By YUCHENG SHI B.Sc, Nanjing Institute of Architectural Engineering, China, 1984 M.A.Sc., Tongji University, China, 1987

Transcription

1 FILTRATION BEHAVIOUR OF NON-WOVEN GEOTEXTILES IN THE GRADIENT RATIO TEST By YUCHENG SHI B.Sc, Nanjing Institute of Architectural Engineering, China, 1984 M.A.Sc., Tongji University, China, 1987 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CIVIL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1993 Yucheng Shi, 1993

2 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or he representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. (Signature) Department of Civil Engineering The University of British Columbia Vancouver, Canada Date )) 0 ) \cl a DE-6 (2/88)

3 Abstract Tests performed in a permeameter have been used to evaluate the Gradient Ratio (GR) test method for steady unidirectional flow through non-woven geotextiles.the test device was modified to include two additional manometer ports that better define the water head distribution the soil,and a energy dissipator that was mounted to prevent disturbance on the top of the soil sample by inlet water at large flow rates. A water pluviation technique was used for reconstitute homogeneous, saturated samples for uniform soils at any targeted density; a slurry deposition technique was used for broadly graded soils. A commercial algaecide liquid was added to, the circulating water to eliminate biological growth. Some physical blinding may develop on the top surface of the soil samples due to fines in the recirculating water, and approaches are reported that mitigate this action. A modified definition of the gradient ratio is proposed. A series of tests was performed on four uniform soils, which have a D85 from 70 Am, with four nonwoven geotextiles, which have a filtration opening size (FOS) from 60 Am to 150 Am. No observations of impending clogging were made, however piping occurred in some tests. A ratio of FOS/D85 < 1.5 has been found to be appropriate effective retention of soils without significant piping. A series of tests was also performed on broadly graded soils, which have a D85 from 57 m to 1400 m and a silt content from 3% to 90%, with one geotextile. Gradient ratio values grater than one were observed, which imply the soilgeotextile composite permeability was less than that of the soil alone. This is attributed to some minor clogging of the geotextile. Soil retention criteria are performed, based on ii

4 geotextile. Soil retention criteria are proposed based on the experimental results, in terms D15 and D50 of the soil and the D5of which is that of the fraction less than the FOS of the geotextile. Tests performed for more than 100 hours show both the gradient ratio and soil permeability are constant at each imposed hydraulic gradient, and it is concluded that a test duration of 24 hours at each gradient is adequate to characterize soil-geotextile performance. Finally, a comprehensive relationship is established that unifies observations of water head distribution, gradient ratio, permeability of the soil and permeability of the soilgeotextile composite. The unified relationship leads to the identification of three zones that describe the distribution of water head in a gradient ratio test. iii

5 Table of contents Abstract ^ ii List of Tables ^ ix List of Figures ^ xi Acknowledgement ^ xv Chapter 1 Introduction ^ Types of geotextile^ Applications of geotextiles in construction ^ Requirements of a geotextile filter ^ Objectives and scope of the study ^ Organization of the thesis ^ 5 Chapter 2 Literature Review ^ Properties of geotextile ^ Pore size opening ^ Direct method ^ Indirect methods ^ Dry sieving ^ Wet sieving ^ Theoretical method ^ 11 iv

6 2.1.2 Permittivity lk ^ Thickness ^ Mass per unit area ^ Filter criteria of geotextile ^ Retention criteria ^ Permeability criteria ^ Clogging resistance criteria ^ Laboratory tests for soil-geotextile compatibility ^ Gradient Ratio (GR) test ^ Long Term Flow (LTF) test ^ Hydraulic Conductivity Ratio (HCR) test ^ Fine Fraction Filtration (F 3) test ^ Dynamic Filtration (DF) test ^ Summary ^ 33 Chapter 3 Gradient Ratio Test Device ^ The apparatus ^ Constant head devices (CHD) ^ Energy dissipator ^ Manometer ports and mounting board ^ Water circulation system ^ Geotextile specimen size ^ 39

7 3.7 Soil sample dimension ^ Some Definitions and calculations ^ Hydraulic gradient ^ Gradient ratio ^ Gradient ratio definition ^ Comparison between ASTM and UBC definitions ^ Permeability ^ Relationship between gradient ratio and permeability ratio ^ 43 Chapter 4 Properties of test materials ^ Introduction ^ Materials used in preliminary tests ^ Properties of soils ^ Properties of geotextiles ^ Properties of uniform soils and geotextiles ^ Properties of soils ^ Properties of geotextiles ^ Properties of broadly-graded soils and geotextile ^ Properties of broadly-graded soils ^ Properties of geotextile ^ 53 Chapter 5 Sample preparation and test procedure ^ 51 vi

8 5.1 Permeameter preassembly ^ Soil sample preparation ^ Saturating the soil ^ Initial height reference ^ Soil placement ^ Top levelling ^ Densification and relative density control ^ Test procedure ^ Permeameter assembly and setup ^ Test operation ^ 57 Chapter 6 Preliminary Tests and Results ^ Introduction ^ Test results and discussion ^ Summary ^ 74 Chapter 7 Filtration Behaviour of Uniform Soils ^ Introduction ^ Water head distribution ^ Permeability of the soil and the system ^ Gradient ratio ^ Soil retention ^ 85 vii

9 7.6. Summary ^ A tri-linear model of water head distribution ^ Geotextile filter design criteria ^ 88 Chapter 8 Filtration Behaviour of broadly graded Soils ^ Introduction ^ Water head distribution ^ Permeability of the soil and the system ^ Gradient ratio ^ Soil retention ^ Summary ^ A tri-linear model of water head distribution ^ Geotextile filter design criteria ^110 Chapter 9 Summary and Conclusions ^ Soil sample preparation and test procedure ^ Interpretation of the gradient ratio test ^ Validation of geotextile filter design criteria ^115 References ^ 118 viii

