Mechanical Behavior of Soil Geotextile Composites: Effect of Soil Type

Transcription

1 Mechanical Behavior of Geotextile Composites: Effect of Type A.I. Droudakis and I.N. Markou Department of Civil Engineering, Democritus University of Thrace, Greece 12 Vas. Sofias str., GR-671 Xanthi, Greece ABSTRACT Triaxial compression tests were conducted on specimens of a granular and a cohesive soil both reinforced with 5 horizontal layers of woven and non-woven geotextiles. The two soils used in this investigation were dry and dense Ottawa 2-3 sand and a fine-grained soil of low plasticity compacted with water content close to optimum water content using energy comparable to that of Standard Proctor compaction test. Stress strain relationship, shear strength and failure mode of reinforced soil specimens are used for the comparison of mechanical behavior of these reinforced soils. It is observed that reinforcement effect on strength and axial strain at failure is greater in sand than in fine-grained soil. This observation can be attributed to the better sand geotextile cooperation in comparison to the insufficient cooperation between fine-grained soil and geotextiles. Triaxial compression tests yielded bilinear failure envelopes for both reinforced soils attributed, however, to different behavior. Introduction Geotextile reinforced soil is used in a large number of applications because of its cost and engineering effectiveness. Free draining granular materials, e.g. sands, are specified as backfill material for reinforced soil structures. The main reasons for use of granular materials instead of cohesive soils are the volume change potential and inherent low strength of cohesive soils which make them unsuitable. However, difficulties are encountered when the available quantity of granular materials is insufficient. Since low-plasticity soils are generally not expansive, they could be used in reinforced soil structures provided the reinforcement could increase the load-bearing capacity of the soil [1]. So, it is of merit to investigate the possibility of using geotextiles as reinforcement of cohesive soils, since they are frequently used to improve the drainage characteristics of these soils. The mechanical behavior of sand geotextile composites has been extensively investigated in the past and several research efforts were based on the results obtained from triaxial compression tests [e.g. 2 7]. On the other hand, a number of investigations of the mechanical behavior of geotextile reinforced cohesive soils was also performed by conducting triaxial compression tests [8 12]. Although the results of the above mentioned investigations have provided valuable information, a comparison between reinforced sand and reinforced cohesive soil has not been reported. Toward this end, triaxial compression tests were conducted in order to compare the mechanical behavior of a granular and a cohesive soil both reinforced with geotextiles and the observed results are presented herein. Materials For the purposes of this investigation, Ottawa 2-3 sand and a fine-grained soil were tested. The properties of soils are presented in Table 1 and their grain size curves are shown in Figure 1. Ottawa 2-3 is a uniform, quartz sand consisting of rounded grains. This sand has angle of internal friction, φ, equal to 36 ο (Table 1) in dry condition and at an average relative density of 84%. The fine-grained soil is a cohesive silty soil of low plasticity and is classified as CL according to Unified Classification System (U.S.C.S.). The compaction characteristics of fine-grained soil were obtained by conducting Standard Proctor compaction test. The shear strength parameters of this soil (Table 1) were determined by conducting Unconsolidated- Undrained (UU) triaxial compression tests on specimens compacted with optimum moisture content and compaction energy equal to that of Standard Proctor compaction test. Three non-woven geotextiles and two woven geotextiles provided by five different manufacturers were used during this investigation. These geotextiles were selected in order to have comparable mass per unit area and to cover a wide range of types of commercially available products. More specifically, one thermally bonded (Typar SF 111), one needle-punched with thermally treated surfaces (Fibertex F 5) and one needle-punched (Polyfelt TS 7) polypropylene non-woven geotextiles as well as one high strength polyester (Bonar 15/6) and one standard grade polypropylene (Thrace Plastics 4) woven geotextiles were tested. These geotextiles are designated as TB, TTS, NP, HS, and SG, respectively. Pertinent geotextile properties, according to the manufacturers, are presented in Table 2.

2 Table 1. properties Specific Gravity, G s Grain Size Distribution Atterberg Limits D max :.85 mm : 32% D 5 :.71 mm Silt: 58% D min :.6 mm Clay: 1% - w L : 45% - w P : 18% - I P : 27% Compaction e max :.77 w opt : 19.5% Characteristics e min :.46 γ dmax : 1.7 g/cm 3 Shear Strength c: kpa c: 72 kpa Parameters φ: 36 ο φ: 17 ο Percent finer (by weight) rg Ottawa 2-3 sand soil Grain size, (mm) Figure 1. Grain size curves of soils Geotextile Manufacturing process * Table 2. Geotextile properties Thickness [mm] Mass per unit area [g/m 2 ] Maximum tensile load [kn/m] Tensile test results Extension at maximum load [%] TB NW TTS NW NP NW / 4 # HS W / 63 # 12 / 11 # SG W / 86 # 2 / 14 # * NW: non-woven, W: woven # Machine direction / Cross machine direction

