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Soil mechanics papers presented at the Building Research Congress
1951
N A T I O N A L R E S E A R C H C O U N C I L
CANADA
A S S O C I A T E COMMITTEE ON S O I E AND SNOW MECHANICS
O t t a w a
N o v e m b e r ,
1952
SOIE
MECHANICS P A P E R S
PRESENTED
AT
THE
B U I L D I N G R E S E A R C H CONGRESS
1951
( R e p r f n t e d ,
b y p e r m i s s f o n , f r o m the
Proceedings
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TABLE
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CONTENTS
B u i l d i n g
F o u n d a t i o n s
i n t h e o r y
a n d p ~ a c t i
ce.K a r l
T e r z a g h f ,
T h e i n f l u e n c e o f modern s o f l
s t u d f e s o n
the c o n s t r u c t i o n
o f f o u n d a t f o n s ,
H , J , B ,
H a r d f n g a n d
R,
G l o s s o p
Some f o u n d a t i o n p r o b l e m s
i n
G r e a t B r i t a f n ,
L,F,
C o o l f n g
S p e c i a l f o u n d a t i o n p r o b l e m s i n
C
m a d
a.R o b e r t
F,
L e g g e t
A p p l i c a t i o n of r e s u l t s o f d e e p
p e n e t r a t i o n t e s t s t o f o u n d a t i o n
p e e r s ,
T
, K O
Huf z f n g a
The b e a r i n g c a p a c f t y o f c l a y s ,
A,W, Skempton
P a g e No,
139
Div.
1
Part
111.
The Influence of Modern Soil Studies
on
the Design and Construction of Foundations
Karl Terzaghi
( P r o fcssor of the Practice of Civil Engineering, Harvard University, Cambridge, Mass., U . S . A .)
BUILDING FOUNDATIONS IN THEORY AND PRACTICE
Introduction
This paper deals with the influence of scientific reasoning on fountlation engineering. Foundations can appropriately be described as a necessary evil. If a building is t o be constructed on an outcrop of sound rock, no foundations are required. Hence, in contrast t o the building itself which satisfies specific needs, appeals t o the zsthetic sense a ~ i d fills its makers with pride, the foundations merely serve as a remedy for the deficiencies of whatever whimsical Nature has provided for the support of the structure at the site which has been selected.
On accouilt of the fact that there i3 no glory attached t o the foundations, and that the sources of success or failure are hidden deep in the ground, building foundations have always been treated as stepchildreii ; and their acts of revenge for the lack of attention can be very embarrassing.
The Egyptian temples still arouse our admiration, three or four thousand years after they were built. Yt:t the design of the foundations was amateurish, and the performance of the foundations is a prolific source of concern for the engineers in charge of maintenance.
In the sixth century A.D. the engineers and architects of the Uyzantine emperor Justinian amazed their contemporaries by the construction of the masonry dome of St. Sophia in Constantinople, with a diameter of one hundred feet. To design this dome without the assistance of applied mechanics was an extraordinary accomplishnient of engineering intuition. The dome was neither too weak nor too strong, and if the founda- tions of the supporting pillars had been adequate the dome would have stood for ever. However, when it carnc t o the design of the foundations, intuition did not operate properly and the dome collapsed repeatedly, during the first centuries of its existence, on account of a progressive outward tilt of the supporting piers. The buttresses which have been added t o the piers t o stop the movement deface the structure.
Modern counterparts t o the inadequate foundations of the Egyptian temples and S t . Sophia are numerous and impressive : Iiailroad Terminal in Le Havre (about 1030). Palais de Justice in Cairo (about 1935), Charity Hospital in Sew Orleans (1938), Office Building Compania Paulista di Seguro, Sao Paulo (1945), and, quite recently, the Norinal School of Mexico City. This structure was completecl about two years ago at an expense of about two million U.S.,4. dollars. From an architectural point of view the building is a masterpiece. The walls are decorated by murals by one of the greatest modern painters of Mexico. Immetliately after the building was completed an
international Engineering Congress was inaugurated in its auditorium, and the members of the congress were impressed by both the architecture and the workmanship. At present, two years after the com- pletion of the structure, the walls have already cracked up t o such an extent, on account of unequal settlements, that it is necessary t o evacuate the structure. I t cannot be re-occupied until the underpinning operations are completed and the defective structural members are repaired.
Most of the members of this congress are primarily interested in the design and the construction of buildings and not of foundations. Hence in their practice they may ,try t o assign the unglamorous occupation of foundation design t o somebody else, reducing their own share t o the request that the foundation " should
not settle." However, the assignment may pass into the hands of a bungler, as it has so many times before. Therefore, the " superstructure-man " should a t least be informed on the hazards involved ; on the progress which has been made during the last decades in the line of reducing the hazards, and the type of research required for further amelioration.
Historical Review
The design of foundations is a branch of civil engin- eering. Experience has shown that most of these branches have passed in succession through two stages, the empirical and the scientific stage, before they reached the present and final one, which may be called the state of maturity.
In the empirical stage, results are obtained by trial and error a t the price of occasional failures. The most important prerequisite for signal success is intuition, and, unfortunately, intuition is not hereditary. Hence progress is intermittent and slow.
In the scientific stage, an attempt is made t o predict results by mathematical reasoning on the basis of test data and a few fundamental relationships. At the outset of this stage the prospects for success appear t o be unlimited, because it takes time until the deficiencies of the procedure come clearly into evidence. The first disappointments and setbacks are the fore- runners of the third stage.
In the third stage, the ardour of the scientist is already tempered by bitter experience. The restrictions which Nature has imposed upon the theoretical approach t o engineering problems are clearly realised, and further progress is made on a semi-empirical basis. A store of knowledge is accumulated which supplements and qualifies the conclusions based on analysis, whereby the status of analysis is gradually reduced from that of a Czsar to that of a member of an advisory committee.