10 List of Tables Table 2.1 A summary on soil retention criteria (modified after Christopher and Fisher, 1992) ^ 16 Table 2.2 A summary of permeability criteria (modified after Christopher and Fisher, 1992) ^ 19 Table 2.3 Laboratory test methods used to assess the soil-geotextile filtration compatibility ^ 33 Table 4.1 The properties of soils used in the preliminary tests ^ 46 Table 4.2 The properties of geotextiles used in the preliminary tests ^ 47 Table 4.3 The properties of uniform soils ^ 48 Table 4.4 The properties of geotextiles tested with uniform soils ^ 49 Table 4.5 The properties of broadly graded sands ^ 50 Table 4.6 The properties of broadly graded silts ^ 52 Table 7.1 Test program on uniform soils ^ 71 Table 7.2 GRunc at 24 hours for each system gradient ^ 83 Table 7.3 Mass of soil passing through geotextile and FOS/D 85 ratio ^ 86 Table 7.4 A comparison of established soil retention criterion with some notable criteria ^ 88 Table 8.1 Test program on the broadly graded soils ^ 94 Table 8.2 GR ubc at 24 hours for each system gradient ^ 107 ix

11 Table 8.3 Mass of soil passing through geotextile and FOS/D 85, FOS/D 50 and FOS/D 15 ratios ^ 108 Table 8.4 Mass of soil passing through geotextile and FOS/D 5of ratio ^109 Table 8.5 A comparison of established soil retention criterion for broadly graded soils with some currently used criteria ^112 x

12 List of Figures Fig.2.1 Scheme of Filtration Opening Size (FOS) apparatus in EPU (After Rollin et al, 1990) ^ 10 Fig.2.2 Filter formation (After Koerner, 1990) ^ 15 Fig.2.3 Methods of clogging and blinding (After Bell & Hicks, 1980) ^ 20 Fig.2.4 Cross-section detail of U.S.Army Corps of Engineering Gradient Ratio (GR) test permeameter (After Haliburton and Wood, 1982) ^ 22 Fig.2.5 Gradient ratio as a function of soil silt content for geotextiles tested (After Haliburton and Wood, 1982) ^ 24 Fig.2.6 Schematic sketch of a long term flow apparatus (After Rollin, 1988) ^ 25 Fig.2.7 Types of filtration behaviours of for soil-geotextile during long term flow (LTF) test (After Rollin, 1985) ^ 26 Fig.2.8 HCR test specimen (After Luettich & Williams, 1989) ^ 28 Fig.2.9 Graphical rests of HCR test (After Luettich and Williams, 1989) ^ 28 Fig.2.10 Schematic fine fraction filtration (F 3) test set-up (After Sansone et a1,1992) 30 Fig.2.11 Schematic diagram of dynamic filtration test set-up (After Narejo et al, 1992) 32 Fig.2.12 Dynamic filtration test results on nonwoven needle punched geotextile. (upper: Fly ash; intermediate: well graded sand; lower: Le Bow soil with fraction less than No.100 sieve) (After Narejo et al, 1992) ^ 33 xi

13 Fig.3.1 Schematic layout of the gradient ratio test apparatus ^ 36 Fig.3.2 The relationship between UBC defined gradient ratio and ASTM defined gradient ratio. (a). a schematic illustration, (b). a correlation based on theoretical calculations ^ 42 Fig.4.1 Particle size distribution curves of soils used in the preliminary tests ^ 46 Fig.4.2 Particle size distribution curves of uniform soils ^ 48 Fig.4.3 Particle size distribution curves of broadly graded sands ^ 51 Fig.4.4 Particle size distribution curves of broadly graded silts. ^ 52 Fig.5.1 Sample preparation technique: (a). Take an initial height reference; (b). Perform water pluviation; (c). Level the top surface by siphoning; (d). Densify the soil sample to desired density. ^ 54 Fig.6.1 Water head distribution versus time (medium sand USPM and normal deaired water) ^ 64 Fig.6.2 Soil permeability k35 and system permeability k 17 versus time (medium sand USPM and normal deaired water) ^ 65 Fig.6.3 Flow rate versus time (medium sand USPM and normal deaired water)..^65 Fig.6.4 Water head distribution versus time (medium sand USPM and water with bleach) ^ 68 Fig.6.5 Soil permeability k35 and system permeability kr, versus time (medium sand USPM and deaired water with bleach) ^ 69 Fig.6.6 Flow rate versus time (medium sand USPM and deaired water with bleach) ^ 69 xii

14 Fig.6.7 Water head distribution versus time (coarse sand USPC and deaired water with bleach) ^ 71 Fig.6.8 Soil permeability k 35 and system permeability kr, versus time (coarse sand USPC and deaired water with bleach) 72 Fig.7.1 Water head distribution versus time (Uniform coarse sand USPC / Trevira 1120 Fig.7.2 Water head distribution versus time (Uniform medium sand USPM / Trevira) ^ 74 Fig.7.3 Water head distribution versus time (Uniform fine sand USPF / Trevira 1120) ^ 75 Fig.7.4 Water head distribution versus time (Uniform silt UMLS / Trevira 1120).^76 Fig.7.5 Soil permeability k 35 and system permeability kr, versus time (Uniform coarse sand USPC / Trevira 1120) ^ 79 Fig.7.6 Soil permeability k 35 and system permeability k 17 versus time (Uniform medium sand USPM / Trevira 1120) ^ 79 Fig.7.7 Soil permeability k35 and system permeability k 17 versus time ( Uniform fine sand USPF / Trevira 1120) ^ 79 Fig.7.8 soil permeability k35 and system permeability kr versus time (Uniform silt UMLS / Trevira 1120) ^ 80 Fig.7.9 Gradient ratio at four different gradients versus time (Uniform medium sand USPM / Trevira 1120) ^ 83 Fig.7.10 Gradient ratio at four different gradients versus time (Uniform fine sand USPF / Trevira 1120) ^ 84

