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MSU Is An Affirmative Action/E qual Opportunity institution
cAcirc\datedue.pm3-p. 1
THE SUB STRUCTURE OF THE MODERN HIGH BUILDING OF CHICAGO.

THESIS FOR DEGREE OF C.E.

WARREN WAYLAND HITCHCOCK,JR.

Re ad Oe

1916.
THESIS
INTRODUCTION.

The rapid growth of the City of Chicago,
and the conditions which have led to confining the
main business section of the city to what is gener-
ally known as the loop district, has caused an ab-
normal advance in the price of real estate in this
location.

This high price of real estate has led to
the construction of extremely high buildings and
also to the installation of two, three and even four
story basements.

This thesis will teke up the discussion of
tne methods of construction of the foundations, base-
ments, retaining walls, basement floors, etc., as
well as the effect of this type of construction upon

existing structures.

eS 104425
SOIL CONDITIONS.

To understand the difficulties of this work,
one must be familiar with the geological formation un-
derlying this portion of Chicago.

Old Chicago, tnat is Chicago before the fire,
was built in a swamp; the soil rising only a few feet
above the surface of the river and lake. The soil was
of blue clay and rather soft.

After the city burned tne rubbish from the
fire and more or less imported material was used to fill
in the streets of this district, raising them to an ele-
vation of ten or twelve feet above the lake level.

In excavating for foundations to bed rock, we
usually find at least five distinct strata of material,
as shown in Figure l.

The upper eight feet, as stated above, is made
up of filled-in material, which is quite pervious.

Below this fill we find a strata of soft blue
clay, about fifty feet in thickness, which rapidly hard-
ens as we pass downward, until it becomes a fairly stiff
hard pan.

Below this hard pan strata we usually find one
or two conditions; the more common form being that the
hard pan rapidly softens, forming a strata of soft clay

for a depth of about sixteen feet, below which is a strata
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of water-bearing sand and gravel extending to bed rock.
This strata is ordinarily about three feet thick.

The bed rock is a hard limestone formation,
more or less fractured and of unknown depth.

In the vicinity of the stock yards, this strata
of rock has been drilled to a depth of several thousand
feet.

It must be borne in mind that the clay strata
above the hard pan will flow under a superimposed load
if not supported laterally.

How to prevent this lateral movement, and the
consequent settlement of surrounding foundations, is the
chief problem encountered in the construction of deep

foundetions.
OLD AND NEW TYPES OF FOUNDATION:-

The older Chicago buildings rest upon floating
foundsetions consisting of a grillage of steel beams en-
cased in concrete.

This type of foundation was found very satis-
factory for the lighter buildings, and in fact, for com-
paratively heavy structures, as long as the soil below
these foundations was not disturbed. The Masonic Temple,
which was the highest building in Chicago at the time of
its construction, rests upon floating foundations, but
due to soil disturbances, such as the construction of
the Illinois tunnel, the sinking of deep foundations in
the vicinity end perhaps to a certain extent to natural

settlement, the foundations have become so badly out of
De

level, that it has been found necessary to carry a large
part of this building on jacks, which can be so adjusted
as to maintain equilibrium.

The same conditions apply to the Great Northern
Hotel building. |

These two buildings taugnt the architect and en-
gineer that the floating founaetion was not suitable for
buildings of this class.

Three other types of foundation have been de-
vised to carry buildings in this locality; the pile foun-
dation, the hard pan caisson and the rock caisson.

The pile foundation is not commonly employed
for structures of extreme height, and will not be treated
in this article.

The hard pan caisson properly designed is suit-
able to carry a building of almost any height, as long as
the soil underlying the hard pan strata is not disturbed,
but as will be more fully explained later, there is danger
of settlement, which will leave the hard pan crust unsup-
ported and allow a settlement which would cause the ruin
of the structure carried upon it.

The most notable exemple of a modern high office
building carried upon hard pan caisson is the Peoples' Gas
Building at Michigan Boulevard and Adam Street. However,
nearly all other large office and merchantile buildings in

the loop district rest upon bed rock caissons.
THE HARD PAN CAISSON.

The hard pen caisson is a cylindrical concrete
shaft extending from the cast base of the steel building
column to a good firm bearing in the hard pan strata, as
shown in Figure 2.

The allowable loading for the column is 400;
per square inch of cross sectional area, and the allowa-
ble loading for the hard pan strata is from six to eignt
tons per square feet of bearing area.

So that the full strength of the shaft may be
effective, the bottom of the caisson is belled out, as
shown in Figure 2, to twice the area of the shaft, making
the bearing area upon the herd pan strata just four times
the shaft area.

The allowable loading for the concrete being
400 lbs. per square inch, the maximum load brought upon
the hard pan will be 100 lbs. per square inch, or 14,400
lbs. per sq- foot. This is equivelent to 7-2 tons or about
the average allowable loading as given under the city build-
ing code.

The methods of construction, specifications for
material, etc., for the hard pan caisson, are exactly the
same as those for the rock caisson, so tnat the following
discussion of “Rock Caissons" may be held to apply to both

down to the hard pan strate.

