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استخراج الکتریکی آبهای زیرزمینی

ELECTROKINETIC GROUNDWATER EXPLORATION:

A NEW GEOPHYSICAL TECHNIQUE

Sukhyoun Kim, Graham Heinson & John Joseph

CRC LEME, School of Earth and Environment Sciences, University of Adelaide, SA, 5005

ABSTRACT

Electrokinetic potentials, sometimes known as streaming potentials, are generated by fluid-flow, heat fluxes,pressure sources and diffusion potentials across boundaries between a fluid electrolyte and mineral grains orrock particles in fractured rocks and porous media. In the electrical double layer model (Stern model, Figure1), there is an immobile layer that is divided into two by the Stern plane. The layer nearest the mineral grainis immobile, and the surface of the mineral grain has a net charge so that it adsorbs cations from the fluid inan electrical double layer. Away from the grain boundaries, the mobile part of the fluid may have a surplus opositive or negative ions, depending on the electrostatic charge distribution at the electrical double layer. The excess density of ionic species can be transported with fluid flow producing an advective electric current. The advective flow of charge is balanced by a return electric current; this process is known as coupled flow. The electric potential in the Stern layer varies as a function of distance. The electric potential at the end of slipping plane (the end of the electrical double layer furthest from the mineral grains) is known as the zetapotential (ζ). To measure such electrokinetic potentials, a good understanding of petrophysical properties of the media, particularly the ζ-potential and electrokinetic coupling-coefficient are required. This research project aims to establish a new technology to measure fluid-flow and infer hydraulic conductivity from the electrokinetic potential in three approaches. Firstly, measurements of surface electrical potentials will be combined with hydraulic pumping tests to determine sub-surface fluid-flow. Measurements will be made using 36 non-polarized electrodes relative to a reference. This setup will allow monitoring of changes in electrokinetic potentials associated with draw-down. Data will be sampled at a rate of 1Hz with a resolution of 0.1 mV. The second step of this research will be the investigation of the relationship between porous media and water-flow in the laboratory. Experimental work will focus on the properties of clay and sand, which form most of the ground in the area along the River Murray. The key part of this experiment will be defining the ζ-potential and electrokinetic coupling coefficient of these porous media. The final aim of this research is to further develop numerical modelling of the electrokinetic potential data to quantitatively

interpret electrokinetic potential data.

INTRODUCTION

The electrokinetic potential occurs by fluid-flow in porous media and in fractured rocks. Interaction between fluid-flow and mineral grains produces an electrokinetic potential (Corwin & Hoover 1979). However, it is not easy to quantitatively interpret electrokinetic potentials because of the complex dependency on parameters such as temperature, pressure, pH and the heterogeneity of porous media in the ground. The theoretical basis of this method was originally described by Overbeek (1952) and Nourbehecht (1963). However, relatively few studies have been carried out to investigate the electrokinetic potential in applied geophysics, and to obtain quantitative interpretation of electrokinetic potentials with numerical modelling (Sill 1983, Wurmstich & Morgan 1994, Titov et al. 2002).

When fluid flows through pores and in fractures, it forms two layers which are separated by a slipping plane as immobile and mobile parts of fluid in the Stern model (Figure 1). The layer in the vicinity of the mineral grain boundary, which is immobile, is divided into two again. Next to the mineral grain boundary is the Stern layer, and the other layer is the diffusion layer. Together, these layers form the electrical double layer (EDL). The interaction between mineral grains and fluid generates the electrokinetic potentials. In the Stern layer, the negatively charged surface of the mineral grain adsorbs positive ions from the fluid in the immediate vicinity of the grain's surface. The diffusion layer consists of surplus positive ions which were attracted but not adsorbed. If the negative charge density is very high at the surface of the mineral grain, the voltage may rise above zero in the Stern layer and will then decrease toward zero as a function of distance in accordance with a Boltzmann distribution of the ions (Overbeek 1952) (Figure 1B). Normally, the surface of silicate mineral grains is negatively charged and attracts cations from the fluid  surrounding the mineral grain (Fitterman 1979). By this phenomenon, so-called electrostatic attraction, a distribution of ions is produced, and thus electrical potentials occur. In a system which consists of mineral grains and fluid, there must be an electrical equilibrium (Ishido & Mizutani 1981). This means the total charge of the system must not change in magnitude. In the electrical double layer, where the voltage approaches zero at the Stern layer, the electrical charge at the hydraulic slipping plane is the zeta-potential (ζ)

as shown in Figure1. Thus, the zeta-potential is directly related to the amount of transported electrical charge and is the magnitude of the potential change in the mobile part of the fluid. Values of zeta-potential and of the electrokinetic coupling coefficient are necessary to estimate the magnitude of electrokinetic phenomena (Ishido & Mizutani 1981).

The convection flow of pore fluid in the diffuse layer produces an advective electric current; such flow yields an electric field which produces a counter electric current (conduction current) through the interface. Assuming that the flow is laminar and the radius of curvature of interface between mineral grains is much bigger than the thickness of the double layer, the advective and conduction currents at equilibrium are equal in magnitude. The total current per unit area itotal in the system can, therefore, be expressed as:۰۰۰

 

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ماسراهای زغال سنگ-Macerals

A maceral is to coal what a mineral is to rock. Rocks are composed of minerals; coal is composed of  macerals. However, minerals often have reasonably well defined chemical and physical characteristics.
Macerals can't quite be put into such neat little boxes. Here's a killer definition of macerals:

Macerals are phytogenetic organic substances or optically homogeneous aggregates of phytogenetic substances possessing distinctive chemical and physical properties." William Spackman

It takes awhile for this definition to sink in. Macerals are the remains of plants and degraded plant materials; they also have some characteristic chemistries and physical attributes. Unfortunately, these characteristics have to be more loosely defined than for minerals. In the coalification section some of the factors that affect the transformation of plant material to coal were given. One was how the type of plant influences the type
of peat. But there is a much finer point to this, which is at the heart of the maceral concept. Not only is the type of plant important, but the type of plant part and how nature modifies it is an important aspect of coal structure. For instance, spores, pollens, and resins often do not degrade as rapidly in a peat as
other organic materials, such as leaves or roots. They are often seen intact, but chemically changed in coal. Depending on microenvironmental conditions, some parts of a fallen log may degrade extensively in a swamp becoming gel-like, while other parts may retain their original cellular structure. All this can be seen in coals. It is these organic remains which we call macerals.

Now we're ready to get down to the heart of what coal is, and one of its big surprises. At the left is a microphotograph of a coal thin section taken in white light. The bar on the bottom right is ten micrometers long. Coal is not just a black featureless rock. In fact, most coal in the world, when cut thin enough, will show predominantly red colors. There certainly is material that is black though. There are also yellows and all the intermediate shades up to red. Sorry, there are no greens or blues (not when observing the coal in white light anyway). Want a hint to tell you if a coal will have a lot of red material? When you break it, does it have areas that have a very shiny texture? Those will be red under the microscope. By the way, the white areas in the picture are pores, or holes were coal was lost making the thin section.

Okay, so what do all these pretty different colors mean"? All the different colored materials are the macerals. Take a close look at the small, short, yellow "lines", blobs, or dumbbells in some of the photos. Those are coalified spores. In the coal thin section, do you see any long orange squiggly lines? Those are coalified plant cuticles. Cuticles are a plant's idea of skin. They prevent moisture loss and resist damage by microorganisms
to the underlying material. It is often found on the leaves of plants. The red material is the remains of the woody tissues of plants, such as the trunk and roots of trees. The black material can be two things:
either minerals or coalified charcoal. The type of material you see in the photos can be found in any coal chunk that you pick up. What will vary is the amounts and types of the different macerals.

Unfortunately, thin sections are not often used in analyzing the types and amounts of macerals in coal. It is very difficult to make thin sections of coal. Coal scientists need some way of rapidly preparing samples for routine examination. An easier technique is to work with polished sections of coal. The surface of many
coals can be polished until it resembles a black mirror. Then reflectance microscopy is used to examine the coal. This type of microscopy is commonly used with opaque samples. Unfortunately, pretty colors are not what you see. The coal is mostly shades of grey. Some macerals do show shades of red and orange, but this
is not in the same sense as with thin sections. 

There have been several attempts to classify the colored components in coal so that scientists can all understand each other. The system that has stuck the longest is the maceral concept, which was defined earlier. Just as there are three major colors for the macerals: yellow, red, and black, so do we have three major maceral groups: liptinite, vitrinite and inertinite.

Macerals are defined by both their color/reflectance and morphology. The two cannot really be separated. It is not always easy to decide which category a maceral is in; even coal petrographers don't always agree. (A coal petrographer is someone who systematizes and classifies coal. Often he or she has a strong background in geology.)

Its not hard to figure out why some macerals have the names they do. Many take the name of the material from which they were derived: sporinite from spores, alginite from algae. However, in some cases its not obvious. Liptinite is derived from the word lipid, which are highly aliphatic materials. i.e. they have a chemical composition similar to waxes or oils. Vitrinite is derived from the word vitreous, because of its glossy appearance in a chunk of fresh coal. Inertinite is derived from inert, because some of these
macerals don't react or change very much when heated.

Sidelight: Who came up with the idea of macerals? Well, here is another surprise. The first person to come up with the concept was Marie Stopes. The system we now use is more elaborate then she envisioned, but the origin is attributed to her. She was a geologist by profession. The surprising part is she is remembered
more for her other escapades. She was an activist for feminine rights in the thirties and forties, and an advocate of birth control. She had a very colorful history, and even spent a fair time in jail because of her militant dedication to her unpopular and radical views for that time.

One feature that is not clear from the Table is that the reflectance and color of a coal changes as its rank increases. In reflected light the vitrinite becomes much brighter. However, at the same time the liptinites, which appear darker than vitrinites in low rank coals, increase their reflectance even more rapidly. Inertinites, which are brighter than vitrinite don't change their reflectance very much.
So in very high rank coals (medium volatile bituminous and above, we can't see the different macerals without very special techniques.

Properties of Some Macerals

In addition to their appearance, macerals have many other different physical and chemical properties. Again though, the properties of macerals change as a function of rank or maturation. They cannot be considered as a single molecular species with a well defined chemical structure. Thus, it is difficult to summarize their properties. In addition, even an approximate chemical makeup of many macerals is unknown. The following Table gives you an idea of some differences for the more common macerals of high volatile bituminous rank.

Maceral	H/C	O/C	frac. aromatic C	Density (g/cc)
Liptinite
Resinite	1.33 - 1.55	0.03 - 0.11		1.01 - 1.15 He
Sporinite	0.92 - 1.13	0.09 - 0.12	0.45 - 0.6	1.15 - 1.25 Aq
Cutinite
Bituminite	1.3 - 1.5	0.14 - 0.17
Alginite	1.1 - 1.4	0.10 - 0.07	0.18	1.01 - 1.15 Aq
Vitrinite	0.39 - 0.9	0.10 - 0.33	0.5 - 0.9
Telocollinite
Desmocollinite
Inertinite
Semifusinite	0.5 - 0.64	0.14 m-0,13		1.28 - 1.50 He
Fusinite	~0.5	~0.13		1.40 - 1.60 He
Micrinite
Macrinite
Inertodetrinite

A Table of properties such as this can be misleading. The ranges given for the maceral properties are affected by several factors. One is the rank of the coal. Most macerals will show a decrease in
H/C, O/C, and N/C ratios as coal rank increases. Density of many macerals goes through an even more complicated change. However, there are also variations in chemistry for macerals of the same rank.
This can be caused by such factors as the conditions of the original peat, and the variation in chemistry of the original plant part. For instance, it is well known that pollens and spores have different
chemical resistances and therefore different structures.

From the Table, coal contains large amounts of aromatic material. In high volatile bituminous coals most of the aromatics moieties are substituted benzenes and naphtalenes containing macromolecules. These macromolecules may have molecular weights in the range of several thousand daltons, but this is not
completely established. As the rank increases the size of the aromatics tends to increase. At the right is a model of bituminous coal. However, there is still much speculation on coal structure.

This quite specific structure of coal does bother some coal scientists, and with our present knowledge of coal chemistry, they prefer a less specific picture of coal similar to that shown below. The numbers represent the number of carbons in a cluster (mostly aromatic). The single lines represent connections between clusters.
Although this structure is based on the properties of a higher rank coal than the previous structure you can see that some relationships are similar. For instance, the idea of linked compact aromatic clusters and the presence of heteroatomes. Nevertheless, this latter representation of a high rank coal does point out that there are many gaps in our understanding of coal's physical and chemical structure. Part of the reason for this uncertainty is that all our evidence for the detailed macromolecular structure of coal is based on both thermal and chemical degradation methods, which themselves have uncertainties and complications in their reaction mechanisms.

میکا- Mica

Mica

Any one of a group of hydrous aluminum silicate minerals with platy morphology and perfect basal (micaceous) cleavage.

Structure

 Sheets can be produced from silicate (SiO4) tetrahedra by having each tetrahedron share each of its three basal oxygen atoms with a different adjacent SiO4 tetrahedron. The overall stoichiometry of these sheets will be (Si2O5)2-. Such sheets can also be considered to be produced by the linking together of double chains of SiO4 tetrahedra such as occur in the amphibole structure. The structures of trioctahedral phyllosilicates can be thought of as being derived from that of brucite [Mg3(OH)6], a three-layer structure in which a layer of octahedrally coordinated magnesium ions (Mg2+) is sandwiched between layers of hydroxyl (OH−) groups. If the sheets described above replace those two OH- layers in brucite, the mineral talc [Mg3(Si4O10)(OH)2] is produced; in this structure the Mg2+ ions remain octahedrally coordinated, this time by the unshared apical oxygen ions of the sheet-forming tetrahedra and by the remaining hydroxyl ions between the sheets. In the micas, the net negative charge on the sheets is increased by the substitution of one aluminum ion (Al3+) for one of every four silicon ions (Si4+), and this is compensated for by the addition of one alkali metal ion (for each Al3+ ion) between the two layers of basal oxygen ions that form the top and bottom of two three-layer sandwiches stacked on top of each other. If potassium ion (K+) is the alkali, the trioctahedral mica phlogopite [KMg3(AlSi3O10)(OH)2] results. The structure of dioctahedral micas is related in an analogous way to that of the three-layer structure of gibbsite [Al2(OH)6], giving such phases as muscovite [KAl2(AlSi3O10)(OH)2]. In the brittle micas, there is a divalent ion (for example calcium, Ca2+) between the sandwiches. Each mica can exist in several structural types or polytypes. This occurs because the sheets can be rotated relative to one another, and different stacking sequences can result from repeating layers that have been rotated by specific amounts at regular intervals, that is, at every so many layers. Micas crystallize in the monoclinic system, but lepidolite can be trigonal. See also: Amphibole; Crystal structure; Lepidolite; Muscovite; Phlogopite; Talc

 

Chemistry

 The most common micas are muscovite [KAl2(AlSi3O10)(OH)2], paragonite [NaAl2-(AlSi3O10)(OH)2], phlogopite [K(Mg,Fe)3(AlSi3O10)-(OH)2], biotite [K(Fe,Mg)3(AlSi3O10)(OH)2], and lepidolite [K(Li,Al)2.5-3.0(Al1.0-0.5Si3.0-3.5O10)(OH)2]. Calcium (Ca), barium (Ba), rubidium (Rb) and cesium (Cs) can substitute for sodium (Na) and potassium (K); manganese (Mn), chromium (Cr), and titanium (Ti) for magnesium (Mg), iron (Fe), and lithium (Li); and fluorine (F) for hydroxyl (OH).

 

Physical properties

 Mica is commonly found as small flakes or lamellar plates without a crystal outline. Muscovite and biotite sometimes occur in thick books, tabular prisms with a hexagonal outline that can be up to several feet across. The prominent basal cleavage is a consequence of the layered crystal structure. Thin cleavage sheets of micas, particularly muscovite and phlogopite, are flexible, elastic, tough, and translucent to transparent (isinglass). They have low electrical and thermal conductivity and high dielectric strength. Percussion figures may be developed on cleavage plates by striking the surface sharply with a dull-pointed tool. This yields a six-rayed star, the rays of which are parallel to certain crystallographic directions.

Micas have Mohs hardnesses of 2–3 and specific gravities of 2.8–3.2. Upon heating in a closed tube, they evolve water. They have a vitreous-to-pearly luster. Muscovite is colorless to pale shades of brown, green, or gray. Paragonite is colorless to pale yellow. Phlogopite is pale yellow to brown. Biotite is dark green, brown, or black. Lepidolite is most often pale lilac, but it can also be colorless, pale yellow, or pale gray. See also: Biotite

 

Occurrence

The three major species, muscovite, biotite, and phlogopite, are widely distributed rock-forming minerals, occurring as essential constituents in a variety of igneous, metamorphic, and sedimentary rocks and in many mineral deposits.

Muscovite is found in regionally metamorphosed aluminous rocks that formed under a wide range of physical conditions. In igneous rocks, it occurs in some types of granites, in aplites, and as books in pegmatites. It is a characteristic phase of greisens which are produced when fluorine and other volatiles are introduced from granitic melts into adjacent rocks. Sericite is the name given to fine-grained white mica, usually muscovite. This mineral is a widespread gangue mineral in many hydrothermal ore deposits, either in the deposits themselves or in the adjacent altered wall rocks. Paragonite occurs in schists, gneisses, quartz veins, and fine-grained sediments. Phlogopite is a product of regional metamorphism of impure magnesian limestone. It is also characteristic of mantle-derived kimberlites and inclusions in kimberlites. Of all the micas, biotite occurs in the widest range of geological settings. It is common in the thermally metamorphosed rocks adjacent to granitic intrusions. In regionally metamorphosed rocks, it occurs in schists with chlorite, garnet, staurolite, kyanite, and sillimanite, although not all of these simultaneously. In plutonic igneous rocks, biotite is most common in intermediate and acid rocks, but it even occurs in some norites. Biotite books are found in pegmatites. Lepidolite is the most common lithium-bearing mineral and occurs almost exclusively in pegmatites with beryl, topaz, tourmaline, and other lithium minerals such as spodumene and amblygonite. It is also found occasionally in granites and aplites. See also: Aplite; Granite; Ore and mineral deposits; Pegmatite

 

Uses

Commercial mica is of two main types: sheet, and scrap or flake. Sheet muscovite, mostly from pegmatites, is used as a dielectric in capacitors and vacuum tubes in electronic equipment. Lower-quality muscovite is used as an insulator in home electrical products such as hot plates, toasters, and irons. Scrap and flake mica is ground for use in coatings on roofing materials and waterproof fabrics, and in paint, wallpaper, joint cement, plastics, cosmetics, well drilling products, and a variety of agricultural products. See also: Electric insulator; Silicate minerals

Bibliography

• W. A. Deer, R. A. Howie, and J. Zussman, An Introduction to the Rock-Forming Minerals, 2d, ed., 1992   Alifazeli = egeology.blogfa.com
• C. Klein and C. S. Hurlbut, Jr., Manual of Mineralogy, 21st ed., rev. 1999  Alifazeli = egeology.blogfa.com
• J. J. Papike, Chemistry of the rock-forming silicates: Multiple-chain, sheet, and framework structures, Rev. Geophys., 26:407--444, 1988 Alifazeli = egeology.blogfa.com
• Alifazeli = egeology.blogfa.com

 

میکا-MICA

MICA

Mica: Micas commonly occur as flakes, books, or sheets. Sheet muscovite (white) mica is used in electronic insulators (mainly in vacuum tubes), ground mica in paint, as joint cement, as a dusting agent, in well-drilling muds, and in plastics, roofing, rubber, and welding rods.

Photo Courtesy of Simon Williams, from 

Mica Schist

Muscovite 

---

Background

Mica is a mineral name given to a group of minerals that are similar in their physical properties and chemical compositions. They are all silicate minerals, which means that chemically they all contain silica(SiO₄). Mineralogists call micas sheet silicates because their molecules combine to form distinct layers. These layers can be seen in muscovite mica specimens because it can be split (mineralogists call this feature cleavage) into very thin, flexible, transparent layers. This physical property is so distinctive that all minerals that cleave in this fashion are said to have micaceous cleavage.

There are 37 different mica minerals. In addition to the silicate tetrahedrons in all micas, Purplelepidolite.

contains the elements potassium, lithium, and aluminum. Black biotite contains potassium, iron, and magnesium. The two micas used as a commodity are: brown mica or phlogopite which contains iron and magnesium; and the "reddish, green, or white (or clear) mica" or muscovite which contains potassium and aluminum.

A typical mica crystal.

Large sheets of muscovite form in igneous rocks. Very large sheets or crystals of muscovite form in a pegmatite. A pegmatite is an extremely slow-cooling igneous rock in which very large crystals can form. Phlogopite generally forms in metamorphic rocks, especially in metamorphosed limestone, although it occasionally forms in igneous rocks, too.

Mica crystals are six-sided. They are fairly light and relatively soft, at 2 to 4 on Mohs' hardness scale for the univalent micas. Sheets and flakes of mica are flexible. Mica is heat-resistant and does not conduct electricity.

Two distinct forms of mica are utilized as a commodity. Scrap and flake mica is mica that either occurs naturally or is ground into very small flakes and pieces. Sheet mica is large pieces of mica that can be cut into various shapes for use in electronics.

Name

The name mica was probably created from the Latin word micare meaning to shine in reference to the shiny luster of the micas. Muscovite is very resistant to heat and electricity. As a result, it was commonly called "Muscovy." This mineral was commonly called Muscovy Glass after the Latin term vitrum Muscoviticum. In 1850, James Dwight Dana formally named this mineral muscovite based on the Latin term. The name phlogopite, named by F.A. Breithaupt in 1841, comes from the Greek word phologopos meaning fiery in reference to the reddish color seen on some specimens of this mica.

Sources

Scrap and flake mica is produced all over the world. In the U.S., scrap and flake mica was produced in Arizona, North Carolina, South Dakota, Georgia, New Mexico and South Carolina. North Carolina's production accounts for half of total U.S. mica production. The flake mica produced in the U.S. comes from several sources: the metamorphic rock called schist as a by-product of processing feldspar and kaolin resources, from placer deposits, and from pegmatites. Canada, India, Finland, and Japan export flake mica to the U.S.

Sheet mica is considerably less abundant than flake and scrap mica. Sheet mica is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are the pegmatite deposits. The United States has limited sheet mica resources. U.S. mining of sheet mica is costly and labor costs are high. As a result, the U.S. imports more than half its sheet mica from India, but also from Belgium, Germany, China, and a few other countries.

Uses

The principal use of ground mica is in gypsum wallboard joint compound, where it acts as a filler and extender, provides a smoother consistency, improves workability, and prevents cracking. In the paint industry, ground mica is used as a pigment extender that also facilitates suspension due to its light weight and platy morphology. The ground mica also reduces checking and chalking, prevents shrinkage and shearing of the paint film, provides increased resistance to water penetration and weathering, and brightens the tone of colored pigments. Ground mica also is used in the well-drilling industry as an additive to drilling ?muds.?

Coarsely ground mica flakes help prevent lost circulation by sealing porous sections of the uncased drill hole. The plastic industry used ground mica as an extender and filler and also as a reinforcing agent. The rubber industry uses ground mica as an inert filler and as a mold lubricant in the manufacture of molded rubber products, including tires.

Sheet mica is used principally in the electronic and electrical industries. The major uses of sheet and block mica are as electrical insulators in electronic equipment, thermal insulation, gauge ?glass?, windows in stove and kerosene heaters, dielectrics in capacitors, decorative panels in lamps and windows, insulation in electric motors and generator armatures, field coil insulation, and magnet and commutator core insulation. Mica is also used as segment plates between copper commutator sections to insulate copper from the steel; phlogopite mica is used because it wears at the same rate as the copper segments.

Substitutes and Alternative Sources

 Ali Fazeli = egeology.blogfa.com

Some lightweight mineral and rock materials, such as vermiculite, diatomite and perlite are similar to micas and can be used in place of mica. A long list of manufactured materials, such as styrene, polyester, Teflon⼯font> , Plexiglass⼯font> , etc., can be used in place of sheet mica in the electronic applications. Paper made from ground mica can be used in place of sheet mica for insulating applications.

  Ali Fazeli = egeology.blogfa.com

Phlogopite crystals from New York

Mica drawings used with permission. Ali Fazeli = egeology.blogfa.com
Ӽ/font> 2001 Darryl Powell

 Ali Fazeli = egeology.blogfa.com Ali Fazeli = egeology.blogfa.com

تونل-Tunnel

Tunnel

 An underground space of substantial length, usually having a tubular shape. Tunnels can be either constructed or natural and are used as passageways, storage areas, carriageways, and utility ducts. They may also be used for mining, water supply, sewerage, flood prevention, and civil defense.

Construction

Tunnels are constructed in numerous ways. Shallow tunnels are usually constructed by burying sections of tunnel structures in trenches dug from the surface. This is a preferred method of tunneling as long as space is available and the operation will not cause disturbance to surface activities. Otherwise, tunnels can be constructed by boring underground. Short tunnels are usually bored manually or by using light machines (such as a roadheader or backhoe). If the ground is too hard to bore, a drill-and-blast method is frequently used. For long tunnels, it is more economical and much faster to use tunneling boring machines which work on the full face (complete diameter of the opening) at the same time. In uniform massive rock formations without fissures or joints, tunnels can be bored without any temporary supports to hold up the tunnel crowns. However, temporary supports are usually required because of the presence of destabilizing fissures and joints in the rock mass (Fig. 1). A layer of shotcrete serves as the primary lining to protect the newly exposed surface and to support the tunnel crowns as well. The shotcrete is frequently reinforced by steel meshes and, if necessary, braced by steel lattices.  See also: Drilling, geotechnical

 

 

Fig. 1  Temporary supports for a four-lane road tunnel. (RESA Engineering Corporation)

 

 

In soft ground, it has become popular to use shield machines for boring and reinforced concrete segments for lining. The largest shield machine ever, 14 m (46 ft) in diameter, was used in constructing the Tokyo Trans-Bay Highway (Tokyo Wan Aqua-Line) between Kawasaki City and Kisarazu City of Japan (completed in 1997). There are various types of shield machines available to serve different purposes. The multiface shield machine (Fig. 2) was first used in constructing the Osaka Business Park subway station in Japan (completed in 1995). The machine is 17 m (56 ft) in width and 7.5 m (25 ft) in height and has three cutters which operate independently. Theoretically, the two side cutters can be detached from the center cutter upon the completion of station excavation, leaving the center cutter to continue boring toward the next station. However, this technique was not applied in this case, and the machine was used for the Osaka Business Park station only.

 

 

Fig. 2  Multiface shield tunneling machine. (Hitachi Zosen Corporation)

 

 

 

 

 

Immersed tunnels

 For tunnels to be constructed across bodies of water, an alternative to boring is to lay tunnel boxes directly on the prepared seabed. These boxes, made of either steel or reinforced concrete, are prepared in dry docks and sealed at their ends by the use of bulkheads. They float as the docks are flooded, and are towed to the site by tugboats. The boxes are then flooded to allow them to sink to the seabed after they are properly positioned. Immersed tunnels are usually buried in shallow trenches dug for this purpose and covered by ballast so they will not be affected by the movement of the water. The joints between tunnel sections are made watertight by using rubber gaskets, and water is pumped out of the tunnel to make it ready for service. Among the numerous immersed tunnels the Øresund Link, completed in 1999 [3.5 km (2.2 mi)], between Denmark and Sweden is second in length to the cross-bay tunnel [5.8 km (3.6 mi)] for the Bay Area Rapid Transit system of San Francisco, California.  See also: Concrete; Steel

 

Microtunnels

 Small tunnels, such as sewer lines and water mains, are usually installed by jacking steel or concrete pipes into the ground (Fig. 3). The soil core inside the tubes can be removed manually or by using “moles,” which are essentially small shield tunneling machines. The alignment of the pipes is continuously monitored and adjusted. With the moles guided by a computerized navigation system, it is possible to align pipes to a precision within 100 mm (4 in.) regardless of length. This technique has been used for jacking pipes as large as 2 m (7 ft) or so in diameter for distances more than 100 m (330 ft). Pipes with smaller sizes can be jacked to distances of more than 300 m (990 ft).

 

 

Fig. 3  Microtunneling and pipe jacking technique.

 

 

 

 Special tunneling techniques

 Auxiliary measures are frequently required to ensure the safety of tunnels during boring in soft ground. Compressed air was used in the past, but is it is seldom used anymore because improper decompression may cause aeroembolism (diver's disease) in workers. Instead, grouting and ground freezing are now preferred. In the Central Artery Project (scheduled to be completed in 2004) in Boston, Massachusetts, ground freezing is carried out to permit three tunnels to be bored under the railway tracks leading to the South Station Railway Terminal. Roughly 1600 freezing pipes are installed to depths varying from 13.7 to 16.8 m (45 to 55 ft), and the volume of frozen soil is more than 60,000 m3 (2,100,000 ft3). This could be the largest undertaking of this nature.

An underpass, scheduled to be completed in 2001, is being constructed beneath the Taipei International Airport by using the Endless-Self-Advancing method. To minimize ground settlements and ensure the safety of air traffic, interlocked steel pipes are first jacked into the ground to form a protective shelter. The soil core inside the shelter is excavated at the rate of 400 mm (16 in.) per lift. Concrete segments are moved one by one into the space created, by jacking the segments behind. Each time only the last segment is jacked forward, and the jacking force is taken by the frictional resistance acting on all the rest of the segments. Cables are anchored to the first and last segments so the force acting on the last segment can be transmitted to the segments in front of it. The movements of these segments resemble the movements of centipedes. In theory, there is no limit on the length of tunnels installed by this method.

 

Longest tunnels

 The longest tunnel of any kind is the New York City/West Delaware water supply tunnel (completed in 1944). It runs for 169 km (105 mi) from the Rondout Reservoir into the Hillview Reservoir in Yonkers, New York. The longest rail tunnel is Seikan Tunnel (53.6 km; 33.3 mi) in Japan (completed in 1985). The longest road tunnel was Saint Gotthard Tunnel (16.9 km; 10.5 mi) in Switzerland (completed in 1980). In mid2001 the title was taken over by the Laerdal Tunnel (24.5 km; 15.2 mi).

The longest undersea tunnel is the Channel Tunnel [49.4 km (30.7 mi), of which 38 km (23.6 mi) is undersea] across the English Strait. It runs from Folkestone in Britain to Calais in France. There are two running tunnels plus one service tunnel in the center, 7.6 and 4.8 m (25 and 16 ft) in internal diameter, respectively. The tunnel was officially inaugurated on May 6, 1994, when the Queen of England and President Mitterrand of France became the first official passengers to pass by train between the two countries. Eurotunnel has a concession from the British and French governments to run the tunnel until 2052. Shuttle trains, carrying up to 180 cars, and freight shuttles, carrying 28 lorries, will take 35 minutes to cross the strait.

Bibliography

•  C. J. Kirkland (ed.), Eurotunnel: Engineering the Channel Tunnel, 1995
• Alifazeli = egeology.blogfa.com
• J. O. Bickel, T. R. Kuesel, and E. H. King (eds.), Tunnel Engineering Handbook, 1995
• Alifazeli = egeology.blogfa.com
• Z. C. Moh et al., Underpass beneath Taipei International Airport, Proceedings of the Conference on New Frontiers and Challenges, Bangkok, Thailand, November 8–12, 1999
• Alifazeli = egeology.blogfa.com
• A. M. M. Wood and A. M. Wood, Tunnelling: Management by Design, 2000
• Alifazeli = egeology.blogfa.com