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R T W E R S E G T T 0 M
O F T H E:
I, A S A. I. I, E S T R E. E. T. T. U N N E L.
Prepared in accordance ºrith
out line previously submitted, as
a candidate for the Degree of C. E.
BY
ſ - HERMAN H. HANINK
|
B.S. in C. E.





SYNOPSIS.
The object of this thesis is to describe
the design and method employed in the construction
of the River Section of the La Salle Street Tunnel,
which passes under the Chicago Piver, Chicago, Illinois.
This tunnel, built for the Chicago Railºrays Company
by the M. H. McGovern Company of Chicago, was finished
in 1912.
The old tunnel, still existing at the time
Work was begun on the new tunnel, is described for the
purpose of exhibiting the reasons for it is removal; it
is then compared with the new tunnel, a general explana-
tion of which follows, together with a brief account of
the method used for water-proofing.
The River Section is described in some detail.
The steel shell is detailed, together with methods of
building and placing the interior and exterior concrete
parts. The excavation of the site of low bring also is
discussed.
The design of the cofferdams is next taken up.
Two dams, one on each side of the river, serve the purpose
of making connections with the land sections, and consist
essentially of two parts. That part directly over the
tiºnnel is constructed of timber, the remaining portion

being made of steel sheet piling which extends the full
width of the street and back into the land. These dams
present some interesting features, especially that of an
unusual head of water (fifty-four feet) against the steel
portion.
An explanation of the foundation for the
tubes, the method of lowering the tunnel, the finis hing
of the concreting and the removing of the Gofferdams,
complete the article.
It is to be hoped that the accompanying
drawings and photographs will assist the reader to
under 5t, and the text.
tº
I M Đ E X.
DESCRIPTIVE.
WATRPRO OFING.
RIVER SECTION – STEEL, SHELL.
Concrete Work in Tºry Dock.
concreting While Floating.
Fºxcavation of Trench.
TEstºn of coºpFBTAM.
Timber Portion.
Hydro-static Pressure on Timber Portion.
Steel Portion.
Hydrostatic Prês Bll re on Steel Portion.
Rracing.
TEMPORARY FOUNDATION FOR TUBES.
LOWERING TUBES.
PHRMANENT FOUNITATION FOR TURES.
CONCF TING BALANCE OF PIV; H SECTION.
REMOVING COFF:FT) AMS.
Page.
15.
16.
20.
2l.
23.
24.
26.
27.
28.


I N T E X
T 0
D R A W T N G S.
Plate.
General Profile of old and New Tunnels with Details. 1.
PRofile and Sections of River Section. 2.
Plan of Cofferdam. 3.
Section of Cofferdam. - - 4.
Elevation of Cofferdam. 5.


I, A S A I, L E S T R H. E. T. T. U N N E I,
R. I V E R S E G T T O N.
—oo Qoo-
DESCRIPTIVE.
The old La Salle Street Tunnel, used for cable
ear service, extended along the center line of La Salle
Street from a point near Randolph Street on the south to
Michigan Street on the north, a distance of 1887 feet.
It was built entirely of brick and completed in 1871 at
a Goºst of $566,000.00. Thes maximum depth of Water over
the River Section was about eighteen feet, which proved
sufficient until the advent of lake carrying vessels of
large tonnage and greater arart. Congress subsequently
- passed an Act declaring the tunnel tº an obstruction to
- navigation", and ordered the City of Chicago to remove it.
Thug was brought about the construction of the
new tunnel for trolley service, with a depth of twenty-
--- seven feet of water over the river section. This tunnel
is a double bore structure, extending from Randolph Street
to Michigan Street, a distance of 2,000 feet. Each bore
accommodates a single track, with ample room for cables


ºf
and conduits. At each end provision has been made
for connection to any future subway, without inter-
ference with street, c ºr service.
The total cost of this fork was about
$1,300,000.00. It seems surprising indeed that this
amount is only slightly greater than twice the cost
of the old tunnel. In place of the brick, always in
a leaky condition, the new tunnel has thick concrete
walls and arch, practically water-proof and is of
considerably larger bore. It also has a well built
and tight river section, in fact, owing to the steel
covering, the dryest section of the tunnel. The
comparison of these tºro tunnels affords one of the
most impressive illustrations of the efficiency Of
modern methods and materials.
As will be noted from the general plan (Pl. 1),
the permanent track is maintained at a three per cent.
grade from the river section toward either end, while
the temporary track is carried as a descending nine
per cent. grade from the present street level to an
intersection with the three per cent... grade. The track
on the nine per cent. grade is carried by temporary
beams, which were originally placed solely as struts
or suppots for the side walls of the tunnel, and the
tracks were to be supported on timber bents spaced

