LOAD TRANSFER MECHANISM IN PULL-OUT TESTS

Transcription

1 Technical Paper by I.M. Alobaidi, D.J. Hoare and G.S. Ghataora LOAD TRANSFER MECHANISM IN PULL-OUT TESTS ABSTRACT: This paper presents a numerical method to predict soil-geotextile interface friction parameters. A strain softening model was used to simulate the relationship between shear stress and horizontal displacement at the soil-geotextile interface. Pullout tests were performed on two types of geotextile, with different tensile stiffnesses, embedded in a granular soil. For each geotextile, pull-out tests were performed at confining pressures of 20, 50, 100, and 200 kpa. It was found that, unless breakage of the geotextile occurs, the peak pull-out force occurs after a small displacement of the free end of the geotextile. At the peak pull-out force, the maximum shear stress occurs near the free end of the geotextile while the shear stress at the loaded end is at or near a residual value. The use of an average interface friction angle overestimates pull-out resistance. The numerical technique developed in this paper provides more accurate values for soil-geotextile interface friction parameters. KEYWORDS: Reinforcement, Pull-out, Geotextile, Friction, Shear stress, Laboratory testing. AUTHORS: I.A. Alobaidi, Research Fellow, D.J. Hoare, Senior Lecturer, and G.S. Ghataora, Lecturer, School of Civil Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom, Telephone: 44/ , Telefax: 44/ /3675. PUBLICATION: Geosynthetics International is published by the Industrial Fabrics Association International, 1801 County Road B West, Roseville, Minnesota , USA, Telephone: 1/ , Telefax: 1/ Geosynthetics International is registered under ISSN DATES: Original manuscript received 8 February 1997, revised version received 6 July 1997 and accepted 21 July Discussion open until 1 May REFERENCE: Alobaidi, I.M., Hoare, D.J. and Ghataora, G.S., 1997, Load Transfer Mechanism in Pull-Out Tests, Geosynthetics International, Vol. 4, No. 5, pp

2 1 INTRODUCTION When designing reinforced soil structures, such as retaining walls and slopes, the values of soil-reinforcement interface shear strength parameters are required. Conventional design uses limit equilibrium methods which assume that all of the points along the reinforcing elements reach the limit state simultaneously. This assumption is more appropriate for the case of metallic reinforcing strips which are virtually inextensible and the displacement at the point of load application at the peak pull-out force is relatively small. Polymeric reinforcement, such as geotextiles and geogrids, require a much larger displacement than metallic reinforcement to develop internal forces. Therefore, the shear stresses developed along polymeric reinforcement are not uniform and the above assumption is not valid. Soil-reinforcement interface strength parameter values may be determined from a pull-out test or a direct shear test; the limit shear resistances obtained from these tests are different (Jewell et al. 1984; McGown 1978; Collios et al. 1980). Pull-out tests have been conducted by many researchers (McGown 1978; Ingold 1983; Juran and Chen 1988; Wilson-Fahmy et al. 1994; Yogarajah and Yeo 1994) and are acknowledged to more closely simulate in situ conditions (Venkatappa and Kate 1990). By measuring the shear stresses along a geotextile in a pull-out test, McGown (1978) found that the shear stresses are not uniform, but concentrate near the end where the pull-out load is applied. Therefore, to determine the interface friction parameters in a pull-out test, a back calculation method is required. In this paper, pull-out test results are presented along with a numerical technique that uses the test results to calculate the soil-reinforcement interface friction parameters. Based on these parameters, a comparison is made between measured and predicted pull-out forces and displacements along geotextiles. A comparison of the friction parameters of the two geotextiles based on the assumption of a uniform shear stress distribution and the predicted parameters is also made. The effect of geotextile extensibility was investigated by employing two geotextiles with different tensile stiffnesses. The laboratory work was undertaken in 1985 by Eltayeb (1986) and was re-analysed by the authors in MATERIALS, EQUIPMENT, AND TESTING All of the tests used Leighton Buzzard sand with a specific gravity of 2.65 and a particle size distribution as shown in Figure 1. Two types of woven geotextile were employed: Geolon 70 and Lotrak 16/15, which are hereafter denoted Geotextile A and B, respectively. The technical data supplied by the manufacturers are summarised in Table 1. The decision to use these particular geotextiles was made on the basis of the type and nature of the laboratory test equipment, and on the need to measure significant deformation at low applied loads. The pull-out test apparatus consisted of a steel box with internal dimensions of 570 mm long, 370 mm wide, and 500 mm deep (Figure 2). A servo-hydraulic testing machine was used to apply a horizontal pull-out displacement at a rate of 1 mm/minute. Dry sand was placed in the box using a sand spreader. A sand density of 510

