Tunnel Structures

Christian Ingerslev

Parsons Brinckerhoff, Inc., New York, NY

Brian Brenner

Department of Civil and Environmental Engineering, Tufts University, Medford, MA

Jaw-Nan Wang

Parsons Brinckerhoff, Inc., New York, NY

Phil Rice

Parsons Brinckerhoff, Inc., New York, NY

Birger Schmidt*

Parsons Brinckerhoff, Inc., New York, NY

28.1 Introduction 28-1

What is a Tunnel? • Fundamental Approach to Underground Design • Cut-and-Cover Tunnels • Bored and Mined Tunnels in Soil or Rock • Immersed and Floating Tunnels • Jacked Tunnels

28.2 Cut-and-Cover Tunnels 28-6

Introduction • Structural Analysis • Methods of Framing • Analysis in Section: Typical Frame and BOEF Methods of Analysis • Loading • Finite Element Analysis • Buoyancy • Evaluation of Construction Impact and Mitigation

28.3 Tunnel Linings for Bored and Mined Tunnels 28-14

Introduction • Mechanized Tunneling through Soil • Linings for Tunnels in Soil • Bored Tunnels in Rock • Sequential Excavation and Support for Rock Tunnels • Selection of Lining System in a Rock Tunnel • Structural Design of Permanent Concrete Linings in Rock • Design of Segmental Concrete Linings

28.4 Immersed and Floating Tunnels 28-25

Introduction • Sizing of Tunnel Sections • Types of Immersed Tunnel • Principles of Design • Analysis • Methods of Constructing Elements: Concrete and Steel • Tunnel Joints • Construction Aspects • Protection against Ship Traffic and Currents

28.5 Seismic Analysis and Design 28-35

Introduction • Performance Record during Earthquakes • Design and Analysis Approach for Ground-Shaking Effects • Tunnel Subjected to Large Displacements • Shaft Structures and Interface Joints

28.6 Tunnel Jacking 28-45

Glossary 28-47

References 28-49

Further Reading 28-49

28.1 Introduction

This chapter deals with structural design and the selection of structural systems for several types of tunnel structure. Methods of construction are important in the selection of structural systems and their details, but space does not permit this chapter to do justice to every complex method of tunnel construction possible in the great variety of geologic environments that exist. The chapter includes methods for seismic analysis of underground structures.

* Deceased.

28.1.1 What is a Tunnel?

Tunnels are located either below ground or below a body of water. Almost all tunnels serve as part of the infrastructure of cities and countries, conveying utilities, water, or sewage, or serving as transportation arteries. Some tunnels serve as storage, and some caverns are used as stadiums, swimming pools, or even for cleaning sewage. In cities, tunnels are often preferred because they can be built without taking up precious surface space. In the open country, tunnels are most likely to be found where it is necessary to traverse obstacles such as mountains, rivers, lakes, and fjords. The depth, cross-section, and alignment of tunnels are determined by the functional requirements of the facility, by geographical and environmental constraints, and by issues of constructibility under the prevailing geotechnical conditions. The three principal types and methods of tunnel construction are

• Cut-and-cover tunnels, built by excavating a trench, constructing the concrete structure in the trench, and covering it up. Methods used include top-down and bottom-up.

• Bored or mined tunnels, excavated through rock either by blasting or by tunnel boring machines, or excavated through soil using shields, mechanized to a greater or lesser extent.

• Immersed or floating tunnels, made from very large precast concrete or concrete-filled steel elements, floated out, and then either lowered into a prepared trench and covered over (immersed), or anchored or supported below the water surface but not buried (floating).

In this chapter a short section on tunnel jacking is included, a procedure used recently in Boston and which is on occasion used in Europe and elsewhere. It is used only for very shallow tunnels when it is vital not to interrupt surface facilities and if other methods are not suitable.

At shallow depth and for short water crossings, the cut-and-cover tunnels are the most economical. Per unit length, immersed or floating tunnels can be more costly than bored or mined tunnels, but because they can often be made shallower and therefore shorter, and because they may serve their function better than other choices, the overall project cost can be less.

