Transcriber’s Notes
Text between _underscores_, =equal signs= and ~tildes~ represents text
printed in italics, bold face and sans-serif, respectively. Small
capitals have been replaced with ALL CAPITALS.
More Transcriber’s Notes may be found at the end of this text.
TUNNELING:
A PRACTICAL TREATISE
BY
CHARLES PRELINI, C. E.
AUTHOR OF “EARTH AND ROCK EXCAVATION,” “DREDGES AND DREDGING,”
“EARTH SLOPES, RETAINING WALLS AND DAMS,” ETC. PROFESSOR
OF CIVIL ENGINEERING IN MANHATTAN COLLEGE,
NEW YORK
_167 ILLUSTRATIONS_
SIXTH EDITION, REVISED AND ENLARGED
[Illustration]
NEW YORK
D. VAN NOSTRAND COMPANY
TWENTY-FIVE PARK PLACE
1912
COPYRIGHT, 1912,
BY
D. VAN NOSTRAND COMPANY
NEW YORK
Stanhope Press
F. H. GILSON COMPANY
BOSTON, U.S.A.
PREFACE TO THE SIXTH EDITION
During the few years that have elapsed since the publication of the
first edition of this work, the art of tunneling through different soils
and especially under large bodies of water, has made considerable
progress. During the last ten years, no less than eight subaqueous
tunnels involving the construction of sixteen tubes have been
constructed for the service of the city of New York alone. The reader
will, no doubt, also recall the tunnels under the Boston Harbor, the St.
Clair, the Charles and Detroit Rivers in our own country as well as the
tunnels under the Thames and the Seine in Europe. Engineers, contractors
and workmen have acquired such experience in these difficult underground
and under-river construction that the work is now undertaken without any
of the fear and hesitation that were associated with the earlier
enterprises.
As entirely new methods have been introduced by professional men, it was
found necessary to arrange the presentation of the subject in this sixth
edition so as to give due prominence to these recent methods.
Besides this, other changes have been made in order to give greater
attention to American method of excavating tunnels through rock and
loose soil. This will explain the treatment of the crown-bar and also
the extensive illustration of the heading and bench method as well as
the drift method of driving tunnels which is followed in the United
States.
Space has also been given to important tunnels recently built mainly for
the purpose of illustrating the various methods discussed in the text
and also to bring out more clearly the characteristics of the different
methods of tunnel excavation.
The author hopes that these added features will meet the present
requirements of engineers and students.
CHARLES PRELINI.
MANHATTAN COLLEGE,
NEW YORK CITY.
CONTENTS
PAGE
INTRODUCTORY--THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING xiii
CHAPTER
I. PRELIMINARY CONSIDERATIONS; CHOICE BETWEEN A TUNNEL AND AN
OPEN CUT; GEOLOGICAL SURVEYS 1
II. METHODS OF DETERMINING THE CENTER LINE AND FORMS AND
DIMENSIONS OF CROSS-SECTION 9
III. EXCAVATING MACHINES AND ROCK DRILLS; EXPLOSIVES AND
BLASTING 22
IV. GENERAL METHODS OF EXCAVATION; SHAFTS; CLASSIFICATION OF
TUNNELS 36
V. METHODS OF TIMBERING OR STRUTTING TUNNELS 47
VI. METHODS OF HAULING IN TUNNELS 59
VII. TYPES OF CENTERS AND MOLDS EMPLOYED IN CONSTRUCTING TUNNEL
LININGS OF MASONRY 66
VIII. METHODS OF LINING TUNNELS 72
IX. TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION;
REPRESENTATIVE MECHANICAL INSTALLATIONS FOR TUNNEL WORK 84
X. TUNNELS THROUGH HARD ROCK (_continued_); EXCAVATION BY
DRIFTS; THE SIMPLON AND MURRAY HILL TUNNELS 102
XI. TUNNELS THROUGH HARD ROCK (_continued_); EXCAVATION BY
HEADINGS 130
XII. EXCAVATING TUNNELS THROUGH SOFT GROUND; GENERAL
DISCUSSION; THE BELGIAN METHOD 143
XIII. THE GERMAN METHOD--EXCAVATING TUNNELS THROUGH SOFT GROUND
(_continued_); BALTIMORE BELT LINE TUNNEL 155
XIV. THE FULL SECTION METHOD OF TUNNELING; ENGLISH METHOD;
AMERICAN METHOD; AUSTRIAN METHOD 166
XV. SPECIAL TREACHEROUS GROUND METHOD; ITALIAN METHOD;
QUICKSAND TUNNELING; PILOT METHOD 182
XVI. OPEN-CUT TUNNELING METHODS; TUNNELS UNDER CITY STREETS;
BOSTON SUBWAY AND NEW YORK RAPID TRANSIT 195
XVII. SUBMARINE TUNNELING; GENERAL DISCUSSION; THE SEVERN TUNNEL 218
XVIII. SUBMARINE TUNNELING (_continued_); THE COMPRESSED AIR
METHOD; THE MILWAUKEE WATER-WORKS TUNNEL 225
XIX. SUBMARINE TUNNELING (_continued_); THE SHIELD SYSTEM 238
XX. SUBMARINE TUNNELING (_continued_); THE SHIELD AND