3 Percentage passing (%) Particle size (mm) Figure 1. Particle size distribution of the Leighton Buzzard sand kg/m 3 was achieved by varying the width of the slit in the spreader and the height of drop. Geotextile extension was measured using a stainless steel, 0.71 mm diameter wire with a Teflon (P.T.F.E) tube sleeve having a bore size of 0.89 mm and a thickness of 3 mm. The wire was hooked and sewed to the geotextile at one end and clipped to a chain outside of the system from the other end. The chain was connected to a potentiometer to measure the movement of the steel wire attached to the geotextile. The pull-out force was measured by a load cell fixed to the clamp holding the geotextile. Eltayeb (1986) provides full details of the test apparatus. A layer of sand was placed using the sand spreader to the level of the slit (approximately 150 mm thick). A geotextile specimen was then clamped at the front of the box and laid on top of the first sand layer. Three wires were hooked and fixed to the geotextile at 50, 100, and 150 mm from the free (embedded) end. The same procedure was then used to place the second 150 mm layer of sand. A vertical normal stress was applied using an air bag. Table 1. Physical properties of the geotextiles used in the present study (data supplied by the manufacturer). Property Geotextile A Geotextile B Polymer Polyester Polypropylene Structure Woven filament Woven, extruded tape Tensile strength (kn/m) Extension at maximum load (%) Mass per unit area (g/m 2 ) Thickness (mm) N/A 0.3 Note: N/A = not applicable. 511

4 (a) Anchored end of geotextile Geotextile clamp Applied force Positions along geotextile where extension was measured Sand Sand Steel box Load cell for measuring the pull-out force R Steel wire for geotextile anchorage Wood blocks Concrete and wood blocks Steel table Stainless steel wires (b) Geotextile specimen 200 mm 200 mm Applied force Moving clamp Fixed clamp mm Sleeved wires for extension measurement in the geotextile Figure 2. The pull-out test apparatus: (a) side view; (b) plan view (Eltayeb 1986). 512

5 For each type of geotextile, two pull-out tests were performed using a sand with a porosity of 35 1% and at vertical stresses of 20, 50, 100, and 200 kpa. Thus, a total of eight pull-out tests were performed on each geotextile. 3 NUMERICAL ANALYSIS 3.1 Load Transfer Model The following differential equation models the tension-displacement relationship of geotextiles (Juran and Chen 1988): τ x = 1 2 te g where: τ x = shear stress between the geotextile and the soil at point x (x = horizontal distance along the geotextile); y = displacement of the geotextile at point x; E g = elastic stiffness of the geotextile; and t = thickness of the geotextile. d 2 y dx Strain Softening Model of the Soil-Geotextile Interface The normal practice in the construction of reinforced soils is to compact the soil in layers; therefore, it is reasonable to assume that the soil is in a dense state. For the dense, granular soil used in the present study, the residual strength is less than the peak strength. The following equation was used to simulate the relationship between shear stress and horizontal displacement at the soil-geotextile interface (Juran et al. 1988): (1) τ(y) = cy σ y y a ( y + b) 2 (2) where: τ(y) = shear stress; y = horizontal displacement; d = thickness of the interface layer (sheared zone); σ y = normal stress; and a is equal to the following expression: a = 4 σ y d G (tan 2 δ p J 2 ) tan δ r (3) where: G = tangent shear modulus at the soil-geotextile interface; δ p = peak soil-geotextile friction angle; δ r = residual soil-geotextile friction angle; and b, c, andj are equal to the following expressions: b = 2 σ y d G (tan δ p J) c = tan δ r J = 1 + (1 tan δ r tan δ p ) 2 (4) (5) (6) 513

