Behaviour of railway track subgrade under cyclic loading

Abstract

The railway track foundation of fine-grained soil subgrade, under repeated loading of cyclic nature, can gradually build up excess pore pressure and result in progressive shear failure at a stress level much lower than the monotonic loading strength of the soil. It is widely accepted that a threshold stress exists, above which, the induced stress generates large shear deformation and; below which, the soil deformation become stabilized irrespective of the number of loading applications. Previous studies into the behaviour of fine-grained subgrade under cyclic loading had mainly involved slow loading rate. The advance in transportation technology and the required efficiency of transporting goods and commuting passengers demands higher train speed and heavier loads. Consequently, better understanding on the behaviour of fine-grained subgrade under higher rate of loading (i.e. cyclic frequency) will be necessary as it influences the build-up of excess pore water pressure and strain accumulation of subgrade soil, hence the stability of the railway track. In addition, current design approaches to the railway tracks foundation of fine-grained subgrade have largely been empirical or semi-empirical unique to a range of geological characteristic, operating and traffic condition. The rationalization and reformulation of design approach may be required if general design means will to be devised.In this research, an in-depth study of undrained triaxial testing on the behaviour of a typical fine-grained reconstituted kaolinite clay soil, simulating the characteristic subgrade responses exhibit under the passing axles / wheel load of the contemporary train speed was performed and discussed. Four series of both static and cyclic undrained triaxial test were conducted. The static undrained triaxial test provides the behavioural benchmark and a prior indication on the maximum stress level from which the cyclic stress level can be apportioned. The cyclic undrained triaxial test simulates the behaviour of clay subgrade under a large number of passing axles/wheel load. The results of the testing described and characterized the stress / strain behaviour for saturated kaolinite clay of various consolidated state and stress history. In particular, it described the general concept of cyclic stress equilibrium state and resilient state, and detailed the characteristic pattern of the line of cyclic stress equilibrium state which dictates the way in identifying the threshold stress (or critical level of cyclic stress). The study demonstrated that, using effective stress analysis, the threshold stress can be obtained for clay under high cyclic loading rate. In conjunction with the stress response, the deformation and resilient characteristics of various series of cyclic undrained triaxial tests carried out on the saturated clay was described. Apart from highlighting and confirming the influence of consolidated state and stress history on the threshold stress and deformation characteristics, influence of the cyclic loading frequency simulating higher train travelling speed was also investigated and examined.Using the framework of critical state soil mechanics, the study described the existence of cyclic stress equilibrium state surface CSESS within the stress state boundary surface SSBS for saturated clay under cyclic undrained compression. A form of analytical / theoretical model was established which enables threshold stress to be ascertained without resort to lengthy laboratory testing at the preliminary design stage of railway track foundation design. Developed from the “original Cam-clay” model and validated by the cyclic triaxial test data on reconstituted kaolin clay, the theoretical model which described the CSESS enables the prediction of threshold stress (or critical level of cyclic stress) of general saturated clay soil under undrained cyclic compression to be made from the fundamental properties of the soil, based on the designed consolidated state and the stress history of subgrade soil.In addition, this study proposed a new approach which rationalized and reformulated the current state of design and management process of railway track substructure involved clay subgrade. The rational approach was developed based on the comprehension of “lower-bound threshold stress” and potential strengthening and densification of fine-grained subgrade soil through progressive and deliberate incremental loading of the track foundation. Concept of “Managing current lower-bound threshold stress” for clay subgrade was elaborated. The readily use of the developed theoretical model and design charts for predicting the threshold stress could offer a key advantage for the new approach over current practices.