GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India.

Tel.022-25767328

email: cejnm@civil.iitb.ac.in

Module - 4 LECTURE- 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 CONTROL

 Conventional graded granular materials are used for filtration and drainage purpose in various projects of civil engineering. Various types of aggregate drains are available.

 The most common form of aggregate filled drains is the French drain which comprises a trench filled with free draining aggregates.

Conventional aggregate filled French drain

 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 fine particles from the base soil and consequently, the aggregates become blocked after some period having no adequate water removal capacity.

 The continued transmission of fines from the base soil to the drainage aggregates is called piping/ clogging. It also causes internal erosion of the base soil.

 The aggregates in a French drain should fulfill the following criterion so as to perform effectively.

 Permeability criteria

 Filtration criteria of base soil

 Uniformity criteria

 Special grading of aggregate is required based on the grading of base soil. The requirements for conventional graded filter design are as follows,

Piping criteria: D_15(filter) ≤ 5 D_85(soil)

Permeability criteria: D_15(filter)≥ 5 D_85(soil) Uniformity: D_50(filter)≤ 25 D_50(soil)

D₁₅ = diameter of soil particles at which 15% by dry weight of the soil particles are finer than that grain size D₈₅ = diameter of soil particles at which 85% by dry weight of the soil particles are finer than that grain size D₅₀ = diameter of soil particles at which 50% by dry weight of the soil particles are finer than that grain size

10 60

u D

c  D

 

60 10

2 30

c D x D

c  D

 Satisfying the drainage criterion for conventional graded filter design is extremely expensive.

 The conditions can easily and cheaply be achieved using a geosynthetic drainage system. It can perform both drainage and filtration.

 Drainage: Geosynthetic allows water to pass along its plane. Transmissivity.

 Filtration: Geosynthetic allows water to pass across its plane, but retain the soil particles. Permittivity.

PERMITTIVITY TRANSMISSIVITY

It occurs across the plane of geosynthetics

It occurs along the plane of geosynthetics

It is useful in filtration function

It is useful in drainage function

Unit is sec^-1 Unit is m²/sec

Geosynthetic for filtration

The filtration function of geosynthetic is illustrated through a simple example, where the liquid tea is filtered through a textile material.

Now a day, geosynthetics are extensively and successfully being used for filtration, drainage and erosion control systems alternative to the traditional granular materials.

Geosynthetic drainage materials alternative to traditional materials

Some differences between conventional aggregates and geosynthetic (CUR/RWS, 1995)

Property Aggregate Geosynthetic

Porosity 25-40 % 75-95 %

Thickness High (> 150 mm) Low (< 50 mm) Capillary rise (h_c) h_c < 500 mm h_c < 50 mm

Compressibility Negligible Medium to high

Tensile strength None Low to high

Transmissivity under

confining 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 alternative to graded granular filter.

 Design criterion for filtration with geotextile is the same as 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 proper opening sizes like the soil has particles and voids.

Grades granular material for filtration

Geosynthetic for filtration

 Soil with larger particle sizes or geotextiles with larger pore sizes allow higher flow rate. The soil with smaller particle sizes or geotextiles with smaller pore sizes allow lower flow rate.

 Soil filter design depends primarily on the size of soil particles. The geotextile filter design depends primarily on the opening sizes of geotextile.

Conventional drainage systems:

Roadway drainage systems

Railway drainage systems

Geosynthetics filter criteria:

 Adequate permeability: Allow the water to flow through the filter into the drain so as no excess hydrostatic pore pressure can build up.

 Retain the soil particles in place and prevent their migration (piping) through the filter.

 If some soil particles move, they must be able to pass through the filter without plugging or clogging.

 Open geotextiles allow the water to pass whereas closed geotextiles 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 reduced

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 fences

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

Geocomposite drainage behind retaining wall

Geosynthetic between earth bank (sub-grade) and rock protection (rip-rap or armour) for separation and erosion

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 than the larger soil particles, soil will not pass by the filter. As a result, a filter bridge will form over the geotextile and retain the soil particles or prevent piping (migration).

Filter bridge formation (After Christopher and Holtz, 1989)

2) If the smaller opening size of geotextile is larger than the size of smaller soil particles, the smaller particles can freely pass through the filter. As a result, the geosynthetic pores will not become clogged or blind

Method of clogging and blinding ( After Bell and Hicks, 1980)

3) Large number of openings in the geosynthetic would be preferable to maintain proper flow as some of the openings may become plugged.

Therefore, we require three criterion for the design of geosynthetics filtration or drainage systems:

 Retention criterion: The geosynthetics must retain the soil

 Permeability criterion: Allow water to pass

 Clogging resistance criterion: The geosynthetic-to-soil long-term flow compatibility should not excessively clog the fabric.

Subsurface Drainage

The geosynthetics can be used as subsurface drainage in pavements, retaining walls and earth dams etc. to replace the graded granular materials as filters in drain (FHWA, 1998).

Steps 1: Check the nature of the project, weather it is critical/ severe or less critical/ severe.

Step 2: Determine the grain size analysis of the soil, calculate C_u = D₆₀/D₁₀.

C_u = co-efficient of uniformity

D₆₀ = size in mm at 60% passing D = size in mm at 10% passing

Step 4: Choose proper drainage aggregates

Step 3: Conduct the permeability test. In absence, use Hazen’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, and D₈₅ = Soil particle size for which 85% are smaller (mm).

The coefficient “B” varies between 1 and 2 depending on the value of uniformity coefficient, C_u.

 For soil ≤ 50% passing the 0.075 mm sieve (i.e. sand and silty sands etc.), ‘B’ value is a function of C_u as shown below.

C_u B

≤ 2 1

2 ≤ C_u ≤ 4 0.5 C_u 4 < C_u < 8 8 / C_u

≥ 8 1

‘B’ value as a function of C_u

 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 geotextile

B = 1.8, O₉₅ ≤ 1.8 D₈₅ for nonwoven geotextile

O₉₅ ≤ 0.3 mm for both woven and nonwoven geotextile

 Nonwoven geotextile generally will retain finer particles than a woven geotextile of the same AOS. Therefore, B = 1 will be more conservative for nonwoven geotextile.

 In absence of detailed design, follow AASHTO M288 standard specification for geotextiles (1997).

Maximum AOS values in relation to percent of in-situ soil passing 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,

k_geotextile ≥ 1 k_soil

For critical and severe applications, k_geotextile ≥ 10 k_soil

Permittivity:

In accordance with AASHTO T88, from the grain size analysis, 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 permittivity

Step 7: Calculate flow capacity requirement

q_required = q_geotextile / (A_g/A_t), or (k_geotextile / t_g) h A_g ≥ q_required

(k_geotextile/ t_g ) = Ψ = permittivity, t_g = geotextile thickness h = average head in field

A_g = geotextile area available for flow (i.e. if 70% of geotextile is covered by the wall of pipe, A_g = 30% of total area), and

A_t = total area of geotextile

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 above criteria. However, tightly woven slit film does not meet the criteria and not recommended for sub-grade drainage applications.

For critical/severe conditions,

 Select the geotextiles that meet the retention and permeability criteria.

 Perform gradient ratio test (ASTM D5101) using on site soil samples. A gradient ratio less than 3 is recommended by the U.S. Army Corps of Engineers (1977) with gap graded soils.

This test is more suitable for sandy and silty soils with coefficient 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.022-25767328

email: cejnm@civil.iitb.ac.in