6 The soil-geotextile interface friction parameters are related to the internal soil (soilsoil) friction parameters as follows: tan δ p = R 1 tan φ p tan δ r = R 2 tan φ r (7) (8) G = R 3 G s (9) where: G s = shear modulus at the soil-soil interface; φ p = peak internal soil friction angle; φ r = residual internal soil friction angle; and R 1, R 2,andR 3 are constants. It is reasonable to assume that the geotextile influences the interface properties in a consistent manner; hence R 1, R 2,andR 3 are assumed to be equal and are subsequently denoted by the parameter R. Combining Equations 1 and 2 yields: d 2 y dx 2 = 2σ y E g cy y a (y + b) 2 (10) Equation 10 was solved numerically using the Runga-Kutta-Nyström method (Kreyszig 1993); a computer program was developed and written to solve the problem. The values of R and, consequently, the interface friction parameters were determined using a trial and error procedure. Adopting an initial value of unity for R, the pull-out force was calculated numerically and compared with the experimental value. Then, the value of R was changed until the numerical and experimental results agreed to within 1%. The interface friction parameters were then calculated using Equations 7, 8, and 9. 4 ANALYSIS OF RESULTS The relationship between the normalised shear stress (with respect to confining pressure) and the horizontal displacement of the sand was obtained using 300 mm 300 mm direct shear tests. The results of these tests are shown in Figure 3 for confining pressures of 50 and 100 kpa. The numerical solution of the relationship between the horizontal displacement and the shear stress was obtained using Equation 2 and is also shown in Figure 3. The values of the parameters in Equation 2 were chosen to provide the best fit to the experimental results. The sand-geotextile interface friction parameters were then derived from the sand-sand parameters using the reduction factor R defined in Section 3. The comparison between the experimental and the numerical pull-out test results are shown in Figures 4 and 5 for Geotextiles A and B, respectively. There is a closer fit between the numerical model and the experimental results at low confining pressures, and the numerical model predicts the displacement of the loaded end at the peak pull-out force quite well. The displacement of the loaded end of Geotextile A and B at the peak pull-out force increases with an increase in the confining pressure. In general, the pre- 514

7 Normalised shear stress, τ(y) / σ y Numerical model Horizontal displacement, y (mm) Figure 3. sand. Stress versus displacement from direct shear tests using the Leighton Buzzard Experimental Numerical Pull-out force (kn/m) σ y =20kPa σ y = 200 kpa Displacement of loaded end, y (mm) Figure 4. Pull-out force versus displacement for Geotextile A. 515

8 Experimental Numerical σ y =20kPa Pull-out force (kn/m) σ y = 200 kpa Displacement of loaded end, y (mm) Figure 5. Pull-out force versus displacement for Geotextile B. dicted pull-out force after peak load is higher than the measured value. This suggests that the residual friction angle ratio, R 2, is smaller than the calculated value for R. The measured and the predicted displacements along the two geotextiles at the peak pull-out force are shown in Figures 6 and 7. Although there is scatter in the experimental results, the general trends of the numerical model results and the experimental results are similar. It can be seen from Figures 6 and 7 that without any breakage of the geotextile, the peak pull-out force took place after a very small displacement of the free end of the geotextile. When the geotextile ruptured (Figure 7, specimen at a confining pressure of 200 kpa), the peak pull-out force occurred at the rupture point before full mobilisation of the shear stresses along the whole length of the geotextile. The shear stresses along Geotextiles A and B at the peak pull-out force are shown in Figures 8 and 9. It is evident that the maximum shear stresses at the peak pull-out force occurred near the free end of the geotextile. The shear stress values at the loaded end are similar to the residual values because only a small displacement is required to mobilise the peak shear stress. Therefore, at the beginning of the pull-out test, the peak shear stress develops at the front (loaded) end of the geotextile, and there are no shear stresses toward the free end of the geotextile. As the frontal displacement increases, the shear stress at the front end reduces to the residual value, while the peak shear stress occurs further away from the loaded end. The variation of the interface friction angles with confining pressure for Geotextile A and B are shown in Figures 10 and 11, respectively. As mentioned in Section 3.2, the peak and residual friction angles of the soil-geotextile interface were estimated using the corresponding values for the soil and the interface friction factor R. An average friction angle was calculated from the test results assuming a uniform shear stress distribu- 516

9 Displacement, y (mm) σ y =20kPa σ y = 200 kpa Experimental Numerical Distance from free end, x (mm) Figure 6. Displacement along Geotextile A at the peak pull-out force. Displacement, y (mm) σ y =20kPa σ y = 200 kpa Experimental Numerical Distance from free end, x (mm) Figure 7. Displacement along Geotextile B at the peak pull-out force. tion. Generally, all of the angles for the stiffer geotextile (Geotextile A) were higher than the corresponding angles for the less stiff geotextile (Geotextile B). For both types of geotextile, the values of the peak, average, and residual friction angles decrease with increasing confining pressure. The average friction angle is closer to the peak angle for 517

10 Shear stress, τ x (kpa) σ y = 200 kpa σ y =20kPa Distance from free end, x (mm) Figure 8. Shear stress along Geotextile A at the peak pull-out force. Shear stress, τ x (kpa) σ y = 200 kpa σ y =20kPa Distance from free end, x (mm) Figure 9. Shear stress along Geotextile B at the peak pull-out force. the less extensible geotextile. This behaviour may be interpreted by referring to Figures 8 and 9 where the peak shear stress zone spreads over a greater length of the geotextile for the stiffer geotextile. As the length of the geotextile increases, the effect of the high shear stress zone decreases and the average friction angle tends to the residual angle: 518

