Shield Construction Techniques in Tunneling

By Kui Chen, Jiangka Wang and Shengjun Jiao

Shield Construction Techniques in Tunnelling presents the latest on this fast, environmentally-friendly and relatively safe construction technique, reflecting on its technical risks and challenges as seen in China. Sections introduce the type of shields, the history of the technique, shielding principles, selection, management, the latest techniques in operation, consider engineering cases, discuss construction in gravel, soft-soil, composite, and rock strata, and present video clips of construction that are accessible through QR codes embedded in the text. The book combines theory and practical experience, giving the reader unique insights into shield equipment and construction techniques.

The shield tunneling technique is being used very widely, particularly in China, which is building urban-rail transit systems at an unparalleled scale and speed. The use of tunneling-shields provides a fast, relatively-safe, and ecologically-friendly method for the construction of tunnels. However, a number of incidents have shown the risks involved in tunnelling through geologically complex areas.

• Gives the principles and practice of shield construction techniques, including shield selection and operation
• Demonstrates the latest technologies in shield construction that can be applied in practice
• Reflects on the technical risks and challenges of shield construction, based on extensive use of the technique for tunnel construction in China
• Discusses challenges in construction in gravel, soft-soil, composite and rock strata
• Provides engineers with applicable insights into shield equipment and construction techniques

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 5, 2021
• ISBN: 9780128201282

Kui Chen

Professor-level, Senior Engineer, National Constructor, and Executive Deputy Director of the National Key Laboratory of Shield and Tunnelling Techniques. He is Adjunct Professor of Shijiazhuang Railway University, North China University of Water Resources and Electric Power, and Henan University of Science and Technology. He has run numerous national projects in China, and collaborated internationally. He is one of the founders of the shield industry in the country. He was awarded first prize from the National Science and Technology Progress Awards for his work in the area, and is a leader in the field.

Book preview

Shield Construction Techniques in Tunnelling

Kui Chen

Jiangka Wang

Shengjun Jiao

Table of Contents

Cover image

Title page

Copyright

Foreword

Preface

Introduction to the second edition

Chapter 1. Introduction to shield machines

1.1. Shield machine and its working principles

1.2. Classification of shield machine

1.3. Introduction of typical shield machines

Questions

Chapter 2. Development and applications of shield machines

2.1. The origin and development of shield machines

2.2. The development and application of shield machines in China

2.3. Industrial model of shield machine

2.4. The development tendency of shield machines

2.5. New technology of shielded method construction[1]

2.6. The development directions of shield machine technology

2.7. Shield machine market prospects

Questions

Chapter 3. Introduction to the shield construction method

3.1. The main methods of tunnel construction[1]

3.2. Basic concept of the shield construction method

3.3. The main technical features of the shield method

3.4. The advantages and disadvantages of the shield method

3.5. Adaptation range of the shield method

3.6. The development history of shield method tunnel abroad[2]

3.7. The development history of shield method tunnel in China

3.8. Introduction of typical shield construction method

Questions

Chapter 4. Shield machine type selection

4.1. Overview

4.2. Principles of shield machine type selection

4.3. The basis of shield machine type selection

4.4. Main steps of shield machine type selection

4.5. Main methods of shield machine type selection

4.6. Selection of shield machine forms

4.7. Cutting wheel structure form selection

4.8. Types of cutting tools and rock-breaking mechanism

4.9. Cutting wheel drive mode selection

4.10. Calculations of main technical parameters

4.11. Selection of construction auxiliary equipment

Questions

Chapter 5. The construction of shield tunnel shafts

5.1. General requirements for shield tunnel shaft

5.2. The construction method of shield machine shafts

5.3. Open caisson shaft construction method

5.4. Construction method of retaining shaft

5.5. Example of vertical shaft engineering

Questions

Chapter 6. Shield method tunnel construction

6.1. Site assembly and commissioning of shield machines

6.2. Site acceptance of shield machine

6.3. Technologies for shield machine launching

6.4. Advance technologies of an EPB machine

6.5. The tunnelling technology of a slurry machine

6.6. Ring building technology[1]

6.7. Back grout filling technology

6.8. Construction measurement

6.9. Technologies for compressed air intervention

6.10. Inspection and replacement of cutting tools

6.11. Construction in special sections and special geologic conditions

6.12. Underground docking technology

6.13. Waterproof technologies of tunnel construction

6.14. U-turn technology of shield machines

6.15. Shield machine arrival technology

6.16. Secondary lining construction technology

6.17. Disassembly and packing of shield machine

6.18. Safety and health of shield construction

6.19. Environmental protection for shield construction

Questions

Chapter 7. Geotechnical problems of shield construction

7.1. Introduction

7.2. Stability issues of shield tunnelling working face

7.3. Geotechnical problems in shield tunnelling

7.4. Deformation and fluidity of soil layers under the tunnel

7.5. Geotechnical problems of shield machine exit/entry sections

7.6. Soil mass reinforcement technology

Questions

Chapter 8. Tunnel lining structure and construction

8.1. Tunnel lining structure

8.2. Primary lining

8.3. Secondary lining

8.4. The structure of segments

8.5. Segment design

Questions

Chapter 9. The technology of segment fabrication

9.1. The allocation of resources

9.2. Preparation work before fabrication

9.3. Main raw materials and standards

9.4. The production technology of segments

Questions

Chapter 10. The management and applications of shield machines (TBM)

10.1. The management of shield machines (TBM)

10.2. The use of shield machine (TBM) equipment

10.3. Shield machine (TBM) maintenance and repair

Questions

References

Index

Copyright

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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ISBN: 978-0-12-820127-5

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Typeset by TNQ Technologies

Foreword

Shield Construction Technology (Second Edition, in China) describes the current situation and prospect of shield machine development, shield machine type selection, shield construction technology, and use and management of shield equipment, and it mainly studies and summarizes the latest technology of shield construction in a sandy pebble stratum represented by the Beijing region, a soft soil stratum represented by the Shanghai region, a composite stratum represented by the Guangzhou region, and a rock stratum represented by the Chongqing region. This combined research and summary is a new achievement not only in China but also in the field of shield tunnel construction in the world. It will play a valuable guiding and reference role in future shield tunnel engineering, shield machine design, and shield construction (especially in China).

