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Subject: Saga in Steel and Concrete - 169-180
Date: Thu, 1 May 2003 09:38:22 -0700


The following selection is taken from "Saga in Steel and Concrete:
Norwegian Engineers in America" by Kenneth Bjork published by the
Norwegian-American Historical Association (NAHA) in 1947. The volume is
still available from NAHA at where you will
also find the first 33 volumes of Studies and Records online. This
chapter is published with the kind permission of NAHA. The book this
selection is drawn from is under copyright and permission has been
granted for educational purposes and it is not to be used in any way for
any commercial venture.

While bridges have evolved from the primitive structures of early man and
have a history spanning every means of transportation, tunnels and
subways belong peculiarly to our own age. They are a part of the story of
the railroad and the motor vehicle --- both of recent origin. In their
construction, therefore, inventiveness and novelty play a part that is a
vital one when no large body of slowly accumulated experience is at hand
to guide the engineer. And the men who pioneered in this branch of
transportation, while still embarking on new projects and meeting
problems peculiar to local conditions, yet recognize that the major
obstacles involved in underground travel have been met and overcome;
their task today is largely one of perfecting and improving tunnels that
were only recently completed. In the saga of tunnel and subway building
American engineers have been conspicuous leaders, though they have
borrowed much from English experience; and among these engineers Olaf
Hoff, Ole Singstad, and Sverre Dahm were pioneers of unquestioned
The underwater tunnel originated in England. The first of its kind was
driven under the Thames about three miles east of Charing Cross and was
completed in 1843 after attempts covering a period of twenty-five years.
The successful engineer, M. I. Brunel, invented a strange device known as
a "shield" which was used to drive tunnels in soft ground. The tunneling
procedure begun by Brunel and developed by P. W. Barlow and J. H.
Greathead can best be described by comparing it with the wood-boring
worm, which in fact was supposed to have suggested the shield method to
Brunel. The worm, as it bores its way through wood, secretes a lining
which prevents the wall of the passageway in back of it from caving in.
In a similar manner the shield leaves behind it a tube of cast iron or
steel, made up of narrow rings which constitute the lining of the
completed tunnel.
The use of compressed air, now a regular feature in shield tunneling, is
credited to Thomas Cochran, who patented the procedure in 1830.
Compressed air is used both for sinking shafts and for tunneling under
water; it permits work to proceed in the dry by holding back the water
that constantly threatens to pour in. Its chief disadvantage is that it
prevents the tunnel laborers or "sand hogs" from working for more than a
few hours at a time.
With the use of the improved Greathead shield and of compressed air,
English engineers were able in 1869 to drive the second Thames tunnel,
the little Tower subway, in the short span of eleven months. In this
project the shield method as it is known today was employed in all its
essentials. {1}
How the shield works can best be understood by observing one in action.
After shafts have been sunk and the shield lowered, it is started on its
course. As the earth is removed in front, the shield is forced ahead by a
row of hydraulic jacks around the end of the finished lining. The outer
rim of the shield is built as a blunt cutting edge that also trims the
tunnel. Running back from the cutting edge is a thick steel cylinder
plate about 15 feet long which supports the freshly tunneled ground
between the finished lining and the cutting edge. When the shield has
been driven forward by the jacks about 2½ feet, a lining ring is put into
place. Jacks are released several at a time, a segment of the ring is
placed by an erector and securely bolted to other segments, and the jacks
are shortened and replaced. This routine is repeated eleven to fifteen
times for one ring, which thus constitutes about 21/2 feet of tunnel. An
interior lining of concrete is later applied. {2}
The shield might seem, from this description, a kind of steel monster
operating without human assistance. In reality it is bustling with life.
Doors can be opened, permitting workers to remove the soft dirt in the
path of the tunnel. As the shield is shoved forward into the muck of the
river bed, ribbons of mud come pouring into the interior. The dirt is
carried back on a conveyor and loaded into dump cars. The working chamber
of the shield is under air pressure sufficient to hold back the rush of
water or dirt while digging is going on. A bulkhead closes the tunnel
near the shore, men and material passing in and out of it through air
In the United States the first two efforts at driving river tunnels were
unsuccessful. Significantly, both failures occurred at points where Hoff,
Singstad, and others later succeeded. The first attempt was under the
Detroit River between Windsor, Canada, and Detroit, where a tunnel was to
serve as a connecting link between the Michigan Central Railroad and the
Great Western of Canada. Begun in 1872 by Ellis Sylvester Chesbrough,
city engineer of Chicago, the tunnel was never completed. The river broke
through, workers were killed, and the backers became discouraged; they
abandoned the project in 1873. {3}
The obstacles at Detroit were overcome by Olaf Hoff when he solved the
practical problems involved in laying a tunnel in " a prepared underwater
trench. He thus contributed to the most revolutionary development in
tunneling since the discovery of the shield and the use of compressed
air. As vice-president and engineer of Butler Brothers-Hoff Company, he
worked out plans that supplemented and altered the bold and original idea
of another engineer, and was later rewarded by seeing his name linked
with the new and ingenious techniques.