between the struts. Upon the writer's suggestion
they were slightly altered in arrangement and design
and made to carry the track also. This portion of
the Tunnel in each approach is single bore. The
walls, however, are in alignment with those of the
double bore, but the arch runs parallel to the
temporary nine per cent. track.
The drainage system consists of eight inch
tile laid along the inverts of both bores and empty-
ing into a sump near the north end of the river section.
A three-foot drain leads to a stump that connects with a
pump well, north of the north Tock, and a five-foot
shaft connects the pump well with a pumping station
on the street level. This shaft, is accessible for
inspection through the wall of the Tunnel at this
point.
WATER PRO OFING.
As shown in sketch on Pl. 1, the original
plan for water proofing called for an eight-inch
course of asphalt and brick extending from track grade
up the Walls and over the arch. This Tſas to be covered
with an eight-inch course of concrete. When the con–
creting was first begun in the open cut from Station
0 + 00 to about Station 3 + 00, this scheme was followed,
but, owing to the slowness with which the work had to be
-----


executed, the Chicago Railways company accepted, as
a substitute, McCormick Integral Water-proofing. This
compound was in the form of a powder and was mixed dry
with the cement and resacked for use. The entire cross-
section of the Tunnel, except below the track grade, was
built with waterproofed cement and paid ror per Square
foot, on the original basis. This then gave a section
of concrete 16 inches thicker than the previous neat
line of monolithic concrete, and at the same time obviated
considerable form work. It is difficult to determine
whether this material actually did waterproof the con–
Crete, or whether the concrete was naturally thick enough
and dense enough to prevent seepage, but the fact remains
that very femſ leaks resulted, and these were easily
stopped by grouting under air pressure.
RIVER SECTION.
STEEL SHELL. — According th the original plans,
the River Section was to have been constructed similarly
to the double bore Land Section. This would have neces-
sitated coffer-damming about half the River to permit a
reasonable extent of Operation. But, owing to the re-
fugal of the War Tepartment to allow this obstruction
to navigation, it was decided to make this portion a
one unit steel tube, equal to the full width of the

|-
|
River, with an interior lining of reinforced concrete.
These tubes do not form complete cylinders, as did the
Tetroit design; neither is there a concrete encasement;
but the two cylinders intersect and are separated by a
vertical concrete wall which contains the longitudinal
steel stiffening truss. The lining of the tubes varies
in thickness from about two feet, at the crown to three
and one-half feet in the invert. The only concrete
placed outside the shell was that used to level off
the top , and that, on the bottom, called the keel.
The steel shell is 278 feet long , 27 feet
high, and 41 feet wide, and is made in plates 3/8 Of
an inch thick, 7 feet 6 inches wide, and about 19 feet
long. The plates are stiffened by 6 by 3 1/2 inch z-bars,
57 feet long, bent to the required radius and Spaced
3 feet and 4 feet 6 inches. Butt splices were used for
longitudinal joints with straps both inside and out.
All circumferential joints were lap spliced and the
edges of the plates were planed to a bevel and caulked.
The top and bottom plates of each bore are
connected by 6 X 6 X 3/8 inch angles, and riveted to
the struts of the longitudinal truss. These struts
are composed of two angles, 5 x 8 1/2 x 3/8 inches,
double laced with 4 x 3/8 inch bars. The gussets are
3/8 inch plates. The diagonal longitudinal bars in
-º
pairs (one on each side of the struts) are 4 1/2 x 3/8
inches, about 24 feet long, and connect with six of the
struts. The tubes are also stiffened by plate and angle
gussets above and below the center wall at intervals
of 7 1/2 feet. The top gussets form a level surface,
but those at the bottom were made with a wide angle to
provide for a drainage pipe.
Near the ends of the tubes provision was made
for connection to a cofferdam by riveting side plates
or fins, with angle stiffeners and with an angle at
the edge oto mesh in with the steel sheeting of the
cofferdam. Within the space reserved for cofferdams,
§t, eel shaft, 5 three feet in diamet, ºr were connected with
each tube, sufficiently high to project out of the water
when the tubes were sunk to place.
Timber buikheads were built at each end of
the tube, and all temporary bracing and concrete for
the keel and center Trall were placed while in dry dock.
The tubes were erected in the ship owners'
dry dock, near Chicago Avenue and North Halstead Street.
The material tº as teamed and transport, ed on scows from
the shop of the company, which fabricated and built
the steel tubes. A derrick, travelling On 8 track
alongside of the dry dock, was used in the erection.
The weight of the steel in the tubes was about 500 tons.
Approximately loſ), 000 rivets were driven.
***

