Cyclic Response of Clay Deposits: Developing a Constitutive Model

This study aims at developing a generalized elasto-plastic constitutive model for clays able to describe stress-strain response, accumulation of permanent deformations, and excess pore pressure in monotonic and cyclic loading. This constitutive model takes advantage of the nonlinear elasticity and bounding surface plasticity concepts to mimic generation of excess pore pressure and plastic deformation within the yield surface upon cycles of unloading and reloading during cyclic excitation. The generalized formulation of the model also facilitates the prediction of multi-directional cyclic response of the fine-grained material. Capabilities of the model are evaluated using the available experimental database on Boston Blue Clay (BBC). The model is successful to mimic a wide range of monotonic drained and undrained stress paths as well as the complicated cyclic response of clays. Implementation of the model in numerical packages will facilitate the simulation of different boundary value problems under various loading conditions.United States National Science Foundation CAREER grant (NSF Award No. CMMI-0449021

The development of shear strain in undrained multi-directional simple shear tests

Actual earthquakes apply horizontal shear forces on soil in a multi-directional manner. The effects of multi-directional seismic loading on the undrained behaviour of medium dense Hostun sand was recently studied with stress-controlled undrained cyclic multi-directional simple shear tests. The development of shear strain in several tests with uni-directional, oval and circular loading paths is compared and the preliminary results are reported here. The general geometry of the strain paths resembles that of the loading paths. But strain paths are diverted when excess pore pressure ratio exceeds a certain threshold (0.6-0.7 in this study). The existence of static shear stresses further complicates the development of shear strain. Downhill shear strain happens no matter what direction the cyclic shearing is applied in. The shear strain in the perpendicular direction is limited if the static shear stress is large enough to eliminate stress reversal. The knowledge regarding the development of shear strain in multi-directional loading conditions remains scarce and more efforts are needed to advance the understanding of this topic

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Undrained sliding resistance of shallow foundations subject to torsion

While the behavior of shallow foundations under vertical load combinations has been the subject of numerous studies, the response of shallow foundations subjected to combined horizontal and torsional loading has received considerably less attention. New offshore applications of shallow foundations for LNG facilities and other subsea structures have underscored the importance of the behavior of shallow embedded foundations subjected to combined in-plane translation and torsion. This study investigates the undrained bearing capacity of rectangular and square shallow foundations under eccentric horizontal loads through comparisons of various limit equilibrium and plastic limit analysis solutions to 3-D finite element solutions. In general, the plastic limit approach considered in this paper agrees well with the finite element solutions, although it has some tendency to over-predict capacity at greater embedment depths. The studies revealed a general insensitivity in the shape of the yield envelope to variations in embedment depth, which permits a simplified analysis suitable for first order estimates of load capacity. The variables considered in this study include footing aspect ratio, embedment depth, and load direction in addition to eccentricity.This is the author's accepted manuscript. The final version can be found published by ASCE in the Journal of Geotechnical and Geoenvironmental Engineering here: http://ascelibrary.org/doi/abs/10.1061/(ASCE)GT.1943-5606.0001138

Theory on Measuring Orientation with MEMS Accelerometers in a Centrifuge

Microelectromechanical systems (MEMS) sensors have become a common part of everyday life and can be found in a number of consumer electronics. Specifically, MEMS accelerometers have become widespread because of their low cost, due to mass production techniques, and ability to sense constant acceleration. This ability allows devices, such as cellular phones, to measure their rotation relative to Earth's gravity. These properties also make MEMS accelerometers an option for measuring the rotation of geo-structures, such as foundations, in the field or in scale model geotechnical centrifuge tests. MEMS accelerometers appear to be especially beneficial for measuring orientation in centrifuge experiments because they are not limited by the design constraints of traditional tilt sensors: a single constant acceleration vector (Earth's gravity). This paper presents the theory behind using single-axis MEMS accelerometers to measure the orientation of an object on a plane of reactive centrifugal acceleration and Earth's gravity within a geotechnical centrifuge. The paper specifically addresses cross-axis sensitivity which can significantly impact measurements and is typically excluded from simpler theories.The authors acknowledge the National Science Foundation, the Network of Earthquake Engineering Simulations (NEES), and the project Capacity and Performance of Foundations for Offshore Wind Towers, Award Number: 1041604. Additionally, we would like to acknowledge the NEES site at Rensselaer Polytechnic Institute.This is the author accepted manuscript. The final version is available from ASCE via http://dx.doi.org/10.1061/9780784479087.24