28.1.2 Fundamental Approach to Underground Design

Underground design must achieve functionality, stability, and safety of the underground openings during construction and for the lifetime of the structure thereafter. There is no recognized U.S. standard, practice, or code for the design of most underground structures. Designers may apply codes such as ACI Codes and Practices for concrete design, but these were developed for structures above ground, not for underground structures, and only parts of these codes apply to underground structures such as cut-and-cover structures. There are five basic steps in the design of underground structures:

1. Define functional requirements, including design life and durability requirements.

2. Acquire and analyze geologic, geotechnical, and cultural data relevant to the design and construction of the underground facilities.

3. Determine plausible and possible modes of failure, including construction events, unsatisfactory long-term performance, failure to meet environmental requirements, and flooding; determine means of analyzing these modes of failure and acquire the necessary data.

4. Establish appropriate method(s) of construction considering the geologic and groundwater information, constructibility, and economy.

5. Establish and design appropriate initial and final ground support and lining systems, considering both ground conditions and the method(s) of construction.

28.1.3 Cut-and-Cover Tunnels 28.1.3.1 Introduction

Cut-and-cover tunnels are by far the most common of all the types of tunnel. They are constructed by one of two main methods. With the "bottom-up" approach, a trench is excavated from the surface, either supporting the sides of the excavation or sloping the sides. The complete structure is then constructed within the excavation; temporary decking may be installed across the top of the excavation to permit early reinstatement of traffic. With ''top-down'' construction, the walls (in narrow trenches) and the tunnel roof are built first and the surface reinstated; thereafter, the tunnel section is excavated down from the underside of the already constructed roof structure, and the tunnel is completed. For both methods, the tunnel structure is usually either cast-in-place concrete or erected steel structural elements with infill concrete. It is unusual for prefabricated elements to be used for cut-and-cover tunnel structures.

The cut-and-cover method is most attractive for relatively shallow tunnel sections. Cut-and-cover construction, particularly when the bottom-up method is used, can be disruptive to both surface traffic and existing facilities even when temporary decking is used over the cut.

28.1.3.2 Trench Excavation

The tunnel trench may be constructed by

• Sloping back the excavation, with little or no excavation support.

• Using the temporary support of excavation walls, such as soldier piles and lagging or steel sheet piles that do not form part of the permanent structure.

• Using excavation support walls that are designed to be part of the permanent structure.

The first option can be the most economical, but it is often impractical due to adjacent structures and facilities or due to depth. Comparing the second and third options, increased cost and staging difficulties for building permanent excavation support walls must be balanced against the simplicity and ease of building a permanent structure within temporary walls.

28.1.3.3 Top-Down Construction

As a cost-saving measure, this method typically uses the excavation support walls as part of the final structure (for e.g., concrete diaphragm walls cast in slurry-filled trenches, known as slurry walls, or precast wall sections lowered into slurry-filled trenches). After the walls are constructed downward from the surface, excavation proceeds down to the underside of roof level, and the roof is installed. The surface is then restored to its final condition, and excavation proceeds below the finished roof.

Top-down construction requires a different set of analysis assumptions and staging approaches than bottom-up construction, whether or not it has temporary decking. For example, the staging must lay out specific glory hole locations for all stages, being holes that provide access for construction, materials, and removal of excavated soil and rock. This type of staging planning can be more complex than cut-and-cover work with temporary decking or an open cut.

28.1.3.4 Groundwater Impacts

It is important to evaluate groundwater impacts for cut-and-cover construction. The evaluation includes

• Analysis to determine impacts on the water table beyond the zone of excavation.

• Analysis of the stability of the cut during construction.

• Evaluation of buoyancy under final conditions.

In all stages of cut-and-cover excavation and construction, the weight of the structure and backfill will tend to be less than the weight of the soil removed. The staging evaluation can be more complicated in areas with high groundwater levels.