–7-
plank, with 12 X 12 timbers 1 foot apart behind them, - º
-
CONCRETE WORK IN DRY TOGK: – After the steel construc-
tion had proceeded sufficiently, a concrete plant 733
erected near one end of the tubes. A 1-yd. mixer TT38
supported under a trestle work, Thich carried a Tº lat-
form from one side of the dry dock to the other side.
The material was brought from storage piles on the
bank in wheel-barrows, and dumped into the mixer hopper
from the platform. At the discharge of the mixer,
runways led through the tubes on each side of the
Center Wall.
The first concret, G Work done ºf a $3 the ext, erior
section beneath the tubes, which were supported on
timbering about 3 feet above the floor of the dry dock,
while the forms for the exterior section of concrete
were supported on short posts under the tubes. The con–
Grete Was then poured through holes in the steel shell.
The work of concreting the center wall was then begun
and the forms were built for it, so as to include a very
Short section of both arch and invert. The forms were
carried up gradually so as to permit dumping of concrete
from wheel-barrows from runways. The runways were at
first placed about 5 fººt above the bottom of the tube
and later at about 5 feet, fºom the top.
The end bulkheads consisted of 3 × 12 inch
and diagonal braces of 10 X 10 timbers. The joints
were caulked with oakum. The tubest were floated in


this condition on November 21, 1910. The weight was
then about 3,000 tons. They were towed to a wharf
near the tunnel site ºthere the depth of the ºrater was
sufficient to permit the completion of the interior
concreting without grounding the tubes.
CONCRETING WHILE FLOATING: — A new concrète arrange-
ment was then devised for carrying on the work of lining
the interior. Two 1-yard mixers were erected on plat—
forms on the tubes at each end. The mixers were placed
on rollers Bo that they could be used to chute concrete
into the manholes at either tube. In this way the long
wheel-barrow haul within the tubes was limited to one-
half the total length of the tube. The shifting of the
mixer ºf as a matter of five minutes' fork for two men.
The two mixers were worked alternately on the two tubes
and concrete was placed at the rate of about 200 yards
in eight hours. -
The form; Were placed first for a section of
the invert about 5 feet in Width. This form consisted
of a longitudinal bulkhead on the outside, embracing
the steps, which were used as a base for the cable
conduits. Above this point the forms were made of
2 X 6 inch shiplap, laid over segmental rings formed
by two thicknesses or 2 X 12 inch plank and shaped to
the radii indicated on the plans.

Excavation of TRENGH:- This work was rendered
most difficult by the depth of the water (26 ft. )
and the depth of the trench ( 54 ft.) below the surface
of the water. The existence of the old tunnel floor
( see Pl. 1 Sec. CC. ) was also a problem. A dipper
dredge was rigged up specially for the work. The
dredge was equipped with a 2-yard dipper bucket on a
72-foot dipper arm and new spuds of extra length were
added. It was at first thought that the dredge could
excavate the trench and old invert without blasting,
but owing to the length of the dipper arm and spuds,
the river current, etc. , this was round impracticable.
A drill boat containing four drills was then placed
over the invert and in a period of about tºo weeks the
invert was broken up. The material was loaded on scows
by the arease, towed out into Lake Michigan and dumped.
Upon the completion of the excavation, final precautions
were taken by making accurate soundings of the trench.
Next a rail was suspended by light cables at an eleva-
tion slightly lower than that at which the tubes were
to be set. This rail was carried across the river, and
all boulders and fragments of bricks, which could possibly
interfere with the correct placing of the tubes, were thus
located.


- —lo-
º-
-->
-
-
DESIGN OF COFFEPTAM.
It Will be remembered that in a previous
paragraph, it was stated that projections of plates,
or fins, had been placed on the tubes for connection
to the cofferdam. The original idea was to lower
that portion of the cofferdam which was directly on
the tubes, into the area between the fins, as a crib
after the tubes were lowered into place. There is
always considerable risk, however, in lowering a crib
of this character accurately into place and securing
a watertight job.
The only alternative was to construct this
Grib on the tubes while they were yet floating, omitting
sufficient concrete lining to off-set the added weight
of the cribs. It was also necessary to determine the
capacity of the tubes to carry these two end loads with-
out, danger of buckling. In this case we have the con-
dition of a beam 278 feet long, uniformly loaded on the
bottom (upward hydrostatic pressure) and a concentrated
load at each end. The center wall, consisting of steel
columns and steel diagonals encased in concrete, acts
as a stiffening web. -
Section pp. (p. 5) shows the percentage of con-
crete placed at this time. This cross-section of concrete,
together with the steel shell and reinforcing system, proved

