On cyclic accumulation models for degradation of railway foundations
Doktorsavhandling, 2023

The degradation of railway foundations due to the repeated loading of traffic induces maintenance and safety obligations for the track operator. One major contributor to track degradation is the accelerated settlement rates in soft soils below the track that lead to alignment issues, especially at stiffness transitions. Furthermore, due to the low stiffness of most soft soils, significant ground vibrations are emitted and lead to low critical train velocities that avoid track resonance. In addition, the dynamic properties of the soil, particularly weakly bonded soft natural clays, are subject to significant alternations over the lifetime of the railway structure due to cyclic traffic loading. The soil/foundation is thus a major source of degradation issues that, as opposed to track and subgrade-related causes, are largely controlled by local site conditions.

This thesis aims at identifying a proper sensitivity analysis method in geotechnical Finite Element Analysis (FEA) for optimal use of advanced constitutive soil models. For this purpose, first the viscoplastic Creep-SCLAY1S model is evaluated for a boundary value problem. The objective was ultimately addressed by implementing two Global Sensitivity Analysis (GSA) methods for quantifying the uncertainties of Creep-SCLAY1S. The common GSA method of Sobol was benchmarked against Experimental design in a lab-scale numerical model of Constant Rate of Strain (CRS). The Sobol method has proven to be computationally expensive for sensitivity analysis of advanced constitutive models using FEA. The spatial sensitivity measures of Sobol and Experimental design indicate that they are not altogether distinct. Thus, Experimental design represents a more feasible approach by using less resources, such as computational time and required storage. Furthermore, temporal Sensitivity Analysis (SA) has demonstrated the importance of the entire time domain spectrum, particularly for factor fixing purposes.

The second part of the study proposes a revised strain accumulation model that has been validated using new data on Swedish natural clay for cyclic loads with low amplitude. The model presented herein offers a strong basis for the accurate prediction of strain accumulation in soft clays beneath embankments subjected to a significant number of loading cycles. In general, the knowledge gained in this research contributes to a better understanding of comprehensive numerical models in Geotechnics and can be a helpful prior to inverse modelling, data assimilation, and Random Finite Element Method (RFEM).

Natural soft clay

Global Sensitivity Analysis

Sobol method

Experimental design

Rate-dependent models

Cyclic accumulation models

Cyclic degradation

Uncertainty analysis

EA, Hörsalsvägen 11, Chalmers
Opponent: Dr. Marti Lloret-Cabot, Department of Engineering, Durham University, United Kingdom


Hossein Tahershamsi

Chalmers, Arkitektur och samhällsbyggnadsteknik, Geologi och geoteknik

Using experimental design to assess rate-dependent numerical models

Soils and Foundations,; Vol. 62(2022)

Artikel i vetenskaplig tidskrift

Towards rigorous boundary value level sensitivity analyses using FEM

IOP Conference Series: Earth and Environmental Science,; Vol. 710(2021)

Paper i proceeding

Tahershamsi, H., Ahmadi Naghadeh, R., Zuada Coelho, B., Dijkstra, J. Low amplitude strain accumulation model for natural soft clays below railways

Railway systems are becoming increasingly popular as a sustainable transportation option, with electric passenger trains offering a way to reduce carbon emissions. However, transitioning to railways on a larger scale requires significant investments in planning, construction, operation and maintenance. To effectively connect urban cities, high-speed tracks are crucial, but it is equally important to improve conventional networks to serve rural areas. This means that both high-speed and slow trains are needed to create a comprehensive railway system.

The natural ground is a critical yet overlooked aspect of railway systems that can lead to degradation. With limited field measurements, the use of models is a preferred method for predicting the behaviour of railway structures over the long term. A model has been developed in this thesis by which to predict strain accumulation in soft natural clays by analysing their behaviour under cyclic loading through laboratory testing.

This thesis also aims to provide an efficient experimentation method using models. Existing models are becoming increasingly complex to be able to capture the physical problems with high precision. Of special value are statistical techniques that are used in this study to carefully assess these complex models.  In addition, these statistical techniques are the very first steps with which to improve the design of railway systems by collecting suitable data. Collecting and studying suitable data can lead to significant improvements in the system, potentially revitalising the railway industry through a chain reaction initiated by quality improvement techniques.

Deformationer på spår-platta grundlagd på pålar i lös lera

Trafikverket (2015/108250), 2016-02-01 -- 2019-02-15.

Quantifying complex track stiffness response

Trafikverket (TRV2016/106277), 2017-08-15 -- 2020-04-30.








Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5276



EA, Hörsalsvägen 11, Chalmers

Opponent: Dr. Marti Lloret-Cabot, Department of Engineering, Durham University, United Kingdom

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