Determination of frictional properties of geotextiles

Geotextiles and Geomembranes 6 (1987) 307-314

Technical Note Determination of Frictional Properties of Geotextiles

A BS T R A C T The influence o f some test conditions on the measured frictional properties o f geotextiles is discussed. Conclusions and suggestions are presented, based on research on s o m e non-woven and woven geotextiles in a small (10 cm x 10 cm ) and large (25 cm x 40 cm ) direct shearbo xes as well as f r o m pull-out tests. Specifications f o r testing frictional properties o f textiles, which have been published in Poland, are also briefly presented.

1 INTRODUCTION A n internationally accepted method for testing frictional properties of geotextiles has not yet been developed. Direct shear tests as well as pull-out tests are used. Differences between methods of evaluation of friction coefficient in soil-geotextile or soil-geomembrane interfaces are sometimes quite significant. Ingold I specifies five basic types of apparatus which can be used for direct shear testing and puts 20 questions which need to be answered before a specification for testing geomembranes is developed. Detailed information concerning test conditions for geotextiles is also required; we present some conclusions and suggestions derived from research on frictional properties of geotextiles.

2 R E G U L A T I O N S P U B L I S H E D IN P O L A N D Two specifications for testing frictional properties of textiles have been published in Poland. (1) Polish Standard 2 defines conditions for evaluating the coefficient of friction between two planar layers of textile material or between textile and 307 Geotextiles and Geomembranes 0266-1144/87/$03-50 © 1987 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain

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plywood (for testing of carpets). The standard is used in the textile industry for acceptance of produced textiles. Figure 1 presents an outline of the test m e t h o d . Dimensions of samples are: width of the lower textile layer, 60 mm; the length of the lower textile layer should enable at least 25 mm displacem e n t of the carriage; base of the carriage, 40 mm x 60 mm. Rate of carriage displacement is 1 mm/s. The test is performed for one normal stress only, with the magnitude of this stress depending on the material tested. This test is not considered adequate for measurement of frictional properties of geotextiles or geomembranes, mainly because of sample dimensions. (2) Figure 2 shows the layout of a test developed in the USSR 3'4 and the test apparatus. The test is a pull-out test. The sides of the sample are fixed in a frame, which prevents necking of the sample. Inside dimensions of the box are 100 m m x 200 mm. The box seems to be too small for testing some very coarse soils, e.g. gravel. The width of geotextile sample should be sufficient to enable it to be clamped in the frame; the length of the sample is not specified. Basic conditions of the test are as follows: duration of application of normal stress before commencement of shear stress is 1 hour; shear is applied in six to eight increments; shear stress is increased after 1 minute when the measured rate of displacement of the sample at point B in Fig. 2 is less than 0.2 mm/min. The test for one normal stress level is finished when m o v e m e n t of point B is uniform or displacement exceeds 10 mm. The test is repeated for a further two or three values of normal stress level which should be related to normal stress levels expected in the field.

3 SUGGESTIONS AND CONCLUSIONS Research has been carried out in the Geotechnical Department of the Technical University of Gdafisk using a large shearbox and the conventional 100 m m x 100 m m shearbox. Direct shear tests were carried out using dif-

Determination of frictional properties of geotextiles

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ferent types of non-woven needle-punched geotextiles and multifilament woven geotextiles in contact with dry sand or clay at one moisture content and density only. In addition pull-out tests were conducted on geotextiles confined in dry sand in the large box. (1) It appears that the direct shear test would be preferable to the pull-out tests as the standard test method since results of the pull-out test are difficult to interpret for the following reasons: (a) Shear stresses are not uniformly distributed before movement of the sample occurs. (b) The part of the sample geotextile between the clamp and soil is, by definition, not confined in soil. For tests on non-woven geotextiles this part of the sample elongates significantly before m o v e m e n t of the geotextile in the soil begins. (c) T h e r e is necking, especially in case of thick and soft non-woven geotextiles. This can be prevented by using special clamps or frames to impose plane strain conditions. (d) The measured horizontal force consists of two components: (i) the force connected with elongation of the free length of geotextile outside the box and (ii) the force connected with the shear forces developed inside the box. Consequently detailed analysis of friction developing at the soil-geotextile interface, especially for non-woven geotextiles, requires results of tensile tests. However, performing pull-out tests can be desirable in some cases, as they are closely connected with the failure mechanism of the structure. Williams & Houlihan 5 express an opinion that since in pull-out tests the horizontal load tends to stretch the fabric, the friction parameters are likely to be lower than measured in direct shear tests. 5 (2) Partially fixed shearboxes were found to be a good tool for determining frictional properties of geotextiles. For this test the geotextile is clamped along the trailing edge of the lower half of the shearbox. In our opinion the part of the sample between the clamp and soil should be as short as possible. O n e possible solution is to fasten the sample to the inside surface of the lower half of the shearbox. Such a solution was used in our large box. At the beginning of some tests performed in the small box under high normal stress on very thick and soft non-woven geotextiles against clay, m o v e m e n t of the upper half of the box did not cause shearing at the soil-geotextile interface but elongation of the 'free' part of the sample. (3) The range of normal stresses used in testing should be related to those in the field. Normal stresses used in our experiments ranged from 25 to 100 kPa. One Polish geotextile was additionally tested under normal stress