The reprinting of this book also reflects the rapid development in the field of shield construction in China. Up to now, hundreds of kilometers of shield projects have been completed in Guangzhou, Beijing, Shanghai, Chongqing, and other places. The large-scale implementation of these projects has greatly enriched and improved China's shield construction technology under various complicated stratum conditions, laying a solid foundation for the future application of shield construction under various complicated stratum conditions.

This book is a rare technical reference book, which can be used as an important reference for colleagues engaged in shield construction and management.

As the book is about to go to press, on behalf of Herrenknecht AG, I would like to express my heartfelt thanks to the authors of the book, China Railway Tunnel Group, and all relevant units, engineers, and technicians involved in the subway construction in Guangzhou, Beijing, Shanghai, and Chongqing.

Chairman of the board of directors of Herrenknecht AG

Preface

The 21st century is an era of great development of tunnels and underground space. China, as the largest market of tunnels and underground engineering constructions in the world, has broad prospects. At present, China's urban rail transit is facing an unprecedented construction climax. As of 2015, there were 39 cities across the country constructing subways. Beijing, Shanghai, Guangzhou, Chongqing, and other cities are still advancing at an investment rate of tens of billions of yuan per year upon opening and operating a number of subway lines. Taking Beijing as an example, its 2020 urban rail transit network consists of 30 lines with a total length of 1177 kilometers. The long-range network consists of 35 lines with a total length of 1524 kilometers. From 2015 to 2021, 12 projects with a total length of 262.9 kilometers and a planned total investment of 212.28 billion yuan will be constructed.

With the improvement of the legal system and regulations in the field of construction engineering in China, the requirements for the comprehensive benefits of construction engineering projects, and the improvement of environmental protection awareness, the requirements for construction technology and management are increasing day by day. Shield tunnelling is generally required in busy urban areas with dense buildings and special geologic terrain sections. Judging from the engineering practice of subway construction in Beijing, Guangzhou, and other places, with the continuous improvement of domestic shield construction technology and domestic shield technology in recent years, shield tunnelling has shown strong advantages and has been applied more and more.

Shield tunnelling has the advantages of little impact on the surrounding environment, a high degree of automation, rapid construction, high quality, high efficiency, safety, and environmental protection. With the development and maturity of shield construction technology in long distance, large diameter, large buried depth, complex geology, and complex cross-section, shield tunnelling has attracted more and more attention and favor. Especially in poor stratum conditions, complex geology, and high groundwater level, shield method has much more obvious advantages. Shield method construction has its own unique technical characteristics. Shield machines are different from conventional equipment, being tailor-made for specific construction applications. The design and construction of shield machines must be closely combined with engineering geology and matched with engineering quantity and economic rationality to give full play to the advantages of shield method in safety, quality, and speed.

This book is reprinted after comprehensive revision based on Shield Construction Technology (First Edition∗). It comprehensively and thoroughly expounds the working principle, type selection, management, and latest technology of shield construction, and it closely combines shield tunnel engineering practices in Beijing, Shanghai, Guangzhou, Chongqing, and other regions and truthfully introduces valuable experiences and lessons of shield construction technology under different geologic conditions. The theory of this book is closely linked with practice, with illustrations and pictures, and the visual effect is dramatic. The content explains the depth of this subject in an easy to understand way, showing the latest technology of shield construction and highlighting its application. It is a masterpiece in the field of shield construction technology and has good guidance and reference for domestic shield construction.

On the occasion of reprinting this book, I would like to extend my sincere greetings to the engineers, technicians, and construction enterprises who have made outstanding contributions to the development of tunnels and underground engineering in China for many years. I would also like to express my gratitude to the editors and authors for their hard work in reprinting this book.

I recommend this book to the personnel engaged in shield machine design, construction, project management, teaching, scientific research, and other work and to general readers. I believe that the publication of this book will definitely play a positive role in promoting the improvement of shield construction technology in China.

Wang Mengshu

Academician of Chinese Academy of Engineering

---

∗ 

This is the first edition of the book published outside of China; it has published as a second edition within China.

Introduction to the second edition∗

The great development of tunnels and underground space has promoted the progress of shield construction technology. The history of shield construction began in Britain and was developed in Japan and Germany. Since 1825, when Brunel began to build the world's first shield tunnel under the Thames River in London, shield construction has had a history of 194 years (2020). China began to use shield construction in the 1950s to build tunnels. Although it started relatively late, it has developed rapidly due to the attention paid to absorbing and adopting advanced technologies and new processes, and referring to and drawing lessons from foreign successful experiences and failures. However, there is still a big gap between China and technologically advanced countries.

Although shield construction has the advantages of fast tunnelling speed, excellent quality, little impact on the surrounding environment, and relatively high construction safety, due to the complexity of shield construction, plus improper type selection and construction methods, engineering accidents often occur. Shield construction is always at risk of thin, weak links such as the complexity of the geology, the inadaptability of shield machines, the limitation of human cognition, the inappropriateness of the scheme and the measures applied, etc. To reflect the new techniques and methods of shield construction as soon as possible, and to make full use of the technical advantages of shield construction, China Railway Tunnel Group Co., Ltd., organized many experts in the field of shield technology to write the book Shield Construction Technology in 2009. Since its publication, the book has been printed five times with a circulation of more than 10,000 copies and is deeply loved by shield machine technicians.

To fully grasp the latest development of shield technology in recent years, the State Key Laboratory of Shield and Tunnelling Technology once again organized relevant technical experts in the field of shield construction to comprehensively revise the Shield Construction Technology published in 2009.

The book is divided into 15 chapters, trying to explain the depth of principle, type selection, management, and latest technology of shield construction in a comprehensive and thorough way. It focuses on the respective characteristics and practical examples of shield (TBM) projects in several representative state-owned cities with typical geologic conditions in China. The specific examples include shield construction in sandy pebble stratums represented by Beijing, shield construction in soft soil stratums represented by Shanghai, shield construction in composite stratums represented by Guangzhou, rock stratum rock tunnelling machines (TBM) represented by Chongqing, and shield construction. The book is illustrated with text and pictures, has visual impact, and is informative and referential.

At present, China's urban rail transit construction is facing an unprecedented climax. As of 2015, 39 cities across the country were building subways. Beijing, Shanghai, Guangzhou, Chongqing, and other cities are still investing tens of billions of yuan per year on the basis of opening and operating several subway lines. Taking Beijing as an example, Beijing's urban rail transit 2020 network consists of 30 lines with a total length of 1177 kilometers. The long-dated planning network consists of 35 lines with a total length of 1524 kilometers. From 2015 to 2021, 12 projects with a total length of 262.9 kilometers and a total planned investment of 212.28 billion yuan will be constructed. This book mainly focuses on the construction technology of shield tunnels in urban subways, and it has important reference value for municipal, highway, and railway tunnel construction.

The Academician of the Chinese Academy of Engineering, Mr. Mengshu Wang, was invited to serve as the consultant of the editorial board. Kui Chen, Executive Director of the State Key Laboratory of Shield and Tunnelling Technology, Jiangka Wang, Deputy General Manager of China Railway Guangzhou Investment Development Co., Ltd., and Shengjun Jiao, President of Shaanxi Railway Institute, Shaanxi, China, were appointed as chief editors.

In the process of writing this book, relevant personnel such as the China Railway 1st Bureau, China Railway 16 Bureau, Beijing Urban Construction Group, Beijing Residential General Group, and Guangzhou Metro Corporation, respectively, provided some parameters and construction experience of shield machines used in the construction of the Beijing and Guangzhou subways. Mr. Guoqin Pan of Shanghai Urban Construction and Design Institute provided the construction data of Shanghai shield tunnels. Mr. Guanjun Zhang of Shanghai Tunnel Engineering Co., Ltd., provided the data of rectangular tunnels and put forward many pertinent opinions or suggestions on the relevant contents of this book. Here are many thanks to all.

Despite giving our best efforts, there may still be many shortcomings and even mistakes in this book, so we earnestly request the reader's criticism and pointing out the mistakes.

State Key Laboratory of Shield Machine and Tunnelling Technology

---

∗ 

This is the first edition of the book published outside of China; it has published as a second edition within China.

Chapter 1: Introduction to shield machines

Abstract

This chapter introduces the shield and its working principle, shield classification, and other contents, focusing on the open shield, compressed air shield, slurry balance shield, earth pressure balance shield, and composite shield, to facilitate readers in understanding the working principles and characteristics of various types of shield machines.

Keywords

Composite shield; Open shield; Shield type; Slurry balanced shield

1.1 Shield machine and its working principles

1.2 Classification of shield machine

1.2.1 Classifications according to excavation face

1.2.2 Classifications according to boring diameter

1.2.3 Classifications according to the supporting method of excavation face

1.2.4 Classification according to the structure between excavation face and the bulkhead of working chamber

1.3 Introduction of typical shield machines

1.3.1 Open-type shield machine

1.3.1.1 Overview

1.3.1.2 Hand-dug shield

1.3.1.3 Semimechanized shield

1.3.1.4 Mechanized shield

1.3.1.5 Extrusion shield

1.3.2 Compressed air shield

1.3.3 Slurry shield

1.3.3.1 The composition of a slurry shield

1.3.3.2 Excavation surface stability mechanization

1.3.3.3 Geologic range of adaptation

1.3.4 Earth pressure balanced shield

1.3.4.1 Overview

1.3.4.2 Basic configuration

1.3.4.3 Stabilization mechanism of excavation face

1.3.4.4 The range of geologic adaptation

1.3.5 Composited shield machine

1.3.5.1 Overview

1.3.5.1 Structure features

1.3.6 Earth pressure balanced shield machine for composited ground

1.3.6.1 Advancing in open mode

1.3.6.2 Advancing in semiopen mode (compressed air mode)

1.3.6.3 Advancing in EPB mode

Questions

1.1. Shield machine and its working principles

A shield machine, also called a shield construction machine in China, is a type of construction machine applied for underground tunnel excavation. It has a metal shell, and components and auxiliary equipment are installed within; under the protection of the shell, the machine carries out soil excavation, muck discharging, machine advancing, segment erection, and other works, and it is used to build the tunnel once formed, as shown in Fig. 1-1.

A shield machine is one kind of special engineering machinery for tunnel excavation. A modern shield machine integrates technologies of mechanics, electrics, hydraulics, sensing, and IT, and it has functions of soil excavation, muck transportation, segmental lining, navigation, alignment correction, etc. Shield machines have been widely used in tunnelling projects of subways, railways, highways, municipal projects, and hydropower engineering.

The working principle of a shield machine is that a steel structure assembly does forward advancing along the tunnel alignment while excavating soil. The shell of the steel structure is called a shield shell and the shield shell plays the role of temporarily supporting unlined tunnel section, bearing earth pressure of surrounding soil and water pressure of underground water, and keeping underground water out. Excavation, discharging of soil, tunnel lining, and other works are carried out under the protection of the shield shell.

Shield or protection refers to the shield shell.

Construction building refers to segment installation.

