The effects of biological and physical processes on the aggregate stability of some weakly structured arable and pasture soils were investigated. Preliminary investigations showed significant correlations between soil organic matter and both wet sieving and turbidimetric methods of aggregate stability; the latter method was chosen on grounds of convenience. Scanning electron microscope pictures showed the presence of both fungi and bacteria when soils were incubated. Growth of fungi, estimated by ergosterol measurement, correspond to temporary stability increases, which could be explained by retention of soil particles within the reticulum of fungal hyphae. The effect disappeared as the fungi were destroyed and replaced by bacteria and actinomycetes. Effects caused by fungi were examined separately, using vancomycin to inhibit bacterial growth, and bacterial effects by using cycloheximide to eliminate fungi. Bacterial growth had little direct effect in stabilising soil aggregates; periodate oxidation showed that polysaccharides produced by bacteria are mainly responsible. To examine the contribution of physical processes to increased stability in remoulded soils biological processes were eliminated by sterilisation. Thixotropic changes made a contribution to age hardening in remoulded aggregates similar in magnitude to biological processes. Thixotropic changes were reversible and accompanied by soil strength and metric water potential changes. Polysaccharides did not contribute to thixotropic aging processes. Remoulded soils were subjected to wetting/drying and freezing/thawing cycles. After 3 to 6 cycles the stability of both sterilised and unsteriliserd soils recovered to that of natural aggregates, suggesting a contribution by thixotropy. Repeated weathering cycles decreased the stability of unsterilised, and more so sterilised, field aggregates suggesting that in the former, bond reformation due to biological activity counteracted the destruction caused by wetting/drying and freezing/thawing.

Typescript (photocopy) / xii, 160 leaves, 11 leaves of plates : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.) Dept. of Soil Science, University of Adelaide, 1982

Thesis (M.S.)--Ohio State University, 1982. / Includes bibliographical references (leaves 119-124). Available online via OhioLINK's ETD Center

Thesis (M.S.)--Oregon State University, 1984. / Typescript (photocopy). Includes bibliographical references (leaves 84-91). Also available on the World Wide Web.

Thesis (M.S.)--Oregon State University, 1981. / Typescript (photocopy). Includes bibliographical references (leaves 105-113). Also available on the World Wide Web.

The results of a study to evaluate the behavior of a flexible culvert with the inclusion of a goefabric above the culvert are presented. In the soil-fabric-culvert system the geofabric becomes an interactive stress-carrying component of the system. Insight into the phenomenon of the stress-attenuation due to the inclusion of a semimechanical reinforcement, like geofabric, is also obtained from this research. The numerical technique of the finite element method is used for the analysis of the soil-fabric-culvert system. In the finite element model, two-dimensional triangular and quadrilateral elements having nonlinear, stress-dependent material properties are used for representing the soil. Beam elements are used to model the culvert, no-compression bar elements are used for the fabric, and two-dimensional interface elements for the contact surfaces between the soil and fabric. Incremental construction sequence and approximate nonlinear geometry are adapted in the analysis. Because of the relatively recent usage of fabrics for engineering purposes, no information regarding their employment as an inclusion in a soil-culvert system is available in the open literature. However, a review of the literature is conducted to bring out the current state of understanding of the behavior of soil-culvert systems without the presence of a fabric. The classical design concepts which reflect the development of the design methodology for flexible culverts are reviewed. The phenomenological concepts of soil-culvert interaction, particularly those of arching in the soil above the culvert and buckling of the soil-surrounded culvert wall are discussed. The burial depths of D/2 and D are considered for the horizontal configuration of the fabric when D = diameter of culvert. A burial depth of 2D is considered for the inclined configuration of the fabric. The following surface concentrated loading conditions are considered for the horizontal configuration of the fabric. Loads of 10 kip are placed symmetrically at S/D = 1/4, 1/2, 1, or 2 and 20 kip load at S/D = 0 (S = distance from vertical centerline of culvert). For the inclined configuration of the fabric above culvert, 50 kip surface concentrated loads are placed at S/D = 0, 1/4, and 1/2 simultaneously. This study shows that the fabric alters the stresses in the soil-fabric-culvert system by two mechanisms: the fabric can carry part of the load in tension and/or it can distribute the load more uniformly over a wider area. Under most practical conditions, the inclusion of fabric causes a significant reduction in the magnitudes of the following culvert design parameters: maximum axial force, maximum moment, vertical crown deflection, and horizontal springline deflection. The presence of fabric is more effective in attenuating the culvert design parameters for the location of surface loads within the horizontal projection of the culvert and for shallow depths of soil cover.

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