Strain Measurement of Geogrids Using a Video ... - CiteSeerX

Geotechnical Testing Journal, Vol. 27, No. 5 Paper ID GTJ11940 Available online at: www.astm.org

Masahiro Shinoda1 and Richard J. Bathurst2

Strain Measurement of Geogrids Using a Video-Extensometer Technique

ABSTRACT: The paper describes a novel technique to record displacements and compute local strains at the surface of typical geogrid soil reinforcement products using a noncontact high resolution digital CCD camera (video-extensometer) technique. Specimens of biaxial polypropylene (PP) geogrid, knitted polyester (PET) geogrid, and uniaxial high-density polyethylene (HDPE) geogrid were subjected to in-isolation wide-width strip tensile loading under constant rate of strain (CRS), constant load (creep), and stress relaxation load paths. The specimens were gripped using a set of split roller clamps. Targets painted on the surface of the specimens were tracked in both vertical and horizontal directions using a commercially available CCD camera with ancillary hardware and software. The paper examines repeatability of the test methodology and demonstrates the ability of the method to record strains at high resolution up to rupture and to identify nonuniform distribution of axial and lateral strains in geogrid specimens. KEYWORDS: measurement

image processing, digital camera, video-extensometer, CCD, geosynthetics, geogrid, reinforcement, tensile testing, strain

Introduction In-isolation tensile loading of geosynthetic reinforcement products according to ASTM Test Method for Tensile Properties of Geotextiles by the Wide-Width Strip Method (D 4595), Determining Tensile Properties of Geogrids by the Single or Multi-Rib Tensile Method (D 6637), and Evaluating the Unconfined Tension Creep Behavior of Geosynthetics (D 5262) is routine practice to characterize the load-strain properties of these materials. Strain measurements are computed from displacements measured over a central portion of the specimen rather than using the crosshead displacement, which may influence computed strains due to slippage of the specimens in the clamps. Clip-on mechanical extensometers are routinely used to measure specimen axial strains. These devices have the disadvantage that their weight and method of attachment can influence the loadstrain response of the test specimen. Furthermore, many mechanical devices have limited travel and must be removed prior to specimen rupture to avoid damage to the extensometer assembly. Perkins and Lapeyre (1997) compared the measured local strains from three different types of mechanical devices attached to 1800-mm-wide by 900-mm-long geosynthetic specimens loaded in a wide-width strip apparatus using drum rollers. They demonstrated that the mechanical devices gave strain readings that varied significantly between devices and with the average strain computed by extensometer points mounted close to the top and bottom of the specimens. Strain gages bonded directly to the surface of a geogrid specimen require careful attention to surface preparation and gluing. In Received April 29, 2003; accepted for publication February 12, 2004; published August 25, 2004. 1 Research Engineer, Railway Technical Research Institute, Foundation and Geotechnical Engineering, Structures Technology Division, 2-8-38, Hikari-cho, Kokubunji-shi, Tokyo, 185-8540, Japan. 2 Professor, GeoEngineering Centre at Queen’s-RMC, Civil Engineering Department, 13 General Crerar, Sawyer Building, Room 2085, Royal Military College of Canada, Kingston, Ontario K7K 7B4.