The means of excavation face stability is the main feature of the working principle of a shield machine, and it is the main difference between a shield machine and a hardrock tunnel boring machine (TBM). In China, normally, only a hardrock tunnel boring machine is called a TBM, and the common definition of TBM is full-face rock tunnel boring machine, where its excavation object is rock formations. The main difference between a rock TBM and a shield machine is that it does not have those functions of supporting tunnel face by bearing slurry pressure, earth pressure, or other means; on the contrary, shield machine construction is mainly composed of the three elements of supporting tunnel face, excavation and soil discharging, segment lining and backfill grouting.

Figure 1-1  The shape and structure of a shield machine.

1.2. Classification of shield machine

1.2.1. Classifications according to excavation face

According to the shape of excavation face, a shield machine can be divided into single-circle shield machine, dual-circle shield machine (multicircle shield machine), and noncircular shield machine (Figs. 1-2–1-4). A multicircle shield machine can be divided into elliptical shield machine, rectangular shield machine (Fig. 1-5), rectangular-like shield machine, horseshoe shield machine (Fig. 1-6), semicircular shield machine, and dual-circular shield machine. Dual-circular shield machine and noncircular shield machine are collectively called abnormity shield machine.

1.2.2. Classifications according to boring diameter

According to the different boring diameters of the shield machines, shield machines can be divided into the following categories: boring diameter from 0.2 to 2  m, called microshield machines; boring diameter from 2 to 2.4  m, called small shield machines; boring diameter from 4.2 to 7  m, called medium shield machines; boring diameter from 7 to 12  m, called large shield machines; and boring diameter above 12  m, called super-large shield machines.

Figure 1-2  Single-circle shield machine.

Figure 1-3  Dual-circle shield machine.

Figure 1-4  Triple-circle shield machine.

Figure 1-5  Rectangular shield machine.

Figure 1-6  Horseshoe shield machine.

1.2.3. Classifications according to the supporting method of excavation face

According to the supporting method of excavation face, shield machines can be mainly divided into five types: natural supporting, mechanical supporting, compressed air supporting, slurry pressure balanced, and earth pressure balanced; see Fig. 1-7.

1.2.4. Classification according to the structure between excavation face and the bulkhead of working chamber

According to the structure between excavation face and the bulkhead wall of the working chamber, shield machines can be divided into three types: fully opened, partially opened, and closed type, where the specific division is referenced in Fig. 1-8.

1.3. Introduction of typical shield machines

1.3.1. Open-type shield machine

1.3.1.1. Overview

Open-type shield machines are divided into fully opened and partially opened types. Fully opened shield machines have no closed pressure compensation system to the tunnel excavation face and cannot resist earth pressure and underground water pressure. According to the excavation methods, fully opened shield machines are divided into the following types:

(1) hand-dug shield

(2) semimechanized (partial section excavation) shield

(3) mechanized (full-face excavation) shield

The fully opened shield, also known as opened working face shield machine, is known by the English name open-face shield machine or OF shield machine. The fully opened shield is generally applicable to surrounding rocks that have good self-stability in the tunnel face. If the natural stability of the construction formation is insufficient, mechanical methods must be applied for stabilizing the rock formation. When the fully opened shield is excavating in formations below the water table or in permeable conditions, the underground water level must be lowered by well-point method, and the foundation can be treated by grouting and freezing method. A fully opened shield machines is suitable for a variety conditions in nonsticky and viscous ground. Its advantage is that it can also be used when the excavation face is partially or fully composed by rocks or boulders. Furthermore, noncircular face excavation is possible by hand or semimechanized method.

Figure 1-7  Classifications according to ground supporting method.

Figure 1-8  Classification of shied machines.

The partially opened shield is also known as the common closed-face shield (CF shield for short), or common extruded shield machine. There are two main types:

① Bulkhead wall is completely closed in front, extruding while advancing; reserve area adjustable soil inlet port, partially extruding and advancing.

② Completely covered grid plates or partially covered by sealing plates in front, adjustable gate may be installed at excavation area, extruding or partially extruding while advancing.

1.3.1.2. Hand-dug shield

Hand-dug shield means a shield machine excavates manually at the tunnel face. A hand-dug shield is a basic form of shield machine. It is open in the front, and tunnel excavation is carried out manually by shovels, air picks, stone crushers, and other excavation tools. The supporting methods of excavation face are normally by means of natural soil piling pressure support and mechanical support plates. According to different geologic conditions, the tunnel face can be fully opened and excavated manually; also, it can be fully or partially supported in front, and to conduct excavation in layers properly according to the self-stability of soil mass, tunnel excavation is followed by tunnel supporting. Excavated soil volume is the total discharged volume of tunnel. This type of shield is convenient to observe ground formations and to remove obstacles, is easy to correct deviation of tunnel alignment, and is simple and economical, but on the other hand, it is high labor intensity and low efficiency; and in case of collapse of the tunnel face, it can easily endanger workers and project safety. In water pressure–bearing ground, it needs to be supplemented with lowering water level, air pressurizing, or soil reinforcement.

This type of shield carries out excavation from top to the bottom, and sequentially changes front supporting jacks during excavation; excavated soil is loaded to the discharging truck by belt conveyor in the invert area. The basic condition of using this type of shield is that there is at least no collapse phenomenon during tunnel excavation, because the front of the shield machine is opened during excavation.

Fig. 1-9 shows a ϕ10.92  m Mitsubishi hand-dug shield with hydraulic telescopic work shelves and chest plate for mechanical support to the working face.

Hand-dug shields can be applied to soil layers from sandy to viscous soil, so they are suitable for relatively complicated ground formations. When an obstacle appears in the excavation face with this type of shield, it can be easily removed because the front is opened. This type of shield is the most economical one because of its low cost and low failure rate.

Because of lower excavation speed of the hand-dug shield, high labor intensity, and high labor costs, this type of shield has been almost obsoleted in developed countries, and it has only been used in exceptional cases, such as short distance excavation (mechanized or semimechanized shield is not economical in this case), obstacles or giant boulders in excavation face, and other occasions.

