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Thin granular fill layers are routinely used to aid the construction of shallow footings seated over undrained soft clay foundations and to increase their load capacity. The influence of time- and strain-dependent reduction in reinforcement stiffness on the bearing capacity and load-settlement response of a footing seated on a thin reinforced granular fill layer over undrained soft clay foundations is examined in this paper using finite-difference method (FDM) numerical models. The time- and strain-dependent stiffness of the reinforcement described by a two-component hyperbolic isochronous tensile load-strain model is shown to influence the bearing capacity and load-settlement response of the reinforced granular base scenario. The additional benefit of a reinforced granular layer diminishes as the time-dependent stiffness of the geosynthetic reinforcement increases. An analytical solution for the ultimate bearing capacity of strip footings seated on thin unreinforced and reinforced granular layers over undrained clay is proposed in this study. The main practical outcome from this study are tables of bearing capacity factors to be used with the analytical solution. The bearing capacity factors were back-calculated from the numerical analyses and account for the influence of rate-dependent properties of geogrid reinforcement materials and clay foundations with soft to very soft undrained shear strength. • Load-settlement and ultimate bearing capacity of strip footing on reinforced granular layer over undrained clay is studied. • Stiffness of the reinforcement is described by a two-component hyperbolic isochronous tensile load-strain model. • Benefit of reinforced granular layer diminishes as time-dependent stiffness of the geosynthetic reinforcement increases. • Analytical solution for ultimate bearing capacity of strip footing on thin reinforced granular layer over clay is proposed. • Bearing capacity factors are back-calculated from numerical simulations using large-strain finite difference method model. [ABSTRACT FROM AUTHOR]

Many analytical and numerical analysis and design methods for geosynthetic-reinforced soil structures require a single-value (constant) estimate of reinforcement stiffness. However, geosynthetic reinforcement products are rate-dependent polymeric materials meaning that they exhibit time and strain-dependent behaviour under load. Hence, the appropriate selection of a constant (elastic) stiffness value requires careful consideration. A simple hyperbolic stiffness model is shown to be a useful approximation to the constant-load isochronous creep-strain behaviour of these materials at low load levels applicable to operational (serviceability) conditions of geosynthetic-reinforced soil structures. A large database of 606 creep tests on 89 different geosynthetic reinforcement products falling within seven different product categories was collected. From these data, isochronous stiffness values were determined for different combinations of duration of loading and strain level. Data from products falling within the same category were collected together to provide approximations linking the isochronous load-strain (creep) stiffness to the ultimate tensile strength of the material. These approximations are useful for analytical and numerical modelling particularly when parametric studies are undertaken to identify the sensitivity of model outcomes to reinforcement stiffness. Finally, three different geosynthetic-reinforced soil application examples are provided to demonstrate the important role of tensile stiffness on analysis and design outcomes. • A simple hyperbolic model is developed for the isochronous load-strain (creep) behaviour of geosynthetic reinforcement materials. • The model is used to estimate reinforcement stiffness at low load levels consistent with operational conditions of reinforced soil systems. • Model parameters are fitted from a large database of creep tests on geosynthetic reinforcement materials from different product categories. • Approximations linking the isochronous load-strain (creep) stiffness to the ultimate tensile strength of the material are provided. • Three different geosynthetic-reinforced soil examples demonstrate the important role of tensile stiffness on analysis and design outcomes. [ABSTRACT FROM AUTHOR]

Abstract: The paper describes two constitutive models for the simulation of expanded polystyrene (EPS) seismic buffer shaking table tests previously reported by the writers. The first model is linear elastic–plastic with Mohr–Coulomb criterion and Rayleigh damping. The second is called the equivalent-linear method (ELM) which incorporates hysteretic load–unload cycles using Masing-type functions and strain-dependent shear modulus and damping ratio functions. The constitutive models were applied to both the geofoam buffer and sand backfill in both approaches. Both models are shown to capture qualitative features of measured peak load-time response of the physical tests. The advanced ELM model gave predictions that were closer to the measured results for the seismic buffer test with the most compressible EPS when model accelerations and total wall forces were greatest. However, the simpler model is judged to be sufficiently accurate for practical purposes when the dynamic force increment is considered only. [Copyright &y& Elsevier]

Abstract: The paper describes the details and verification of a FLAC numerical code that was used to simulate the results of an experimental program of reduced-scale wall models constructed with a seismic geofoam buffer inclusion and loaded using a large shaking table. Five physical tests constructed with expanded polystyrene (EPS) geofoam materials having different modulus values and a control case with no geofoam inclusion were carried out. The paper shows that the numerical model was able to capture the trend in earth forces with increasing base acceleration for all six models and in most cases quantitative dynamic load–time response of the numerical simulations was in good agreement with measured values. [Copyright &y& Elsevier]