Scielo RSS <![CDATA[Journal of the South African Institution of Civil Engineering]]> vol. 59 num. 3 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<b>Centrifuge modelling of railway embankments under static and cyclic loading</b>]]> With past embankment failures on the heavy haul coal export railway line between Ermelo and Richards Bay in South Africa, a study was conducted focusing on the modelling of embankments in a geotechnical centrifuge. This was done in order to investigate the influence of static and cyclic loading on settlement, and to evaluate the potential failure of the embankment under the loading conditions. The effect that moisture has on the slope stability was also investigated. A suitable loading system was developed that could be used to apply static and cyclic loading to a model embankment in the centrifuge. Embankment models representing a standard slope and using one material type with varying moisture content were built and tested in the centrifuge. Clayey sand material was sampled from one of the sites where a failure occurred on the coal line and was used for the tests. The loading system simulated the loading created by a heavy haul coal train with a 26 t per axle load along the length of an embankment. The results from the tests conducted in the geotechnical centrifuge indicated that settlement resulting from the cyclic loading was on average 67% higher than that of the static loading when considering loaded time. It was observed that the tests with increased moisture content exhibited significantly reduced stability. A completely developed slip surface failure was not observed. However, cracks formed at the crest of the embankment along its length, which indicated a downward shift of material. The research therefore concluded that cyclic loading on railway embankments increases permanent vertical settlement of the embankment compared to static loading. As the moisture content was increased for the different tests, there was a clear increase in crack development at the crest of the embankment along its length. <![CDATA[<b>Interpreting DPSH penetration values in sand soils</b>]]> Site investigations to classify the underlying soil for geotechnical purposes often rely on in-situ penetrometer tests. Two common tests used in southern Africa are the Standard Penetration Test (SPT) and Dynamic Probe Super Heavy (DPSH) test. Although the specific work per blow is essentially the same in both tests, the resulting penetration values are not equivalent. The DPSH tends to be more variable than the SPT and has higher blow counts. A comparison of SPT and DPSH penetration values at a series of strata below sites has been undertaken. From this, new relative density descriptor boundaries, based on DPSH penetration values, are suggested for sand soils. <![CDATA[<b>Seepage column hydraulic conductivity tests in the geotechnical centrifuge</b>]]> Provided that inter-particle flow remains laminar, hydraulic conductivity tests can be carried out in a centrifuge to accelerate flow, allowing the hydraulic conductivity of relatively impervious materials to be measured within a reasonable time. It is well documented that the inter-particle flow velocity in the centrifuge increases linearly with acceleration, and a debate in the literature deals with whether hydraulic conductivity also scales with acceleration or not. A number of hydraulic conductivity tests were carried out using seepage columns in the geotechnical centrifuge in which pore pressures were recorded within the samples during testing. When hydraulic conductivity is calculated from the hydrostatic potentials measured during testing, the hydraulic conductivity is found to be independent of the imposed acceleration. It is therefore advocated that the hydrostatic potential is scaled in the centrifuge rather than the hydraulic conductivity. It must therefore be recognised that the hydraulic gradient used in the conductivity calculation does not remain constant, but changes with the imposed acceleration. <![CDATA[<b>Exploring the relationship between vertical and lateral forces, speed and superelevation in railway curves</b>]]> The research described in this paper is based on an experiment which involved running a test train through a curve at various speeds, changing the cant of the curve by tamping and repeating the train runs. The cant was changed due to high wheel wear rates. The curve already had a cant deficiency, and this cant deficiency was subsequently increased by reducing the curve's cant. Assessing the before and after tamping test data validated the existence of the expected relationships between the vertical and lateral rail forces, the speed and the cant. The change in cant had a minimal effect on the magnitude of the vertical forces, although a transfer of loading between the high and low legs did occur. The theory indicates that the 14% reduction in cant in this curve, given all of the other curve characteristics, should have resulted in an increase in the lateral forces. There was, however, a roughly 50% reduction in the maximum lateral forces, after the cant had been reduced, which can be explained from a train dynamics point of view. In addition, there was an increase in safety, due to a reduced derailment ratio at this curve's normal operating speed of 85 km/h. It is not unreasonable to presume that a 50% reduction in the maximum lateral forces could lead to a halving of the wear rate of the rail and wheels in this curve, with similar results to be expected in other curves on the rail network. <![CDATA[<b>Alternative wall-to-slab connection systems in reinforced concrete structures</b>]]> In many reinforced concrete structures the walls precede the construction of the connecting floors. A system is, therefore, required to connect the floors to the already cast walls. There are many different floor-to-wall connection systems available in South Africa, but their behaviour and capacity are not always fully understood, especially when the moment capacity of the joint is to be utilised. This study focuses on four systems: continuous starter-bars, pre-bent site-installed starter-bars, pre-assembled starter-bars and cast-in anchors with mechanical couplers. The design procedure for the continuous starter-bar system is well understood and documented in design codes, but not enough information is available on the design procedure for the other systems. Certain practical aspects of the installation process are also not fully understood. Cold-bending and straightening of the starter-bars are inevitable in both bend-out systems. Previous research shows that this cold-working of the reinforcement can reduce the yield stress and E-modulus of the steel. In order to investigate these findings, a series of tensile tests are conducted. The results indicate that a significant reduction can be expected in both the yield stress and modulus of elasticity of the steel. Low-cycle fatigue tests further suggest that cold-bent steel also has a reduced ductility. The tensile tests are followed by the construction and testing of the systems in full-scale wall-to-slab connections. The effect of the cold-bending on the starter-bars is clearly visible, as both the responses of the bend-out systems are less satisfying than the results from the continuous starter-bar system. The experimental phase is followed by numerical analysis of the connection systems. The finite element analyses show that the structural performance is significantly more sensitive to a reduction in the yield stress of the starter-bars, than to the use of a lower concrete grade. It is concluded that all the alternative connection systems can be implemented successfully in a moment-fixed wall-to-slab connection, but that the site-installed bend-out system is the preferred method. However, in order to ensure that the system performs on the same level as conventional systems, it is recommended that the design should be conducted with a set of modified steel properties to allow for the negative effect of the cold-working on the starter-bars. <![CDATA[<b>Sewer network design: Heuristic algorithm for hydraulic optimisation</b>]]> For a given sewer network layout and choice of pipe material, the total installed cost of the network is determined mainly by the pipe diameters and slopes. Hydraulic design optimisation is the task of determining suitable pipe diameters and slopes so as to minimise the installed cost of the network. This is a complex problem for which numerous solution approaches have been proposed. Recently the use of metaheuristic algorithms, like Ant Colony Optimisation (ACO) for example, has gained popularity, and they perform well for a given static layout. However, their computational complexity precludes their use in simultaneous layout and hydraulic optimisation, where a complete hydraulic optimisation has to be performed for each layout. This paper proposes a computationally efficient method for near optimal hydraulic design of a gravity sewer network. It makes use of required minimum slope information to heuristically determine optimal pipe sizes and slopes. The method is used to solve two benchmark problems and is shown to obtain good solutions while being computationally extremely efficient. Therefore it is ideally suited to be used in combination with a metaheuristic algorithm aimed at optimising the network layout. <![CDATA[<b>Soft water attack on concrete tunnel linings in the Ingula pumped storage hydro-power scheme: Assessment of concrete resistance and protection</b>]]> The resistance of the concrete tunnel linings at the Ingula Pumped Storage Scheme (IPSS) to soft water attack was investigated. Concrete disc specimens obtained from cores taken from selected locations of the tunnel lining were exposed to a simulated soft water attack testing regime in a site laboratory using the actual (soft) water to which the tunnel linings will be exposed when in service. The assessment was carried out for around four months. The aggressivity of the soft water was quantified using Basson Indices. The results showed that the water is readily aggressive and will lead to corrosion of the concrete tunnel lining. The experimental work showed the average surface rate loss of the linings to be approximately 1.1 mm/a, which, if sustained in the concrete linings, would amount to an average loss of about 10 mm in a 10-year period. This will result in increased surface roughness and hence higher head losses due to increased friction coefficients. It was recommended that a protective epoxy coating be applied on the tunnel lining to limit the reduction in power generation efficiencies.