INVESTIGATION OF GEOGRID REINFORCED SOIL WITH LARGE SCALE ELEMENT TESTING

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1 Second Pan American Geosynthetics Conference & Exhibition GeoAmericas 212 Lima, Perú - May 212 INVESTIGATION OF GEOGRID REINFORCED SOIL WITH LARGE SCALE ELEMENT TESTING F. Jacobs, Geotechnical Engineering, RWTH Aachen University, Germany, A. Ruiken, Geotechnical Engineering, RWTH Aachen University, Germany, M. Ziegler, Geotechnical Engineering, RWTH Aachen University, Germany. ABSTRACT The compound behavior of geogrid reinforced soil is very complex as two completely different materials are combined. Interaction mechanisms between geogrid and soil, e.g. surface friction and particle interlocking contribute to tensile and confining effects in the composite material. To investigate the complex stress-strain behavior of geogrid reinforced soil and to account for the large size of geogrid apertures, a highly sophisticated apparatus with innovative instrumentation has been developed at RWTH Aachen University. It allows to carry out large scale biaxial compression tests under a constant confining pressure. The stress-strain behavior of the specimens tested proves the well-known increase in strength of the reinforced soil compared with those of the unreinforced soil. A transparent glass side wall of the laboratory stand allows to obtain the particle deformation and rotations with the Digital Image Correlation (DIC) method. The kinematic behavior of the tests is linked to the observed stress-strain behavior. 1. INTRODUCTION The contribution of geogrids to the strength of the composite material geogrid reinforced soil is a well-known and established fact. There is a still growing field of applications for geogrid reinforced soil and many researchers seek to improve the understanding of those with the help of model tests, e.g. Bathurst (29) and Ruiken et al. (21) recently investigated geogrid reinforced retaining walls. To investigate the complex characteristics of geogrid reinforced soil in principle, element tests are an appropriate tool for the analysis of the stress-strain behavior. Reinforced soil was investigated by Broms (1977) and Chandrasekaran et al. (1989) with triaxial tests and by McGown et al. (1978) with plane strain tests, just to mention some early works. Considering the effects of the geogrids three-dimensional geometry and the ratio of the geogrid aperture to the soil s particle size, non-scaled tests are essential. Therefore, a sophisticated laboratory apparatus has been designed to carry out large scale biaxial compression tests under plane strain conditions with a constant confining pressure. Due to the presence of geogrid layers and the large dimensions stresses and strains within the specimen are not homogeneous. However, the presented test is referred to as an element test. The aim is to carry out tests with well defined and simple boundary conditions rather than to investigate a specific reinforced structure. This allows for the calibration of a mechanical model that can be easily transferred to problems with differing geometries. This paper presents results of the first series of biaxial compression tests with unreinforced and reinforced soil. 2. LABORATORY TEST SETUP 2.1 Testing Apparatus and Test Procedure Figure 1 shows the apparatus that has been constructed to carry out large scale tests. The results presented in this paper have been obtained from testing of unreinforced specimens and of those reinforced with one and two geogrid layers, respectively. Specimen configurations and the confining stress are also shown in Figure 1. The confining stress has been applied via vacuum to ensure a pressure that is constant upon all element sides. Axial loading has been realized with a stiff plate at a constant strain rate. During compression the soil could be monitored through a transparent side wall. To allow for the condition of plane strain, this glass wall had a thickness of 16 mm reducing its deflection to less than.1 mm. Therefore, horizontal deformations have only occurred in one direction, i.e. the direction of the tensile members of the geogrids. The interfaces between the specimen and the top and bottom plates have been lubricated with a combination of a thin latex membrane and silicone grease. For this combination Tatsuoka and Haibara (1985) determined a contact friction angle of approximately 2. For the sake of transparency, the glass wall has not been lubricated in this manner, but a low friction angle could still be obtained.