addition, strain gages will typically debond from the geogrid surface at large strains prior to specimen rupture. In most cases, local strains recorded by the strain gage are often different from the average strain in the reinforcement and, therefore, require a separate calibration exercise to establish the relationship between strain gage reading and average reinforcement strain over one or more geogrid apertures (Bathurst et al. 2002). Various noncontacting extension measuring systems, such as proximity transducers and laser extensometers have become available in recent years for materials testing. However, proximity gages are suited only for very small displacement measurements. In addition, a single proximity transducer is required for each monitoring point, and a supporting structure is required close to the test specimen (Tatsuoka and Shibuya 1992). To the best of the writers’ knowledge these devices have not been used for tensile testing of geosynthetic materials. Laser extensometers can be mounted at a distance from the test apparatus, but a single device can record only one strain location on the test specimen (Skochdopole et al. 2000; Hirakawa et al. 2002). Leshchinsky and Fowler (1990) used a photogrammetric technique to measure local strain of geotextiles during wide-width strip tensile loading. This method has the disadvantage that a continuous strain measurement is not practical, and some skill is required to interpret the photogrammetric records. High-resolution CCD (Charged Coupled Device) cameras have been used for noncontact measurement of displacements in 2-D particulate media (e.g., Paikowsky and Xi 2000; Gachet et al. 2003). Jones (2000) reported the use of a CCD camera-based apparatus as a video-extensometer to compute axial strains from multiple targets attached to the surface of three different geotextile specimens and compared strain measurements to strains inferred from conventional clip-on extensometer devices. Jones demonstrated that the video-extensometer technique offers ease of use, flexibility, and high resolution of strain measurements. Recker et al. (2001) used a video-extensometer device to measure crosshead movement for tensile specimens of PP and PET geogrid products in a study

C 2004 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. Copyright


1

2 GEOTECHNICAL TESTING JOURNAL focused on a comparison of rupture loads and their variation using ISO Test Method for Wide Width Tensile Test of Geotextiles (DIN EN ISO 10319), ASTM D 4595, and Geosynthetic Research Institute (GRI) methods of test. The current investigation examines the use of a similar CCD camera video-extensometer with ancillary hardware and software to track axial and lateral strains at multiple locations on the surface of three typical geogrid soil reinforcement products. An advantage of this approach is that local displacements of geogrid points can be tracked at locations well removed from the specimen clamps and local strains computed from the relative displacement of target pairs. The general approach eliminates the disadvantages noted for the contact devices, strain gages, and photogrammetric methods described above. The specimens were subjected to in-isolation wide-width strip tensile loading under constant rate of strain (CRS), constant load (creep), and stress relaxation load paths. The current study is the first reported use of the CCD camera videoextensometer technique to investigate the variation in local loadstrain-time response of typical geogrid soil reinforcement products under different loading conditions using conventional in-isolation tensile test methods. A novel application of the technique that is demonstrated in the paper is the measurement of lateral strains that may develop in some geogrid products under axial extension.

Geogrid Specimens Specimens of biaxial polypropylene (PP) geogrid, knitted polyester (PET) geogrid, and uniaxial high-density polyethylene (HDPE) geogrid were used in the current investigation. These materials are typical geogrid reinforcement products used in soil reinforcement applications. Material properties are summarized in Table 1 and have been taken from manufacturers’ literature. Test Method

FIG. 1—General arrangement of wide-width strip tensile loading apparatus, test specimens, and strain monitoring points on specimens. Notes: LVDT-extensometer and video-extensometer methods used separately; letters on specimens denote video-extensometer targets; targets on HDPE specimens placed vertically from midpoint of transverse member to midpoint of longitudinal members. (a) Cross section view of test apparatus, (b) Polypropylene (PP) specimen, (c) Polyester (PET) specimen, (d) Highdensity polyethylene (HDPE) specimen.

Loading System An MTS servo-controlled hydraulic actuator with a stroke of 160 mm and rated to 100 kN was used to carry out the tensile load tests. The controller for this actuator provided strain rate and load control functions and the ability to switch between modes without interruption. Split roller clamps described by Bathurst and TABLE 1—Geogrid properties.

Property Polymer type Structure

Materials PP punched sheet and drawn NA 215

Coating Mass/unit area (g/m2 ) Aperture size (mm) Machine direction 25 Cross-machine direction 33 Thickness (mm) at longitudinal member 1.0 at junction 2.9 Wide-width tensile strength (kN/m) at 5 % strain 8.3 Ultimate 12.5

PET knitted PVC 114

HDPE punched sheet and drawn uncoated NA

27 22

140 15 (maximum)

1.1 1.2

1.0 2.7

4.4 17.5

35.7 68.9

Notes: NA = Not available; PP = Polypropylene; PET = Polyester; HDPE = High-density polyethylene; PVC = Polyvinyl chloride. Source—manufacturers’ literature.

