geosynthetics engineering: in theory and … 15.pdf · geosynthetics engineering: in theory and...

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Module - 4LECTURE- 15

Geosynthetics for filtrations, drainages and erosion control systems


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

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.


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)


D15 = diameter of soil particles at which 15% by dryweight of the soil particles are finer than that grain size





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.


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


Geosynthetic for filtration

The filtration function of geosynthetic is illustrated through asimple example, where the liquid tea is filtered through atextile material.


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


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


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.


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.


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.


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):


Geosynthetic wrapped trench drain beneath pavement edge drain


Geosynthetics in drainage blanket


Geosynthetic as chimney drain and toe drain in earth-rock fill dam to control seepage


Geosynthetics wraps around interceptor, surface and toe drain to control surface erosion and provide stable slopes


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


Traditional drainage system replaced by geosynthetics sheet drain


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.


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


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).


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


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).


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):


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


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.


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).


Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


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