Assessment Of The Efficiency Of Embedded Walls To Mitigate Ground Deformations Induced By Tunnelling

Abstract

O aumento da população nos grandes centros das cidades resultou em uma maior demanda para os sistemas de trânsito. A utilização de túneis foi mostrado como uma ótima solução para melhorar essa crescente demanda por transporte. No entanto, seu uso pode induzir movimentos significativos na superfície do solo, o que pode danificar estruturas localizadas nas proximidades. Existem várias técnicas desenvolvidas para mitigar a magnitude desses movimentos. O objetivo deste estudo é focar em uma medida de mitigação: o uso de cortinas no solo para um caso de um túnel superficial. Tendo como referência o caso da Secção 63 da Linha Verde da rede do Metro de Lisboa, foram realizadas análises numéricas utilizando o software RS2 v2019, assumindo condições de deformação plana. Um estudo paramétrico foi realizado alterando a localização, profundidade e rigidez da cortina. A influência do fator de alívio do estresse em sua eficiência também foi investigada e discutida. Duas relações sem dimensões (local e global), os parâmetros de eficiência, foram empregados para avaliar o desempenho da cortina. A magnitude do impacto da cortina no suporte do túnel também foi investigada. A capacidade da cortina em modificar os assentamentos na superfície de forma favorável foi observada neste estudo específico. Além disso, foi observado um impacto quase insignificante nas forças que atuam no suporte do túnel.This dissertation incorporates 7 chapters:Chapter 1 is an introductory chapter that brings attention to the unavoidable growth of traffic rates in city centres. It highlights that tunnelling is a solution in the modern world to expand mass transportation. However, it also describes the consequences of tunnelling in heavily dense areas by describing three case histories. Among these, two of them suffered a visible impact, while the other one mitigation measures were taken in order to protect the structure. Chapter 2 gives an overview of terms and explanations of general considerations for short-term ground movements for shallow tunnels in soft ground. It depicts two conventional tunnelling methods in urban areas. It reviews the empirical methods from the literature employed to estimate displacements induced by tunnelling. Broadly, this chapter explores some mitigation measures typically taken to mitigate ground movements induced by tunnelling by giving some examples of applications in the past. Chapter 3 gives a concise bibliography review of studies performed and describes three case histories of using an embedded wall as a mitigation measure of settlements on the ground surface. It also describes in details two reference studies, Bilotta (2008) and Rampello et al. (2019) that this mitigation technique was employed. It explores and discusses the results of centrifuge tests and numerical analyses. Additionally, it describes the use of two dimensionless parameters termed as local and global efficiency to assess the efficacy of the embedded wall in mitigating settlements behind it. The results obtained and the suggestions made from the two studies were discussed and criticized. Chapter 4 presents the case of study of this dissertation, section 63, which is located between “Cais do Sodre” and “Baixa-Chiado” stations, from the Greenline Lisbon metro network. It briefly describes the background history and reviews with the evolution over time of the metro lines construction. It also describes the construction method employed. The geological and geotechnical conditions from this section are also discussed by describing the instrumentation survey and monitoring plan performed in the field between sections P4 and P13 (instrumented sections within section 63) Chapter 5 is dedicated to exploring in details the influence of using an embedded wall in the settlement trough by a two-dimensional finite element analysis by using RS2 v2019 software. It describes how the numerical analysis was performed in greenfield, and in the presence of the embedded wall (reference model) for the instrumented section P9. The general conditions of the study, the geometry of the problem, the mesh, the boundary conditions, the constitutive model, the construction sequence, and the geotechnical parameters adopted also are described. It is included the calibration of the numerical model in order to replicate the instrumented sections P9 and P10A, which involves the stress relief methodology based on the reference results measured in the field. The forces mobilised along the tunnel lining and the embedded wall are also depicted. This chapter also studies the efficiency of the wall by using a local efficiency parameter and describes the implementation of an additional methodology by using a dimensionless parameter that involves assessing the global efficiency of the embedded wall in the settlement trough.Chapter 6 describes three parametrical studies performed in this investigation by exploring the main factors that could affect the performance of the embe

This dissertation incorporates 7 chapters:Chapter 1 is an introductory chapter that brings attention to the unavoidable growth of traffic rates in city centres. It highlights that tunnelling is a solution in the modern world to expand mass transportation. However, it also describes the consequences of tunnelling in heavily dense areas by describing three case histories. Among these, two of them suffered a visible impact, while the other one mitigation measures were taken in order to protect the structure. Chapter 2 gives an overview of terms and explanations of general considerations for short-term ground movements for shallow tunnels in soft ground. It depicts two conventional tunnelling methods in urban areas. It reviews the empirical methods from the literature employed to estimate displacements induced by tunnelling. Broadly, this chapter explores some mitigation measures typically taken to mitigate ground movements induced by tunnelling by giving some examples of applications in the past. Chapter 3 gives a concise bibliography review of studies performed and describes three case histories of using an embedded wall as a mitigation measure of settlements on the ground surface. It also describes in details two reference studies, Bilotta (2008) and Rampello et al. (2019) that this mitigation technique was employed. It explores and discusses the results of centrifuge tests and numerical analyses. Additionally, it describes the use of two dimensionless parameters termed as local and global efficiency to assess the efficacy of the embedded wall in mitigating settlements behind it. The results obtained and the suggestions made from the two studies were discussed and criticized. Chapter 4 presents the case of study of this dissertation, section 63, which is located between “Cais do Sodre” and “Baixa-Chiado” stations, from the Greenline Lisbon metro network. It briefly describes the background history and reviews with the evolution over time of the metro lines construction. It also describes the construction method employed. The geological and geotechnical conditions from this section are also discussed by describing the instrumentation survey and monitoring plan performed in the field between sections P4 and P13 (instrumented sections within section 63) Chapter 5 is dedicated to exploring in details the influence of using an embedded wall in the settlement trough by a two-dimensional finite element analysis by using RS2 v2019 software. It describes how the numerical analysis was performed in greenfield, and in the presence of the embedded wall (reference model) for the instrumented section P9. The general conditions of the study, the geometry of the problem, the mesh, the boundary conditions, the constitutive model, the construction sequence, and the geotechnical parameters adopted also are described. It is included the calibration of the numerical model in order to replicate the instrumented sections P9 and P10A, which involves the stress relief methodology based on the reference results measured in the field. The forces mobilised along the tunnel lining and the embedded wall are also depicted. This chapter also studies the efficiency of the wall by using a local efficiency parameter and describes the implementation of an additional methodology by using a dimensionless parameter that involves assessing the global efficiency of the embedded wall in the settlement trough.Chapter 6 describes three parametrical studies performed in this investigation by exploring the main factors that could affect the performance of the embedded wall in the problem. In that, sixty-six numerical analyses were computed is investigated the influence of the location, depth, and stiffness of the embedded wall on local and global efficiency parameters. In addition to that, the impact of distinct stress relief factor on the efficiency parameters is explored. This chapter also explores and discusses the impact of using an embedded wall in the maximum forces acting along the lining. The forces induced along the embedded wall after the tunnel excavation is also investigated for a better understanding of the soil-structure interaction.Chapter 7 contains the conclusions and the main findings of the project. It considers how the project carried out varied from the previous studies performed. It also contains recommendations for future research.