TESTS; -

It is ordinerily specified thet the bottom of
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each hard pan caisson shall be tested by drilling a hole
at least eight feet deep with a common wood auger to as-
certain if the hard pan strata is of sufficient depth.
When the depth of the hard pan has been deter-
mined, this test is only required for a portion of the
holes, and as soon as good firm material is reached the

caisson is immediately filled.

SIZES OF CAISSONS:-=

Caissons cannot be built readily of a smaller
diameter than 34 feet, as a man cannot handle the materi-
al in a smaller opening than this.

There is nothing to limit tne size except the
danger of deformation due to side pressure, and the fact
that for a large caisson a longer time is required to con-
plete a section and thus allow more time for settlement.

The points will be discussed further under "Rock

Caissons".
THE ROCK CAISSON.

The "Rock Caisson" differs from the hard pan
caisson only inasmuch as it passes through the hard pan
end underlying clay, to the bed rocx, which as shown in
Figure 1, lies at a depth of ebout one hundred feet be-
low the street grade. The "Rock Caisson" has no bell as
the allowable loading. On the limestone rock is 400
pounds per square inch, or the same as the concrete pier.

The diameter of the concrete column is, there-
fore, the same throughout its length.

Under present conditions existing in the loop
district, the hard pan caisson is the only safe founda-
tion as it reaches below all movable strata of earth and,
therefore, eliminates all danger of settlement. Practi-
cally all of the modern buildings in the loop district

are carried upon this type of foundation.

SIZE OF CAISSONS:-

The minimum diameter of the Rock Caisson is
fixed by some architects at four feet and eight inches.

The opening must be large enough for two men to
work at the same time, for as soon as the water bearing
strata is reached the digging must be daone very rapidly.
The maximum size is limited only by the load they are de-
Signed to carry.

The largest caisson to come under the observa-
tion of the writer is under Mandel Brothers Merchantile
Building on State Street, and is 13 feet, 4 inches in di-
ameter.
LAGGING:-=

The lagging used in sheeting the caisson should
consist of sound lumber three inches thick and six inches
wide, matched and beveled. This should be cut in sections
three feet and four inches long when used in the ordinary
soft clay soil, and of shorter length as required in soft
ground wnere caving may occur or where the clay is of such
a nature that swelling or bulging of the sides takes place
within a short period after the digging is extended below
a completed section of lagging.

This bulging indicates a lateral movement of
the soft clay strata due to the superimposed loading which
it carries, and as this movement and consequent settlement
of surrounding foundations is exactly whet we wish to avoia,
the legging must be placed and this movement stopped as
quickly as possible.

When the herd pan is reached, lighter lagging may
be used.

Lagging 2 inches thick, 6 inches wide and 5 feet,
4 inches long, is ordinarily used and in some cases the hard
pan has been found so dense and dry that no lagging has been

used.
CAISSON RINGS:-

The lagging is held in place by caisson rings,
such as are shown in Figures 4 and 5.
These rings are forged from wrought iron and made

in segments, which bolt together as shown. The ring wher
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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10.
bolted together must form a perfect circle of the exact
diameter of the finished caisson and be free from all
defects.

The size of the cross section of the ring
varies with the diameter. The smaller sizes are made of
material 2 inches wide and 3/8 of an inch thick, while
for the larger sizes material 4 inches wide and 1 inch

thick is used.
THE CONCRETE: -

The concrete should be of the best quality in
every respect, and mixed in the following proportion; 1
part cement, 2 parts sand and 4 parts washed gravel or
crushed stone.

The specifications for the above material should
conform to the rules adopted by the American Society for
Testing Materials.

No cement should be used until approved by a
competent tester.

During the winter months sand and gravel are
hard to obtain, and in many cases it has been found neces-
sary to substitute limestone screenings for sand.

Concrete made from crushed limestone and lime-
stone screenings has been found to give fully as good, if

not better, results than that made from sand and gravel.
ll.

THE COFFER DAM.

Ordinarily the digging of the caisson is begun
at the basement floor level of an existing building, or
in the open basement of a previously existing building.

Here is usually found about three feet of com-
paratively pervious material. To prevent the surfece
water from seeping through this strata and running down
the inside of the caisson, a cofferdam is constructed
around the top of the opening.

Tne most common form of specification states
that a set of lagging five feet long shall be driven so
as to form a complete circle the exact size of the caisson
end two feet outside of this ring another set of lagging
shall be driven.

The space between these two rings shall be ex-
cavated to the impervious blue clay and the opening re-
filled with blue clay which shall be tamped and puddled
to form a water tight wall, which shall extend to a few
inches above the general ground level.

The space within the inner set of lagging is
then excavated and two caisson rings placed in position
and wedged tightly against the lagging.

The more rational method of constructing the
coffer dam and the one commonly employed is as follows:
A circular pit is excavated, having a diameter three or
four feet greater than that of the required caisson.

