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Settlement studies on the Mt. Sinai Hospital, Toronto
Crawford, C. B.; Burn, K. N.
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CANADA
DIVISION OF BUILDING RESEARCH
SETTLEMENT STUDlES
on
the
Mt.
Sinai Hospital,
REPRINTED FllONI
'I'HE
ENGINEERING JOURNAL. VOL. 45,
12.
DECEMBER
1962. I-'. 31-37
ION VOL. -16 SO. #5, 1063, 'p'igc
!5H
ESEARCI-I PAPER NO.
178
This publication i s being distributed by the Division of Building Research of the N a - tional Research Council. It should not be reproduced in whole or in part, without permission of the original publisher. The Division would be glad to be of assistance i n obtaining such permission.
Publications of the Division of Building Research may be obtained by mailing the a p - propriate remittance, (a Bank, Express, or Post Office Money Order or a cheque made pay- able at par in Ottawa, to the Receiver General of Canada, credit National Research Council) to the National Research Council, Ottawa. Stamps are not acceptable.
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Mt.
Sinai Hospital,
HE CONSTRUCTION of any
T l
aige
. structure on soil without the
benefit of piles or caissons to transfer
the building loads clown to bedrock
or other solid strata is sometimes
carried out with much apprehension.
Nevertheless, the savings to be gained
by avoiding the use of deep founda-
tion units is always an attractive
possibility. In an effort to assist in the
accumulation of documented esperi-
ence on the performance of large
buildings thus founded on soil, the
Division of Building Research of the
National Research Council has co-
operated with design engineers and
builders in measuring building settle-
ments. This report of observations on
the Mt. Sinai Hospital in Toronto
from the beginning of constructioil
in
1950 through several years of
operation is the result of such a
co-operativc effort.
Geology and Site Conditions
The southern half of the city of
Toronto is located on the bed of
glacial Lake Iroquois; the shoreline
of the glacial lake lies about three
miles north of the Toronto waterfront.
Near the old shoreline, the Lake
Iroquois beach sands, several feet
Toronto
C.
R.
Crawford,
Hend, Soil Mechanics Section,
Dicision of Building Reserirch,
Nationol Research Council, Ottnttia.
I(.
N . Burn,
Soil Afechnnics Section,
1lil;ision of Building Research,
iYntionnl Research Council, 0ttau;o.
ut 5 5 ft. thick
north from the Toronto waterfront.
stratified sand, silt and clay. The the soil layers. TiVhile the foundatio
The depth of overburden increases
upper 2 5 ft. of soil is the Sunnybrook
excavation for the building
was
beiu
correspondingly so that the ground
till,
a
massive silty till of Wisconsin
carried out, undisturbed block an
surface rises at a rate of a little more
age. The southein limit of Lake
tube samples were obtained fro1
than
1 ft. in 100 ft. from the shores
Iroquois sancl is in the vicinity.
various locations at the bottom of the
of Lake Ontario to the old shoreline
At the time of construction of the
excavation (El. 302). T h e material
confirmed the suspicion that pervious
met at elevations ranging from 260
cavation averaged
5%
gravel size, 45
soil strata (perhaps of the interglacial
to 265 ft.
sand, 35% silt, and 15% clay sizc. The
beds) carried water under artesian
Standard penetration tests (with a
specific gravity of solids was 2.74.
145 lb. weight falling 3 0 in. on
a
Consolidation tests were made on
opportunity for detailed mapping
of
per foot ranged from 7 to 16, indicat-
sure is
2
T S F (tons/sq/ft). The com-
0.8
TSF. This stress was removed
stations or to close the survey. For
during a six-week period cncli~lg April
other areas, an accuracy of
t0.02,
in.
15, 1950.
During the next six months
is claimecl.
the soil was reloaded to about
0.50
After three or four years of obser-
I I
L---J
TSF or to about
30%
of the total
vations, it became apparent that the
building load. At this point
33
special
total settlement of the building would
o 50 100plugs were installed in the basement
be about
11
in
;considerably less than
F E E T
columns. On October
11,
1950,
the
hncl becn expected. For such a small
initial level survey was carried out.
settlement the exterior surveys were
Fig.