15 Fig.7.11 Gradient ratio at four different gradients versus time (Uniform silt UMLS /Trevira 1120) ^ 84 Fig.7.12 A tri-linear distribution of water head for the uniform soils ^ 87 Fig.8.1 Water head distribution versus time (Broadly graded silty sand BSM15 / Polyfelt TS550) ^ 96 Fig.8.2 Water head distribution versus time (Broadly graded silty sand BSM47 / Polyfelt TS550) ^ 97 Fig.8.3 Water head distribution versus time (Poorly graded sand BSPO3 / Polyfelt TS550) ^ 98 Fig.8.4 Water head distribution versus time (Broadly graded sandy silt BML56 / Polyfelt TS550) ^ 99 Fig.8.5 Water head distribution versus time (Broadly graded silt with sand BML74 / Polyfelt TS550) ^ 100 Fig.8.6 Water head distribution versus time (Broadly graded silt BML90 / Polyfelt TS550) ^ 101 Fig.8.7 Soil permeability k 35 and system permeability k 17 versus time ( Broadly graded sands) ^ 103 Fig.8.8 Soil permeability k35 and system permeability kr versus time (Broadly graded silts) ^ 103 Fig.8.9 Gradient ratio versus time (Broadly graded sands / Polyfelt TS550) Fig.8.10 Gradient ratio versus time (Broadly graded silts / Polyfelt TS550) Fig.8.11 A plot of gradient ratio against silt content ^105 ^105 ^106 xiv

16 Fig.8.12 A tri-linear distribution of water head for the broadly graded soils ^111 Fig.9.1 Schematic sketch showing three zones for a unified interpretation of the gradient ratio test ^117 xv

17 Acknowledgement I wish to express my gratitude to my academic supervisors, Dr. R.J. Fannin and Dr.Y.P. Vaid, for their support, encouragement, patience, and valuable discussions throughout this research, and their critical review of the manuscript. The assistance of the Civil Engineering Workshop and expert technical support of Art Brookes and Fred Zurkirchen in fabricating the equipment is gratefully acknowledged. Helpful discussions with Hongbo Xin, a visiting engineer, and the assistance of my colleagues, especially Guoxi Wu, Thomas Joyis, Harvey Choy, Raju Muthu and S Sivathayalan are greatly appreciated. This work was funded by grants from the Natural Science and Engineering Research Council of Canada. Ontario Ministry of Transportation supplied the geotextiles samples for testing. Finally, I am deeply indebted to my wife Yaozhen who suffered separation for over one year and sacrificed much everything for unification and companionship. I also wish to express special thanks to my parents for their love, and understanding of my desire to study overseas. This thesis is dedicated to them. xv

18 1 Chapter 1 Introduction 1.1 Types of geotextile Geosynthetics and related products have become an increasingly important construction material in civil engineering, with many areas of application in geotechnical and environmental engineering. The major types of geosynthetics are geotextiles, geogrids, geonets, geomembrances, and geocomposites. Geotextiles, the largest group of geosynthetics, are generally classified according to their manufacturing process. They are typically made from polypropylene and polyester. The two most common types are nonwoven and woven materials. Nonwoven geotextiles are formed from fibers arranged in an oriented or random pattern, and in a planar structure. The fibers are bonded together using either a needlepunching (mechanical bonding), heat-bonding (thermal bonding), or chemical bonding processes. Woven geotextiles are composed of two sets of parallel yarns that are systematically interfaced to form a planar structure. The yarn is made from one or several fibers, and may be a monofilament, multifilament, spun, silt film, or fibrillated yarn. 1.2 Applications of geotextiles in construction Geotextiles are employed in a wide variety applications in engineering practice. The fabric is considered to mobilize one ore more of four basic functions: 1. Separation: the placement of a flexible barrier between two dissimilar materials

19 Chapter 1 Introduction^ 2 prevents intermixing and reserves the integrity and properties of both materials. An example is the use of geotextiles as a separator in road construction between the subgrade soil and the subbase or aggregate layer. 2. Reinforcement: the introduction of an element that may improve the system strength through bonding with the soil, which is poor in tension but good in compression, and going into tension. An example is the use of geotextiles in reinforced soil slopes and walls, and the stabilization of embankments. 3. Filtration: the geotextile retains soil on the upstream side but allows for free liquid flow without significant loss of soil across the plane of the fabric and maintains a stable equilibrium. The filtration of a geotextile is similar to that of a granular soil filter. 4. Drainage: the geotextile accommodates free flow of liquid without significant loss of soil in the plane of the fabric and again maintains a stable equilibrium. An example is the use of a geotextile around prefabricated vertical (wick) drains (PVD) to accelerate the drainage of soft soil and activate re-consolation, thus improving the strength of the soils. Often, while one or more of these four functions are mobilized, there will be a dominant function that is addressed in design and specification. 1.3 Requirements of a geotextile filter In filtration, the geotextile acts like a granular soil filter. The main requirements for successful performance are soil retention, permeability, clogging resistance, and strength. With respect to soil retention, the openings of geotextiles must be small enough to

20 Chapter 1 Introduction^ 3 retain the soil on the upstream side of the fabric. The intent is to retain the coarser soil fraction which in turn acts to stabilize the finer particles. There are a number of approaches used in design, all of which compare a characteristic soil particle size to the characteristic opening size of the geotextile. The most common soil retention criteria are in the form of: 095<1 RDSS where: 095 = opening size of the geotextiles (95% of openings are the same size or smaller); D85 = diameter of soil for which 85%, by weight, is smaller than or equal to that diameter; Xi, = dimensionless coefficient, generally in the range of 1 to 3, depending on the soil and geotextile properties. With respect to permeability, the basic requirement is that cross-plane permeability of the geotextile must be and remain greater than that of the adjacent soil. To provide some assurance, the common approach is to evaluate the permeability of geotextile and permeability of soil in isolation, and compare them using a criterion of the following general form: kg>1 As^(1.2)

21 Chapter 1 Introduction^ 4 where: kg = permeability (hydraulic conductivity) of geotextile; kg = permeability of soil; Xp = dimensionless coefficient. With respect to clogging resistance, the issue is one of will the fabric completely clog, such that the flow of liquid through it will be unacceptably restricted. Such compatibility is normally assessed by taking a soil sample and candidate geotextile and testing them in the laboratory using a test such as the gradient ratio test or hydraulic conductivity test, and evaluating head losses occurring during fluid flow. With respect to strength and survivability, the requirement is to provide a geotextile that is strong enough to withstand construction and in-service loads. Typically strength is reported according to standard test methods. Requirements for design are established with reference to these test methods, and taking into consideration for example the installation stresses, aggregate types (i.e., sharp angular aggregate or smooth graded surfaces having no sharp angular projections), and degree of compaction. Under each class of working condition, it is required that the above four strengths and elongation meet a certain of minimum criterion. Additional requirements of a geotextile filter may include chemical compatibility, durability (especially U.V. degradation) and type of polymer, depending on the application. 1.4 Objectives and scope of the study This study assesses the soil -geotextile compatibility of nonwoven geotextiles in

22 Chapter 1 Introduction^ 5 filtration, with reference to the gradient ratio test. Specially, the objectives are to: 1. Review existing approaches for specification of geotextiles and associated design criteria for filtration. A large number of filter design criteria exist, which have developed from different test methods, and are not quite consist with each other. The approaches are compared and contrasted. 2. Critically evaluate the gradient ratio test with respect to the equipment, sample preparation and test procedure, and approach for interpretation of results. Use preliminary test results to improve the test procedure, and develop an approach for reconstitution of saturated, homogeneous, reproducible uniformly graded and broadly graded soil samples. 3. Perform a series of gradient ratio tests on several uniformly graded soils and nonwoven geotextiles, to assess the compatibility of the soils and geotextiles. Use the results to critically evaluate existing filter design criteria for such uniform soils. 4. Perform a series of gradient ratio tests on serval broadly graded soil and one nonwoven geotextile, to assess soil-geotextile compatibility in similar manner to that for the uniform soils 1.5 Organization of the thesis The thesis consists of five parts: Part 1, comprising Chapter 1 and Chapter 2, introduces the background of the research, reviews the state of the art of geotextile filter design, and presents the objectives of this program of research.

23 Chapter 1 Introduction^ 6 Part 2, comprising Chapter 3, Chapter 4, and Chapter 5, describes the UBC gradient ratio test device, the materials used in testing, the sample preparation technique and test procedure, and reports definitions of gradient ratio, permeability of the soil and soilgeotextile composite that are relevant to interpretation of the test results. Part 3, Chapter 6, presents the preliminary test results and analyses. It describes the early test observations, the setup and control of the test conditions, and the confirmation of test repeatability. Part 4, comprising Chapter 7 and Chapter 8, presents the gradient ratio test program, results and analyses for both uniformly graded soils and broadly graded soils. In this part, the existing geotextile filter design criteria are critically evaluated and some modifications to the criteria proposed based on interpretation of the experimental results. Part 5, Chapter 9, summarizes modifications to the gradient ratio test device and its associated interpretation, and unifies the relationship between water head distribution, permeability and gradient ratio.

24 -7 Chapter 2 Literature Review 2.1 Properties of geotextile Properties of geotextiles are measured either by an index test or a performance test. Index tests are used to measure properties such as pore size opening and cross-plane permeability (hydraulic properties), thickness and mass per unit area (physical properties), and strength (mechanical properties). Performance tests, such as the gradient ratio (GR) and hydraulic conductivity ratio (HCR) tests, are used to evaluate the soil-geotextile interaction under conditions similar to those of the fluid installation. The major index properties of geotextiles which are relevant to filtration applications are the pore size opening, coefficient of cross-plane permeability or permittivity, thickness, and mass per unit area Pore size opening The characteristic pore size opening is an important parameter used for the selection of geotextile filter. While most design criteria consider both the opening size and permeability (Christopher and Holtz 1985, Saathoff 1986, Faure 1986), because permeability is related to the porometry of a fabric (Mlynarek 1985, Dierickx 1986), the pore size distribution is the controlling parameter. Several techniques have been developed to measure the opening size of a geotextile, and are classified as direct, theoretical and indirect methods by Rollin (1986). A general discussion of techniques for determination of opening size follows, with emphasis placed