Hoff entered upon his work at Detroit after an extensive and brilliant
career. He was born at Smaalenene in 1859 and graduated from
Christiania's Technical College with the highest honors ever granted by
that institution. He left for America in 1879 and entered the Keystone
Bridge Company as assistant foreman in one of the firm's Pittsburgh
shops. Transferred to the drafting room, he was rapidly promoted to
assistant chief engineer. After a period with the New York Central and
Hudson River Railroad, 1901-05, he formed a new company in New York with
Butler Brothers of St. Paul to develop his plans for laying the
subaqueous tunnel at Detroit. Hoff was to supervise the work of
construction if his firm received the contract and to have a financial
interest in the company. {4}
The Michigan Central Railroad, after vainly trying for years to reach an
agreement with the Grand Trunk (formerly the Great Western) Railway for
jointly constructing a bridge or tunnel entrance into Detroit from
Canada, finally decided to build a double-track tunnel under the Detroit
River for its own use. All through traffic on the Michigan Central, Grand
Trunk, and Pere Marquette was iming ferried across the river that divides
Windsor, Ontario, from Detroit-at no little expense and inconvenience to
the railroads. In the late 1860's the Michigan Central and Great Western
lines had agreed to build a tunnel jointly and for this purpose had
organized the Detroit River Transit Company, which was to own and operate
the underwater connection. It was shortly thereafter that Chesbrough was
employed to drive a tunnel by means of the familiar shield method. {5}
Early in the twentieth century, interest once more focused on the need of
a tunnel which would not only provide a Ø between Canada and the United
States but, more significantly, facilitate an uninterrupted rail
connection between East and West by way of Detroit.
In 1904, William J. Wilgus, then vice-president of the New York Central,
anticipating the successful electrification of his railroad's terminals
in New York City, suggested the feasibility of an electrically operated
tunnel under the Detroit River. Subsequent discussion favored a tunnel to
consist of two separate and single-track tubes, making use of electricity
as a driving power. Shortly thereafter the Detroit River Tunnel Company
was organized, and in July, 1905, an advisory board of engineers,
composed of Howard A. Carson, W. S. Kinnear, and Wilgus, who was
chairman, was engaged to plan construction and electrification. Kinnear
was charged with local authority as chief engineer, a position which he
occupied very competently, and Benjamin Douglas had direct supervision of
construction proper.
By the fall of 1905, the usual surveys and borings had been completed and
the alignment and profile of the tunnel had been determined. The next
problem facing the board was the choice of one of four suggested types of
construction. Wilgus said:
[It] was found that if possible some other method than the usual
compressed air shield-driven type should be employed, in the interest of
life and health of workers and time and expense of construction. To that
end Mr. Howard A. Carson suggested the use of precast pipe sections laid
in a dredged trench. This did not appeal to me because of anticipated
difficulty in effecting tight joints under water and securing continuity
of support.... The idea came to me of lowering forms in sections in a
prepared trench, around which concrete deposited from the water surface
by means of the tremie [pipe] would harden and seal all joints, thus
enabling the interiors to be pumped out successively and the concrete
lining laid in the dry.
Wilgus prepared sketches of the tunneling method based on this idea,
expanded and drew these to scale, made estimates of cost, and submitted
the scheme to his colleagues on the board and to other engineers,
including Olaf Hoff.
