Some information and design guide about prefabricated vertical drains

1. Technical Presentations
Prefabricated vertical drains for
Soil Consolidation
Vertical drain design
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2. What is a PVD
• Prefabricated Vertical Drain - PVD
• Typically 95 -100 mm wide by 3 - 5 mm thick
• Synthetic core wrapped with geotextile
• Many types of core
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3. PVDs shorten drainage path
• 90% Consolidation time reduced from >15 years to 1 year
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4. Why use PVD over sand drain
• Installation of PVDs typically 6,000 linear meters per day and result in
a lower project cost.
• No risk of PVDs breaking during installation - sand drains can have
discontinuities if mandril is withdrawn too fast.
• No risk of shear failure of PVDs during settlement - sand drains are
vulnerable to shear failure during settlement.
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5. 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 ∅ 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 are consistent factory produced whereas sand drains are
subject to quality variance of naturally occurring sands.
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6. Vertical drain design
• Terzaghi T90 time factor = 0.848 while assuming soft clay with ch = 2 m²/year:
• without PVD settlement for U = 90%:
T90 d2 0.848 x 10²
t = ------------ = ------------------ = 42 years
cv 2
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7. Vertical drain design
• by using:
- Colbonddrain CX1000
- 1.6 m triangular centers
90% consolidations
in 12 months.
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8. 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
π
• Delft laboratory finds f = -------
4
2b π b
=> d = ------ x ------ = ----
π 4 2
where d = equivalent diameter of PVD
b = width of PVD
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9. Drain spacing
• triangular spacing standard
π D2 1
---------- = ---- S2 √ 3
4 2
D=S
√ 2√3
---------
π
= 1 . 05 S
• for a square grid :
D = 1.128 S
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10. Kjellman formula
D2
• t = ---------
8 Ch
[ D 3
]
ln ( ---- ) - ----
d 4
1
ln ----------
1 - Uh
• where:
t = consolation 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
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11. Discharge capacity
• maximum flow observed from PVD = 5 x 10-6 m3/s = 158 m3/year.
Hydraulic gradient approximately 0.1
• reduction in discharge capacity from :
– deformation and creep of filter into core
– permeability reduction due to clogging of filter and core
– bending and kinking of PVD during settlement
– pressure on PVD
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12. Discharge capacity
• qw = Q / i Darcy’s Law (valid for laminar flow only)
where qw is constant: qw ≥ 140 x 10-6 m3/s from test ASTM D4716
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13. Discharge capacity
• Effect of i on qw : plot of discharge against hydraulic gradient at 360 kPa
confining pressure for filament core PVD.
70
60
Discharge,Q (ml/s)
50
40
30
20
10
0
0 0.2 0.4 0.6 0.8 1 1.2
Hydraulic gradient, i
Q
qw = --------- Maximum actual gradient 0.1
i
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14. Disclaimer
The technical data set forth in this slideshow reflect our
best knowledge at the time of issue. The slideshow is
subject to changes pursuant to new developments and
findings, and a similar reservation applies to the
properties of the products described. We do not
undertake any liability for results by usage of these
products and information. We do not take any
responsibilities. This slideshow is only for general
information.
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