Handbook of Tunnel Engineering I: Structures and Methods: 1

Handbook of Tunnel Engineering I: Structures and Methods

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Often, in cases where tunnels are to be driven into rock of questionable competence, very small tunnels are driven first and then carefully enlarged and supported during the enlarging process. Wahlstrom, The Mechanics of it all. First of all, it is not possible to cover all of the mechanics of tunnel design and construction in a short paper. Dozens of books hundreds of pages long have been devoted to this. However, there are a few basic concepts which apply to the design process, and I will cover those, then a brief description of the tunnel design process itself..

The two concepts in mechanics which most apply to tunnelling are stress and strain.

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Stress may simply be thought of as a force applied on a body, and strain may simply be thought of as that body's deformational response to the stress. For instance, consider placing a heavy book on top of a grape. The stress is the force of gravity and the mass of the book, and the strain is the reaction of the grape to the stress, namely, flattening. Staining of the book by the grape juice is a chemical process, and is not considered in this paper.

Stress and strain models are used by most tunnel engineers to determine the feasibility of a particular excavation. However, recent models have begun to move away from a stress-and-strain focus. The focus of some of the newer models is the distortional strain energy stored in the rock masses.

The reason for this is that energy is a scalar, and thus has no direction, so that the analysis may be performed without regard to the directions of the stress and strain acting on the rock body. Matsumoto and Nishioka, However, this paper will not go into the discussion of these methods, primarily because they have not been tested to the extent of the classical mechanics models, and also because the author has difficulty understanding them.

In the consideration of stress and strain, the dynamic nature of a material can be put into three classes of ideal behavior, and all real materials behave in some combination of the three. Hookian solid-body elastic behavior: In elastic behavior, the strain is completely proportional to the stress applied, so that a plot of stress vs. Venant's solid-body plastic behavior: In plastic behavior, an applied stress will not result in any strain until a certain stress is reached yield point. At this point, only strain increases.

Reduction of the stress to below the yield stress will result in the cessation of the strain. Newtonian liquid viscous behavior: In viscous behavior, the rate of strain is proportional to the stress applied. That is, as the stress applied increases, the deformation does not increase, but the rate at which the body deforms does. With respect to these three ideals, no rock behaves perfectly in these manners, but rather in some combination of elastic, plastic, and viscous. A potentially important quality of some materials, notably glasses, is fragility. A material exhibiting fragile behavior will shatter while still in the range of elasticity.

Matsumoto and Nishioka, However, many materials which would normally behave in a fragile manner while being compressed from one direction will flow in a ductile manner when compressed from all three directions. The term "rock strength" is comprised of a number of different types of properties. Rocks are often tested for strength by use of a uniaxial compression test, in which a cylinder of rock is placed in a vise and compressed along its length.

This is a measure of compressive strength. Other tests may measure tensile strength the rock's ability to withstand being pulled apart , flexural strength the ability to withstand bending , unconfined shear strength the rock's ability to withstand being cut in two , or triaxial compressive strength. In triaxial compression tests, the rock is confined in a pressurized fluid, which compresses it, imitating the confining pressure of rock at depth.

In these triaxial tests, the failure pressure of the rock is normally quite higher than under uniaxial stress. During the triaxial compression test, if even a slight sideways pressure is imposed on the rock, a shear failure occurs. Matsumoto and Nishioka, The greater the confining pressure on the sides, the greater the maximum stress at failure, and the less failure that will occur.

One curious phenomenon in some rocks is that of strain hardening. Certain plastic or ductile materials may, when subjected to a certain degree of deformation, develop increasing strength. This is occasionally observed in underground openings, especially when plastic rocks with low initial strength become stiffer and more resistant to strain dislocation after a period of time Wahlstrom, Rocks around a tunnel are considered to be under triaxial compression. This leads to a very dangerous problem: If the rocks are under triaxial compression, and the confining rock on one side is removed as the tunnel is driven, there will exist a severe stress gradient on the rocks.

The rocks which make up the tunnel wall and roof will be subjected to high stresses on the one side, but the counteracting force is no longer present, as those rocks are no longer present. This situation can, in deep mines, lead to rock bursts, where large portions of the tunnel walls can suddenly and violently explode in seismic events which are often measured at -2 to 4 on the Richter scale Jha and Chouhan, So where does this bring us?