3.
Foundation plan and surveystations.
Stresses in the ground under the main
not being carried out with sufficient
wing (survey station
9)
at various
accuiacy so it was necessary to make
stages of construction are shown in
a spcclal stucly of the probable errors.
pression index is
0.14.
The uncon-
Fig.
4.
The excavation and reloading
After caleful study of the instrument,
fined compression strengths of speci-
schedule of areas
3
and
4
are shown
bcnchinalk, ancl previous survey re-
mens cut from the same block sample
in Fig.
5.
sults, it was conclucled that t h e pos-
average
1.7
TSF.
sible error in the optical surveys might
During the summer
of1960,
Settlement Measurementsbe as much as
k0.06
in. and possibly
D. G. HLlbley, Soil Engineer, Toronto
Following the initial survey of the
~1little more when carried out under
Transit Commission, killclly providecl building in October,
1930,
furthei
adveise weather conditions.
nclisturbed block
samples obtainecl surveys were made at intervals of
Since
1956,
a special precise optical
I
cxcavations in the SLlllllyl~~oOk
till
about six months until
1954,
ancl
lekelling instrument has been used
One block (Sample
10'7-3)
was ob-
then again in
1956
and
1958.
The
ancl suiveys wcre macle only uiider
tainccl from a depth
of
5
ft. (El.
298)
interior surveys were made using a
(deal weather conditions. From these
at a location about
100
ycl. nolth
ofwater-tube level which was patterned
obscnations the reliability of the
the hospital on Univelsity Avenue, after an apparatus clevelopecl by
bcnchmarlt was clefinitely established.
Compression tests on specimens cut
from this block are reported in Table
I. Classification tests confirm that thc
Fig.4.
Stresses beneath station 9.character of the block is similar to
that of the material sampled in thc
hospital e x c a v a t i o
11(wr,
=
30,
top
=
17)
but it containecl fewer
stones. Consoliclation tests confirmed
the consoliclation c h a r a c t c r i s t i c s
shown in Fig.
1.
BASE
OF
MAT
Foundation Pressures
The Mt. Sinai Hospital is a multi-
storey steel flame builcling resting on
-
a reinforced concrete mat. Building
+
loads arc caiiied to the mat through
four lines of columns at 19-ft. spacing,
5
one line at each exterior wall and
one
4?6
ft. from either sidc of the
ccntie lines of each wing of the
a
builcliilg. A front view of the com-
pleted building is shown in Fig.
2.
The general outline of the mat
foundation is shown in Fig.
3.
I t is
divided into four areas in which the
mat thickness varies from
28 to
34
in., the total pressure on the soil
varies from
0.75
to
1.7
TSF. Basecl
on the assumption that only an aver-
age of
25%
of the total design live
load is active at the founclation level,
less than
10%
of the total pressures
is due to live load. While area
1
now
carries only
0.75
TSF, it is designccl
VERTICAL EFFECTIVE STRESS (T.S.F.)
I I I I I I I I
I
Fig.
5. Loadi~ig and settlement curves.
Because of the agreement between
repeated surveys in 1956 and in 1958
tile results are consideled to be accur-
ate to
f0.02 in. Outsicle surveys only
were made in 1960 and 1961 and
these, together with the surveys of
1956 and 1958, confirmed that move-
mcnt of reference point 3 3 is practi-
cally completed.
Time-settlement curves of several
typical interior stations (as located
on Fig.
3) are shown on Fig.
5
to-
gether with the estimatecl uilloacliilg
and reloading at founclation levcl.
Settlement contours computed from
the last survey are sho\vn in Fig. 6.
Settlenlent Analysis
The most significant result of this
study is that the measured settle-
ment of the structure is much less
than would be estimated from consoli-
dation test results. Total settlement
under the centre of the main wing
of the building since the start of mea-
surerncnts has amountcd to less than
0.6 in. The amount of rebound and
recompression before measuiements
were begun is unkno\vii but is con-
sidered to be small. According to the
laboratory compression curve (Fig.