In the empirical stage of foundation engineering the relations between cause and consequence received
Buildin,g Research Co~tgress, 1951 no attention whatsoever. As a matter of fact, in the
Middle Ages, the foundation was not even recognised as a specific part of the buildings. The lowest part of every wall was flared out as a matter of routine, and the base of the flared-out part was established on the bottom of a trench with a depth of several feet. If the earth exposed on the bottom of the trench inspired confidence it was not touched a t all, and if it appeared t o be weak it was reinforced by driving wooden piles into it.
Towards the end of the nineteenth century, when science began to invade the various branches of engin- eering, it was realised by some advanced members of the profession that the width of the footi~igs should be adapted in some fashion to the nature of the soil exposed on the bottom of the trench. This was accomplished by assigning t o the principal types of soils " allowable bearing values," but no attempt
was made t o correlate the unit load with settiements. I t was simply claimed-in the spirit of the empirical stage-that a building does not settle a t all unless the unit load on the base of the footings is greater than the allowable bearing value, and the others believed the claim. The capacity for faith, in the empirical stage, is almost unlimited.
The transition of foundation engineering from the empirical into the scientific stage lagged far behind the corresponding transition in all the other branches of civil engineering, for the simple reason that founda- tions are a means and not an end. The transition started some thirty-five years ago, though many engineers are not yet aware of it. I t was prompted by increasing evidence that the gospel of the " allowable
soil pressures " does not live up t o its promises. The scientific stage of foundation engineering came into existence as soon as it was realised by a foundation engineer (and not by a physicist) that ever-y load produces settlement, regardless of what the " allowable bearing value " of the subsoil may be. The settlement depends on many factors other than the contact pressure on the base of the footings and the type of soil in contact with the base. These factors include the soil profile t o a considerable depth below the base, and the dimensions of the loaded area. Therefore, a foundation cannot adequately be designed unless the relation between unit load and settlement is taken into con- sideration. Once this obvious fact was recognised, the artificial concept of the " allowable bearing value "
was replaced by the concept of " allonrable settlement,"
mineers and the efforts of a whole generation of en,' were concentrated on the development of reliable methods for predicting settlement on the basis of the results of laboratory tests.
As a result of these research activities, the relations between surface loads and the corresponding stresses and strains in the loaded soil were determined ; the causes of the progressive settlement of buildings above clay strata became known ; satisfactory techniques for sampling and testing were developed, and the theoretical problems involved in settlement forecasts were solved.
All this was accomplished within a quarter of a century, between about 1915 and 1940. During the same period, the first attempts were made t o apply the findings t o the design of foundations. Thus it was realised that the costs of accurate settlement forecasts are commonly out of proportion t o the practical value of the results. In most cases a crude estimate is all t h a t is needed. Therefore, the efforts of the research workers gradually turned from the exploration of the fundamental relationships t o the task of correlating settlement in a semi-empirical manner with the results
of simple laboratory or field tests, such as penetration tests in drillholes. The next step was to verify the conclusions baseci on the test results by observations on full-sized structures, and t o determine the errors involved in the use of the semi-empirical procedures. Investigations of this lci~ld are symptomatic for the last or mature stage in the development of an engineering discipline. The indications of growing maturity can be recognised in every one of the different branches of foundation engineering.
Spread Footings
As a result of theoretical and experimental investiga- tions covering a period of several decades, it is now known that the settlement of a footing due to a given load per unit of area of its base is a cornplex function of the dimensions of the base and of the compressibility, permeability and Poisson's number of al! the soil strata located between the base and a depth wliich is a t least equal t o three times the width of the base. 111 other words, the relationship between unlt load and s e t t l e n e i ~ t is very complicated. .4n accurate forecast ok the settlement of a single footing supported by natural ground, on the basis of the results of soil tests, would be a full-time job for an exceptionally competent research engineer, backed by a sponsor who does not count the costs. On a foundation job in\.olving the design of many footings wlth different sizes, it would be necessary t o assign one such engineer, together with his sponsor, to each one of the footings, because as a rule the mechanical properties of the subsoil change from point t o point, a t least in a vertical direction.
On account of the complexity of the relations involved, scientific research in the realm of spread footings did not yield any results of immediate practical usefulness. However, it cleared the field of deep-rooted super- stitions, and disclosed the type and relative importance of the factors which determine the settlement of the footings. Expedient and yet adequate ,procedures for footing design were subsequently developed by radical simplification of the real relationships, and by adapting the methods of subsoil exploration t o the different types of soil.
The design of the footings for the Shamrock Hotel, a 22-storey reinforced concrete structure in Houston, Texas, is an example of a technique which can be used, if the subsoil consists of stiff, but non-homogenous clay. The building covers an area of 275 by 200 ft., and it rests on 250 spread footings. The subs011 consists t o a depth of a t least several hundred feet of stiff Pleistocene clay with sandy layers and a few layers of fine, silty sand. The test borings furnished continuous, undisturbed sanlples t o a depth of 50 ft. below the surface. The base of the footings is located between depths of 13 and 30 ft. below the surface. Between these two levels the liquid limit of the clay varied between 26 and 60, the natural water content between 17 and 29 and the unconfined compressive strength between 2 . 2 and 3 . 3 tonslsq. ft. I t was obviously impracticable to find out whlch ones of the 250 footings are located on the least, and which ones on the most compressible parts of the clay deposit. Therefore, it was arbitrarily assumed that the largest footings rest Gn the most compressible parts of the clay stratum. The structure could be damaged by differential settlement only. An upper limiting value for the differential settlement was obtained on the basis of the fact that the difference between the settlement of any two adjacent columns will hardly be greater than the settlement so which would ensue if the ground carried no load other than a single one of the heaviest
footings. The design load was chosen such that the computed settlement s, of this one footing does not exceed
2
in. The settlement computation was made on the basis of the results of soil tests which were perfnrmed many years ago in connection with another job in the proximity of the site of the new hotel. No attempt was made to evaluate the maximum settlement, because it was known from observations on other structures in Housron that it will not exceed a few inches.If the subsoil of a footing foundation is cohesionless, the costs of securing undisturbed samples in adequate number are likely to be prohibitive, and the soil tests on such samples require very elaborate equipment. Theref ore, se\-era1 p e t hods have been developed for securing information on the compressibility of cohesion- less soils by means of penetration tests in the drillhole. Without such tests or equivalent subsoil investigations, the real settlement of the footings may be many times greater than the anticipated ones. The following incident is an example.
