Sensitive soils of the Puketoka Formation, Hamilton, New Zealand
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/14686
Sensitive soils are known to cause significant issues in the infrastructure and civil engineering industry, and are prone to sudden, oftentimes catastrophic failures following long periods of stability. The Walton Subgroup soils deposited through volcanic activity and partially reworked by the ancestral Waikato River are identified within the Hamilton Basin. These materials are of interest to the engineering community, and consultancy groups in the Hamilton area, due to their identified sensitivity. The Puketoka Formation was focused on in this study. Sampling of material was undertaken at two active construction sites in Hamilton: Dixon Road Roundabout, and Temple View Urban Development. A series of classification tests were completed to determine the samples’ mineralogical and geotechnical characteristics. Mineralogical testing found that all the materials were silt dominated (55.06 – 81.12 %), with sub-ordinate clay (13.10 – 18.85 %) and sand (5.56 – 25.35 %) fractions. The moisture contents were variable (49 – 65 %). The field moist bulk density varied throughout the samples (1069 – 1576 kg/m3), as did the oven dry bulk densities (645 – 1057 kg/m3). The porosity as a result also varied (36 – 51 %). The void ratios followed the same trend as the porosity values (0.56 – 0.88). The particle densities of the materials also showed variation (1935 – 2487 kg/m3). The activities of all three samples were variable, ranging from inactive (0.60) to active (1.79). The X-Ray Diffraction (XRD) analysis determined that the main clay mineral present within the samples was a kaolinite group clay, either halloysite or kaolinite. The Fieldes & Perrott NaF test to determine the presence of allophane was also completed on the samples. Both tests were negative, confirming the dominance of halloysite. Bulk analysis also determined other constituents of the samples to include quartz, plagioclase, augite and volcanic glass. Scanning Electron Microscopy (SEM) indicated the main halloysite morphologies present to be a combination of short and long tubules, spheroids, plates and book formations. The microstructure was analysed, and the main clay minerals, grains, microaggregates, and pores within the materials described. The microfabric was consistent for all three samples, with each showing an abundance of small pores (< 20 µm). These pores hold significant amounts of water in not readily accessible locations, leading to the highly porous, but poorly permeable nature of the materials. DRAB1 and TV1 showed microstructures consistent with Huppert (1986) and Beattie’s (1990) ‘matrix’ structures, whilst TV2 showed a more dominant ‘skeletal’ to ‘matrix’ structure. Atterberg limits testing found that the samples had a high liquid limit (59 – 75 %) and plastic limit (41 – 53 %), but a variable plasticity index (8 – 34 %) and liquidity index (0.1 – 2). This placed all three samples within the high plasticity “MH” category upon the Casagrande chart. Consolidated undrained triaxial testing of DRAB1 and TV1 indicated a characteristic normally consolidated response to shear, with the majority of cores showing shear band formation. Only TV1 at 170 kPa displayed an intermediate failure type, potentially occurring due to a combination of material being tested, confining pressure, and the low shear rate allowing time for pressure dissipation. The percentage of axial strain the cores failed at varied (1.81 – 16.35 %), and the peak deviator stress (129 – 277 kPa) and pore water pressure values (717.2 – 879.7 kPa) tended to increase as the confining pressures increased. Graphical outputs show a moderate strain, contractive (MSC) style of failure. The effective shear strength values produced were in keeping with the varying ranges of previously published values (effective cohesion: 0 – 37 kPa, effective friction angle: 27 - 38°). The permeability for all three samples was estimated to be ~10-9 – 10-10. Thin section analyses of failed core sections for DRAB1 at 270 kPa confining pressure and TV1 at 130 and 230 kPa confining pressures found that some degree of shear formation and propagation occurred. The calculated R’ angle for DRAB1 was closely observed in the thin section, however was not for TV1. This was hypothesised to have occurred due to a true shear band failure not occurring, rather the development of intermediate to juvenile shear bands within the soil. The sensitivity of the Puketoka Formation and wider Walton Subgroup is hypothesised to have been derived from the reworking and weathering the materials experienced. This in turn influenced the porosity and permeability of the materials, which I believe govern the sensitivity of the soils to a degree. Therefore, I believe the Puketoka Formation materials are representative of a silt-dominated, minimally reworked deposit of volcanic origin, that showed appreciable differences compared with the previously studied Tauranga soils. The remediation strategies used in the engineering industry were also analysed, and the suitability of the strategies in the context of the Puketoka Formation discussed. This assessment provides an overview of why traditional strategies for remediation may not be beneficial in some cases for the Walton Subgroup materials, which could be of benefit to key engineering groups that encounter these soil materials. The results of this study provide a set of data which can be used as an initial reference point. The findings and hypotheses made could be of use to key groups including local engineering consultancies, who handle these materials regularly.
The University of Waikato
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