Browsing by Author "Asante, Samuel Yaw"

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    Alternate methods to determine the microstructure of collapsible soils
    (Stellenbosch : Stellenbosch University, 2015-12) Asante, Samuel Yaw; Fouche, Nanine; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Collapsible soil is one of the most widely distributed problematic soils in the world including South Africa. Extreme leaching and erosion of the colloidal matter and fine particles creates a structure similar to a honeycomb within the microstructure of these soils, leading to the formation of a collapsible grain structure. Upon wetting and under additional loading these soils undergo a significant decrease in volume resulting in severe damage to structures. In South Africa the collapse phenomenon, which is regarded a geotechnical hazard, was first identified in the 1950’s. According to Rogers (1995), a geotechnical engineer needs to be able to identify these soils by examining in detail the properties of collapsible soils by listing the features commonly associated with it; which includes:  An open soil structure;  A high void ratio;  A low dry density;  A high porosity;  Geologically young or recently altered deposit;  High sensitivity and  Low inter-particle bond strength. The first four features suggested by Rogers (1995) suggest that the collapse phenomenon is directly controlled by the microstructure of these collapsible soils. From Rogers’s definition, it can be concluded that the microstructure of collapsible soils are governed by the following: porosity, pore size distribution, grain size distribution, pore fluid content, and ions on the grain and in pores (mechanical properties). Each of these microstructure properties can be examined and/or determined by laboratory testing or field observation. For this reason, the collapse behaviour of two soil types (reworked residual granite and residual Malmesbury shale) within the Stellenbosch municipal area was investigated by examining the microstructure and mechanical behavior of these soils. Alternate methods (CT-scanning and scanning electron microscopy) as well as conventional laboratory tests were applied. The aims of the study was achieved by developing a soil testing method using x-ray computed tomography (CT-scanning) and scanning electron microscopy (SEM) to determine the porosity, void ratio, particle size distribution, particle shape, and pore size distribution of residual soils. In order to achieve this, the VGStudio Max version 2.2 coupled together with Avizo Fire image analysis software version 8.0 were used in filtering and classification and distribution of voids, and particle size distribution within the soil microstructure. The image analysis was achieved by examining three dimensional (3D) and two dimensional (2D) X-ray images obtained using a General Electric Phoenix VTomeX L240 X-ray micro computed tomography scanner (microCT) and ZEISS EVO MA15 scanning electron microscope. From the image analysis, it was found that substantial volumetric changes (settlement) occur within the macropores of a potentially collapsible soil. The measured particle size distribution (PSD) by CT-scanning compared relatively well with the mechanical sieving method, although a few discrepancies were noted between the two methods. The image analysis from the SEM 2D images revealed that the particle morphology and mineralogy contributed greatly to the degree of collapse. The PSD from SEM images using imageJ (image analysis software) was not possible due to the bleeding effect of fine to medium-sized particles. It can thus be concluded that CT-scanning and SEM are good alternative methods to investigate the microstructure of soils; and further research in this regard is indicated.