1)
a representative sample of the sub-
soil would compress about l"/ovhen
loaded over a range equivalent to
that from thc first survey until the
end of construction, i.e., 0.5 to 1.7
TSF. If an average compressioil of
this amount occurred over a 40 ft.
layer the total deflection would
amount to about
5
in. Since the pre-
consolidation load has not been ex-
ceeded and loads were applied rather
slowly it is improbable that cxcess
pore pressures were developed ~ v i t h
the usual time lag in deform a t'
ion.
There may, however, have been a
plastic or secondary consolidation
type of compression occurring under
loads of this magnitude. Time-settle-
ment observations (Fig.
5)
tend to
confirm these deductions.
Owing to the marked similarity
1)ctween the shapes of the time curves
of loading and of settlement (Fig. 5)
it is apparent that most of the settle-
ment is directly proportional to the
applied load, and is therefore of an
elastic nature. I t is possible then, to
compute the average modulus of
elasticity of the subsoil using the
classical equation:
wherc
p
= settlement
q
=
increase in pressure
B
=
width of loaded area
p
=
Poisson's ratio
E
=
Modulus of elasticity
1,
=
Influence value computed by
Steinbrenner.s
The, computed modulus is affected
greatly by the chosen influence value
I,. It, in turn, depends directly on the
thickness of soil and on Poisson's ratio.
The total thickness of the soil is well
established but there is little doubt
that the modulus of the lower Illinoian
till is much greater than that of the
ory results are availabl
it is necessary to compensate for t
lower soil by using a reclucecl thi
ness of soil in the computation.
illustrate this effect, computations
were made assuming soil thicknesses
of 30 ft. and 40 ft.
In computing the elastic compres-
sion of a saturated soil it is commonly
assumed that no volume change
occurs and that Poisson's ratio equal
0.5. This is a questionable assumpti
because Poisson's ratio is thought
depend on the rate of loading and
the applicd stress level. Theref
the modulus has been computed
various values of Poisson's ratio. These
computations were checked using in
fluence charts developed by Ne
mark.5 The effect of embedment1
w
found to influence the classical ca
by only about 2%.
The results of these computation
are shown in Table 11. It is of intere
to compare the modulus compute
from the results of the full-scale load-
ing
with values determined from
laboratory tests. The initial tangent
modulus determined from the stress
strain relationships of
unconfine
specimens cut from the original bloc
sample (Sample
23-5, Table I) av
aged
50
TSF. The initial tangent m
ules
for
unconfined
compression
tests on the recently obtained sample
of Sunnybrook till (Sample 102-3,
Table I ) averaged 130 TSF. The
higher value is probably due to the
lower stone content.
It has been suggested that the
moclul~~s
of elasticity should be de-
termined from the stress-strain rela-
lationships of specimens subjected to
triaxial compression tests. Detailed
studies have shown further improve-
ment when axial loads are cycled.12
Fig.
6 .
Final settlement contours.
n
CONTOUR INTERVAL = 0 . 0 5 INCH
Uriconfincd Compression Tests
19.0 1.7 0 0
17.5 3.3 0 0
T a g e stone
111 sperin~en
Consolidated-Uliclrained Compression Test
TABLE 11
Colnputed hiodulus of Elasticity of Subsoil
Depth
o fLa!jer,
ft.
30I
760 8Modzllus of E h s t ~ c i t j / ,
I'SF
p = 0.5 250 440 p = 0.4 -120 -- p = 0.3 540 620pt for the first one or
systems were judged, in addition, t
Poisson's ratio to
each stress level, when
be much more costly than cither
quite wide, the unload-
mat or spreacl footings.
right of them. There Lvas very little
Primarily it was thought clesirable to
Ack~~olowledgementsquestioil about the computecl value of
distribute the building load through a
the modulus of elasticity because most
relatively rigid mat in order to reduce
plots were straight lines and those
the effects of kilowil variatioils in the
that curved slightly had distinct
compressibility of the subsoil. As a
straight sections. Complete clestruc-
result, the main wing of the Sick
on spread footings.
