PVDs shorten drainage path • 90% Consolidation time reduced from >15 years to 1 year
www.geosyntheticsworld.com
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No risk of PVDs breaking during installation - sand drains can have discontinuities if mandril is withdrawn too fast.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.
www.geosyntheticsworld.com
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.
www.geosyntheticsworld.com
Vertical drain design
• Terzaghi T90 time factor = 0.848 while assuming soft clay with ch = 2 m²/year: • withou withoutt PVD PVD settl settleme ement nt for for U = 90%: 90%: T90 d2 0.848 x 10² t = ------------ = ------------------ = 42 years cv 2 www.geosyntheticsworld.com
Vertical drain design • by using: - Colbonddrain CX1000 - 1.6 m triangular centers
90% consolidations in 12 months.
www.geosyntheticsworld.com
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
π 2b b => d = ------ x ------ = ---π 4 2 where d = equivalent diameter of PVD b = width of PVD www.geosyntheticsworld.com
Drain spacing
•
triangular spacing standard
π D2
1 ---------- = ---- S 2 √ 3 4 2 D=S
•
√
2√3 ---------
π
= 1 . 05 S
for a square grid : D = 1.128 S
www.geosyntheticsworld.com
Kjellman formula
•
•
[
D2 t = --------8 Ch where: t = D = d = Ch =
]
D 3 ln ( ---- ) - ---d 4
1 ln ---------1 - Uh
con consol solation period (years) ars) diam diamet eter er of drain drained ed soil soil cyli cylind nder er (m) (m) equi equiva vale lent nt diam diamet eter er of drai drain n (m) (m) horizontal consolidation coefficient
(m2/year) Uh = average horizontal consolidation degree
www.geosyntheticsworld.com
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 c ore – permeability reduction due to clogging of filter and core – bending and kinking of PVD during settlement – pressure on PVD
www.geosyntheticsworld.com
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
www.geosyntheticsworld.com
Discharge capacity • Effect of i on q w : plot of discharge against hydraulic gradient at 360 kPa confining pressure for filament core PVD. 70 ) s 60 / l m 50 ( Q , e 40 g r a h 30 c s i D 20
10 0 0
0.2
0.4
0.6
0.8
1
1.2
Hydraulic gradient, i
Q qw = --------i
Maximum actual gradient 0.1
www.geosyntheticsworld.com
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.
