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Mount Moorosi Village, located within the Senqu River Valley in Lesotho, has experienced significant damage to its buildings over the last several years, due to severe cracking. The cause of these cracks has been attributed to the local soil, which presents itself as an ideal building and founding material, while it may in fact be to the contrary. A previous investigation by Damane (2019), into the underlying soil profile of Mount Moorosi Village concluded that the village was underlain by a top layer of silty-sandy loess, a windblown soil of at least 3 m in thickness, with the majority of its grains falling within the siltsized particle range. The material demonstrated significant warping potential as well as a high hydrocollapse potential. To attempt to mitigate the hydrocollapse potential of the soil, linear low-density polyethylene (LLDPE) pellets, made from recycled plastic waste, were blended with the soil at concentrations of 3, 6 and 9 % by mass, before being subjected to pycnometer, oedometer and triaxial tests, to determine the specific gravity of the blended soil as well as the settlement and shear strength parameters, respectively. Test results showed that the addition of the LLDPE chips, at 6 % concentration by mass, resulted in the maximum improvement in both the settlement and shear strength parameters. The soil's collapse and hydrocollapse coefficients were reduced by 54 and 40 % respectively, with the overall classification of the soil going from ‘severe' to ‘moderately severe'. The cohesion of the soil was improved by 8 and 400 % for field and saturated moisture contents, respectively. The internal angle of friction of the soil was increased by 97 and 150 % for field and saturated moisture contents, respectively. Collapse and settlement calculations, conducted on Settle3D, showed that the soil's overall settlement was reduced by 34 and 54 % for field moisture and saturated soil conditions, respectively. The total settlement of the strip foundations with their current dimensions, however, was still outside of the acceptable ranges. The total settlement was lowered to within acceptable range of under 50 mm when the width of the strip footings was widened to 750 mm, and their depth increased to 200 mm.

Plastic bottles were first used commercially in 1947 but remained relatively expensive until the early 1960s when high-density polyethylene was introduced, with its attractive characteristics such as being strong, lightweight, durable, cheap, and resistance to breakage. Decomposition of plastic bottles or other plastic products can last from 400 to 1000 years; before this process happens, the plastic waste becomes a problem to the environment continuing to clog our waterways, forest, oceans and others natural habitats. As the capacity of landfills decrease and urbanization leads to rapid growth rates in the human population, either in Africa or any part of the world, this concern brought forward the need for this study. The research aimed to present an end-use solution for plastic bottles by investigating the feasibility of utilizing the plastic bottles as reinforcing elements in problematic soils encountered in the construction industry. In South Africa, plastic bottle waste has continued to increase despite efforts by government in the form of new waste legislation and taxes on plastic bottles. Hence, there is a need to find alternative uses for plastic bottle waste. The use of plastic bottle waste shreds as a soil reinforcement material in geotechnical engineering applications can help mitigate the disposal problems associated with plastics. In this study, a series of direct shear tests were conducted to examine the effect of plastic waste shredded pieces on the engineering properties of Cape Flats and Klipheuwel Sand. The shredded plastic bottles that were used for this study were sourced from Kaytech (supplier and manufacturer of Geosynthetics) in South Africa. The research was done to utilize this plastic through the inclusion of shredded plastic bottles as a form of soil reinforcement. The effects of introducing polyethylene shreds cut from used plastic bottles on the settlement parameters were investigated. It was found that presence of plastic shreds improved the shear strength parameters of the sand soil and they tend to improve further with increasing in plastic shred dosage. The cohesion reached its maximum value for both sands at a shred dosage of 30% by dry mass of the soil.

Soil reinforcement is an ancient technique which involves the addition of tensile elements like plastics in the soil to increase its engineering properties like shear strength, settlement, cohesion and bearing capacity. In consideration of this, a series of triaxial tests were undertaken to investigate the reinforcing effect of High-Density Polyethylene (HDPE) plastic material in Cape Flats sand, predominant in the Western Cape region of South Africa. Plastic strips of various lengths were randomly included to the soil at different concentrations to form a homogenous soil-plastic composite specimen prepared at varying compactive effort. Using a split mould, cylindrical specimens of 50 mm diameter and 100 mm height were prepared using the dry tamping technique. The test specimens were compacted to achieve target average dry densities of the composite sample. The plastic strip reinforcement parameters comprised of 7.5 mm to 30 mm lengths, and concentrations of 0.1 % to 0.3 % by weight of dry sand. Triaxial compression tests were performed using confining pressures of 50 kPa, 100 kPa, 200 kPa, 300 kPa and 400 kPa at a shear rate of 0.075 %/min, and to a maximum strain of 10 %. Laboratory results favourably suggest that there is an improvement in the soil shear strength properties due to these inclusions. The friction angle increased up to a peak value on varying plastic strip length and concentration, beyond which further addition of plastic material led to a reduction in the friction angle. The greatest friction angle was reported at plastic strip length and content of 15 mm and 0.2 % respectively. Additionally, the results indicate that a higher compactive effort leads to a greater increase in friction angle of the soil. The existence of a critical confining stress was observed from triaxial test results on soil-plastic composites. This threshold limit was influenced significantly by the plastic inclusions, and the range of confining stresses. Consequently, a bilinear failure envelope was reported in reinforced samples while unreinforced specimens realised a linear relationship. The Mohr-Coulomb failure line above the critical confining pressure almost paralleled the unreinforced linear relationship. An embankment model was developed using Slide Modeler software and the factor of safety of slope was analysed with unreinforced and reinforced backfill subjected to static and dynamic loading. It was observed that the safety factor increased due to polyethylene strip inclusions. Therefore, the proposed technique will find potential practical applicability in low-cost embankment or road construction.

