Jacobsz, Schalk Willem2021-11-022021-11-0220212020*A2021http://hdl.handle.net/2263/82460Thesis (PhD)--University of Pretoria, 2020.In geotechnical engineering, the soil type which has had some of the most severe economic implications is that of swelling clays. Swelling or expansive clays form a subgroup within the field of unsaturated soils which exhibit large volumetric changes upon wetting and drying, thereby causing severe distress to structures. The aim of this study was to use a combination of element testing and centrifuge modelling to investigate certain aspects affecting the behaviour of piled foundations. To perform the necessary centrifuge modelling, a sample preparation procedure was developed whereby the aim was to prepare samples which retained some degree of fissuring. Samples with such a fabric type facilitate a more rapid ingress of moisture. Element testing was then carried out to investigate any difference in the mechanical or soil water retention properties of the laboratory compacted specimens and undisturbed intact samples. The mechanical properties of primary interest were the pressure required to prevent swell, as well as the clay’s heave potential at various applied stresses. The results of the element testing conducted illustrated that the properties of interest remained, for all practical purposes, unchanged between statically compacted and intact undisturbed samples. Consolidation testing was also performed on various statically compacted and undisturbed samples. Additionally, the intrinsic clay properties were established from a test on a reconstituted specimen and were found to conform well to the framework outlined by Burland (1990). Comparisons of the consolidation tests on statically compacted and undisturbed samples with the reconstituted specimen allowed for soil structure and swell induced-softening to be analysed. This analysis revealed that only undisturbed samples which were inundated at relatively high stresses existed in structure permitted space upon further loading. This result illustrated the effect of swelling on soil structure and highlighted how, for the stress range applicable to this study, both undisturbed and statically compacted samples exist in permissible stress states after swelling. Furthermore, for samples which had undergone swell, yielding was found to occur at significantly lower stresses than what would be predicted for unstructured soils. More specifically, yielding occurred well within permissible stress space, rather than on the intrinsic compression line defined by the reconstituted consolidation test. This finding is consistent with the swell-induced softening described by the constitutive framework considered in this study. The centrifuge modelling conducted illustrated that the sample and preparation procedure utilised, facilitated the ingress of moisture, such that a significant magnitude of swell could be achieved within a practical time frame. This allowed for various aspects affecting the performance of piled foundations to be evaluated before (at the clay’s in-situ moisture content) and after a targeted value of swell was achieved. A series of ‘pull-out’ tests were conducted to evaluate the shaft capacity of short length piles at various depths in a clay profile, before and after achieving the targeted swell. The piles used for this evaluation were bored piles manufactured from rapid hardening grout. From this series of testing it was found that on average, the shaft capacity of piles reduced after achieving the targeted magnitude of swell. Exceptions to this trend were found at greater depths in the profile where shaft resistances increased. A final centrifuge model was conducted to evaluate the development of lateral pressure against a pile throughout a swell process. This test included an aluminium pile anchored at its base, instrumented with lateral load cells. The development of lateral pressures was monitored up until the targeted magnitude of swell was achieved. This test illustrated a distinct initial increase in lateral pressure, followed by a more gradual reduction in pressure as swell progressed. This result helps clarify some discrepancies in the literature on the effect of soil swell on the shaft capacity of piled foundations. If testing is performed fairly early in the swell process where lateral pressures are increasing, it is possible that an increase in shaft capacity will be measured. However, if shaft capacity is assessed at relatively large magnitudes of heave (as was done in this study), reductions in shaft capacity are likely to be measured.© 2021 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria.UCTDThesis