—ll-
ample to carry these added loads ºrithout danger of
buckling. The center of gravity of the cross-section
was raised but slightly, and the tubes were at all
times in stable equilibrium.
TIMBER PORTION:- That part of the cofferdam,
---
which was built on the tubes before being submerged,
was to be constructed of 12 X 12 inch timbers, and
was designed on this basis. The projecting diaphragm
plates on the tubes determined the dimensions of this
º
as 11 ft. 6 in. X 39 ft. on the outside. The water to ),
º
at 30 ft., giving 3 ft. as a safety in case of a rise
be 27 ft. above the tubes, the height was determined
in the Water.
The longitudinal timbers were placed directly
on one another, and of convenient lengths to locate splices
at the centers of the uprights, which were also 12 X 12
inch X 30 reet. The end timbers were placed so as to
leave alternate Spaces open in order to permit free con-
nection with the remainder of the dam, insuring a good
union of clay and also acting as a web between the two
walls. The longitudinal timbers were drift bolted to
each other at 3 foot intervals with 18 X 3/4 inch pointed
spikes without heads; a 5/8 inch hole Was first arilled
through one timber and then the spike Tas driven into
the next timber.
It was now necessary properly to locate the
upright timbers and determine the size and number of

tie rods. Owing to the fact that the 3 foot steel
shafts are within this area, it was decided to place
uprights on the center line and two other pairs, whose
tie rods would just clear these shafts, thus making
three pairs of uprights. The ends of the dam require
no wales or tie rods, owing to its construction as
described above.
Tn computing the number and size of tie rods
(see Pl. 4), the weight of the submerged clay was assumed
to be eighty pounds per cubic foot. There is , of course,
considerable variation of opinion as to the prºgures
arising from cofferdam filling. However, the classe lºss
of clay used in this case, led to the assumption that
its angle of repose Will be represented by a slope of
3 1/2 to 1. rººm. formula
P = 1/2 whº tan? (45°– 1/2 ; ) = 0.57 x 1/2 whº
the unit pressure at any élevation normal to the wall
will be 57% of the vertical head of clay. With clay
at 80 pounds per cubic foot and h equal to 27 feet we
find the pressure at the bottom to be
57% X 80 x 27 = 1231 pounds per square foot. -
With this quantity as the base of a triangle and the º
apex at datum, the pressures at various elevations
were scaled and the rods spaced accordingly. It will
be noted from the drawing (Pl. 5) that the tie rods be-
tween the two center uprights support a distance of
5 ft. 6 in. to each side. All the tie rods were made



—13–
from 1 1/2 inch stock, upset at each end and with
double nuts. The two bottom courses of timbers, being
securely anchored to the concrete, required no tie rods.
The following table gives the general
information concerning one crib:
Total feet B. M. 42703
Weight of Timber 80.2 tong.
Displacement of Timber 106.9 tº
Buoyancy of Timber 26.7 tº
This indicates that it was necessary to pro-
vide for the buoyancy of the timber. All asſistance
of the drift bolts and anchors was neglected, and a
1 1/2 inch rod was placed at each corner, as shown on
- Pl. 4 and 5. These were securely anchored in the con-
crete on the tubes and provided ºrith turri buckles for
tightening. This arrangement also aided mat 3rially in
holding the cribs in alignment during the process of
towing and lowering. To help stiffen the dam, l? X 12 in.
cross timbers were placed at various points between the
uprights, as buttresses in tightening the rods.
In view of the fact that these large weights
Öf the dams were to be placed on the ends of the tubes,
it was necessary to provide suitable foundations for
them. At this time, as before noted, the interior (3 OH-
crete arch had not been finished. The absence of the
concrete would have exposed portions of the steel shell


-14-
to this weight, which might have buckled them. As
* -
--
* *
-
"--
-
--
---
º
---
---
º
shown on Pl. 4. and 5. the space between the fins was
filled with concrete, the tºro overhanging volumes
balancing eabh other by means of one inch reinforcing
rods inserted in the concrete. These rods were con–
tinuous over the top of the tubes and acted as saddles.
The top of the concrete was carefully smoothed off level
with the crown of the tube, leaving a projection of
plates 8 inches above it. Also at this time were set
the 3/4 inch anchor bolts, as shown on the drawings.
Upon the completion of this concreting, the timber work
was commenced, all timber being handled With a delerick
scow, which also furnished compressed, air for the drills.
work accordingly went forward rapidly.
It was now necessary to provide for the hydro-
static pressure that would come against the dam when the
water had be ºn removed from one side. The plan of opera—
tion was as follows:
When the tubes were lowered to position and
the filling of the tofferdam was finished, both the
timber portion and the steel portion, a complete set
of bracing made of 12 X 12 inch timbers fas set (floating )
and carefully Wedged against the dam and the land at Water
level. The water was then pumped out in five foot stages
and after each stage another set of bracing was placed
and wedged, and so on down to Elevation -86. This placed
an increasing head of water after each stage. In order