2 σ1 σ1 σ1 2 cm 4 cm geogrid 8 cm σ3 = 2.5 kpa geogrid 4 cm 4 cm 2 cm Figure 1. Laboratory apparatus for plane strain biaxial compression tests with a sketch of the three presented geogrid arrangements. 2.2 Material Used The soil used in the tests was a dry uniform-graded medium sand, classified as SP according to the Unified Soil Classification System. It has been deposited approximately with 11%-Proctor density (D = 1.74 t/m³) corresponding to a relative density of ID = 89 % using a rainfall technique, already applied by Ruiken et al. (21). As tensile reinforcement layers a biaxial geogrid with flat bars and welded junctions has been used. The geogrid s stress-strain behavior is bilinear with an average tensile stiffness for strains between and 2 % of J-2% = 7 kn/m. 2.3 General Instrumentation The prescribed vertical displacement has been determined as a mean value of four vertical displacement transducers that have been located at the corners of the loading plate to notice any inclination of the plate. Meanwhile, the vertical axial force has been applied with a hydraulic plunger and recorded with a standard load cell. Additionally, lateral specimen deformations have been recorded by a system of horizontal freely sliding metal bars those position has been registered by photography once a minute (Figures 1 and 5). This has facilitated a simple determination of the lateral strains along the height of the soil sample. 2.4 Visualization of Kinematics with the Digital Image Correlation (DIC) Method During loading photographs of the σ2-surface of the specimens have been made with a remote controlled digital camera (Figure 1). The displacements and rotations of the soil particles have then been determined with a subsequent computer based processing of the photographs with the DIC method. The resulting plots of particle displacements and rotations give an insight of the kinematic behavior of the unreinforced and reinforced specimens under biaxial compression and help to investigate the reinforcing effect of the geogrids. 2.5 High Resolution Stress Measurement To determine the actual pressure distribution between the stiff loading plate and the specimen at all stages during testing flexible, grid-based, tactile pressure sensors have been used. These foil sensors have been integrated in the test setup covering the complete cross-sectional area of the specimen ( 1-surface). They consist of conductive paths that run crosswise, an intermediate layer of a pressure-sensitive ink, and a synthetic lamination for protection (Macintyre, 211). The electric resistance of the intersecting points of the conductive paths varies with the applied pressure. Applying an

3 electric voltage and measuring the electric current the change in electric resistance has been received. Calibrating certain resistances with pressure levels, the normal contact pressure has been obtained. The sensors used in the tests had a resolution of 1 value/cm² and were designed for a nominal stress of 3 PSI (27 kpa). However, lowering the sensitivity they could be calibrated for stresses up to 565 kpa. To compensate for slight differences between the measured cell values of a foil sensor, in addition to calibration, each foil sensor had to be equilibrated before the tests according to Tekscan (26). 3. EXPERIMENTAL RESULTS 3.1 Stress-Strain Behavior The resulting global vertical stress σ 1 over vertical strain ε 1 is shown in Figure 2a for unreinforced and reinforced soil tested with a confining stress σ 3 = 2.5 kpa. Due to the large specimen dimensions of 8 x 8 x 45 cm³ in combination with this low confining stress, the soil s weight could not be neglected in the vertical direction. It has been accounted for as an additional vertical stress calculated with the soil s weight (γ soil) at medium height of the specimen as in Eq.1. σ 1 = γ soil H / 2 (1 - ε 1) [1] Although the condition of a homogeneous stress state in the soil element cannot be met exactly because of the inclusion of geogrid layers and the large scale dimensions, global element stress values are being compared here. These have been obtained by dividing the measured axial load by the mean cross-sectional area of the specimen. The results presented in Figure 2a indicate a significant increase in strength if the soil has been reinforced with geogrids that have been tensioned during axial loading. With a higher reinforcement ratio this effect clearly increases as one reinforcement layer leads to an enhancement of 255 % and two reinforcement layers lead to an enhancement of 45 %, comparing