COMPRESSED AIR METHOD; THE HUDSON RIVER TUNNEL OF THE
PENNSYLVANIA RAILROAD 263
XXI. SUBMARINE TUNNELING (_continued_); TUNNELS AT VERY SHALLOW
DEPTH; THE COFFERDAM METHOD; THE PNEUMATIC CAISSON METHOD;
THE JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND 281
XXII. ACCIDENTS AND REPAIRS IN TUNNELS DURING AND AFTER
CONSTRUCTION 301
XXIII. RELINING TIMBER-LINED TUNNELS WITH MASONRY 315
XXIV. THE VENTILATION AND LIGHTING OF TUNNELS DURING
CONSTRUCTION 325
XXV. THE COST OF TUNNEL EXCAVATION AND THE TIME REQUIRED FOR
WORK 336
LIST OF ILLUSTRATIONS
FIGURE PAGE
1. Diagram Showing Manner of Lining in Rectilinear Tunnels 10
2. B. R. Value’s Device for Locating the Center Line Inside of
a Tunnel 11
3. Triangulation System for Establishing the Center Line of the
St. Gothard Tunnel 12
4. Method of Transferring the Center Line down Center Shafts 13
5. Method of Transferring the Center Line down the Side Shafts 14
6. Method of Laying out the Center Line of Curvilinear Tunnels 15
7. Diagram of Polycentric Sectional Profile 19
8, 9 and 10. Typical Sectional Profiles for Tunnel 20
11. Soft Ground Bucket Excavating Machine; Central London
Underground Railway 22
12. Column Mounting for Percussion Drill; Ingersoll Sargent Drill
Co. 26
13. Sketch of Diamond Drill Bit 27
14. Diagram Showing Sequence of Excavation for St. Gothard
Tunnel 36
15. Diagram Showing Manner of Determining Correspondence of
Excavation to Sectional Profile 38
16. Polar Protractor for Determining Profile of Excavated Cross-
Section 39
17. Joining Tunnel Struts by Halving 48
18. Round Timber Post and Cap Bearing 48
19. Ceiling Strutting for Tunnel Roofs 49
20. Ceiling Strutting with Side Post Supports 49
21. Sill, Side Post and Cap Cross Frame Strutting 49
22. Reinforced Cross Frame Strutting for Treacherous Materials 49
23. Longitudinal Poling-Board System of Roof Strutting 50
24. Transverse Poling-Board System of Roof Strutting 50
25. Shaft with Single Transverse Strutting 52
26. Rectangular Frame Strutting for Shafts 53
27. Reinforced Rectangular Frame Strutting for Shafts in
Treacherous Materials 53
28. Strutting of Timber Posts and Railway Rail Caps 56
29. Strutting Made Entirely of Railway Rails 56
30. Rziha’s Combined Strutting and Centering of Cast Iron 57
31. Cast-Iron Segment of Rziha’s Strutting and Centering 57
32. Cast-Iron Segmental Strutting for Shafts 58
33. Platform Car for Tunnel Work 59
34. Iron Dump-Car for Tunnel Work 60
35. Wooden Dump-Car for Tunnel Work 60
36. Box-Car for Tunnel Work 61
37. Elevator Car for Tunnel Shafts 65
38. Ground Mold for Constructing Tunnel Invert Masonry 67
39. Combined Ground Mold and Leading Frame for Invert and Side
Wall Masonry 67
40. Leading Frame for Constructing Side Wall Masonry 68
41. Plank Center for Constructing the Roof Arch 69
42. Trussed Center for Constructing the Roof Arch 70
43 and 44. A Typical Form of Timber Lining for Tunnels 73
45. Diagram Showing Forms adopted for Side-Wall Foundations 76
46 and 47. Transverse Sections of Tunnels Showing Methods for
Increasing the Thickness of the Lining at Different Points 79
48. Refuge Niche in St. Gothard Tunnel 81
49. East Portal of Hoosac Tunnel 82
50, 51 and 52. Arrangement of Drill Holes in the Heading of
Turchino Tunnel 91
53 and 54. Arrangement of Drill Holes in the Heading of the Fort
George Tunnel 91
55. Diagram Showing Sequence of Excavations in Drift Method of
Tunneling Rock 102
56. Sketches Showing Sequence of Work in Excavating and Lining
the Simplon Tunnel 111
57. General Details of the Brandt Rotary Drills Employed at the
Simplon Tunnel 112
58. Sequence of Excavation in the Murray Hill Tunnel 124
59. Traveling Platform for the Excavation of the Upper Side of
the Murray Hill Tunnel 125
60. Timbering Used in the Murray Hill Tunnel 126
61. Diagram Showing Sequence of Excavation in Heading Method of
Tunneling Rock 132
62. Method of Strutting Roof, St. Gothard Tunnel 135
63. Sketch Showing Arrangement of Tracks, St. Gothard Tunnel 135
64. Arrangement of Drill Holes in the Fort George Tunnel 137
65. Longitudinal Section of the Heading and Bench Excavation at
the Fort George Tunnel 137
66. Diagram Showing the Arrangement of Drill Holes in the
Heading and Bench of the Gallitsin Tunnel 140