Determination or frictional properties of geotextiles

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200 kPa, but this appeared to be too high for the non-woven geotextiles which were tested. The incremental angle of bond stress obtained for sand was found to decrease with increasing normal stress. Similar results have been obtained 6 for normal stresses from 25 to 75 kPa. (4) The size of the shearbox affects values of the efficiency coefficient. Angles of bond stress, 8, obtained for the same sand, geotextile and normal stress in the small box always gave smaller values of efficiency coefficient

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Determination of frictional properties of geotextiles

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than the large box. This is reflected in Figs 3 and 4. Many researchers confirm that small samples give lower values of efficiency coefficient. The difference between the maximum shear stress obtained from tests on the same geotextile and sand in the large and small box increased as the normal stress increased. This was more distinct for smooth geotextiles such as woven multifilament (Fig. 3). Experiments 7 demonstrating smaller values of shear stress in a small box ( 5 c m x 5 c m ) than in a large box (30 cm x 30 cm) under the same normal stress, showed that both shear stresses became equal for normal stresses exceeding 700 kPa. In our research programme, tests under such large normal stresses were not included. (5) The size of a shearbox should be related to the maximum particle size of the soil used. In our opinion however it should also be related to the shearbox displacement at which the maximum shear stress is indicated. Shearbox displacements at the maximum shear stress in our tests on dry sand were 18-35 m m in the large box and 4--11 mm in the small box. The displacement at which peak shear stress occurred was also found to increase as the geotextile became thicker and softer. For soft clay the displacements at the m a x i m u m shear stress were approximately 1.5-2 times greater. This excluded tests on some very thick and soft non-woven geotextiles in the small box, although the soil was fine. The suggestion that the shearbox should have length (in the direction of shearing) eight to 10 times bigger than m a x i m u m displacement seems to be reasonable. Thickness of the lower half of the shearbox should guarantee that contact area between soil and geotextile does not decrease during the test. (6) Tests were performed at a constant rate of shearbox displacement. This was 6 m m / m i n for the large box and 1 mm/min for the small box. Since the length of the samples, in the direction of shearing, used in the large box was 400 ram, this rate of displacement gives rise to a rate of strain of 1-5%/min. The gearing of the boxes did not allow tests with the same rate of strain in both cases. The large box is now being redesigned to allow displacem e n t rates of 1 m m / m i n and less. The displacement rate for tests in the small box was chosen to give a rate of strain as close as possible to that in the large box. For the small box this was l%/min. Comparison of tests performed in the large and small box on different geotextiles and dry sand led to the following conclusions. (a) For smooth geotextiles, for example woven multifilament, values of strain obtained from both tests did not differ significantly for a given normal stress; however, the difference in the values of absolute displacement was significant, (b) As the thickness of the geotextile increased and surface smoothness decreased, for example thick non-woven geotextiles, the difference

314

E. Dernbicki, J. A lenowicz b e t w e e n values of absolute displacement at maximum shear stress from both tests decreased but the strain differed more significantly. The plots of shear stress (Figs 3 and 4), T, versus strain, s/lo, for a Swedish woven multifilament geotextile 'AB Foderv~rnader' and a Polish non-woven needle-punched geotextile 'Anilana' are a good illustration of this phenomenon.

(7) For soft and thick geotextiles values of efficiency coefficient obtained from experiments were high, in some cases slightly exceeding 1-00. (8) The degree of compaction specified for laboratory tests should be related to the degree of compaction required on site. This means that the density specified should be at least 95% of the maximum density from the normal Proctor test. On the other hand in some specific cases research on the influence of density on frictional properties would be desirable. (9) The number of tests performed on clay to date is too small to allow conclusions to be drawn. With cohesive soils difficulties increase, compared with tests on granular soils.

REFERENCES 1. Ingold, T. S., Measurement of frictional properties, RILEM 103-MHG Committee Meeting, September 1987, Mechanical and Hydraulic Properties of Geomembranes. 2. PN-74/P-04615 Textiles. Determination of Friction CoeJficient. 3. Guidelines for Increasing Bearing Capacity of Embankments with Use of. Synthetic Fabrics, CMEA, Sofia 1985 (in Russian). 4. Guidelines for Building Embankments on Weak Subsoil with Use of" Synthetic Fabrics, Roads and Bridges Research Institute, Warsaw, 1986 (in Polish). 5. Williams, N. D. & Houlihan, M., Evaluation of friction coefficients between geomembranes, geotextiles and related products. 3rd International Conference on Geotextiles, Vienna, 3(1986) 891-6. 6. Collios, A., Loi d'interaction m6canique sol-g6otextile. Th6se pr6sent6e ~t L'Universit6 Scientifique et M6dicale de Grenoble pour obt~nir le titre de Docteur--Ing6nieur, Grenoble, Juin 1981. 7. Degoutte, G. & Mathieu, G., Etude experimentale du frottement solmembranes et sol-g6otextiles ~ l'aide d'une boite de Casagrande de 30 × 30 c m 2. 3rd International Conference on Geotextiles, Vienna, 3 (1986) 791-6.

Eugeniusz Dembicki & Jacek Alenowicz Technical University o f Gdahsk, Ma]akowskiego 11/12, 80-952 Gdatisk, Poland (Received 4 December 1987; accepted 2 February 1988)