11 Tan δ Peak Average Residual Normal stress, σ y (kpa) Figure 10. Soil-Geotextile A interface friction angles. Tan δ Peak Average Residual Normal stress, σ y (kpa) Figure 11. Soil-Geotextile B interface friction angles. this has an important practical implication. The average friction angle obtained from the pull-out tests cannot be directly applied in practical design because it overestimates the peak pull-out resistance. However, the use of peak and residual friction angles ob- 519

12 tained by numerical analyses of pull-out tests can provide more realistic results irrespective of the length of the geotextile. 5 CONCLUSIONS The following conclusions and observations are made: S A numerical technique was developed to determine soil-geotextile interface friction parameters from a pull-out test. The model closely predicts the measured displacement of the loaded-end at the peak pull-out force. S Unless breakage of the geotextile occurred, the peak pull-out force took place after a small displacement of the free end of the geotextile. S At the peak pull-out force, the maximum shear stress occurred near the free end of the geotextile, while the shear stress at the loaded end was at or near the residual value. S The use of average interface friction angles overestimates the pull-out resistance when geotextiles longer than those used in the laboratory experiments described in the present study are used. The adoption of the model developed in this paper can overcome this problem and provide more realistic predictions of the soil-geotextile pull-out capacity. REFERENCES Collios, A., Delmas, P., Gourc, J.P., and Giroud, J.P., 1980, Experiments on Soil Reinforcement With Geotextiles,The Use of Geotextile for Soil Improvements, ASCE National Convention, Portland, Oregon, USA, April 1980, pp Eltayeb, I.M., 1986, Some Aspects of the Behaviour of Geotextile Reinforcement in Sand, Ph.D. Thesis, The University of Birmingham, United Kingdom, 272 p. Ingold, T.S., 1983, Laboratory Pull-Out Testing of Grid Reinforcement in Sand, Geotechnical Testing Journal, Vol. 6, No. 3, pp Jewell, R.A., Milligan G.W.E., Sarsby, R.W. and DuBois, D., 1984, Interaction Between Soil and Geogrids, Polymer Grid Reinforcement, Thomas Telford, 1985, Proceedings of a conference held in London, United Kingdom, March 1984, pp Juran, I. and Chen, C.L., 1988, Soil-Geotextile Pull-Out Interaction Properties: Testing and Interpretation, Transportation Research Record, No. 1188, pp Juran, I., Ider, M.H., Chen, C.L. and Guermazi, A., 1988, Numerical Analysis of the Response of Reinforced Soils to Direct Shearing: Part 2, International Journal for Numerical Methods in Geomechanics,Vol. 12,No. 2, pp Kreyszig, E., 1993, Advanced Engineering Mathematics, John Wiley & Sons Inc., New York, New York, USA, 1271 p. 520

13 McGown, A., 1978, The Properties of Non-Woven Fabrics Presently Identified as Being Important in Public Works Applications, Proceedings of INDEX 78 Congress, European Disposables and Non-wovens Association, Congress held in Amsterdam, The Netherlands, Session I, April 1978, pp Wilson-Fahmy, R.F., Koerner, R.M. and Sansone, J.L., 1994, Experimental Behaviour of Polymeric Geogrids in Pull-Out, Journal of Geotechnical Engineering, Vol. 120, No. 4, pp Venkatappa, R.G. and Kate, J.M., 1990, Interface Friction Evaluation of Some Indian Geotextiles, Proceedings of the Fourth International Conference on Geotextiles, Geomembranes and Related Products, Balkema, Vol. 2, The Hague, The Netherlands, p Yogarajah, I. and Yeo, K.C., 1994, Finite Element Modelling of Pull-Out tests With Load and Strain Measurements, Geotextiles and Geomembranes, Vol. 13, No. 1, pp NOTATIONS a b c d E g G G s J Basic SI units are given in parentheses. = constant (m) = constant (m) = constant (m) = thickness of interface layer (sheared zone) (m) = elastic stiffness of geotextile (Pa) = tangent shear modulus at soil-geotextile interface (Pa) = shear modulus at soil-soil interface (Pa) = constant (dimensionless) R, R 1, R 2, R 3 = reduction factors (dimensionless) t = thickness of the geotextile (m) x = horizontal distance along the geotextile (m) y = horizontal displacement of the geotextile at point x (m) δ p = peak soil-geotextile interface friction angle (_) δ r = residual soil-geotextile interface friction angle (_) σ y = normal stress (Pa) τ x = shear stress between the geotextile and soil at point x (Pa) τ(y) = shear stress (Pa) φ p = peak internal soil friction angle (_) φ r = residual internal soil friction angle (_) 521