-15–
to shorten the time of this pressure and hasten the
pumping, four tiers of this bracing were built, tied
together, and forced into place with rock ballast.
This method of procedure was made possible because of
the accuracy with which the steel sheet piling had
been driven, and 51so because of the shape of the
enclosure. (See Pl. 3 showing off-set in sheeting. )
HYDRO-STATIC PRESSURE ON TIMBER PORTION.
In anticipation of the hydro-static pressure
against the dam, as the water in the enclosure was
lowered, six I beams were set in a vertical position
on the enclosure side of the dam, as shown on Pl. 4.
These Tere securely anchored to the concrete on top
Of the tube and temporarily fastened to the top with
clamps. After lowering, the bracing was wedged directly
against these beams.
It was found that, as the water was pumped
out of the enclosure, the maximum stress in the beams
(19,000 pounds per square inch) developed When the
water reached the elevation of –10, and the lowest tier
of bracing at elevation —5. Under this condition, the
beams had a span of 22 feet, Tith a unit pressure of
625 pounds per square foot on the river side of the dam.
This unit, pressure was considered constant for the full
span of 22 feet, although as a matter of fact, this was
not quite true of the top 5 feet. It will be noted

—lé-
from the drawing, that the two I beams in the center
of the dam, support a distance of 7 ft. 6 in. to each
side. The three pairs of T beams were of the same
size, being 24 inch beams at 80 pounds.
The maximum reaction at the low ºr end of
the center pair of I beams occurs when the water in
the enclosure is lowered to elevation –15, and the
span of the beams is 17 feet, that is, the distance
from the lowest tier of bracing to the top of the
tubes. This reaction is equal approximately to 119,000
pounds, and for this stress three 2 inch rods were used
as anchors. Detail “BB” P1. 5 shows clearly the arrange-
ment of these rods, which ran back through the dam,
slightly pitched down, to an anchorage in the back web.
They thus pulled against the concrete, which was banked
over the pods between the two ºrebs. Temporary fastenings
and spreaders Tºrºe employed to keep the beams parallel
under loºd. Precatutions were taken to insure uniform
loading by driving thin Wedges for the entire height of
the bººm; .
STEFL PORTION OF COFFERDAM: The remaining portion -
of the cofferdam consisted entirely of steel sheet piling,
in lengths of 45 feet to 65 feet. This piling was of º
the type known as “Friestedt Symmetrical Piling w, consist—

-17–
ing in this case of a 15-inch channel at 33 pounds and
two z bars, 4 1/8 x 3/8 inch at 9.2 pounds. This type
of sheeting has an average section modulus per horizontal
foot, of enclosure equal to 12.93.
P1.3 shows in full lines the sheeting which
had been driven prior to lowering the tubes. It was
necessary to have this sheeting in place before submersion
of the tubes in order to gain access for driving with a
floating pile arivor. Triving was next started from each
corner of the timber dam, pockets being formed of approx–
imately the same width as the timber portion, and tied in
with that portion previously driven. Thus it was necessary
to have two laps in the falls of the pockets, a 5 shoºn
on Pl. 3. All driving was done with a 4700 pound Arnott
Steam Hammer, but, as the penetration was inconsiderable,
the driving was easy. -
All the pieces were provided with open holes
ror 1. 1/2 inch rods, spaced at 6 foot centers (see P1.5. ).
Upon the completion of these pockets, divers were set to
Tork installing the tie rods. This work, of course, had
to be left entirely to the divers, but they performed
their york well. All the rods fºre provided ºrith 18 inches
of thread at each end, and were of lengths suitable to
various dimensions. All the rods passed through Wales
made of two 6-inch channels with separators and were pro-
-13–
vided with 3/4-inch plate washers and double nuts at
each end (see Pl. 3 and 5).
As shown on Pl. 5, there remained after driving
all the sheeting, large holes under the tubes between the
first steel sheet piling adjacent to the timber portion of
the dam and the lower quandrant of each tube. Owing to
the extreme depth here, it was impossible to use clay
filling, which would at once squeeze out. The following
method was used for closing these gaps.
The space beneath the tubes and between the
diaphragma was first thoroughly cleaned of all slime
that had accumulated down to good hard clay. This was
2
effected by means of pumping with a six-inch Morris centri-
fugal, steam driven pump, which was mounted on the timber
º
º
dam, the auction leading straight down to within about
5 feet of the bottom, then elbowing underneath the tubes
with rubber hose. While the pump was running, two divers
kept up a brisk stirring, so that this space was cleaned
in a satisfactory manner. Next, under each diaphragm,
as shown on Pl. 5, was built up a rowſ of bags filled about
three-fourths full of cement, and these were thoroughly
packed and tamped, effectually closing off the rockets.
Two-inch grout pipes were set, leading from
the top of the dam doºrn and under the tubes and ending
in well points, the pipes being of various lengths.
Around the pipes was carefully placed crushed stone,
1 inch to 1 1/2 inches in size, to fill up the entire