The consensus was that my method, though bold, was practicable. The Board
thereupon voted to include it, as well as Mr. Carson's and the compressed
air shield-driven methods, in the requests issued for bids on alternative
designs. Each bidder was required to submit supplemental plans by him
deemed necessary to more clearly explain the manner in which he proposed
to carry out the work in conformity with the method of his selection. The
lowest acceptable bidder proved to be the Butler Brothers Construction
Company.... Its proposition was based on the employment of the method of
which I was the inventor, and was accompanied by the required
supplemental plans, prepared by Mr. Hoff, illustrative of the ingenious
manner in which the contractor proposed to build, transport, deposit,
join and surround the forms in the prepared trench, all in conformity
with the tunnel specifications I had prepared. . . . I took out a patent
on my invention, in the application for which I was joined by Mr. Carson,
and a free license thereunder was given the tunnel company. . . . The
idea was presented to the world. {6}
Before the contract was closed Butler Brothers-Hoff Company asked for and
received protection against any claims that might arise from the use of
the Wilgus design, and the patent indemnity clause in the contract was
accordingly modified.
The revolutionary scheme proposed by Wilgus was known as Design A and
that of Carson as Design B; Design C was a modification in details of
Design A, while the compressed airshield method was labeled Design D. The
contractors were given the interesting option of "selecting any one of
the four methods for the subaqueous work, or submitting entirely new
designs, or modifications of those suggested, restricted only to a
compliance with certain conditions regulating stability, clearances,
workmanship, etc." The plan worked out by Hoff was a "modification of
Design A, embodying some of the elements of Design C, accompanied by a
large amount of detail covering the methods to be used in the prosecution
of the work." The contract was "unique, particularly with reference to
the subaqueous section, leaving the working out of details to the
ingenuity of the contractor." {7} Work was begun on October 1, 1906, and
completed July 1, 1910.
Briefly considered, the novel underwater portion of the Detroit Tunnel
was laid in the following manner: A trench was first dredged in the river
bottom and supports placed in it to receive twin-tubed steel forms in
sections, each about 260 feet in length, that were built and launched in
a shipyard, towed like barges to a position above the trench, lowered
into place, and connected together by divers. Wooden sides and cross
diaphragms of steel restrained the concrete which was later poured around
the forms through pipes, or tremies, from a floating concreting plant
anchored in the river. After several lengths of tunnel had been laid,
they were unwatered, leaks were stopped, and an inner layer of concrete,
reinforced with steel rods, was added in the dry and without the use of
compressed air. The combination of surrounding concrete and the firm
lining inside prevented water seepage and provided resistance against the
shock of trains passing over rails and ties that rested directly on the
underlying concrete. In this manner it was possible to enlarge the
diameter of the tunnel from 18 to 20 feet. {8}
The tunnel, once construction was begun, attracted considerable
attention, the Engineering News in the fall of 1907 designating it the
"most novel and interesting tunnel works now in progress." The technical
journals called special attention to the length of the river portion,
which measured over 2,600 feet, the difficulties of construction inherent
in the project, and the methods of setting grillage and depositing the
exterior concrete which made of the completed tunnel one great monolithic
mass. Both Wilgus and Kinnear in their exhaustive accounts paid tribute
to the skill of the contractors who gave form to an idea. In the
discussion that followed the Wilgus paper, E. W. Moir said that his
British firm, S. Pearson and Son, which had tendered a bid, would have
made a "handsome profit at their price if they had been as clever as the
firm who obtained the work" and added that since the author of the paper
"gave such great credit to the contracting staff," he was "sorry to see
that the names of individual members of it did not appear." These
sentiments were echoed by E. W. Monkhouse, who tried to place himself
mentally in the position of the one laying sections 260 feet long on the
grillage in moving water and then joining these one to another with great
accuracy. {9} August Gundersen, Hoff's chief assistant, went much farther
--- indeed too far --- in stating that "none of the four designs
submitted by the Board was used in the actual work," for they "did not
solve the old question `how to build a subaqueous tunnel in an excavated
trench.' The first solution of this problem as carried out at Detroit, is
entirely due to the ability of Mr. Hoff." {10}
It is well known that contractors regularly work out their own methods to
put into effect a given design, within the limits of specifications and
always with an eye to reducing costs. In the case of the Detroit Tunnel,
however, the engineer of the contracting firm went much further --- both
because of the freedom afforded by the tunnel company and the very
newness of the design that was employed. Several "ingenious measures"
were credited to Hoff by Wilgus: "(a) the bracing of the forms to prevent
distortion in launching, towing and lowering into place; (b) the use of
outside planks attached to the forms to minimize the quantity of
tremie-placed concrete; (c) the employment of air cylinders to regulate
the lowering and placing of the forms; and (d) the adoption of devices
for drawing the forms together in the bottom of the trench --- all means
which this contractor deemed necessary for accomplishing the desired
purpose set forth in the contract at the least possible cost to it!" {11}

In the light of these and of innumerable other statements, both published
and unpublished, concerning the Detroit project, it is of prime
importance that Hoff be given the opportunity to speak for himself.