The actual three-dimensional mathematical modeling of the design of a tunnel is a very complicated process, involving a fair amount of differential equations. For instance, a three-dimensional model would contain the three normal strains, the three shear strains, the three normal stresses, the three shear stresses, and the three deformations or displacements measured in Cartesian space. These fifteen unknowns can translate into 36 elastic constants. Mahtab and Grasso, However, by making a very simple assumption, it is possible to use a two-dimensional mathematical model, which greatly simplifies the process and works quite well.

The basic assumption made is that all the stresses involved are either in or parallel to the plane of interest xy, for example. There are a number of different solutions out there, but a good number of them make use of the following assumptions: Plane strain can be illustrated by a long, cylindrical opening, which is conveniently what a tunnel is.

If z represents the axis of the opening, the displacements of all the points in the plane of the cross-section x-y plane are not zero, but the shear strains.

With the conditions thus, the Hooke's Law becomes:. Thus, using the above equations, in conjunction with rock mechanics testing and perhaps in-situ testing of the stress conditions underground, the tunnel engineer could approximate the existing stresses on the rocks at depth, and can estimate the potential strains which may result from excavating a tunnel. From these, the engineer can determine the following:.

The feasibility of the project, given the rocks' potential to remain competent. The costs of the project, in terms of time, labor, and structural support. The type and amount of artificial support needed for stability. The optimum geometry for the tunnel, based on rock qualities. These equations, are theoretical. Nature, unfortunately, has an ability to and a habit of throwing kinks into well-laid plans.

This section will demonstrate a number of difficulties associated with excavation. When any underground excavation is made in an already weak rock, it often serves to further weaken the rock above it. This is due to a combination of 1 the excavation activity itself primarily blasting weakening the surrounding rocks as a result of shock waves and 2 the removal of supporting rock from underneath a large mass of rock.

The net effect of this, weakening of rock and also giving it a place to go, is a movement toward the tunnel from above, which wraps around the tunnel and forms wedges of material which press in on the sides of the tunnel Terzaghi, As Figure 4 illustrates, the rock in area abcd, loosened by the excavation, is attempting to move downward, and is being resisted by friction on surfaces ac and bd. The effect of this is the transfer of a large amount of the overburden, W 1 , onto the abutments of the tunnel. This tunnel will require steel supports, and they will be supporting a load equivalent to Hp, which will depend on the characteristics of the rock mass and the dimensions of the tunnel.

As was mentioned earlier, tunnels are sometimes cut with flat roofs when excavated in strongly laminated rock, in order to take advantage of the rock's tendency to separate on those planes. However, it is rarely the case that laminated rocks are found in a pristine state with their bedding planes parallel and horizontal. Inclined bedding planes in stratified rocks pose a great problem, in that there is a great tendency for the rock to move along the bedding planes and thus slide into the tunnel, as as indicated in Figure 5, modified from Terzaghi The steep angle of the bedding planes with respect to the tunnel will result in the wedge-shaped rock body aed sliding into the tunnel and putting stress on the support ac.

The lateral force, P, can be estimated from the mass of the sliding rock body and the angle which it makes with the tunnel support. According to King , the load on the tunnel and supports depends on the strike and dip of the strata, and tunnels will have to be cut more narrowly in the event of steeply inclined stratified rocks.

In general, rocks have a high resistance to crushing. King states that the walls of a tunnel will not fail as a result of compression except at great depth - more than feet m for softer sandstones and more than 19, feet m for the strongest rocks. However, the rocks are still under an immense amount of stress, and the rock left standing after tunnel or cavern excavation must bear a greater load than before, as illustrated in Figure 4 from Terzaghi.

This point is further illustrated in Figure 6, from Herget The grid lines represent the principal plane-strain stresses around a circular tunnel after excavation. The crowding of the trajectories at the sides indicates an increase in compression, and the widening at the top and bottom indicates a decrease in compressive stress. This implies that the walls of a tunnel, not the roof, would be more susceptible to failure. This is the mechanical basis of the rockburst phenomenon.

