Measurement of stress - strain relationships in soil systems usually incorporate varying degrees of uncertainty. These uncertainties arise from laboratory testing mechanisms, sampling disturbances, errors performed by operators or technicians performing the tests, etc. Currently, deformation analyses have been modeled using several deterministic techniques. However, because of the uncertainties involved, there is a need to adapt these numerical methods into probabilistic models. This thesis develops a probabilistic constitutive model based on a variable moduli deterministic technique. First-order, second-moment stochastic methods are used to estimate a mean stress - strain curve and its ±1 standard deviation from raw data obtained on nearly identical, remolded sand samples. Probabilistic estimations for Bulk and Shear moduli are determined from the estimated mean curves and are used to develop a probabilistic constitutive model. Through the use of a probabilistic constitutive matrix, a stochastic equation is produced which can relate strains to any stress state imposed on a particular soil. This is verified through an example.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 104-109). Also available in electronic version.

Thesis (Ph. D.)--Oregon State University, 2003. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

Not available / text

Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

A constitutive model based on rate-independent elastoplasticity concepts is developed and used to simulate the behavior of geologic materials under arbitrary three-dimensional stress paths. The model accounts for various factors such as friction, stress path and stress history that influence the behavior of geologic materials. A hierarchical approach is adopted whereby models of progressively increasing sophistication are developed from a basic isotropic-hardening associative model. Nonassociativeness is introduced as correction or perturbation to the basic model. Deviation of normality of the plastic strain increments to the yield surface F is captured through nonassociativeness. The plastic potential Q is obtained by applying a correction to F. This simplified approach restricts the number of extra parameters required to define the plastic potential Q. The material constants associated with the model are identified, and they are evaluated for three different sands (Leighton Buzzard, Munich and McCormick Ranch). The model is then verified by comparing predictions with laboratory tests from which the constants were found, and typical tests not used for finding the constants. The effect of varying initial density of a material on the stress-strain and volumetric response is investigated. An empirical relation is proposed, whereby one parameter is modified based on the initial density, such that improved predictions can be obtained without increasing the total number of parameters. Implementation of the nonassociative model in a finite element program to solve boundary value problems leads to a nonsymmetric stiffness matrix. Besides, using a nonsymmetric solver, three numerical schemes are investigated. The idea of the schemes is to modify the stiffness matrix such that a symmetric equation solver can be used. Prediction of stress-strain, volumetric response and CPU time for different schemes are compared with the predictions obtained using the nonsymmetric solver. The nonsymmetric equation solver used less CPU time and the solutions were more accurate. Based on the above findings, a soil-footing system is analyzed using the finite element techniques. The associative and nonassociative models are used to predict the behavior. For the nonassociative model, solution is obtained by using a nonsymmetric solver. Results obtained from both models are compared with a model footing test performed in the laboratory.

The behavior of porous media subjected to any arbitrary loading is a complex phenomenon due to the coupled nature of the problem. Proper understanding of this coupled behavior is essential in dealing with many of the geotechnical engineering problems. A very general three-dimensional formulation of such a coupled problem was first reported by Biot; however, a two-dimensional idealization of the theory is used here with extension to nonlinear material behavior. A finite element computer code is developed to analyze the response of coupled systems subjected to both static and dynamic excitations. The code can also be used to solve problems involving only solid media by suppressing the presence of fluid. The generalized anisotropic hardening model is implemented into the finite element procedure to characterize nonlinear material behavior throughout the realm of its deformation process. Both drained and undrained conditions are considered in order to verify the performance of the model in capturing material behavior. Three different materials are considered for this purpose. The predictions obtained using the anisotropic model for both drained and undrained condition yield satisfactory comparison with observed behavior. The finite element procedure is verified by solving several problems involving undrained, consolidation and dynamic responses of coupled system. Good agreements are found between numerical and analytical results. Further verification of the computer code and the material model is performed by solving two boundary value problems. For this purpose, a laboratory pressuremeter test subjected to quasi-static loading condition and a building foundation system subjected to rapid earthquake excitation were analyzed. The results of this research have provided an improved understanding of coupled behavior of porous media. The procedure developed here can be effectively used under a wide range of loading conditions varying from very slow quasi-static to very rapid earthquake excitations.

