Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 4 SOIL SETTLEMENT
Percent consolidation U
0
20
40
60
80
U T v
10 0.0077
20 0.0314
30 0.0707
40 0.126
50 0.196
60 0.286
70 0.403
80 0.567
90 0.848
100 Infinity
100
0 0.2 0.4 0.6 0.8
Time factor T v
Figure 4.6 Average Degree of Consolidation U versus Time Factor, Tv under Various Drainage
Conditions
Note that the longest drainage distance, H d
of a soil layer confined by more permeable layers on
both ends is equal to one-half of the layer thickness. When confined by a more permeable layer on
one side and an impermeable boundary on the other side, the longest drainage distance is equal to
the layer thickness. The value of the dimensionless time factor Tv may be determined from Table
4.6 for any average degree of consolidation. U. The actual time, t, it takes for this percent of
consolidation to occur is a function of the boundary drainage conditions, i.e., the longest distance to
a drainage boundary, as indicated by Equation 4.8. By using the normalized time factor, Tv,
settlement time can be computed for various percentages of settlement due to primary
consolidation, to develop a predicted settlement-time curve. A typical settlement-time curve for a
clay deposit under an embankment loading is shown in Figure 4.6
Coefficient of consolidation, c v can be obtained from laboratory consolidation test data. Two
graphical procedures are commonly used for this i.e. the logarithm-of-time method (log t) proposed
by Casagrande and Fadum (1940) and the square-root-of-time method proposed by Taylor (1948).
These methods are can be found in various textbooks such as Holtz and Kovacs, and Soil and
Foundations, FHWA.
4-8 March 2009