The International Information Center for Geotechnical Engineers

Prefabricated Vertical Drains

 

 

Effectiveness of Prefabricated Vertical Drains (PVDs)

 

In order for effective expedition of the consolidation process, the PVDs must accept water from the surrounding soil, and discharge it.  The initial design conditions and changes in these factors throughout the length of the project will affect the consolidation rate/success of the project.  

 

Water Flow into Drain

 

Hydraulic Conductivity

The hydraulic conductivity of the soil surrounding the drain will control the rate at which water can enter the drain and relieve pore water pressures in the soil.  

 

Smear Zone

The installation of PVDs require the use of a steel mandrel, which firmly clamps the drain during its insertion into the ground.  Accompanying the mandrel, is an anchor plate fixed to the bottom of the drain, which serves to prevent soil from entering the drain and to keep the drain in place upon removing the mandrel. Below is a simple diagram of the anchoring system [11].

 

 

[K] Drain, Mandrel and Anchor Plate

 

As the mandrel is pushed downward and upon its removal once the drain is in place, it disturbs the soil surrounding the drain producing shear strains and displacement that decrease its hydraulic conductivity.

 

Minimizing the mandrel cross sectional area will reduce potential for soil displacement and disturbance. It may be suitable to taper the mandrel tip, as long as the stiffness is not sacrificed. For soil profiles with many different layers, large kh/kv ratios, the laying may enhance horizontal permeability. It is possible to retard the lateral seepage of pore water into the drains by smearing of pervious and less pervious layers. Static pushing is preferred to drive or vibrating the mandrel in sensitive soil, but may cause buckling or wobbling of the mandrel.  An idealization of the mandrel disturbance area is shown in Figure L.

 

 

 [L] Approximation of the Disturbed Zone Around the Mandrel

 

Although there have been numerous studies conducted to assess properties of the smear zone including range, shape, and effect on hydraulic conductivity, there is no precise consensus among researchers.  Nevertheless, some generalities are listed below:

The larger the mandrel, the larger the smear zone.

The shape of the mandrel affects the shape of the smear zone

Square/Circular Mandrel – Square/Circular Smear Zone

Rectangular Mandrel – Ellipsoidal Smear Zone

The outer boundary of the Smear Zone has been found to range from 4-18 times the equivalent mandrel radius.

The overlap of smear zones from adjacent drains complicates smear zone calculations further [3]

 

 

 

Discharge Capacity

 

Design

The design discharge capacity of the drain is a function of its cross-sectional area (core available for flow).  This capacity can and will likely be reduced immediately upon installation and over the lifetime of the project.

 

Installation

 

The installation of the drains is a rough process and is not only destructive of the installation equipment itself (mandrels often need be replaced over the course of a project, and installation equipment repaired), but is also presents the critical case for the mechanical properties of the drain.  Grab and puncture tests per ASTM standard of the geosynthetic material are important in this regard [7].

 

Reduction With Time

 

Over the lifetime of the project a number of factors can reduce the discharge capacity of the drain and slow down the consolidation process.

 

Clogging

 

Clogging of the PVD can significantly reduce the discharge capacity of the drain.  A filter with appropriate properties (such as AOS – Apparent Opening Size, per ASTM D 4751) will pass water, while retaining clay particles and limit clogging.  The larger the drainage channel (cross-sectional area of the drain), the less the discharge capacity will be effected by clogging, all else held equal [8].

 

 Bending/Kinking

 

As the soil consolidates, reducing the thickness of the clay layer, the drain undergoes bending and/or kinking deformation.  The figure below idealizes some of the possible bending/kinking mechanisms [1].

 

 

[M] Various Possible Configurations for Vertical Accommodation to Soil Settlement

 

Nguyen &Hung (2010), tested a variety of PVDs, and took photos of the deformations.  They appear in Figure N.

 

 [N] Deformation Patterns

 

Whether the drain kinks or bends depends on the flexibility of the drain and modulus of the surrounding clay [4].  A more flexible drain core will lead to greater reduction in discharge capacity [14].  Furthermore, sharp kinks will reduce flow through the drain more than gradual bends. [1]

 

Lateral Earth Pressures

 

Lateral earth pressures can cause the filter of the drain to pass into the core in a punching manner, reducing flow [11].

 

Biological Degradation

 

Biological and chemical activity can reduce discharge capacity as well [11]

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