Shear strength of soil is characterized by cohesion, angle of internal friction and dilatation. The first two parameters mentioned primarily define the soil's ability to resist shear stress underspecified load. These parameters can be determined by tests conducted either in the laboratory or the field for use in design of geotechnical structures. Some of the tests in the laboratory to determine the shear strength of soil, include triaxial, ring shear, torsional shear, direct shear and direct simple shear. Direct shear test is the most widely used geotechnical shear device due to its simplicity, however, the test suffers from stress inhomogeneity. Direct simple shear apparatus was developed because of the shortcomings in the direct shear test. In these two tests, different shearing conditions are applied to soil samples. For the direct shear test, shearing occurs at a predetermined center of the specimen which may not be the weakest plane of the soil while indirect simple shear, the entire specimen distorts without the formation of single shearing surface. The mode of shearing established in the direct simple shear device is similar to that which occurs around the shaft of a pile. In contrast to the extensive geotechnical application of direct shear test, limited information exists on direct simple shear test. This thesis endeavours to establish the relationship between the two tests by undertaking extensive testing to obtain a better understanding of direct simple shear test as used for testing local soils and to determine a correlation with the results from direct shear tests. A series of shear tests were undertaken on Klipheuwel sand, Kaolin clay and composite of the sand and clay using universal shear device. The soils were mixed with water in percentages of 5,10, 15, 20 and 25%, and the composite with clay percentages of 10, 25, 50 and 75%, to investigate the impact of water and clay on the shear parameters determined from the two tests. The results showed that direct shear test gives higher shear strength when compared to direct simple shear test under the same soil condition. The addition of water, and clay, generally reduced the internal friction angle of sand for both tests. Furthermore, increase in cohesion was observed with the addition of water to Kaolin clay for the direct simple shear test and the reverse was true in the direct shear test. The correlation factors developed in this study for the direct simple shear test could be used to refine the results from the direct shear test.

Densification of soil during construction of earth structures is achieved through the process of compaction by application of mechanical energy to obtain the required engineering properties of the soil for a particular project such as hydraulic conductivity, soil strength and compressibility. These properties are dependent on attainment of high compaction densities normally achieved at specific moisture contents for a given compactive effort. The optimum moisture content and maximum dry density for a particular soil is determined by means of Proctor tests in the laboratory. A relative compaction index is then used to correlate the laboratory values with the field compaction values obtained using in-situ tests. The Sand Cone (SC) and Nuclear Density Gauge (NDG) are the common field tests used to the dry density and moisture content of the soil for purposes of quality control of the compaction process. The sand cone is a laborious test that involves excavation of part of the compacted layer and requires a 24-hour waiting period to obtain the moisture content of the soil through the laboratory oven method. The NDG on the other hand is less laborious, however it uses a radioactive source that is a potential health hazard and therefore requires strict handling, storage and maintenance of the equipment to maintain safety standards. The Electrical Density Gauge (EDG) is an alternative in-situ test that is quicker, safer and easier to maintain since it uses electric current to measure the compaction characteristics of the soil. The objective of the study was to determine the repeatability, accuracy and applicability of the EDG on South African soils by comparing its measurements for dry density and moisture content in the laboratory and in the field to the results from the sand cone and oven method. In the laboratory, a clean sand and a clayey sand were tested at the optimum moisture content and at ± 3% of the optimum moisture content. The soils were compacted to 200 mm using the RT74 rammer and the compaction values first tested using the EDG then followed by the sand cone test at the centre of the EDG test spot. The moisture content of the excavated sample from the sand cone test was determined using the oven method. For the field tests, the compaction characteristics of a sandy gravel and three uniformly graded sands were tested in-situ using the EDG followed by the sand cone test. Overall, the EDG measurements were repeatable based on test-retest comparison of the paired measurements. EDG results for moisture content were consistent with the values obtained from the laboratory oven method especially in the uniformly graded sands. However, the density measurements differed from the results of the sand cone test, which was considered the reference test for determination of field soil density. It is recommended that the EDG calibration relationship for bulk density be revised in order to improve the accuracy of the density measurements.

Includes bibliographical references., End-of-life tyres are a disposal problem resulting from the large volumes produced worldwide every year. Waste tyres are difficult to manage because of their sheer volume and the potential impacts on human health and environment. These discarded tyres, currently stockpiled and dumped in the open, are a source of fire hazards and provide a prolific breeding ground for mosquitoes and other pests. The use of waste tyre shreds as fill material in geotechnical applications can help to mitigate the waste tyre disposal problems. Specifically, when tyre shreds are used as lightweight fill material in the construction of highway embankments, a considerable volume of waste tyres is consumed. With regard to this, an investigation into tyre shreds mixed with sandy soils of South Africa was undertaken to assess the shear strength behaviour of the sand-tyre shred composite and to propose an alternative use of the scrap tyres produced every year.