Year

2012

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Railways have technical and economic advantages, and therefore, railway traffic has gained a specific and irreplaceable position in recent years. With this background, and to satisfy their rapid development, railways will inevitably be constructed on soft soil subgrade such as soft clay subgrade. It is desired to understand the behaviour of soft clay subgrade subjected to cyclic loads when a new rail track is designed or an existing one is under maintenance. When the soft clay subgrade is subjected to the cyclic loading, excess pore pressures and axial strains keep developing with the increasing number of cycles, resulting in a decreased bearing capacity of the subgrade and excessive settlement. To improve the subgrade, prefabricated vertical band drains (PVDs) is used increasingly in popularity among a variety of techniques. With the installation of PVDs, a short radial drainage path is introduced to dissipate the excess pore pressure so that the soft clay subgrade becomes more stable subjected to train loads. This thesis covers the laboratory tests and numerical modelling. The behaviour of soft clays under cyclic loading with or without radial drainage is investigated.

A series of undrained cyclic triaxial tests were conducted on specimens of reconstituted Kaolinite. Three cyclic stress ratios and four loading frequencies were used in the laboratory tests to study the performance of soft subgrade soil subjected to cyclic loading. The effects of cyclic stress ratio and loading frequency on the generation of excess pore pressures and axial strains were investigated. The roles of cyclic stress ratio and loading frequency play in influencing the stability of soft clay subgrade were discussed. The effect of strain rate for a stress controlled test on the performance of soft clays under cyclic loading was investigated. The relationship between the strain rate and either cyclic stress ratio or loading frequency were figured out, and in this way, the influence of cyclic stress ratio and loading frequency on the progressive shear failure and excessive plastic deformation were justified. Furthermore, due to a high strain rate varying from 150 to 550 %/h for a stress controlled test, the slope of q/p' at failure for cyclic loading could increase compared to the condition of monotonic loading.

An undrained cyclic model has been proposed based on the modified Cam-clay model. A modified yield surface function during elastic unloading was proposed to capture the soil behaviour under cyclic loading. Only two additional cyclic degradation parameters ξ1 and ξ2 were introduced to present the yield surface during elastic unloading, in addition to the parameters adopted in the modified Cam-clay model. These two cyclic degradation parameters controlled how much the size of the yield surface would shrink during an elastic unloading and how this reduction in the size of the yield surface would change with loading cycles. This model was verified against the laboratory experiments conducted. The cyclic degradation parameters ξ1 and ξ2 from the back calculation indicated that ξ1 was a soil property which was independent of both the cyclic stress ratio and loading frequency, while ξ2 depended on the loading frequency. This undrained cyclic model was further analysed by investigating the effects of consolidation stress ratio, parameters ξ1 and ξ2 on the predictions of the proposed model.

Large scale cyclic triaxial tests were conducted on specimens of reconstituted Kaolinite using the cylindrical dynamic triaxial equipment (accommodating 300 mm diameter and 600 mm height samples) which was designed and built at the University of Wollongong. To allow for radial drainage during and after the cyclic tests, a single PVD was installed in the centre of the soil cylinder. The effectiveness of radial drainage in dissipating the excess pore pressure was examined. It was found that for a high cyclic stress ratio, the radial drainage decelerated the rate of excess pore pressure build up to its critical value, so the soil could undergo more loading cycles prior to failure. With a low cyclic stress ratio, radial drainage could prevent the excess pore pressure from accumulating to its critical value, so the soil would not fail. The test results also suggested that for newly constructed railway lines, a train with a lower speed is preferred initially, until the track becomes stable for the next loading stage.

A radial consolidation model under cyclic loading has been proposed to capture the behaviour of soft clays subjected to cyclic loading when radial drainage is allowed during the loading period. This was achieved by combining the theory of radial consolidation with the undrained cyclic model. The effects of the stress history and dissipation of excess pore pressure on the generation of excess pore pressure were considered in the proposed model. This model was verified against the large scale cyclic triaxial tests conducted at the University of Wollongong. Analysis of this new model was carried out to investigate the effects of coefficient of radial consolidation, parameters ξ1 and ξ2 on the predictions of the proposed model. This model was further verified against a field case history in Sandgate, between Maitland and Newcastle. It is indicated that the stability of the soft soil subgrade could be improved effectively by the installation of PVDs.

FoR codes (2008)

0905 CIVIL ENGINEERING, 090501 Civil Geotechnical Engineering

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.