Of all the hazards associated with mining, rockbursts are perhaps the most terrifying. A rockburst is the sudden, violent dislocation of slabs of rock in a tunnel, usually from the walls, but also potentially from the roof or even floor. Considered to be a "mining-induced seismic event," a rockburst can release enormous amounts of energy, and some have been measured at 4 on the Richter scale Jha and Chouhan, and one rockburst was recorded by a seismological station miles distant. Obert and Duvall, The danger is obvious and quantifiable: Obert and Duvall, The above link blue "rockburst" illustrates the before and after of a rockburst in the Kolar gold-field.

What are the causes of rockburst? One of the primary causes is obviously stress. The forces necessary to shatter tons of rock require the input of stress.

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The other primary factor is the rock type. An interesting point about rock bursts is that they do not occur in weak rocks.

It is thought that the pressures which can cause a rockburst are slowly released in the weaker rocks by semiplastic adjustments. Wahlstrom, The rocks affected are nearly always hard, strong, and brittle. Obert and Duvall, All other things being equal, the weakest rocks would be the least likely to burst, because they would reach their failure point far before they could store enough strain energy to produce a violent failure.

It appears that the lithostatic pressure of depth is not in general sufficient to produce the amount of stress necessary for a rockburst. What appears to also be required is a mechanism for producing a localized increase of the stress of the rock. The possibilities are numerous, including dike intrusion, faulting, and many others. Figure 7, taken from Wahlstrom , illustrates three of the many possibilites. The quartzites of the Revett Formation in the Coeur d'Alene mining district are stressed by a myriad of faults, two of which the right-lateral strike-slip Osburn and Placer Creek faults trace for over kilometers.

Once the major conditions are met, rockbursts still require a mechanism for failure. This so-called "crack kinking" is shown in Figure 8 below, with both an inclined crack which has kinked as well as a pore which has begun cracking under stress. In the presence of a free surface in this case, the tunnel wall , the cracks growing parallel to the free surface are affected by the free surface, and grow unstably after reaching a certain length.

Dyskin and Germanovich, a This instability of crack propagation can result in the separation of thin layers of rock from the rock mass and produce spalling. By itself, a single crack probably won't be responsible for a failure, but through the stress-induced crack growth, interaction of the cracks may cause the rock to reach a level of instablility where it will fail.

As observed by Nemat-Nasser and Horii in their experiments with compression of resin, the presence of a free surface had the effect of inducing crack-kinking away from the free surface. The cracks then followed more or less the free-surface contour. In rock, this would have the effect of forming slabs of rock parallel to the wall, which may then buckle or explosively break in a rockburst.

Nemat-Nasser and Horii, click here to see it. Rockbursts are, however, mining-induced events. Were it not for the removal of rock, the rock mass would stay perfectly happy at depth. Excavation causes a large stress gradient and the potential for release of the rock's stored strain energy.

The rate of release of the strain energy is important. A gradual release may be perfectly safe, whereas the violent releases of energy are what we call rockbursts. Figure 9, taken from Whittaker, et. Sequences 1 and 3 both result in the release of energy quickly, which makes them more susceptible to rockburst than Sequence 3, which has a more uniformly gradual release of energy.

Special tunnels, such as wildlife crossings , are built to allow wildlife to cross human-made barriers safely. Tunnels can be connected together in tunnel networks. A tunnel is relatively long and narrow; the length is often much greater than twice the diameter , although similar shorter excavations can be constructed, such as cross passages between tunnels.

The definition of what constitutes a tunnel can vary widely from source to source. In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called a subway , while an underground railway system is differently named in different cities, the " Underground " or the " Tube " in London , the " Subway " in Glasgow , and the " Metro " in Newcastle. The place where a road, railway, canal or watercourse passes under a footpath, cycleway, or another road or railway is most commonly called a bridge or, if passing under a canal, an aqueduct.

Where it is important to stress that it is passing underneath, it may be called an underpass , though the official term when passing under a railway is an underbridge. A longer underpass containing a road, canal or railway is normally called a "tunnel", whether or not it passes under another item of infrastructure. An underpass of any length under a river is also usually called a "tunnel", whatever mode of transport it is for. In the US, the term "subway" means an underground rapid transit system, and the term pedestrian underpass is used for a passage beneath a barrier.