-19-
pocket as high as the top of the bags of cement. After
this had been finished, purs cement grout was forced
by means of air pressure, 80 to 100 pounds, into the
crushed stone. At first it could not be determined
whether concrete was being produced, but after a few
trials, it could be seen from bubbles and the resistance
to the air pressure, when it was time to stop forcing
the rout through each pipe. generally grout #35 force
through each pipe until plugged. After twenty-four hours,
upon inspection, the concrete was hard and satisfactory.
This process of filling in with stons and
grouting was carried on up in 3-foot stages as shown
on Pl. 5. This method not only effectually stopped
any danger of leaks through this spandrel, but also
gave firm anchorage to the sheet piling, and reduced
its weakness due to extreme length by greatly stiffen-
ing it.
Before producing concrete by this method,
experiments had been made ºrith Tremies, but owing to
the horizontal distance which the concrete had to be
forced this method was found to be unsatisfactory.
In connection with the Tremie method, an attempt was
made to overcome this horizontal travel of the concrete
by means of a slight air pressure on the pipe, but this
invariably ended in a sudden spouting of the concrete,
causing a loss of most of the cement.
The method used as described above was found
*** -

–20–
very satisfactory, although somewhat slow. This grout-
ing method was suggested by a number of well points
which had been used for pumping along sewer trenches
excavated in sand. It was found inadvisable to use
any sand in the grout, as it clogged between the stones
and bloºked the circulation of the grout.
HYDRO-STATIC PREssung on STEEL poRrron.
As in the cage of the timber portion of £he
dam, there is an increasing hydro-static pressure ana
a corresponding decrease of span, as the water in the
enclosure is lottered. The inside and outside toys of
piling exert an equal resistance to this pressure,
which, exerted against the dam, can be aiviaea equally
between these two rows, and it was a case of investigat—
ing the strength of the piling to resist bending moment
33 & beam; the sheet piling in this case serving the
§3rºle as the T beams on the timber portion of the dam
previously described.
The maximum stress was developed. When the
water was lowered to elevation -15 and the span became
26 feet. This theoretical stress (about 36,000 pounds
per square inch) is, for severºl reasons, much greate ºr
than the actual stress. In the first place, the pockets
formed by the steel sheet piling are comparatively narrow,
and are rigidly supported, or fixed, at elevation -36.
Also the first few sheeting adjacent to the timber portion
of the dam, and those adjacent to the single row of piling
–21– --- *"
running back to the shore, are gripped throughout their
entire length to practically rigid bodies. This side
restraint is transmitted throughout the oute. and inner
facing of the pocket, thus greatly starreñº ºne walls.
In fact the river side of the pooket presents a steel
diaphragm supported on all four eases, and * , practically
impossible to obtain an accurate idea of the actual stresses
involved. Any assistance from the natural inertia of the
mass of clay in the pocket to resist the hydro-static
pressure is also neglected.
During the entire time of pumping out the
enclosure, the dam was carefully observed for the purpose
of detecting any possible signs of weakness. By reason
of the rapidity with which the bracing was installed and
wedged, it was not considered that there was any great
risk at any time.
corºppam BRACIng: owing to the constricted area
of the enclosure in Thich the bracing was situated and
the spacing of 5 foot centers, the stresses involved
were comparat, ively small. This system of bracing was
practically the same in the rest of the work G# the - -
land section, a portion of mion was done in an open ---
cut. The main thing to guard against was the pressures
coming from the sides, caused by the weight of large

