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GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE
Prof. J. N. Mandal
Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: cejnm@civil.iitb.ac.in
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Module - 4LECTURE- 15
Geosynthetics for filtrations, drainages and erosion control systems
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
OUTLINE
INTRODUCTION
MECHANISM OF FILTRATION FUNCTION
SUBSURFACE DRAINAGE
DESIGN FOR FILTRATION
MECHANISM OF DRAINAGE FUNCTION
GEOCOMPOSITE PREFABRICATED HIGHWAY EDGE
DRAINS
GEOSYNTHETICS WRAPPED FRENCH DRAIN
PREFABRICATED GEOCOMPOSITE LATERAL DRAIN
GEOSYNTHETICS FOR EROSION CONTROLProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Conventional graded granular materials are used forfiltration and drainage purpose in various projects of civilengineering. Various types of aggregate drains are available.
The most common form of aggregate filled drains is theFrench drain which comprises a trench filled with freedraining aggregates.
Conventional aggregate filled French drainProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
French drains can serve the following purposes:
It can collect surface water run-off from top of the drain Control ground water flow Lower the high ground water table
Ground water flows towards the drain carrying some fineparticles from the base soil and consequently, theaggregates become blocked after some period having noadequate water removal capacity.
The continued transmission of fines from the base soil tothe drainage aggregates is called piping/ clogging. It alsocauses internal erosion of the base soil.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
The aggregates in a French drain should fulfill thefollowing criterion so as to perform effectively.
Permeability criteria Filtration criteria of base soil Uniformity criteria
Special grading of aggregate is required based on thegrading of base soil. The requirements for conventionalgraded filter design are as follows,Piping criteria: D15(filter) ≤ 5 D85(soil)
Permeability criteria: D15(filter)≥ 5 D85(soil)
Uniformity: D50(filter)≤ 25 D50(soil)
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
D15 = diameter of soil particles at which 15% by dryweight of the soil particles are finer than that grain size
D85 = diameter of soil particles at which 85% by dryweight of the soil particles are finer than that grain size
D50 = diameter of soil particles at which 50% by dryweight of the soil particles are finer than that grain size
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
10
60u D
Dc
6010
230
c DxDDc
Satisfying the drainage criterion for conventionalgraded filter design is extremely expensive.
The conditions can easily and cheaply be achievedusing a geosynthetic drainage system. It can performboth drainage and filtration.
Drainage: Geosynthetic allows water to pass along itsplane. Transmissivity.
Filtration: Geosynthetic allows water to pass across itsplane, but retain the soil particles. Permittivity.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
PERMITTIVITY TRANSMISSIVITY
It occurs across theplane of geosynthetics
It occurs along theplane of geosynthetics
It is useful in filtrationfunction
It is useful in drainagefunction
Unit is sec-1 Unit is m2/sec
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetic for filtration
The filtration function of geosynthetic is illustrated through asimple example, where the liquid tea is filtered through atextile material.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Now a day, geosynthetics are extensively and successfullybeing used for filtration, drainage and erosion controlsystems alternative to the traditional granular materials.
Geosynthetic drainage materials alternative to traditional materials
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Some differences between conventional aggregates andgeosynthetic (CUR/RWS, 1995)
Property Aggregate GeosyntheticPorosity 25-40 % 75-95 %
Thickness High (> 150 mm) Low (< 50 mm)Capillary rise (hc) hc < 500 mm hc < 50 mmCompressibility Negligible Medium to highTensile strength None Low to high
Transmissivity underconfining stress Invariable Variable
Uniformity Variable gradation
Factory controlled production
Installation Compaction needed
Seaming of the joint easily
Risk damage None Puncture and tearing may occur
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
MECHANISM OF FILTRATION FUNCTION
Geosynthetics can perform effectively as the alternativeto graded granular filter.
Design criterion for filtration with geotextile is the sameas the designing with graded granular filter.
