Tunnels and Underground Cities. Engineering and Innovation Meet Archaeology, Architecture and Art

Volume 8: Public Communication And Awareness / Risk Management, Contracts And Financial Aspects

Daniele Peila, Giulia Viggiani, Tarcisio Celestino

• Publisher: CRC Press
• Year: 2020
• Topic: Technology & Engineering
• eBook ISBN:
  9781000175578
• Subtopic: City Planning & Urban Development
• Index: Technology & Engineering

Tunnels and Underground Cities: Engineering and Innovation meet Archaeology, Architecture and Art. Volume 8: Public Communication and Awareness / Risk Management, Contracts and Financial Aspects contains the contributions presented in the eponymous Technical Sessions during the World Tunnel Congress 2019 (Naples, Italy, 3-9 May 2019).

The use of underground space is continuing to grow, due to global urbanization, public demand for efficient transportation, and energy saving, production and distribution. The growing need for space at ground level, along with its continuous value increase and the challenges of energy saving and achieving sustainable development objectives, demand greater and better use of the underground space to ensure that it supports sustainable, resilient and more liveable cities.

The contributions cover a wide range of topics, and is a valuable reference text for tunnelling specialists, owners, engineers, archaeologists, architects, policy makers, decision takers and others involved in underground planning, design and building around the world, and for academics who are interested in underground constructions and geotechnics.

Risk management, contracts and financial aspects

Shallow TBM launch – alternative to cut and cover

W. Angerer

Jacobs, Dubai, UAE

A.M. Haimoni

Dubai Roads and Transport Authority, Dubai, UAE

A. Ozturk

Jacobs, Dubai, UAE

V. Tellioglu

Expolink Alliance, Dubai, UAE

ABSTRACT: This paper describes the decision-making process and implementation of a value engineering alternative that allowed a shallow launch of TBM on Route2020 Metro program in Dubai. This replaces a 300m long cut and cover tunnel included in the reference design. The TBM launch was originally targeted at the end of the cut and cover tunnel with 12m cover to the tunnel alignment. After award of the construction works, the client, contractor and designer investigated a value engineering alternative to launch the TBM from the open cut section, hence, replacing the cut and cover tunnel with a bored tunnel. The intent of the shallow launch was to de-risk the challenging construction program by reducing the construction works required prior to TBM delivery. Ultimately, the TBM was successfully launched with only 2.0m of cover. The paper will present an outline on the risk management, engineered mitigation measures together with a review of performance during construction.

1 INTRODUCTION

Dubai’s award in 2013 of the World Exposition (Expo 2020) served as the catalyst for the Route 2020 project, a new metro route connecting the existing Dubai Red Line with the Expo site in the south of the city. Route 2020 is the first major rail extension to the existing Dubai Metro rail network. The project consists of approximately 15km of metro alignment which consist of 12km of viaduct and 3.2km of tunnel. It also features seven LEED Gold certified metro stations consisting of one interchange station, three elevated stations, two underground stations and a signature Iconic Station at the Expo Site as well as the supply of 50 trainsets, of which 15 are needed for the new line and 35 to increase capacity on the existing network. The new line will establish a direct metro connection between Dubai Airport, the City and the Expo Site. The number of passengers to use Route 2020 is expected to reach 125 thousand commuters per day in year 2020 and will rise to 275 thousand commuters by year 2030.

In June 2016, Dubai RTA (Roads and Transport Authority) awarded the design and build contract for the construction works to Expolink, an Alliance that consists of Alstom (France) in partnership with the Civil Works Joint venture of Acciona (Spain) and Gulermak (Turkey). The detailed design for the civil works was awarded to Jacobs Engineering by the Civil Works Joint Venture.

Figure 1. Route alignment.

Tunnelling on the 2.4km bored tunnel between Discovery Gardens and Green Community was formally launched on October 24th 2017. Tunnelling was successfully completed on 11th June 2018. This paper outlines some of the major decisions taken in relation to the shallow launch of the Tunnel Boring Machine (TBM) in Discovery Gardens.

2 PROJECT INITIATION

At the start of the project the Client, the Contractor and his Designer jointly reviewed the launch arrangement of the TBM in Discovery Gardens. Due to the tight construction timeline, a special focus centered on the reference design to identify if any improvements could be achieved to safeguard the critical activity of the TBM drive within the works construction program.

The reference design provisioned a classic cut and cover ramp approach leading to the TBM launch site. This consists of approximately 560m of below grade ramp section, of which 300m would be executed as cut and cover. Two TBM supply shafts were provisioned as shown in Figure 2.

Figure 2. Launch approach in reference design.