When the blue clay is reached, e ring properly
le.

boltea is placed in the bottom of tnis pit and raised a
few inches above the ground by blocking, es indicated in
Figure 7.

The workmen then set lagging in place around
the outside of this ring, and when the circle is complete
it is held in place by a rope or wire, whicn encircles it
like the hoop of a barrel.

Another ring is now placed about one foot below
the top, and after tne rings have been checked to see that
they are properly centered with respect to tne location of
the caisson center the opening outside of the lagging is
refillea with blue clay which is thoroughly tamped and
puddled. The caisson top now has the appearance as indi-
cated in Figure 3.

The entire top is now covered with a platform
consisting of two layers of two incn planking, leaving an
opening eighteen inches square in its center, through
wnich the excavated material is removed. This opening
shoulda have a curb extending four inches above tne planx-
ing, 80 as to prevent eartn and other substences from fall-

ing down tne sinaft.
13.

EQUIPMENT FOR THE REMOVAL OF EARTH FROM THE CAISSON.

HAND DERRICK: -

The equipment used for handling the excavated
material depends upon the size, depth, number and gener-
al location of the caissons.

Where only a few small hard pan caissons are
to be sunk, the earth is ordinarily removed with a hand
windlass erected as shown in Figure 6.

The bucket used for handling the material is
constructed from heavy sheet metal and is about 20 inches
high and 18 inches in diameter.

The hook by which the rope is attached to the
bale should be at least 16 inches long, so as to reduce
the danger of the bale slipping out and allowing the bucket
to fall.

The rope used is the best, one inch manilla, and
Old rope should not be used in any instance.

The one thing to be xept uppermost in the mind of
the rigger who erects the equipment for caisson digging, as
well as that of the operator, is that every chance for drop-
ping the bucket down the shaft must be eliminated, as tnere
is little chance for the caisson digger to escape death if

tne bucket drops.
THE HOISTING ENGINE: -

Where the work is of a greater scope, some mechnan-

ical power must be used as the hand derrick is not economi-
Hand Derrick For Raising
Caisson Bucket

 
14.

cal and when the caisson reaches water bearing s3s0il cannot

oe operated rapidly enough to dispose of the water, to say

nothing of nandling that and the excavated soil at the same
time.

Where circumstances permit of its use, the hoist-
ing engine is the most economical power unit that can be
employed. The ideal place for its use is when the caissons
are being sunk in an open lot, although it is often employed
wnere the work is being carried on in a basement.

The proper method or rigging the engine is to
place it at the end of a row of caissons, as indicated in
Figure 7. The smaller drum is used as a driving pulley,
and an endless belt of wire cable extends from this along
the line of caissons.

Over each caisson is erected a short piece of
shafting carrying a hollow faced pulley, or “nigger head"
at each end. Tne shaft is held in place by a derrick sim-
ilar to that used for the hand windlass; one pulley being
placed directly over the center of the caisson opening and
those at the other end of the shaft being set in a direct
line from the drum of the hoisting engine.

The wire rope belt is given one complete turn
around this drum, and another about each of the driving
pulleys at the caisson. In this way the pulley over the
opening is constantly rotated.

The operator at the caisson gives the rope whicn
is attached to the bucket two or three turns around this
pulley, and when he wishes to raise tne bucket draws in
the rope hand over hand. The friction vetween rope and

nulley transmits tne vower tn raise tne bucket.
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15.

In lowering the bucket the operator allows tne

rope to slide freely over the pulley.
THE ELECTRIC HOIST: -

Where the above method cannot be employed to ad-
vantage, a portable electric hoist is used. In this case
the rope from the bucket is passed through a pulley over
the caisson opening and carried directly to the drum of
the hoist.

Tae disadvantage of this metnod is that a sepa-
rate hoist must be installed for each caisson that is being
worked and a hoisting engineer employed to operate it. How-
ever, it has the advantage of greater speed of operation,
and when a neavy flow of water is encountered it is often
found to advantage to discontinue the use of the Hoisting

Engine and complete the work with the electric hoist.
THE FRICTION MACHINE:

To the writer's knowledge, there has only been
One special type of machine built to handle this class of
work, and that is an electrically driven friction machine
put out by the Thomas Elevator Company.

This macnine was so designed that the rope from
the caisson bucket passed through a pulley above the
caisson opening; then by a system of pulleys, into the ma-
chine and around a grooved pulley about eighteen inches in
diameter; thence, over a small idler attached to a friction
clutch which operated the pulley,

From this idler the rope returned to a second

pulley over the caisson.
 

16.

To raise the bucket the operator pulled downward
on tne rope. This forced the clutch lever downward and the
large pulley began to revolve. The operator drew in the
rope hand over hand.

Theoretically, he should exert only enough force
upon the rope to hold the clutch in operation, but owing
to poor mechanical design it was found to be a hard machine
to operate, and there was also trouble due to the cutting
ot the rope and thus endangering the diggers.

Each machine was equipped to operate eight caissons
and the manufacturers built two of these outfits which were
rented by the contractors.