67. Diagram Showing Modification of the Heading and Bench Method 140
68 and 68A. Diagrams Showing Sequence of Excavation in the
Belgian Method 145
69. Sketch Showing Radial Roof Strutting, Belgian Method 147
70. Sketch Showing Roof Arch Center, Belgian Method 147
71. Sketch Showing Method of Underpinning Roof Arch with the
Side Wall Masonry 149
72. Longitudinal Section Showing Construction by the Belgian
Method 149
73. Diagram Showing Sequence of Excavation in Modified Belgian
Method 152
74. Sketch Showing Failure of Roof Arch by Opening at Crown 153
75. Sketch Showing Methods of Repairing Roof Arch Failures 154
76. Diagrams Showing Sequence of Excavation in German Method of
Tunneling 155
77. Diagram Showing Sequence of Excavation in Water Bearing
Material, German Method 156
78. Sketch Showing Work of Excavating and Timbering Drifts and
Headings 157
79. Sketch Showing Method of Roof Strutting 157
80. Sketch Showing Roof Arch Centers and Arch Construction 158
81. Sketch Showing Method of Excavating and Strutting Baltimore
Belt Line Tunnel 162
82. Roof Arch Construction with Timber Centers, Baltimore Belt
Line Tunnel 163
83. Roof Arch Construction with Iron Centers, Baltimore Belt
Line Tunnel 164
84. Diagram Showing Sequence of Excavation in English Method of
Tunneling 167
85. Sketches Showing Construction of Strutting, English Method 168
86 and 87. Sketches of Typical Timber Roof-Arch Centers, English
Method 169
88. Sequence of Excavation in the American Method 172
89. Strutting the Heading in the American Method 172
90. Temporary Timbering of the Roof in the American Method 173
91. Showing Crown Bars Supported by Segmental Arches 173
92. Transversal and Longitudinal Section of a Tunnel Excavated
and Strutted According to the American Method 174
93 and 94. Diagrams Showing Sequence of Excavation in Austrian
Method of Tunneling 177
95, 96 and 97. Sketches Showing Construction of Strutting,
Austrian Method 178
98. Sketch Showing Manner of Constructing the Lining Masonry,
Austrian Method 179
99. Diagram Showing Sequence of Excavation in Italian Method of
Tunneling 183
100. Sketch Showing Strutting for Lower Part of Section 183
101 and 101A. Sketches Showing Construction of Centers, Italian
Method 184
102. Sketch Showing Invert and Foundation Masonry, Italian Method. 185
103. Sketch Showing Longitudinal Section of a Tunnel under
Construction, Italian Method 186
104. Sketch Showing Sequence of Excavation, Stazza Tunnel 186
105. Sketch Showing Method of Strutting First Drift, Stazza
Tunnel 187
106 and 107. Sketches Showing Temporary Strutting Arch
Construction, Stazza Tunnel 187
108. Sketch Showing Preliminary Drainage Galleries, Quicksand
Method 190
109. Sketch Showing Construction of Roof Strutting, Quicksand
Method 190
110. Sketch Showing Construction of Masonry Lining, Quicksand
Method 191
111. Sketch Showing Pilot Method of Tunneling 193
112. Diagram Showing Sequence of Construction in Open-Cut Tunnels 197
113. Sketch Showing Method of Timbering Open-Cut Tunnels, Double
Parallel Trench Method 198
114. Side-Wall Foundation Construction Open-Cut Tunnels 198
115. Wide-Arch Section, Boston Subway 204
116. Double-Barrel Section, Boston Subway 205
117. Four-Track Rectangular Section, Boston Subway 206
118. Section Showing Slice Method of Construction, Boston Subway 206
119. Double-Track Section, New York Rapid Transit Railway 212
120. Park Avenue Deep Tunnel Construction, New York Rapid Transit
Railway 214
121. Harlem River Tunnel, New York Rapid Transit Railway 215
122. Sketch Showing Underground Stream, Milwaukee Water-Works
Tunnel 229
123. Sketch Showing Methods of Lining, Milwaukee Water-Works
Tunnel 232
124. Longitudinal Section of Brunel’s Shield, First Thames Tunnel 241
125. First Shield Invented by Barlow 242
126. Second Shield Invented by Barlow 243
127. Shield Suggested by Greathead for the Proposed North and
South Woolwich Subway 245
128. Beach’s Shield Used on Broadway Pneumatic Railway Tunnel 245
129. Shield for City and South London Railway 246
130. Shield for St. Clair River Tunnel 247
131. Shield for Blackwall Tunnel 248
132. Elliptical Shield for Clichy Sewer Tunnel, Paris 249
133. Semi-Elliptical Shield for Clichy Sewer Tunnel 250
134. Roof Shield for Boston Subway 251