its peak stresses with those of the unreinforced specimens. The development of the tangent modulus of the three above tests is shown in Figure 2b. As can be seen, the stiffness of the unreinforced sample is the highest at the beginning of the test, commonly known as the initial stiffness E. Right after the start it is decreasing until the peak stress has been reached. The initial stiffness of the reinforced specimens is very similar to those of the unreinforced sample. However, right after the start of the tests it is increasing due to the activation of the reinforcement layers verticalstress σ 1 [kpa] tangent modulus E [kpa] ,max 1,max 1,max a) verticalstrainε 1 [%] b) verticalstrainε 1 [%] Figure 2. Results from plane strain compression tests of an unreinforced specimen and those of specimens reinforced with one or two layers of geogrid. As the soil is a dense sand all tests exhibit a peak and a residual stress. Consistently, the corresponding volumetric strain curves evaluated in Figure 3 show the dilatancy behavior of a dense sand. All tests show a contraction at the beginning of the compression. While the unreinforced sand changes to dilative behavior just after the start of the test, the higher the reinforcement ratio the more contractant the sample reacts. This can be explained by two effects, also observed by Peng et al. (2): First, due to the tensile reinforcement layers local confining pressures in the sand can be higher than those globally applied via cell pressure, and therefore, higher vertical strains are locally possible and lead to more contraction. Second, dilative behavior is reduced by the tensile reinforcement restricting horizontal deformations, as can be seen again in Figures 5 and 6.

4 3. volumetric strain ε V [%] verticalstrainε 1 [%] Figure 3. Development of the volumetric strain of an unreinforced and of reinforced tests during compression. 3.2 Kinematic Behavior The resulting plots of total and horizontal particle displacements and particle rotations evaluated with the help of the DIC method are shown in Figure 4 for a compression state of ε 1 = 7 % for an unreinforced test on the left, for a test reinforced with one layer in the middle, and for a test reinforced with two layers on the right. The particle rotations of the unreinforced specimen suggest that during the course of loading three main soil wedges develop. The left wedge slides down and sideways, before the right part of the sample moves to the right, while a central wedge remains almost without any movement. This can also be observed looking at the horizontal and total particle displacements. Total particle deformation max Horizontal particle deformation Particle rotations Figure 4. Particle deformations and rotations for a vertical strain of ε 1 = 7 % evaluated with the DIC method for a unreinforced test (left), a test reinforced with one geogrid (center), and a test reinforced with two geogrids (right).

5 Now, comparing particle movements and rotations of the reinforced tests with the kinematic behavior described above, it can be seen that the geogrids clearly influence the evolution and form of the soil wedges, and therefore, have a strong impact on the kinematics of the compression tests. Generally, instead of only having three main kinematic bodies, the geogrid layers force the soil to develop more zones of shearing. This leads to a higher resistance of the whole specimen. Looking closely at the particle rotations in the upper left part of the test with one layer of reinforcement in Figure 4, four shear bands can be seen that all start at the left corner of the geogrid. The first of these shear bands has appeared at a compression state of ε 1 = 2 % and is the one most inclined with an angle of 64 to the horizontal. Forced by the movement of the loading plate the other mentioned shear bands have developed with decreasing angles until all four shear bands have evolved at the displayed state of ε 1 = 7 %. The latest and least inclined shear band has an angle of only 45. As it appears, it causes less resistance to decrease the angle of a shear band than to run through the geogrid, and therefore, the integration of a reinforcement layer increases the strength of the specimen. The specimen reinforced with two geogrid layers has had the highest strength of the presented tests. This can be explained looking at the total particle movements in Figure 4. Here, all three illustrated tests have a central soil wedge that almost has not moved. The motionless soil parts of the unreinforced test and those of the test reinforced with one geogrid layer cover the complete specimens height. However, for the sample with two geogrids only below the upper reinforcement layer a wedge of unmoved soil remains: the two geogrid layers constrict the whole specimen, and thus force a higher degree of soil to be moved. In total, it can be said that the higher the reinforcement ratio, the more zones of dissimilar movement have been developed and hence, all the more total resistance against compression has been activated. 