The above machines were used on several large con-
tracts, but the owners recognized then poor mechanical design
and took them back to their shops to have them rebuilt, and
it is hoped that they will give better results when they
again appear.

If this machine is so constructed as to eliminate
the defects pointed out above, it will be the most efficient
and economical method for nandling caisson excavation in the

basements of old buildings.
 

17.
DISPOSAL OF EXCAVATED MATERIAL.

Owing to the extreme congestion of traffic within
the loop district, very little of the excavated material is
removed by wagon.

When for any reason this method must be resorted
to, a brick noist is installed near the sidewalk line and
running from the general lot level to a platform built to
a height of six or eight feet above the street. }

All material is loaded into wheelbarrows, which
are elevated to the platform and dumped directly into wagons.

The more convenient method of disposal is through
the Illinois tunnel. This tunnel lies about 40 feet below
the street level, and for the removal of excavated material
for caisson and general excavation a tube about 20 inches in
diameter is forced upward at an angle of about forty-five
degrees until it reaches the general level of the lot where
the work is to be carried on.

This tube is placed either from the main bore of
the tunnel, or where the tunnel traffic is heavy from a branch
tunnel driven into a side street, and in many instances where
the new building is to be served by the tunnel tne bore is
extended directly under the lot and the pipe driven from this
extension to the surface.

In all cases the pipe shoulda be given an easy slope,
so that the material dumped will not strike the cars with
too high a velocity.

The material is conveyed by cars to the disposal

station of the Illinois Tunnel Company, where it is loaded
 

18.

Figure 8 shows the arrangement of the tunnel con-
nection to old basement floor level, as installed for re-
moving caisson excavation from Rothschild and Company's
building at State and Van Buren streets.

When the tunnel extension is driven under the lot,
great care must be exercised so that it will not interfere
witn caisson work, and it is required that all tunnel exten-
sion shall be driven under air pressure to avoid settlement

of surrounding buildings.
 

 

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19.

SINKING AND LAGGING THE CAISSON.

THE UPPER CLAY STRATA:

When the coffer-dam has veen constructed, as
shown in Figure 3, and the opening covered with the plank
top previously described, the digging operation is begun.

The opening should be dug to a depth equal to
the length of a set of lagging and cut down plumb with
the inner face of the lagging, as indicated in Figure 9.

When the proper depth has been reached, a plumb
bob is lowered from a point above which marks the caisson
center, and a stake driven in the bottom of the hole. A
nail is then driven into the stake to locate the exact
center, and allowed to project about 14 inches.

The digger is now provided with a gauge of a
length equal to the radius of the caisson. One end of the
gauge is determined by a hole through the stick of which
the gauge is constructed.

This hole is placed over the nail which marks
the center point and the bottom of the hole trimmed to the
exact required diameter.

The sides are then trimmed so as to form a per-
fectly vertical face, flush with the back of the lag.ing,
as shown in Figure 10.

A caisson ring is now lowered, bolted and raised
upon a set of blocks about eight inches high, as indicated

in Figure 7.

The lageing is now placed back of this ring, a
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20.

piece at a time, and driven to place until the circle is
complete. Another ring is now lowered, bolted and driven
into place about eight inches below the top of the set of
lagging.

In case the ring does not fit tight against the
lagging at all points, wedges should be driven back of the
ring to insure a solid bearing at all points.

In case the lagging does not have a bearing at
all points, tne voids are ordinarily packed with clay, al-
though most specifications state that if the trimming is
not properly done the opening must be trimmed large enough
for a double set of lagging, and this requirement should be
enforced wnere the voids behind the lagging are large.

It must always be kept in mind that the object to
be attained is that of sinking the caisson with a minimum
amount of soil movement. Therefore, a firm bearing between
the lagging and sides of the excavation must be maintained.

Only a few cases are found in this locality where
the soil is so plastic that swelling or bulging takes place
during the digging for a set of lagging, and where this does
taxe place shorter lengths of lagzsing must be used so as to
reduce the time which the soil is unsupported lateraly.

it is absolutely necessary that when once a caisson
is begun, the work must be rushed as rapidly as possible by
three shifts of workmen, so that the caisson is not allowed
to stand unworked until the hard pan strata is reached.

With ordinary precaution this soil gives no trouble
from rising upward from the bottom of the excavation when

the work is pushed with the ordinary rapidity.
el.

THE HARD PAN STRATA:-

When the nard pan is reached the thickness of
the lagging is reduced from three inches to two inches
and the length increased from three feet to five feet
and four inches.

Less care is required in trimming the opening
in the hard pan caisson, as this strata is of such un-
yielding material that the settlement, if any, is extreme-

ly slight.
SOFT CLAY OR SAND STRATA:-

Below the hard pan strata we find different con-
ditions, depending upon the locality. For instance, at
the Mandel Brothers’ building on State street, immediate-
ly below tne hard pan strata was a course black sand.

This sand strata extended to bed rock at a depth
of about twenty feet below the hard pan.

At the Rothschilds’ building, further south on
State street, the hard pan gradually became softer until
it became almost a mud at a distance of ten or twelve feet
above bed rock.

Whatever tne soil condition may be, the depth of
soft material must be found as near as possible.

The fact that the bed rock lies at an almost uni-
form depth below the street line, enables the contractor
to: make a close estimate of this depth from the point when

tne soft digging begins.
22.

As soon as soft material is reacned, tne caisson
is belled out, as shown in Figure ll.

The additional radius at the bottom of the bell
being made a little more than the thickness of the required
numoer of sets of lagging to reach the rock.

When the bell is completed a ring is placed at
the bottom small enough to allow a set of lagging to be
driven between it and the bottom of the bell. This set of
lagging is driven as the excavating proceeds in exactly
the same way any sheet piling is driven for trench work.
When the proper depth is reached the second ring is placed.

After this set of lagging is in place and driven
its full depth, another set is placed within it and driven
down just as tne first set was placed within the bell.

This process is repeated until bed rock is reached.
The diameter of the caisson at the bottom, how-

ever, must not be less than the specified diameter.
WATER BEARING STRATA:-=

When the water bearing strata is reached, a rapid
method of handling material must be employed.

Ordinarily an electric hoist is installed, as
previously described. The water must be removed with the
bucket as well as the solid material. A number of pumps
have been tried out for this kind of work, but none of them

have been found successful.
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23.

DIFFICULT DIGGING.

SLIPPING DOWN OF THE LAGGING;

As soon as the soft material beneath the hard
pan strata is reached, more or less of the soft materi-
al continually works through the openings in the lagging.

This is particularly true of sand. During the
caisson excavation for Mandel Brothers' merchantile
building, on State street, so much of this material
worked into the caisson and was removed tnat the lagging
settled, leaving an opening several feet wide immediate-
ly below the hard pan strata.

The movement of this material was so cereat
that in several cases the opening extended from one cais-
son to the next. In One instance a man passed between
two caissons beneath the hard pan strata.

To prevent undue distortion of the lagging
under tne above conditions, it is held as near as possi-
ble to its original position by lacing. This is done by
spiking 2 x 4 inch strips vertically against the lagging.

Another method used when the above does not
prove sufficiently secure, is to run heavy steel cable
from the lower sets of lagcsing to some secure fastening
at the surface.

In the case mentioned above, the two systems
combined failed to prevent the formation of openings as

great as three feet. This, however, does no harm if the
24.

settlement is vertical and the caisson thus retains its
correct position.

In filling the caisson this opening should be
left so that the concrete can flow into the space between
the hard pan and the underlying strata.

The contractor, however, is adverse to supply-
ing the necessary concrete, and as this is a difticult
point for the inspector to watch it is the opinion of the
writer that many of these large openings are left beneath
the hard pan strata.

The fact cannot be too strongly emphasized that
in the course of time the hard pan strata will settle until
all voids beneath it are filled, carrying witn it all foun-
aations not reaching bed rock.

In some cases the lagring anda rings may become
so distorted and out of line tnat it becomes necessary to
remove the lagging and refill the caisson.

This should be done by removing tne lagzing one
set at a time, beginning at the bottom. As soon as one
set of lag;ing is removed, tne hole should be refilled witn
sand to the bottom of the set above.

This operation is repeated until all lagging
Wnicn is badly out of line or seems to be in a dangerous
condition is removed.

The caisson is now redug. Luring the redigging
every precaution must be taken to avoid a repitition of
tne settlement, as the conditions will always be found

nerder to contend witn than before the soil was disturbed.
25.

BOULDERS: -

In some localities the lower strata of ground
contains many boulders. This was found especially true
of the caissons beneath the Rothschilds and Company's
building, at State and Van Buren streets.

Two boulders were encountered in the hard pan
strata which entirely covered the cross sectional area
of the caisson. It was necessary to break these up by
the use of a drill and a plug and feather. The drill was
Operated by compressed air.

The boulders found at this depth are of granite
formation and have no well defined lines of cleavage. It
is, therefore, a slow process to break them up, for as a
rule only small portions are removed at each operation.

Another large boulder was found near the bottom
of a caisson, as shown in Figure l2.

In this case tne sand and mud was removed as
thoroughly as possible and all spaces beneath the boulder
were careiully packed with neat cement end then the caisson
filled in the ordinary manner, leaving the boulder project-
ing into the side of tne caisson.

No explosives can be used in breaking up these
boulders, because of the danger to tne nearby foundations,
and as a rule tne old buildings are supported on shoring
during the foundation construction, and this fact alone
eliminates any possibility of blasting.

TESTING THE ROCK.
standard specifications state tnat wnen bed rocx

1s reached, alternate caissons shall be drilled to a depth
jantai ite

Boulder.

 

 
26.

of eight feet to insure a soliag foundation.

The character of the rock, nowever, nas become
30 detinitely established that tne test is not ordinarily
insisted upon for the full number of caissons.

