Technical Notes


Thrace-LINQ TECH NOTE #9
FILTRATION DESIGN:
A LOOK AT THE STATE OF THE PRACTICE

Introduction

In the past 15 years, Thrace-LINQ, Inc., has seen a massive transformation away from graded granular filters to soil/geotextile filter systems in the construction industry. This transformation in the geotechnical, transportation and environmental industries is due in large part because geotextiles are easier to install than graded granular filters, are quality controlled materials with controlled opening sizes, and are extremely cost effective compared to graded granular filters.

Geotextiles are the most versatile of all geosynthetic materials. Geotextiles can serve any of five geosynthetic functions. The five primary geosynthetic functions being separation, reinforcement, filtration, drainage and moisture barrier (if impregnated).

It is the goal of Thrace-LINQ to selectively present the state of the practice of filtration design criteria for soil/geotextile filter systems. Filtration with regard to geotextiles is defined as follows:
"the process of retaining soil in place while allowing water to pass from soil." (1)
The design criteria presented, will only utilize geotextile properties obtained from standardized test methods, that are generic to all geotextiles.

The Design Objectives Harry Cedergren points out in his book, Seepage Drainage and Flow Nets:
"Properly designed filters and drains are essential for the safety and economy of essentially all civil engineering facilities exposed to the damaging actions of water in their foundations or other supporting soil or rock formations......of great importance in drainage design is the need for developing systems capable of removing all the water that reaches them without excessive head buildup and without clogging or piping." (2)
Keeping the above comment in mind, we can establish the important objectives of our filtration criteria:

  • soil retention to control piping
  • sufficient water passage capability to handle excess hydrostatic pore pressure
  • long term effectiveness (ability of the filtration system to resist clogging)
  • survivability and durability of geotextile

These four objectives are critical to any filtration criteria, whether a soil/geotextile system or graded granular filter is used.

Soil Retention to Control Piping

Stable soils are composed of ranges of particle sizes, and if pore spaces in any filtration system are small enough to hold the 85% size (D85) of adjacent soils in place, the finer soil particles will also be held in place. (2) Soil retention is critical because if finer soil particles pass through the fabric, leaving larger soil voids behind, the liquid velocity increases until the soil structure begins to collapse (piping phenomena). (3)

It is vital that one completely understand the soil properties because the selection of an appropriate geotextile and overall filtration design is contingent upon the soil properties. Soils that are gap graded and/or cohesionless can be considered unstable and special filtration design criteria should be used. Unstable soils may require an additional soil filter used with an opened up geotextile (i.e., large percent open area for wovens) that will allow passage of some of the soil particles (leaving questions of piping once again). (3)

As outlined in his paper, "Filter Criteria for Geotextiles", Giroud's retention criterion is conservative and valid. His retention criterion, outlined in Table 1, takes into account various assumptions that need to be made: (4)

  • relevant soil stability properties are cohesion and density
  • soil particles are considered spherical and geotextile openings circular
  • steady-state flow is assumed
  • only soil particles directly in front of filtering openings and soil particles directly behind them are mobile

This logical retention standard depends on the relative density (ID) and the coefficient of uniformity (Cu) as determining factors in soil stability. Giroud's table is as follows: (4)

Table 1
Retention Criterion


  Linear Coefficient of Uniformity of the Soil

 

Relative Density 1 < Cu < 3 Cu 3

Loose Soil

ID < 35% O95 < Cud50 O95 < (9/Cu)d50

Medium Dense Soil

35% < Id < 65% O95 < 1.5Cud50 O95 < (13.5/Cu)d50

Dense Soil

ID > 65% O95 < 2Cud50 O95 < (18/Cu)d50

When Giroud graphs his retention criterion and compares it to the 1977 retention criterion developed by the Corps of Engineers (COE) where O95 < d85, two interesting problems surface:

  • for small values of Cu, the COE O95 > d85 criterion gives one too small a value for O95, possibly inadvertently increasing hydrostatic pore pressure
  • for large value of Cu, the COE O95 < d85 criterion gives one too large a value of O95, increasing the risk of piping

