Scielo RSS <![CDATA[Journal of the South African Institution of Civil Engineering]]> vol. 58 num. 1 lang. es <![CDATA[SciELO Logo]]> <![CDATA[<b>Are estimates of catchment response time inconsistent as used in current flood hydrology practice in South Africa?</b>]]> Catchment response time parameters are one of the primary inputs required when design floods, especially in ungauged catchments, need to be estimated. The time of concentration (T C) is the most frequently used time parameter in flood hydrology practice, and continues to find application in both event-based methods and continuous hydrological models. Despite the widespread use of the T C, a unique working definition and equation(s) are currently lacking in South Africa. This paper presents the results of the direct and indirect T Cestimation for three sets of catchments, which highlight their inherent variability and inconsistencies. These case studies demonstrate that estimates of T C, using different equations, may differ from one another by up to 800%. As a consequence of this high variability and uncertainty, we recommend that, for design hydrology and calibration purposes, observed T Cvalues should be estimated using both the average catchment T Cvalue, which is based on the event means, and a linear catchment response function. This approach is not only practical, but also proved to be objective and consistent in the study areas investigated in this paper. <![CDATA[<b>A comparison of technical and practical aspects of Eurocode 3-1-1 and SANS 10162-1 hot-rolled steelwork design codes</b>]]> In South Africa engineers are starting to use the Eurocode guidelines for steelwork design, and it is important to understand the implications and differences in results that are obtained when applying the different codes. This paper presents a comparison between the Eurocode 1993-1-1:2005 and SANS 10162-1:2005 hot-rolled steelwork design codes. Numerical comparisons of predicted member design strengths for the important modes of failure and the complexity of calculations are presented, along with considerations regarding the parameters used in design. The following are explicitly shown for both codes: (a) differences in the classification of commonly used H, I, PFC and equal L sections, (b) differences in tension resistance calculations, (c) comparisons of all axial buckling curves, (d) calculations for a selection of members in flexural buckling which have different classifications, and (e) a summary of the shear resistances of commonly used H and I sections. It is shown that, on average, Eurocode 3 predicts higher member design strengths than the SANS 10162 code for most failure modes, primarily because of material partial safety factors closer to unity, less conservative buckling curves and the consideration of plastic resistance of sections. These EC3 design capacities can be higher by up to 11% for tension, 35% in compression, 31% in bending and 51% in shear, although there are cases where strengths of up to 33% lower were calculated, such as for an IPE AA-200 in shear. Results are influenced by design geometric tolerances, which are based on section classifications. The Eurocode's equations and design methodologies are more complex and computationally demanding. Since South Africa has started moving in the direction of adapting or adopting Eurocodes with the SANS 10160 Loading Code (from EN 1) and SANS 10100 Structural Concrete Code (from EN 2), it should be considered whether or not the steelwork code should be adopted or adapted in a similar fashion in the future. <![CDATA[<b>The identification and treatment of poor durability Karoo dolerite base course aggregate - evidence from case studies</b>]]> The Karoo Supergroup covers approximately 57% of South Africa's surface area and the sedimentary rocks therein generally do not yield acceptable pavement aggregates. The Karoo Dolerite Suite (intrusions) present in these sedimentary units have successfully been used as pavement aggregate sources, but numerous cases of premature pavement failure due to alleged rapid degradation of the dolerite have been reported. Durability tests are included in basic or mafic igneous rock aggregate specifications, but rapid pavement failures continue to occur. A study was recently undertaken to identify cases where degradation of Karoo dolerite was the cause of pavement failure. A secondary objective of the study was to determine if any observed degradation could have been identified using currently specified or alternative testing methods. Three such case study sites are presented in this article and the properties of their materials compared to those from five non-problematic dolerite materials. It is shown that the poor performance of the case study materials was likely due to the poor durability of the materials, manifesting as a reduction in resistance to abrasion and attrition. The identification of the observed poor durability could not have been performed accurately using only the currently specified test specifications. Alternative tests that allow an accurate differentiation to be made were, however, identified and, based on the results, tentative limits set. Additionally it was shown that modification of problematic Karoo dolerite base course materials, by applying lime at a rate less than the initial consumption of lime, can be successful in preventing further rapid pavement failures. <![CDATA[<b>Disintegration of concrete construction induced by acid mine drainage attack</b>]]> This paper presents findings from microanalytical investigation conducted on disintegrated concrete that had been used to construct a weir within a coal mine in South Africa. The concrete was in contact with polluted mine water, commonly referred to as acid mine drainage (AMD). Accordingly, the weir had been exposed to AMD decant which led to disintegration of concrete due to chemical attack. Investigations were conducted by optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry (EDX), and X-ray diffraction (XRD). The field samples examined consisted of soft, broken concrete chunks and a whitish powdery substance that had crystallised and formed a surface coating on certain cracked locations on the deteriorated concrete. No evidence of pyrite oxidation was found in the investigation. The observed deterioration is discussed in relation to acid attack mechanism and its possible co-existence with external sulphate attack process. <![CDATA[<b>Hydraulic model study of the blowback behaviour of the bottom outlet of the Berg River Dam, South Africa</b>]]> The Berg River Dam is equipped with the first multi-level draw-off environmental flood release outlet in South Africa and can release flows up to about 200 m³/s. The outlet is controlled by a radial gate at the outlet end, and is protected by a vertical emergency gate near the inlet end. Commissioning tests of the emergency gate in 2008 found that large volumes of air were expelled, instead of the expected air entrainment into the air vent, designed to reduce expected negative pressures in the conduit during emergency gate closure. This paper describes the testing of a 1:14 physical model representing the outlet works of the Berg River Dam to determine the reasons for the unexpected release of air from the outlet work's air vent, as observed in the field during the commissioning tests of the emergency gate in the outlet conduit. Simulations of continuous gate closure on the as-built physical model of the Berg River Dam outlet showed predominant inflow of air into the air vent during emergency gate closure, with intermittent short duration high-speed air releases during the stages of emergency gate openings between 37% and 25% open. The problem was determined to be one of intermittent air blowback from the outlet conduit via the air vent during the latter stage, rather than continuous air release for all stages of the gate opening operation. The cause of the blowback was found to be the constriction of flow due to a reduction in the conduit cross-section at the radial gate chamber located at the downstream end of the outlet conduit. <![CDATA[<b>Electricity generation as a beneficial post-closure land use option for a dormant tailings storage facility</b>]]> Dormant tailings storage facilities (TSFs) have negative effects on their surrounding environments and communities. This study is aimed at determining the financial and practical feasibility of generating energy from the ERGO TSF site, near Brakpan, Johannesburg, as a beneficial post-closure land use option. The beneficial post-closure options investigated were rainwater harvesting and electricity generation from wind power, photo voltaic solar power and a pump storage system constructed on the TSF. Wind power generation and rainwater harvesting from the site were found to be unviable. It was found that a 470 MW (peak capacity) solar photo voltaic plant on top of the ERGO TSF will provide the best solution, both from a practical and financial point of view, yielding a potential internal rate of return (IRR) of 10.7% over 50 years. A pump storage system yields a maximum IRR of 10.3%, but with a substantially smaller generation capacity of approximately 80 MW. <![CDATA[<b>The effects of lubricant and tendon mass variances on the coefficient of friction in unbonded post-tensioning tendons</b>]]> In unbonded post-tensioning tendons, the coefficient of friction varies from one design standard to another. This variation is caused by the large number of complex factors that must be considered in design. These factors include the thickness of lubricant or grease present in the system, clearance between the strand and the plastic sheath, and surface characteristics of the strand and plastic sheath. In order to determine the effect of these factors a series of friction tests were performed on two different diameters of strand, namely 12.7 mm and 15.24 mm diameters. Through a regression analysis, it was found that the frictional force decreases with an increase in the thickness of the grease, and that friction increases with an increase in the mass of the strand. The degree of friction was found to be dependent on the surface characteristics of the strand and plastic sheath, clearance between the plastic sheath and the strand, and the extrusion process of the plastic sheath.