It was noticed that therc was
a
The performance of the foundatioil
pointed
tendency for the value of modulus of
for the Mt. Sinai Hospital is illustratecl
surveys
carried
Outelasticity to increase ~ v i t h
each stress
by the settlement contours show11 on
by
W'
''Scllriever)
'low 'lea''of
the
water content.
c l a t i o i ~ . ~ ~
Caissons were unacceptable
triaxial shear tests in which axial
clue to ailticipatecl grouncl-water prob-
stress is cycled.
ETTLEMENT STUDIES ON
HE
MOUNT SINAI HOSPITAL,
ORONTO
.
B. Crawfordead, Soil Mechanics Section, ivision of Building Research, ational Research Council, Ottawa
K. N.
BurnSoil Mechanics Section, Division of Building Research, National Research Council, Ottawa The Engineering Journal,
December, 1962, page
3
1 Discussion by Dr.G. G.
Myerhof Head, Department of Civil Engi- neering, Nova Scotia Technical College, HalifaxThe authors' interesting paper indicates some of the difficulties involved in estimating the settlement of structures on raft foundations. They find that standard consolidation tests considerably overesti- mated the settlement and concluded that the observed movements are mainly of an elastic nature. However, for the relatively slow rate of loading of the s t r ~ ~ c t u r e ex- tending over a period of more than two years, appreciable consolidation settlement would occur concurrently with the immediate settlement, and both types of movement are expected to be of the same order of magnitude. This is shown by the authors' deduction of a Poisson's ratio of about one- third, compared with a value of one-half corresponding to fully saturated soils sub- jected to elastic movements. Moreover, the average initial tangent n~odulus of elasticity of the present soil for rapid loading is likely to have been greater than the value of about 600 tons sq. ft. deduced from laboratory tests if the influence of the rate of loading had been determined from separate specimens instead of using the same samples for this investigation (Fig. 7 ) . Finally, it is believed that the relatively wide foundation raft compared with the thickness of the compressible stratum pro- duces considerable lateral restraint and thus reduces the elastic movements as shown in previous papers by the writer.l.2
settlement has been small (not greater than aboal 0.6 i~lches), and the strains imposed upon the structure by the characteristically dish-shnpecl settlement pattern have been so small that no apparent damage to the building is recorded, or is likely to occur. The mat foundation chosen for the struc- ture appears to have been a wise selection. The chief values of this paper are th:~t it places on record one more case history on settlements, and it focuses attention o n some of the unresolved questions in se:lle- ment analysis.
The writer was particularly interesred in the process of reasoning that the authors seem to have followed. If this has been interpreted correctly, it seems to run as follows: The measured settlements are ap- preciably less than the estimated ones; the settlement appears to have occurred quickly, alrnosi simultaneously with load application; this suggests that the settle- ment is largely elastic; perhaps we can compute settlements close to the actual ones if we use a sufficiently high modulus of elasticity and elastic theory only.
The writer wishes to make it clear that he does not disagree with some of this reasoning. I-Iowever, it would seem to be
a n over-simplification of the settlement process, and one which does not contribute to our understanding of the problem. The similarity between the time-settlement and the load-application curves could suggest a n elastic phenomenon, but it should be pointed
O L I ~ that the construction period for this
building was relatively lengthy, and our k~iowledge of the soil property known as the coefficient of consolidation (C,.) is still rather limited. The probability that some of the settlement is due to consolidation is further reinforced by a consideration of the loading conditions and the "p-e" curve. It is quite possible that no swelling occurred during unloading of the foundation, and, if so, the rebound and recompression would be entirely elastic up until the condition of zero net load was re-established. As soon as a positive net load was applied, and if the soil was completely saturated, primary consolidation should have begun to occur, even though it might have been at a fast rate. It is the writer's belief that the settle- ment the authors term 'elastic' is only partly elastic and the remainder is in nature what has hitherto been termed pri- mary consolidation.
The description of the testing to ascertain thc modulus of elasticity of the soil and of the results obtained, is very interesting, and
REFERENCES the correct evaluation of this property is essential for the furtherance of settlement
Eirgz.