Fortunately, he has left a record of his work in the form of a letter
published in the Transactions of the American Society of Civil Engineers:
When the firm' with which the writer was connected [Ho ff wrote in
February, 1906] received invitations to submit proposals for the
construction of the Detroit River Tunnel, he immediately and with
assiduity set to work on this intensely interesting problem. At that time
he had no knowledge of the numerous patented inventions for building
subaqueous tunnels in a trench in the bottom of a river or waterway. . .
. Later, having occasion to look the matter up, he was surprised to find
a number of patents on such tunnels, mostly impracticable schemes, of
doubtful merit, not one of which was ever carried out.
Hoff then proceeds to discuss the designs submitted for bids, stating
that all of them contemplated a tunnel two feet smaller in diameter than
the one actually built. Analyzing the designs, he also shows their
defects. "The result of the foregoing analysis," he concludes, "was the
gradual development of the design submitted by the writer, which,
together with the specifications and the accompanying proposal, was
accepted by the Board of Engineers, and according to which the tunnel was
The first object sought in working out his plans, Hoff tells us, "was the
elimination of compressed air, with its attendant cost and restrictions
in prosecuting the work." He felt, however, that he should be ready to
use it in case the outer concreting should be a failure. "The initial
step toward the accomplishment of this was a tube of steel of sufficient
strength in itself or in connection with the exterior concrete, to resist
the water pressure and effectively to prevent its ingress into the
tunnel. . . . This shell, at the same time, would constitute an inner
form for the exterior concrete."
The next step was "to reduce the exterior concrete to a definite
quantity-the minimum required-without filling up the whole trench, thus
saving a large item of cost. A little study and a few calculations soon
demonstrated that this minimum would be the quantity necessary to
overcome the buoyancy of the mass in the trench, when the tubes were
unwatered, and prevent them from floating up again." Hoff then had to
secure his outer form to the steel shell. A solid steel plate seemed to
him to be the proper solution, since this would divide the tubes into
compartments, which could be filled with concrete, one at a time. "Thus
the diaphragms were developed, together with the pocket or compartment
principle, to which, in the writer's judgment, the success attained at
Detroit is to be attributed. The concrete was discharged through the
tremies under a head; its lateral flow was confined to the exterior sides
of the compartment, and thus it was forced under the steel tubes,
affording them a reliable and satisfactory bearing."
One peculiarity of Hoff's tunnel system is that "the load on the bottom
of the trench during construction will be as great as, or greater than,
the maximum load of the completed tunnel when in use. In other words, the
weight of the water inside the tubes is equal to or greater than, the
weight of the concrete lining and the live load." Because of this and his
uncertainty as to the quality of the concrete, he increased the thickness
above the tubes more than was necessary. The quality of the concrete, "to
the very top surface," proved to be good.
"Of the greatest importance," Hoff continues, "was the problem of
lowering the tubes into the trench and keeping them always under absolute
control. To this end the four air cylinders were devised, and served the
purpose most successfully. They were of such size as to have a combined
buoyancy, when submerged, slightly in excess of that required to hold a
tunnel section in suspension." Water was admitted to the center or
adjustment compartment of each cylinder to lower the mass into the
When the tubes had been sunk in the trench and concrete placed under them
at the ends and at least at one point in the middle, the air cylinders
could be released. "This was done by first filling them with water, which
caused the weight of the tubes to be gradually transferred from the
cylinders above to the concrete below." Then the cylinders were brought
to the surface by forcing air into the center compartments and by using
derricks. Hoff explains how the tubes were held against the river current
during the sinking. The plans originally called for anchors of concrete
planted in the bed of. the river, but the superintendent of construction
thought ordinary anchors would serve. The superintendent's plan failing,
Hoff was forced to use concrete slabs in a hole dredged out in the river
bottom; clay was filled in on the top.
The time needed "for taking a section from its moorings, placing it in
position over the trench, attaching it to the anchor lines, filling the
tubes, adjusting the air cylinders, lowering the section to the bottom of
the trench and pulling it home, so that the keys could be inserted in the
pilot pins, thus locking the sections together . . . took from, say 8
A.M. until 8 or 9 P.M., after the first two or three sections had been
sunk. The lining up of the tubes at the outer end, and the bolting up of
the flange connections, could then be commenced the next day." The
bolting process, performed by divers, generally required two days, since
there were about 50 bolts to the joint, or 100 bolts to a section.
