1,720,991 research outputs found
Comparison of shear strength properties of textured polyethylene geomembrane interfaces in landfill liner systems
Full text linkThe interface shear strength between geomembranes and geosynthetics is a critical factor governing the stability of slopes that incorporate geosynthetics. In order to better characterise the shear properties of geomembranes, a wide-range of shear strength friction measurements were conducted. This dissertation presents the results of a study that examined interface shear strength parameters of textured high density polyethylene geomembranes (HDPE) and textured low linear density polyethylene (LLDPE) geomembranes sheared against different geosynthetics; geotextiles, geogrid and geosynthetic clay liners (GCLs), typically used in South African landfill base liners and capping systems. Tests were performed using a modified 305 x 305 mm x 100 mm large direct shear box over a range of normal pressures of 25, 50, 100, 150, 200 and 300 kPa. Shear rates of 0.1 mm/min and 1 mm/min were used for geomembrane/ GCL and geomembrane/ geosynthetic interfaces respectively. Results indicated that LLDPE and HDPE geomembranes sheared against various geosynthetic combinations produced different friction characteristics which resulted in varying performance patterns. HDPE geomembrane surfaces mainly experienced conventional linear failure envelopes when sheared with different geosynthetics. However, LLDPE geomembrane interfaces showed that the linear failure envelopes did not always give the best representation of the shear stress and normal stress relationship for sheared interfaces. These geomembrane shear strength envelopes could be described as linear until a critical confining stress in the range of 100 kPa to 150 kPa was attained, therefore making the failure envelopes bilinear. A comparison of the linear and bilinear failure envelopes showed that a bilinear failure envelope was a more appropriate approximation over large normal stress ranges. A bilinear relationship resulted in higher interface friction angles and low apparent adhesion parameters being achieved at normal stresses less than 100 or 150 kPa. While low interface friction angles and large adhesion values were produced above the critical confining pressure. Although HDPE geomembrane interfaces indicated larger stiffness and rigidity at early shear, it was observed that LLDPE geomembrane/ geosynthetic interfaces presented larger factor of safety values when compared to HDPE geomembranes sheared against majority of the geosynthetics. These results were produced when friction parameters generated from this study were applied to practical design examples of landfill base liner and capping systems. From these observations several practical recommendations were generated to assist professionals to choose suitable materials during design
An introduction to geotechnical design of South African wind turbine gravity foundations
No full textWith the increasing pressure on global governments to pursue more green and renewable energy production measures, wind based solutions have progressed into one of the most dominant development areas in the global renewable energy sector. In South Africa, with notable deficiencies in reliable energy supply, a number of wind projects have been planned in order to relieve the pressure on the nation's volatile reserves. With a lack of exposure to the complexities of wind turbine foundation design in Africa, this research aimed to present a methodology for the geotechnical design of gravity footings for these structures, specific to SA soil conditions and policies. To understand the implications of the main aim of the study, the current scope for renewable energy project uptake in South Africa was summarized, highlighting the scope and growth potential legislated by the Renewable Energy Independent Power Producer Procurement Programme. This summary indicates the current development corridors for wind projects that fall along the Eastern and South West coasts of the country and discusses the economics of wind farm ventures and their inherent ability to attract local and international investment. Additionally to this, topics including a basic introduction to turbine mechanics, tower and foundation types, and the effect of loading actions on the dynamic soil reactions, were presented. This was concluded by discussing gravity footings in context to other foundation types, and their advantages for use in these types of developments. With this understanding, the main research outcome was addressed by selecting three representative sites from each of the major wind development corridors, and using them as practical examples. These were resultantly named the Western Cape, Eastern Cape and Karoo sites. Soil profiles and properties were assumed based on site investigation data from real projects from each of these corridors and this data was compiled, discussed and used in the planning of three designs for each respective site. In this way, a geotechnical methodology was created addressing the critical criteria that require consideration for the construction of turbine base structures. These considerations included appropriate site investigation methods particularly suited for wind turbine foundations, such as Continuous Surface Wave testing, as well as bearing capacity calculations according to theories suggested by the DNV/Risφ (2002) guidelines and site-specific bearing capacity theories. Settlement concerns were addressed through the analysis of immediate elastic settlement beneath a foundation using a general elastic solution, a non-linear stepwise method as well as the computer software, Settle 3D. Unique to wind structures, the criterion of soil stiffness was considered in order address the structure's global resistance to rotation, caused by the high overturning moments inherent in these systems. The effect that the calculated finite soil stiffness has on the assumptions in computing the natural frequency of the system was also investigated. Finally, additional concerns such as the effect of gapping, issues with designing on pedogenic soils such as calcrete, as well as the use of the finite element method in the planning of turbine foundations, were discussed. In concluding the study, a general design process for engineers tasked with planning gravity footings for wind turbines subject to local soil conditions was presented