135. Transversal and Longitudinal Section of Prelini’s Shield 252
136. Elevation and Section of Hydraulic Jack, East River Gas
Tunnel 260
137. Cast-Iron Lining, St. Clair River Tunnel 262
138. General Elevations and Sections of Shields 270
139. Plan and Elevation of First Bulkhead Wall in South Tube,
Manhattan 273
140. Typical Cross-Sections of One Tube of Pennsylvania Railroad
Tunnel under the Hudson River 278
141. Sections of Cofferdam, Van Buren St. Tunnel, Chicago 283
142. Showing Working Platforms and Piles Sunk in Dredged Channel 286
143. Showing Sheeting-Piles for the Sides of the Caisson and
Trussed Beam for the Roof 287
144. Showing the Caisson with the Working-Chamber 287
145. Showing the Tunnel Constructed within the Caisson 289
146. Showing Sides of the Caisson and Supports for the Roof 290
147. Showing the Roof of the Caisson Formed by the Upper Half of
the Tunnel 291
148. Showing the Tunnel Completed by Building the Lower Half
within the Caisson 292
149. Transversal Section of the Caissons for the Tunnel under the
Seine River 294
150. Showing the Joining of the Caissons at the Pont Mirabeau
Tunnel under the Seine River 295
151. Cross-Sections and Plans of the Detroit River Tunnel 298
152. Tunneling through Caved Material by Heading 306
153. Tunneling through Caved Material by Drifts 307
154 and 155. Filling in Roof Cavity Formed by Falling Material 307
156. Timbering to Prevent Landslides at Portal 308
157. Shortening Tunnel Crushed by Landslide at Portal 308
158. Extending Tunnel through Landslide at Portal 309
159 and 160. Relining Timber-Lined Tunnel 316
161. Relining Timber-Lined Tunnel, Great Northern Ry 317
162. Relining Timber-Lined Tunnel, Great Northern Ry 318
163. Relining Timber-Lined Tunnel, Great Northern Ry 319
164. Construction of Centering Mullan Tunnel 320
165. Centering Mullan Tunnel 321
166. Relining Timber-Lined Tunnel, Norfolk & Western Ry 322
167. Relining Timber-Lined Tunnel, Norfolk & Western Ry 323
INTRODUCTION
THE HISTORICAL DEVELOPMENT OF TUNNEL BUILDING.
A tunnel, defined as an engineering structure, is an artificial gallery,
passage, or roadway beneath the ground, under the bed of a stream, or
through a hill or mountain. The art of tunneling has been known to man
since very ancient times. A Theban king on ascending the throne began at
once to drive the long, narrow passage or tunnel leading to the inner
chamber or sepulcher of the rock-cut tomb which was to form his final
resting-place. Some of these rock-cut galleries of the ancient Egyptian
kings were over 750 ft. long. Similar rock-cut tunneling work was
performed by the Nubians and Indians in building their temples, by the
Aztecs in America, and in fact by most of the ancient civilized peoples.
The first built-up tunnels of which there are any existing records were
those constructed by the Assyrians. The vaulted drain or passage under
the southeast palace of Nimrud, built by Shalmaneser II. (860-824 B.C.),
is in all essentials a true soft-ground tunnel, with a masonry lining. A
much better example, however, is the tunnel under the Euphrates River,
which may quite accurately be claimed as the first submarine tunnel of
which there exists any record. It was, however, built under the dry bed
of the river, the waters of which were temporarily diverted, and then
turned back into their normal channel after the tunnel work was
completed, thus making it a true submarine tunnel only when finished.
The Euphrates River tunnel was built through soft ground, and was lined
with brick masonry, having interior dimensions of 12 ft. in width and
15 ft. in height.
Only hand labor was employed by these ancient peoples in their tunnel
work. In soft ground the tools used were the pick and shovels, or
scoops. For rock work they possessed a greater range of appliances.
Research has shown that among the Egyptians, by whom the art of
quarrying was highly developed, use was made of tube drills and saws
provided with cutting edges of corundum or other hard, gritty material.
The usual tools for rock work were, however, the hammer, the chisel, and
wedges; and the excellence and magnitude of the works accomplished by
these limited appliances attest the unlimited time and labor which must
have been available for their accomplishment.
The Romans should doubtless rank as the greatest tunnel builders of
antiquity, in the number, magnitude, and useful character of their
works, and in the improvements which they devised in the methods of
tunnel building. They introduced fire as an agent for hastening the
breaking down of the rock, and also developed the familiar principle of
prosecuting the work at several points at once by means of shafts. In
their use of fire the Romans simply took practical advantage of the
familiar fact that when a heated rock is suddenly cooled it cracks and
breaks so that its excavation becomes comparatively easy. Their method
of operation was simply to build large fires in front of the rock to be
broken down, and when it had reached a high temperature to cool it
suddenly by throwing water upon the hot surface. The Romans were also
aware that vinegar affected calcareous rock, and in excavating tunnels
through this material it was a common practice with them to substitute
vinegar for water as the cooling agent, and thus to attack the rock both
chemically and mechanically. It is hardly necessary to say that this
method of excavation was very severe on the workmen because of the heat
and foul gases generated. This was, however, a matter of small concern
to the builders, since the work was usually performed by slaves and
prisoners of war, who perished by thousands. To be sentenced to labor on
Roman tunnel works was thus one of the severest penalties to which a
slave or prisoner could be condemned. They were places of suffering and
death as are to-day the Spanish mercury mines.
Besides their use of fire as an excavating agent, the Romans possessed a
very perfect knowledge of the use of vertical shafts in order to
prosecute the excavation at several different points simultaneously.
Pliny is authority[1] for the statement that in the excavation of the
tunnel for the drainage of Lake Fucino forty shafts and a number of
inclined galleries were sunk along its length of 3¹⁄₂ miles, some of the
shafts being 400 ft. in depth. The spoil was hoisted out of these shafts
in copper pails of about ten gallons’ capacity by windlasses.
[1] “Tunneling,” Encly. Brit., 1889, vol. xxiii., p. 623.
The Roman tunnels were designed for public utility. Among those which
are most notable in this respect, as well as for being fine examples of
tunnel work, may be mentioned the numerous conduits driven through the
calcareous rock between Subiaco and Tivoli to carry to Rome the pure
water from the mountains above Subiaco. This work was done under the
Consul Marcius. The longest of the Roman tunnels is the one built to
drain Lake Fucino, as mentioned above. This tunnel was designed to have
a section of 6 ft. × 10 ft.; but its actual dimensions are not uniform.
It was driven through calcareous rock, and it is stated that 30,000 men
were employed for eleven years in its construction. The tunnels which
have been mentioned, being designed for conduits, were of small section;
but the Romans also built tunnels of larger sections at numerous points
along their magnificent roads. One of the most notable of these is that
which gives the road between Naples and Pozzuoli passage through the
Posilipo hills. It is excavated through volcanic tufa, and is about 3000
ft. long and 25 ft. wide, with a section of the form of a pointed arch.
In order to facilitate the illumination of this tunnel, its floor and
roof were made gradually converging from the ends toward the middle; at
the entrances the section was 75 ft. high, while at the center it was
only 22 ft. high. This double funnel-like construction caused the rays
of light entering the tunnel to concentrate as they approached the
center, and thus to improve the natural illumination. The tunnel is on a
grade. It was probably excavated during the time of Augustus, although
some authorities place its construction at an earlier date.
During the Middle Ages the art of tunnel building was practiced for
military purposes, but seldom for the public need and comfort. Mention
is made of the fact that in 1450 Anne of Lusignan commenced the
construction of a road tunnel under the Col di Tenda in the Piedmontese
Alps to afford better communication between Nice and Genoa; but on
account of its many difficulties the work was never completed, although
it was several times abandoned and resumed. For the most part,
therefore, the tunnel work of the Middle Ages was intended for the
purposes and necessities of war. Every castle had its private
underground passage from the central tower or keep to some distant
concealed place to permit the escape of the family and its retainers in
case of the victory of the enemy, and, during the defense, to allow of
sorties and the entrance of supplies.
The tunnel builders of the Middle Ages added little to the knowledge of
their art. Indeed, until the 17th century and the invention of gunpowder
no practical improvement was made in the tunneling methods of the
Romans. Engravings of mining operations in that century show that
underground excavation was accomplished by the pick or the hammer and
chisel, and that wood fires were lighted at the ends of the headings to
split and soften the rocks in advance. Although gunpowder had been
previously employed in mining, the first important use of it in tunnel
work was at Malpas, France, in 1679-81, in the tunnel for the Languedoc
Canal. This tunnel was 510 ft. long, 22 ft. wide, and 29 ft. high, and
was excavated through tufa. It was left unlined for seven years, and
then was lined with masonry.
With the advent of gunpowder and canal building the first strong impetus
was given to tunnel building, in its modern sense, as a commercial and
public utilitarian construction, since the days of the Roman Empire.
Canal tunnels of notable size were excavated in France and England
during the last half of the 17th century. These were all rock or
hard-ground tunnels. Indeed, previous to 1800 the soft-ground tunnel was
beyond the courage of engineer except in sections of such small size
that the work better deserves to be called a drift or heading than a
tunnel. In 1803, however, a tunnel 24 ft. wide was excavated through
soft soil for the St. Quentin Canal in France. Timbering or strutting
was employed to support the walls and roof of the excavation as fast as
the earth was removed, and the masonry lining was built closely
following it. From the experience gained in this tunnel were developed
the various systems of soft-ground subterranean tunneling since
employed.
