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building.



94 UNDERPINNING TRINITY VESTRY BUILDING

Fig. 16 is a view inside the completed east-iron lining of one of the
tunnels. It shows the concrete key of the inside lining which was
jacked into place in advance of the general lining.

The work of underpinning and of transferring the load to the new
piers caused a settlement of the building, as shown by the surveys, as
follows :

At N.E. Corner, on Vesey Street 1^ in.

^' N.W. - •' '• - IfV "

" center of Churchyard side 1^ "

'• " " Church Street side 2^ "

" S.E. Corner, on Fulton Street 2^| ''

a S.^Y. '' '!• " . " 2? ''

■ ^ A few cracks of minor importance developed, and the plaster
spalled in some places.

As shown by the survey figures, the settlement was very uniform.
This even settlement, and the slight damage to the building, resulted
from digging the pits Avithout regularity, that is, by not excavating
them from one end of the building toward the other.

The contract price for this section of the subway, extending from a
point on Broadway, just north of Barclay Street, to Church Street, a
distance of 1 030 ft., was $982 740.

The work of underpinning was commenced in June, 1913. The
longitudinal girders along the wall footings and the cross-girders were
all placed by February, 1914. Both tunnels were completed beneath
the building by June, 1914. The back-filling under the building was
completed and the basement floor and rooms restored by October,
1914, or 17 months after the active work of iinderpinning was started.
The final restoration of the property was delayed by the work under
Church Street, requiring the adjustment of street steam-supply pipes,
telephone ducts, sewers, etc., until April, 1916.

The contractors were Frederick L. Cranford, Inc., and the work
was done under the personal direction of Mr. F. L. Cranford and
J. C. Meem, M. Am. Soc. C. E. ; the Superintendent on the work
was Mr. H. L. Robinson, and the Assistant Engineers were Messrs. H.
P. Moran, in charge of the office, and W. McI. Wolfe, Assistant Engi-
neer on field work. For the Public Service Commission, Alfred Craven,




Fig. 11. — View in Advance Heading of Tunnel Under St. Paul's Churchyard.




Fig. 12. — Tunnel, Jills Breaking Into Space Under Trinity Vestry Building.




,-■- ^





Fig. 15. — East Tunnel at Fulton Street. Footing Course of Building
IS Shown Overhead.




Fig. 16. — View Showing Concrete Key on Inside of Completed
Cast-Iron Lining.



. / . , UNDEEPINNING TRINITY VESTRY BUILDING 101

M. Am. Soc. C. E., was Chief Engineer; Robert Ridgway, M. Am. Soc.
C. E., Engineer of Subway Construction; and Mr. Jesse O. Shipman,
Division Engineer. The writer was Consulting Engineer for the
Corporation of Trinity Church, and was assisted in supervising the
work by D. C. Johnson, Assoc. M. Am. Soc. C. E., and Mr. W. E. W.
Moore. The photographs were taken by Mr. Edwin Levick.



102 Discussioisr : underpinning trinity vestry building
DISCUSSION



Mr. James C. Meem,* M. Am. Soc. C. E. (by letter). — The author has

Meem. covered the subject of this paper so ably and fully that very little
remains to be said, except in reference to the elaboration of certain
points which he has mentioned.

The sectional roof shield method of tunneling has been used on
many important pieces of work, and is doubtless familiar to many,
but the use of the "pilot-girder" in connection therewith is new.
This was made of two lattice-girders, 4 ft. high, cross-tied and braced
to form a continuous box-girder, the rear members, as stated, being
carried forward and placed in position as required. The outer chords
were made longer than the inner ones, in order to accommodate it to
the curvature of the tunnel. The office of this girder was to carry the
falsework over the bottom excavation between the heading and the
completed section of the tunnel, and it proved to be a very satisfactory
and efficient substitute for the arched timber method which had gen-
erally been used in connection with the sectional shield.

Owing to the shortness of the radius of curvature of this tunnel
and the extreme accuracy with which the rings had to be set (the
contractor was limited absolutely to a variation of not more than
2 in. at any point), it is doubtful whether any other method of tunnel-
ing could have been used so successfully and advantageously.

In the matter of the concrete key in the roof, which is shown in
Fig. 16, very satisfactory results were obtained by filling beforehand
these spaces in the cast-iron ring above the key. Heretofore, as far
as the writer knows, no satisfactory method of filling these spaces after
placing the key has been found, except by grouting. This key was
placed by using a form with a movable bottom; on this the concrete
was placed and then the whole mass was jacked into the roof space
and maintained until it had set.

In the matter of the very limited dependence to be placed on
skin friction resistance of piles, as noted by the author, the writer
agrees very heartily. He believes that skin friction resistance is often
mixed up with bearing resistance, and that, as a factor by itself, it
has very little value, especially with smooth-bore piles. To assume a
value of, for instance, 1 000 lb. per sq. ft. for skin friction resistance,
as many engineers often do, is to assume that the pile withstands an
external pressure of approximately 2 500 lb. per sq. ft. It is improbable,
however, except in cases of very long piles in deep fluid strata, that
piles are ever subjected to a pressure of more than a few pounds per
square foot, and if this is true, skin friction resistance, of course, can
be only a fraction of this pressure, so that, as stated, it may be neglected

* Brooklyn, N. Y.



DISCUSSION" : UNDERPINNING TRINITY VESTRY BUILDING 103

in nearly all cases. The pile, undoubtedly, derives its real support Mr.
from the so-called ''bulb of pressure" at its base. ®^™*

Elias Cahn,* Assoc. M. Am. Soc. C. E. (by letter). — The writer Mr.
has read this paper with great interest, particularly the part relating ^"^°*
to the tests on concrete piles, and considers the Profession greatly
indebted to the author for their publication. It is a pity, however,
that these valuable tests were not extended to include the influ-
ence of other factors bearing on the supporting power of these
piles and their settlement imder load, no less important than
those considered. Time is one of these factors, and should not be neg-
lected in dealing with earth, because stresses travel so slowly through
this medium. All the tests, particularly No. 5, give indications of
this, but the time taken in any of them, about 1 hour, is very far from
sufficient to develop this influence fvdly. The depth to which a pile
is driven is also an important factor.

The following, therefore, are suggested as additions to the author's
conclusions :

(8) The settlement accompanying the development of a certain
pressure will be greater, the longer the time taken to develop this
pressure.

(9) A pile will settle under the, action of a constant pressure at a
rate and for a time depending on the pressure, the area of the base
of the pile, the kind of material which the pile penetrates, and the
surcharge over the horizontal plane through the bottom of the pile.

The earth immediately under the pile, when subjected to the
maximum pressure of the jack, as is clearly indicated by the rebound,
is in a state of excessive compression, which undoubtedly decreases
in all directions from this point. There will be a tendency to equalize
this compression in time, thereby reducing that immediately vmder
the pile, and either decreasing its resistance or causing further settle-
ment.

The conditions under which equilibrium is finally obtained and the
total settlement in time under a definite pressure or load, are the deter-
minants of what the safe load should be, rather than the pressure it
is possible to develop with the jack in a short time.

In Conclusion (5) no mention is made of the effect, on its sup-
porting power, of the depth to which the pile is driven. This depth,
for any given earth, determines the compression that the material
under the pile will stand permanently, and therefore the load it
will support.

The practice of considering the safe load as one-half the maximum
pressure obtained with the jack, therefore, is not logical unless the
depth of penetration and the kind of earth as well as the area of the

* New York City.



104 DISCUSSION : underpinning trinity vestry building

Mr. base of the pile are considered in determining the pressure to be
. Cahn. obtained with the jack. Aside from any difference in the skin friction,
a pile driven 30 ft. into the earth would have a greater supporting
power than if driven only 15 ft., though the same pressure is obtained
with a jack in the manner described in the paper, simply because the
earth at the lower level will stand permanently a greater degree of
compression than at the upper level.

The author has done more than his share in conducting the tests
recorded, and it is to be hoped that those who may have the opportunity
in the future will continue where he has left off, clearing up this
matter of the effect of time and of depth of penetration on the settle-
ment of a pile imder a constant pressure after the earth under it has
been compressed by a far greater pressure.

Mr. T. Kennard Thomson,* M. Am. See. C. E. — It i8 a pleasure to

Thomson. ^^ ^ well-designed piece of work well carried out, and then well
described; and the Society is once more indebted to Mr. Parsons.