Cai (1995) were used to grip the geogrid specimens (Fig. 1a). The roller clamps are solid steel and 75 mm in diameter. The specimens are secured using a steel platen with two threaded 12-mmdiameter bolts. The linear voltage differential transducer (LVDT) extensometer clamps illustrated in the figure were used in previous CRS load-extension tests reported by Bathurst et al. (2001) and carried out on the same PET and PP materials used in the current investigation. A load cell mounted between the actuator piston and the top-loading clamp was used to record the tensile load during each test. CCD Camera Video-Extensometer System and Specimen Target Tracking A commercially available system comprising a CCD monochrome video camera, PC interface card, and matching software was used in this investigation. The interface card is manufactured by Messphysik GmbH of Austria and comes with the application software “Dot Measurements for Windows” (Messphysik 1997). The interface card is used to capture camera frames in an 8-bit digital format while generating a 640 × 480 pixel image on a PC color monitor. The gray scale of each pixel is resolved in 256 shades. The interface card is also configured to record up to 16 channels of external analog signal input at 16-bit resolution and 2 output analog channels at l6-bit resolution. One input channel was

SHINODA AND BATHURST ON VIDEO-EXTENSOMETER TECHNIQUE

used to acquire a synchronized load cell reading during specimen loading. The software program tracks the displacement of multiple targets by tracking the contrast of the corresponding gray scale pixels on the digitized camera images in the target region. The target or targets are selected by the operator using the video image of the test specimen displayed on the PC monitor at the beginning of the test. The video-extensometer software converts the trajectory of monitored pixels into X and Y displacement components of the corresponding monitoring point. Individual digitized frame images of test specimens can also be taken manually or at preselected intervals determined by the operator and stored as bit maps (640 × 480 pixel size). A description of the theory behind digital image processing using a CCD camera method can be found in the textbook by Castleman (1996). In this investigation, the targets comprised 2-mm-diameter solid circles painted on to the black surface of the geogrid specimens with reflective typewriter “white-out” paint. The system used in this investigation is capable of simultaneously tracking up to 100 targets at a frequency of 25 Hz and plotting the displacements and computed strains against load in real time. The resolution of the displacement measurements is a function of the distance to the target points, level of contrast of the targets, lens focal length (25 mm for the camera used in this investigation), and field of view. For the test setup and lens used in this investigation, the optimum camera distance was 1 m from the specimen, which gave a field of view of 150 mm and an image resolution of less than 2 µm. A larger field of view is possible using a lens with a shorter focal length. The axis of the camera was oriented perpendicular to the plane of the test specimen in each test. The specimens were illuminated with an arc lamp oriented at an angle to the plane of the specimen to improve target contrast. Based on the technical data sheets supplied by the manufacturer for the lens, hardware, software, and the camera-specimen setup used in this investigation, the accuracy of recorded strains is estimated to be ±0.005, ±0.01, and ±0.02 % of the measured strain value for strain levels of 10, 20, and 30 %, respectively. Specimen Preparation Test specimens were mounted in the split roller clamps shown in the schematics of Fig. 1. The specimen length for the videoextensometer test specimens was 200 mm. The white targets were painted on the longitudinal members of the specimens at the locations shown in the figure diagrams. Next, the target points were located on the PC monitor and the self-weight of the lower clamp (0.2 kN) used as a preload to remove any initial slack in the test specimen. All tests were carried out at a temperature of 20 ±2◦ C consistent with related ASTM specifications. A number of tensile tests on PP and PET specimens from a prior investigation are reported later in this paper. However, all specimens reported in this paper were tested from the same roll and trimmed from the same set of longitudinal members. The earlier tests were carried out using a pair of aluminum clamps and two LVDT extensometers (Fig. la). The video-extensometer tests reported here were executed without any clip-on extensometers to avoid influencing specimen load-strain response.

3

FIG. 2—Load-extension-time paths for tensile tests. (a) Constant rate of strain (CRS) test, (b) Constant load (creep) test, (c) Sequential CRS and stress relaxation test.

repeatability. Polymer-based materials of the type investigated here exhibit pronounced visco-elastic-plastic behavior. Hence, additional CRS tests were carried out at strain rates of 0.1, 1.0, 10, and 100 % strain/min. up to rupture to investigate the influence of strain rate on load-extension behavior. Constant load (creep) tests (ASTM D 5262) were carried out by holding the tensile load at a predetermined target value (Fig. 2b). The initial target load was achieved by a constant rate of loading (CRL) sequence applied at 0.5 kN/min. Finally, the load-strain-time response of specimens during a 24-h-duration stress relaxation period carried out in general conformance with ASTM Test Method for Stress Relaxation for Materials and Structure (E 328) was also investigated (Fig. 2c). The target strain value was achieved at the end of an initial CRS loading sequence carried out at 10 % strain/min. These tests are part of a larger study that was undertaken to investigate the influence of load path history on the load-strain-time response of typical geogrid soil reinforcement products. For example, geogrid reinforcement materials in earth structures may experience a loading history in the field characterized by rapid loading during construction, followed by loading that falls between the idealized cases of constant load (creep) and stress relaxation (i.e., constant strain condition) (Walters et al. 2002). Strain Monitoring

Load Paths Different load paths were applied to the specimens. Reference conventional CRS tests (ASTM D 4595) at 10 % strain/min. were carried out to rupture on specimens of each material type (Fig. 2a). Three tests were carried out to investigate test procedure

Multiple targets were painted on the surface of the geogrid specimens at the locations shown in Fig. 1b, c, and d. Unless noted otherwise, axial strains were calculated using displacement measurements for targets “a” and “b” and lateral strains using targets“a” and “d” located close to the center of PET and PP specimens

4 GEOTECHNICAL TESTING JOURNAL (Fig. 1b and c). In the current investigation, six targets were placed on the surface of the HDPE specimens in the pattern illustrated in Fig. 1d. The targets “a” and “d” were located at approximately the midpoint of the longitudinal members between transverse members. This array of targets allowed average strains corresponding to one aperture spacing in longitudinal and transverse directions to be calculated, as well as to investigate the distribution of lateral strains between longitudinal members. Test Results

FIG. 3—Load-strain response from CRS tests (10 % strain/min.).

Figure 3 shows the results of three repeat CRS tests carried out on specimens of each PP, PET, and HDPE material. The curves show that the test method gives repeatable load-strain results between tests. The difference in shape of the curves and the magnitude of strain at rupture is distinctly different between the three specimen types. For example, the PP geogrid exhibited the largest strain values at rupture (about 25 %) and the HDPE material the least (about 10 %). The PET geogrid specimens exhibited a sigmoidal shape that is characteristic of woven and knitted multifilament PET geogrid products. Axial strains recorded between adjacent pairs of targets on two different longitudinal members are plotted in Fig. 4. The PP

FIG. 4—Variation of axial tensile strains during CRS tests. (a) PP geogrid specimen (10 % strain/min.), (b) PET geogrid specimen (10 % strain/min.), (c) HDPE geogrid specimen (10 % strain/min.), (d) HDPE geogrid specimen (0.1 % strain/min.).

SHINODA AND BATHURST ON VIDEO-EXTENSOMETER TECHNIQUE

5

FIG. 5—Variation of lateral strains during CRS tests. (a) PP geogrid specimen (10 % strain/min.), (b) PET geogrid specimen (10 % strain/min.), (c) HDPE geogrid specimen (10 % strain/min.), (d) HDPE geogrid specimen (0.1 % strain/min.).

geogrid exhibited the greatest consistency in strain measurements at nominal identical locations (Fig. 4a). The PET specimen showed some variability that may be due to the more complex mechanical structure of the knitted multifilament product (Fig. 4b). Detectable differences in the distribution of vertical strain were recorded for the HDPE geogrid specimen (Fig. 4c). Nonuniform strain distribution in thick uniaxial drawn HDPE geogrids of the type tested here has also been noted by McGown et al. (1994) and is due to local variation in the cross-sectional area and local modulus of the drawn polymer material. The influence of variation in polymer rheological properties as a result of the drawing process used to manufacture the HDPE material is highlighted by comparing CRS test results run at 10 % strain/min. (Fig. 4c) and 0.1 % strain/min. (Fig. 4d). In both tests, the stiffness of the longitudinal member in the region between the transverse members is markedly higher and the strain at rupture is less than the comparable load-extension response recorded in the vicinity of the junctions where the molecular structure is less oriented. The difference in local load-strain response is more pronounced for the slower test (0.1 % strain/min.) than the test carried out at 10 % strain/min. An advantage of the test equipment employed in this investigation is that lateral displacements of test specimen targets can be

tracked and lateral strains computed. Figure 5a shows lateral strains recorded over gage lengths corresponding to one aperture width of the PP material. The uniformity of lateral strains is apparent in the data. Lateral strains are negligible up to about 5 % tensile strain of the specimen. Thereafter, lateral strains (contraction or necking) are pronounced with values at rupture equal to about 40 % of the tensile strain values at rupture. Lateral strains recorded for the PET geogrid are negligible over the entire range of data in Fig. 5b. The trend in nonuniform distribution of strains noted in the direction of loading for the HDPE specimens appears again in the data for lateral strains (Fig. 5c). The largest contractive strains occur in the region of the junctions, and the least strain occurs at the measurement points located closest to the center of the longitudinal members. The magnitude of lateral contractive strains increased when the CRS tests were carried out at 0.l % strain/min. (Fig. 5d). However, it should be noted that in both HDPE test series, short specimen lengths were used. Hence, the magnitude of the lateral strains measured closest to the clamps may have been attenuated due to friction between the roller clamps and the longitudinal members. However, if it is assumed that this attenuation is negligible, then contraction of the specimen across the open apertures does not occur until about 5 % axial strain. The contractive behavior for the polyolefin geogrids in

6 GEOTECHNICAL TESTING JOURNAL specimens were single strand tests carried out on specimens 300 mm long. The longer specimen lengths were required to accommodate the 100-mm distance between LVDT clamps. The data in Fig. 6a shows that both sets of curves have similar trends. However, there is a difference in average strain recorded by the LVDT clamps, strains recorded over the monitored aperture distances, and in rupture behavior (particularly strains at rupture). Furthermore, there is not a systematic trend in the curves for each data type at similar strain rates. A possible explanation for the discrepancy is slip of the PP longitudinal members between the aluminum brackets that were used to support the pair of LVDT devices. A similar comparison is shown in Fig. 6b using PET geogrid specimens. Again, the trends in the two data sets are similar, but there is a detectable decrease in the apparent stiffness of the specimens using the LVDT clamps, particularly at higher load levels, which may be due to slip of the single strand PET longitudinal members in the LVDT clamps. Potential slip of geosynthetic specimens has already been mentioned as a disadvantage of mechanical clip-on extensometer devices. Constant Load (Creep) Testing Typical constant load (creep) test results are plotted in Fig. 7 for the three specimen types. The specimens were loaded to 60 % of the average rupture load (Tmax ) determined from the reference CRS tests carried out at 10 % strain/min. The strain readings in Fig. 7b and 7c are taken with respect to the beginning of the constant load increment. Lateral creep strains versus axial creep strains are plotted in Fig. 7c. The data in this figure illustrate that lateral creep strains (contraction) are not significant for the knitted polyester geogrid, but are of the same magnitude as the axial creep strain recorded for the HDPE specimen. This result for the HDPE material is consistent with the results of the slow rate of loading CRS test (0.1 % strain/min.) that showed that most of the contraction occurs in the transverse members, where the polymer chains are less aligned (Fig. 5d), and the contractive strains in the transverse members are close to values recorded in the direction of loading. FIG. 6—Axial load-strain response during CRS tests using contact LVDT-extensometers and noncontact video-extensometer methods. (a) PP geogrid specimens, (b) PET geogrid specimens.

this investigation is not unexpected, since contraction of these materials is observed during the drawing process that is used to stretch the materials during manufacture and, thereby, align the polymer molecular chains in preferential directions. This drawing process can be understood to be restarted at large axial strains in these tests, at which time further lateral contraction of the specimen occurs.

Influence of Rate of Strain and Comparison of Local Strain Measurements and LVDT Extensometer Strains Figure 6 shows CRS load-extension curves for specimens of PP and PET geogrid carried out at different strain rates. In general, the PP geogrid specimens showed lower secant stiffness values with decreasing rate of strain loading (Fig. 6a). The load-extension response of PET specimens was essentially rate-of-strain independent. A similar conclusion has been reported by Bathurst and Cai (1994), who carried out CRS tests on similar PET geogrids over a range of strain rates varying from 1 to 10 % strain/min. Superimposed on the figures are data from CRS tests using LVDT extensometers to measure specimen strain. The PP specimens in these tests were 200 mm wide by 300 mm long. The PET

Stress Relaxation Typical stress relaxation test results for specimens of the three geogrid materials investigated are shown in Fig. 8. The breakpoints in the curves of Fig. 8a correspond to the beginning of the stress relaxation stage, which was initiated by holding the clamp crosshead in a fixed position after ramping to the target load level using a constant rate of strain load path. Figure 8b shows normalized load reduction with time taken with respect to start of the stress relaxation stage. The internal strain response between target points “a” and “c” in the specimens was monitored using the video-extensometer. It is interesting to note that while the boundaries of the test specimens were fixed, there were internal adjustments in load-strain over the monitored lengths of PP and HDPE specimens, which resulted in a deviation of the tests from an ideal fixed strain boundary condition. This highlights the challenge of carrying out relaxation tests on drawn polyolefin geogrid products with the expectation of developing a constant strain condition because of the influence of nonuniform geometry and polymer mechanical properties over the length of the specimens and slip in the clamps. Conclusions The paper reports a novel technique to record displacements and compute local strains at the surface of typical geogrid reinforcement

SHINODA AND BATHURST ON VIDEO-EXTENSOMETER TECHNIQUE

7

FIG. 8—Load-strain-time response for sequential CRS and stress relaxation tests. (a) Load-strain response, (b) Normalized load versus time during relaxation stage.

FIG. 7—Load-strain-time response for constant load (creep) tests. (a) Load-strain response, (b) Axial strain versus time during constant load stage, (c) Lateral strain versus axial strain during creep loading.

products using a noncontact high resolution digital CCD camera (video-extensometer) system. The technique is demonstrated for different load paths using conventional in-isolation tensile test methods. The technique has the advantage over clip-on mechanical

devices that may slip on the test specimen and, therefore, underestimate reinforcement stiffness. The methodology and multiple-target tracking capability allows the internal magnitude and distribution of strains in both axial and transverse directions to be recorded accurately and in real time. Example test results presented in this paper highlight that local axial reinforcement strains for uniaxial drawn HDPE geogrids are nonuniform at large axial strains and the magnitude of nonuniformity is sensitive to rate of loading. This observation has important implications for the selection of appropriate stiffness values to be used in the back-analysis of loads in reinforcement using strain readings from in-isolation and in-soil monitoring (Walters et al. 2002). Finally, local lateral contractions of HDPE and PP test specimens were recorded, particularly at large axial strains, which have implications to soil-geogrid interaction mechanisms assumed for reinforced soil structures.

Acknowledgments The first author is grateful for the financial support from Integrated Geotechnical Institute Limited of Tokyo, Japan to carry out the work described in this paper while a visiting research fellow