The footings for the columns of a one-storey factory building in Denver, Colorado, were designed on the assumption that the " allowable bearing value " for
the underlying sand is 2 tonsisq. f t . The unit pressure on the.sancl due to dead load only was 0.95 tonslsq. ft. When the total unit load increased for the first time on account of a heavy snowfall to 1 . 4 tons/sq. ft., w1iic:h was still well below the " allowable bearing
value," the columns settled by amounts ranging between . -
3
and 34 in.At a later dare, 13. B. Peck made penetration tests in the same sand stratum preparato;y t o the design of the footing foundation of another structure. H e found that the structure of the sand ranged between "loose " and " very loose," and that it varied erratically in both vertical and horizontal directions. This observation explained the unsatisfactory per- formance of the foundation of the older structure, and furnished the data for the rational design of the footings of the new one.
In the humid tropics, crystalline rocks like granite or meta~norphic schists are likely t o be decomposed t o great depth and transformed into a clay-like substance. Proceeding from the surface in a downward direction three different layers are encountered, a layer of topsoil or creep material, a layer of soft decom- posed rock, and a layer of hard decomposed rock. Below the layer of topsoil, the original structure of the rock is still preserved with all its details, but the liquid limit may be as high as 50 and the porosity as high as 35 per cent. 'The base of the footings is customarily established on the surface of the hard decomposed rock or, if this surface is located at great depth, on the surface of the soft decomposetl rock. The boundary between the layers is commonly very une\:en. In Brazil, the position of the boundaries is ascertained by measuring the rate of progress of wash- borings. Each layer corresponds to a definite range of the rate. The method is satisfactory, provided the washborings are made in strict compliance with standard specifications concerning the water pressure and the manipulation of the wash pipe.
If footings are to be constructed on cohesive soil it is necessary t o determine whether or not the properties of the soil located beneath the level of the base of the footings are subject t o seasonal variations. I;ortunately, significant variations below a depth of four or five feet are rather rare, but there are esceptiolls. Solrle twenty-five years ago, a Middle Western institute of
higher leariling decided to investigate the influence of the size of loaded areas on the settlement of footings.
The project was sponsored by a government agency. The footings were established at a depth of several feet below the original ground surface on a gentle ridge during the winter months, and t h e loads were applied. In the spring it was found that some of the largest footings, carrying heavy loads, were located above the level which they occupied before the load was applied. This observation indicated that the clay soil composing the hill was subject t o seasonal volume changes t o considerable depth.
The most conspicuous seasonal variations of soil properties to considerable depth were observed in loess regions, and in areas located above highly colloidal and heavily pre-compressed clays in regions with marked contrasts between dry and wet seasons.
The strength of loess decreases rapidly with increasing moisture content. Hence, during the rainy season t h e bearing capacity of loess may be very much smaller than during tlie dry season. Some twenty years ago a large coal bin was built near Bobriki, south of Moscow in U.S.S.R. The footings rested on loess, and they were designed on the basis of an " allowable bearing value " of three tons per sq. ft. This value was obtained by means of a loading test performed during the summer. In the fall when the autumn rains started, the bin was not yet completed and the load on the foundations did not yet exceed a small fraction of the design load. Nevertheless, the structure was so severely damaged by unequal settlement that it hacl t o be abandoned. Examples of the settlement of structures on loess due to leakage into the loess from sources located within the buildings were described by 0. K. Peck and R. B. Peck in the Proceedingsof the Second International Conference on Soil Mechanics.
I n order t o avoid miscalculation of the bearing capacity of loess, the material should be tested a t the highest water conterlt which it may acquire after the structure has been erected, and provision must be made that the water content will never exceed this value on account of leakage from defective pipe lines or other man-made sources.
In regions with well-defined dry and wet seasons such as central Texas, parts of South Africa anti the central plains of Burma, buildings with spread footings on highly colloidal and lieavily pre-compressed clay are likely t o be damaged by differential swelling due t o the accuniulation of moisture beneath the areas covered by the buildings, or b l ~ differential shrinkage in exceptionally dry seasons. Damage of t.his kind can be avoided by carrying the foundations to the lower boundary of the zone subject t o seasonal variations, or else by maintaining free circulatioil of air between the ground surface and the base of the building. Both procedures have been successfully used in Texas. I t may also be possible in some cases t o prevent, or a t least t o reduce differential heave, by giving t o the base of the footings such tlimensions that the pressure on the soil under [lead load alone is equal t o or greater than the swelling pressure of the clay.
These examples illustrate the prescnt status of our methods for coping with the problem of footing design. The " fundamental research " concerni~lg the factors
which determine the settlement of spread footings was practically completed inany years ago, but the proceclures for adapting our theoretical knowledge t o the practical requirements are still in an experimental stage. 7'he tlevelopment work can only be carried out in the iielcl in connection with foundation jobs, and the relati~re value of tlie results obtained can be judged only on thc basis of well-documented case records, accompanied by the records of reliable settlement observations.
Building Research Congress, 7957
Raft Foundations
Once the mechanics of settlement were revealed by soil mechanics research, it became evident that the settlement of a raft foundation does not depend on the weight of the building which is supported by the raft. I t depends only on the difference between this weight and the weight of the soil, solid and water combined, which was removed prior t o the construction of the raft, provided the heave produced by the excavation was inconsequential.
A few engineers gifted with exceptional common sense itnew this fact long before soil mechanics came into existence. The design of the foundations of t h e Albion Mills in London by John Rennie, towards t h e end of the eighteenth century, and that of a few other old structures resting on very soft ground were based on the principle of keeping the structure " afloat."