Ty,Y{r$fv
s~~heCl-'$?,!in~ro~f2
analysis work. In the writer's opinion, how-wales ~ ~E ~ ~ . , ~ 1951, t . vol, 67, p, 53. cver. there is one other difficult area of 2. G. G. Myerhof and T. K. Chaplin, "The settlement prediction which is exemplified
and Bearing Capacity of by the Mt. Sinai case, and that is the pre- Cohesive Layers". Brit. J1. App. Physics,
1953, vol. 4, p. 20. diction of consolidation settlements when the applied loads are in close proximity to
Discussion by D.
H.
MacDonald Director, H.G.
Acres&
Company Limited, Niagara Falls, Ont.This paper describes one of the very few settlement studies that have been carried out on the compressible glacial soils of the T o ~ o n t o region. The structure in question is irregularly shaped and imposes com- paratively light loads at a shallow depth on a moderately over-consolidated clay till by means of n reinforced concrete mat. I n consequence of this the nlaximum total
the preconsolidation pressure. Such is the case at Mt. Sinai and it makes the correct evaluation of the compressibility very dif- ficult.
In the light of these comments it would bc very interesting to learn whether any attempt has been made to estimate the settlements using one of the methods that combines elastic and consolidation move- ments, such as that suggested by Skempton and Bjerrum in 1957.1 In closing, the writer would like to commend the authors for contributing this interesting settlement rccortl to the literature on the subject.
Discussion b y Wilson
It is, unfortunately, very rare that slgnels and builders of structures think spending comparatively small amounts money to ensure that the behaviour loundations can be determined after t structure has been built.
I am sule that a great deal of very val able information in the science of soils i formation can be obtained in this ma by the expenditure of very little mone
Many buildings on three sides of Mt. Sinai Hospital, and close to it, been built since the latter was constru in 1950. Franki of Canada Limited, whom I am associated, has executed t ~ a c t s on excavated caissons down to roc lor a n extension to the Sick Children' Hospital across University Avenue to east; 500 University Avenue, just t south; and the School of Nursing, i ately to the west; among several close by.
The caissons on these projects from 30 in. to 6 ft. in diameter and enlarged bases, up to 12 ft. in dia These caissons are based o n the sh which is loaded to 25 tons per square f o At the time the Mt. Sinai Hospital wa bu~lt, excavated caissons cost nearly doubl their present cost. Approximate calculation carried out o n the basis of the informatio contained in the paper indicate that th slab and caissons would cost approximate1 the same at today's prices, so that th large savings incurred at the time of con- struction would no longer reply.
Based on the above experience and o n many borings taken in conjunction with these projects, the validity of the assump- tion that an accurate estimate of settle- ment co~lld be obtained by applying the modulus of elasticity of the upper soil over a depth of 30 ft. is questioned. There
IS only 12 ft. to 15 ft. of the upper
Sunnybrook Till of which the properties are known, then some 15 ft. of Toronto FOI- mation which is practically incompressible, and 15 ft. of the very dense York Till which is also incompressible. I t is con- sidered that if the elastic properties for the upper soil are used a maximum equivalent depth of 15 ft. would be more appropliate.
Authorsr Reply
The authors appreciate the critical com- ments of the discussers which have helped to draw attention to some of the un- certainties in predicting small movements of foundations. Only by observing full- scale settlements in the field and attempting to apply a rational analysis will the ac- curacy of the pred~ctions improve. An at- tempt was made in the paper to demonstrate the difficulty of assessing settlement by conventional consolidation theory in the recompression range of stresses, and atten- t ~ o n was drawn to possible merits in the elastic theory analys~s. Because of the com- ments made in the discussion, a further clarification of the authors' reasoning is warranted.
The pressure ~mposed on the subsoil by Mt. Sinai hospltal is less than the pre-
er of soil compresse tal of 0.6 in.
his
is the approximateion under the hospital. If the subsoil compressed uniformly, this would represent a compression of about 0.0014 in.