Phosphorus (P) limitations to agricultural productivity commonly occur in Australian soils and have largely been overcome by the use of inorganic fertilisers. However, studies have shown that most of the P taken up by plants is from native P pools. The turnover of P and native soil organic matter may be strongly affected by drying and rewetting (DRW). Rewetting dry soil results in a pulse of respiration activity and available nutrients. In Mediterranean-type climates surface soils naturally undergo recurrent DRW cycles. In southern Australia, soils experience DRW due to erratic rainfall within the growing season, and short, high intensity thunderstorms also during summer periods. The principal objective of this thesis was to determine the significance of dry-rewet events, for altering P availability and cycling in agricultural soils in Australia. Soils representing a wide range of soil types and climatic zones of southern Australia, showed large flushes in carbon (C) mineralisation after a single DRW event. For some soils these were comparable with reported values, however large variability in flush size between soils was observed. Soils that commonly experience DRW did not appear to be more resilient to DRW than soils from areas with fewer DRW events. Even when soils had relatively small respiration flushes, as a result of low soil organic matter, a high proportion of the soil C was mineralised after rewetting. Soil physiochemical properties (total C, total N, organic C, humus, microbial biomass P, organic P, sand and silt) were correlated to the size of the flush, hence nutrient availability and soil texture appear to primarily determine flush size. Therefore, the influence of climate on DRW may relate to determining the quantity of organic matter and microbial biomass that is available for turnover. Different size and composition of the microbial biomass within the same soil matrix were achieved by adding three different C substrates (glucose, starch and cellulose at 2.5 g kg-¹) at 5 times over 25 weeks. The treatments showed disparate responses to DRW, due to greater biomass (larger flushes) and effects of community composition, highlighting the central role of the soil microbes in DRW processes. When subjected to multiple DRW events these soils showed smaller rewetting respiration flushes with subsequent rewetting events. In contrast, the amount of P released after rewetting was the same. This study showed that increases in P after rewetting were transient and rapid immobilisation of P by microbes occurred, which may limit the availability to plants. The composition of the microbial community was changed by DRW with a reduction in fungi and gram negative bacteria, showing that certain species are more susceptible to DRW than others. Closer investigation at 2 hourly intervals after rewetting confirmed the transient nature of P flushes. The response in microbial respiration after rewetting was immediate, with the highest activity occurring within the first 2 h. Phosphorus availability was increased by DRW but remained stable over the following 48 h incubation period. The study highlights the rapid nature of changes in available nutrients after rewetting. Furthermore, while potentially only a small component of the P flush that occurred, the DRW soil had higher levels of P than most incubated soil at 48 h, this would be potentially available for plant uptake or movement with the soil solution. Long-term water regimes (continuously moist or air-dry, or DRW occurring at different times during incubation) that were imposed on two soils from different climatic regions over a 14 wk period, did not alter available nutrient (P and C) pools or the size of the microbial biomass. However, these long-term water regimes determined the respiration response of the soils to experimental DRW. The largest flushes occurred in the treatment with the longest dry period, and confirm findings of reported studies that the response of a soil at rewetting is determined by the length of the period that it is dried. Microbial biomass was little affected by experimental DRW, but showed large changes in C:P ratio. Thus, changes in physiological state or community composition may be more affected by DRW than the size of the microbial biomass. Microbial communities were altered by DRW irrespective of climatic history (warm wet summer and temperate Mediterranean), however these changes were not related to specific groups of organisms. In addition, the disparate respiration responses and inhibition of phosphatase by DRW, indicate that functional changes may be induced by DRW but can not be sufficiently explained by quantifying available nutrient pools or the microbial biomass. The use of wheat seedlings bio-indicators of P availability after the long-term water regimes, confirmed that plant available P was altered by DRW, indicated by differences in growth, although the large variability in seedling growth made it difficult to quantify these differences. However, the distribution of labile P, available at planting, in soil and plant pools at harvest, showed that long-term water regimes increased P allocation in plant tissue in one soil and decreased it in another. Furthermore, only a small fraction of the labile P present at planting was taken up by plants, which confirms the superior ability of soil microbes to immobilise P that is released by DRW. Nevertheless, since the long-term water regimes increased P availability, this may be transported via surface water or leaching. DRW is important for C and P turnover in soils of southern Australia. However, P flushes occur rapidly after rewetting and are transient. Therefore, DRW appears to have only minor consequences for P availability to plants. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1321018 / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2008

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Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1990. / Includes bibliographical references (leaves 137-150) / Microfiche. / xiii, 150 leaves, bound ill. 29 cm