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

Chapter 8 GROUND IMPROVEMENT
APPENDIX 8A: DESIGN OF VERTICAL DRAINAGE SYSTEM
The principal objective of soil pre consolidation, with or without PVD, is to achieve a desired degree
of consolidation within a specified period of time. The design of pre consolidation with PVDs requires
the evaluation of drain and soil properties (both separately and as a system) as well as the effects of
installation.
For one dimensional consolidation with drains, only consolidation due to one dimensional (vertical)
seepage to natural drainage boundaries is considered. The degree of consolidation can be measured
by the ration of the settlement at any time to the total primary settlement that will (or is expected
to) occur. This ratio is referred to as Ū, the average degree of consolidation.
By definition, one dimensional consolidation is considered to result from vertical drainage only, but
consolidation theory can be applied to horizontal or radial drainage as well. Depending on the
boundary conditions consolidation may occur due to concurrent vertical and horizontal drainage. The
average degree of consolidation, Ū, can be calculated from the vertical, horizontal or combined
drainage depending on the situation considered.
With Vertical drains the overall average degree of consolidation, Ū, is the result of the combined
effects of the horizontal (radial) and vertical drainage. The combined effect is given by:-
Ū = 1 – ( 1 – Ū h ) (1 – Ū v ) (8.1)
where,
Ū = overall average degree of consolidation
Ū h = average degree of consolidation due to horizontal (radial) Drainage
Ū v = average degree of consolidation due to vertical drainage.
The graph of Ū vs log time for both the vertical and horizontal drainage in shown in Figure 8.11 and
Figure 8.12 respectively.
March 2009 8A-1