# ASTM D7178-16e1

Designation: D7178 − 16´1Standard Practice forDetermining the Number of Constrictions “m” of Non-Woven Geotextiles as a Complementary Filtration Property1This standard is issued under the fixed designation D7178; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.ε1NOTE—A legend was added editorially to Eq X1.4 in X1.1.2.2 in July 2016.1. Scope1.1 This practice describes the procedure used along withexisting test method to determine the number of constrictionsm of mechanically bonded non-woven geotextiles, based onthickness, mass per unit area and fiber properties1.2 The number of constrictions is a property of non-wovengeotextiles, which is complementary to opening size to predicttheir filtration behavior. It can be used to differentiate non-woven geotextiles with similar opening sizes but differentstructures (thickness, weight, fiber diameter, etc.). However,more research is needed to assess its significance whencomparing two products with different opening sizes.1.3 Consideration of the number of constriction is relevantin filtration applications where piping or clogging concerns areto be controlled with a high level of confidence, that is, forfilters applications in critical soils.1.4 This standard is for design purposes only and is notintended for quality control purposes.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D1577 Test Methods for Linear Density of Textile FibersD4439 Terminology for GeosyntheticsD4751 Test Method for DeterminingApparent Opening Sizeof a GeotextileD5199 Test Method for Measuring the Nominal Thicknessof GeosyntheticsD5261 Test Method for Measuring Mass per Unit Area ofGeotextilesD6767 Test Method for Pore Size Characteristics of Geotex-tiles by Capillary Flow Test3. Terminology3.1 Definitions—For definitions of terms used in this testmethod, refer to Terminology D4439.3.2 Definitions of Terms Specific to This Standard:3.2.1 constriction—in the non-woven geotextile, a “win-dow” delimited by three or more fibers, through which soilparticles could migrate.3.2.2 filtration paths—under the forces induced by fluidflows, soil particles may travel in the geotextile filter alongfiltration paths. Each of these paths is composed of a sequenceof constrictions of various size and shape.3.2.3 number of constrictions, m—average number of con-strictions for a filtration path.3.2.4 fiber count, T—a measure of the linear density of thefiber expressed in tex, where 1 tex = 10-6kg/m.3.2.4.1 Discussion—The fiber count is sometime expressedin “Denier” (equivalent to the weight in grams of a theoretical9000 meters long fiber). The value in “Tex” can be obtainedfrom the value in Denier considering that 1 Denier = 9 Tex.3.2.5 opening size—largest significant opening of a non-woven geotextile as measured using Test Method D6767.NOTE 1—Although Test Method D4751 (Apparent Opening Size) iswidely used to characterize geotextiles, it may often not be sufficient foradvanced filtration investigations such as those requiring consideration ofthe number of constriction as a significant parameter. The “bubble-point”measurement technique proposed in Test Method D6767 shall thus bepreferred to AOS per Test Method D4751.4. Summary of Practice4.1 The physical properties of the geotextile are evaluatedaccording to specific procedures and the number of constrictionm is determined based on Eq 1.1This practice is under the jurisdiction of ASTM Committee D35 on Geosyn-thetics and is the direct responsibility of Subcommittee D35.03 on Permeability andFiltration.Current edition approved June 1, 2016. Published June 2016. Originallypublished 2006. Last previous version approved 2011 as D7178 – 06 (2011). DOI:10.1520/D7178-16E01.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States15. Significance and Use5.1 This practice provides a calculation method for deter-mining the number of constrictions m of a non-woven geotex-tile (or of a layer of a composite material). This standard is notapplicable to woven geotextiles, knitted geotextiles, heat-bonded geotextiles or any other type of geosynthetic.5.2 The number of constriction represents the number of“windows” delimited by three or more fibers, in which soilparticles could migrate. In that regard, it can be basicallydefined by the following equation: m5tdcwhere t is thethickness and dcthe average distance between two constric-tions. This value has been found to be relevant to explain thedifferent filtration behaviors of non-woven geotextiles withsimilar opening sizes but different structures for various soilconditions (see Appendix X1 for details).5.3 This value will be used in filtration research to evaluatethe prediction of filtration efficiency and effectiveness ofvarious non-woven geotextiles with similar opening sizes (TestMethod D6767).5.4 Interpretation of the significance of m as calculatedusing this standard shall be done with care as some non-wovenstructures may not reflect the hypothesis used to establish theproposed equation (see Appendix X1 for details).6. Procedure6.1 Condition specimens at 23 6 2°C and 65 % relativehumidity for not less than 24 h.6.2 Determine the mass per unit area and thickness of thegeotextile according to Test Methods D5261 and D5199.NOTE 2—Although the thickness of non-woven geotextiles is influencedby the normal load, the number of constriction shall be calculatedconsidering the geotextile thickness under 2 kPa for standardizationpurpose. Practically, the number of constriction is not influenced by thethickness as it represents the structure of the non-woven (number of“windows” delimited by three or more fibers, in which soil particles couldmigrate as defined in 5.2), which does not depend on the normal load.6.3 Determine the fiber count of the fibers per Test MethodsD1577 using the data available from MQA or supplierscertificate. Report values by classes of average fiber countassociated to the type of polymer as well as the percentage ofeach class found in the geotextile sample (that is, respectivepercentages of polypropylene / xx tex, polypropylene / yy tex,polyester / zz tex, etc.).6.3.1 Calculate the number of constrictions m of the