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Sensitivity and clay mineralogy of weathered tephra-derived soil materials in the Tauranga region

Soil sensitivity is defined as the ratio of peak to remoulded shear strength. Problem soil materials are those that show large strength losses on disruption, resulting in catastrophic failure, liquefaction and long run-out distances. This study focussed on sensitive, weathered, mainly tephra-fall derived soils of mid-Pleistocene age in the Tauranga region. The liquefiable character of these soils is well known, but little detailed study has been directed towards the reasons for sensitivity. The objective of this work was to examine soil sensitivity by investigating geomechanical properties, clay mineralogy, and microfabric, and to determine how these factors combine to develop sensitivity. To achieve these objectives a combination of both field and laboratory investigations was undertaken. Field investigations indicated that sensitive soils are common in the Tauranga region. Sampling was undertaken at sites in Tauriko and Otumoetai. Selected samples ranged across high (76) and low (≈8) field sensitivity. Stratigraphically, samples from Otumoetai lie below the Rangitawa Tephra (ca. 0.34 Ma), and those from Tauriko underlie the Te Ranga Ignimbrite (ca. 0.27 Ma). Geomechanical investigation revealed that the sensitive soils had high moisture contents (gt; 60 %), low dry bulk density (lt; 966kg m-3), and high porosity (gt; 60 %). Liquidity index values ranged between 0.27 and 2.41. Plasticity index values ranged from 13.2 % to 42.7 %, with all samples plotting below the A line. Strength tests indicated effective friction angles from 25.7 to 38.5 , effective cohesion from 4.7 kPa to 34.5 kPa, residual friction angles of 19.34 to 33.18 , and cohesions of 0 kPa to 4.87 kPa. Remoulded vane shear strengths ranged between 1 kPa and 36 kPa. Clay minerals were dominantly hydrated halloysite. Scanning electron microscopy indicated that clay morphology was in the form of hollow tubes, spheres, plates, and platy vermiforms ('books'). Tubes and spheres represent characteristic forms of halloysite in soils, plates are less common, and books have never previously been observed. Hence, these books represent a new morphology for halloysite. Individual plates in each of the books appear to show structural Fe enrichment (~5.2 %). This enrichment indicates that Fe had replaced Al in octahedral positions reducing the mismatch with the tetrahedral sheet, lessening layer curvature and thus generating flat plates. All microfabrics were continuous with larger sand and silt grains supported in a background of clay minerals. Microfabrics ranged from extremely open with components being loosely packed to those which were dense and tightly packed. A feature common to all structural types was an abundance of extremely small pores (lt; 20 μm) which are capable of tightly retaining water. The loosely packed microfabrics had void ratios that allowed moisture content to exceed liquid limits, producing a liquidity index gt; 1. These open microfabrics are probably a result of quick burial by subsequent pyroclastic beds; hence weathering to clays occurred as a process of subsurface diagenesis. Dense microfabrics with low void ratios and high liquid limits did not have liquidity indexes gt; 1. These dense microfabrics arose as a result of the deposits being at, or near, the land surface for a considerable time, thus allowing strong pedogenic processes to occur, which promoted clay formation and clay migration (illuviation) and reduced void ratios. Liquidity index was a major control on remoulded strength and sensitivity. Liquidity index is controlled by clay type and content, void ratio, and natural moisture content. When remoulded, structures with natural moisture contents exceeding the liquid limit release a large amount of water, which both dilutes the plasticity of binding clays and supports grains and broken aggregates of clay, allowing the material to flow. The development of sensitivity with low remoulded strength requires a number of factors. These include: a void ratio that is sufficiently high to allow natural moisture content to exceed the liquid limit; the presence of halloysite, which encourages samples to retain a coherent structure when saturated and to ensure the liquid limit remains sufficiently low so that it can be exceeded by natural moisture content; and a saturated environment, which ensures the liquid limit is exceeded.
Type of thesis
Wyatt, J. B. (2009). Sensitivity and clay mineralogy of weathered tephra-derived soil materials in the Tauranga region (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/4362
The University of Waikato
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