It was by the development of the steam railway, however, that the art of
tunneling was to be brought into its present prominence. In 1820-26 two
tunnels were built on the Liverpool & Manchester Ry. in England. This
was the beginning of the rapid development which has made the tunnel one
of the most familiar of engineering structures. The first railway tunnel
in the United States was built on the Alleghany & Portage R. R. in
Pennsylvania in 1831-33; and the first canal tunnel had been completed
about 13 years previously (1818-21) by the Schuylkill Navigation Co.,
near Auburn, Pa. It would be interesting and instructive in many
respects to follow the rise and progress of tunnel construction in
detail since the construction of these earlier examples, but all that
may be said here is that it was identical with that of the railway.
The art of tunneling entered its last and greatest phase with the
construction of the Mont Cenis tunnel in Europe and the Hoosac tunnel in
America, which works established the utility of machine rock-drills and
high explosives. The Mont Cenis tunnel was built to facilitate railway
communication between Italy and France, or more properly between
Piedmont and Savoy, the two parts of the kingdom of Victor Emmanuel II.,
separated by the Alps. It is 7.6 miles long, and passes under the Col di
Fréjus near Mont Cenis. Sommeiller, Grattoni, and Grandis were the
engineers of this great undertaking, which was begun in 1857, and
finished in 1872. It was from the close study of the various
difficulties, the great length of the tunnel, and the desire of the
engineers to finish it quickly, that all the different improvements were
developed which marked this work as a notable step in the advance of the
art of tunneling. Thus the first power-drill ever used in tunnel work
was devised by Sommeiller. In addition, compressed air as a motive power
for drills, aspirators to suck the foul air from the excavation, air
compressors, turbines, etc., found at Mont Cenis their first application
to tunnel construction. This important rôle played by the Mont Cenis
tunnel in Europe in introducing modern methods had its counterpart in
America in the Hoosac tunnel completed in 1875. In this work there were
used for the first time in America power rock-drills, air compressors,
nitro-glycerine, electricity for firing blasts, etc.
There remains now to be noted only the final development in the art of
soft-ground submarine tunneling, namely, the use of the shield and metal
lining. The shield was invented and first used by Sir Isambard Brunel in
excavating the tunnel under the River Thames at London, which was begun
in 1825, and finished in 1841. In 1869 Peter William Barlow used an iron
lining in connection with a shield in driving the second tunnel under
the Thames at London. From these inventions has grown up one of the most
notable systems of tunneling now practiced, which is commonly known as
the shield system.
In closing this brief review of the development of modern methods of
tunneling, to the presentation of which the remainder of this book is
devoted, mention should be made of a form of motive power which promises
many opportunities for development in tunnel construction. Electricity
has long been employed for blasting and illuminating purposes in tunnel
work. It remains to be extended to other uses. For hauling and for
operating certain classes of hoisting and excavating machinery it is one
of the most convenient forms of power available to the engineer. Its
successful application to rock-drills is another promising field. For
operating ventilating fans it promises unusual usefulness.
TUNNELING
CHAPTER I.
PRELIMINARY CONSIDERATIONS. CHOICE BETWEEN A TUNNEL AND OPEN CUT.
GEOLOGICAL SURVEYS.
CHOICE BETWEEN A TUNNEL AND AN OPEN CUT.
When a railway line is to be carried across a range of mountains or
hills, the first question which arises is whether it is better to
construct a tunnel or to make such a détour as will enable the
obstruction to be passed with ordinary surface construction. The answer
to this question depends upon the comparative cost of construction and
maintenance, and upon the relative commercial and structural advantages
and disadvantages of the two methods. In favor of the open road there
are its smaller cost and the decreased time required in its
construction. These mean that less capital will be required, and that
the road will sooner be able to earn something for its builders. Against
the open road there are: its greater length and consequently its heavier
running expenses; the greater amount of rolling-stock required to
operate it; the heavy expense of maintaining a mountain road; and the
necessity of employing larger locomotives, with the increased expenses
which they entail. In favor of the tunnel there are: the shortening of
the road, with the consequent decrease in the operating expenses and
amount of rolling-stock required; the smaller cost of maintenance,
owing to the protection of the track from snow and rain and other
natural influences causing deterioration; and the decreased cost of
hauling due to the lighter grades. Against the tunnel, there are its
enormous cost as compared with an open road and the great length of time
required to construct it.
To determine in any particular case whether a tunnel or an open road is
best, requires a careful integration of all the factors mentioned. It
may be asserted in a general way, however, that the enormous advance
made in the art of tunnel building has done much to lessen the strength
of the principal objections to tunnels, namely, their great cost and the
length of time required for their construction. Where the choice lies
between a tunnel or a long détour with heavy grades it is sooner or
later almost always decided in favor of a tunnel. When, however, the
conditions are such that the choice lies between a tunnel or a heavy
open cut with the same grades the problem of deciding between the two
solutions is a more difficult one.