Preparations for field testing for the performance validation of piled wind turbine foundations in expansive clays
Full text linkField-testing of piles in an expansive clay is required for the validation of the design criteria for wind turbine foundations in such soil profiles. The test programme includes lateral load tests to investigate the horizontal stiffness of the soil and vertical plug pull-out tests to examine the axial capacity of pile-soil interface. These tests will be conducted in both dry and wet conditions to study the range of expected responses of the pile and evaluate how the changing soil moisture condition affects the pile behaviour. In addition, an instrumented pile socketed into bedrock will be used to determine the strain induced in the pile due to soil heave as the soil saturation levels increase. For the execution of these tests, a large open area with expansive clay of a sufficient depth is required; a stable stratum below the clay is desirable for the socketing of the long piles. An important component of the investigation is the characterisation of the expansive clay profile. A preliminary site investigation has been carried out on the field-testing site which will be supported by further laboratory testing. Details of the soil profile and site characterisation which will be used in the future analyses of the field-testing results are presented
Shear strength behaviour of sugarcane bagasse reinforced soils
No full textSugarcane is considered as the most abundant plant based crop grown in the tropics and part of the temperate climates. Its by-product, sugarcane bagasse, constitutes 30% of the total production. In the past, it was considered as waste material but contemporaries through innovative research projects over the years have found uses for it. Among these projects is soil reinforcement, which provides an alternative application to industrial by-products and natural fibres as a way of reducing their environmental footprints and contributing to sustainable geotechnics. Although bagasse morphological composition contains structural elements ideal for reinforcement and composite materials, it has received little research as a standalone reinforcement material. Because of this, a direct shear test was therefore initiated to establish the usefulness of using sugarcane bagasse as a soil reinforcement material by comparing the extent of shear strength and stiffness response due to its inclusion to unreinforced soil. Three different types of bagasse, fibre, millrun and pith, were added to unreinforced soil in percentage by weight content of 0.3 - 1.7. The bagasse was added to Klipheuwel sand, Cape Flats sand and Kaolin Clay at both dry and moist conditions. In addition, durability studies involving 12 cycles of wetting and drying, and soaking for a period of 14 days were constituted
Analytical and numerical study of dolomite sinkholes in Centurion South Africa
No full textIncludes bibliographical references.Sinkholes encompass the withdrawal of shallow sediment into deep hollow compartments located in karstic stratum. These subsequent surface openings associated with karst geology have inhibited multiple infrastructure developments. Sinkholes are triggered by the alteration of the existing groundwater level which erodes weathered altered dolomite (WAD) residuum into karst cavities. Substantial literature has explored the stability of sinkholes, with reliance on limit analysis and empirical data, to quantify the strength of the porous karst residuum. In this study, the appraisal of sinkhole propagation was facilitated with the geological data acquired along the Gautrain route through Centurion, South Africa. Sinkhole development was analysed through analytical theories and the application of numerical methods. The analytical study conceptualized the ‘angle of draw’ of dolomite overburden layers into cavities, with Terzaghi’s arching in soil equation. The analytical results illustrated constant vertical drawdown in the WAD and incremental cavity propagation in the frictional chert residuum
Plane strain viscoplastic modelling in vacuum consolidation
Full text linkThe mechanism involved in ground improvements with vacuum assisted prefabricated vertical
drains (PVDs) for road embankments is essentially 3-dimensional (3-D) but could reasonably well approximated
as axisymmetric. In this context, axisymmetric unit cell modelling generally provides a very good representation
for Finite Element (FE) modelling. However, it carries significant limitations in terms of the scope
of the analysis. Hence, to get an understanding of the overall deformational behaviour of the foundation soil
being improved, a full scale Plane Strain (PS) FE model would be necessary. In this paper, conversion of axisymmetric
unit cell to an equivalent PS model is carried in the context of a vacuum consolidation project.