3 Experimental Procedures Conventional laboratory triaxial compression equipment without modifications was used to conduct tests on geotextile reinforced soil specimens, in order to investigate the mechanical behavior of composite materials. The cylindrical specimens prepared, had a diameter of 5 mm and an overall height of 16 mm. Five geotextile discs having a diameter equal to the diameter of the specimen and placed at equal distances perpendicular to the axis of the specimen, were used for soil reinforcement. A schematic representation of the reinforced soil specimens is shown in Figure 2a. Specimen configurations same as that of Figure 2a have been used previously [13, 14] in laboratory investigations using triaxial compression tests. The reinforced sand specimens (Figure 2b) were prepared with compaction of dry sand using a special hand operated tamper and extreme care was taken in order to produce sand layers with constant density. All tests were conducted at a relative density of sand between 84% and 94%, with cell pressure, σ 3, equal to 5, 1, 2, 4 and 6 kpa and at a constant axial displacement rate of.57 %/min. The fine-grained soil was compacted using energy comparable to that of Standard Proctor compaction test, with water content ranging from 18.3% to 19.%. As it can be seen in Figure 2c, all soil layers had the appropriate thickness at the end of the compaction process, indicating a uniform distribution of the compaction energy. fine-grained soil specimens had saturation ratio, void ratio and dry unit weight values ranging from 75% to 9%, from.57 to.68 and from 1.61 g/cm 3 to 1.72 g/cm 3, respectively. The observed differences in the parameter values mentioned above can be attributed to the compressibility of geotextiles used, since it is believed that high geotextile compressibility leads to a decrease in compaction energy and, as a result, to a decrease in compaction degree of the soil mass. All tests on reinforced fine-grained soil specimens were unconsolidated-undrained (UU) and conducted with cell pressures, σ 3, equal to 1, 25, 5, 1 and 2 kpa and at a constant axial displacement rate of.57 %/min, which corresponds to undrained loading conditions. (a) (b) (c) Figure 2. soil specimens: (a) schematic representation, (b) reinforced sand, (c) reinforced fine-grained soil Stress Strain Relationship Typical stress strain curves obtained by triaxial compression testing of sand and fine-grained soil reinforced with the same geotextile, are presented in Figures 3a and 3b, respectively. It can be observed that reinforced sand presents a maximum value of deviator stress (failure deviator stress) followed by a considerable decrease in deviator stress. On the contrary, reinforced fine-grained soil presents either a peak deviator stress (failure deviator stress) followed by a negligible decrease in deviator stress or a continuous increase in deviator stress as the axial strain increases. Accordingly, in tests not presenting a maximum deviator stress, the failure of specimen and, as a result, the failure deviator stress were defined to correspond to a value of axial strain equal to 2%. Stress strain curve of reinforced fine-grained soil having similar shape to the ones presented herein, were reported by Fabian and Fourie [12]. It is also observed (Figure 3a) that an increase in cell pressure, σ 3, causes an increase in failure deviator stress of reinforced sand. This behavior was also observed in fine-grained soil specimens tested with cell pressures up to 5 kpa. On the other hand, reinforced fine-grained soil specimens tested with cell pressures greater than 5 kpa, present either negligible increase or no increase or even a decrease in failure deviator stress as cell pressure increases (Figure 3b). As an exception, fine-grained soil specimens reinforced with HS geotextile show the same behavior with reinforced sand.

4 In order to quantify and compare the deformability of reinforced soils, the axial strain ratio, ε fr /ε fu, defined as the ratio of the axial strains at failure of reinforced and unreinforced soil for the same cell pressure, is used. The ε fu, ε fr and ε fr /ε fu values obtained, are presented in Table 3. It can be seen that unreinforced fine-grained soil presents higher values of axial strain at failure than unreinforced sand. For this reason, the values of ε fr /ε fu ratio of fine-grained soil are lower than the ones of sand, although reinforced fine-grained soil presents higher values of axial strain at failure than reinforced sand. Therefore, it is concluded that the effect of reinforcement on the axial strain at failure is greater in sand than in fine-grained soil. As regards the effect of geotextile type and properties on the deformability of reinforced soil, it is evident that it depends on soil type, as well. More specifically, reinforced sand with geotextiles of higher compressibility (NP and TTS) presents higher ε fr values than sand reinforced with the other geotextiles (Table 3). This influence is not observed in reinforced fine-grained soil, where the axial strain at failure is generally independent of geotextile type. 25 (a) 5 1 (b) Deviator stress, σ1-σ3 (kpa) σ 3, (kpa) Ottawa 2-3 sand TB geotextile Deviator stress, σ1-σ3 (kpa) σ 3, (kpa) 1 soil TB geotextile Axial strain, ε (%) Axial strain, ε (%) Figure 3. Typical stress strain curves of (a) reinforced sand and (b) reinforced fine-grained soil Table 3. Values of axial strain at failure and axial strain ratio σ 3 TB NP TTS HS SG [kpa] ε fu [%] ε fr [%] ε fr /ε fu ε fr [%] ε fr /ε fu ε fr [%] ε fr /ε fu ε fr [%] ε fr /ε fu ε fr [%] ε fr /ε fu Shear Strength Shown in Figure 4 are the failure envelopes of unreinforced and reinforced sand and fine-grained soil. It can be observed that triaxial compression tests yielded bilinear failure envelopes for reinforced sand which is in agreement with the observations of other investigators [2, 3]. This bilinear form is attributed to a change of the interaction behavior at the sand geotextile interface. More specifically, the part of failure envelope before the break point corresponds to failure of the composite