In countries with undeveloped technology and low labor costs, a hand-dug shield is also used for excavation of long tunnels. Without auxiliary measures, a hand-dug shield is only applicable to sufficiently self-stable surrounding rock in the excavation face; for unstable surrounding rock and permeable formations, it can be used with auxiliary construction methods, such as compressed air, lowering underground water level, chemical grouting, and other means of ground stabilization. According to actual conditions, compressed air construction method, ground treatment, lowering underground water level, and other measures can be performed during construction processes.

Figure 1-9  A Mitsubishi φ10.92   m hand-dug shield.

The hand-dug shield is not necessarily a circular section; rectangular or horseshoe sections are also possible.

1.3.1.3. Semimechanized shield

Due to the very low excavation speed and poor working conditions of a hand-dug shield, the semimechanized shield was developed. A semimechanized shield is a form between a hand-dug shield and mechanized shield, but closer to the hand-dug shield. On the base of an open shield, devices of soil excavation and discharging are installed for replacing manual labor, so a semimechanized shield has man-power savings, high efficiency, and other characteristics. Mechanical soil excavation devices can move back and forth, left and right, and up and down. It could be an excavator type, roadheader type, or exchangeable between excavator and roadheader, or combination of excavator and roadheader. The shield top is the same as a hand-dug shield, equipped with an active forward shield, face supporting jacks, and so on.

Excavation and discharge of semimechanized shields (Fig. 1-10) are carried out by special machines, such as hydraulic excavators or roadheaders and other excavation machines, and they are equipped with belt conveyor or screw conveyor and other discharge machinery, or equipped with dig-load machinery that has both functions of digging and discharging. The safety of workers must be fully considered and ensured during construction, and low-noise equipment shall be used. To prevent collapsing of the excavation face, the shield machine shall be equipped with an active forward shield and half-moon-shaped jacks; usually hydraulically operated chest plates are used, where chest plates are placed in a separate area or are used as auxiliary support to the tunnel working face in the surrounding area of the shield skin. A semimechanical shield is suitable for soil mainly consisting of diluvium sand, gravel, consolidated silt, or clay. It can also be applied for soft diluvium, but compressed air construction method shall be applied as well, or other auxiliary measures such as lowering groundwater level and ground treatment.

Figure 1-10  Semimechanized shield.

Figure 1-11  ECL shield and construction.

The excavation devices of the semimechanized shield have the following forms:

① equipped with excavator and roadheader, etc., in the lower half of the working face;

② equipped with excavator in the top half of the working face and roadheader in the lower half;

③ equipped with roadheader in the center;

④ equipped with excavator in the center.

A semimechanized shield is more suitable for good formation than a hand-dug shield. Form ① is suitable for formations where the excavation face needs to be supported, and forms ② to ④ are suitable for self-stable formations. Form ② is mostly suitable for mezzanines of subclay and gravel. Form ③ is mostly suitable for consolidated clay or hard sand layers. Form ④ is mostly suitable for clay and mixed gravel layers.

A semimechanized shield is also suitable for excavating noncircular section tunnels. The shield shown in Fig. 1-11 is an ECL shield machine used by the Japanese railway construction company Takasaki Construction Bureau for the construction of Hokuriku Shinkansen. The tunnel section is horseshoe-shaped, the tunnel length is 3580  m, and the soil is soft to medium-hard rock. An extruded concrete lining was synchronized during tunnel excavation.

ECL is an abbreviation for extruded concrete lining, which replaces traditional segment lining with site cast-in-situ concrete. ECL shield construction method does excavation and lining at the same time, without using conventional segments, but extrudes concrete into the void between rock and inner molds at the time of advancing to build a concrete lining combined closely with the surrounding rock. Because of prompt cast-in-situ concrete lining, segment installation, simultaneous backfilling, and other works of conventional shield construction are not necessary anymore.

1.3.1.4. Mechanized shield

When the ground formation is stable itself or after using auxiliary measures, a cutting wheel that is suitable for the shield diameter can be installed to carry out full-face opened mechanized excavation. The mechanical shield (Fig. 1-12) is a type of shield machine that uses a rotating cutting wheel and is closely attached to the excavation face for full-face excavation; known as the fully opened mechanized shield machine, it is equipped with a rotating cutting wheel in the front, which increases the excavation capacity of the shield machine. The excavated materials are loaded onto a belt conveyor by rotary buckets and chutes. The tunnel excavation and soil discharging can be continuously carried out. It is suitable for ground with the same conditions as hand-dug and semimechanical shields.

Figure 1-12  Mechanized shield.

The excavation mechanism of mechanized shields is mostly using a cutting wheel, which could be single-axis form, doubled rotation form, multiaxis form, etc., and single-axis form is the most popular one.

In addition to improving working environment and labor savings, a mechanized shield can also significantly improve advance speed and shorten construction duration. Compared to hand-dug and semimechanized shields, the cost of the mechanized shield is higher. If the tunnel is short, it would not be economical.

1.3.1.5. Extrusion shield

An extrusion shield is also known as blind-type shield in Japan. An extrusion shield creates strong disturbance to the surrounding soil while extruding and advancing, and large changes of upwarping and settlement of ground surface can also easily occur, so it is not suitable for areas where there are buildings on the ground surface.

An extrusion shield is only suitable for soft clay and silt ground formations with poor self-stability and high liquidity, and it is not suitable for high sand content ground formations and hard ground formations. If the liquidity index is too high and the fluidity is too large, then a stable excavation face cannot be obtained. Because applicable geology range is so limited, it is rarely used at present. The types of extrusion shield are mainly cover plate, screw-discharged, and grid-extrusion types.

Figure 1-13  A Mitsubishi ϕ6.32   m extrusion shield.

(1) Cover plate extrusion shield

The excavation face is completely closed by cover plates, and only a small area is covered by an adjustable discharge plate. The front of the shield structure intrudes into soil and moves forward, and the soil that is pushed by the shield becomes plasticized and flowable and then is discharged through an adjustable plate. The stability of the excavation face is realized by balancing the thrusting force of jacks and earth pressure of the excavation face with adjusting the opening size of the cover plate and soil resistance of discharging. Fig. 1-13 shows a Φ6.32  m extruded shield of Mitsubishi.

(2) Screw conveyor discharged extrusion shield

The excavation face is closed by using sealing plates, and the front of the shield penetrates into the ground formation and advances forward, where the soil that is pushed by the shield becomes plasticized and flowable and is then discharged by screw conveyor. The stability of the excavation face is achieved by balancing the thrusting force of jacks and earth pressure of the excavation face with adjusting rotation speed of the screw conveyor and the opening rate of screw conveyor discharge gate. The principle is shown in Fig. 1-14.

(3) Grid-extrusion shield

A grid-extrusion shield is often used in soft soil formations in Shanghai. Its feature is that the incoming soil volume is close to or equal to the discharged soil volume, and it often has a character of partial extrusion. A steel plate grid is installed in the font of the shield, so it can cut soil during advancing and stabilize the excavation face when advancing is stopped. The cut soil can be transported out by turning-plate, belt conveyor, skips, or hydraulic machinery. With careful construction in suitable ground formation, the settlement of ground surface can be controlled to medium or minor levels by using this type of shield machine. In case of constructing in ground formations with water content, dewatering needs to be supplemented.

Figure 1-14  Extrusion shield.

A grid-extrusion shield extrudes the front soil mass into small pieces by using the grid in the cutting area of shield, and it balances the side pressure of front soil formation with the friction resistances between the side surfaces of the cutting ring, closing plates, grid plates, and the soil mass to achieve stability of the excavation face. It has advantages such as a simple structure, it is easy to operate, and has convenience of obstacle removal in the front.

A grid-extrusion shield has small openings in the front grid, and it is suitable for construction in soft clay layers, when the excavation face has partial fine sand layers, and the front soil mass can be stabilized by means of local pressure method in the excavation chamber. Depending on the means of soil discharging, grid-extrusion shields can be categorized into two types: dry discharged and hydraulic discharged. Fig. 1-15 shows a hydraulic-type grid-extrusion shield.

Figure 1-15  Hydraulic-type grid-extrusion shield.

1.3.2. Compressed air shield

It has a long history where compressed air is used to suppress intrusion of underground water. As early as in 1828, when Isambard Brunel encountered intrusions of large volumes of water during the construction of the Thames Tunnel, Calladon had proposed the use of compressed air. In 1886, this method was introduced in shield construction for the first time by James Greathead.

The principle of a compressed air shield is that the static pressure of underground water is balanced by air pressure, so it is also known as the air pressure balance shield, or APB shield. But air pressure cannot directly oppose soil pressure, so soil pressure is borne by natural or mechanical support.

A compressed air shield is suitable for clay, sandy clay, and soft ground formations with water content. This type of shield machine includes all shield machines that use compressed air as supporting material, and it can be hand-dug or mechanical form, and the excavation sections can be partial or full face. In an early time, when a compressed air shield is under construction, a long working chamber shall be sealed between excavation face and waterproof tunnel section during tunnelling, so most workers are often in compressed air. The later developed compressed air shield is that where only the excavation chamber bears the pressure, which is known as a local air pressure shield; in Japan, it is known as limited compressed air shield This kind of shield is equipped with a sealing bulkhead that can seal up a pressurized working chamber and isolate it from the completed tunnel section, and equipment can be safely operated under atmospheric pressure. Fig. 1-16 shows a ϕ5.25  m compressed air shield of Mitsubishi. This ϕ5.25  m compressed air shield discharges soil via a ball valve–type rotary hopper, and it maintains the stability of excavation face pressure at the same time. Fig. 1-17 shows a site photo of soil discharge by a ball valve–type rotary hopper.

Figure 1-16  ϕ5. 25   m compressed air shield.

Figure 1-17  Soil discharge by a ball valve–type rotary hopper.

The pressure of compressed air should be higher than or equal to the water pressure at the bottom of the working face because water pressure has a distinct gradient, so the surplus pressure at the top forces the air entering the ground formation. When soil particles are out of balance due to air flow, the working face with shallow overburden may be leaking and lead to an eruption which may cause disastrous consequences. The compressed air shield has been replaced by the slurry shield because of the risk of eruptions and poor working conditions.

1.3.3. Slurry shield

1.3.3.1. The composition of a slurry shield

The slurry shield is also known as a slurry pressure balanced shield and called SPB shield for short. A slurry shield is a mechanical type shield which is installed with a bulkhead wall at the front, and equipped with a cutting wheel, feed and discharge pipelines for transportation of slurry and thrust cylinders for the propulsion of the shield. Slurry treatment facilities are installed on the ground surface.

A slurry shield includes the following five systems:

① excavation system, which excavates the entire excavation face using a cutting wheel while advancing;

② slurry circulation system, which can adjust the properties of slurry and transport it to the excavation face and maintain the stability of the excavation face;

③ integrated management system, which can manage the state of slurry discharge, slurry pressures, and the operation of slurry treatment facilities synthetically;

④ slurry separation system;

⑤ synchronous grouting system.

A slurry shield adjusts and controls the pressure of slurry by circulating the volume of suspension liquid and uses bentonite suspension (commonly known as slurry) as supporting material. Slurry is pumped into an excavation chamber to form impermeable filtercake on the excavation face, so earth pressure and water pressure on the excavation face are balanced by the tensile force of the filtercake, and then the stability of the excavation face is maintained. Excavated material is transported to the ground surface in slurry form, which will be separated by slurry treatment facilities, and then its quality will be regulated before transporting to the excavation face again.

The development of slurry shield has three technical development systems, i.e., the Japanese system, the British system, and the German system. Until now, there have been only two major development systems, i.e., the Japanese and the German systems. Based on the Japanese slurry shield, an earth pressure balanced shield was developed, while the German slurry shield pushed development of the mix shield. The major difference between German and Japanese systems is that a compressed air chamber is set up in the slurry chamber in the German system, but it is all slurry in a Japanese system.

(1) The Japanese system

Direct controlled type of slurry shield (Fig. 1-18) is generally used in Japan. Slurry balance mode is employed for the slurry system of direct controlled slurry shield. The procedure is as follows: fresh slurry is pumped into the slurry chamber of the shield machine from a slurry regulation tank on the surface by feed pump(s), mixed with excavated material to become a thick slurry, and then transported to a slurry separation plant on the surface by discharge pump(s); after separation and disposal of soil slag, thin slurry flows into a regulation tank and is pumped into slurry circulation again after adjusting its density and concentration. The slurry pressure of the slurry chamber can be regulated by adjusting the revolving speed of the feed pump or the opening of control valves. Because the feed pump is normally installed on the ground surface, the long controlling distance generates a delayed effect, and it is inconvenient to control the slurry pressure in the slurry chamber, so adjusting the opening of the control valve is often used for slurry pressure adjustment.

Figure 1-18  Direct controlled type of slurry shield(the Japanese system).

(2) The German system

Indirect controlled type of slurry shield (Fig. 1-19) is generally used in Germany; its working character of slurry system is combined pressure controlled by dual circuits of slurry and compressed air, so it is also known as the D mode or air pressure composited mode.

Figure 1-19  Indirect controlled type of slurry shield(the German system).

Air pressure composited mode slurry shield is installed with a bulkhead with an opening on the bottom in a slurry chamber, and it is filled with pressurized slurry in front of and the bottom half behind the bulkhead, where the remaining half behind the bulkhead is filled with compressed air, which forms an air buffer, and air pressure acts on the contact surface between air and slurry behind the bulkhead. Because the air and slurry pressure on the contact surface are the same, the corresponding slurry support pressure can be determined and maintained on the excavation surface by adjusting the pressure of compressed air. When a shield machine is advancing, sometimes, due to the loss of slurry, or the change of advance speed, the slurry volumes of feeding and discharging will lose balance, so the air–slurry contacting face will appear as a fluctuation phenomenon up and down. To maintain the stability of the slurry pressure in the excavation face, the revolving speed of the slurry feed pump needs to be operated according to the change of slurry level to restore slurry level back to the set position. In other words, the output volume of the slurry feed pump increases when the level of slurry decreases, but it decreases when the level of slurry increases. In addition, the position limiting switches are set in the highest and lowest levels of slurry. When slurry level reaches the highest position, the slurry feed pump stops; when slurry level drops down to the lowest position, the slurry discharge pump stops. Due to the elasticity effect of the air buffer, when the slurry level fluctuates, there is no obvious impact on the pressure of the supporting slurry.

Compared to direct controlled slurry shield, the controlling operation of indirect controlled slurry shield is simpler, and the support of the soil on the excavation face is more stable and more beneficial for controlling ground settlement.

1.3.3.2. Excavation surface stability mechanization

(1) The mechanism of filtercake formation

For a slurry shield, filtercake on the excavation face is formed via a properly pressurized slurry in the slurry chamber and supports the soil mass of the excavation face. The cutting wheel cuts off the filtercake of the soil face to mix with slurry and then becomes a high-density slurry that is transported to the ground surface by discharge pump and pipeline for separation treatment.

In the theory of slurry balancing, the formation of filtercake is essential. When slurry pressure is higher than the pressure of the groundwater, slurry will permeate into the soil according to Darcy's law, and it forms a certain proportion of suspension particles among soil clearances, where those particles will be captured and accumulated on the contact surface between soil and slurry, and then filtercake is formed. The thickness of filtercake will increase as time goes by, and the resistance of permeation is gradually enhanced. When the resistance of slurry is far higher than the frontal soil pressure, the balance effect of slurry forms.

(2) Basic elements of filtercake formation

The mechanism of excavation face stabilization during the construction of a slurry shield is as follows: slurry pressure resists earth pressure and water pressure of the excavation face for maintaining the stability of the excavation face and controlling the deformation of the excavation face and the settlement of ground foundation; impermeable filtercake is formed on the excavation face to ensure that slurry pressure effectively acts onto the excavation face. From the theory of slurry balancing, it is obvious that forming impermeable filtercake as soon as possible is a very important step in slurry shield construction.

In the excavation face, with constant penetration of pressurized slurry into the soil mass, the fine particles of slurry fill out soil clearances, so impermeable filtercake is formed. Since the pressure loss of the excavation face is reduced because of formed filtercake, the slurry pressure can effectively act on the excavation face, so deformation and collapse of the excavation face is prevented, and the stability of the excavation face is ensured. Therefore, controlling the pressure of earth and water and controlling the quality of slurry are two important subjects in slurry shield construction.

To maintain the stability of the excavation face, filtercake must be formed quickly and reliably, so slurry pressure can act effectively on the excavation face. Therefore, slurry should have the following characteristics:

① The density of slurry: To maintain the stability of the excavation face, the deformation of the excavation face should be controlled to the minimum, so the density of slurry should be relatively high. Theoretically speaking, the increase of slurry density can make the yielding value of slurry increase, in the meantime enhancing the stability of filtercake. Experiments prove that slurry with high density can produce high-quality filtercake, and the best slurry density is the same density of the excavated soil. However, a high density of slurry will cause overload of slurry pump(s) and difficulties of slurry treatment; although a low density of slurry will reduce the load of slurry pump(s), but the filtercake forming will be slow because the water permeability of soil is increased, which is unfavorable for the stability of the excavation face. Therefore, while selecting the density of slurry, earth formation structure must be fully considered. The capacity of equipment should also be considered while ensuring the stability of the excavation face.

② Sand content: In highly permeable soil mass, the speed of filtercake formation has a close connection with the maximum particle size of sand and sand content (weight of sand/weight of clay) of slurry because sand particles have the function of filling the clearances of a soil mass. To fulfill its role, the particle size of sand shall be bigger than the clearance in the soil mass, and sand content shall be moderated.

③ The viscosity of slurry: Slurry must have proper viscosity for the following effects: to prevent settlement of clay and sand particles of slurry at the bottom area of the slurry chamber, to maintain the stability of the excavation face, to increase the viscosity and resistance for avoiding loss of slurry, to transport excavated soil as fluid mixture, and separate it via slurry treatment facilities.

④ Slurry pressure: Once the soil mass is excavated by a shield machine, its original stress is released, and that will produce deformation of the excavation face toward the direction of stress releasing. At this time, to control the settlement of ground and maintain the stability of the excavation face, an equivalent force of the releasing stress shall be placed onto the excavation face. For a slurry shield machine, slurry pressure is used for neutralizing the stress release of the excavation face.

The permeated volume increases along with the increase of slurry pressure, but its increase is much less than the increase of pressure, and increase of slurry pressure will increase the effective supporting pressure to the excavation face. Therefore, under the condition of high-quality slurry in the excavation face, increasing slurry pressure will improve the stability of the excavation face. While making a decision on slurry pressure, water pressure, soil pressure, and reserved pressure of the excavation face should be primarily considered.

(3) The interactions between advance speed and filtercake

During normal advancing of a slurry shield, cutting tools do not cut soil mass directly, but cut the filtercake formed in front of the cutting wheel. At the moment right after one layer of filtercake is cut, another layer of filtercake is formed. Because the rotating speed of the cutting wheel is in a certain range of value, and the maximum thrust speed of shield machine is also limited to a certain value, the advance speed is only related to the cutting depth into the soil mass; it is not related to the filtercake. However, during abnormal advancing of a slurry shield, especially when the quality and pressure of slurry fail to meet the designed requirements, it will take a long time to form filtercake, which will limit the advance speed of a shield machine. With high-quality slurry, time for forming filtercake is about 1–2  s.

1.3.3.3. Geologic range of adaptation

The slurry shield was originally designed to use in special formations with interlaced alluvial clay and diluvial sand, and due to the obvious effect of slurry to the excavation face, it is also applicable in all soft silt, loose sand, gravel, and pebble gravel layers and interlayering of gravel and hard soil.

At present, the geologic application range of the slurry shield method is expanding, even in deteriorative construction environments, with existence of underground water or under other undesirable conditions, because of the existence of corresponding treatment methods, allowing slurry shield to adapt to almost all kinds of ground formations.

(1) Viscous soil

Viscous soil is formed from the minerals of clay after a mutually electrochemical combination. Its appearance looks like metamorphic agar block, and the force from the gravity and pressure of slurry easily forms the stability of an excavation face, no matter how weak the clay layer is, so it is suitable for slurry shield construction method. Slurry shield is also applicable for construction in silty soil formation.

(2) Sand layers

Because of the leakage of slurry in dry sand layers, keeping the pressure and stability of the excavation face is not possible. Usually, in alluvial layers with a certain amount of silt and clay, there will be the existence of a certain amount of water content. It is very rare that the ground formation contains only fine sand, and dry and loose sand is very rare as well. As most of the friction angles in sand layers are around 28 degrees, in most cases the stability of the excavation face can be maintained by using slurry pressure. With other shield construction methods, it is very difficult to maintain soil formation stability in loose sand layers with high water content, so slurry shield shall be chosen, and slurry density, viscosity, and pressure of slurry shall be increased.

(3) Gravel layers

Regarding gravel layers with high water content but no adhesive silt and clay and no large size gravels, slurry shield can be used for construction, and a stone crusher shall be installed in the slurry chamber.

(4) Shell layer

Most shell layers contain water and exist in soil mass, and they are relatively harder than the gravel layer. It is more difficult to maintain the stability of the excavation face, but with a large diameter, a slurry shield can adapt to this kind of stratum much better.

Slurry shields can be applied to all kinds of geologic soil formations, and they are particularly effective for soil formation that has difficulties in keeping excavation face stability, and they can also overcome various difficulties caused by ground surface conditions and underground conditions, for instance a tunnel where a river or sea or other waters or road and building is above, or a location where settlement is essential and must be controlled to the minimum. In these places, slurry shield is all applicable and effective in both engineering and economics.

1.3.4. Earth pressure balanced shield

1.3.4.1. Overview

Earth pressure balanced shield is called EPB shield for short. EPB shield has a bulkhead in the front of the shield structure, and the excavation chamber and screw conveyor, which is used for discharging soil, are filled with excavated material. The pushing force of thrust cylinders pressurize the excavated material in the excavation chamber, and the earth pressure supports the excavation face and keeps it stable. The supporting material of an EPB shield is the soil itself. An EPB shield consists of shield structure, cutting wheel, main drive, screw conveyor, belt conveyor, segment erector, man lock, hydraulic system, etc., as shown in Fig. 1-20.

The working principle of an EPB shield is as follows: the cutting wheel rotates and cuts the soil of the excavation face. The crushed soil enters the excavation chamber through the openings of the cutting wheel, falls on the bottom of the excavation chamber, and is transported to the belt conveyor through the screw conveyor. It is unloaded into muck skips that are parked on rails. The shield machine advances by the pushing force of thrust cylinders. The shield structure plays a temporary supporting role for the unlined tunnel and bears the earth pressure of surrounding soil and the water pressure of underground water, and keeps underground water out of the shield shell. Excavation, soil discharging, lining, and other works