Showing posts with label Soil Consolidation. Show all posts
Showing posts with label Soil Consolidation. Show all posts

Anchorage of wickdrains

During the installation of prefabricated wick drains (Prefabricated vertical drains) for soil consolidation you need some materials under the mandrel (lance) to hold the drain in the soil when you pull the mandril back.
So you have to use anchor plates or anchor rods.
Both is OK depending on site conditions.
I find the plate more easy to use.
When the mandril is pushed in the soil the plate holds the drain in the soil.

Standards for prefabricated vertical drains (wickdrains)

ISO 12958:1999    Geotextiles and geotextile-related products -- Determination of water flow capacity in their plane
ISO 10319:1993   Geotextiles -- Wide-width tensile test
ISO 11058:1999   Geotextiles and geotextile-related products -- Determination of water permeability characteristics normal to the plane, without load
ISO 12956:1999 Geotextiles and geotextile-related products -- Determination of the characteristic opening size

Below there is a typical datasheet of a wickdrain (prefabricated vertical drain) for soil consolidation. The standards used are listed.

Why use PVD over sand drain

Why use PVD (Prefabricated vertical drain) over sand drain
  • Installation of PVDs typically 6,000 linear meters per day (depending on the soil and the machine used it changes from 2000 to 8000) and result in a lower project cost.
  • No (or less) risk of PVDs breaking during installation - sand drains can have discontinuities if mandril is withdrawn too fast.
  • No (or less) risk of shear failure of PVDs during settlement - sand drains are vulnerable to shear failure during settlement.

Principle installation of PVD

Principle installation of Prefabricated Vertical drains
  • Specialized rig
  • Mandrill supported inside a leader held by crane or hydraulic excavator
  • Mandrill pushed into soil by chains/cables driven by hydraulic engines.
  • PVD installed inside a mandrill
  • Rhombic mandrill less disturbance (Often rectangular mandrill)

What is a PVD ?

  • Prefabricated Vertical Drain - PVD
  • Also called as wickdrain.
  • Used for soil consolidation, settlement instead of sand drains.
  • Typically 95 -100 mm wide by 3 - 5 mm thick
  • Synthetic core wrapped with geotextile
  • The core may be produced from different polymers (PVC, PP, PE or mixtures ...)
  • The geotextile jacket type can differ depending on the producer.
  • There are also many core types in the market.

A Soil consolidation project

In this project about 700.000 meters of wickdrains (prefabricated vertical drains-PVD) were specified.

Working with sand drains need more time and work.

The soil should be consolidated quickly, and the road is near the sea.

It was like draining the sea.

At the top of the soft soil a sand layer was placed to move the machines and also for drainage.

The contractor preferred a special soil consolidation composite - Colbonddrain.

At the top of the soil there were a 1 meter thick soil which was difficult to penetrate in some areas. The bottom was very soft.

A simple wickdrain installation equipment attached to an excavator worked well.

The wick drains installed upto 18 meters depth.

At the end the consolidation was done succesfully.

Why use PVD over sand drain

  • PVD’s have high discharge capacities, typically 30 x 10-6 m3/sec to
    90 x 10-6 m3/sec compared to a Ø 0.35 sand drain with a discharge capacity of 20 x 10-6 m3/sec (Van Santvoort, 1994).

  • When installed with purpose designed mandril, smear effects are much smaller for PVDs than for the large diameter sand drains. Zone of smear is directly proportional to the diameter of mandril used for installation.

  • PVD’s (prefabricated vertical drains) are consistent factory produced whereas sand drains are subject to quality variance of naturally occurring sands.

Calculation of band drains (PVD) 4 - Drain spacing

1)triangular spacing standard

2)for a square grid :
D = 1.128 S

S : spacing of drains
D : diameter of the equivalent cylindrical column of soil, drained by each drain.
For more information about the calculation of prefabricated vertical drains (wickdrains) please also see other posts.

Calculation of PVD's 3 - Kjellman formula

t = consolidation period (years)
D = diameter of drained soil cylinder (m)
d = equivalent diameter of drain (m)
Ch = horizontal consolidation coefficient (m2/year)
Uh = average horizontal consolidation degree

Calculation of PVD's 2 - Equivalent PVD diameter

  • calculation assumes PVD cylindrical and draining effect dependent on periphery
  • PVD effective periphery is 2 x width x f, where f is a correction factor allowing for:
    less favorable inflow to possible disturbance & smear effect to soil during installation

d = equivalent diameter of PVD
b = width of PVD

Calculation of Prefabricated wick drains 1

An assessment of the effect of vertical drains on the consolidation process may be made using the method proposed by Kjellman.
This method is based on the assumption that the drains are placed in a regular pattern and that each drain serves a cylindrical soil column of exactly the same length as the drain (colbonddrain) . it is assumed that the increase in vertical load is evenly distributed over the consolidation area and that horizontal layers remain horizontal throughout the consolidation process. It is been assumed
that the permeability of the cohesive soil remains unchanged throughout the consolidation period and that the internal resistance of the drain is negligible. Practical experiments have shown that although these assumptions are not always applicable, in majority of cases their influence on the ultimate results is of little significance. The exception is where drains without a "high flow" core are used for thick highly compressible strata. In such cases the internal resistance of the drain can have a substantial effect on the consolidation period.

Soil consolidation

Civil Engineers, planners and contractors aim for direct economic highways, airports and other traffic routes using natural resources as efficiently as possible in their construction: this often means having to build on poor soils and sometimes even to reclaim land. In many cases, the sub-grade is too weak to allow construction to proceed at a normal rate. Soft, cohesive soils are very slow to consolidate. 90% settlement often takes 20-25 years without the use of Prefabricated Vertical Drains (PVD's).
Prefabricated vertical drain (PVD), provides an economical solution to this issue. The principle feature of prefabricated vertical drain (Colbonddrain) lies in providing much shorter and highly permeable drainage paths for the dissipation of excess porewater pressure, allowing 90% consolidation in a time period of months, rather than years.
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