–22-
buildings and other unknown conditions. Since in a
large system of bracing failure often occurs by up-
heaving, it was necessary to watch carefully for
indications of this, but it was not necessary at any
time to provide ballast.
TEMPORARY FOUNDATION FOR TURES :- As shown on Pl. 2
provision was made for a temporary foundation to take
the Weight of the tubes until such time as a permanent
- is:
foundation could be made. For the temporary foundation
two landing platforms were built, each 8 feet by 13 feet
arid about 50 feet, from the ends of the tube. These were
constructed of concrete and were merely a changed section
or the regular keel. The elevation of the platforms when
submerged was –54.0, and they landed on clumps of piles,
which offered the proper resistance to the weight of the
tube.
Rather than drive long piling and saw them off
to the required elevation, which would have been a tedious
undertaking in that depth of water, maple rollers 8 inches
in diameter, sharpened at, one end. and 5 to 6 feet long,
Were used. In order to arive them and to determine their
carrying capacity, a 9-inch wrought-iron pipe filled with
§§hº was used as a follower. This pipe, 60 feet long,
was provided at the bottom with a core of iron, inserted


so as to leave a projection of pipe of about 1 foot,
into which the roller was set.
in place while being lowered by
glamping device, which could be
water surface by means of a cord, when the pile was
in place.
plain white marks to indicate the exact elevation of
the top of the pile, and convenient and accurate gauges
showing water elevation were near at hand.
into place, was tapped lightly with a drop hammer until
in each case by measuring directly from a point on
the shore.
of excavation, the resistance to driving varied slightly,
but in every case it was more than equal to the weight
of the follower and hammer.
each pile is merely equal to this load, not including
driving, and the water displacement being deducted,
each pile has the following carrying capacity:
–23–
Weight of follower—-2040 pounds
º * Sand------ 3.11.0
Trori ºore----------- 1090
Hämmer—---------------2000
T8:40
T}isplacement---------- 1677
Net------------5533 T
The roller was held
means of a simple
loosened from the
The pipe was also accurately gauged with
The pile, with the follower, being lowered
to grade, accurate location of the pile being obtained
Owing to the irregularity in the surface
On the assumption that
ºt
º
º
ºt
º
*
----
-
Forty-two piles were driven in all, giving
a total resistance, or carrying capacity, of 138 tons,
which is very conservative in viewſ of the fact that all
frictional resistance obtained from driving has been
neglected. Since 100 tons of water ballast was determined
upon as the weight to be used in lowering the tubes, this
capacity was ample.
LOWERING TUBES:- The timber dams being completed
and the landing piles in place, the tubes were ready
to be lowered. The method used was simple, Two pairs
of girders 75 feet in span ſtere placed across the tubes,
a pair at each end, supported on scows, to which ºf ere
attached cable slings, wrapped about the tubes and -*.
fastened to the shell to prevent creeping. These girders
We're designed to carry a load of 60 tons per pair, or
120 tons, this being slightly in excess of the require-
ment, .
On each scow was placed a steam hoisting engine
fed with steam from tugs standing by, which did the
|
towing to the site of lowering. the cables wrapped
around the tubes led over the sheaves set on the girders
and from there to the drum of the hoisting engines.
In order to avoid unevenness during lowering,
while letting in the Water ballast, each tube was divided


into three compartments, as shown on Pl. 2. The inlet
of water into these compartments was controlled on the
tops of the adjacent stack, so that any compartment
could be flooded at Will. Also suitable duplex pumps
were installed in the tubes to remove water when necessary.
Twenty-four hours previous to the time of
lowering the tubes, the current in the river was checked
by closing the gates at Lockport, Ill., at which point
the sanitary District of Chicago maintain a dań and
hydro-electric plant. With the current thus reduced, it
was necessary to have only murricient Control of the
tubes to check any tendency to lose alignment from other
causes. For this purpose eight cables were used, three
running upstream, three down, and one at each end. The
cables all led to electric hoisting engines, furnished
by the Street Railway Company, which were fed from nearby
trolley lines. Breakwaters at the bridges and docks made
suitable locations for these engines. As this, together
with pre-arranged signals by megaphone, gave complete
control of the tubes at all times, they were lowered to
exact alignment arid grade without difficulty. From
observations of the gauges on the bulkheads between the
compartment, 3 for water ballast, it Was found that iss;
than 100 tons of water were used in lowering.

