Browsing by Author "Mong, Danica"
Now showing 1 - 1 of 1
Results Per Page
- ItemThe Disintegration characteristics of residual granite soil(Stellenbosch : Stellenbosch University, 2022-04) Mong, Danica; Fouche, Nanine; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Plutons of the Cape Granite Suite are largely limited to the south-western Cape with exception of occurrences near George and in Namaqualand (Brink, 1981). The composition and texture of Cape granitoids are very complex and variable, depending on the location of the plutons. Problems associated with residual granite soil from the Cape Granite Suite include the presence of a collapsible grain structure. In partially saturated residual granite soils, quartz particles have colloidal coatings around individual grains, contributing to a seemingly high strength soil, and when saturated under load, the colloidal bridges become unstable, causing particles to pack into a denser state, leading to sudden settlement of the foundation. This research contributes towards classifying and characterising decomposed granite soil, from the Cape Granite Suite, in terms of its physical properties and engineering behaviour. The soil was classified based on its grading properties, Atterberg limits, maximum dry density, and optimum moisture content, and characterised based on triaxial compression test results. The triaxial compression test results provided shear strength properties, volumetric behaviour during shearing, critical state properties, and stress paths. Focus was placed on the influence of water on the behaviour of decomposed granite soil during different stages of the triaxial compression test, and on individual grains and/or aggregated particles which resulted in particle breakage after triaxial shearing. The amount of particle breakage was determined from the relative breakage of particles after triaxial shearing. Classification properties primarily depended on the degree of weathering for decomposed granite soil. The particle size distribution (PSD) shows that the decomposed granite soil ranges from gravel to clay size particles with the greatest amount of particles present in the medium- sand and fine-sand size ranges. Decomposed granite soil has a low plasticity with a typical fines content (<0.075mm) of around 15%. Descriptions for some of the soil properties are given below: • The mineralogy consists of quartz, plagioclase, K-feldspar, mica, kaolinite, and calcite. • SM “poorly graded silty sand” material (USCS), Class A-2-4 material (AASHTO system), and G8 quality “gravel soil” (TRH14 system). • Maximum dry density of 2120.5 kg/m3 by using modified AASHTO compaction effort, and an optimum moisture content of 8.5%. The mechanical behaviour of decomposed granite soil is evaluated through an isotropic compression and swelling test, and triaxial compression tests on saturated, dry, partially saturated, flooded, and air-dried specimens. Saturated results show that the soil conforms to the critical state Modified Cam-Clay model with a friction parameter, M = 1.41, an isotropic normal compression line λ = 0.073 and N = 1.88, and an unload reload line κ = 0.012. Partially saturated specimens (Sr = 20% - 80%) had similar strength and stiffness results as the saturated specimens indicating relatively small pore water suctions, and flooded specimens had a decrease in strength. Air-dried specimens reflected the effects of seasonal wetting and drying. Interparticle bonds gave the soil a cohesive component of strength whereas saturation of the soil caused the collapsible grain structure to collapse and cause a significant decrease in strength. Particle breakage of decomposed granite soil increased with increasing confining pressure and also with the presence of water. CT scans showed that larger particles primarily underwent particle degradation and that the process of particle breakage was mainly due to the separation of particle amalgams and breakage along pre-existing fissures.