Foundation soft soil is modelled using an elastic-viscoplastic (EVP) model which accounts for the time dependent
behaviour of soft clay. Results of this PS conversion is compared with the axisymmetric FE solution. Stability
of the embankment is also analysed using maximum lateral displacements
Investigating the Dynamic Soil Behaviour of Cape Flat Sands Under Earthquake Cyclic Loading
Full text linkMost geotechnical structures are surrounded by partially or unsaturated soils, which can greatly affect their dynamic properties, such as shear modulus and damping ratio. Therefore, this necessitates an inclusive methodology to consider and incorporate the degree of saturation in seismic analyses to ensure that seismic designs are not overly conservative based on local soil conditions. In Cape Town, South Africa, limited soil characterization on cyclic testing has been executed on Cape Flat Sands, although the area is in a sensitive seismic region of magnitude ML 6.1 - 7.5. As a result, geotechnical engineers may be using dynamic correlations that are too conservative due to limited assumptions about soil type, earthquake response, relative density, plasticity, confining pressure, and saturation level. The cyclic properties of the Cape Flat Sands were evaluated at ten different cyclic axial strains (0.005%, 0.01%, 0.02%, 0.05%, 0.1%, 0.2%, 0.4%, 0.6%, 0.8% and 1.0%), three relative densities (loose dense, medium dense, and dense), and five saturation levels (0%, 25%, 50%, 75% and 100%). The tests were conducted using cyclic triaxial equipment, and soil classification tests were performed according to the latest standards of the American Society for Testing Materials (ASTM). The Unified Soil Classification System classified the Cape Flat Sands as poorly graded sand. The results of the undrained cyclic triaxial tests, which measured shear modulus, damping ratio, and excess pore pressure ratio, are also included. Based on the results, correlations were developed to determine the relationship between the damping ratio and normalized shear modulus. Additionally, the applicability of existing dynamic models was evaluated by superimposing them on the test data for shear modulus, damping ratio, and excess pore pressure ratio. To identify the factors and their interactions that affected the values of the shear modulus, damping ratio, and excess pore pressure ratio, MANOVA was conducted. Repeatability and quality control of results was carried out to ensure that the results were precise and reproducible using the same test standard and operator. A ground response and free vibration analysis of a five-storied building were executed using ProShake software and Plaxis 2D software. This was achieved using the normalized shear modulus and damping ratio with shear strain curves against a soil profile obtained along the R300 road within the Cape Flats region. A water table underlay this soil profile at 2.45m depth, enabling varying saturation states to be adopted. The Nahanni (1985)-RSN497 ground motion from the PEER NGA ground database was adapted for dynamic analysis. The results of the ground response analysis, free vibration and earthquake analysis showed that the different saturation states, relative density, and equivalent dynamic properties influenced the behavior of the ground motion parameters for each respective sandy layer. This study has provided invaluable insights into the dynamic properties of Cape Flat Sands under earthquake cyclic loading and it has also highlighted specific areas that require further research to enhance its understanding
Investigation into the use of waste tyre shreds in reinforcement of sandy soils in South Africa
No full textIncludes 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
A numerical study of the suitability of rigid inclusion ground reinforcement beneath caisson quay walls
Full text linkThe objective of this study was to determine whether rigid inclusions are suitable for reinforcement of the foundation of a caisson quay wall functioning as a container terminal. Apart from their brittle behaviour under lateral loading, rigid inclusions are well suited to the large uniform loads and stringent post-construction deflection tolerances associated with container terminal structures. Their inherent strength and stiffness means they have certain advantages over other stiffening columns commonly used for ground reinforcement in port expansion projects. Their mechanical properties allow construction to unrestricted heights at any construction rate and, in theory, RIs can be applied to all soil types. Additionally the locations of many ports coincide with rivers, deltas and estuaries which are associated with poor soil conditions often requiring ground improvement. Their suitability is of practical significance to port planners and engineers who are faced with the challenge of providing satisfactory foundation performance that is cost effective. The addition of RI ground reinforcement for this structural application would allow for greater flexibility in meeting these challenges. The literature review for this study was broad in its scope with emphasis placed on describing the mechanics of the problem, analysis methods and suitable installation methods for execution in the marine environment. One of the key outcomes of the literature review was identifying the problem of lateral loading due to "free-field" lateral ground movements. In light of this, suitable strategies for limiting and accommodating lateral loading of the RIs were proposed. A numerical study of the proposed ground improvement scheme was undertaken using the 3D finite element method. The key model outputs were caisson deflections and RI forces, moments and stresses, for the various simulated construction phases up to operational conditions. The model results were assessed in terms of the key foundation performance criteria which were related to STS crane rail tolerances and limiting tensile stresses in the RIs. This study found that for a firm clay subsoil condition the proposed RI ground reinforcement scheme met the foundation performance criteria for this structural application provided (i) strategies to limit lateral loading were implemented and (ii) the RIs were reinforced over the length where they were not fully compressed. While this study provided insights into the behaviour of RIs for this structural application, ultimately suitability is a function of range of factors, in addition to the limited technical performance criteria derived for this study
- …
