A Study On Rheologic Properties Of Geogrids And Long-Term Performance Of Reinforced Earth Walls

As a natural geomaterial, soil is of weaker tension or virtually no tension strength. As the steel bars are embedded in concrete to form reinforced concrete and the high tensile strength of the steel bars are utilized to improve the tensile behaviour of concrete structures, a certain reinforcements can be put into or laid in soil mass properly, the frictional interaction and locking interaction between the soil and reinforcement can carry tensile stresses in soil mass, enhance deformation modulus of soil mass, then effectively improve strength and deformation properties of soil mass to a certain extent. The high molecular polymers or steel bars are usually taken as reinforcements and are generally called as geosynthetics. Such geosynthetics can be employed to improve soft grounds, to reinforce soil slopes. In addition, the retaining structures can take the advantage of high tensile strength of geosynthetics to form flexible reinforced earth retaining wall structures. These soil structures or foundations reinforced by geosynthetics are called as reinforced earth structures. As large number of engineering constructions such as roads, railway subgrades, embankments and foundations are built under poor geological or geotechnical conditions, earth reinforcement techniques have been widely applied in geotechnical engineering practice due to their good performance in construction efficiency, cost and safety. Nevertheless, the deformation behaviour and working mechanism of reinforced earth structures have not well understood and rational analysis theory and design methods for these structures are not available. The theoretical study cannot fulfill the rapid and wide requirements in engineering design and construction. Moreover, as mixture made of the high molecular polymers, the geosynthetics usually display noticeable rheologic behaviour under long-term application of tensile forces at a given environmental temperature. Such creep characteristics will affect the long-term performance of reinforced earth structures. As consequence, an intensive study is required for rationally evaluating the long-term performance of reinforced earth structures when rheologic behaviour of geosynthetics is taken into consideration. Therefore, in this dissertation, the geogrid is taken as a typical geosynthetics and the retaining wall reinforced by geogrids is taken as a representative reinforced earth structures. The creep behaviour and visco constitutive model of geogrids are studied through experimental creep tests in laboratory. At the same time, the numerical analyses of long-term performance of geogrid-reinforced earth retaining walls are conducted based on nonlinear FEM. The main investigations and achievements are composed of following portions.1. Upper-bound limit analysis for stability of soil slope reinforced by uniformly-distributed geogrids: Based on fundamental theory of limit analysis of plasticity, the upper bound procedure is employed in limit analysis of stability of slope reinforced by uniformly distributed geogrids. The upper-bound theorem is applied for the reinforced soil slopes to solve for critical height and stability number. The numerical results are compared with the corresponding solutions for non-reinforced soil slopes. Based on parametric study, effects of slope angle, surface inclination, frictional angle of soil and tensile strength of geogrids on stability number are examined for the reinforced soil slope. Furthermore, the local enhancement effect of soil strength induced by reinforcements are taken into account by the equivalent cohesion which take replace of the additional effect of reinforcements on soil in the limit-equilibrium state, the upper-bound limit analysis of plasticity is performed to define the minimum tensilestrength of geogrids to mobilized for stability. The dimensionless tensile-strengthk / parameter, i.e., */?, is used to represent the effect of mobilized tensile strength ofgeogrids. Based on comparative study, the effects of both soil strength and geometry of slope on the mobilized tensile strength are investigated. Furthermore, under the condition that a portion of reinforcements loss its function due to their pullout failure, the minimum length of the reinforcements to resist overall collapse are defined and the dependency of this minimum length on soil strength and slope geometry is discussed.2. A calculation method of micro-stress of composite media of reinforcement and soil considering rheology of geogrids: The geogrid-reinforced soil mass is regarded as a macro-homogeneous and anisotropic composite medium which is composed of the elasto-plastic soil and visco-elastic geogrids. The soil in the composite mass is of elasto-plastic behavior and its strength obeys the Mohr-Coulomb's criterion of failure while the creep behavior of geogrids is represented by the three-parameter rheological model consisting of a series of a Kevin model and an elastic spring. The formulations for evaluating micro-stress of geogrid-reinforced soil which can reproduce the combined effects of micro-stresses of geogrid reinforcements and surrounding soils is established on the basis of fundamental of mechanics of composite material in this study. The effect of rheological behavior of geogrids on micro-stress of reinforced soils is duly taken into consideration. The effects of various parameters associated with rheological behavior of geogrids and deformational and strength characteristics of surrounding soils on micro-stress of reinforced soils are investigated through parametric studies.3. A study on creep behaviour and visco-elasticity constitutive relationship of geogrids based on experimental tests: In order to consider the effect of creep characteristics of geogrids on long-term deformation and stability of geogrid-reinforced earth structures, a comparative study on the creep behaviour of geogrids under different combinations of the externally-applied load levels and environmental temperature is made based on a series of experimental tests in special-purpose laboratory established in Qingdao Estong Geogrid Co. Ltd. On the basis of the test results experimentally achieved under different conditions, the general characteristics of variations of long-term strength of geogrids with time are examined for a given environmental temperature. The creep test data obtained for different temperatures are correlated to the creep behaviour for a given temperature by the time shift factor and then all the revised data are used for curve fitting to determine the empirical equation for computing long-term strength and the creep reduction factor of geogrids. The proposed model will offer an instructive guidance for the analysis and evaluation of long-term performance of geogrid-reinforced earth structures while the creep behavior of geogrids is taken account. Furthermore, two types of empirical constitutive models including an exponential-type model and a hyperbolic-type model based on visco-elasticity are proposed for geogrids to reproduce the nonlinear creep properties of geogrids, then rational procedures for defining the relevant parameters respectively by experimental tests are presented. In addition, both proposed models are verified by comparing the prediction given by the models with experimental results. Finally, the effect of environment temperatures and loading levels on these model parameters is examined and empirical relationship for defining model parameters based on temperature and loading levels is given.4. Numerical analyses of long-term performance of geogrids reinforced retaining earth walls considering both rheological behaviour of backfill soils and creep property of geogrids based on visco-elasto-plasticity theory by nonlinear FEM: In order to evaluate the long-term performance of the reinforced earth structures, nonlinear finite element method is developed and numerically implemented for evaluating the long-term performance of geogrid-reinforced retaining earth walls. In the propose method, both rheological behaviour of backfill soils and creep property of geogrids are simultaneously taken into account while the layer-by-layer construction process is simulated by incremental method and interaction effects between geogrid reinforcements and soils, panel and backfills as well between panels are considered duly by Goodman's joint elements. The visco strains induced by rheology of soils and creep of geogrids are taken as initial strains and transferred into equivalent nodal loads. Nonlinear computation is numerically implemented by hybrid procedure which combines the incremental method with iteration scheme based on initial-strains and unbalanced forces. A case study for the Denver test wall of clay backfills is performed by using the proposed method and the computational parameters required in the computational model are specified or calibrated based on the fundamental tests results of backfills and geotextiles used in the test wall. The computed results are compared with the measured data achieved in the tests and the computational results obtained by other numerical methods. The reliability and rationality of the proposed method are verified through such a comprehensive comparative study for the Denver test wall. It is shown that the numerical results given by the proposed method can well agree with test results in general and the proposed method can reproduce time-dependent behaviour of deformations and stresses in the subsoils and backfills and panel and pullouts and strains in the reinforcements induced by the creep or rheology of soils and geotextiles. Furthermore, for a given reinforced earth wall, a comparative study is conducted to examine the effects of filling process, geogrid length and spacing on long-term performance of geogrid-reinforced earth retaining walls. It is shown that the filling process has an appreciable influence on both lateral deformations of panel and pullout forces or strains of geogrid as well as horizontal and vertical displacements of subsoils. The geogrid used for reinforcement will induce the redistribution of stresses in backfill. The lateral deformations of panel and pullout forces or strains of geogrids will attain a stable stage after a certain period. The numerical results and conclusions gained from this study will offer an instructive guidance for the analysis and evaluation of long-term deformation properties of geogrid-reinforced earth structures.