As Giroud pointed out when he presented his paper in Las Vegas, his retention criterion is probably too restrictive for highly cohesive soils. (5) However, cohesive soils are not usually problematic, and Giroud's retention criterion handles worst case (i.e., cohesionless) conditions. Also, the resultant AOS design value does not require and should not have a factor of safety. If one is unable to perform the necessary (and relatively inexpensive) soil property evaluations, one may look to a simplified soil retention criterion presented by Task Force 25 and part of the AASHTO M288 geotextile specification. (6)

  1. Soils with 50% or less particles by weight passing U.S. No. 200 Sieve: AOS Number of Fabric 30 sieve
  2. Soils with more than 50% particles by weight passing U.S. No. 200 Sieve: AOS Number of Fabric 50 sieve

An important area of concern is the use of the dry sieving test method, ASTM D-4751, to determine the minimum average roll value of the geotextile. This is the current, accepted test method to measure the O95 of the soil. The test method has several drawbacks. (3)

  • poor reproducibility (temperature, humidity, bead size variation, etc.)
  • trapping of beads within some geotextiles
  • test is only capable of providing O95 opening

Some current design methodologies have suggested using ASTM D-4751 to provide an AOS value that measures the smallest opening of the geotextile to provide an AOS range. The idea of this upper limit sieve size is to ensure that the permeability of the entire filtration system is maintained over time (prevent possible clogging). Using an upper limit sieve size for a water passage requirement is using the wrong criterion and test method. For water passage, one should use appropriate permittivity values; also an upper limit sieve size is beyond the scope of ASTM D-4751. ASTM D-4751 was originally a Corps of Engineers test method adapted to woven geotextiles where more uniform openings occur than nonwoven geotextiles. With any nonwoven's random pore size, ASTM D-4751 is unable to provide a clear picture of this pore spectrum. ASTM D-4751 is set up to provide the largest opening size of a geotextile, and loses its effectiveness as an index test method at about the 80 to 100 U.S. Standard Sieve. (7) Also, ASTM D-4751 is designed to give one a value for soil retention only.

If one is looking for movement of particles through a geotextile to promote performance, one is asking a fabric to perform as a filter itself, and this indefinite particle movement through the geotextile (selection most often results with a woven geotextile with a high percent open area) should only be considered where gap graded and/or cohesionless soils are present. With particle movement through the soil/geotextile filter system, one also needs to address upstream piping and downstream drain clogging. For long term performance, the filtering system development between a geotextile and an adjacent stable soil performs the filtering function for fine particle movement. From a design standpoint, one wants as many possible paths per unit area available for water to pass through the soil/geotextile system. The adjacent soil will compromise a percentage of these paths once the filtering mechanism is developed. From a conservative design approach, the more paths available per unit area, the healthier the filtration system over time. For soil retention criterion only, a designer should look to ASTM D-4751 to provide him with a relevant O95 of the geotextile to compare with his soil properties.

Sufficient Water Passage Capability

All voids in soils are connected to neighboring voids, making flow possible through the densest of natural soils. (8) In the 1850's H. Darcy came up with the following equation for fluid flow through soils:

Q = KiA
where
Q = the rate of flow K = coefficient of permeability i = the gradient A = the total cross sectional area of the sample
Darcy's equation is the foundation for determining sufficient water passage capability of a soil/geotextile filter system. At Harvard University (1940), Bertram with the advice of Terzaghi and Casagrande validated Terzaghi's filter design criterion: (2)

D15 (of filter) / D85 (of soil) <4 to 5> D15 (of filter) / D15 (of soil)

where the right half of the equation may be stated as follows:
The 15% size (D15) of a filter material should be at least four or five times the 15% size (D15) of protected soil.
This establishes a lower limit pore size for the filter system. However, Cedergren clearly points out that using a lower limit pore size is incorrect; filtration design must be analyzed hydraulically (Darcy's Law) to establish their capabilities for meeting discharge needs. (2)

The adjacent soil's hydraulic properties govern the initial and long term filtration behavior of a soil/geotextile filter system. Work performed by Koerner and Ko (9) and Wayne and Koerner (10) show that the soil's coefficient of permeability (K) determines the soil/geotextile filtration system's long term permeability. For applications deemed critical by the designer, soil permeability should be measured in accordance with ASTM D-5084. For non-critical applications, the following may be used. (11)

Table 2


Typical Soil Permeabilities
Soil Type Permeability Coefficient k (cm/sec)

Uniform, course sand

0.4

Uniform, medium sand

0.1

Clean well-graded sand and gravel

0.01

Uniform, fine sand

0.004

Well-graded, silty sand and gravel

0.0004

Silty sand

0.0001

Uniform silt

0.00005

Sandy clay

0.000005

Silty clay

0.000001

Clay

0.0000001

Colloidal clay

0.000000001

Selecting a Geotextile Permittivity

Having established the necessary soil and application parameters, one need now select an acceptable geotextile permittivity. Permittivity is defined as:
"for a geotextile, the volumetric flow rate of water per unit cross-section area, per unit head, under laminar flow conditions, in the normal direction through the geotextile." (1)
A geotextile's ultimate permittivity, Yult, can be determined using ASTM D-4491. This permittivity result is often misused to develop a geotextile permeability value dependent upon an irrelevant geotextile thickness value. (13) Bhatia (14), Rollin, Mlynarek and Bolduc (15), and others have commented on the complete lack of a role thickness plays in soil/geotextile filtration design. Nonwovens of various thicknesses exhibit similar frequent and variable openings that may be a consideration for a designer looking for good filtration. As Giroud points out,
"In practice, (permittivity) can be used with all types of geotextiles because permittivity is easy to measure for all geotextiles" (5)
J. Richard Bell (16), Koerner (3) and others also support the use of permittivity for design. Using Darcy's Law, Koerner has developed the following required soil permittivity (3):
Yr = q / Dh A
Where
Yr = required permittivity q = flow rate Dh = head loss A = area of fabric
One can determine the required permittivity using flow net analysis. Harry Cedergren provides one the basis for performing flow net analysis in his text, "Seepage, Drainage, and Flow Nets." The following equation is presented:
Yg F.S. Yr (3)
where
Yg = design allowable geotextile permittivity F.S. = Factor of safety Yr = required permittivity
As with other construction materials, the factor of safety is based on experience. A factor of safety of 5 is recommended for geotextile wrapped underdrains and erosion control structures. The required permittivity, where a long flow path exists through the soil, may not be accurate; however, the required permittivity at the point where the soil meets the geotextile is accurate. This required permittivity is the best tool for a geotechnical engineer to use when designing soil/geotextile filtration systems.

The reasons are twofold:

  • there really is no such thing as a permeability of a geotextile (a two-dimensional structure) that can be used in soil/geotextile filtration design equations.
  • the permeability of a soil is a prescribed constant based on an average head loss. The required soil permittivity at the soil/geotextile interface is a more accurate tool for soil/geotextile filtration design.

Long Term Effectiveness (Resistance to Clogging)

The long term performance of soil/geotextile filtration systems is critical. Quantifying long term performance is the big question mark. The ultimate long term performance concern is potential clogging of the geotextile. The best way of answering this concern is to do a long term filtration test (ASTM D-1987) with the native soil and the geotextile. Koerner and Ko (9) point out that once soil/fabric filter systems are established, the slope of the flow curve should be near zero. This near zero slope indicates stabilization of the soil/geotextile filter system. Recent work at the Geosynthetic Research Institute suggests using a drainage correction factor (DCF) along with long term flow tests where one compares the total area to the available area for flow into the drainage system. Long term flow tests are not always practical, but are the best available methods to determine long term effectiveness. If time is a concern, one may want to conduct a gradient ratio test (ASTM D-5101) that will compare a specific soil hydraulic gradient to the hydraulic gradient through the soil/geotextile system.

Promising new filtration design criteria based on pore size distribution that addresses both soil retention and clogging by Fischer, Christopher, Holtz (17) cannot be considered until standardized tests are established to determine the O15 and O50 of the geotextile. In general, after meeting the above outlined soil retention and water passage requirements, one must expect the geotextile to have a sufficient number of pores so that cross plane flow can be maintained even if a percentage of the pores become clogged during the life of the soil/geotextile filter system.