8 GEOTECHNICAL TESTING JOURNAL with the GeoEngineering Centre at Queen’s-RMC at the Royal Military College of Canada, Kingston, Ontario, Canada. Financial support was also provided by the Natural Sciences and Engineering Research Council of Canada and the Department of National Defence (Canada) in the form of equipment and operating grants awarded to the second author. References Bathurst, R. J., Allen, T. M., and Walters, D. L., 2002, “ShortTerm Strain and Deformation Behavior of Geosynthetic Walls at Working Stress Conditions,” Geosynthetics International, Vol. 9, Nos. 5–6, pp. 451–482. Bathurst, R. J. and Cai, Z., 1994, “In-isolation Cyclic LoadExtension Behavior of Two Geogrids,” Geosynthetics International, Vol. 1, No. 1, pp. 1–19. Bathurst, R. J., Walters, D. L., Hatami., K., and Allen, T. M., 2001, “Full-Scale Performance Testing and Numerical Modelling of Reinforced Soil Retaining Walls,” Special Plenary Lecture: International Symposium on Earth Reinforcement, IS Kyushu 2001, Fukuoka, Japan, 14 November 2001, Vol. 2, pp. 777–799. Castleman, K. R., 1996, “Digital Image Processing,” Englewood Cliffs, NJ, Prentice Hall. Gachet, P., Klubertanz, G., Vulliet, L., and Lalout, L., 2003, “Interfacial Behavior of Unsaturated Soil with Small-Scale Models and Use of Image Processing Techniques,” Geotechnical Testing Journal, Vol. 26, No. 1, pp. 12–21. Hirakawa, D., Uchimura, T., Shibata, Y., and Tatsuoka, F., 2002, “Time-Dependent Deformation of Geosynthetics and Geosynthetic-Reinforcement Soil Structures,”Proceedings of the 7th International Conference on Geosynthetics, Nice, France, October 2002, Vol. 4, pp. 1427–1430. Jones, D., 2000, “Wide-Width Geotextile Testing with Video Extensometry,” Grips, Clamps, Clamping Techniques, and Strain Measurement for Testing of Geosynthetics, ASTM STP 1379, P. E. Stevenson, Ed., ASTM International, West Conshohocken, PA, pp. 83–88.

Leshchinsky, D. and Fowler, J., 1990, “Laboratory Measurement of Load-Elongation Relationship of High-Strength Geotextiles,” Geotextiles and Geomembranes, Vol. 9, pp. 145–164. McGown, A., Yogarajah, I., Andrawes, K. Z., and Saad, M. A., l994, “Strain Behaviour of Polymeric Geogrids Subjected to Sustained and Repeated Loading in Air and in Soil,” Geosynthetics International, Vol. 1, No. 3, pp. 341–355. Messphysik, 1997, “Videoextensometer for Windows,” Manual No. 197, Messphysik Materials Testing, Altenmarkt 180, A-8280 Fuerstenfeld, Austria. Paikowsky, G. and Xi, F., 2000, “Particle Motion Tracking Utilizing a High-Resolution Digital CCD Camera,” Geotechnical Testing Journal, Vol. 23, No. 1, pp. 123–134. Perkins, S. W. and Lapeyre, J. A., 1997, “In-Isolation Strain Measurement of Geosynthetics in Wide-Width Strip Tension Test,” Geosynthetics International, Vol. 4, No. 1, pp. 11–32. Recker, Ch., Elias, J. M., Schroer, S., and Muller-Rochholz, J., 2001, “Untersuchungen der Hochstzugkraft im Breitstreifenund Einzelstrangversuch nach DIN EN ISO 10319, ASTM D 4595 und GRI 1 an geschweiBten und gewebten PP und PET Geogittern,” 7. lnformations- und Vortragstagung uber “Kunststoffe in der Geotechnik,” Munich, March 2001, pp. 163– 167. Skochdopole, T. R., Cassady, L., Pihs, D., and Stevenson, P. E., 2000, “Comparative Study of Roller and Wedge Grips for Tensile Testing of High Strength Fabrics with Laser Extensometry: Comparisons to LVDT and Crosshead Extension,” Grips, Clamps, Clamping Techniques, and Strain Measurement for Testing of Geosynthetics, ASTM STP 1379, P. E. Stevenson, Ed., pp. 68–79. Tatsuoka, F. and Shibuya, S., l992, “Deformation Characteristics of Soil and Rocks from Field and Laboratory Tests,” Report of the Institute of Industrial Science of the University of Tokyo. Walters, D., Allen, T. M., and Bathurst, R. J., 2002, “Conversion of Geosynthetic Strain to Load using Reinforcement Stiffness,” Geosynthetics International, Vol. 9, Nos. 5–6, pp. 483–523.