Rail station platforms may be connected by pedestrian tunnels or footbridges. Much of the early technology of tunneling evolved from mining and military engineering. The etymology of the terms "mining" for mineral extraction or for siege attacks , "military engineering", and " civil engineering " reveals these deep historic connections. Predecessors of modern tunnels were adits to transport water for irrigation or drinking, and sewerage. The first Qanats are known from before B. A major tunnel project must start with a comprehensive investigation of ground conditions by collecting samples from boreholes and by other geophysical techniques.

An informed choice can then be made of machinery and methods for excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions. In planning the route, the horizontal and vertical alignments can be selected to make use of the best ground and water conditions. It is common practice to locate a tunnel deeper than otherwise would be required, in order to excavate through solid rock or other material that is easier to support during construction. Conventional desk and preliminary site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground.

This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel or "drift tunnel" may be driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically should unexpected conditions be met, and it can be incorporated into the final tunnel or used as a backup or emergency escape passage. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face. For water crossings, a tunnel is generally more costly to construct than a bridge.

However, navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel. Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as Manhattan and urban Hong Kong , this is a strong factor in favor of a tunnel. Boston's Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront.

The Queensway Tunnel under the River Mersey at Liverpool was chosen over a massively high bridge for defense reasons; it was feared that aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the Kingsway Tunnel under the Mersey. In Hampton Roads, Virginia , tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges might prevent US Navy vessels from leaving Naval Station Norfolk.

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather, and shipping during construction as in the Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in Virginia.

There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in Switzerland in One of the worst railway disasters ever, the Balvano train disaster , was caused by a train stalling in the Armi tunnel in Italy in , killing passengers. Designers try to reduce these risks by installing emergency ventilation systems or isolated emergency escape tunnels parallel to the main passage.

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Government funds are often required for the creation of tunnels. Civil engineers usually use project management techniques for developing a major structure. Understanding the amount of time the project requires, and the amount of labor and materials needed is a crucial part of project planning. Also, the land needed for excavation and construction staging, and the proper machinery must be selected. Large infrastructure projects require millions or even billions of dollars, involving long-term financing, usually through issuance of bonds.

The costs and benefits for an infrastructure such as a tunnel must be identified. However, the Port Authority of New York and New Jersey was not aware of this bill and had not asked for a grant for such a project. Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.

There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally a tube can be sunk into a body of water, which is called an immersed tunnel. Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. Shallow tunnels are often of the cut-and-cover type if under water, of the immersed-tube type , while deep tunnels are excavated, often using a tunnelling shield.

For intermediate levels, both methods are possible. Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube station in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral, owing to the sheer size of the excavation.

This contrasts with many traditional stations on London Underground , where bored tunnels were used for stations and passenger access. Nevertheless, the original parts of the London Underground network, the Metropolitan and District Railways, were constructed using cut-and-cover. These lines pre-dated electric traction and the proximity to the surface was useful to ventilate the inevitable smoke and steam.

A major disadvantage of cut-and-cover is the widespread disruption generated at the surface level during construction. This, and the availability of electric traction, brought about London Underground's switch to bored tunnels at a deeper level towards the end of the 19th century. Tunnel boring machines TBMs and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as quick and cost effective alternative to laying surface rails and roads.

Expensive compulsory purchase of buildings and land, with potentially lengthy planning inquiries, is eliminated. Disadvantages of TBMs arise from their usually large size — the difficulty of transporting the large TBM to the site of tunnel construction, or alternatively the high cost of assembling the TBM on-site, often within the confines of the tunnel being constructed. There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground.

Some types of TBMs, the bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, allowing them to be used in difficult conditions below the water table. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurised compartment, but may occasionally have to enter that compartment to renew or repair the cutters.

This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. The borehole has a diameter of 8. All of these machines were built at least partly by Herrenknecht. Clay-kicking is a specialised method developed in the United Kingdom of digging tunnels in strong clay-based soil structures. Unlike previous manual methods of using mattocks which relied on the soil structure to be hard, clay-kicking was relatively silent and hence did not harm soft clay-based structures.