Yet the number of engineers who grasped this principle and dared t o take advantage of it was very small. As late as 1926 the writer met stubborn resistance when he tried t o persuade one of his clients, the president of a prominent firm of consulting engineers, t o omit the piles beneath a raft-supported structure, the weight of which was almost exactly equal t o that of the displaced mass of water and soil.
The settlement of a raft-supported structure with a weight equal t o that of the displaced materials is roughly equal to the distance through which the soil rises a t the level of the base of the future raft while the excavation is being made. Therefore, the excavation should not be carried beyond the depth a t which the weight of the surrounding soil may produce an important upward movement of the soil located beneath the excavation. The methods for estimating heave in advance of excavation on the basis of the results of soil tests are rather inaccurate. Therefore, it has become customary t o verify the results of the estimates during the excavation operations by periodic levels on a set of underground reference points located beneath the level of the bottom of the excavation. The results of such heave observations are documents of general interest and should be published.
If the weight of the excavated materials is smaller than the weight of the structures, the difference between the two weights can be assigned either t o the soil or t o piles. By driving the piles prior t o excavation, the heave of the bottom of the excavation can be con- siderably reduced.
Outstanding examples of skilful utilisation of the principle of " floating " and " semi-floating " foundations can be found in Mexico City. The subsoil of this city, down t o a depth of about zoo ft., contains thick layers of highly colloidal clay with water contents u p t o 300 per cent. of the dry weight. By systematic soil investigations it was found that these clays do not start t o consolidate unless the unit load on the strata is increased by very roughly 0 . 5 tons/sq. ft. beyond the overburden pressure. This critical load is referred t o as the " breaking point " of the structure of the clay. On account of the relatively important difference between breaking point and overburden pressure, tall and heavy buildings can be constructed in Mexico City without the risk of important settlement. This is done by giving t o the sub-basements such a depth that the ultimate load on the subsoil of the raft is slightly smaller than the load corresponding t o the breaking point. The breaking point is determined prior t o construction by standard consolidation tests on un- disturbed samples.
The subsoil of Mexico City contains several layers of water-bearing sand. If the excavation for a sub-
basement is made, the water table must be lowered t o a level below the bottom of the excavation. Otherwise the heave may be excessive. Both soil mechanics and experience have shown that the process of drainage causes a bowl-shaped st.itlement ot the ground surface surrounding the seat of tht: pumping operations. The settlement a t the centre of the bowl depends on the thickness and compressibility of the silt or clay strata located below the original water level, and on the distance through which the water level is lowered. In Mexico these strata are very compressible. Therefore, the settlement may be important enough t o damage existing structures located in the proximity of the drained area.
An original and successful method for preventing such damage has recently been used by L. Zeevaert in Mexico City, in connection with the excavation of the sub-basement for a 44-storey office building. The depth of the excavation was about 40 f r . , and the base of the footings of t h e adjacent buildings is located a t a considerable height above the bottom of the excavation. I n order t o prevent settlement due t o the consolidation of the highly compressible subsoil of the existing footings by drainage, the water which is pumped out from beneath t h e excavation is injected under pressure into t h e subsoil of the footings.
All these important and original applications of soil mechanics t o the solution of exceptionally difficult foundation problems were conceived and put into practice by Mexican engineers, after their interest in the young science had been aroused. Yet, a t the same time, and a t the very seat of their activities, spectacular failures like the differential settlement of the Normal School have occurred, because from time t o time even the foundations of important structures are still designed without taking advantage of what the best informed local engineers have known for many years. Similar incongruities may be encountered in almost every part of the world. They are due t o the fact t h a t it takes time for newly acquired knowledge to percolate from the points of origin within the individual engin- eering communities t o the outlying districts and t o rise, by capillarity, t o the peaks of administration.
Pile Foundations
During the empirical stage of foundation engineering it was sincerely believed that the settlement of a pile foundation depends exclusively on the load per pile. Hence +.he settlement of the pile foundation was assumed t o be equal t o the settlement of the test pile under the design load.
When the other branches of civil engineering entered, one by one, the scientific stage, foundation engineers made a pseudo-scientific attempt t o simplify and improve the methods of the design of pile foundations by the derivation of the so-called pile-formula. These equations represent the relation between the penetration of the last blows of the hammer and the ultimate bearing capacity of the pile.
The design of pile foundations did not enter a truly scientific stage until it was realised that the ratio between the settlement of a pile foundation and that of a simple pile acted upon by the design load can have almost any value. This is due t o the fact t h a t the settlement of a n individual pile depends only on the nature of the soil in direct contact with the pile, whereas the settlement of a pile foundation also depends on the number of piles and on the compressibility of the strata located between the level of the points of the piles and the surface of the bedrock. If a single pile is loaded with one half of its ultimate bearing
capacity, its settlement never exceeds half an inch. Commonly it is very much smaller. However, if each one of five hundred piles is assigned a load equal t o one half of its ultimate bearing capacity, the settlement of tlie foundation may have any value between half an inch and several feet, depending on the nature of the soil strzta located beneath the points of the piles.
Modern methods for the design of pile foundations take into consideration all the factors which determine the settlement of pile foundations, including the compressibility of the soil located below the points of the piles. Hence these methods can be considered reliable, provided the subsoil of the proposed foundation has been adequately explored. On the other hand, if the subsoil conditions have been misiudaed. for , " , instance, on account of inadequate sampling operations, erroneous inter~olation between drillholes. or lack of care in the examination of the soil samples, the difference between anticipated and real settlement can be distressingly important, in spite of our advanced knowledge of the mechanics of the settlement of pile foundations. This is due t o the fact that the correct interpretation of erroneous d a t a has no advantage over the erroneous interpretation of accurate data which has been practised so extensively during the empirical stage of foundation engineering. The con- sequences of misjudging the soil conditions are illustrated by the following incident.