25 Chapter 2 Literature Review^ 8 on the apparent opening size (AOS) and the filtration opening size (FOS) since these are most commonly used in design. There is no universally standardized test method and the opening size values determined by different methods are not consistent with each other. Therefore, filter criteria for a certain geotextile are a function of these different test methods and the preferred design approach Direct method This early method involved measurement of the pore size openings by projecting a magnified image of the geotextile on to a screen and measuring the aperture size (Calhoun, 1972). It is acceptable for geotextiles which have relatively large and uniform pores (i.e. wovens), but is difficult to use with geotextiles which have relatively small and varying pores (i.e. nonwovens), and has since been replaced by other indirect methods Indirect methods Most indirect methods involve measurement of the pore size openings using a reverse sieving technique. The procedure involves the sieving particles of known size through a geotextile screen of unknown apertures. The mass of particles of each size fraction that is either retained on, or passing through, the geotextile is recorded and used to establish the gradation of apertures in the geotextile. The methods include both dry sieving and wet sieving techniques. Two different types of particles have been used with these reverse sieving techniques. Ogink (1975), and Schoher and Teindl (1979), advocated the use of sand particles of predetermined sizes. Other researchers, such as Makeand (1977), used spherical glass beads

26 Chapter 2 Literature Review^ 9 (ballotinis) of predetermined sizes. In both cases there is a limit to the minimum size of particle which can be used. Very fine particles may not be used because interparticle attractive forces begin to influence the results Dry sieving Apparent Opening Size, (AOS): Minnitti (1980) developed the AOS procedure, which supersedes the EOS procedure and is now standardized (ASTM D ). It involves sieving spherical glass beads of known diameter through the geotextile. A 200 mm diameter geotextile specimen is mounted in a sieve frame, and 50 g of sample glass beads are placed on its surface. The geotextile and frame are then shaken laterally for 10 minutes causing the beads to pass through the test specimen. The procedure is repeated on the same specimen with ballotinis of increasing size until 5% or less of the beads pass through. The AOS is the size of ballotinis at which 5 % or less pass, which implies 95% of the pore size openings are the same size or smaller. AOS is a value that is often reported in the technical literature. It is widely used in engineering practice, particularly in design of filters. However, it is an index test, and different countries have adopted variations to the dry sieving technique, therefore its relevance to practical applications depends on engineering experience and appropriate empirical specification criteria Wet sieving Filtration Opening Size (FOS): A wet sieving technique was developed in in France by Fayoux (1977), based on a hydrodynamic approach. The filtration opening size

27 Chapter 2 Literature Review ^ 10 Fig.2.1 Scheme of Filtration Opening Size (FOS) apparatus in EPU (After Rollin et al, 1990) (FOS) (CAN/CGSB No.10) of a geotextile is determined from the gradation of glass beads which have passed through the geotextile under the action of water flow resulting from the immersion and withdrawal of the geotextile in a water tank. The geotextile specimen is secured between open grids at the bottom of a cylindrical basket that is 300 mm in diameter. Glass beads (5.64 kg/m) are placed on the geotextile surface, and the system is alternatively immersed in the tank as shown schematically in Fig The glass beads are a well graded mixture, with a maximum bead size greater than twice the expected FOS of the geotextile, and a coefficient of uniformity (C u) greater than 4. Immersion of the geotextile is to a depth of 100 mm and the withdrawal period allows the cylinder to empty: the minimum number of cycles is The value of the FOS is the 95% (by mass) bead diameter of ballotinis which have passed through the geotextile under the hydrodynamic action, which again implies that 95 % of the pore size openings are the same size or smaller.

28 Chapter 2 Literature Review^ 11 The FOS procedure is also an index test method. The major advantage is that, in comparison to the AOS, it better simulates the operating condition in the field, and for these reasons the FOS is gaining increased acceptance. Again, like the AOS, some countries have adopted variations to the wet sieving technique, such as the Franzius Institute Hannover where the effective opening size of a geotextile, dw, is determined by wet sieving quartz sand of known composition 15 minutes in a sieve machine with a vertical amplitude of 3 mm at a frequency of 50 Hz Theoretical methods Some attempts have been made to predict the pore size opening distribution of a geotextile, though they have not been used in engineering practice. Faure et al (1986) examined the probability of a particle passing through a geotextile and established a relationship between the opening size and the geotextile parameters such as porosity, fibre diameter and thickness which ascribe the structure of the geotextile. Similarly, Mlynarek (1985) developed a simple formulae to approximate the equivalent opening size of nonwoven fabrics by using the following equation: D=3.3dn^ (2.1)

29 Chapter 2 Literature Review^ 12 where: D = equivalent opening size, /Am d = fibre diameter, Am n = porosity of the nonwoven fabric Lombard and Rollin (1986) analyzed the structure of nonwoven heat-bonded geotextiles using a geometrical probability approach. While an expression of the pore size distribution of these fabrics was derived, it was concluded that the structure and the hydraulic behaviour of these materials cannot be adequately described by a unique model Permittivity V/ Permittivity describes the cross-plane permeability of geotextile fabric, and is defined as the volumetric flow rate of water per unit cross sectional area per unit head under laminar flow conditions, in the normal direction. The permittivity is related to a coefficient of permeability by: Iir - kltg^(2.3) where:tg = thickness of the geotextile, m k = cross-plane permeability ASTM (D ) has established a standard test method for determining the permittivity of geotextiles using either a constant head or falling head test procedure. The constant head test is used when the flow rate of water through the geotextile is so large that it is difficult to obtain readings of head change versus time in the falling head test. Data have shown agreement between the two methods.