The clay-kicker lies on a plank at a degree angle away from the working face and inserts a tool with a cup-like rounded end with the feet. Turning the tool manually, the kicker extracts a section of soil, which is then placed on the waste extract. Used in Victorian civil engineering, the method found favour in the renewal of Britain's ancient sewerage systems, by not having to remove all property or infrastructure to create a small tunnel system.

The method was virtually silent and so not susceptible to listening methods of detection. A temporary access shaft is sometimes necessary during the excavation of a tunnel. They are usually circular and go straight down until they reach the level at which the tunnel is going to be built. A shaft normally has concrete walls and is usually built to be permanent. Once the access shafts are complete, TBMs are lowered to the bottom and excavation can start.

Shafts are the main entrance in and out of the tunnel until the project is completed. If a tunnel is going to be long, multiple shafts at various locations may be bored so that entrance to the tunnel is closer to the unexcavated area. Once construction is complete, construction access shafts are often used as ventilation shafts , and may also be used as emergency exits.

The New Austrian Tunneling Method NATM was developed in the s and is the best known of a number of engineering practices that use calculated and empirical measurements to provide safe support to the tunnel lining.

The main idea of this method is to use the geological stress of the surrounding rock mass to stabilize the tunnel, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the supports. Based on geotechnical measurements, an optimal cross section is computed. The excavation is protected by a layer of sprayed concrete, commonly referred to as shotcrete. Other support measures can include steel arches, rockbolts and mesh.

Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibres being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's deformation. By special monitoring the NATM method is flexible, even at surprising changes of the geomechanical rock consistency during the tunneling work. The measured rock properties lead to appropriate tools for tunnel strengthening.

Handbook of Tunnel Engineering

In the last decades also soft ground excavations up to 10 kilometres 6. In pipe jacking , hydraulic jacks are used to push specially made pipes through the ground behind a TBM or shield. This method is commonly used to create tunnels under existing structures, such as roads or railways. Tunnels constructed by pipe jacking are normally small diameter bores with a maximum size of around 3. Box jacking is similar to pipe jacking, but instead of jacking tubes, a box-shaped tunnel is used. A cutting head is normally used at the front of the box being jacked, and spoil removal is normally by excavator from within the box.

Recent developments of the Jacked Arch and Jacked deck have enabled longer and larger structures to be installed to close accuracy. The m long 20m clear span underpass below the high speed rail lines at Cliffsend in Kent, UK is an example of this technique [ citation needed ]. Submerged floating tunnels are a novel approach under consideration; however, no such tunnels have been constructed to date.

During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove excavated spoil , often narrow gauge so that it can be double track to allow the operation of empty and loaded trains at the same time. The temporary way is replaced by the permanent way at completion, thus explaining the term "Perway". An open building pit consists of a horizontal and a vertical boundary that keeps groundwater and soil out of the pit.

There are several potential alternatives and combinations for horizontal and vertical building pit boundaries. The most important difference with cut-and-cover is that the open building pit is muted after tunnel construction; no roof is placed.

In Turkey, the Eurasia Tunnel under the Bosphorus , opened in , has at its core a 5. Although each level offers a physical height of 2. Each level was built with a three-lane roadway, but only two lanes per level are used — the third serves as a hard shoulder within the tunnel. The A86 Duplex is Europe's longest double-deck tunnel. In Shanghai , China, a 2. In each tube of the Fuxing Road Tunnel both decks are for motor vehicles.

In each direction, only cars and taxis travel on the 2. In the Netherlands, a 2. The two lower tubes of the tunnel carry the A2 motorway , which originates in Amsterdam, through the city; and the two upper tubes take the N2 regional highway for local traffic. The Alaskan Way Viaduct replacement tunnel is a bored road tunnel that is under construction since in the city of Seattle in the U.

New York City 's 63rd Street Tunnel under the East River , between the boroughs of Manhattan and Queens , was intended to carry subway trains on the upper level and Long Island Rail Road commuter trains on the lower level.

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