The International Information Center for Geotechnical Engineers

Prefabricated Vertical Drains

 

Design Parameters

The design of PVDs requires an evaluation of design parameters including soil and drain properties as well as the effects of PVD installation. The size of the project can influence the amount of effort needed for evaluation.

This section refers to the following project descriptions as defined by Rixner et al. (1986):

 

Project Category

Description

A

 

Basically uniform soil (no varving, low to moderate sensitivity)

Simple construction (no staged loading)

PVDs few in number (length < 18m (60ft))

 

B

 

Increased complexity, Intermediate

 

C

 

Unusual soils (varved, or high sensitivity)

Staged loading, complicated construction

PVDs numerous in number (length > 18m (60ft))

 

 

Soil Properties

All soil property values used in design of wick drains should be evaluated at the maximum vertical effective stress that shall be applied to the compressible soil in the field.

 

Coefficient of Consolidation for Horizontal Drainage (ch) & Coefficient of Permeability for Horizontal Seepage (kh

Category A projects may use ch conservatively estimated as approximately cv (kh/kv = 1) measured in lab from one-dimensional consolidation tests (ASTM D2435). There are estimates for kh, but field and lab measurements should be taken to compare and adjust the value [9].

 (8)

Category C projects should have more accurately estimated values for ch using any variation of methods. In-situ piezometer probes and analysis of pore pressure dissipation curves may be of reference. In-situ determination of kh by small-scale pumping tests in piezometers or self-boring permeameters can be used with laboratory mv values to form a relationship for ch [9].

 (9)

Where:
ϒw = unit weight of water
Mv = coefficient of volume change

It is generally recommended to employ conventional consolidation tests to measure cv combined with field and laboratory investigations to estimate kh/kv and then evaluate ch via the Category A equation.


Coefficient of Permeability in the Horizontal Direction in the Disturbed Soil (ks)

The Ratio of kh/ks range 1-5, and is expected to vary with soil sensitivity and the presence or absence of soil macrofabric [9].

Drain Influence Zone

The drain influence zone (D) is a function of drain spacing (S) only. Vertical drains are commonly installed in square of triangular patterns. Square pattern layouts have greater ease and control in the field. However the triangular patterns are preferred to provide more uniform consolidation between drains. Shown below are diagrams of the square and triangular patterns [9].

 

Pattern

D as a function of S

Square

D = 1.13*S

Triangular

D = 1.05*S

 



 [J] Installation Patterns

 

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