I
in thick (size of laboratory pecimen). Obviously there is little room or experimental error if a realistic evalua- ion of the soil is to be obtained.Several factors influence the test result. One of the no st important is the fit of the specimen against the porous loading plates and in the consolidation ring. A second factor is the reduction of co~npression resistance due to disturbance. T o reduce these influences the laboratory specimen is often loaded to overburden pressure, unloaded and reloaded, the reloading curve being considered the more realistic one to Another factor that may be peculiar to the laboratory specimen is the compression of air or gas which has conie out of solu- tion due to stress reductions by sampling and has allowed the soil to expand without taking up water. This phenomenon is not conipletely understood. It is apparent that the specimen will be recompressed at least to its original volume under applied ovcr- burden pressure but it is equally apparent that if pore pressure is reduced to zcro after each load increment the gas will re-form into bubblcs. This process may have an effect equivalent lo pumping water out of These are some of the factors that are thought to contribute to thc unrealistic estimate of 5 in. of consolidation settlement based on an average of several loading curves. On the basis of cyclical oedometer tests made on this soil, the estimated con- solidation settlement can be reduced by one-third to about 3.3 in. Applying the procedures of Skempton and B j e r r ~ ~ m , as suggested by Dr. MacDonald, a further reduction to about 2.2 in. is possible. The resulting figure is still about four times the actual settlement. If elastic settlement is even one-half of the total, the prediction from the consolidation test is in error by
could be maintained under pressure equiva- lent to overburden pressure (1.7 T S F ) and this was expected to return any free air into solution. Further, the specimen could be load-cycled many times without drainage in order to ensure proper seating of loading caps. This resulted in a more realistic modulus of elasticity, much greater than previously obtained.
Using this modulus (600 TSF) the elastic compression of the subsoil under applied loads from the beginning of settlement observations is computed and shown in Table 111. These computed values can be compared with the total settlement of less than l/z in. observed at the end of loading. As noted previously, the 30 ft. depth is considered to be most realistic because it compensates for the less compressible material at lower depths. If Poisson's ratio is assumed to be equal to 0.5, the maximum elastic settlement is only 40% of the measured settlement. If Poisson's ratio is assumed to be equal to 0.3 the computed elastic settlement accounts for about 90% of the measured settlement.
Both Dr. MacDonald and Dr. Meyerhof point out that due to the slow rate of loading an appreciable amount of consoli- dation could occur during construction. Dr. MacDonald suggests that this should be "primary consolidation." The authors were inclined to the view that "it is improbable that excess pore pressurcs were developed" and suggest t h a ~ "a plastic or secondary consolidation tvpe of compression" may have occurred. In the absence of pore pres- sure measurement neither interpretation can be proved. Obviously some of the settlement must be due to creep as illustrated by continuing settlement after completion of the building (Fig. 5 ) .
If as Dr. Meyerliof suggests, the modulus of elasticity is even greater than 600 T S F and if lateral restraint in this case is a significant factor, then the computed settle- ments in Table 111 would be correspond- ingly smaller. The crilical question is the assumed value of Poisson's ratio because this
Poisson's ratlo increase
of the order of 0 2 at low stresses t o mo than 0.5 at very high stresses." T h e stress beneath the Mt. Sinai hospital are considere to be in the low stress range, about on quarter to one-third of the ultimate strengt 1~ is interesting to note that the Poisson I-atio for ice, when loaded elastically, ma
be as low as one-third [Gold 1960). If th ice creeps as well, the apparent Poisson' ratio will increase to 0.5 or greater depend- ing on the geometrical arrangement of individual ice crystals. Under creep condi- tions the deformation is not entirely elastic so the elastic theory cannot be applied by itself.
The authors know of no satisfactory experimental deterniinations of Poisson's ratio for soil. This is clearly a fertile field for research. In the meantime the observa- tions on other materials scarcely confirm the usual assumptions for soil. Until more refined measurements of both elastic and plastic properties of soils are available, engineers will find it necessary to design, in the soil recompression stress range, on basis of experience gained through car field observations.
REFERENCE
Gold. L. W. (1960). Discussion to "Micro- craclts and the static and dynamic con- stants of annealed and heavily cold-worked metals." British Journal of Applied Physics.
Vol. 11, November, 1960, p. 522-523.