Where the drill test is not applied, the in-
spector tests the rock by striking the surface with a pick.

If the bed rock has been reached it will give a
sharp ringing sound, but if only a layer of shell rock
that has been uncovered it will give a dull hollow sound
due to the layer of mud veneath it. Experience alone
teaches the inspector how to distinguish between tne two
conditions.

FILLING THE CAISSON.

As soon as bed rock has been uncovered, tested
and approved, the caisson should be filled as rapidly as
possible to such a heighth that water cannot force its
way up through the concrete. Ten or twelve feet of con-
crete will seal the caisson properly under most conditions.

In many cases there is from two to six teet of
broken snell rock overlying the bed rock, and in some
cases the bed rock itself is very rough. These conditions
make it very difficult to drive tne sheeting close enough
to prevent the mud from running into the caisson, and in
some cases it is necessary to leave large openings around
the pieces of broken rock near the bottom.

vnen the rock has. been carefully cleaned, sacxs
of cement snould be lowered and packed into these openings
t9 hold vack tne soil, and if there is still any great

amount of soil flowing in a few sacxs of cement should be
27.6
@s possibile with the foreign material.
AS much well mixed concrete as possible should
be at nand in wheelbarrows or cars to be dumped into the

caisson as soon as the diggers are taken out.
LETHODS OF FILLING: -

There are two general methods used for filling
the caisson.

One is to hang a pipe about eight inches in di-
ameter, so tnat it reaches witnin a few feet of the bottom
of the caisson as shown in figure.

The other is to dump the concrete directly from

the wneelbarrow or car down to the shaft.

THE PIPE METHOD:-

The pipe method was introduced to overcome the
danger of tne separation of the concrete materials due to
the great heignth of fall, but it brought with it so many
new disadvantages that it has been discarded by nearly
all of the leading architects.

One of the great disadvantages of using the pipe
is that so much time is spent installing it, after the
dGigsers leave the caisson, that a large amount of water
collects into which the concrete must be poured. This in
itself offsets any advantage that the pipe might give in
the lower portion of the caisson.

Even atter the caisson has been filled above the
water line, the pipe sand its hangings make it very diffi-
cult to remove tne caisson rings.

Provided all difficulties might be overcome, the

fact remains that no better concrete is secured in this
way than by the direct method of filling.

DIRECT METHOD: -

Where tne concrete is dumped directly into the
caisson without a pipe, there are two points which must
be carefully watched or separation is sure to result.

First, the concrete must be properly mixed.

To be properly mixed for this work, the concrete must be
of such a consistancy tnat when dumped it will remain in
one solid mass until it strikes the bottom of the caisson.

The second important point is to have the con-
crete handled in such a manner tnat it does not come in
contact witn the side of the caisson during its descent.
This is accomplished by building a wooden hopper with
steep sides over the caisson top into which the concrete
is dumped.

With a little care this hopper can be 80 ar-
ranged that none of the concrete will be thrown against
the sides of the caisson.

Old nard pan caissons which were poured in this
way to Support certain parts of the boiler room of the
zandel Building, were removed during the reconstruction
and the concrete was found to be as near perfect as it can

be mage in practice.

THE FILLING OPERATION:

As previously explained, as soon as the caisson
bottom is made ready tne filling should begin at once and

be carried to a point above the water line. in fact, the
29.

filling should be continued as rapidly as is consistant with
the following method.

All caisson rings must be left in place in the
lower part of the caisson. The number of rings depends en-
tirely upon the soil conditions.

In some localities all rings can be removed after
eight or ten feet of concrete has been placed, and under
Otner conditions, particularly in sand, it has been found
necessary to place as much as twenty feet of concrete before
removing rings. The caisson is, therefore, filled to a
point just beneath the lower ring of the first set of lag-
ging, where it is deemed safe to remove the rings. The
workmen descend, unbolt and remove the first ring, which is
indicated as Ring 1 in Figure 13. The filling then contin-
ues until a point is reached immediately below Ring 2.

Ring 2 is now removed and the filling proceeds to the bottom
of Ring 3. This method is continued until a point is
reached immediately below the grillage or reinforced top of

the caisson.

CAISSON TOPS.

THE GRANITE CAP;

some architects specify tnat a granite concrete
cap eighteen inches thick shall be placed imnediately below
tne grillage for the purpose of distributing the pressure
from tne column more uniformly to tie concrete.

This cap is mede of granite screening and cement
mixed in the proportion of two parts granite screenings

ana one part cement. Most architects, however, have reacned
 
30.

the conclusion that the grillage or reinforced top is suf-

ficient for this purpose in itself.
THE GRILLAGE TOP;

The grillage top is mace up by placing a number
of tiers of I beams across the top of the caisson in oppo-
Site directions. The number and size of the beams depend
upon the loading, but for some of tne larger caissons as

nmeany @s six courses of twelve inch beams ere used.
PLACING THR GRILLAGE:

Tne first course of grillage beams should be
placed on metal blocks, which will raise them about two
inches above the hardened concrete, and as soon as this
set is pleced ell openings between ena beneath the steel
should be tnoroughly filled with a thin grout made up of
two parts granite screenings and one part cement.

The second set of beams should tnen be placea
at right angles to the first and this set grouted in as
above. |

It is to be emphasized that each set must be
placed ana grouted independently; otherwise, the voids
will not be filled.

One contractor, at least, made the mistake of
setting all the grillage and then doing the grouting at
one operation, but upon examination tne result wes found
to be so poor that he was requirea to remove the work.

The elevation for setting the grillage beams

must be so computed that the top of the last set of beams
31.

will be about two inches below the botton of the cast iron

shoe carrying the steel column.
THE REINFORCED TOP:

When steel reinforcing is used, the top of the
caisson is designed as a hooped column and carried to an

elevation two inches below the cast iron column base.
SETTING THE BASE.

When the grouting around the grillage is thore-
oughly set, the cast iron vase is lowered and blocked up
in position; when the center end elevation have been
checked the base is grouted into place.

A grout consisting of one part cement and two
perts granite screenings is used beneath the shoe.

All openings in the base are now filled with
concrete and the entire shoe is encased with four inches
of concrete ss specified for caisson filling.

Figure 14 shows a caisson top completed and nav-
ing the steel column in place.

The basement columns are made up in lengtn suf-
ficient to reach the first floor of the building, and as
soon as the caissons ere completed the columns are set and
the first floor beams put in place.

To furnish additional rigidity, tne bottom of
the column is braced securely against the caisson lagging.

As far as the csisson work is concerned, the
general basement excavation can be begun. However, before
tnis is done the retaining walls must be in place.

Retaining wall construction will be taken up in
 
&® separate article.

 

32.
33:

BULKHEADING THE CAISSON.

When large caissons are sunk and allowed to stand
open for a great length of time, the lateral pressure be-
comes so great as to deform the rings and some iiore secure
method must be resorted to in order thet settlement may be
prevented.

This is more commonly true of tne large caissons
carrying a party wall, where the ground is heavily loaded
by an adjoining building. The caisson must be left open
for a considerable time from the top of the concrete which
carries the new steel to the surface of the original ground.

As soon as the rings show signs of deformation,
the caisson should be bulkheaded. The bulkheading consists
ot two segments cut from eight inch by eight inch timbers,
set as shown in Figure 15, and forced in place against the
caisson lagging by six inch by six inch drums.

The basement retaining wall, as has been hereto-
fore noted for many of the basements beneath the Chicago
buildings, heve a depth as great as fifty feet.

It is spparent that witn the soil conditions and
heavy loedings that it would be practically impossible to

complete the basement retaining walls by the ordinery metnods.

THE RETAINING WALL.
THE TRENCH METHOD:

Where the trench method is used the walls are com-
pletea before any of the general basement excavations are

begun.
 

 

 

 

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34.

Tne best specifications require that the wall
shall be built in sections of length equal to the dis-
tance between column centers, and that no two adjacent
sections of trench shall be open at the same time.

A coffer-dam is built along the top of the
trench in the same manner as around the top of the cais-
son and a set of lagging put in pace.

The lagging for the trench work is exactly the
seme as that for caissons, except that it is not cut on
@ bevel. Two walling strips consisting of eight inch by
eight inch timbers are placed against each side of the
sneeting and forced to a tight bearing ageinst the lagging
by drums, as shown in Figure 16.

The digging is now done for the second set of
lagging and the same method is followed as for caisson work.
That is, the rough digging should be carried down flush
with the inside of the lagging and when the proper depth is
reached tne bank should be carefully trinmuea back flush with
the outside of the lagging.

The second set of lagging is now placed and braced
in the same way as the first. This operation is continued
until the bottom of the trench is reached.

For the ordinery basement, tne retaining well is
of suificient depth to reach the hard pan strata. Even if
a few feet of additional digging must be done, it is better
to extend the wall to the hard pan strata than to leave it
with a bearing upon the soft clay.

As soon as the bottom is reached and approved, a
few inches of concrete should be placed in the trench to

furnish secure tooting for the workmen and then the wall re-
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inforcing should be put in place.
THE REINFORCING: -

The walls are ordinarily reinforced with I beans,
placed vertically as shown in Figure 17.

The length of the beems is just sufficient to
reach from one floor to the next, so that the wall really
forms a vertical reinforced slab from floor to floor.

Rod reinforcing is sometimes used, but for the

heavier work the beam reinforcing is the more practical.
FILLING THE TRENCH;

There are two methods used for filling the wall
trenches. The older method is practically the same as that
for caisson work.

When the reinforcing is in place, the concrete is
poured to the bottom of the first set of drums and allowed
to set for eight hours, after which this set of bracing is
removed and the concrete poured to tne bottom of the second
set, this operation being continued until the wall is com-
pleted.

Where tne wall is to be faced with tile and plas-
tered, tne concrete is poured directly against the lagging,
but in many cases the concrete is to act as the finished
basement wall and a smooth form must be provided.

