Centrifuge modelling of railway embankments under static and cyclic loading

Abstract

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.

Fly-by-Pi: Open source closed-loop control for geotechnical centrifuge testing applications

“…Despite advances in numerical modelling, physical models serve as an external calibration measure to validate numerical results especially for experiments that present complex interaction phenomena not suited for current software applications. Active areas of research interest pursued at the University of Pretoria include modelling of pipe deflection as a function of sinkhole development [2], cavity propagation [3] in sinkhole and cave mining applications [4], small strain stiffness measurements [5], cyclic loading of railway embankments [6] and ultra-thin continuously reinforced concrete pavements (UTCRCP) [7].…”

The influence of soil suctions on the deformation characteristics of railway formation materials

“…The physical model used in the experiment consisted of a 500 mm x 500 mm strongbox with fully frictional sidewalls filled with compacted soil in 4 layers of 50 mm thick each. The total formation depth of 200 mm was chosen to correspond with a single layer of the 26 t/axle structural layer specification (Transnet, 2006). This model was instrumented to monitor pore pressures/suctions using tensiometers and surface deflection using 4 linear variable differential transformers (LVDTs).…”

“…The formation was constructed layer by layer from material that was pre-wetted to the targeted moisture content. These layers were compacted to 95% Mod AASHTO as per the South African railway formations specification (Transnet, 2006). A fitted plastic sheet with openings for instrumentation and the loading block was placed over the finished formation to reduce moisture loss through the surface.…”

“…The results of these tests are summarized in Table 5. Figure 3 presents the grading of both materials within the grading envelope recommended by Transnet (2006). Gradings were obtained using the hydrometer method to determine the finer portion of the grading curve in line with TMH1: Method A6 (NITRR, 1986).…”

“…The design of railway formations has traditionally relied on empirical and catalogue design methods that prescribe a required formation (Li and Selig, 1998;Transnet, 2006;UIC, 1994). This formation structure may then be defined in terms of the number of layers required, their thicknesses and types of materials used.…”