30 Chapter 2 Literature Review^ 13 In addition to the experimental determination of the permittivity and a coefficient of permeability, some theoretical formulas have been developed. Lewandowski & Mlynarek (1985) derived an expression relating the intrinsic permeability (K) of a nonwoven geotextile with the porosity (n), fibre diameter (d) and a shape factor (A) determined experimentally, where: K^n3 d2 ^ (2.4) 16(1 -n)2 A In design, emphasis is placed on relating the permeability of the geotextiles to that of the adjacent soil Thickness Since most geotextiles are compressible, thickness is dependent on the applied pressure. The standard test method ASTM (D1777); CGSB(CAN/CGSB No.3) for measurement of the thickness of geotextile requires the nominal thickness be measured and reported as that between parallel plane surfaces under a pressure of 2.0 kpa. Thickness is believed to be an indirect measure of other physical properties of a geotextile, and has been used to predict opening size (Gourc and Faure, 1990) Mass per unit area Mass per unit area is an important property for quality control of geotextiles during manufacturing (Adolphe, 1993), and one that may also be used to predict opening size. The

31 Chapter 2 Literature Review^ 14 standard test method, ASTM (D1910) or CGSB (CAN/CGSB No.2-M85), involves cutting specimens of known minimum dimensions, from the fabric and determining their masses. In addition to the conventional method, Adolphe (1990) presents a non-destructive and accurate analysis of the distribution mass per unit area of the geotextiles that is based on a laser image analysis. 2.2 Filter criteria of geotextile A filter should prevent excessive migration of soil particles from the retained soil, while at the same time allowing unrestricted flow of liquid from the soil. The filtration function is therefore dependent on two apparently conflicting requirements: 1). The filter must retain the soil, which implies that the size of the filter pore spaces or openings should be smaller than a specified maximum value; and 2). The filter must be permeable enough to allow relatively unimpeded flow into the drainage medium, which implies that the size of the filter pore spaces or openings should be larger than a specified minimum value. In addition to the retention and permeability requirements, an anti-clogging criterion is suggested to ensure that if soil particles block or clog a few openings of the fabric, the compatibility of soil and geotextile are such that the composite permeability is not significantly impaired Retention criteria

32 Chapter 2 Literature Review^ 15 Retention design for geotextiles has been developed from existing soil filter criteria, and with reference to extensive laboratory and field experience. Like soil filters, the geotextile filter is generally selected such that enough larger soil particles are retained to Well-distributed particles Filter formation Bridging network Fig.2.2 Filter formation (After Koerner, 1990) develop a soil bridge leading to the development of a stable soil structure which is able to prevent further migration, as illustrated in Fig Some soil particles may actually be designed to pass through to prevent clogging (Christopher & Holtz, 1985). The filter design criteria are usually expressed using the ratio of pore size opening (FOS or AOS, etc) to soil particle diameter (D), at various particle diameters (in terms of percent passing). D85 represents the diameter of soil particles for which 85 %, by weight, of the soil particles are less or equal to that diameter. The ratios are commonly compared using D85, D50, D15. Therefore, the retention criteria for geotextile filters in a drainage system can be generally

33 Chapter 2 Literature Review^ 16 expressed by the following equation: 095<).^ (2.5) where: 095 = pore opening size of geotextile, it can be FOS, AOS, etc. XR = dimensionless coefficient for retention Retention criteria have been reviewed by Rankilor (1981), Christopher & Holtz (1985), Williams & Luettich (1990), Fischer et al (1990), and Christoper & Fischer (1992). A comparative review of the more notable criteria for steady state flow is presented by Rankilor (1981), and Fischer et al (1990), and is summarized in Table 2.1. Table 2.1 A summary on soil retention criteria (modified after Christopher and Fisher, 1992) Source Criterion Remarks USFHWA(1986) CFEM(1992) Ontario Ministry of Transportation(undated) Calhoun(1972) AOS/D B AOS/D, 5 B FOS/D 85 <1.5 FOS/D 85 <3 FOS <1.0D and 40iim or 0.5D <FOS 095/D 85 C mm B=1 (C 9 52 or..8); B=0.5C (25C...4); B=8/C, (4<C <8);soils with 550% passing No.200 sieve B=1.8 for nonwovens; B=1 for wovens; soils with.50% passing No.200 sieve For uniform soils, excluded gap-graded soils For well graded soils,excluded gap-graded soils Nonwoven or woven, strong preference for nonwoven Wovens, soils with 50% passing No.200 mesh Wovens, cohesive soils

34 Chapter 2 Literature Review^ 17 Zitscher(1975) from Rankilor (1981) 050/D5, /D Wovens, soils with C u 52, D 50 = mm Nonwovens, cohesive soils Ogink(1975) 090/D ,/D Wovens Nonwovens Sweetland(1977) 0, 5/D /D85 51 Nonwovens, soils with Cu =1.5 Nonwovens, soils with Cu =4 Rankilor(1981) 0,/D /D, Nonwovens, soils with D mm Nonwovens, soils with D mm Millar(1980) 050/D85 51 Wovens and nonwovens Giroud(1982) 095/D50 5(9-18)C. Depend on soil Cu,Assumes fines in soil migrate for larger C u Carroll(1983) 095/D wovens and nonwovens FCGG (1986) Of/D Depend on soil type, compaction, hydraulic and application conditions Fisher et al(1990) 050/D ,0/A /D Based on geotextile pore size distribution, dependent on C u of soil Bhatia(1990) FOS/D Based on long term filtration tests using gap-graded soils Faure et al (1986) FOS/D85 < FOS <D with critical conditions with lesser critical conditions Based on filtration tests