When liquid or water flows across the plane of geotextile,it is called filtration.
Geotextile is made of filaments or yarns with properopening sizes like the soil has particles and voids.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Grades granular material for filtration
Geosynthetic for filtration
Soil with larger particle sizes or geotextiles with largerpore sizes allow higher flow rate. The soil with smallerparticle sizes or geotextiles with smaller pore sizes allowlower flow rate.
Soil filter design depends primarily on the size of soilparticles. The geotextile filter design depends primarily onthe opening sizes of geotextile.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Conventional drainage systems:
Roadway drainage systems
Railway drainage systemsProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetics filter criteria:
Adequate permeability: Allow the water to flow throughthe filter into the drain so as no excess hydrostatic porepressure can build up.
Retain the soil particles in place and prevent theirmigration (piping) through the filter.
If some soil particles move, they must be able to passthrough the filter without plugging or clogging.
Open geotextiles allow the water to pass whereas closedgeotextiles retain the soil.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Advantages:
Size of the drain can be reduced.
Quantity of aggregate can also be reduced.
Excavation of soil can be reduced.
Perforated pipe may not be required.
Prevent contamination and segregation of aggregate
Cost of construction can be reducedProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Applications:
Trench drains
Pavement base course or edge drains
Blanket drains
Interception drains
Retaining walls and bridge abutments
Chimney and toe drains for earth dams and levees
Erosion control, and
Silt fencesProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetic encapsulating the drainage granular fill in a trench drain to prevent soil from migrating into the
aggregate
Geosynthetic drainage applications (After FHWA, 1990):
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetic wrapped trench drain beneath pavement edge drain
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetics in drainage blanket
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetic as chimney drain and toe drain in earth-rock fill dam to control seepage
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetics wraps around interceptor, surface and toe drain to control surface erosion and provide stable slopes
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetic placed behind the retaining walls and bridge abutments to separate the drainage aggregates from
backfill soilProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geocomposite drainage behind retaining wallProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Geosynthetic between earth bank (sub-grade) and rock protection (rip-rap or armour) for separation and erosion
controlProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Silt fence made of geotextile to block the silt transported by water current and protect the construction site
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Traditional drainage system replaced by geosynthetics sheet drain
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
There are mainly three filtration concepts:
1) If the largest opening size of geotextile is smaller thanthe larger soil particles, soil will not pass by the filter. As aresult, a filter bridge will form over the geotextile and retainthe soil particles or prevent piping (migration).
Filter bridge formation (After Christopher and Holtz, 1989)Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
2) If the smaller opening size of geotextile is larger than thesize of smaller soil particles, the smaller particles can freelypass through the filter. As a result, the geosynthetic poreswill not become clogged or blind
Method of clogging and blinding ( After Bell and Hicks, 1980)Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
3) Large number of openings in the geosynthetic would bepreferable to maintain proper flow as some of the openingsmay become plugged.
Therefore, we require three criterion for the design ofgeosynthetics filtration or drainage systems:
Retention criterion: The geosynthetics must retain the soil
Permeability criterion: Allow water to pass
Clogging resistance criterion: The geosynthetic-to-soillong-term flow compatibility should not excessively clog thefabric.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Subsurface Drainage
The geosynthetics can be used as subsurface drainage inpavements, retaining walls and earth dams etc. to replacethe graded granular materials as filters in drain (FHWA,1998).
Steps 1: Check the nature of the project, weather it iscritical/ severe or less critical/ severe.
Step 2: Determine the grain size analysis of the soil,calculate Cu = D60/D10.
Cu = co-efficient of uniformityD60 = size in mm at 60% passingD10 = size in mm at 10% passing
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Step 4: Choose proper drainage aggregates
Step 3: Conduct the permeability test. In absence, useHazen’s formula
k = (D10)2 k = coefficient of permeability (cm/sec)
Step 5: Check the suitability of geotextile
a) Retention Criteria for Steady state flow condition:
O95 ≤ B. D85 (B = 1 for conservative design)
O95 = AOS = Opening size of the geotextile for which 95% are smaller (mm), B = Dimensionless coefficient, andD85 = Soil particle size for which 85% are smaller (mm).
