pvd soil consolidation design

of 14www.geosyntheticsworld.com

Prefabricated vertical drains for

Soil Consolidation

Vertical drain design

Technical Presentations


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


PVDs shorten drainage path

• 90% Consolidation time reduced from >15 years to 1 year


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• No risk of PVDs breaking during installation - sand drains can have discontinuities if mandril is withdrawn too fast.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.


Why use PVD over sand drain

• PVD’s have high discharge capacities, typically 30 x 10-6 m3/sec to90 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.



• 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



• by using: - Colbonddrain CX1000 - 1.6 m triangular centers

90% consolidations in 12 months.


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


Drain spacing

• triangular spacing standard

π D2 1 ---------- = ---- S2 √ 3 4 2

2 √ 3D = S --------- = 1 . 05 S

π

• for a square grid :D = 1.128 S

√


Kjellman formula

D2 D 3 1• t = --------- ln ( ---- ) - ---- ln ---------- 8 Ch d 4 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

[ ]


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


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


Qqw = --------- Maximum actual gradient 0.1

i

Discharge capacity

• Effect of i on qw : plot of discharge against hydraulic gradient at 360 kPa confining pressure for filament core PVD.

0 0.2 0.4 0.6 0.8 1 1.20

10

20

30

40

50

60

70

Dis

char

ge,Q

(ml/s

)

Hydraulic gradient, i


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.

pvd soil consolidation design

Business

sand drains

shear failure

mandril

installation

pvds

pvd

settlement

www