It is generally assumed that when the cut required will have a vertical
depth exceeding 60 ft. it is less expensive to build a tunnel unless the
excavated material is needed for a nearby embankment or fill. This rule
is not absolute, but varies according to local conditions. For instance,
in materials of rigid and unyielding character, such as rock, the
practical limit to the depth of a cut goes far beyond that point at
which a tunnel would be more economical according to the above rule. In
soils of a yielding character, on the other hand, the very flat slope
required for stability adds greatly to the cost of making a cut.
It may be noted in closing that the same rule may be employed in
determining the location of the ends of the tunnel, for assuming that it
is more convenient to excavate a tunnel than an open cut when the depth
exceeds 60 ft., then the open cut approaches should extend into the
mountain- or hill-sides only to the points where the surface is 60 ft.
above grade, and there the tunnel should begin. If, therefore, we draw
on the longitudinal profile of the tunnel a line parallel to the plane
of the tracks, and 60 ft. above it, this line will cut the surface at
the points where the open-cut approaches should cease and the tunnel
begin. This is a rule-of-thumb determination at the best, and requires
judgment in its use. Should the ground surface, for example, rise only a
few feet above the 60 ft. line for any distance, it is obviously better
to continue the open cut than to tunnel.
THE METHOD AND PURPOSE OF GEOLOGICAL SURVEYS.
When it has been decided to build a tunnel, the first duty of the
engineer is to make an accurate geological survey of the locality. From
this survey the material penetrated, the form of section and kind of
strutting to be used, the best form of lining to be adopted, the cost of
excavation, and various other facts, are to be deduced. In small tunnels
the geological knowledge of the engineer should enable him to construct
a geological map of the locality, or this knowledge may be had in many
cases by consulting the geological maps issued by the State or general
government surveys. When, however, the tunnel is to be of great length,
it may be necessary to call in the assistance of a professional
geologist in order to reconstruct accurately the interior of the
mountain and thereby to ascertain beforehand the different strata and
materials to be excavated, thus obtaining the data for calculating both
the time and cost of excavating the tunnel.
The geological survey should enable the engineer to determine, (1) the
character of the material and its force of cohesion, (2) the inclination
of the different strata, and (3) the presence of water.
=Character of Material.=--The character of the material through which
the proposed tunnel will penetrate is best ascertained by means of
diamond rock-drills. These machines bore an annular hole, and take away
a core for the whole depth of the boring, thus giving a perfect
geological section showing the character, succession, and exact
thickness of the strata. By making such borings at different points
along the center line of the projected tunnel, and comparing the
relative sequence and thickness of the different strata shown by the
cores, the geological formation of the mountain may be determined quite
exactly. Where it is difficult or impracticable to make diamond drill
borings on account of the depth of the mountain above the tunnel, or
because of its inaccessibility, the engineer must resort to other
methods of observation.
The present forms of mountains or hills are due to weathering, or the
action of the destructive atmospheric influences upon the original
material. From the manner in which the mountain or hill has resisted
weathering, therefore, may be deduced in a general way both the nature
and consistency of the materials of which it is composed. Thus we shall
generally find mountains or hills of rounded outlines to consist of soft
rocks or loose soils, while under very steep and crested mountains hard
rock usually exists. To the general knowledge of the nature of its
interior thus afforded by the exterior form of the mountain, the
engineer must add such information as the surface outcroppings and other
local evidences permit.
For the purposes of the tunnel builder we may first classify all
materials as either, (1) hard rock, (2) soft rock, or (3) soft soil.
Hard rocks are those having sufficient cohesion to stand vertically when
cut to any depth. Many of the primary rocks, like granite, gneiss,
feldspar, and basalt, belong to this class, but others of the same group
are affected by the atmosphere, moisture, and frost, which gradually
disintegrate them. They are also often found interspersed with pyrites,
whose well-known tendency to disintegrate upon exposure to air
introduces another destructive agency. For these reasons we may divide
hard rocks into two sub-classes; viz., hard rocks unaffected by the
atmosphere, and those affected by it. This distinction is chiefly
important in tunneling as determining whether or not a lining will be
required.
Soft rocks, as the term implies, are those in which the force of
cohesion is less than in hard rocks, and which in consequence offer less
resistance to attacks tending to break down their original structure.
They are always affected by the atmosphere. Sandstones, laminated clay
shales, mica-schists, and all schistose stones, chalk and some volcanic
rocks, can be classified in this group. Soft rocks require to be
supported by timbering during excavation, and need to be protected by a
strong lining to exclude the air, and to support the vertical pressures,
and prevent the fall of fragments.