geo-textile using Eq 1 (dimensionless value). Result must berounded to the closest unit.mi5123 =π·µi·ti3(kpk·Œ1Tk(1)where:i = specimen number,mi= number of constriction for the geotextile specimen i,µi= mass per unit area of the geotextile specimen i (g/m2)as measured in 6.2,ti= thickness of the geotextile specimen i (mm) as mea-sured in 6.2,k = class of fibers with a given fiber count and type ofpolymer,pk= percentage (in decimal unit, that is, 10 % = 0.1) offibers from a class of fibers k, andTk= fiber count (tex) associated to a class of fibers k asmeasured in 6.3.NOTE 3—If the fiber count Tkis available in Denier, divide the availablevalue by 9 to obtain the value in Tex.7. Report7.1 Report the following information:7.1.1 State that the number of constriction was calculated asdirected in Practice D7178;7.1.2 Complete geotextile identification;7.1.3 Statement of conditioning;7.1.4 Thickness and mass per unit area of the geotextile:individual values and average;7.1.5 Fiber count distribution / polymer as evaluated in 6.3.If it was assumed that only one type of polymer was used tomanufacture the geotextile, state this on report;7.1.6 Number of constriction as calculated in 6.3.1; and7.1.7 Report any deviation from the described standardpractice.8. Precision and Bias8.1 Precision and bias has not been established yet.9. Keywords9.1 clogging; constriction; filtration; geotextile; mechani-cally bonded; non-woven; number of constrictions; openingsize; pipingD7178 − 16´12APPENDIX(Nonmandatory Information)X1. PHYSICAL SIGNIFICANCE OF THE NUMBER OF CONSTRICTIONX1.1 Alternate Equations Describing the Number ofConstrictionX1.1.1 The number of constriction represents the number of“windows” delimited by three or more fibers, in which soilparticles could migrate. In that regard, it can be basicallydefined by the following equation:m 5tdc(X1.1)where:t = the geotextile thickness, anddc= the average distance between two constrictions.X1.1.2 Given that the average distance between two con-strictions can be expressed by the following equation (1):3dc5df=1 2 n(X1.2)X1.1.2.1 The porosity of the non-woven geotextile by:n 5 1 2µρ·t(X1.3)where:µ = the mass per unit area,t = the thickness of the non-woven geotextile, andρ = the polymer specific gravity.X1.1.2.2 Another expression of the number of constrictionof non-woven geotextiles is as follows:m 5 Œµ·tρ·dF2(X1.4)where:dF= fiber diameter.X1.1.3 Fiber count expressed as T5Sπ·dF24D·ρ, it is alsopossible to define the number of constriction of non-wovengeotextiles as follows:m 5 Œπ·µ·t4T(X1.5)where:π = 3.14,µ = mass per unit area,t = thickness, andT = fiber count.X1.2 Constrictions and Filtration Opening Size Distribu-tionsX1.2.1 A conceptual definition of the constrictions andfiltration opening size distributions of all the filtration paths ofa geotextile is presented in Fig. X1.1 (2). Given that thefiltration openings are defined as the smallest constrictions ofthe filtration paths, their distribution in size will be proportion-ally smaller than the constrictions one. By definition, thesmallest opening size (O0) could not be smaller than thesmallest constriction (C0).X1.3 Significance of the Number of Constrictions withRegard to Opening Size Distribution and Soil Fil-trationX1.3.1 Non-woven geotextiles filters having similar Filtra-tion Opening Size (which may be considered equal to O100) butdifferent constriction numbers may exhibit significantly differ-ent filtration behavior for specific situations.X1.3.2 Fig. X1.2 (2) presents the constrictions and openingssizes distributions of two geotextiles having an identicalopening size (O100), but where the number of constrictions offilterAis smaller than the one of filter B.As a consequence, themaximum constriction size C100of filter A is also smaller thanthe one of filter B.X1.3.3 Using the constriction and opening size distributioncurves, it is possible to estimate the probability that a given soilparticle will be retained in or on the geotextile, or be piped:X1.3.3.1 For a soil particle diameter d2larger than the filteropening size O100, the probability to be trapped into thegeotextile increases as the number of constrictions increases.So filter B is more likely to trap soil particles with a diameterd2, while these particles will be retained on the surface of filterA. If the particle particles with a diameter d2is part of the soilskeleton, this skeleton will not be stable, the particles closer tothe filter being likely to move downward into the geotextilestructure. For two geotextiles presenting the same opening sizebut different numbers of constrictions used to filter a soil madeof a skeleton and movable particles, the geotextile presenting ahigh number of constrictions could thus lead to an unstable3The boldface numbers in parentheses refer to the list of references at the end ofthis standard.C: constrictionsO: openingsFIG. X1.1 Constriction and Filtration Size Distribution Curves (2)D7178 − 16´13behavior of the soil, the soil skeleton integrity being notproperly supported in critical situations and/or soil / geotextilecombinations.X1.3.3.2 For a soil particle diameter d1lower than thegeotextile opening size O100, the probability to be washed outthrough the geotextile increases as the m value increases. Onthe other hand, this increased probability to be retained on alow m geotextile compared to a high m geotextile couldeventually lead to the development of the blinding mechanismin critical situations and/or soil / geotextile combinations.X1.3.4 At the time of preparation of this standard, there wasno general agreement regarding the limits that shall be consid-ered for filtration design.REFERENCES(1) Giroud, J. P., “Granular Filters and Geotextile Filters,” KeynoteLecture, Proceedings of Geofilters ’96, Montréal, Québec, Canada,Ecole Polytechnique Montréal, 1996, pp. 565-680.(2) Giroud, J. P., “Geotextile Filters: Reliable Design or through the ASTM website(www.astm.org). Permission rights to photocopy the standard may also be secured from the Copyright Clearance Center, 222Rosewood Drive, Danvers, MA 01923, Tel: (978) 646-2600; http://www.copyright.com/D7178 − 16´15