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
The coefficient “B” varies between 1 and 2 depending on thevalue of uniformity coefficient, Cu.
For soil ≤ 50% passing the 0.075 mm sieve (i.e. sand andsilty sands etc.), ‘B’ value is a function of Cu as shown below.
Cu B
≤ 2 1
2 ≤ Cu ≤ 4 0.5 Cu
4 < Cu < 8 8 / Cu
≥ 8 1
‘B’ value as a function of Cu
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
With soil more than 50% passing the 0.075 mm sieve (i.e.silts and clays), ‘B’ depends on the type of geotextile.
B = 1, O95 ≤ D85 for woven geotextileB = 1.8, O95 ≤ 1.8 D85 for nonwoven geotextileO95 ≤ 0.3 mm for both woven and nonwoven geotextile
Nonwoven geotextile generally will retain finer particlesthan a woven geotextile of the same AOS. Therefore, B = 1will be more conservative for nonwoven geotextile.
In absence of detailed design, follow AASHTO M288standard specification for geotextiles (1997).
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Maximum AOS values in relation to percent of in-situ soilpassing the 0.075 mm sieve,
1. 0.43 mm for less than 15% passing
2. 0.25 mm for 15 -50% passing, and
3. 0.22 mm for more than 50% passing
If the plasticity index is greater than 7 for cohesive soils,O95 = AOS = 0.3 mm (maximum).
AASHTO M288 standard specification for geotextiles(1997):
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
b) Retention Criteria for Dynamic Flow:
AOS or O95 ≤ 0.5 D85
Step 6: Determine the permeability/ permittivity of geotextile.
Permeability:
For less critical and less severe applications,
kgeotextile ≥ 1 ksoil
For critical and severe applications,
kgeotextile ≥ 10 ksoil
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Permittivity:
In accordance with AASHTO T88, from the grain sizeanalysis, for percent in-situ passing 0.075 mm sieve,
Ψ ≥ 0.5 sec-1 for < 15% passing 0.075 mm
Ψ ≥ 0.2 sec-1 for 15 to 50% passing 0.075 mm
Ψ ≥ 0.1 sec-1 for more than 50% passing 0.075 mm
Ψ = Geotextile permittivityProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Step 7: Calculate flow capacity requirement
qrequired = qgeotextile / (Ag/At), or
(kgeotextile / tg) h Ag ≥ qrequired
(kgeotextile/ tg ) = Ψ = permittivity, tg = geotextile thickness
h = average head in field
Ag = geotextile area available for flow (i.e. if 70% ofgeotextile is covered by the wall of pipe, Ag = 30% of totalarea), and
At = total area of geotextileProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Step 8: Determine the clogging resistance criteria.
For less critical and less severe conditions,
O95(geotextile) ≥ 3 D15(soil) for Cu > 3
Nonwoven: Porosity (geotextile) ≥ 50%
Woven: Percent Open Area (POA) ≥ 4%
Most woven monofilaments geotextile can meet the abovecriteria. However, tightly woven slit film does not meet thecriteria and not recommended for sub-grade drainageapplications.
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
For critical/severe conditions,
Select the geotextiles that meet the retention andpermeability criteria.
Perform gradient ratio test (ASTM D5101) using on sitesoil samples. A gradient ratio less than 3 is recommendedby the U.S. Army Corps of Engineers (1977) with gapgraded soils.
This test is more suitable for sandy and silty soils withcoefficient of permeability (k) ≥ 10-7 m/s.
If k < 10-7 m/s., use hydraulic conductivity ratio (HCR)test (ASTM D 5567).
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
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Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay
Prof. J. N. Mandal
Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: cejnm@civil.iitb.ac.in
Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay