Filteration of Soil Using Geosynthetics.pptx

Filtration of Soils
Using Geosynthetics
Prof. Samirsinh P Parmar
Mail: samirddu@gmail.com
Asst. Professor, Department of Civil Engineering,
Faculty of Technology,
Dharmsinh Desai University, Nadiad-387001
Gujarat, INDIA

Outline
• Introduction
• Types of filters
• Problems in filter design
• Experimental Observations
• Summary
Reinforced Earth & Geotextile, M.tech-geotech, SPP-CL-
DDU, Nadiad
2

Simple explanation of the filter concept
• Tea filter should
liquid
allow the
tea while
passage of
retaining the tea powder
• If the openings in the filter are
large, tea powder will flow
through, which is undesirable
• If the filter openings are too
small, liquid may not strain
through freely, which is also not
desirable
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Filtration
• Equilibrium soil to geotextile system that allows for adequate liquid
flow with limited soil loss across the plane of the geotextile over a
service lifetime compatible with the application under consideration.
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Functions of a Filter
• Retain particles of the base soil to be filtered
• Avoid piping
• Allow free flow of water
- upstream of the filter
Avoid external clogging
- through the filter
- Avoid internal clogging
(With unstable soils)
• Survive construction and environmental stresses
• Function can be provided by either natural
aggregates or by Geotextiles
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100
90
80
70
60
50
40
30
20
10
0
0.01
Percent
finer
0.1 particle size
1 (mm)
2
10
D85
D15
D
D
cu
10
60

D10 D60
D30
cc

D60
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Conventional granular filters
The water flow from base soil through the filter should
not result in loss of fines from the soil (piping) or if any
fine soil particles pass through they should not clog the
filter during the life time. The permeability of the filter should
be high.
Retention criteria:
Permeability criteria:
 5(to prevent piping)
(soil)
( filter)
d85
15
d
 5  20
(soil)
( filter)
d15
15
d
In most cases, it is not possible to directly satisfy the above criteria. In such
cases, graded filters are provided, i.e. layers of different size soil particles
gradually increasing in size away from base soil.
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Base soil
Flow
direction
Pore pressure dissipation through soil
Larger particles have
larger inter-particle
space
Conceptual representation of filtration phenomenon
Filter layer(s)
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Conceptual representation of filtration
phenomenon
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Conceptual representation of filtration
phenomenon
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Water bubbles coming out of stagnated water on a pavement
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Geotextiles as filters
Thick geotextile used
for cushion and
drainage
Thin geotextile – used
as separator, filtration
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woven non-woven
Natural geotextiles made of jute
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Comparison between conventional &
geotextile filters
Item conventional Geosynthetic
Thickness High (> 150 mm) Very low (3 – 10 mm)
Porosity 25 to 50% 75 to 90%
T
ensile strength None Low to high
Compressibility Very low Function of normal
pressure
Availability as per
design
Depends on quarry Manufactured under
controlled conditions
Capillary raise Important (hc <
500mm)
Not significant (hc<50
mm)
Risk of damage None Installation damage
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Similarities
Internal clogging by finer particles
Biological clogging (warm climates)
Clogging due to Precipitation of salts
Clogging in environmental applications
Frost susceptibility
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Equipments used for Apparent Opening Size test
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0
5
10
15
10 100
percent
finer
particle size (microns)
Typical data from dry sieve analysis on a woven geotextile
O90 = 150microns
O95 = 230 microns
O90 O95
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Geotextile filter in uniformly
graded soil
Geotextile filter in
well graded soils
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Filter in cohesive soil – no piping due to bridging due to cohesive
strength – in such cases, pore opening could be larger
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Flow around a geotextile immediately after installation
Fine soil particles piping out initially
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CLOGGING
• Filling up of the voids in the textile with solid particles progressively until
the passage of water is slowed down
• Clogging reduces the hydraulic conductivity Internal clogging –
by mineral particles, by precipitation and chemical deposition
of particles from salt/iron laden water by biological growth in
aerobic conditions & warm climates
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Geotextile Selection for granular soils
Piping limit:
O90  D85 of soil
Permeability limit:
O90  D15 of soil
and O90  0.05 mm
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Minimum water flow rate
• Up to Apparent Opening Size (O90) < 75, flow rate required = 30
lit/m2/s under 100 mm head
• For higher O90 sizes, the flow rate linearly increases up to about
100 lit/m2/s under 100 mm head for large opening size of 800
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Geotextile for Clay Soils
Piping Limit
O90  0.12 mm
Permeability Limit
O90  0.05 mm and minimum water permeability
of 30 litres/m2/sec under 100 mm head
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Permeability criterion
Giroud (1985)
For granular filters kgf  10 ks
For geotextile filters
kg  0.1 ks as tg  tgs/100 (theoretically for similar headloss)
 For critical applications, kg > 10ks where failure may result
in loss of life& result in structural damage
 For non-critical applications, kg > ks
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Long term flow compatibility
 What is the compatibility between the geotextile
and the soil at the given site.
 Will the geotextile eventually clog?
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Geotextile Selection for steady-state flow
More than 20% clay (d20 < 2)
 Non-dispersive soil O95 < 210
 Dispersive soil use 75 to 150 mm fine sand between base soil and
geotextile and then design the geotextile as a filter for the sand
Less than 20% clay (d20 > 2)
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Geotextile for dynamic flow conditions
More than 30% clay fraction (d30 < 0.002 mm) Non-
dispersive soil –
O95 < 10d50; O95 < d90 and O95 < 0.1mm
Dispersive soil
Use 75 to 100 mm fine sand layer below geotextile and
design the filter for this sand layer
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Stable or unstable soils ?
• Internally Stable soils – soils with uniform gradation or linearly
graded broad gradation (well graded) soils
• Unstable soils – subject to internal movement of soil particles, e.g.
gap graded soils
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Dutch guidelines for filter
O90 < d90 for woven geotextiles
O90 < 2 d90 for nonwoven geotextiles
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German Working Group 14
recommendations
Dw = apparent opening size from wet sieve test Problem soils are as
follows:
Fine grained soils with a plasticity index less than 0.15 Soils whose d50 lies
between 0.02 and 0.10 mm
Soils with a uniformity coefficient less than 15 which also contains clays or silts
Soil Description Geotextile criteria
d40 < 0.06 mm, stable soil Dw < 10d50 and Dw > 2d90
d40 < 0.06 mm, problem soil Dw<10d50 and Dw < d90
d40 > 0.06 mm, stable soil Dw<5 d10(U)1/2 and Dw < 2d90
d40 > 0.06 mm, problem soil Dw<5d10(U)1/2 and Dw<d90
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Drainage ditches construction
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Applications:
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Filteration of Soil Using Geosynthetics.pptx

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