Whenever the writer sees a "jack" under a building he thinks of
the comments of an eminent lawyer who was wont to laugh at engi-
neers for their careless use of the English language, for he claimed
that they nearly always referred to the process of "jacking up the
building"; whereas, they did not do anything of the sort, as the
method was to put a jack between the base of a building and a post
or sill resting on the ground and then, by operating the jack, force the
post or sill into the ground, keeping the building itself exactly where
it was, instead of "jacking it up".

Sentimental reasons against disturbing the church building and
graveyard undoubtedly entered into the selection of this design.
The contract price, as stated by the author, was $982 740 for 1 000 ft.
Otherwise, it might have been cheaper to tear down the old building
instead of underpinning it. However, the occupants of the building
and countless thousands of pedestrians gained by the lack of disturb-
ance of surface conditions.

It was fortunate in this case that two of the most dangerous
elements, often encountered in underpinning operations, were absent —
that is, water, and bad design or construction of the building to be
underpinned.

The speaker recalls one building which caused him much worry,
due to its very defective — to say the least — construction. It was some
twelve stories high; the two lower floors were supported by cast-iron
and steel columns of various shapes and the floors above were carried
by brick walls. The first joint uncovered disclosed the fact that two
20-in. beams did not rest on the shelf brackets of the cast-iron columns,
as there was at least a 2-in. space between the bottom of the I-beams



• New York City.



DISCUSSION : UNDERPINNING TRINITY VESTRY BUILDING 105

and the top of the shelf, and although there had been provision for Mr.
several bolts to connect the flange of the columns to the webs of the '"'^o'^o*''
beams, only one bolt, for each beam, had been put in place — so all
the support which the heavy 20-in. I-beams had was one |-in. bolt
each. If a safe or any other heavy article had ever been moved over
these beams, there would have undoubtedly been a bad collapse.

Needless to say, the disclosure of such an unsafe condition in the
first joint uncovered caused much worry about the possible condition
of the many joints in the building, which it was not possible to vmcover
for examination. However, the building, which rested on quicksand,
at the surface of the ground-water, was safely underpinned by forcing
cylindrical caissons to bed-rock from 60 to 70 ft. below the water
surface, through New York quicksand.

The speaker has underpinned walls eighteen stories high, by putting
cylindrical caissons down by the Breuchaud method, through 70 ft.
of water and quicksand, and then constructing a cellar for the new
building 35 ft. under water.*

In underpinning a six-story building by jacking down 16-in. pipes
until they were supposed to have reached hardpan (as a matter of fact
they offered so much resistance to the jacks that the weight of the
old building was really taken off the old foundations, as shown by hori-
zontal cracks in the brickwork), it was found, after all the sand had
been washed out of the cylinders and they had been filled with good
concrete, which should have made the underpinning strong enough
to carry very much heavier loads than those to which they were
subjected (for the building was practically raised by jacking against
the empty pipes, that is, before they had been filled with concrete)
that the cylinders actually settled when the pneumatic caissons were
being sunk near-by, thus loosening up the soil arormd the pipes and
thereby destroying the friction. It would seem, therefore, that
dependence cannot be placed on such pipes when the surrounding mate-
rial is likely to be disturbed.

The author states that in the Trinity Vestry Building the friction
of the material around the pipes was disregarded, and that the pipes
were treated as short columns having all the load transmitted to the
base, but these pipes ' were comparatively short. The length of such
cylinders is often 30 ft. or more; they are too long to be considered as
short columns, and offer very considerable resistance, due to the friction
of the earth.

Owing to this lack of reliability, the speaker does not usually
recommend underpinning with cylinders, unless they are large enough
to permit a man to enter them to excavate the material and then fill
them with concrete. A cylinder having an outside diameter of 30 in.
is about the smallest that can be used for this purpose.

*A full description of this work was published in Engineering News, March 28th,
1901.



106 DISCUSSION : undeepinning trinity vestry building

It pays to use an ample thickness of metal in the cylinders, for
the speaker has seen them made so thin that it was impossible to
see the bottom of the excavation from the top, owing to the warping
of the pipes while being jacked down.

On removing some old buildings on Wall Street, New York City,
which, about 14 years previously, had been underpinned by 14-in.
pipes jacked down and filled with the best of concrete (paper bags
having been used there also), except for the bottom 2 ft. of the cylinder,
it was found that it had been impossible to pump out all the sand, and
the concrete had probably set without thoroughly compressing the sand
in the lower 2 ft. of the pipe.

This, of course, would allow the building to settle at least a few
inches, if it ever happened that the friction of the soil against the
pipe was insufficient to carry the load, as is often the case when the
soil is disturbed.

In underpinning, as in other foundation work, no hard-and-fast
rules can be laid down, as each building requires a treatment of its
own.

The author refers to the rebound of the cylinders after the jacks
were removed, and the method taken to prevent it. The usual method,
however, is to use steel wedges, either between steel plates and beams,
or between granite wedging blocks.

The speaker tested some open concrete caissons for a bridge in
New Jersey a few years ago, and measured the rebound. The cylin-
ders were of reinforced concrete 6 ft. 6 in. in diameter, having an
open well, about 3 ft. 6 in. in diameter from the bottom to the top,
to permit the material to be excavated by clam-shell buckets.

After the caisson had penetrated some 75 ft., and was resting
on what was supposed to be fairly good sand, it was filled solid with
concrete to the top, which was 87 ft. from the cutting edge, making
a solid concrete pier, 6 ft. 6 in. in diameter, from 12 ft. above ground
to the cutting edge.

After this had stood for some time, a load of steel rails was grad-
ually applied imtil a maximum of 10.24 tons per sq. ft. on the base of
the pier was attained.

A gradual settlement of f in. resulted, and, when the load was
removed, the rebound was i in., leaving a supposed permanent settle-
ment of i in. As this settlement had been caused by applying 10.24
tons per sq. ft., and as the total weight of the bridge, with its live
load, would not exceed 6 tons per sq. ft., it would seem that the piers
would be perfectly safe. As a matter of fact, however, the jar of
the trains started fresh settlement in a number of the piers, and this
continued for about one year, when the maximum of 6 in. was reached.
This was about 5 years ago; and since then there has been no trouble.



DISCUSSION : UNDEKPIXNING TRINITY VESTRY BUILDING 107

Strange to say, the piers which settled were those which were supposed Mr.
to rest on the best material. Thomson,

The speaker once constructed a tunnel 7 ft. in diameter, by the
poling-board method, under Cedar Street, ISTew York City, and then
ran a branch tunnel 4 ft. in diameter for 104 ft. up the street. The
7-ft. tunnel was just below the sewers and other pipes, and just above
the water line at the bottom, as about 6 in. of very damp sand had
to be handled. The work was done so carefully that there was abso-
lutely no settlement in the street, and, as a matter of fact, nobody
except those concerned in the construction knew that the tunnel
had been built.

It might be interesting to mention the fact that in Michigan, where
there are mining shafts 4 000 ft. or more in depth, it has been noted
that heavy weights dropped from the top of the shaft never reach
the bottom, as the revolution of the earth causes them to strike the
side (always the same side) of the shaft, and become lodged there.
This was first noticed when a very heavy weight was accidentally
dropped from the top and it was feared that the men in the shaft
must have been killed, but the men did not even know that anything ;^'

had fallen, because it never reached the bottom.

James F. Fouhy,* Esq. — The speaker would like to know Mr. Mr.
Parsons' opinion as to whether or not the rebound of the piles was due ^°^^^-
to looseness at the joints of the pile sections. Such rebound can hardly
be attributed to loose joints when the piles are concreted. The speaker
has tested numerous piles recently before placing the concrete filling. '
The sections were from 12 to 16 in. in diameter, ," in. thick, and
from 7 to 12 ft. long. Tests up to 40 tons were made with piles in
clay, and 80 tons with piles on rock. In all the tests, the reboimd was
noted, and it is not unlikely that play at the joints was responsible
therefor. ■'

Charles Kufus Harte,! M. Am. Soc. C. E. — The speaker would Mr.
like to know whether the rebound occurred immediately after the ^^'^^•
pressure was removed, or whether it was some time afterward that
the piles showed a tendency to come up?

In connecting the Providence Division of the New York, New
Haven and Hartford Railroad with the South Station, at Boston,
Mass., in 1889, the necessity of maintaining traffic on the important
streets crossed led to the use of a pile-driver with very short leaders,
and sectional piles, which were made by sawing ordinary piles into
10-ft. lengths and fitting the lower end of each piece with a wrought-
iron ferrule about 12 in. long, and a central dowel of 2-in. pipe. A
special driving head, consisting of a heavy ferrule with a short section

* New York City.
t New Haven, Conn.