35 Chapter 2 Literature Review^ Permeability criteria All permeability criteria are based on the principle that the geotextile must have a sufficiently high permeability to avoid any development of excess pore water pressure behind it when installed and operating. Some design methodologies are based on the argument that the geotextile needs to be no more permeable than the retained soils (e.g., Schober & Teindl, 1979; Wates, 1980; Carroll, 1983; Christopher & Holtz,1985). The assumption is simple and rational, since flow should not be impeded at the soil-geotextile interface if the permeabilities are at least equal. Carroll (1983) and Christopher & Holtz (1985) further recommend that the permeability of the geotextile be increased by a factor of safety equal to 10 for critical projects (e.g. where failure could result in significant damage or loss of life or where repair costs would approach installation costs) and for severe soils and hydraulic conditions (i.e. soils which are prone to piping and exposed to high hydraulic gradients). In addition, for well designed geotextile filters used in severe soil and hydraulic conditions, clogging has been shown to cause approximately an order of magnitude decrease in the geotextile permeability. Therefore, increasing the permeability initially will provide some assurance that the geotextile has the required permeability over its lifetime(carroll, 1983). The permeability criteria for geotextile filters is generally expressed as: kg>1 pk,^ (2.6)

36 Chapter 2 Literature Review^ 19 where: kg = the permeability of geotextile filter kg = the permeability of soil AR = the dimensionless coefficient for retention Permeability criteria have been proposed by Giroud (1982, 1988), USFHWA (1985), the French Committee on Geotextiles and Geomembranes (FCGG), and reviewed by Williams & Luettich (1990), Christoper & Holtz (1992), Fischer et al (1990). A summary of the more commonly used criteria for various flow conditions is presented in Table 2.2. Table 2.2 A summary of permeability criteria (modified after Christopher and Fisher, 1992) Source Criterion Remarks USFHWA (1985), Calhoun(1972), Wates(1980), Carroll(1983), Haliburton et al(1982), USFHWA(1985), Carroll(1982) kg >k, kg > 10k, Steady state flow, noncritical application and nonsevere soil conditions Critical applications and severe soil or hydraulic conditions CFEM(1992) FCGG (1986) Giroud (1982) Koerner (1990) kg > k, kg > 10k, Based on sk with st >10"k, kg >0.1kg For retention of clean medium to coarse sands For retention of fines Critical 10 5k,; less critical 104k,; clean sands 103k, No factor of safety Factor of safety FS based on application and soil conditions

37 Chapter 2 Literature Review^ Clogging resistance criteria By definition, clogging is the result of fine particles penetrating the geotextile and blocking off pore channels or caking on the upstream side of the geotextile thereby reducing its permeability, as illustrated in Fig.2.3. As such, clogging is closely related to the Blinding 00) Fabric Clogging by Particle Deposition Fig.2.3 Methods of clogging and blinding (After Bell & Hicks, 1980) permeability criteria. However, it was shown by Carroll (1983) that satisfying retention and permeability criteria would not necessarily provide for a complete filter design, as the system could still fail by clogging. The findings by Carroll are rational considering the basis for retention and permeability criteria, and the mechanisms that would cause clogging. Recall that the opening size to retain the soil is based on a large characteristic opening size of the geotextile (i.e. maximum opening size criterion) with no restriction on the smaller pore size openings or the percentage of the volume of openings (porosity) it should represent. An exception to this approach is the Ontario Ministry of Transportation, see Table 2.1. With regard to the permeability criteria, a geotextile with a very small porosity may still achieve a permeability greater than that of the soil, especially for fine grained soils.

38 Chapter 2 Literature Review^ 21 The evaluation of clogging resistance involves performance of a filtration test on the site specific soils in order to evaluate the clogging potential. Several tests have been proposed, including the gradient ratio (GR) test (Calhoun, 1972) for which ASTM has established a standard (ASTM D5101). This procedure was actually used for the selection of the first geotextiles in the early 1960s, where numerous fabrics were evaluated for a range of soil conditions, to select the fabric with the best clogging resistance (Pollici, 1961; Barrett, 1966). Another test method is the long term flow (LTF) test, where the variation of flow rate is measured with time (Koerner & Ko,1982). The hydraulic conductivity ratio (HCR) test, developed recently by Luettich and Williams (1989), is believed to better simulate the field stress conditions, and therefore provide a better understanding of soilgeotextile interactions. The Fine fraction filtration test (F 3) is another test in which the ability of fine soils to pass through a geotextile without clogging it is evaluated (Hoover, 1982), and dynamic filtration (DF) is used to evaluate some particular cases where dynamic pulsing is involved. These tests are described in detail in the following sections. 2.3 Laboratory tests for soil-geotextile compatibility Gradient Ratio (GR) test Gradient ratio (GR) test, see Fig.2.4, was first initiated by Calhoun (1972) of the U.S. Army Corps of Engineers. The permeameter has an inside diameter of 10 cm ( 4 inches), and the soil sample which is approximately 10 cm long is placed on the geotextile which is supported by a mesh screen. During permeation of water through the soil-geotextile

39 Chapter 2 Literature Review^ 22 Fig.2.4 Cross-section detail of U.S.Army Corps of Engineering Gradient Ratio (GR) test permeameter (After Haliburton and Wood, 1982) composite system, the variation of water flow and water head at different locations through the manometer ports are measured with time. The hydraulic gradient of imposed on the system is increased incrementally from 1 to 10. The test is run for 24 hours at each hydraulic gradient. Calhoun (1972) found that when the gradient ratio value (defined as the gradient through the lower 1 inch (25 mm) of soil plus geotextile, divided by the gradient through the adjacent 2 inches (51 mm) of soil ) is greater than 3, clogging is likely to occur in the field. Haliburton and Wood (1982) performed GR analyses on four woven and two nonwoven geotextiles and established a relationship between gradient ratio and silt content, see Fig.2.5.