5 6 TB Geotextile 6 TTS Geotextile (a) (b) 6 6 NP Geotextile 6 SG Geotextile (c) (d) 6 6 HS Geotextile (e) 6 Figure 4. Failure envelopes of reinforced and unreinforced soils

6 material by slippage of the geotextile with regard to the surrounding sand. The part of failure envelope after the break point corresponds to failure caused by excessive deformation during which the geotextile is stretched in unison with the surrounding sand. The break point of failure envelopes of reinforced sand corresponds to critical values of normal stress, σ vcr, ranging from 257 kpa to 29 kpa. It can be clearly seen (Figure 4) that reinforced sand presents higher shear strength than unreinforced sand, for all the geotextiles tested. fine-grained soil also presents bilinear failure envelopes (Figure 4). With the exception of soil reinforced with HS geotextile, the part of failure envelopes after the break point is horizontal showing that shear strength of reinforced fine-grained soil is independent of the applied normal stress. This behavior is similar to that of fully saturated fine-grained soils loaded under undrained conditions. Therefore, the bilinear form of these failure envelopes is possibly attributed to a change of draining conditions in reinforced soil as the applied normal stress increases. Curved and bilinear failure envelopes were also reported by Athanasopoulos [15] for reinforced fine-grained soil, indicating a continuous transition from drained to undrained behavior as normal stress is increased. The break point of failure envelopes of reinforced fine-grained soil corresponds to normal stress values ranging from 133 kpa to 28 kpa. However, these critical values of normal stress should not be taken to represent the transition from slippage failure to stretching failure of the reinforcement but appear to represent transition from drained to undrained behavior. It is also observed (Figure 4) that reinforced fine-grained soil does not always present higher shear strength than unreinforced soil. The strength ratio, S R, defined as the ratio of failure deviator stress of reinforced soil to the failure deviator stress of unreinforced soil for the same cell pressure, is used for the quantification of the strength increase due to reinforcement of soils. As it can be seen in Figure 5, the S R values range from 2.17 to 4.62 and from.94 to 1.84 for reinforced sand and fine-grained soil, respectively. It is evident that the reinforcement effect on strength is greater in sand than in fine-grained soil. Reported S R values by Ingold [1] and Fabian and Fourie [12] for clays reinforced with geotextiles, are generally lower than 2 and, therefore, are in good agreement with the values presented herein. Strength ratios of sand decrease as cell pressure increases and strength ratios of fine-grained soil attain to maximum values for cell pressures between 5 and 1 kpa (Figure 5). This difference in behavior is justified by the positions of failure envelopes of reinforced soils relative to those of unreinforced soils (Figure 4) Strength Ratio, SR Ottawa Cell Pressure, σ 3 (kpa) Figure 5. Strength ratio values Failure Mode of Specimens Shown in Figure 6 are typical forms of specimens of reinforced sand and fine-grained soil, respectively, after triaxial compression testing. The failure mode of reinforced soil specimens generally consisted of bulging of soil between geotextile layers. In reinforced sand (Figure 6a), bulges and geotextile discs closer to the mid-height of the specimen were greater in diameter than the ones closer to the loading surfaces of the specimen. The increase in diameter of geotextile layers indicates good sand geotextile cooperation. On the other hand, reinforced fine-grained soil specimens (Figure 6b) presented a nearly uniform diameter increase in all soil layers and no remarkable diameter change in geotextile layers, indicating an insufficient cooperation between fine-grained soil and geotextiles. The better sand geotextile cooperation possibly justifies the higher strength increases observed in reinforced sand compared to those obtained from reinforced fine-grained soil (Figure 5).