Soft soils are composed of detrital materials, having so little cohesion
that they may be excavated without the use of explosives. Tunnels
excavated through these soils must be strongly timbered during
excavation to support the vertical pressure and prevent caving; and they
also always require a strong lining. Gravel, sand, shale, clay,
quicksand, and peat are the soft soils generally encountered in the
excavation of tunnels. Gravels and dry sand are the strongest and
firmest; shales are very firm, but they possess the great defect of
being liable to swell in the presence of water or merely by exposure to
the air, to such an extent that they have been known to crush the
timbering built to support them. Quicksand and peat are proverbially
treacherous materials. Clays are sometimes firm and tenacious, but when
laminated and in the presence of water are among the most treacherous
soils. Laminated clays may be described as ordinary clays altered by
chemical and mechanical agencies, and several modifications of the same
structure are often found in the same locality. They are composed of
laminæ of lenticular form separated by smooth surfaces and easily
detached from each other. Laminated clays generally have a dark color,
red, ocher or greenish blue, and are very often found alternating with
strata of stiatites or calcareous material. For purposes of construction
they have been divided into three varieties.
Laminated clays of the first variety are those which alternate with
calcareous strata and are not so greatly altered as to lose their
original stratification. Laminated clays of the second variety are those
in which the calcareous strata are broken and reduced to small pieces,
but in which the former structure is not completely destroyed; the clay
is not reduced to a humid state. Laminated clays of the third variety
are those in which the clay by the force of continued disturbance, and
in the presence of water, has become plastic. Laminated clays are very
treacherous soils; quicksand and peat may be classed, as regards their
treacherous nature, among the laminated clays of the third variety.
=Inclination of Strata.=--Knowing the inclination of the strata, or the
angle which they make with the horizon, it is easy to determine where
they intersect the vertical plane of the tunnel passing through the
center line, thus giving to a certain extent a knowledge of the
different strata which will be met in the excavation. On the inclination
of the strata depend: (1) The cost of the excavation; the blasting, for
instance, will be more efficient if the rocks are attacked perpendicular
to the stratification; (2) The character of the timbering or strutting;
the tendency of the rock to fall is greater if the strata are horizontal
than if they are vertical; (3) The character and thickness of the
lining; horizontal strata are in the weakest position to resist the
vertical pressure from the load above when deprived of the supporting
rock below, while vertical strata, when penetrated, act as a sort of
arch to support the pressure of the load above. The foregoing remarks
apply only to hard or soft rock materials.
In detrital formations the inclination of the strata is an important
consideration, because of the unsymmetrical pressures developed. In
excavating a tunnel through soft soil whose strata are inclined at 30°
to the horizon, for instance, the tunnel will cut these strata at an
angle of 30°. By the excavation the natural equilibrium of the soil is
disturbed, and while the earth tends to fall and settle on both sides at
an angle depending upon the friction and cohesion of the material, this
angle will be much greater on one side than on the other because of the
inclination of the strata; and hence the prism of falling earth on one
side is greater than on the other, and consequently the pressures are
different, or in other words, they are unsymmetrical. These
unsymmetrical pressures are usually easily taken care of as far as the
lining is concerned, but they may cause serious cave-ins and badly
distort the strutting. Caving-in during excavation may be prevented by
cutting the materials according to their natural slope; but the
distortion of the strutting is a more serious problem to handle, and one
which oftentimes requires the utmost vigilance and care to prevent
serious trouble.
=Presence of Water.=--An idea of the likelihood of finding water in the
tunnel may be obtained by studying the hydrographic basin of the
locality. From it the source and direction of the springs, creeks,
ravines, etc., can be traced, and from the geological map it can be seen
where the strata bearing these waters meet the center line. Not only
ought the surface water to be attentively studied, but underground
springs, which are frequently encountered in the excavation of tunnels,
require careful attention. Both the surface and underground waters
follow the pervious strata, and are diverted by impervious strata. Rocks
generally may be classed as impervious; but they contain crevices and
faults, which often allow water to pass through them; and it is,
therefore, not uncommon to encounter large quantities of water in
excavating tunnels through rock. As a rule, water will be found under
high mountains, which comes from the melted ice and snow percolating
through the rock crevices.
Some detrital soils, like gravel and sand, are pervious, and others,
like clay and shale, are impervious. Detrital soils lying above clay are
almost certain to carry water just above the clay stratum. In tunnel
work, therefore, when the excavation keeps well within the clay stratum,
little trouble is likely to be had from water; should, however, the
excavation cut the clay surface and enter the pervious material above,
water is quite certain to be encountered. The quantity of water
encountered in any case depends upon the presence of high mountains near
by, and upon other circumstances which will attract the attention of the
engineer.
A knowledge of the pressure of the water is desirable. This may be
obtained by observing closely its source and the character of the strata
through which it passes. Water coming to the excavation through rock
crevices will lose little of its pressure by friction, while that which
has passed some distance through sand will have lost a great deal of its
pressure by friction. Water bearing sand, and, in fact, any water
bearing detrital material, has its fluidity increased by water pressure;
and when this reaches the point where flow results, trouble ensues. The
streams of water met in the construction of the St. Gothard tunnel had
sufficient pressure to carry away timber and materials.