This design had a significant number of interfaces with existing utilities and a complex construction program with numerous interdependencies. The Contractor studied this section with different construction programs depending on the particular construction approaches and identified significant risks.

As such an alternative to the reference design consisting of a shallow launch of the TBM was brought into consideration.

Figure 3. Alternative Launch of TBM.

The alternative design was aiming at reducing the works required ahead of the TBM delivery. In this approach the cut and cover section would be replaced with a bored tunnel that is constructed with minimum cover requirements. The utilities in this section would therefore remain largely unaffected and the critical activity on the construction program seemed improved. Furthermore, the utilization of the expensive TBM equipment is improved and the major utility corridor crossing the rail alignment would remain available for future usage of the local utility network providers. Therefore, this option was seen to offer many advantages and investigated in more detail.

3 RISKS AND OPPORTUNITIES

As a first task a risk and opportunities register was established for both options. The Contractor, as the owner of the project delivery risks, led the development. The below highlights the major considerations that were taken into account:

3.1 Reference Design

The design offered the following opportunities

• Construction ahead of TBM delivery possible although on the critical path
• Final structure provides more space for Mechanical, Electrical and Plant equipment (MEP)
• Reduced geotechnical risk profile

However, the design was perceived to pose the following risks

• Multiple stakeholder approvals required
• Three major utility diversions
• Stakeholder approval is on the critical path of the project
• Large construction area required close to residential buildings
• Complex construction works that lie on the critical path of the project
• Reduced ventilation performance
• Complex dewatering requirements
• Complex construction sequencing to account for the requirements of each utility
• Utility corridor largely compromised for future development (major utility corridor crossing alignment)
• Permanent diaphragm wall details are not seen as reliable as segmental lining

3.2 Alternative Design

The alternative design offered the following opportunities

• Major utility diversions can be avoided
• Reduced stakeholder approval risk
• Utility corridor remains available for future developments
• Ventilation performance will be improved, which is also an advantage for LEED certification
• Start of the open cut works and the provision of the required protection works for the TBM launch provides more float on critical path of the project
• Reduction of above ground works which will result in reduced noise pollution
• Reduction in dewatering requirements
• Lower Carbon Footprint due to reduced concrete quantity, which is also an advantage for LEED certification
• Reduced soil subsidence from tunnelling compared to cut and cover and decrease in influence zone of settlement

These opportunities are compared with the following risks

• Water mains running close to tunnel crown (require diversion)
• Increased geotechnical/tunnelling risk due to shallow cover
• Increase duration of TBM tunnelling works due to additional length of 390m to replace the cut and cover length present in the original design.

As can be seen in the above, the range of risks and opportunities affect all contractual partners on the project. However, all parties concluded that the Alternative Launch method represents a viable approach that may bring many benifets.

As such a working group to study the Alternative Launch was established which included members from the Client (and his representative), the Contractor and the Designer. The working group was tasked with the investigation and development of mitigation measures for the percieved risks related to the alternative approach prior to deciding to proceed.

4 MITIGATION MEASURES

4.1 Structural and Geotechnical Design

To mitigate against the risk of collapse the working group recommended a range of structural/geotechnical works that are required to be completed prior to TBM launch. These works are shown indicatively in Figure 4 and consist of the following

• Soil Improvement
• Construction of an inclined structural concrete slab – to remain as a permanent structure
• Diaphragm walls
• Geotechnical protection works – concrete slab – temporary works for protection purposes only

Figure 4. 3D Model of Alternative Launch (continued pile wall under concrete slab not shown for clarity).

4.1.1 Soil Improvement

The ground investigation identified a layer of approximately 3m deep Aeolian sands present at the TBM launch location. This layer of sands is at the start of the TBM launch and has to be improved in order to allow the TBM to launch without completing a full EPB (earth pressure balance) chamber. It facilitates the build-up of face pressure.

Figure 5. Soil Improvement.

A further soil improvement is implemented at the interface between inclined structural slab and protection slab. This interface is seen critical, as from this point on the geotechnical stability to the ground is solely reliant on the support provided by the TBM. The soil improvement is executed by the provision of soil replacement, with a target to give the material a temporary cohesion or cementation that allows excavation of the ground underneath the structural protection slab without any risk of localized instability during TBM passage. The geotechnical property targets were designed to achieve a near homogeneous material over the full face of the TBM cutterhead, to facilitate optimal steering control of the TBM. Low strength material is preferred compared to higher strength values fo...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgements
• WTC 2019 Congress Organization
• Public communication and awareness
• Risk management, contracts and financial aspects
• Author Index