108 DISCUSSION : underpinning trinity vestry building

Mr. of pile, received the direct blow of the drop-hammer and protected the
^'^ ^' upper end of each section ; as soon as a section was flush with the
ground, this head was removed, a new section fitted on it, the head
put on the upper end, and the driving continued until the pile was
seated in the hard clay from 40 to 60 ft. below the surface. The
material overlying the clay was salt marsh mud stiffened by the ashes,
earth, and other filling material with which the Back Bay district
had been reclaimed; it flowed laterally and vertically as displaced by
the piles, forming a mound in some cases 2 or 3 ft. higher than the
original surface, and, through the skin friction, opened the joints
of the piles already driven, causing the tops to come up in some cases
nearly 1 ft.

This adjustment of pressure extended over some little time, but,
as a rule, if the piling of a group was tapped by the driver 48 hours
after the last pile was driven, there was no further movement.

With sand, however, as in Mr. Parsons' case, the speaker would
not expect such delayed action. The Boston fill had decided elasticity,
which here would be lacking.
Mr Joseph A. A. Connelly,* Assoc. M. Am. Soc. C. E. — If the speaker

understood Mr. Parsons correctly, he has stated that the load used in
testing the piles was from 40 to 50% in excess of the required load.
He has also stated that the pile was jacked down under this excess
load, was held in place by the jack, and that the load of the 8ui)er-
structure was then wedged hard down on the pile, so as to prevent any
motion. The speaker presumes the motion referred to was the rebound
of the pile.

Assuming the required load on the pile to be 40 tons, the load under
which the pile would be jacked down and held in place would be 60
tons. The speaker would like to know how a pile under a pressure of
60 tons can be held in place by a superstructure load of 40 tons. It is
very probable that even 40 tons is in excess of the load which the pile
actually receives from the building, while the 60 tons is actually placed
on the pile by the jack.

It has been suggested that the rebound may be due to motion at
the joints of the pile, but, in the speaker's opinion, this is not the case.
On work of which he has had charge, steel casings, filled with concrete,
were driven to a depth of 34 ft. These casings were made up of
seventeen 2-ft. sections, and thus there were sixteen joints in each
pile. As in the cases mentioned by the author, the piles rebounded
about i in. when the jacks were released. If all this rebound was due
to motion at the joints, and was considered to be equally divided, the
movement at each joint would be ^V i^- J this, in the speaker's opinion,
is too small to lend much support to the theory that pile rebound is due
to joint motion.

• Ne-w York City.



{ discussion: underpinning trinity vestry building 109

A statement has also been made that a pressure of 77 tons was sus- Mr.
tained by an empty pile made up of short steel sections; this pile °^^^ ^'
resistance, under conditions such as those outlined in the paper, seems
to be high; one would be led to suspect that an obstruction had been
encountered.

J. S. Branne,* M. Am. Soc. C. E. — The speaker wishes to ask Mr. Mr.
Parsons' opinion as to the behavior of the steel-cased concrete piles.

It is stated in the paper that the piles rebounded after releasing the
pressure exerted by the jack which had forced them down. It seems
to the speaker that this rebound is due, in a large measure, to lack of
skin friction in these piles — the surface being a smooth steel plate —
and that this would not occur in piles with a rough surface, as in a
concrete pile cast and seasoned before driving, or even in a wooden
pile, the latter having more or less knots and irregularities. The ma-
terial through which the piles were forced is stated to have been coarse
sand, and this should have oifered quite some resistance to rebound.
The speaker believes that, in a soft clay soil, there would be a rebound,
due to the elasticity of the soil and the lubricating influence of the
moisture in the clay.

Referring to the jill frames which were driven forward with jacks,
the speaker would like to know how large a section of frame was
pushed forward at one time, that is, how wide a section; and also how
much pressure was required to push such section forward? In case
of an unusual obstruction, as, for example, a large boulder, the speaker
supposes that such obstruction would be removed by the laborers with
picks and shovels.

A. W. BuEL,* M. Am. Soc. C. E.— The speaker would like to know Mr.
whether it is possible that a part of the rebound is due to the elastic ^"^'*



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