28.1.4 Bored and Mined Tunnels in Soil or Rock

Transportation tunnels in built-up areas are often constructed by tunneling methods such as machine boring or hand mining. Tunneling can cause much less disruption at the surface, especially of local utilities, and is less expensive for deep tunnels. It is common practice to provide at least one diameter of soil or rock cover, though shallower cover is possible.

Depending on functional requirements, bored or mined tunnels come in almost all sizes, ranging from some 2 m in diameter for medium-sized water and sewage conveyance up to about 20 m in diameter for rail stations and some hydropower plant caverns. While rock tunnels excavated by blasting methods can be built to almost any size and shape, tunnels in soil are usually built using a shield, currently to a maximum circular diameter of about 15 m, although larger-diameter machines are being designed. Most rapid transit tunnels are single-track tunnels with a diameter of about 6 m, but a few have been built as double-track tunnels with a diameter of about 10 m. Even smaller tunnels (microtunnels) are used for utilities, but they may be constructed differently.

Functional and environmental requirements determine basic tunnel support and lining requirements such as shape and size, smoothness, water tightness, and durability. Some tunnels through high-quality rock are unlined or only furnished with nominal support consisting of rock bolts, wire mesh, or shotcrete. At the other extreme, transportation tunnels through soft ground below the groundwater table are furnished with durable, watertight linings, generally designed at present as moment resistant reinforced concrete linings with waterproofing membranes. Many sewer tunnels are furnished with corrosion resistant internal linings.

28.1.5 Immersed and Floating Tunnels

28.1.5.1 Immersed Tunnels

Whenever there is a need to cross water, an immersed tunnel should be considered. Besides their use as road and rail tunnels, many have been built for utilities or pedestrian-only use. Immersed tunnels may appear complex to those unfamiliar with the technology because of the operations in and over water that they entail (Figure 28.1). In reality though, the technique is often less risky than bored tunneling, and tunnel element manufacture can be better controlled since it is precast in the dry. As a result, immersed tunnels are nearly always much more watertight and therefore drier than bored tunnels.

Immersed tunnels offer a number of special advantages:

• They do not have to have circular cross-sections unless the external pressure is particularly high. Almost any cross-section can be accommodated, making immersed tunnels particularly attractive for wide highways and combined road or rail tunnels.

• In contrast to a bored tunnel, they can be placed immediately beneath a waterway with a nominal cover of only 1.5 to 2.0 m of backfill to provide protection against dropping anchors or sinking ships. Because they are shallower, this allows immersed tunnel approaches to be shorter and gradients to be flatter, which is an advantage for all tunnels, but especially so for rail tunnels.

• They can be constructed in ground conditions that would make bored tunneling difficult or expensive, such as the soft alluvial deposits characteristic of large river estuaries.

Immersed tunnels have been located in both freshwater and seawater, in hot and cold climates, in busy waterways, and in earthquake zones. Alignments do not need to be straight, so that the tunnels can be designed to suit design speeds, existing land use, topography, and connections to the existing road or rail system. The ideal alignment for an immersed tunnel may not coincide with the ideal alignment for a bridge. Soft ground is ideal for immersed tunnels, but if excavation into rock is required, the high cost of rock excavation under water could make other tunneling methods more cost-effective.

Immersed tunnels are not suitable for every situation. However, if there is water to cross or water can be used to float-in a precast structure, they usually present a feasible alternative to bored tunnels at a comparable or better price. A catalog of existing immersed tunnels is given in Ref. [1].

28.1.5.2 Floating Tunnels

As water depth increases, particularly if the waterway is narrow, tunnels may become excessively long when compared to the width of the crossing. Using shallow alignments, a new option has been developed in which the tunnel is exposed within the water column, resulting in much shorter

FIGURE 28.1 Lowering an immersed tunnel element into the excavated trench (© Parsons Brinckerhoff, Inc. 1967, with permission).

tunnels. Such tunnels are known as floating tunnels and appear to be viable using available technology; designs for several locations are well advanced, although none has yet been constructed. While very short floating tunnels might not need intermediate supports, longer nonbuoyant tunnels might be supported by columns up from the bed (like an underwater bridge) or suspended from pontoons on the surface. Conversely, anchors might hold down a permanently buoyant tunnel (similar to tension-leg oil platform technology). Further information on floating tunnels can be found in Ref. [1].