In the absence of long term flow tests and the gradient ratio test, this may be dealt with by asking for a minimum porosity of 30% for nonwovens and minimum 4% to 6% open area for wovens (3). Porosity is defined as the ratio of void volume to total volume, and Percent Open Area is a comparison of the total open area to the total sample area. It is important to note that needlepunched geotextiles' pore structure and porosity are sensitive to applied normal loads. Both porosity and percent open area are easily determined with available, standardized testing procedures. Neither porosity nor percent open area should be used alone to determine the filtration design requirements of the geotextile; neither address the uniformity of the geotextile. Without first addressing soil retention and water passage requirements, porosity and percent open area are meaningless.

Survivability and Durability of Geotextile

We have focused on the soil/geotextile filter system without regard to initial and long term stresses on the geotextile. Index strength values such as grab tensile (ASTM D-4632) and Trapezoid Tear (ASTM D-4533) should be selected such that the geotextile survives installation. AASHTO M288 has established survivability strength criteria among others for the general separation, drainage, and erosion control applications. (6)

Also, Koerner has established certain factors of safety that deal with soil clogging (FSsc), creep reduction of void space (FScr), intrusion into voids (FSint), and chemical (FScc) and biological clogging (FSbc) whose numerical value is determined by the application. (3) By applying the factors of safety to a geotextile's ultimate permittivity as determined by ASTM D-4491, one can develop a design allowable permittivity. Koerner's formula to determine a design allowable permittivity helps determine specific application requirements while also helping address the long term clogging issue. The equation is as follows:

Yg = Yult = 1 / (FSsc x FScr x FSint x FScc x FSbc)

The following default factors of safety are suggested (3):

Table 4
Default Factors of Safety


Application Soil Clogging and binding Creep Reduction of voids Intrusion into voids Chemical clogging Biological clogging

retaining wall filters

3 1.75 1.1 1.1 1.15

underdrain filters

3 1.25 1.1 1.35 1.35

erosion control filters

3 1.25 1.1 1.1 1.35

landfill filters

3 1.75 1.1 1.35 2.25

gravity drainage

3 2.5 1.1 1.35 1.35

pressure drainage

2.5 2.5 1.1 1.2 1.2

Durability of geotextiles over time is another important consideration for long term effectiveness. For example, polypropylenes and polyesters are long chain polymers where structural damage can be initiated by heat and/or light with time by the development of free radicals. The presence of oxygen is a fundamental requirement for breakdown to occur. Oxygen content in soils is quite low and continues to diminish exponentially the deeper one goes below the surface. Current stabilizer packages such as HALS (hindered amine light stabilizers) can effectively quench any free radical development. Also, extremely high pH levels (pH > 10) can have deteriorating effects on some polymer families. Fortunately, high pH levels in soils are rarely seen, and excavations of sites utilizing geotextiles for over twenty years have shown little to no polymer degradation. To ensure one addresses geotextile durability, one should keep exposure to ultraviolet rays to a minimum (for example, cover geotextile within 14 days), and be familiar with a geotextile's stabilizer package.

Table 5


Existing Geotextile Retention Criteria (19)
Source Criterion Remarks

Calhoun

O95/D85 1 Wovens, soils with <50% passing #200 sieve
O95 0.2 Wovens, cohesive soils

Zitscher

O50/D50 1.7 - 2.7 Wovens, soils with Cu 2, D50 = 0.1 to 0.2 mm
O50/D50 25 - 37 Nonwovens, cohesive soils

Ogink

O90/D90 1 Wovens
O90/D90 1.8 Nonwovens

Sweetland

O15/D85 1 Nonwovens, soils with Cu = 1.5
O15/D15 1 Nonwovens, soils with Cu = 4.0

Rankilor

O50/D85 1 Nonwovens & soils with 0.02<D85<0.25
O15/D15 1 Nonwovens & soils with D850.25

Schober and Teindl

O90/D50 2.5 - 4.5 Wovens and thin nonwovens, Cu dependent
O90/D50 4.5 - 7.5 Thick nonwovens, Cu dependent

Millar, Ho & Turnbull

O50/D85 1 Wovens & nonwovens

Giroud

O95/D85 [(9-18)/Cu] Dependent on Cu of soil and assumes fines will migrate for large/low Cu soils

Carroll

O95/D85 2-3 Wovens & nonwovens

Christopher & Holtz

O95/D85 1 -2 Dependent on Cu and soil type
O95/D15 1 If dynamic soil condition
O50/D85 0.5 where cyclical flow conditions exist

French Committee on GT and GM

Of/D85 0.38 - 1.25 Dependent on soil type, compaction, and hydraulic conditions of the site

Fischer, Christopher and Holtz

O50/D85 0.8 O50/D15 1.8 - 7.0 O50/D50 0.8 - 2.0 Based on geotextile pore size distribution and dependent on Cu of the soil

where:
O95 = 95% opening size of geotextile O50 = 50% opening size of geotextile D85 = grain size in millimeters of 15 percent finer by weight Cu = D60/D10

Table 6


Existing Geotextile Water Passage Criteria (19)
Source Criterion Remarks

Calhoun, Schober and Teindl, Wates, Carroll, Haliburton, et.al., Christopher and Holtz

kf ks steady state flow condition, noncritical application, and nonsevere soil condition.

Carroll, Christopher and Holtz

kf 10 ks critical applications and severe soil or hydraulic conditions

Giroud

kf iks upgraded criteria to include gradients

French Committee on Geotextiles and Geomembranes clean sand, 1 x 103

Based on permittivity Y with Y > (1x 103 to 1x 105) ks critical condition, 1 x 105 noncritical condition, 1 x 104

Koerner

Yallowable FSxYrequired factor of safety (FS) based on application and soil conditions

where:
kf = Permeability of the geotextile ks = Permeability of the soil

Table 7


Clogging Criteria (19)
Application Suggestion Source

Critical or Severe Condition

Perform soil/geotextile filtration tests Calhoun, Haliburton, et.al., Giroud, Carroll, Christopher and Holtz, Koerner

Less Critical or Non Severe Condition  

Perform soil/geotextile filtration tests Haliburton, et.al.
Spec. min pore size O95 3D15 for Cu 3 Christopher and Holtz
Of 4D15 French Committee on GT and GM
O15/D15 0.8 to 1.2 Fischer, et.al.
O50/D50 0.2 to 1.0 Fischer, et.al
Spec. min open area For woven GT: POA 4% to 6% Calhoun and Koerner
For nonwoven GT: Porosity 30% to 40% Christopher and Holtz & Koerner

where:

O95 = 95% opening size of geotextile O50 = 50% opening size of geotextile O15 = 15% opening size of geotextile Of = average opening size of fabric or geotextile D15 = grain size in millimeters of 15 percent finer by weight D50 = grain size in millimeters of 50 percent finer by weight Cu = D60/D10 POA = Percent Open Area

Table 8


AASHTO M288 Separation Strength Requirements
Property Test Method High Survivability Level Medium Survivability Level
Grab Strength (lbs.) ASTM D 4632 270/180 180/115
Elongation (%) ASTM D 4632 <50% / >50% <50% / >50%
Seam Strength (lbs.) ASTM D 4632 240/160 160/105
Puncture Strength (lbs.) ASTM D 4833 100/75 70/40
Trapezoid Tear (lbs.) ASTM D 4533 100/75 70/40

IMPLEMENTING THE STATE OF THE PRACTICE
DESIGN EXAMPLES


A Typical Underdrain

We need to design a trench drain that consists of a geotextile, open graded aggregate surrounding a 4" perforated pipe along a section of paved roadway with an unpaved shoulder. The trench drain will be directly beneath an 18" aggregate layer. The native soil is a sandy silt with a Cu of 6, a D50 of .1 mm and a K = 7.5 x 10-5 cm/sec. The most permeable section of subgrade aggregate is an open graded stone (OGS) and has a flow capacity of 1500 ft/day, or .017 ft/sec. The geotextile will also act as a permeable separator underneath the entire roadway.

