Alternate methods to determine the microstructure of collapsible soils

Asante, Samuel Yaw (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

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.

AFRIKAANSE OPSOMMING: Swigbare grond is die mees algemene soort problematiese grond in die wêreld insluitende Suid-Afrika. Ernstige uitloging en erosie van die lymagtige stof en die fyner partikels skep ’n struktuur wat soortgelyk is aan ’n heuningkoek in die mikrostruktuur van hierdie grondtipes. Dit lei tot die formasie van ’n korrelstruktuur wat geneig is om in te sak. As dit nat word en as daar addisionele las op die grond geplaas word, verminder die volume van die grond, wat lei tot skade aan die struktuur. Hierdie verskynsel, wat as ’n geotegniese gevaar beskou word, is vir die eerste keer in die vyfigerjare geïdentifiseer. Volgens Rogers (1995) moet ’n geotegniese ingenieur hierdie tipe grond kan identifiseer deur die kenmerke van grond wat geneig is om in te sak, te identifiseer en die kenmerke wat daarmee geassosieer word, te lys. Hierdie kenmerke sluit die volgende in:  ’n Oop grondstruktuur;  ’n Hoë tussenkorrelporie-verhouding;  ’n Lae droë digtheid;  Hoë poreusheid;  Geologiese jong of onlangs veranderde neersetting;  Hoë sensitiwiteit en  Lae interpartikelverbindingsterkte Die eerste vier kenmerke wat deur Rogers (1995) genoem word, wys daarop dat die swigversakking direk van die mikrostruktuur van die grond afhang. Volgens Rogers se definisie kan daar afgelei word dat die mikrostruktuur van grond wat swigbaar is, afhang van die volgende: poreusheid, die verspreiding van die grootte van die porieë, die verspreiding van greingroottes, die vloeistof inhoud van die porieë, en die ione op die korrels en in die openinge (meganiese kenmerke). Elkeen van hierdie mikrostruktuur-kenmerke kan deur laboratoriumtoetsing of veldwaarneming ondersoek en/of vasgestel word. Daarom is die swigversakking van twee grondtipes (residuele graniet en residuele Malmesbury skalie) in die Stellenbosch munisipale gebied ondersoek. Die mikrostruktuur en die meganiese gedrag van die grondtipes is ondersoek. Alternatiewe metodes (CT-skandering en skanderingselektronmikroskopie, SEM) asook konvensionele laboratorium-toetsing is gebruik. Die doel van hierdie studie is bereik deur ’n grondtoetsingmetode te ontwikkel wat x-straal berekende tomografie (CT skandering) en SEM insluit. Die doel was om die poreusheid, die openingverhouding, die verspreiding van partikelgroottes, die vorm van die partikels en die verspreiding van die openinggrootte vas te stel. Om hierdie doel te bereik is die VGStudio Max weergawe 2.2 tesame met “Avizo Fire image analysis” sagteware weergawe 8.0 gebruik vir die filtrering en klassifikasie van openinge en die verspreiding van partikelgrootte in die mikrostruktuur van die grond. Die beeldontleding is gedoen deur drie-dimensionele (3D) en twee-dimensionele (2D) X-straal beelde te ondersoek wat met ’n General Electric Phoenix VTomeX L240 X-straal mikro berekende tomografiese skandeerder (microCT) en ’n ZEISS EVO MA15 elektronmikroskoop gedoen is. Daar is deur middel van hierdie ontleding bevind dat redelike groot veranderinge in volume plaasvind in die openinge van ‘n potensieël swigbare grond. Die verspreiding van partikel-grootte soos deur skandering gemeet vergelyk redelik goed met die bevindings van die meganiese sifmetode, maar daar is wel ’n paar verskille in die bevindinge. Die ontleding van die SEM 2D-beelde toon dat die partikelmorfologie en mineralogie grootliks bydra tot die mate van insakking. Die PSD van die SEM beelde deur die gebruik van beeld-ontledingsagteware was weens die bloei-effek van fyn tot mediumgrootte partikels, nie moontlik nie. Daar is dus bevind dat CT skandering en SEM goeie alternatiewe metodes is vir die ondersoek van die mikrostruktuur van grond en ander soortgelyke navorsing.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97864
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