Fracture of bio-cemented sands

Bio-chemical reactions enable the production of biomimetic materials such as sandstones. In the present study, microbiologically-induced calcium carbonate precipitation (MICP) is used to manufacture laboratory-scale specimens for fracture toughness measurement. The mode I and mixed-mode fracture toughnesses are measured as a function of cementation, and are correlated with strength, permeability and porosity. A micromechanical model is developed to predict the dependence of mode I fracture toughness upon the degree of cementation. In addition, the role of the crack tip $T$-stress in dictating kink angle and toughness is determined for mixed mode loading. At a sufficiently low degree of cementation, the zone of microcracking in the vicinity of the crack tip is sufficiently large for a crack tip $K$-field to cease to exist and for crack kinking theory to not apply. The interplay between cementation and fracture properties of sedimentary rocks is explained; this understanding underpins a wide range of rock fracture phenomena including hydraulic fracture

Exploring the lateral capacity of squat piles in soft clay through geotechnical centrifuge modelling

© 2017 IEEE. Many offshore structures currently in use are supported by piles with large length-to-diameter aspect ratios, because it is well known that such foundations can hold large forces and moments. In environments where long piles are not suitable, structures will use foundations with very low aspect ratios such as skirts and mats. Capacity of long piles has been studied for decades and is well documented, whilst more recent tests have also addressed the behaviour of skirts, mats, and other low-aspect ratio foundations. The vertical and lateral capacity of mid-size foundations, with aspect ratios between one and five, has generally been thought too low for the requirements of most offshore structures. However, in recent years, structures of increasingly different shapes and sizes have been used in offshore environments, such as water-based renewable energy sources or marginal oil and gas platforms. In many of these cases, the usage of a low aspect ratio foundation could significantly reduce installation and transportation costs. Limited studies have been performed on such foundations, and most of the existing work uses only analytical and numerical solutions. Geotechnical centrifuge tests and corresponding numerical analyses were started at Texas A&M University and were continued at the University of Cambridge on the lateral capacity of piles with an aspect ratio of two in normally consolidated clay. Piles were loaded under both pure rotation and a mix of rotation and translation. This work is relevant to offshore structures requiring foundations that are strong but easily installed and cost-efficient, specifically structures secured with piles that experience point loads either through or above the water. It is also of interest for structures in difficult environments, such as areas too shallow or sedimentary for long piles or too fragile for skirts and mats.National Science Foundation (USA), the National Secretary of Science and Technology (Panama

A generalized Drucker–Prager viscoplastic yield surface model for asphalt concrete

A Generalized Drucker-Prager (GD-P) viscoplastic yield surface model was developed and validated for asphalt concrete. The GD-P model was formulated based on fabric tensor modified stresses to consider the material inherent anisotropy. A smooth and convex octahedral yield surface function was developed in the GD-P model to characterize the full range of the internal friction angles from 0 to 90 degrees. In contrast, the existing Extended Drucker-Prager (ED-P) was demonstrated to be applicable only for a material that has an internal friction angle less than 22 degrees. Laboratory tests were performed to evaluate the anisotropic effect and to validate the GD-P model. Results indicated that 1) the yield stresses of an isotropic yield surface model are greater in compression and less in extension than that of an anisotropic model, which can result in an under-prediction of the viscoplastic deformation; and 2) the yield stresses predicted by the GD-P model matched well with the experimental results of the octahedral shear strength tests at different normal and confining stresses. By contrast, the ED-P model over-predicted the octahedral yield stresses, which can lead to an under-prediction of the permanent deformation. In summary, the rutting depth of an asphalt pavement would be underestimated without considering anisotropy and convexity of the yield surface for asphalt concrete. The proposed GD-P model was demonstrated to be capable of overcoming these limitations of the existing yield surface models for the asphalt concrete.Financial support was provided by the U.S. Department of Transportation (USDOT) and the Texas state general revenue funds through Southwest Region University Transportation Center (SWUTC No. 600451-00006). The validation shear tests of this study are based upon the work supported by the National Science Foundation under Grant No. 0943140.This is the accepted manuscript version. The final version is available from Springer at http://dx.doi.org/10.1617/s11527-014-0425-1