A few years ago an important structure was erected on the gentle slope of a shallow valley in one of the suburbs of New York. The site was investigated by means of rzo standard exploratory borings to bedrock. The drillholes were spaced j o feet both ways. According t o the boring records, the rock bottom of the valley was covered with a thick layer of sand and gravel which, in turn, was overlaid in succession by soft clay. peat and recent fill. Therefore, it was decided t o establish the foundation on point-bearing concrete piles t o be driven into the sand and gravel stratum. The piles were driven with a heavy steam hammer, and driving was continued until the penetration under the last eight blows became less than one inch. The design load was 30 tons per pile. . -
During the pile driving operations it was noticed that the elevations a t which the piles of the same cluster met refusal varied b y amounts up t o thirteen feet. This fact did not receive any attention because i t was reasoned that it does not make any difference how deep the piles go provided they penetrate the gravel stratum and meet refusal. However, when the building was almost completed the first settlement cracks appeared, and during the next four years the middle part of the south wall, oriented at right angles t o the axis of the valley, had gone down by about ten inches with reference t o its ends. The maximum settle- ment of the south wall amounted t o about three inches. The owners claimed that the settlement was due t o inadequate bearing capacity of the piles. In order t o find out whether this accusation was justified, the heads of the piles supporting one of the largest footings were exposed by excavation. Some of the piles were cut off and loading tests were performed by inserting hydraulic jacks between the base of the footing and the upper ends of the several piles. The loading tests furnished the following result : None of the piles moved with reference to the base of the footing under a pressure of less than roo tons, and two of the piles could not even be moved by a pressure of zoo tons. Prior t o the test, none of the piles had carried a load of more than z j tons. Yet the cluster had settled eight inches and it continued t o settle during the loatling tests.
The test results, combined with various other indica- tions, led t o the corlclusion that the homogenous sand and gravel stratum shown in the offirial boring records did not exist. I t s place was occupied by a very much thicker layer of clay, interspersed with lenses of sand and gravel. The clay lncated between the lenses of sand and gi-avel had escaped the attention of the man in charge of the soil exploration. Very few, if any, of the piles had approached the base of this stratum. Most of them had met refusal in the gravel pockets close t o the upper boundary. This was shown by the driving records of the different piles in each cluster. The longest piles had crossed in succession two or three hard " layers " separated frorn each other by
soft material, whereas the shortest ones met refusal in the first gravel pocket they met. The progressive settlement was due t o the gradual consolidation of the layers of clay located between the gravel pockets which contained the points of the piles.
As long as the prevalent standards of boring, sampling and testing still permit such flagrant oversights, occasional failures of pile foundations will occur, in spite of the fact that the mechanics of the settlement of pile foundations are already clearly understood.
The only important controversial issue connected with pile foundations is the effect on the consistency of the clay of driving piles into clay. Some engineers claim that the effect is inconsequential. Others maintain that the penetration of the pile, combined with the vibrations set up by the falling hammer, destroy the structure of the clay completely and inaugurate a new process of coilsolidation which drags the piles in a downward direction. Hence, it is said, once the piles are driven into the clay, they would settle on account of their contact with the " remoulded"
mass cf clav even if thev are not loaded a t all.
Experience indicates that some clays behave in accordance with the opinion of the first group of engineers, whereas others oblige the second group. The soft glacial clays of Detroit, Michigan, appear to be almost unaffected by pile driving operations. On the other hand, the soft organic clay of Abbotsinch, west of Glasgow in Scotland, was practically liquefied to a distance of many feet from the seat of pile driving operations. When the driving of t h e second cluster of piles was started, at the site for a pile f o u d a t i o n , the piles of the first cluster started to rise. To prevent the rise it was necessary t o load them. However, we do not yet know whether the re-consolidation of this or of similar clays is associated with a significant volume decrease. This important question can be answered only by systematic levels of the ground surface during and after pile driving operations, or by settlement observations on groups of piles which have been driven but not loaded. Such investigations have not yet been made.
Regional Subsidences
Wherever cities spread over areas located above compressible soil strata, the settlements of the individual structures merge and produce a regional subsidence similar t o the subsidence caused b y a continuous but unequally distributed surcharge.
The first publication describing in detail a subsidence of this kind dealt wit11 the results of observations in Cambridge, Mass. In this city the re-survey of the sewer system in 1897 disclosed the existence of a bowl of subsidence with a diameter of about one mile and a maximum depth of about 2 f t . , located beneath the central part of the municipal area. Subsequent surveys showed that the depth of tlie bowl was still increasing
Building Research Congress, 7957
a t a rate of about one inch per year. The subsidence was ascribed exclusively t o the weight of the buildings occupying the central area, and it was believed that the weight caused the clay to escape in lateral directions towards the broad channel of the Charles River. The real cause of the subsidence was not even suspected, because the process of gradual consolidation of clay under constant load was not then known.
Regional subsidences have also been disclosed by periodic precision levels in the London area. Between
1865 and 1932 the maximum subsidence increased by about one foot. While fountlation engineering was still in the empirical stage, the subsidence was ascribed to various mechanical causes, such as loss of ground due to the removal of sand by pumping through the water supply wells. In 1942, when the principles of soil mechanics were already established, G. Wilson demonstrated conclusively that the settlement repre- sented the result of progressive consolidation of the London clay, due to application of weight in the form of buildings and reduction of hydrostatic uplift by lowering the water table.
In Shanghai, tlie regional subsidences due to loading combined with lowering the water table assumed such proportions that they attracted the attention of local engineers at an early date. They formed one of tlie principal incentives for the organisation of a Foundation Research Committee by the Engineering Society of China after the first world war. The subsidence observations were rather difficult and time-consuming, because there are no two benchmarks in the entire region which retain their position with reference to each other. After lengthy experimentation it was decided to refer all elevations to the head of a well with a depth of about 700 ft., which subsides less than any of the other benchmarks.