Although most immersed tunnels are built for water depths between 5 and 20 m, schemes have been postulated for depths up to 100 m. Such great depths could be avoided by using floating tunnel technology, though if submarines ply the waters, the risk of collision might preclude the use of a floating tunnel or at the very least will complicate the design.

28.1.6 Jacked Tunnels

Tunnel jacking is a specialized technique for constructing a concrete tunnel, typically rectangular, under surface areas that are being used for facilities considered critical to the degree that interrupting them would result in serious impacts, not the least of which are significant costs. Examples of such critical surface facilities include operating railway tracks, major roadways, and airport runways. The technique is generally applied in soft ground for relatively short tunnel sections and shallow depth of cover. The method has been employed to construct tunnels of varying uses, including pedestrian passageways, flood control structures, and roadways.

The tunnel jacking method is costly and is not normally considered the preferred technique for constructing an underground crossing. Conventional mined tunneling would be the method of choice if the depth of cover were sufficient to limit the influence of excavation on the surface facilities to acceptable or manageable levels. For shallow tunnels where mined tunneling would be too risky, cut-and-cover methods would be preferred. However, when the costs of shutting down critical surface facilities completely or rerouting or relocating them or providing substitute services for them for the duration of the construction period are taken into account, tunnel jacking may become the most economical method.

28.2 Cut-and-Cover Tunnels 28.2.1 Introduction

For structural evaluation of cut-and-cover tunnel structures, a spectrum can be considered where cut-and-cover tunnels fall between immersed and bored tunnels. Immersed tunnels are largely made of prefabricated structural sections, and their structural analysis resembles the more traditional approach of loads applied to a structure; construction staging must be carefully considered as the tunnel is floated and immersed. Bored tunnels can be thought of as reinforced openings in a competent subgrade material; their structural analysis is usually not a problem of loads applied to a structure, but one of considering how the soil behaves once a gap is constructed and then lined. Analysis of cut-and-cover tunnels will typically consider both of the following features:

• Soil-structure interaction.

• The traditional approach of loads applied to a structural model.

Computer modeling is now able to handle sophisticated soil-structure interaction analysis of cut-and-cover tunnel construction. As the software becomes easier to use and more flexible, more sophisticated analyses of tunnel sections are being performed, as discussed in Section 28.2.4.

An initial distinction can be made between tunnels for which support of excavation (SOE) walls are temporary and those for which cofferdam walls are incorporated into the final structure. In the past, construction walls such as sheet piling and lagging were ''throw-away'' temporary walls. More recently, however, some engineers have specified that the construction excavation walls be used as part of the final tunnel section. Examples include conventional concrete slurry walls, soldier pile tremie concrete walls, and tangent and secant pile walls. There are important differences in terms of construction issues between incorporating the excavation support or not, with the focus here being on structural analysis and design characteristics. Figure 28.2 shows a schematic of the two different types of section.

When the SOE walls are temporary, the tunnel box is assumed to experience mostly final loading conditions. However, when the SOE walls are included as a part of the final structure, the design loading condition are much more time dependent. The designer will need to consider not just the final loading conditions but also all of the states of stress experienced by the cofferdam walls as they are constructed, braced or tied back, and finally connected to the tunnel as a whole. Many of these stresses are locked-in stresses to be superimposed on later conditions.

The evaluation of impacts on existing facilities is another complication for cut-and-cover tunnel analysis that is not as much of a concern for bored and immersed tunnels. Of the three tunnel types, cut-and-cover tunnels are the most likely to be built adjacent to existing buildings, utilities, and other facilities, so the nature of the tunneling process can lead to significant impacts on existing structures.

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