Zum Spannungs-Dehnungsverhalten des Verbundbaustoffs „geogitterbewehrter Boden”

The effectiveness of geogrids as means of soil reinforcement is well known. Due to complex interaction mechanisms between soil and geogrids, however, the stress strain behaviour of the composite material “geogrid reinforced soil” is still hard to predict. The identification of major material parameters and the description of their effect to the compound behaviour is therefore of great importance. Within this thesis geogrid reinforced soil has been investigated in the laboratory with large scale “element” tests, such as triaxial and biaxial compression tests. Especially the apparatus developed for the plane strain testing of 800mm wide specimens allows testing of soil reinforced with common geogrids with aperture widths of 30 to 40mm. Test series have been carried out to investigate the effect of certain material parameters, i.e. soil density, stress level and tensile stiffness of the geogrids, to the composite material behaviour. Thereby attention has been paid to vary only one parameter at a time to allow the exact assignment of the observed effects to the changes in the stress strain behaviour of the compound material. In contrast to the stress strain behaviour of unreinforced soil test results have shown an initially increasing stiffness of the composite material under vertical compression. This is caused by a gradual activation of the geogrids due to the lateral specimen extension. Furthermore, the reinforcing or confining effect of the geogrids has been identified to be higher at lower stress levels where there is only little confinement within the soil itself. A good compaction of the filling soil as well as a higher tensile stiffness have shown to lead to an increase of both the stiffness and the bearing capacity of the reinforced soil. Apart from the possibility of testing of large specimens the apparatus that has been constructed specifically for the investigation of the reinforced soil under plane strain conditions allows the determination of the kinematic behaviour of the reinforced soil. This is achieved by recording the movement of soil particles through a transparent side wall of the test apparatus during testing and a subsequent evaluation using the digital image correlation (DIC) method. Evaluating displacements and rotations of soil particles the development of arching effects and shear zones within the soil has been visualized. Following the idea of the “confining effect concept” the corresponding stress paths of the materials investigated with biaxial compression tests have been determined. From the equations given in this thesis the additional confining pressure due to the geogrid activation can be calculated for any loading state as a function of the tensile stiffness of the geogrids employed in the tests. In addition to the experimental tests numerical calculations of the biaxial compression tests have been carried out using the finite element method. The calculation results show an excellent match of the kinematic behaviour of unreinforced and geogrid reinforced specimens determined with the lab tests. Using a constitutive law with stress dependant description of the soil stiffness even the initial increase of the stiffness of the composite material during loading could be modeled. Calculations with a constant stiffness value, however, lead to a significant overestimation of the deformations of the composite material at the peak stage. Apart from the fundamental “element”-testing of the composite material also model tests have been carried out to investigate the development of earth pressure and connection loads within the reinforced soil. The cross section of the specimens investigated was 1000mm x 1000mm (H x W). The results have shown a significant reduction of the earth pressure even below the active earth pressure of the unreinforced soil. It has to mentioned that the earth pressure reduction occurred already at very small displacements of the wall facing, similar to or even less as those required for the development of the active earth pressure. Furthermore, it has been shown that the decrease of the earth pressure was apparent under the topmost reinforcement layer, independent of the height of this layer within the specimen. Due to the holistic consideration of stresses and deformations within the reinforced soil and the determination of the relationship between stresses, strains and reinforcement activation the results presented in this thesis constitute a further step towards understanding and describing the mechanical behaviour of the composite material geogrid reinforced soil

Zum Spannungs-Dehnungsverhalten des Verbundbaustoffs „geogitterbewehrter Boden”

The effectiveness of geogrids as means of soil reinforcement is well known. Due to complex interaction mechanisms between soil and geogrids, however, the stress strain behaviour of the composite material “geogrid reinforced soil” is still hard to predict. The identification of major material parameters and the description of their effect to the compound behaviour is therefore of great importance. Within this thesis geogrid reinforced soil has been investigated in the laboratory with large scale “element” tests, such as triaxial and biaxial compression tests. Especially the apparatus developed for the plane strain testing of 800mm wide specimens allows testing of soil reinforced with common geogrids with aperture widths of 30 to 40mm. Test series have been carried out to investigate the effect of certain material parameters, i.e. soil density, stress level and tensile stiffness of the geogrids, to the composite material behaviour. Thereby attention has been paid to vary only one parameter at a time to allow the exact assignment of the observed effects to the changes in the stress strain behaviour of the compound material. In contrast to the stress strain behaviour of unreinforced soil test results have shown an initially increasing stiffness of the composite material under vertical compression. This is caused by a gradual activation of the geogrids due to the lateral specimen extension. Furthermore, the reinforcing or confining effect of the geogrids has been identified to be higher at lower stress levels where there is only little confinement within the soil itself. A good compaction of the filling soil as well as a higher tensile stiffness have shown to lead to an increase of both the stiffness and the bearing capacity of the reinforced soil. Apart from the possibility of testing of large specimens the apparatus that has been constructed specifically for the investigation of the reinforced soil under plane strain conditions allows the determination of the kinematic behaviour of the reinforced soil. This is achieved by recording the movement of soil particles through a transparent side wall of the test apparatus during testing and a subsequent evaluation using the digital image correlation (DIC) method. Evaluating displacements and rotations of soil particles the development of arching effects and shear zones within the soil has been visualized. Following the idea of the “confining effect concept” the corresponding stress paths of the materials investigated with biaxial compression tests have been determined. From the equations given in this thesis the additional confining pressure due to the geogrid activation can be calculated for any loading state as a function of the tensile stiffness of the geogrids employed in the tests. In addition to the experimental tests numerical calculations of the biaxial compression tests have been carried out using the finite element method. The calculation results show an excellent match of the kinematic behaviour of unreinforced and geogrid reinforced specimens determined with the lab tests. Using a constitutive law with stress dependant description of the soil stiffness even the initial increase of the stiffness of the composite material during loading could be modeled. Calculations with a constant stiffness value, however, lead to a significant overestimation of the deformations of the composite material at the peak stage. Apart from the fundamental “element”-testing of the composite material also model tests have been carried out to investigate the development of earth pressure and connection loads within the reinforced soil. The cross section of the specimens investigated was 1000mm x 1000mm (H x W). The results have shown a significant reduction of the earth pressure even below the active earth pressure of the unreinforced soil. It has to mentioned that the earth pressure reduction occurred already at very small displacements of the wall facing, similar to or even less as those required for the development of the active earth pressure. Furthermore, it has been shown that the decrease of the earth pressure was apparent under the topmost reinforcement layer, independent of the height of this layer within the specimen. Due to the holistic consideration of stresses and deformations within the reinforced soil and the determination of the relationship between stresses, strains and reinforcement activation the results presented in this thesis constitute a further step towards understanding and describing the mechanical behaviour of the composite material geogrid reinforced soil