40 Chapter 2 Literature Review^ 23 The gradient ratio values were found to increase slowly with increasing soil silt content until a value of approximately 3 was obtained, and then increase rapidly with further small increases in soil silt content. Based on their observations, the rational for maximum acceptable value of 3 was strengthened. Further work on the GR test (Dierickx 1986), Scott (1980), Greenway et al (1986) has established that the water head distribution, which describes the head loss throughout the soil-geotextile composite specimen, is of great significance to interpretation of the test results. The GR test is intended to simulate the soil-geotextile interaction in the field. The shortcoming of test is that it is unable to impose the confining stress conditions in the field. Although the GR test has been under development for over twenty years, there is no extensive database of experimental results, and the gradient ratio value of 3, which is currently accepted as a maximum allowable value in the geotextile filter design, is based on an interpretation of a few tests. The focus of this thesis is to critically evaluate the GR test and assess criteria for soil-geotextile compatibility.

41 ^ Chapter 2 Literature Review^ T^I^ 1^1 ^TYPAR 3401 (24 5%)^POLY-FILTER X _ 9 MIRAFI 500X (46 5%)^(76 30%) 8 BIDIM C-34 7 OTTAWA SAND AND VICKSBURG SILT LOESS 6 SOIL MIXTURE U.S. ARMY CORPS OF ENGINEERS MAX. ACCEPTABLE VALUE POLY-FI LTER GBH ^i 10^15^20^25^30^35^40 SOIL SILT CONTENT, (%) Fig.2.5 Gradient ratio as a function of soil silt content for geotextiles tested (After Haliburton and Wood, 1982) Long Term Flow (LTF) test A long term flow test has been performed by several researchers(rollin et al, 1983, Mlynarek, 1983, Faure, 1988, Mlynarek 1991), though without any standardization of the size of the soil and geotextile sample, soil preparation techniques and imposed hydraulic gradients. The device used at the Ecole Polytechnique Montreal is shown schematically in Fig.2.6. A test cylinder (2.52 cm diameter) containing approximately 8 g of soil is placed inside a larger diameter cylinder and above the geotextile specimen. The permeability of the system, and the mass of soil retained on a filter paper, are used to assess the interaction between geotextile filter and soil. The test is run until a constant flow rate is obtained, which implies an equilibrium condition. Koerner and Ko (1982) suggests that for silty soils, the test be conducted for at least one hundred hours to ensure a stable filter formation. In

42 Chapter 2 Literature Review^ 25 contrast, Rollin (1988) observes the most critical period is immediately following the Fig.2.6 Schematic sketch of a long term flow apparatus (After Rollin, 1988) initiation of flow and concludes a long term flow test need not exceed 24 hours. A standard test method for conducting long term flow test has not been established. Rollin (1985) observes three different types of long term flow curve, as shows in Fig The first type curve depicts the geotextile as an excellent filter where the particles moving towards the textile are stopped and a cake formation at the soil-geotextile interface. The permeability of system decreases with time to reach a stable constant value. The second curve indicates the result of a continued loss of soil particles: the permeability increases as more fine soil particles pass through the geotextile. The third curve shows development of a stable condition follows loss of soil particles and then a return to the same behaviour shown in the first type. Like the GR test, the LTF test is intended to simulate soilgeotextile interaction, but again does not reproduce stress conditions in the field. It is better

43 Chapter 2 Literature Review^ 26 Fig.2.7 Types of filtration behaviours of for soil-geotextile during long term flow (LTF) test (After Rollin, 1985)

44 Chapter 2 Literature Review^ 27 suited to the collection of soil that has passed through the geotextile, but provides no information on the distribution of water head (and therefore internal hydraulic gradients) and has a very small sample size Hydraulic Conductivity Ratio (HCR) test The hydraulic conductivity ratio (HCR) test is a test method that is intended to overcome some limitations of the GR test. It is performed in a triaxial permeability device, on a soil sample approximately 3 cm in diameter, that is prepared using standard laboratory or field sampling techniques. The soil to be tested is overlain by the candidate geotextile filter, as shown in Fig.2.8. The hydraulic conductivity of the parent soil, Ic s, is initially measured by permeating a relatively small amount of liquid through the system from top to bottom of the sample (i.e., through the geotextile, then the soil). The flow direction is then reversed, such that the permeant flows first through the soil, then across the soil-geotextile interface, and finally through the geotextile. The hydraulic conductivity of the soil-geotextile composite is defined as ksg. The HCR may then be defined as the ratio of the stabilized value of ksg to the hydraulic conductivity of the soil lc, (Luettich and Williams, 1989). The test is terminated after the quantity of flow exceeds approximately 20 pore volumes of the soil. Idealized results of an HCR test for four different geotextiles against the same soil is shown in Fig.2.9. (Luettich and Williams, 1989). The horizontal portion of each curve, for flow volumes of less than one pore volume, is the hydraulic conductivity of the soil, lc s. After one pore volume of flow, the flow direction is reversed. The hydraulic conductivity

45 Chapter 2 Literature Review^ 28 Fig.2.8 HCR test specimen (After Luettich & Williams, 1989) across the soil-geotextile interface, ksg, then decreased as a function of the flow volume for geotextiles A, B, and C, and increases slightly for geotextile D. The HCR values for geotextiles A, B, C, and D are 0.75, 0.40, 0.10, and 1.0 respectively. The quantity of fine grained soil passing through the geotextile into the effluent reservoir may be observed by visual inspection. If the effluent remains cloudy after more than one or two pore volumes have passed across the soil geotextile interface, it is inferred that piping is occurring through the geotextile. Piping is often associated with an increase in HCR value with flow volume, and an absolute value that may exceed one. The HCR test is intended to better simulate field conditions. Since it is conducted in a triaxial cell, it is feasible to apply a back pressure to fully saturate the soil sample, and to apply a cell pressure that simulates the field stress condition. These are the major advantages of the HCR test over other tests, such as the GR test. However, a limitation of