By far the most spectacular regional subsidence is going on in Mexico City. This is due to the fact that the clay layers located between the ground surface in this city and a depth of about 300 ft. below the surface are by far the most compressible clay strata which have so far been encountered in any part of the world. According to P. Cuevas, N. Carrillo and L. Zeevaert, who have made thorough studies of the settlement phenomena in Mexico City, the maximum subsidence zo date is of the order of magnitude of 50 feet. I t developed in the course of somewhat less than ten centuries.
At the sites which were previously occupied by heavy Aztec temples the clay is pre-consolidated, but the bowls produced by the subsidence were filled in. Hence when a new cycle of consolidation was initiated, a t the end of the nineteenth century, by pumping water from the sand strata located between the clay layers, the sites of the temples sank less than the surrounding terrain and developed into gentle hills. If the middle part of a long building is located at the top of such a hill, the building gradually breaks like a beam, by bending. The subsidence due to pumping alone produced differential settlements by amounts u p to ten feet. The effect of these movements on public utilities such as the sewer system, is disastrous. Hence, Mexico City will be compelled t o set an example for coping with the consequences of important regional subsidences and, owing to their active participation in soil mechanics research, the Mexican engineers are exceptionally well prepared to do so.
If the seat of a regional subsidence is located a t great depth, the vertical downward movement of the ground surface is associated with important horizontal compressive strains in the top layer of the subsoil of the central part of the bottom of the bowl. Buried
structures, such as tunnel conduits or box-shaped sub-basements are acted upon by horizontal pressures of increasing intensity. Such conditions prevail a t present on Terminal Island in California. The progressive subsidence is probabll- due to the removal of oil. The centre of the subsidence is !mated at a depth of about 3,000 ft.. and the diameter of the bowl of subsidence is about three miles. The maximum subsidence to date is eleven feet. A description of the effects of the subsidence on the buried portions of the steam power plants located at the bottom of the bowl of the subsidence was recently published by \V. L. Chadwick in CIVIL ENGINEERING (U.S.A.). A S U C C ~ S S ~ ~ ~ theoretical
investigatioii of the relations between vertical sub- sidence and horizontal strain, due to a pressure decline in the interstitial liquid of the rocks at great depth was made by N. Carrillo, and the results will be published before long.
Conclusions
The preceding review of the interaction between soil mechanics and foundation engineering has shown that foundation engineering has definitely passed from the scientific state into that of maturity. The time is gone when contributions of great practical importance could still be made by pure reasoning at the desk or by small-scale laboratory tests. As a matter of fact, perusing the field, one gets the impression that research has outdistanced practical application, and that the gap between theory and practice still widens. Intricate mathematical investigations are still being performed on the subject of the influence of elastic anisotropy on the stress distribution in the subsoil of loaded footings. The errors due to neglecting this influence can hardly exceed thirty per cent. On the other hand, we learn from time to time about recently constructed, expensive buildings which must be under- pinned on account of excessive settlement. The necessity for underpinning indicates that the settlement is a t least ten times greater than the designer anticipated. The investigations of the relation between the pile penetration under blow and the ultimate bearing capacity of the individual pile have reached an un- precedented level of refinement ; but on the job, few engineers are inquisitive enough t o attempt correlation between the boring and the pile-penetration records. If the piles of an individual cluster meet refusal a t very different depths, they hardly care about the cause upless it is explained t o them, a t a later date, by the expert witness of the owners before the courts.
If the soil conditions are clear and simple, an elementary knowledge of soil mechanics, combined with the most primitive methods of dry-sample borings, is nowadays sufficient to prevent flagrantly erroneous forecasts. Such conditions prevail, for instance, if the site for a structure is located above continuous layers of soft clay with well-defined upper and lower boundaries. During the empirical stage of foundation engineering, subsoil conditions of this type were responsible for some of the most spectacular cases of settlement on record, including most of the cases listed at the outset of this paper. The recurrence of foundation defects under simple conditions in recent years is due merely to the fact that some foundation engineers are still in the empirical stage. The advance- ment into the higher stages is a voluntary and not a compulsory act.
Unfortunately, in practice, clear and simple soil conditions are rather uncommon ; and if the soil con- ditions are complex, the advanced state of soil mechanics is of no avail, unless the engineer in charge of the soil
exploration is fully aware of the virtues and deficiencies of the different techniques, and capable of adapting these techniques to the local soil conditions and the exigencies of the job.
Since there is an infinite variety of subsoil patterns and conditions of saturation, the use of the different methods of subsoil exploration cannot be standardised, but the methods themselves still leave a wide margin for improvement, as far as expediency and reliability are concerned. Hence it appears that the development of these techniques and the comparison between forecast and actual performance of the foundation constitutes at present, and for many years to come, one of the most important subjects of research in the realm of
building foundations. These methods include penetra- tion tests in the drillhole, vane tests and unconfined compression tests on the job by means of portable apparatus. Other equally important topics for further research are the effect of pile penetration on the consistency of clays, the " breaking point " of the structure of soft clays, and the laws which determine the rate of settlement of structures above clay due to secondary time-effects. All these investigations can be performed only in the field, at appropriately selected sites. The journal GEOTECHNIQUE and the Proceedings
of the International Conferences on Soil Mechanics and Foundation Engineering serve as clearing houses for the findings.
146 Building Research Congress, 1951
T h e Influence of Modern Soil Studies
on
the Construction of Foundations
H. J. B. Harding and R. Glossop
(Directors, Johvz Mowlem & Co. Ltd., London)
1. The Development of Soil Mechanics
Although several important contributions to soil mechanics were made during the 18th and 19th centuries, it may be said without exaggeration, of Great Britain, that until recently the approach t o most foundation problems was empirical. That such an approach should have persisted for so long was due partly t o the fact that very difficult foundation conditions are comparatively rare in this country, and partly t o the wisdom and commonsense of the engineers en- trusted with major works during the 19th century, who insisted on detailed site investigations and made use of full-scale loading tests ; as, for example, Sir John Hawkshaw, who placed test loads of 850 tons on the cylinders of Cannon Street Bridge1.
About 1935, failures, particularly those of retaining walls on stiff fissured clays, drew attention t o the work in soil mechanics then proceeding in Europe and the United States of America; this work was nothing less than the application of scientific method t o the whole range of engineering soil problems. Three stages marked its progress. The first was mathematical, and although advances were made on a broad front, work on five problems proved of immediate value in practice ; these were, the problem of the distribution of stress in the soil beneath a loaded footing, the theory of the consolidation of clay, the theory of the ultimate bearing capacity of a foundation, and the application of flow net analysis to the flow of water in granular soils and methods of calculating the stability of clay slopes.
The application of these and other theories in practice required a knowledge of the physical constants involved, and led naturally to the second stage, the measurement of the mechanical properties of soils. Though some data was available from other sciences, it soon became evident that soils must be investigated and classified from an engineering point of view, and moreover, t h a t their properties were so variable that every site must be treated as a separate problem, and that new methods of sampling and testing must be developed ; thus the modern technique of site investigation and laboratory testing were evolved. The third stage of development lay in tests and observations on actual structures, and in the analysis of failures. Obviously, a background of theory and a knowledge of soil properties was here essential, but as this background has been built up, field observations have become of increasing value. I t will suffice t o mention the analysis of the failure of slopes, records of consolidation settlements, loading tests, and failures of foundations.
I t is convenient t o divide foundations into shallow foundations and deep foundations; the latter are defined as those in which the ratio of the depth of the bearing surface to the minimum plan dimensioil is less than I.
Since the construction of shallow foundations is generally a simple matter, this paper deals chiefly with deep foundations, such as may be required for heavy structures on sites where ground of adequate bearing
capacity occurs a t some depth below the surface; for subterranean structures such'as basements, air raid shelters, docks, pump houses, etc. ; as protection against scour, as for example in the case of bridge piers and abutments, and where a structure must be carried down t o impermeable ground as in the cut-offs of dams.
The construction of deep foundations may demand the use of one or more of various mechanical devices. such as piles, sheet piles, cofferdams, monoliths, caissons, etc., and/or of geotechnical processes whereby the properties of the soil are improved, the invention of which cannot in all instances be directly ascribed t o the influence of modern soil studies.
Nevertheless, improved methods of site investigation, and the better knowledge of the mechanical properties of soil, have influenced this branch of engineering b y aiding in the correct choice of method, for each of these methods, however ingenious, applies only t o a limited range of soil conditions, and may be ineffective or even useless if employed under other circumstances. Mistakes have been common in the past, and have been due t o a tendency to treat foundation engineering from a structural or mechanical point of view, without realising that the nature of the soil is the essential factor ; indeed, it may be said that the majority of engineering failures can be attributed to inadequate site investigation, due in some cases t o ignorance and in others t o false economy. I t is evident, therefore, that the first step in con- sidering the design and construction of any foundation is a detailed site investigation, which will disclose the various strata present, and their disposal both laterally and in depth, together with a sufficient number of tests t o measure mechanical properties of the soil.
2. Site Investigation
Much has been done in Great Britain during the last ten years to improve and standardise methods of site investigation, by the introduction of a simple classification of soils, by improved methods of taking undisturbed samples, and by the development of soil testing methods, both in the laboratory and i n silu.
I n the past, t h e description of soils was often left t o boring foremen, obscure dialect terms were much used, and there was no general agreement as to what was meant, even by such common phrases as " Fine
Sand." A simple eclectic system for the classification and description of soils was worked out b y a panel of the Code of Practice Committee on Site Investigation2, and although it may fall short of that perfection which some desire, it is now in general use, and has been found satisfactory and accepted by practical engineers The classification is based primarily on particle size and the main divisions in it, corresponding to important changes in the engineering characteristics of the soil, and hence related t o the choice of processes (Fig. I ) .
1 0 0
Fig. 2.-Sampling tool for cohesive soils
1 0 0
7 5
5 0
25
0
of 27 per cent., and gives satisfactory results in all but the most sensitive soils. It is simple, robust, easily and cheaply made, and in it samples can be transported and storc-ld for months, without the use of a humid room.
/'
The problem of taking undisturbed samples of sand has recently been solvedS by the " Compressed Air Sampler " (Fig. 3) ; this is far simpler and less costly
than methods using freezing, bitumen emulsion injections etc., and it has been adopted in many parts of the world.
-
I
S A N D eo C L A Y S I L T G R A V E L bOULDERSI
A R T I F I C I A L CEMENTINGFig. 1.-Tentative limits of application of various geotechnlcal processes (afler Glossop and Skenrpton)
METHODS OF SAMPLING disturbed sarnples are usually stored in airtight con-
soil samples taken from the auger and stored in tainers. Satisfactory methods of taking undisturbed a wooden box soon lose all resemblance to their natural sa'n~les have been evO1ved~ for and s t a t e ; however, such methods are now rare, and ~ " n d s . The type of sampler generally used in
re at
Britain for clav is shown in Fig. 2 ; it has an area ratioSOIL TESTING
The development of soil testing methods in any country is influenced by the geology4. I n Britain, firm saturated clays are of common occurrence ; to these the @ = o analysis is applicable, and accordingly,
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GRAVEL 6 0 C L A Y SOULOERSGROUND WATER LOWERING AND COMPRESSED AIR
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0.001 0.06 f nBuilding Research Congress, 1957 ,< .L, U S ' . . I,.
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Fig. 3.-Compressed air sampler (after Bishop)
Fig. 4.-Borehole record sheet
the unconfined compression test has been much used,
and convenient apparatus devised6.
Stiff fissured clays are widespread in the London Basin, and for these, the quick or undrained triaxial test, originally developed in t h e United States for frictional soils, was found to be best suited for foundation prob- lems6.
Turning to in
situ
tests, the Dutch deep sounding technique has long been used for piled foundations in sand, but the most important recent development has been that of the vane test, first invented for shallow " trafficability " tests7, but later adopted with great success to the measurement of the shear strength of sensitive clays in deptha. This technique has proved most valuable, as soft recent and highly sensitive clays occur in all the estuaries of Britain.The systematic recording of borehole data and of test results receive much attention in the Code of Practice, and convenient graphical methods are generally used (Fig. 4).
3. Shallow Foundations
Shallow foundations may be divided into footings, strip footings and rafts : since little excavation is required, their actual construction is a simple matter.
In recent years, soil studies have altered our whole approach t o their design. Thus, the relation between ultimate bearing capacity and shear strength for a clay soil has been clearly established, and the choice of a suitable factor of safety to fix the safe bearing capacity has been rationalisede, and it is now known that, in the case of sands, the ultimate bearing capacity is a function of the plan dimensions of the foundation. The danger of extrapolating from small-scale field loading tests t o large raft foundations, in cases where compressible strata may exist in depth, is also now generally
appreciated.
Perhaps the most important effect of recent work, so far as shallow foundations are concerned, has been t o draw attention t o the enormous amount of damage t o house property, due to the seasonal shrinkage and swelling of clay soils, and to the effect of the transpiration of plants in the neighbourhood of buildingslO.
An interesting problem is that of structures of considerable size, b i t of no great value in themselves, such as large tanks, where these must be placed upon a great depth of soft soil, such as estuarine clay. The cost of deep piled foundations is then out of proportion t o the value of the structure itself, but it is difficult to devise a shallow foundation which will spread the load to a safe figure, since a raft must then be made sufficiently deep and heavy t o withstand considerable bending forces. An ingenious structural solution problem has recently been inventedm. I n it, the raft takes the form of a concrete dome with heavy peripheral reinforcement ; the stresses in the dome are thus all compressive, and no bending moments occur, and the spread of the foundation can be safely increased, without undue increase in the weight of the foundation
Fig. 6.-Durley dome under construction showing reinforcement
structure itself. No shuttering is required, since in excavation the ground is excavated t o the required shape for the underside of the dome (Figs. 5 and 6).
4. Deep Foundations
From the point of view of construction, deep founda- tions may be considered as those which require excavation, and piled foundations (other than bored piles), which do not. The effect of soil studies may be divided into the application of theoretical soil mechanics to the execution of such works, and t o the use of geotechnical processes (Table I).
A. OPEN EXCAVATION WITHOUT SUPPORT
Open excavation may be carried out either below water by dredging, and here only the problem of the stability of slopes may arise, or in the dry, when the following soil problems may be found t o occur.
(a) The Stability of Slopes irl. Clay
Where the site permits of an open excavation with battered sides, the only serious problem is that of the safe slope permissible for any depth of dig, for in cohesive soils there is a critical height for any degree of slope, and if this is exceeded, a slip will develop by shear failure on an approximately cylindrical surface. The circular arc method of analysis enables the critical height for any slope t o be determined, if the shear
CLEAN GRANULAR FILL
i
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.- 4J!i
:
5
M * .- 0 >-
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93
- - .----- -Constructional device I Soils
!
Open excavation with battered Clay slopes.
I
- -- W - Associated soil mechanics problemsThe application of devices a n d processes t o the
Silt
Sand
construction of deep foundations o
P
h3
4
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Lo CO n 2 w2
3'2
CO Lo Los
YField and laboratory tests required
Boring, undisturbed s a m p l i n g , s h e a r strength measurz- ments, specific weigkt of soil.
Borings, undisturbed samples, specific weight of soil. Obtain lower limit of 0 from triaxial tests on re- compacted samples. Undisturbed samples, permeability tests. Borings, undisturbed s a m p l e s , s h e a r strength measure- ments.
Borings, sizing analy- ses. Undisturbed s a m p l e s , s h e a r strength measure- ments.
Supported excavations with ver- tical sides, timbered trenches, steel sheet piled cofferdams, etc.
Monoliths, cylinders and bored piles.
Caissons and cylinders.
Methods of analysis Geotechnical processes
i
I
RemarksNote :-In timber excavations, sup- port is placed as excavation proceeds ; with sheet piles these are driven to full depth before excavation is started.
Depth of pile penetration is critical, preferably into an impermeable stratum.
These methods resemble each other in principle, in that excavation is carried out under water, and the hole is supported by structure which sinks under its own weight
as excavation proceeds.
For lateral pressures, use Terraghi and Peck's trapezoidal distribution1'. For bottom heave
op. cit. l*
Total pressure calcula- ted from Rankine formula. Use tra-
pezoidal distribution of stress1*.
Investigate stability by flow net analysis. Use McNamee's charts."
I
For very soft clays use vane test, and treat as heavy liquids for lateral pressure cal- culations.
Consider use of ground- water lowering to reduce pile penetra- tion.
Silicate injections have been used to seal end of tube, to enable bored piles to be concreted in the dry. Use of thixotropic clay grout has been suggested to reduce skin friction. Stability of slopes
1
Stability of slopes. Inflows of water. Clay andsilt. Bottom heave.
Sand.
Piping and inflows of
Boring, undisturbed s a m p l i n g , s h e a r strength measure- ments.
Borings, undisturbed samples, sizing analy- ses, permeability. ground water level observations. Borings, permeability,
sizing analyses, ground water level observations.
All soils.
All soils.
Use vane test for soft clays, unconfined compression test for firm clays and undrained triaxial test for stiff clays.
Put down borings to explore for sand strata with artesian water below excavation level. In most cases some form of
cofferdam will be needed.
Graded gravel filters may provc useful as a safety device. Useof Taylor'schartsl~.
Fellenius graphical method for complex soil profiles1*. Ground-water lowering calculations. Use of Forcheimer a n d Sichardt equations17 Skin friction. Extent of losses of compressed air, skin friction.
Not applicable.
Consider useof vacuum drainage or electro-os- mosis, with g a v e l filters as safety measure.
Drainage by sumps, deep wells, shallow wells or well points.
