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Volume 58, Issue 2, 2016
Development of a practical methodology for the analysis of gravity dams using the non-linear finite element methodSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 58, pp 2 –13 (2016) http://dx.doi.org/http://dx.doi.org/10.17159/2309-8775/2016/v58n2a1More Less
For many decades the 'classical' method has been used to design gravity dams. This method is based on the Bernoulli shallow beam theory. The finite element method (FEM) has become a powerful tool for the dam design engineer. The FEM can deal with material properties, temperatures and dynamic load conditions, which the classical method cannot analyse. The FEM facilitates the design and optimisation of new dams and the back analysis of existing dams. However, the linear elastic FEM has a limitation in that computed stresses are sensitive to mesh density at 'singularity points'. Various methods have been proposed to deal with this problem. In this paper the Drucker-Prager non-linear finite element method (DP NL FEM) yield model is presented as a method to overcome the problem of the stress peaks at singularity points, and to produce more realistic stresses at the base of the dam wall. The fundamentals of the DP NL FEM are presented. Benchmark studies of this method demonstrate the method's viability to deal with zones in a structure with stresses beyond the elastic limit where yielding of the material occurs. A case study of a completed gravity dam is analysed, comparing several analysis techniques. The service and extreme load cases are investigated. Different material properties for the concrete and rock, including weathered material along the base of the wall, are considered. The application and merits of the DP NL FEM are presented. The calculation of the critical factor of safety against sliding is done with a more realistic determination of the conditions along the base of the wall.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 58, pp 14 –24 (2016) http://dx.doi.org/http://dx.doi.org/10.17159/2309-8775/2016/v58n2a2More Less
The estimation of clay fraction is important for predicting the engineering properties of a soil. SANS 3001 GR3 (SANS 2011) specifies a procedure for clay fraction determination using a hydrometer. It has long been suspected that there may be flaws in this approach. Some of the possible sources of error have been suggested, but little or no change has been made in the standard procedures for assessment of clay fraction in well over half a century. This paper deals with a microscopic examination of some typical South African clayey soils to assess the adequacy of dispersion and possible consequences for clay fraction determination in currently specified hydrometer procedures. Clays are examined both with and without dispersant, and with and without labelling of clay minerals using an exchangeable cation dye.
A qualitative model study on the effect of geosynthetic foundation reinforcement in sand overlying very soft claySource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 58, pp 25 –34 (2016) http://dx.doi.org/http://dx.doi.org/10.17159/2309-8775/2016/v58n2a3More Less
A centrifuge model study was carried out investigating the effects of the inclusion of geogrid reinforcement in a sand layer constructed over a thin, very soft clay layer to facilitate the construction of warehouse floors supporting product stockpiles. The use of geogrid reinforcement allowed larger surcharge loads to be applied for a given amount of settlement by allowing load to be spread over the footprint area of the geogrid due to the tension membrane effect and its associated benefits. A lower-bound calculation of the load at which the tensile reinforcement starts to function is presented. Upper-bound solutions were found to overestimate allowable surcharge loads, as they are associated with unacceptable deformations. The reinforcing effect of the geogrid was found to have been fully mobilised at 5% vertical compression of the clay layer.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 58, pp 35 –41 (2016) http://dx.doi.org/http://dx.doi.org/10.17159/2309-8775/2016/v58n2a4More Less
Under-sleeper pads (USPs), typically made from polyurethane, are used by railways in certain parts of the world to reduce ballast settlement and consequently lengthen the ballast tamping cycle. The rationale behind this relatively new addition to the conventional ballasted track structure is that the pad increases the contact area between the angular ballast particles and the underside of the concrete sleeper, with the effect that ballast breakdown and total track settlement are reduced. This paper describes two experiments on the effects of USPs on four aspects of sleeper-ballast interaction, namely contact area, contact pressure, ballast settlement and ballast breakdown. Static and dynamic tests up to 1 million loading cycles were performed under controlled laboratory conditions on concrete sleepers with and without USPs. Sophisticated pressure sensors revealed an increase in contact area from 12% to 35% for static loading tests, and from 8% to 20% for dynamic tests, with a resulting 70% reduction in contact pressure. In addition, a 44% reduction in ballast settlement and a 23% reduction in ballast breakdown were achieved by the introduction of USPs. In conclusion it is argued that the introduction of USPs specifically on heavy-haul lines would offer significant advantages with respect to ballast settlement and breakdown. These advantages are most likely to lengthen general ballast tamping and screening cycles, resulting in significant life cycle cost savings.
Stormwater pond efficiency determinations with the effluent probability method : the use of mass versus concentration parametersSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 58, pp 42 –48 (2016) http://dx.doi.org/http://dx.doi.org/10.17159/2309-8775/2016/v58n2a5More Less
Use of the concentration parameter has in the past generally been favoured in stormwater structure efficiency determinations, and specifically with use of the Effluent Probability Method. However, efficiency is by definition related to the amount of substance removed within a stormwater structure, and concentration, being a mathematical construct, cannot be "removed". The purpose of this study was to investigate whether there is substantive proof for the use of substance concentration as a proxy for mass in the Effluent Probability Method to ascertain the amount of substance removed by a structure, i.e. its efficiency. Theoretical considerations and results of data analyses did not support this, and it is therefore recommended that mass, and not concentration, be used in efficiency determinations with the Effluent Probability Method.
Stormwater pond metals and solids removal efficiency determination with the effluent probability method : a novel classification systemSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 58, pp 49 –55 (2016) http://dx.doi.org/http://dx.doi.org/10.17159/2309-8775/2016/v58n2a6More Less
This technical note contains findings from a larger research project into the design of stormwater detention and retention ponds for removal of metals and solids. The project required a standard system with which case studies obtained from the International Stormwater BMP Database could be compared in terms of efficiency. A novel classification system for use with the Effluent Probability Method, designed for this purpose, was developed and is presented here.