Richard J. Bathurst

Behaviour of geosynthetic reinforced soil retaining walls using the finite element method

Abstract The Paper describes finite element models that are used to simulate the behaviour of two carefully constructed and monitored large-scale geosynthetic reinforced soil retaining walls. The walls were constructed using a dense sand fill and layers of extensible polymeric (geosynthetic) reinforcement attached to two very different facing treatments. The model walls were taken to collapse using a series of uniform surcharge loads applied at the sand fill surface. The Paper demonstrates that correct modelling of the dilatant behaviour of the sand soil is required to give accurate predictions of wall performance. A modified form of hyperbolic constitutive model that includes a dilation parameter is adopted to model the behaviour of the granular soil. Mechanical properties of the constituent components of the large-scale physical models are established using standard laboratory tests including constant load tests on the polymeric reinforcement from which isochronous load-strain-time data is developed. The results of analyses show that the finite element model, constitutive models and implementation reported in this study can accurately predict all important features of wall performance.

NUMERICAL SIMULATION OF IDEALIZED GRANULAR ASSEMBLIES WITH PLANE ELLIPTICAL PARTICLES

Abstract The paper presents the development of algorithms that have been implemented in a computer program used to simulate the performance of idealized granular systems composed of elliptical-shaped particles. The work is an extension of numerical simulation methods which have been successfully used in micromechanics research on disk-shaped or polygon-shaped particles by the authors and others. The simulation of elliptical-shaped particles offers the possibility to explore the influence of particle shape on the micromechanical behaviour of plane assemblies of particles and the stress-strain behaviour of these systems at the macro scale. Typical results of simulation runs are presented to illustrate the importance of particle shape on the macroscopic stress-strain response of plane systems.

Numerical Study of Reinforced Soil Segmental Walls Using Three Different Constitutive Soil Models

A numerical finite-difference method (FLAC) model was used to investigate the influence of constitutive soil model on predicted response of two full-scale reinforced soil walls during construction and surcharge loading. One wall was reinforced with a relatively extensible polymeric geogrid and the other with a relatively stiff welded wire mesh. The backfill sand was modeled using three different constitutive soil models varying as follows with respect to increasing complexity: linear elastic-plastic Mohr-Coulomb, modified Duncan-Chang hyperbolic model, and Lades single hardening model. Calculated results were compared against toe footing loads, foundation pressures, facing displacements, connection loads, and reinforcement strains. In general, predictions were within measurement accuracy for the end-of-construction and surcharge load levels corresponding to working stress conditions. However, the modified Duncan-Chang model which explicitly considers plane strain boundary conditions is a good compromise between prediction accuracy and availability of parameters from conventional triaxial compression testing. The results of this investigation give confidence that numerical FLAC models using this simple soil constitutive model are adequate to predict the performance of reinforced soil walls under typical operational conditions provided that the soil reinforcement, interfaces, boundaries, construction sequence, and soil compaction are modeled correctly. Further improvement of predictions using more sophisticated soil models is not guaranteed.

NOTE ON A RANDOM ISOTROPIC GRANULAR MATERIAL WITH NEGATIVE POISSON'S RATIO

Abstract Poissons ratio in generalized Hookes law for an isotropic continuum is subject to the restriction − 1 ≤ v≤ 0.5 . With the exception of recently developed low-density polymer foams, solids including granular materials have not been known to exhibit negative Poissons ratio. This paper is concerned with the verification of constitutive stress-strain relationships proposed by Rothenburg (Micromechanics of idealized granular systems. Ph.D. thesis, Carleton University, 1980) which describe macroscopic behaviour of idealized bonded granular materials by elastic parameters ( K and v ) that are formulated explicitly in terms of microstructural parameters such as interparticle stiffness, contact density and average interparticle distance. The theory includes an expression for Poissons Ratio which is a function only of the ratio of tangential (shear) to normal contact stiffness λ A negative Poissons ratio is predicted for both planar and three-dimensional random isotropic systems when the tangential stiffness is greater than the normal stiffness (i.e. λ > 1 ). The results of numerical simulation of bonded disc assemblies used to verify constitutive relationships show that systems with λ > 1 do exhibit negative Poissqns ratio. Similar theoretical developments are summarized for three-dimensional random isotropic assemblies of bonded spheres and an analogous expression for Poissons ratio is presented for these systems. It is noted that while a negative Poissons ratio is theoretically possible, the micromechanieal condition for λ > 1 is physically unlikely for particles of natural materials.

SEISMIC RESPONSE ANALYSIS OF GEOSYNTHETIC REINFORCED SOIL SEGMENTAL RETAINING WALLS BY FINITE ELEMENT METHOD

Abstract The paper presents the results of a finite element analysis of the dynamic response of a geosynthetic reinforced soil retaining wall that is constructed with dry-stacked modular concrete blocks as the facia system. In the finite element model, the cyclic shear behavior of the backfill soil is described by a hyperbolic stress-strain relationship with Masing hysteretic unload-reload behavior. The reinforcement material is modelled using a similar hysteretic model which takes into account the measured response of cyclic load-extension tests performed on unconfined geogrid specimens in the laboratory. Interface shear between wall components is simulated using slip elements. The results of finite element analyses giving the seismic response of a typical geogrid reinforced segmental retaining wall subjected to prescribed acceleration records are presented. The results of analyses highlight the influence of dynamic loading on: (1) wall displacement; (2) cumulative interface shear force and displacement between facing units; (3) tensile forces developed in the reinforcement and; (4) acceleration response over the height of the wall. A number of implications to the design of these structures are identified based on the results of these simulations.

Effect of structural design on fundamental frequency of reinforced-soil retaining walls

The results of a numerical study on the influence of a number of structural design parameters on the fundamental frequency of reinforced-soil retaining wall models are presented and discussed. The design parameters in the study include the wall height, backfill width, reinforcement stiffness, reinforcement length, backfill friction angle and toe restraint condition. The intensity of ground motion, characterized by peak ground acceleration, is also included in the study as an additional parameter. The study shows that the fundamental frequency of reinforced-soil wall models with sufficiently wide backfill subjected to moderately strong vibrations can be estimated with reasonable accuracy from a few available formulae based on linear elastic wave theory using the shear wave speed in the backfill and the wall height. Numerical analyses showed no significant influence of the reinforcement stiffness, reinforcement length or toe restraint condition on the fundamental frequency of wall models. The strength of the granular backfill, characterized by its friction angle, also did not show any observable effect on the fundamental frequency of the reinforced-soil retaining wall. However, the resonance frequencies of wall models were dependent on the ground motion intensity and to a lesser extent, on the width to height ratio of the backfill.

Deterministic sliding block methods for estimating seismic displacements of earth structures

A review and quantitative comparison of existing deterministic sliding block methods for predicting permanent displacements of earth structures subjected to seismic loading is presented. The reviewed sliding block methods are divided into two main groups based on the characteristic earthquake parameters referenced in each method. One group uses the maximum horizontal ground acceleration and velocity, and the other uses the maximum horizontal ground acceleration and the predominant period of the acceleration spectrum. Displacement functions published by previous authors are reformulated to give common non-dimensionalized displacement functions of the critical acceleration ratio which are then used to compare the different methods for the estimate of permanent seismic displacement of soil structures. The results show that despite the fact that the different methods were formulated using a wide range of earthquake records and different characteristic seismic parameters, permanent displacement values predicted using these methods fall within a reasonably narrow band. Selected acceleration data from three recent earthquakes that occurred in California are used to evaluate and compare the accuracy of the reviewed displacement methods for practical applications.

A Transparent Sand for Geotechnical Laboratory Modeling

The paper describes a new transparent granular soil that can be used for laboratory geotechnical modeling purposes. The transparent soil consists of fused quartz particles in combination with a mixture of two mineral oils as pore fluid. The solid particles and the matching liquid have the same refractive index. The soil has important advantages with respect to transparency, stability, health safety, and utility over glass and silica gel materials. The transparent soil is also inexpensive compared to silica gel-fluid materials that have been used in the past. Conventional laboratory shear box, triaxial compression, and permeability tests were carried out to demonstrate that the mechanical properties and hydraulic permeability of the transparent soil are typical of granular soils with angular particles.

Influence of toe restraint on reinforced soil segmental walls

A verified fast Lagrangian analysis of continua (FLAC) numerical model is used to investigate the influence of horizontal toe stiffness on the performance of reinforced soil segmental retaining walls under working stress (operational) conditions. Results of full-scale shear testing of the interface between the bottom of a typical modular block and concrete or crushed stone levelling pads are used to back-calculate toe stiffness values. The results of numerical simulations demonstrate that toe resistance at the base of a reinforced soil segmental retaining wall can generate a significant portion of the resistance to horizontal earth loads in these systems. This partially explains why reinforcement loads under working stress conditions are typically overestimated using current limit equilibrium-based design methods. Other parameters investigated are wall height, interface shear stiffness between blocks, wall facing batter, reinforcement stiffness, and reinforcement spacing. Computed reinforcement loads are ...

Numerical Investigation of Controlled Yielding of Soil-Retaining Wall Structures

Abstract Geosynthetic materials used as compressible layers against rigid wall structures can be employed to induce controlled yielding of the retained soil backfill during construction, and thereby ensure minimum lateral earth pressures on the structure. This paper describes two sets of numerical simulations that were carried out to model the controlled yielding concept. The simulations used a finite-element method together with a hyperbolic constitutive soil model. The first set of simulations was used to validate the FEM approach by comparing in the literature. The second of simulations was carried out to generate preliminary design charts for the selection of stiffness and thickness of compressible layers placed against rigid walls retaining well-graded sand backfills compacted to a range of densities.