A) Check soil retention

Using Giroud's retention criterion, we look at Table 1.
We assume a medium dense, compacted native soil, Id 60% O95 < (13.5/Cu) d50 O95 < (13.5/6) .1 O95 < .23 mm or approximately #70 sieve
B) Check water passage focusing on subbase aggregate,
Yreqd = q / Dh A Yreqd = .017 / [(1.5) (1 x 1)] = .01 sec-1
to determine the design allowable permittivity of the geotextile,
Yg 5 Yreqd Yg = .05 sec-1
to determine what is an acceptable Yult of geotextile as determined by ASTM D-4491,
Yg Yult = 1 / FSsc x FScr x FSint x FScc x FSbc
For suggested factors of safeties, refer to Table 4.
Yg Yult = 1 / 3 x 1.25 x 1.1 x 1.35 x 1.35 Yult Yg = (7.52) Yult .376 sec-1 (round up to .4 sec-1)
C) Check long term effectiveness (resistance to clogging):
Method #1: Perform long term flow tests using ASTM D-1987 with site specific soils. Method #2: Perform ASTM D-5101 Gradient Ratio test. We have time so we perform long term flow test with potential geotextiles FSfilter clogging = Ksystem allowable / (Krequired) (DCF) determine Ksystem allowable from test (Ktest) Krequired = from highway overland flow model or flow net analysis DCF = drainage correction factor where, DCF = total flow area / drain flow area DCF for highway drains is in the range of 1-10 so, DCF = 10 Ktest = 2 x 10-3 cm/sec (hypothetical) FSfilter clogging = 2 x 10-3 / (7.5 x 10-5) (10) FSfilter clogging = 2.7 OK, Also, slope of flow curve is near zero(4), OK
D) Check survivability and durability criteria:

Project installation requirements:

    • subbase aggregate will be compacted to required density with vibratory rollers
    • open graded aggregate backfill for edge drain is angular stone less than 1"
    • maximum drop height is 1 foot
    • compaction equipment for edge drain is lightweight
    • trench is 18" in depth

So, use the AASHTO criteria for medium survivability(Table 8), incorporating UV exposure requirements. Final specification requirements for geotextile:

The geotextile for the road underdrain trench shall meet either of the following minimum average roll value requirements:

Table 9


Property ASTM Method Property Value
Grab Elongation (%) ASTM D-4632 50 <50
Grab Tensile (lbs.) ASTM D-4632 115 180
Trapezoid Tear (lbs.) ASTM D-4533 40 70
Puncture (lbs.) ASTM D-4833 40 70
AOS (U.S. standard sieve) ASTM D-4751 #70 #70
Permittivity (sec-1) ASTM D-4491 0.4 0.4
Note: Product must be covered within 14 days of placement (reduce UV exposure).(20) Product shall be Thrace-LINQ 140EX or equivalent. An Erosion Control via Rip-Rap Structure A designer wants to incorporate a geotextile into his erosion control structure. The structure will be sloped 3/1, and wave action forces will be present. The geotextile will be placed directly over the native soil (silty sand) where Cu = 5.5, the d50 is estimated to be .17 mm, the DR = 50%, and the porosity is .3. A 6 inch layer of 2" aggregate is going to be placed directly on top of the geotextile to serve as a pore water dissipator and as protection for the geotextiles during rip-rap placement. We'll assume 3 foot tides are present. A) Check soil retention using Giroud's retention criterion (Table 1):
O95 < (13.5/Cu) d50 O95 < (13.5/5.5) .17 O95 < 0.42 mm or approximately #40 sieve
B) Check water passage The 3 foot tidal lag is the critical event to check water passage capability (reversing flow conditions). Therefore, to estimate the maximum flow rate during a 1 hour period due to the tidal lag, refer to the water profile in Figure 1.
qmax = [(150 x 3 x 1) / 1] x .3 = 135 ft/hr-ft. slope qmax = 0.0375 ft/sec-ft. slope Yreqd = q / DHA = .0375 / (3 (9.49x1)) Note: 9.49 is the affected area of geotextile Yreqd =1.3 x 10-3 sec-1 per foot of slope Yg 5 Yreqd
referring to Table 2,
Yg 5 (1.3 x 10-3) Yg 0.007 sec-1 (round to .01)
to determine an acceptable Yult of geotextile as determined by ASTM D-4491, refer to Table 4:
Yg Yult = 1 / FSsc x FScr x FSint x FScc FSbc Yg Yult 1 / 3 x 1.25 x 1.1 x 1.1 x 1.35 Yult Yg (6.13) Yult (.01 sec-1) (6.13) Yult .061 sec-1 (round to .1)
C). Check long term effectiveness (resistance to clogging): There is no time for long term flow tests and no funding available for the gradient ratio test. We look at the soil which is determined to be somewhat cohesionless, so we add porosity or percent open area (POA) as an additional requirement to prevent clogging.
for nonwovens, porosity 30% for wovens, POA 4%
Note: While this situation is unfortunately the norm, these suggested default values for porosity and POA are the least effective ways of answering long term effectiveness concerns.

D) Check survivability and durability criteria.

Project installation requirements:

  • 6" of 1"- aggregate will protect geotextile from rip-rap placement
  • maximum drop height of rip-rap is 3 foot
  • rip-rap weight will average 250 lbs
  • small construction equipment will be used sparingly during installation
  • subgrade fairly firm when saturated

So, use the AASHTO criteria for medium survivability(Table 8), incorporating UV exposure requirements. Final specification requirements for geotextile: The geotextile for the erosion control structure shall meet the following minimum average roll value requirements:

Table 10


Property ASTM Method Property Value
Grab Elongation (%) ASTM D-4632 50 <50
Grab Tensile (lbs.) ASTM D-4632 115 180
Trapezoid Tear (lbs.) ASTM D-4533 40 70
Puncture (lbs.) ASTM D-4833 40 70
AOS (U.S. standard sieve) ASTM D-4751 #40 #40
Permittivity (sec-1) ASTM D-4491 0.1 0.1

Note: Product must also meet either a minimum porosity of 30% (nonwovens) or have a minimum percent open area of 4% (wovens). Product must be covered within 14 days of placement (reduce UV exposure)

Product shall be Thrace-LINQ 140EX, GTF-400EO or equivalent.

Conclusions

With the transformation away from graded granular filters to soil/geotextile filter systems, engineers have had to familiarize themselves with a new construction product as well as a new set of design criteria. Executing a complete geotextile design for filtration involves a four step process of designing for retention, water passage capability, long term effectiveness and survivability and durability of a candidate geotextile. Contact your Thrace-LINQ technical representative at 1-800-543-9966 for assistance.


Bibliography
(1) A Design Primer: Geotextiles and Related Materials, Industrial Fabrics Association International, St. Paul, MN, 1992, pp. A-2, A-4.
(2) Cedergren, Harry R., Seepage, Drainage, and Flow Nets, John Wiley & Sons, New York, 1989, pp. 151, 154, 156, 165.
(3) Koerner, Robert M., Designing with Geosynthetics, Prentice-Hall, Englewood Cliffs, NJ, 1990, pp. 51-52, 86, 121, 123, 219-220.
(4) Giroud, Jean-Pierre, "Filter Criteria for Geotextiles", Second International Conference on Geotextiles, Las Vegas, NV, 1982, pp. 103-108.
(5) Giroud, Jean-Pierre, Geotextiles and Geomembranes: Definitions, Properties and Design, Industrial Fabrics Association International, 1984, pp. 37-38.
(6) Standard Specification for Geotextiles: AASHTO M288-90, Federal Highway Administration, 1990, pp. 689-692.
(7) Hoover, Thomas P., Nonwoven Geotextile Fabrics: Evaluation and Specification for Subdrainage Filtration, California Department of Transportation, Report No. FHWA/CA/TL-81/11, Sacramento, CA, 1981, pp. 9.
(8) Lambe, T. William and Whitman, Robert V., Soil Mechanics, John Wiley & Sons, New York, 1969, pp. 251.
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(10) Wayne, Mark H. and Koerner, Robert M., "Correlation Between Long Term Flow Testing and Current Geotextile Filtration Design Practice", Geosynthetics '93 Conference Proceedings, Volume 1, Industrial Fabrics Association International, St. Paul, MN, 1993, pp. 501-517. (11) Jumikis, Alfreds R., Thermal Geotechnics, Rutgers University Press, New Brunswick, NJ, 1977, pp 71.
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