3.3 Lateral specimen deformations The lateral deformations have been determined along the height of the specimen with a system of sliding metal bars. The photographs of the σ 3 -surfaces of the specimens displayed in Figure 5 visualize the confining effect of the geogrids. In reinforced specimens shear bands have tended to end at geogrid layers so that there has been a high relative displacement between the soil above and below a geogrid. In Figure 5b, showing a photograph of the σ 3-surface of a specimen reinforced with one geogrid layer, the upper part has clearly deformed more than the lower soil wedge. In the test with two reinforcement layers (Figure 5c), the relative displacement between soil and geogrid has not been as large but is still visible. a) b) c) Figure 5. Photographs of the σ 3-surfaces of an unreinforced specimen and those of reinforced specimen. Figures a and c show the system of freely sliding metal bars for the measurement of the lateral deformations. Imagining looking at the soil sample from the same perspective as in Figure 1 (σ 2-direction), Figure 6 illustrates the horizontal strain ε 3 along the height of the specimens. Here, the differently colored areas represent strains at different states of vertical compression. Now, looking at the positions of the left edge of the same unreinforced and reinforced specimens as in Figure 5, the above described observations can be confirmed. For all tests, there is a relatively even lateral deformation up to a vertical compression of ε 1 = 2 %. At higher compression states, shear bands have developed and cause larger relative movements of soil wedges. However, at the height of the geogrid layers the lateral expansion is restricted. The horizontal deformations measured with this simple system are in good agreement with the particle deformations visualized with photographs of the σ 2-surface and the DIC method (Figure 4).

6 ε 1 = 2 % ε 1 = 5 % ε 1 = 7 % ε 3 [%] ε 3 [%] Figure 6: Horizontal deformation expressed as lateral strain ε 3 for three compression states measured with a simple analog system. 3.4 Pressure Distribution underneath the Loading Plate The vertical stresses between the loading plate and the upper σ 1 -surface of the specimens have been measured with sensitive foil sensors. The integration of these vertical pressures corresponded to the force measured with a standard load cell, validating the measurements of the foil sensors. The resulting vertical stress distribution of an unreinforced specimen and a sketch of the position of the integrated foil sensor are illustrated in Figure 7. The left figure shows the stress distribution before the peak stress is reached (ε 1 =.8 %), while the right figure is a snap-shot of the distribution when the maximum vertical stress is transmitted (ε 1 = 1.1 %). The obtained stress distributions are relatively even in σ 2-direction. In particular, due to a low contact friction angle between glass and soil no exceptional increase of the vertical stress is observed along the glass side walls. Therefore, it can be said that the plane strain condition has been obtained to a satisfying degree. Comparing the stress distribution of the left figure with those at the peak stress, a gap in the distribution has appeared. This suggests the evolution of separate zones in the specimen below. vertical stress [kpa] 2 1 ε 1 =.8% gap ε 1 =1.1% σ 1 =max [cm] -2 2 σ 3 Position of stress measurement σ 1 8 cm Figure 7. Vertical stress distribution between an unreinforced specimen and the loading plate for two compression states. To further investigate the just mentioned behavior, the distribution of the vertical stresses of a test reinforced with two geogrid layers is compared with the corresponding particle rotations evaluated with the DIC method in Figure 8. Here,

