Lime stabilisation is a versatile technique applied during earthworks operations. Modern soil recycling units are much more efficient at pulverising fill material and intermixing the added binder/water than machinery available 20 years ago. While supplier innovation adds flexibility to the site working method, specifications have not been sufficiently updated to permit optimal application. This review paper details the physico-chemical changes instigated through the lime-clay soil reaction, updating previous reviews. It aims to assist scientific debate, current practitioners and future specification changes. For example, the application of the minimum 24 h mellowing periods (mandatory to UK specifications) with high reactivity, quicklime powders is concluded to cause increased air voids in the compacted fill.Increased air voids are associated with reduced long-term strength and potential volume change from water ingress, which is of particular concern for sulfate swelling. Shorter mellowing periods and/or use of hydrated lime may lesson this issue; however, a 'one size fits all' approach is discouraged in preference to site-specific methodologies refined to suit the fill material and project requirements. The discussion also summarises working methods which may lower the risk of sulfate swell and defines areas requiring further practical research.
The vibro stone column (VSC) technique is a ground improvement method used to enhance the load-bearing capacity. This paper provides an overview of the VSC technique and its application in railway trackbed stabilization (TBS). The VSC technique involves installing columns of good quality stone into the soil using a vibrating probe, creating both a strong column and radial stiffening of the adjacent compressed soil; a composite ground improvement system. The paper discusses the benefits of the VSC technique, such as its efficiency, versatility, and cost-effectiveness, and its limitations in TBS. Additionally, the paper presents the results of a site trial that utilized a vision-based monitoring system to measure the effectiveness of the VSC technique in improving the trackbed stiffness. The results demonstrated that the VSC technique can be considered a reliable TBS system to improve the stiffness of subgrade and sub-ballast layers in railway trackbeds, reducing the risk of trackbed settlement and extending the life of the track. The paper concludes by summarizing the importance of VSC in railway TBS and highlighting its potential for future ground improvement projects. The use of a vision-based monitoring system further enhances the effectiveness of the VSC technique, providing real-time monitoring and analysis of trackbed conditions, enabling better decision-making and improving the accuracy, reliability, and efficiency of the TBS technique.
Railway transitions from cut to fill are locations across which significant differential settlements may develop. Compounded by similar abrupt changes to subgrade stiffness, accelerated track movement during high speed (HS) train passage may cause tracksubstructure deterioration and instability. This paper considers a foreseeable scenario in UK rail engineering with transition from unweathered Mercia Mudstone (MMG) to MMG cohesive fill. Separate analysis of (1) differential settlement using one-dimensional oedometer consolidation methods and (2) track bed movements using a three-dimensional Finite Element Analysis (FEA) with moving load were undertaken. This included comparison of untreated and lime treated embankment fill material with parameters for each taken from laboratory and field test data. Results showed a difference in settlement of 26.6mm across the modelled cut to 8metre fill transition giving differential settlement for untreated fill that was too high to meet literature criteria of <20mm over 20m. However, 1.5% lime treatment of the fill causes significant reduction to both consolidation settlement and track movement under dynamic loading to meet the serviceability criteria. Consideration of the full settlement profile across the transition has identified that the Rate of Change (ROC) of settlement is maximum at the start of the fill zone and the ROC in settlement could be a more relevant measure of what a moving train would experience with a sudden unloading/loading action. It is concluded that future work including a coupled FEA analysis, including consolidation and then subsequent stages modelling the resulting amplification of moving loads across the settled profile would give stronger understanding of how differential settlement causes rail level movement from HS traffic. This would help confirm how best to apply differential settlement criteria in geotechnical design of transitions and whether ROC in settlement (e.g. 1mm per 1m) is more informative than a settlement range across a longer fixed distance.
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