7 (a) (b) Figure 6. Typical specimens of reinforced (a) sand and (b) fine-grained soil after triaxial compression testing Conclusions Based on the results of this investigation and within the limitations posed by the number of tests conducted and the materials used, the following conclusions may be advanced: The triaxial compression tests yielded bilinear failure envelopes for geotextile reinforced sand and fine-grained soil. In reinforced sand, the bilinear form is attributed to a change of the interaction behavior at the sand geotextile interface, while in reinforced fine-grained soil, is possibly attributed to a transition from drained to undrained behavior. Values of strength ratio range from 2.17 to 4.62 and from.94 to 1.84 for reinforced sand and fine-grained soil, respectively, indicating that the reinforcement effect on strength is greater in sand than in fine-grained soil. Values of axial strain ratio at failure range from.99 to 6.6 and from.78 to 2.46 for reinforced sand and fine-grained soil, respectively, indicating that the reinforcement effect on the axial strain at failure is greater in sand than in finegrained soil. Failure modes of reinforced soil specimens indicate that geotextiles cooperate more effectively with sand than with finegrained soil. The better sand geotextile cooperation possibly justifies the higher strength increases observed in reinforced sand compared to those obtained from reinforced fine-grained soil. The suitability of sands for use as backfill materials in reinforced soil structures is confirmed, while the use of finegrained soils of low plasticity in reinforced soil structures requires very careful consideration. Acknowledgments The Research Committee of Democritus University of Thrace provided financing for the research effort reported herein. This financial support is gratefully acknowledged. Thanks are expressed to Mr. G. Maroulas for his contribution to the construction of the fine-grained soil specimen preparation equipment. Part of the triaxial compression tests described in this paper, were conducted by Mr. G. Sirkelis and Mr. Ch. Ioannou whose careful work is acknowledged. References 1. Fourie, A.B. and Fabian, A.G., Laboratory Determination of Clay-Geotextile Interaction, Geotextiles and Geomembranes, 6, (1987). 2. Gray, D.H., Athanasopoulos, G.A. and Ohashi, H., Internal / External Fabric Reinforcement of, Proceedings, 2 nd International Conference on Geotextiles, Las Vegas, U.S.A., 3, (1982). 3. Gray, D.H. and Al-Refeai, T., Behavior of Fabric- vs. Fiber-, Journal of Geotechnical Engineering, 112, (1986). 4. Baykal, G., Guler E. and Akkol, O., Comparison of Woven and Nonwoven Geotextile Reinforcement Using Stress Path Tests, Proceedings, International Symposium on Earth Reinforcement Practice, Fukuoka, Japan, 1, (1992). 5. Ashmawy, A.K. and Bourdeau P.L., Effect of Geotextile Reinforcement on the Stress-Strain and Volumetric Behavior of, Proceedings, 6 th International Conference on Geosynthetics, Atlanta, U.S.A., 2, (1998).

8 6. Haeri, S.M., Noorzad, R. and Oskoorouchi, A.M., Effect of Geotextile Reinforcement on the Mechanical Behavior of, Geotextiles and Geomembranes, 18, (2). 7. Wu, J.H., Wang, D.Q. and Wang, L.J., Experimental Study on Geosynthetic Reinforcement, Proceedings, 7 th International Conference on Geosynthetics, Nice, France, 4, (22). 8. Ingold T.S. and Miller, K.S., The Behavior of Geotextile Clay Subject to Undrained Loading, Proceedings, 2 nd International Conference on Geotextiles, Las Vegas, U.S.A., 3, (1982). 9. Christie, I.F., Economic and Technical Aspects of Embankments with Fabric, Proceedings, 2 nd International Conference on Geotextiles, Las Vegas, U.S.A, 3, (1982). 1. Ingold, T.S., Clay Subject to Undrained Triaxial Loading, Journal of Geotechnical Engineering, 19, (1983). 11. Ingold, T.S., Fully and Partially Saturated Clay under Undrained Axisymmetric Loading, Ground Engineering, 18, (1985). 12. Fabian, A.G. and Fourie, A.B., Performance of Geotextile Clay Samples in Undrained Triaxial Tests Geotextiles and Geomembranes, 4, (1986). 13. Atmatzidis, D.K and Athanasopoulos, G.A., Geotextile Friction Angle by Conventional Shear Testing, Proceedings, 13 th International Conference on Mechanics and Foundation Engineering, New Delhi, India, 3, (1994). 14. Markou, I. and Droudakis, A., Effect of Triaxial Compression Testing Factors on - Geotextile Interface Friction, Proceedings, 8 th International Conference on Geosynthetics, Yokohama, Japan, 4, (26). 15. Athanasopoulos, G.A., Results of Direct Shear Tests on Geotextile Cohesive, Geotextiles and Geomembranes, 14, (1996).