A significant innovation was made at Detroit in placing concrete under
water by means of the tremie. Properly constructed and operated, Hoff
explains, concrete may be placed so that "the great mass of it will not
come in contact with the water at all, after the first surface of
concrete has been formed. This is accomplished by mixing it so wet that
the mouth of the tremie at all times is buried in it, thus sealing the
end of the pipe and controlling the flow by raising or lowering the
tremie in the concrete, and by confining its lateral flow in compartments
which are filled one at a time, the concrete all the time seeking its
level within the compartment." {13}
In some cases the bottom of the trench became soft when excavated. To
secure a proper foundation for the tubes where this occurred, wooden
sheeting was driven down into the dirt and the width of the base was
increased. Tremies were then lowered, under their own weight of from 7 to
8 tons, as far as they would go through the soft clay; "they would
generally go down to within 1 or 2 ft. of the rock bottom at a depth of
from 85 to 90 ft. below the surface of the river. A little extra force
was used to put them down as far as possible, and concrete was then
deposited until it reached the underside of the diaphragms. In this
manner --- a series of piers, 6 ft. in diameter, was built up under the
tubes, two to a pocket longitudinally of the tunnel, and three rows, one
for each tremie; that is, there was one row under the center wall and one
under each of the side-walls of the tunnel."

<1> Kirby and Laurson, Modern Civil Engineering, 171-174.
<2> S. A. Thoresen, "Tunnel Lining of Welded Steel," in Iron Age, 125:989
(April 3, 1930).
<3> Kirby and Laurson, Modern Civil Engineering, 175. For the detailed
story of discouragement and defeat, see E. S. Chesbrough, "Sketch of the
Plans and Progress of the Detroit River Tunnel," in American Society of
Civil Engineers, Transactions, 2:85-91 (1872-1874); and "Detroit River
Tunnel," in Transactions, 2:233-238.
<4> American Society of Civil Engineers, Transactions, 89:1623 (1926);
Who's Who in America, 13:1590 (Chicago, 1924-25); Nordisk tidende,
November 10, 1921; Minneapolis tidende, December 26, 1924; Morgenbladet
(Christiania), October 26, 1913; Norwegian-American Technical Journal,
vol. 1, no. 4, p. 5 (December, 1928) ; Harper's Weekly, 56:22 (March 23,
1912); Norwegian-American (Northfield), April 25, 1913; and information
received from F. J. Vea of Madison, Wisconsin, a brother-in-law of Hoff.
<5> The Detroit River Tunnel of the Michigan Central," in Railroad
Gazette, 40:149-152 (February 16, 1906). This is the first installment of
a serial record.
<6> A letter to the writer from Wilgus, August 13, 1945, thus takes one
behind the scenes and tells more vividly than the official records the
origins of the Detroit tunnel idea. Statements in the Wilgus letter have
been checked against documents in the possession of the Engineering
Societies Library, New York City, by Harrison W. Craver, director. See
also Wilgus, "The Detroit River Tunnel, between Detroit, Michigan, and
Windsor, Canada," in Institution of Civil Engineers (London), Minutes of
Proceedings, 185:2-36 (1911)
<7> See Wilson Sherman Kinnear, "The Detroit River Tunnel," in American
Society of Civil Engineers, Transactions, 74:288-356 (December, 1911).
The quotations are from pages 303, 304, and 356.
<8> Other accounts of the Detroit Tunnel are James C. Mills, "The Detroit
River Tunnel," in Cassier's Magazine, 33:337-349 (January, 1908) ;
Engineering News, 58:453-455 (October 31, 1907); and Engineering Record,
60:678-680, 719-722 (December 18 and 25, 1909).
<9> See "Discussion" following the Wilgus paper, Institution of Civil
Engineers, Proceedings, 185:45-64 (1911).
<10> See "Discussion" following a paper by Kinnear in American Society of
Civil Engineers, Proceedings, 37:1169 (1911).
<11> Wilgus to the writer, September 22, 1945. Italics are Wilgus'.
<12> Vol. 74, p. 361-878 (December, 1911). The letter follows Kinnear's
paper on the Detroit Tunnel.
<13> In another article by Hoff, "Laying Concrete under Water in the
Detroit River Tunnel," in Engineering News, 63:318-341 (March 17, 1910),
reasons are given for the success of the method of concrete laying at

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