Scielo RSS <![CDATA[Journal of the South African Institution of Civil Engineering]]> vol. 59 num. 2 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Characterisation of rigid polyurethane foam-reinforced ballast through cyclic loading box tests</b>]]> As train speeds and heavy haul axle loads constantly increase due to market demands, so do the stresses and strains experienced by track structures. This is especially true for track transitions that generate high dynamic forces on both the track and vehicles because of poor vertical track geometry and/or differing track stiffness values on either side of the track transition. Reducing differential settlement between the two track structures at a track transition is one method of improving the life of the track, and increasing maintenance intervals. In this study, rigid polyurethane foam was used to reinforce ballast. Tests were conducted using a dynamic hydraulic load frame and a single sleeper in a large ballast box subjected to heavy haul axle loads. Unreinforced, reinforced and 50% reinforced ballast layers of 300 mm depth were tested to approximately 5 000 000 load cycles. The results showed that rigid polyurethane foam-reinforced ballast exhibited in the order of 60% less settlement for a fully reinforced layer, and 42% less settlement for a half reinforced layer. The use of rigid polyurethane foam (RPF) to reinforce ballast has a number of benefits that could result in better track geometry and longer maintenance cycles, in turn resulting in lower life cycle costs. <![CDATA[<b>Fatigue behaviour in full-scale laboratory tests of a composite deck slab with PBL reinforcement</b>]]> Studies on the fatigue behaviour of composite deck slabs are relatively few. To assess the fatigue performance of a composite deck slab at specific design loads, and to provide a reference for its design in fatigue, two full-scale models A and B of a composite deck slab were developed, comprising steel plates and steel-fibre-reinforced concrete slabs. These models will be a useful reference for experiments and design, and for developing codes. In this study we carried out fatigue experiments and focused on the fatigue performance of a composite deck slab in a column area, and the positive and negative bending moments. For the entire fatigue loading cycle, the overall performance of Models A and B was good, the overall stiffness of the composite deck slab was rarely attenuated, the stress levels in the steel members in relation to the fracture strength were not significant, and the steel member was in the stage of flexible work. Through comprehensive tests of Models A and B it was found that the original design exhibits good fatigue performance and meets the design requirements. The research results provide a basis for the design of a composite bridge deck slab in fatigue. <![CDATA[<b>The influence of foundation stiffness on the behaviour of surface strip foundations on sand</b>]]> The objective of this study was to determine whether the contact stress distribution underneath a strip footing on dense cohesionless sand can be predicted as a function of the properties of the footing and that of the underlying sand. Together, the footing and the underlying sand form a foundation system which can be classified in terms of relative stiffness. Centrifuge model tests were conducted on seven aluminium footings of various depths and hence stiffness, and one reinforced concrete footing, during which the qualitative variation in contact stress distribution underneath the footing was captured with the use of Tekscan™ pressure sensors. It was found that, as long as the relative stiffness of the foundation classifies as "stiff", the contact stress distribution underneath the footing is approximately uniform. As the relative foundation stiffness is reduced, the contact stress underneath the footing edge as a percentage of the maximum stress occurring underneath the column, reduces. In the case of a semi-flexible footing tested, zero pressure was measured underneath the footing edge in response to loading of the footing. An expression is presented to allow the contact stress under the edge to be estimated as a function of the relative stiffness of the foundation. This allows the contact stress distribution underneath semi-stiff footings to be estimated. The stiffness of the model reinforced concrete footing tested reduced considerably due to cracking during loading. It is recommended that footings be provided with sufficient stiffness to classify as stiff. A modification of the benchmark value for the minimum relative stiffness classifying as stiff is proposed, based on the results of the centrifuge tests. <![CDATA[<b>Revised regional sediment yield prediction methodology for ungauged catchments in South Africa</b>]]> This paper presents the research on the revision of the regional probabilistic method (Rooseboom et al 1992) for the prediction of catchment sediment yields in South Africa. The determination of sediment yields using probabilistic or empirical methods is suitable for ungauged catchments in the absence of observed data. The prediction of sediment loads is a key component in the quest to deal with reservoir and river sedimentation which is a potential threat to the sustainability of water resources in southern Africa. The revision was necessitated by increased sediment data availability and improved data analysis tools. Ten new sediment yield regions were demarcated in South Africa and Lesotho. Two analytical approaches were developed, namely probabilistic and empirical. The probabilistic approach is applicable in sediment yield Regions 3, 6 and 9. The empirical approach is applicable in sediment yield Regions 1, 2, 4, 5, 7 and 8. The estimation of sediment yields in Region 10 (Lesotho Highlands) needs to be based on direct measurements and locally observed data since no adequate analysis of sediment loads was possible due to limited data. GIS and electronic portable document file (pdf) copies of maps were produced for the retrieval of catchment data. <![CDATA[<b>The two-points condensation technique (TPC) for detection of structural damage due to vibration</b>]]> In recent years, damage detection, as determined by variations in the dynamic characteristics or response of structures, has received considerable attention in the literature. This paper proposes a new damage identification technique that identifies damage location. A methodology termed the Two-Points Condensation Technique (TPC) is presented. It uses identification of stiffness matrix terms to assess damage, based on the incomplete measurement of captured vibration test data. This study identifies damage using free vibration test data in the time domain. Most other techniques used at present are based on data in the frequency domain. The TPC method uses a set of matrices by reducing the structural system to a two-degrees-of-freedom system and then compares the identified coefficients of the stiffness matrices with the coefficients of the theoretically condensed stiffness matrices. The damage location is obtained by observing the change in value of the stiffness coefficients of the two-degrees-of-freedom systems. For the computation, an optimisation uses a program written in MATLAB code. The code can be executed both under the MATLAB and Octave environments. The TPC technique is applied to experimental data obtained from a steel beam model structure after introducing a thickness change in one element. Two case studies are considered. In both cases, the method accurately detects the damage, and determines its location. In addition, the results illustrate that observing changes in the stiffness matrix coefficients can be a useful tool for monitoring structural health. As the procedure proposed here is in a time domain, to eliminate time-consuming calculations this procedure is suitable for structures that are not continuously monitored, but are monitored within scheduled time periods. <![CDATA[<b>The application of continuous surface wave testing for settlement analysis with reference to a full-scale load test for a bridge at Pont Melin, Wales, UK</b>]]> The presence of a significant thickness of soft silts beneath the site of a proposed new road bridge caused concerns over the suitability of shallow foundations. Despite extensive conventional ground investigations, it was not possible to obtain reliable stiffness data, and so a full-scale, fully instrumented load test was undertaken to better determine ground stiffness. The data obtained was used in conjunction with sophisticated numerical analysis to demonstrate that the settlement of shallow foundations would be acceptable. Following completion of the bridge construction, Continuous Surface Wave testing was undertaken and a range of simple non-linear elastic settlement analyses conducted to model the behaviour of the load test. Through these analyses this paper demonstrates the benefit of using continuous surface wave ground stiffness testing in conjunction with simple analysis methods. It underlines the importance of obtaining high-quality stiffness data to accurately predict settlement of shallow foundations. <![CDATA[<b>Effect of the minimum void ratio on the vertical intercept of the steady state line of non-plastic soils</b>]]> The steady state line (SSL) plays a key role in understanding and modelling the mechanical response of soils. Accordingly, understanding how the SSL correlates to soil index properties is of primary importance. A previous study reported that the vertical location of the SSL (Γ1) in void ratio (e) versus mean effective stress (ρ') space is correlated to the minimum void ratio (e min). However, the correlation only included soils with narrow particle size distributions (PSD) and low fines content (FC). In the current study, published data corresponding to 30 non-plastic soils were re-processed to further explore the applicability of the r1-e min correlation. The results indicate that the r1-e min correlation is linear (R² = 0.85) and valid regardless of the coefficient of uniformity (Cu), FC, and particle shape. The r1-e min dataset presented herein was also compared to a previously published dataset, and good agreement was observed. It is proposed that the r1-e mjn correlation can be very useful to understand how the Γ1 of different non-plastic soils compare to one another, and to minimise the extent of triaxial testing required when characterising a soil deposit from an SSL standpoint. Limitations of the r1-e min correlation are also discussed.