective stress paths. The calculations are best set
out in tabular form:
