Prof. A.K.Choudhary Professor & Ex Head

Department of Civil Engineering, NIT, Jamshedpur

A prefabricated vertical drain is an elongated channelled flexible plastic core sheet with horizontal channel that provide continuous longitudinally extending drain channels.

It is provided with a series of closely spaced reliefs (longitudinally extending surfaces ) for inhibiting tendencies of the core to horizontally bend and fold or collapse.

First Installation: California (1934) - 60 mm diameter sand drains at a spacing of 3m c/c

1950: Jetted sand drains (Netherlands), but had problems of additional costs of large jetting pumps and difficulties in driving out large quantities of water

Till 1970: Installation was mainly using close-ended mandrels, producing a large smear zone

1970: First use of smaller diameter band-shaped cardboard wicks by KJELLMAN

Recent trends: Use of Prefabricated Vertical Drains (PVD) using man-made fabrics like Polyethylene, Polyvinyl chloride, Polyester & Polypropylenes

Upcoming trends: PVD having starch based plastic cores , and Electro-Conductive PVD

Creation of closely-spaced artificial vertical drainage paths to which the pore water can flow

Decreasing the consolidation time to a matter of months.

These artificial drainage paths consist of a central plastic core

Functions Free-draining water channel surrounded by a thin geotextile filter jacket.

At the same time decreases the risk of slip plane failure, with unrestricted fill placement

• Two types of PVD :- Sandwick

Band Drain (Geo Drain)

Smaller in diameter than sand drain .

Pre-packed in filter fabric & shaped like a stocking.

Diameter ranges from 60 to 100 mm.

A typical wick drain is approximately 4 inches wide, 1/8 inch thick, and comes in rolls up to 1,000 feet in length.

It is used for moderate depth of about 12 to 15 m .

Installation of wick drain by pushing hydraulically or vibrated into ground by a hollow steel mandrel

Looping of wick material through an anchor at the base of mandrel which holds the drain securely in place as the mandrel is extracted

Equipments used :- I. Mandrel II. Crawler Excavator III. Cranes IV. Heavy chains V. Cable VI. Gear System

Used for warehouses, storage tanks, residential and retail structures, roadways, airport runways industrial facilities, port and marine construction

Quick consolidation for soft ground underlying fills and earthen features such as berms, dikes, levees, and embankments

Do not required drill

Faster installation

Available in the form of reels

Effective for larger depth

Effective for thick clay deposit

Cost is 1/3 rd of sand drain

These are in band shape having thickness 3 to 5 mm and 100 mm wide

These are normally manufactured with corrogated synthetic core enclosed by geotextile filter

It is also known as Strip drains or Geo drains

It is advisable to use for greater depth (for greater than 15 m)

First placing of the sand blanket. The band drains are installed in a triangular pattern with

1.5 m c/c spacing. They are pre-cut to an estimated length and anchor shoes

are attached. These are used to fix the band drain at their base. The band drains are first pulled into a hollow mandrel,

which is then drawn into the seabed by an electrical vibro-hammer.

The band drains penetrates through the sand blanket and the deposit layers such that an effective passage could be created for water flow

In comparison with geotextile laying and sand blanket placing, band drain installation is more time-consuming but requires less working space.

I. Derrick barges with a pair of mandrels

II. A steel panel were used to install the drains.

III. Crawler Excavator

IV. Cranes

V. Heavy chains

VI. Cable and gear system.

Low cost

Faster installation

Ensure drainage continuity

Result in clean site

Light weighing of installaion equipments

Negligeble subsoil disturbance

The least possible disturbance of soil layers

Guaranteed water discharge due to high permeability

Possibility of adapting the core and filter to the nature of the soil

High installation speed; 4.000 to 8.000 meters per day per unit

A shorter consolidation period by using close spacing

No water required for installation

Drains can be installed to a depth of 40 m or deeper

Easy to control installation Proven performance under varied conditions

Case study done by Chai and Miura at SAGA AIRPORT

JAPAN

Problem Faced:-soil consist of fines and prone to excessive differential settlement.

IN-SITU TEST:-Bore hole test and S.P.T. test were conducted.

SOLUTION:-Among various alternatives,PVD’s were found to be more effective due to it’s high drainage capacity.

Vertical drains are generally used in cases where the compression of the soil is dominated by primary consolidation.

Vertical drains accelerate the rate of consolidation and the rate of settlement.

It leads to the considerable reduction in the construction and maintenance costs.

Installation of mandrel leads to an extensive zone of disturbance, and considerable increase of pore water pressure, thus more attention is to be given to the equipment and installation procedure.

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