-26-
PERMAN ENT FOUNDATION : - In yieſ of the fact, that
the tubes when in position are ent, irely below the general
elevation of the river bottom and in a comparatively
narrow trench, sand is as guitable a mat, 3rial for a
foundation as could be used, and is also the easiest
and cheapest to install. This operation of placing
a sand foundation was started at, once after the tubes
had been set.
For this purpose two steel sand carriers of
the Lake Sand Company of Chicago, each of 600 cubic
yards capacity, were used. These boats are provided
With centrifugal sand pumps, With which they load them—
selves in the lake. They are divided into six compart-
ments, each, Then loaded, with a port hole inst above
water line. To these port holes was secured an eight
inch spiral riveted pipe, provided with a suitable intake
hopper, the pipe leading straight down tangent to the
tube as shown in Pi. 2. The pump was then started, dis-
charging its water in a spray into the compartment at
hand, causing a rapid flow of sand down the pipe and
Ander the tube. This method worked successfully and
about 3,000 yards of sand were placed in this way.
During this operation it was necessary only to
maintain careful inspection and see that at no time was
there a greater amount of sand on one side of the tunnel
than on the other, as an unbalanced wedging effect of
the sand might have caused the tubes to shift. To safe- i.
º

-27-
guard against this danger, water wras let into the tubes
as the sand foundation increased in height. Precautions
were taken against allowing any sand to run near the ends
of the tubes ºthere the cofferdams were to be built, later
on, as that might have caused $3rious leaks.
CONCRETING BALANCE or FIVER SECTION.
As noted before, there still remained a ſortion
of the interior concrete lining to be placed. Thus far
in concreting in the tubes, there had always been sufficient
head room to permit the use of wheel-barrows, but these
were not now available, as the remaining portion was at
the top of the arch.
Att, ention had at this time bee called to a
concrète mixer which was able to mix and deliver a small
batch of concrete (one-third cubic yard) by Pneumatic
pressure. This machine consisted of a guitable receptacle,
egg shapūd, with the pointed and down. On the top was
a steel trap door, operated by means of a lever, which
normally hung down into the mixer. The required amount
of stone, Band, cement and water was dumped into the
hopper, the trap was then shut, and the machine connected
to the receiving tank of an air compressor (120 pounds).
When the gauge on the mixer tank indicat, 3d 80 pounds,
the valve in the discharge line was opened and the charge
immediately started through the line, which led from the
machine, located on the north dock, down the 3-foot
–23–
stack and along the arch of the tube to the point of
º
delivery. In this way it was possible to deliver concrete
ºf .
throughout the entire length of the tube from one set-up
of the mixer, and the results were in every way equal to
the other cond Tºet, a previously placed. The discharge line
consisted of 6-inch spiral riveted pipe, which, however,
was not very suitable because of the rivet heads. For
all elbows and turns, sweeps were used, with a 5-foot
radius instead of regular elbows, in order to cut down
frictional resistance. -
All mixing of ingredients took place ºfhile the
material was passing through the line, tests having been
made both with concrete and mixed ºolored poºrders. All
tests showed a perfect mix, and the concrete was up to
all the standards of the specifications.
REMOVING GoFFERTAM:- After the connection of
the land and river sections was completed, newſ dock
*falls wºre constructed of timber, which rested on the
land section just ahead of the end of the tubes. The
river face of these docks was about 3 feet from the
inside row of sheet piling of the cofferdam. As the
timber ºork on these docks progressed upward, the bracing
to the cofferdams was removed and replaced with short
strut 5 to the new dock. After the completion of the
docks, removal of the cofferdams was commenced. Owing


-29–
º
to the concrete placed in the bottoms of the pockets
between the sheet piling, it was impossible to pull
the inside Tour. This rotºſ was then ºut off at elevation
–30 by means of the oxygen acetylene torch. This burning
was accomplished with little difficulty, as practically
no water seeped through the cut to affect the flame.
The next step ſtas the removal of the timber
portion, which was done with a pile driver, by merely
lifting the tiers of timber one from the other, slings
being attached by a diver. The clay filling was exca wated
and removed as it, slid into the river and hauled away
in scows.
The outside row of steel sheet piling was then
pulled out, after which the inside row, which had been
burned off, was removed. The greatest difficulty en-
countered was in removing the outside row, owing to
its adhesion to the concrete and the great friction
between the individual pieces. Before this sheeting
was pulled, the outstanding ends of all tie rods were
sheared off by dropping on them a length of steel
sheeting sharpened at one end. By skillful manipula-
tion of the pile driver, all pieces were finally
removed.


Pº RSONNEL.
The steel tubes described above were designed
by E. G. & R. M. Shankland, Consulting Engineers, Chicago,
Illinois; the land section of the tunnel was designed
by the Board of Supervising Engineers, who represent
the City's interest in the operation of all surface
lines in Chicago. All work on the tunnel was subject
to the inspection of both the Chicago Railways Co. and
the Board.
During the readjustment of the design of the
cofferdams and the lowering of the tubes, the writer
acted as assistant to the Chief Engineer, Mr. J. Z.
Murphy, of the Chicago Railways Company, and later as
Chief Engineer for the contractor in charge of the
River Section.



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