This will be discussed under the heading of "Forms’.

THE SECOND LETHOD OF FILLING:
The second method has two distinct advantages over

tne first.
 

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The more important is that the lateral pressure
can be taken care of in such a way as to reduce the danger
to the trench bracing giving way, and the second improve-
ment being that tne trench can be filled in one operation,
thus decreasing the amount of lost time during the concret-
ing operation.

When the trench is to be filled continuously, it
is dug to a width considerable greater than that of the
finished wall, so that the form for the finished face can
be erected within the trench, as shown in Figure 1d.

The form is built complete for the front face of
the well and then each drum is boxed in in such a manner
that the box can be removed after the basement excavation
is completed and the forms removed from the face of the
wall.

The steel is then placed and the concrete poured
to the finished top of the wall. This method eliminates
tne necessity of disturbing any trench bracing until the
concrete in the wall has become thoroughly set.

The wall must be designed with such a thickness
that tne waling timbers may be left in place and concreted

into the back of the wall.
FORMS:

When the first method of filling is used, the
forms are built up in sections within the trench as the
drums are removed for concreting, and by this method it
is not possible to get as true a surface as when the form

is erected as a unit.
 

 

37:

The proper form material is two inch dressed and
matched pine supported by 2 x 6 inch studding, placed at
intervels not to exceed three feet along the length of the
wall. These strips in turn are braced securely against

the lagging.
BACK FILLING:

As shown in Figure 18, the wall is extended di-
rectly through the caisson top, so that when the wall is
completed there is a crescent shaped opening between the
wall and the caisson lagging which must be backfilled.

The standard specifications require that as the
bracing and bulkheads are removed from this opening, the
space shall be filled with sand placed in layers and
flooded with water to be sure of filling all voids.

This back filling must be done very thoroughly,
or settlement will result.

Another method has been used by some contractors
in preference to following that specified, and for some
cases at least, nas been found cheaper for the contractor
and nas given better satisfaction to the architect. This
is to fill the space with a very lean and wet concrete.

In the case of one job inspected by the writer,
tne back filling was done with a grout made from limestone
screenings and cement mixed in the proportion of one sack
cement to a # yard batch of screenings.

This will make a concrete sufficiently strong,
so tnat when properly set all danger of settlement due to

the gradual filling in of the voids left around the bracing
 

 

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due to careless filling is eliminated, and if at some
future time a deep basement is built back of this wall
the concrete filling will not be so hard as to cause

any difficulty in excavating to the face of the wall.
 

39:

BASEMENT EXCAVATION.

 

when the retaining walls are completed and the
basement columns and first floor beams set in place, the
general basement excavation is begun. This excavation
is carried down to a point imnediately below the level
of the first basement floor.

All forms and bracing are removed from the face
of the retaining wall down to this point, but great care
must be exercised to see that the wall is left properly
braced below this point so that there is no danger of the
high lateral pressure forcing the wall inward and causing
a horizontal crack at the point where the first and second
set of reinforcing meet. That is at the basement floor
line.

In some cases where the pressure is extremely
high, temporary timber bracing is placed from the wall to
the columns and carried from column to column until the
opposite wall is reached.

The steel floor beams for the first basement
floor are now put in place end the reinforced concrete
floor poured.

When this concrete floor has thoroughly set,
the lower end of the first set of reinforcing beams and
tne upper end of the second set are rigidly held in place
and the entire structure is thoroughly braced.

The excavation is now carried down to a point

celow the level of the second basement floor and the
40.

floor beams and concrete floor placed. This operation is
repeated until the bottom of the basement is reached.
Beneath the lower basement floor heavy concrete
struts sbout two feet square are placed in such a way that
they support the bottom of the column and the top of the
caisson. These struts run from column to column in each
direction across the building, and act as a support to the
bottom section of the retaining walls and also prevent any
lateral shifting of the base of the columns when their

loading is brought upon them.
FLOORS .

All basement floors except the lowest are of
heavy construction. The ordinary thickness is about eight
inches and heavily reinforced; their office being not only
to act as a floor but to support the retaining walls and
also furnish rigidity to the entire structure.

The lower floor is usually plain concrete six
inches thick placed upon a bed of cinders. The struts

furnish the necessary bracing at this point.
41.

THE COST OF CONSTRUCTION.

The average prices as used for a basis of esti-
mating the cost of construction for besement work are as
follows:

Caisson, including excavation anda concrete -
fifty cents per cubic foot.

Retaining walls, exclusive of reinforcing and
bracing, twenty-eight cents per cubic foot.

Basement excavation, $3-00 per cubic yard.

Steel work, $8.00 per ton for erection, in ad-
dition to the price of steel delivered at the Michigan
Central yard on Lake Street. The usual price of steel
delivered at this point in the year 1912 was $40.00 per
ton.

The above price for retaining walls does not
include tne bracing of the retaining wall trenches, as
this is ordinarily done by the shoring contractors and
included in a lump sum bid for all shoring work required

for the entire job.
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