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Title: Suction drain as a novel low-carbon ground improvement technique
Author: Martini, Michela
ISNI:       0000 0004 8502 8124
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2018
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The undrained shear strength is the most important parameter in the analysis of tunnel face stability in clayey soils. Soft clays possess very low undrained shear strength and ground improvement techniques need to be implemented prior to tunnel excavation. Jet grouting and fiberglass reinforcement are the most common soil reinforcement methods used in tunnelling to ensure stability. Ground freezing is also used as temporary structural support and/or to exclude groundwater from the excavation until construction of the final lining provides permanent support. These techniques leave chemical residues and spoils into the soil or into the groundwater, slow down construction process due to the need of cleaning up the soil after construction, and have high costs of implementation. Cristelo et al., (2015) show that around 80% of the carbon dioxide emissions of cement-based mixtures for jet grouting is generated by the material used. The technology hereafter conceptualised and studied, the suction drain, utilises compressed air in lieu of cement-based mixture into the ground, hence it has a high margin for reducing the carbon dioxide footprint to the current ground stability techniques. This study presents the suction drain as an innovative and low-carbon technique for temporary stabilisation of geo structures in soft clayey soils. Based on suction generated into the ground by the evaporation from pre-drilled holes, this technique aims to enhance the undrained shear strength in soft clayey by reducing the soil water content. The goal of this study is to investigate the capability of the suction drain in enhancing the undrained shear strength of clayey soils. The objectives of this study are: 1. Understanding and modelling the evaporation-induced water flow that is generated by a tangential airflow in a confined space (as occurs in the suction drain); 2. Testing and validating the suction drain at mock-up laboratory scale level; 3. Investigating the capability of the suction drain to reduce soil water content via field trial. This thesis is structured in papers and includes four chapters as follows: Chapter 2 illustrates the background in tunnelling and water evaporation. The ground improvement techniques currently used in tunnelling, the tunnelling construction techniques, and the theoretical and empirical approaches used to evaluate the tunnel face stability are summarised in this chapter. An insight into moist air turbulent flow and of evaporation in open air is also presented. Chapter 3 focuses on the development of a model that allows estimating the water evaporation rate of the soil exposed to a tangential airflow in a confined space. An evaporation model is required to prescribe the air flow characteristics in terms of air velocity and relative humidity in the implementation of the suction drain. Chapter 4 investigates the suction drain model at mock-up laboratory scale level. An experimental investigation at laboratory scale was conducted to assess the capability of the suction drain in reducing soil water content of the surrounding soil. A numerical application of the suction drain is finally presented to appreciate the enhancement of tunnel face stability that can be potentially achieved following the decrease of soil water content generated by the suction drain. Chapter 5 deals with the validation of the suction drain at the field scale. The field installation and the field procedure are described in this chapter. Numerical analyses based on the hydro-mechanical characterisation of the soil material were carried out to interpret the experimental field data and to validate the field test. Appendix A.1 shows the results of the experimental investigation that was carried out for developing the model described in Chapter 2.
Supervisor: Tarantino, Alessandro Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral