Browsing by Author "Schoeman, Jurgens Johannes"
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- ItemThe Liquefaction potential of soils on the Cape Flats established by means of empirical correlation(Stellenbosch : Stellenbosch University, 2018-12) Schoeman, Jurgens Johannes; Fouche, N.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Soil Liquefaction is a problem associated mostly with earthquakes and occurs in areas with relatively shallow water tables. During an earthquake, the soil loses strength and stiffness due to the shaking. This results in settlement of silty sands, forcing groundwater out of its pores and up to the surface. The soil is put into a temporary state where the structure of the soil is distressed, resulting in soil particles losing contact and ultimately behaving like a liquid. The Cape Flats, situated in the Western Cape of South Africa, is an area known for flooding in low lying areas during the winter months, especially in informal settlements. The majority of the area is characterised by flat plains, and heavy winter rain cannot drain away due to the soil being saturated and draining canals usually blocked or absent. The soil in this area is also predominantly fine and medium grained silty sand, mostly cohesionless and uncemented, and potentially prone to liquefaction if ground shaking occurs. The Milnerton Fault Line runs through the Cape Flats and was the cause of the large destructive earthquake that struck Cape Town in 1809 and the years thereafter. Volcanoes of bursting sand and mud were reportedly seen all over the low-lying areas. Therefore, it is important to know whether liquefaction can occur in the Cape Flats during seismic events. Empirical correlation methods from proven studies were used to determine if the soils from the Cape Flats are susceptible to liquefaction during shear, induced by earthquakes. The testing methods used were the Piezocone Penetrometer and the Standard Penetration Test. Critical State concepts were also applied. Determining areas where the soils might be potentially dilative or contractive were established from literature. Soils that are potentially contractive are generally loose and potentially liquefiable. Soils that are dilative in nature are generally dense and should resist flow liquefaction. The use of the Dynamic Probe Super Heavy (DPSH) test was also included in the study due to limited access of SPT testing done on sands within the study area. The DPSH test also provides an equivalent SPT-N value and was essentially used in the same calculations. Results from the SPT and CPT tests indicated that the majority of soils in the study area are susceptible to liquefaction within the first 1.0 – 2.0 metres below ground. Soil samples obtained from within the study area as well as pre-graded soil samples were tested on a custom made vibrating table. Samples were graded to form uniformly graded, gap graded and well graded samples. The soils were saturated to 100% and vibrated at accelerations of 0.15g and 0.25g. These are Peak Ground Accelerations that can be expected for an earthquake in South Africa. The fact that denser soils have a larger or longer resistance to liquefaction was also proven in the laboratory on the vibrating table. Samples that were in a loose state liquefied much quicker compared to samples that were compacted and denser. Well graded and gap graded samples also resisted liquefaction for longer compared to that of uniformly graded samples.