7 the vertical pressures are illustrated in two dimensions as the average of all values in σ 2-direction. In the left figure for a compression of ε 1 = 3 %, the central part of the specimen has not yet been disturbed by any shear zones. Here, most of the vertical compression load is transmitted. Left and right of this central body the shown vertical pressure decreases sharply as sliding wedges inhibit that a higher load is transferred (represented by the light grey shading). The arrow points at a small gap in the pressure line that is caused by a shear band reaching the top of the specimen. At this point at already ε 1 = 3 %, the sample s resistance against compression is reduced. When the shear band has grown so that a complete wedge of soil is sliding, the transmitted force decreases to a minimum as can be seen in the right figure for a compression state of ε 1 = 6.2 %. Again in this figure, two shear bands end in the middle of the specimen s top surface so that a gap occurs in the pressure line. This suggests that in a shear zone the local vertical stress decreases. In total, these two-dimensional plots validate the particle rotations obtained with the DIC method. ε 1 = 3. % vertical stress [kpa] ε 1 = 6.2 % Figure 8. Comparison of contact pressures between soil and loading plate with particle rotations evaluated with the DIC method. 4. SUMMARY AND CONCLUSION In this paper, the results of first plane strain compression tests have been presented. In terms of the stress-strain behavior they clearly show the strengthening effect of the geogrids. The following can be derived: - A laboratory apparatus has been developed that permits to carry out large scale biaxial compression tests under repeatable conditions. - Several measurement methods have been applied to record the behavior of the reinforced soil. Measurements of particular instruments have been compared and have shown a satisfying correlation: The horizontal deformations measured with a simple system have correlated well with those evaluated with the DIC method. The sum of the vertical pressures determined with sensitive foil sensors has corresponded to the force measured with a standard load cell validating the prior calibration of the foil sensors. The measured vertical pressures have shown the same positions of shear bands underneath the loading plate as the displayed particle rotations, and therefore have confirmed the kinematics visualized with the DIC method. - While the applied load has been converted into an average global stress, foil sensors have actually measured local stresses, and thus are an excellent tool for calibration of numerical models that will be developed to determine the stress distributions within the specimens. - The redundant measurements of displacements and rotations, forces and stresses admit an integral and holistic approach to the investigation of the stress-strain behavior of geogrid reinforced soil. After promising results of a first series of tests, parameters, e.g the geogrid tensile stiffness and the vertical spacing between geogrid layers, can now be varied. In total, the obtained test results provide a good data base for the calibration of a future mechanical model.

8 ACKNOWLEDGEMENTS The authors would like to thank Naue GmbH & Co. KG and Colbond bv for providing the geogrids and for their financial support. Special thanks go to the Geosynthetic Institute (GSI), Pennsylvania, USA for supporting part of this work with its fellowship program. REFERENCES Bathurst, R.J. (29), Recent Developments in Reinforced Soil Wall Testing, Analysis and Design, GIGSA GeoAfrica 29, Cape Town, South Africa. Broms, B.B. (1977), Triaxial tests with fabric reinforced soil, Proc. Int. Conf. on the Use of Fabrics in Geotechnics, 3: , Ecole Nationale des Ponts et Chaussées, Paris, France. Chandrasekaran, B., Broms, B.B. and Wong, K.S. (1989), Strength of Fabric Reinforced Sand Under Axisymmetric Loading, Geotextiles and Geomembranes, 8: Macintyre, L. (211), New calibration method for I-scan sensors to enable the precise measurement of pressures delivered by pressure garments, Burns, 37: McGown, A., Andrawes, K.Z. and Al-Hasani, M.M. (1978), Effect of inclusion properties on the behaviour of sand, Géotechnique 28.3: Tekscan (26), I-Scan User Manual, Tekscan Inc., South Bosten, USA. Peng, F.-L., Kotake, N., Tatsuoka, F., Hirakawa, D. and Tanaka, T. (2), Plane strain compression behaviour of geogrid-reinforced sand and its numerical analysis, Soils and Foundations, 4.3: Ruiken, A., Ziegler, M., Ehrenberg, H. and Höhny, S. (21), Determination of the soil confining effect of geogrids, From Research to Design in European Practice 21, Bratislava, Slovak Republic. Tatsuoka, F. and Haibara, O. (1985), Shear resistance between sand and smooth or lubricated surfaces, Soils and Foundations, 25.1: