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Constitutive models constructed within the combined framework of kinematic hardening and bounding surface plasticity have proved to be successful in describing the rate-independent deformation of soils under non-monotonic histories of stress or strain. Most soils show some rate-dependence of their deformation characteristics, and it is important for the constitutive models to be able to reproduce rate- or time-dependent patterns of response. This paper explores a constitutive modelling approach that combines multiple viscoplastic mechanisms contributing to the overall rate-sensitive deformation of a soil. A simple viscoplastic extension of an inviscid kinematic hardening model incorporates two viscoplastic mechanisms applying an overstress formulation to a ‘consolidation surface’ and a ‘recent stress history surface’. Depending on the current stress state and the relative ‘strength’ of the two mechanisms, the viscoplastic mechanisms may collaborate or compete with each other. This modelling approach is shown to be able to reproduce many observed patterns of rate-dependent response of soils. 

This work focuses on the numerical simulation of multi-plate anchor systems (e.g., helical anchors) in sand subjected to vertical loading. In assessing the stiffness and capacity of these multi-plate anchor systems, full awareness of the abilities and limitations of the various analysis methods must be understood. This work first summarizes studies completed by others and then goes on to assess the failure mechanisms of multi-plate anchors in sand and the influence of (1) plate width-to-depth ratio, (2) number of plates, and (3) relative positioning of plates. The analysis makes use of (1) conventional limit analysis, (2) so-called modified limit analysis that employs reduced strength parameters to account for the influence of soil dilatancy, and (3) the displacement-based finite element method, which considers elastic as well as plastic deformation leading to failure. The work critically reflects on limitations in the current analysis methods for helical ground anchors. 

A perceived advantage of screw-type foundations is the ability infer aspects of foundation performance from quantities measured or observed during installation, especially the installation torque. A particular concept widely used in practice is to correlate installation torque to ultimate capacity. This notion has proven useful as a field verification technique despite the absence of validated models that relate key variables of interest, such as installation torque, axial (crowd) force, geometrical parameters, and soil strength. This paper considers previous work by the co-authors and collaborators on analytical, numerical, and physical modelling of screw piles to relate the quantities measured or controlled during installation (e.g., installation torque) to the ultimate capacity and soil strength. Attention is given to saturated clay as a particular soil type amenable to simplified analysis. An analytical model for a single-helix pile is considered as a means of directly relating the ultimate capacity and undrained shear strength to the installation torque, crowd force, plate pitch, plate diameter, shaft diameter, installation depth, and surface roughness. The connection between the installation variables and ultimate capacity—and the sensitivity to crowd force in particular, a quantity that is typically not measured during field installations—is also discussed. The theoretical predictions are compared against data obtained from small-scale laboratory experiments that suggest the installation torque relates to the remolded strength of the soil. 

Simulation of plowing and cutting processes in soils is challenging and time-consuming due to large deformations and contact interactions. Recent studies on sand have suggested that a simplified, efficient approach based on incremental plastic analysis can capture the essential physics and features of the problem. The present study refines this technique by enhancing the kinematics and implementing a more sophisticated material law. The effects of hardening and softening, as well as dilatancy and compaction, are introduced. With the modified model, it is observed in the case of hardening (compaction) that the occurrence of multiple successive shear bands at variable locations gives the appearance of continuous shearing in the final pattern of deformation. This is markedly different from the previously predicted response in the case of softening (dilatancy), where shear bands appear at distinct locations and transition from one discrete location to the next. The computed results are compared with preliminary experimental data gathered in the Soil-Structure and Soil-Machine Interaction Laboratory (SSI-SMI Laboratory) at Northwestern University. 

Accurate estimation of soil mechanical properties represents a crucial step for most engineering applications. Both in situ and laboratory testing fundamentally rest on mechanically deforming (actuating) the material and simultaneously measuring its response in terms of displacements and stresses (reactions). Facing this widely adopted scheme, key questions remain unanswered: 1) what is the optimal type and/or mode of actuation that can most effectively extract soil properties; 2) what types of measurements are most useful for inferring material constants? As a first step in the investigation of these questions, an inverse model for the direct simple shear (DSS) test is constructed, wherein measurable responses are used to back-calculate soil properties. Specimens with two different aspect ratios are considered to study the influence of the deformation mode. The effect of the choice of measurements (i.e., which displacements and/or stresses are observed) is explored by assessing inverse model performance considering the DSS test as a boundary value problem, with variable displacement and stress fields, versus the conventional interpretation as an elemental test. Parameter sensitivities and correlation coefficients are employed as quantifiable metrics to compare material characterization based on different aspect ratios and types of measurements, and to interpret the performance of inverse analysis.