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Fachbereich Bauingenieurwesen

Study Entrapment Risks of Shielded Tunnel Boring Machines

Abstract

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Application of the Shielded Tunnel Boring Machines (TBM) has become very popular in recent years, due to the flexibility of these machines in coping with various ground conditions. However, there is an intrinsic entrapment risk associated with tunnel construction when a shielded machine passes through rock mass with squeezing behaviour. This project aims to provide a framework for systematically risk analysis of shielded TBM tunnelling based on 3D numerical analysis and measurement data and presents a methodology for statistical analysis of data to determine the inputs for the probabilistic models. This project also intends to offer a discussion and a novel shape of Ground Reaction Curve (GRC) in use for tunnelling by shielded TBMs in weak ground conditions.

Keywords

Shielded Tunnel Boring Machines (TBMs); Numerical Modelling; Ground Reaction Curve; Risk Analysis

Motivation

The shielded TBMs are among the technically most sophisticated construction machines in use by the tunnelling industry. Despite successful use of shielded TBMs in many projects, the presence of shield and limited access to the excavated walls for observation of ground conditions make the machine susceptible to entrapment in deep rock tunnels under high stresses experiencing high convergence. This will be more challenging when machine passes through rock mass with squeezing behaviour [1]. In some cases, TBM may get stuck in the complicated geological structures, which includes jamming of the shield and back-up, blocking of the cutter-head and even loss of a machine, which requires manual excavation to release the machine. This is a time consuming, costly, unsafe, slow, and labour intensive work that should be avoided as much as possible.

Tunnelling by a shielded TBM and jamming of the shield
Tunnelling by a shielded TBM and jamming of the shield

To avoid entrapment risks, ground behaviour should be recognised in a correct manner. The Ground Reaction Curve (GRC), which describes the relationship between the decreasing of inner pressure and the increasing of radial displacement of tunnel wall [2], is generally used in conventional tunnelling projects for evaluation of ground behaviour and pressure around and on the tunnel linings. However, the result of the preliminary study has indicated that the shape of GRC for shielded machines will be different from the conventional tunnelling method.

Estimated shape for GRC for tunnelling by a single shield TBM
Estimated shape for GRC for tunnelling by a single shield TBM

Furthermore, the majority of existing risk analysis systems deal only with the effects of random geological and construction uncertainties on time and cost of construction and are often developed for conventional tunnelling methods [3]. There are other sources of risks in relation to shielded TBM tunnelling, not considered in many of risk analysis systems, which are related to machine and performance scenarios that can have a major impact on the tunnel process.

Objective

The overall objective of this project is the systematic evaluation of ground behaviour for a tunnel construction by using the shielded TBMs and exhibition a comprehensive risk analysis for mechanised tunnelling by considering all of the main machine and tunnel factors. The results of the study will be confirmed with the measurement data from the real tunnelling projects. The project also offers some recommendation for evaluation of the possibility of shield entrapment based on the results of empirical and analytical calculation of GRC for mechanised tunnelling for future projects.

Significance and Innovation

There are several studies about the application and evaluation entrapment risks of shielded TBMs in weak grounds in the literature. However, these studies seem to have inadequacies that should be revaluated. In numerical simulation of tunnels that are excavated by using the shielded TBMs, the exact 3D numerical simulations for evaluation of machines in squeezing grounds in order to evaluation of ground convergence, the contact pressure between shield and ground and also the interaction between the segmental lining and backfilling are scarce. Furthermore, the GRC method for assessment of ground behaviour in tunnelling by shielded machine was not given in the literature. Additionally, considering all of main TBM, tunnel and performance parameters on statistical calculations and systematically risk analysis of tunnelling by shielded TBMs based on 3D numerical simulation and measurement data are not provided in the literature.

Preliminary 3D numerical simulation of tunnelling by a single shield TBM
Preliminary 3D numerical simulation of tunnelling by a single shield TBM

Methodology

The methodology for this project consists of two stages. In the first stage, numerical simulations for prediction of ground pressure on TBM shields and lining through adverse rock conditions for different types of shielded machines will be developed. Furthermore, ground behaviour around and along a tunnelling with shielded TBMs will be presented by ground reaction curves (GRC). Some real cases will be applied to the simulation model for verification of the results. Effects of various operational parameters such as tunnel geometry, size, depth, shield length and thickness, overcut and configuration of the shields on the GRC will be discussed.
In the second stage, a methodology to systematically assess and manage the risks associated with tunnelling by using shielded TBMs will be offered. The model will include utilised machine main components, machine performance data and ground properties for performing a risk analysis. The developed risk model in this project will be based on DNB and Monte Carlo methods because of their ability to combine domain knowledge with data, encode dependencies among variables, and their ability to learn causal relationships.

Time Schedule

Duration for the proposed project is estimated for 22 months from 01.12.2015 to 30.09.2017.

More Information

Project Title Study Entrapment Risks of Shielded Tunnel Boring Machines
Financed by Alexander von Humboldt Foundation (AvH)
Person in Charge Project Supervisor: Prof. Dr. Jürgen Schmitt
Project Manager: Dr. Rohola Hasanpour
 References [1] Hasanpour R., Advance numerical simulation of tunneling by using a double shield TBM. Computers and Geotechnics 57, 37–52, (2014).
[2] Carranza-Torres, C., Fairhurst, C., Application of convergence-confinement method of tunnel design to rock masses that satisfy the Hoek–Brown failure criterion. Tunnelling and Underground Space Technology 15 (2), 187–213, (2000)
[3] Sousa, R.L., Einstein, H.H., Risk analysis during tunnel construction using Bayesian Networks: Porto Metro case study. Tunnelling and Underground Space Technology 27, 86–100, (2012).
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