Proceedings of the Eleventh Canadian Soil Mechanics Conference

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·PROCEEDINGS OF THE

ELEVENTH CANADIAN SOIL MECHANICS CONFERENCE

DECEMBER

9

AND

10, 1957

Technical Memorandum No.

53

Ottawa

FOREWORD

These proceedings are a record of the Eleventh Canadian Soil Mechanics Conference held in Ottawa on

December

9

and

10, 1957.

The conference is sponsored by

the Soil Mechanics Subcommittee of the Associate Committee on Soil and Snow Mechanics of the National Research Council.

The first day was devoted to problems aS50ciated

with pile foundations. Following a key paper on "Some

Aspects of Pile Foundations" and a general discussion, a five-member panel under the direction of Professor D.F. Coates discussed the question of the "Bearing Capacity of Piles".

A symposium on frost action, held during the second day, contained contributions ranging in content from basic phenomena to field practice.

In keeping with the policy of the Associate Committee on publication, authors are encouraged to publish their

complete papers in recognized scientific journals. Summaries

of these papers are, however, contained in the proceedings

together with information on where the paper will be published. The Soil Mechanics Subcommittee wishes to express

its appreciation to all those who participated in the

TABLE OF CONTENTS Session of December

9

Section 1 Section 2 Section

3

Section

4

Address of Welcome by ｾｾＮ R.F. Legget

Some Aspects of Pile Foundations by I.F. Morrison and S.R. Sinclair Introductory Remarks to the Panel

Discussion on the Soil Bearing Capacity of Piles by D.F. Coates, Panel Moderator

Statements by the members of the Panel on the Soil Bearing Capacity of Piles: 1

3

17

20 22

18

(a) What is Pile Failure - With Respect to the Soil by

G.G. Heyerhof

(b) Cast-in-place Piles by C. Ant enbring

(c) Reaction of Clays to the Driving of Friction Piles by E.1. Rubinsky

(d) Considerations of the Ultimate Bearing Capacity of Deep Concreted Footings Installed in Granular

Soils by L.A. Fraikin 27

Section

5

(e) The Treatment of Piles in the National Building Code of

Canada,

1953

by N.D. Lea

Discussion on the SolI Bearing Capacity of Piles:

(a) Prepared by G.G. Meyerhof

(b) Prepared by H.A. Nelson Holland (c) Prepared by A.E. Macdonald

(d) General Discussion

31

33

34

35

36

TABLE OF CO!JTENTS (continued)

Section b Concluding Remarks to the Panel

Discussion on the Soil Bearing Capacity of Piles by D.F. Coates

Panel Moderator

₃₈

Session of December 10 Section 7 Section 8 ｾ･｣ｴｩｯｮ

9

Section 10

ｓｵｾｾ｡ｲｹ of Basic Concepts of Frost

Action in Soils by E. Penner

Summary of Frost Action and Railroad Maintenance in the

Labrador Peninsula by R.W. Pryer The Rational Design of Flexible

Pavements to Resist Detrimental Frost Action by F..B. Wilkins Summary of The Use of Calcium

Lignosulphonate for the Prevention of Frost Heaves by S.R. Annett

39

40

64

Section 11 Summary of:

(a) Frost Action Damage to a

Cold Storage Plant at Cornwall,

Ontario by J.J. Hamilton

65

(b) Considerations of the Thermal Regime Beneath a Cold Storage

Ｑｾ｡ｲ･ｨｯｵｳ･ by D.C. Pearce 66

Section 12

Section

13

ｾ･｣ｴｩｯｮ

14

General Discussion of Frost Action The Fourth International Soil

Mechanics Conference, London,

England, August

1957

by R.F. Legget

Regional Reports for

1957:

(a) Montreal Soil Mechanics Group Activities by D.F. Coates (b) Soil Mechanics Pctivities in

the Atlantic Provinces by G.G. Meyerhof

67

68

70

Section

14

(continued)

Section

15

TABLE OF ｃｏｾｾｅｎｔｓ (continued)

(c) Ottawa Soil Mechanics Group

Activities by D.A. Lane

(d) British Columbia Soils Group

Activities by A. Peebles

(e) Toronto Soil Mechanics Group

Activities by V. Milligan

(f) Soil Mechanics Activities in

the Prairie Provinces by

S.R. Sinclair

Appendix A: List of those present

at the Eleventh Soil Mechanics Conference

72

73

74

75

SESSION OF DECEMBER

9, 1957

Section 1

Address of ｗ･ｬ｣ｯｭ･ｾｾｾＮ R.F. Legget

Mr. R.F. Legget, Chairman of the Associate Committee

on Soil and Snow }wchanics, opened the conference by extending a welcome on behalf of the National Research Council and in particular, the Associate Committee on Soil and Snow Mechanics. He referred to the Tenth Conference and noted that its success was probably responsible for the remarkable attendance at this Eleventh Conference, an attendance so large that the place of meeting had to be moved from the Building Research Centre to the Radio and Electrical Engineering Building through the courtesy of Dr. B.G. Ballard, Vice-President of the National Research Council.

The absence of some leading figures in the field of Canadian soil mechanics was noted, the speaker referring especially to Prof. I.F. Morrison (University of Alberta),

Dr. C.R. Young (University of Toronto), Dean R.M. Hardy

(University of Alberta), Dr. ｎＮｾｲＮ Radforth (McMaster

Univer-sity), and Dr. Norman McLeod (Imperial Oil Limited, Toronto)

-the last three members being prevented from attending by -the pressure of other business.

ｶｾＮ Legget expressed his thanks to those actually

responsible for the organizing of the conference, mentioning

in particular

Mr.

Peterson, Chairman of the Subcommittee on

Soil Mechanics, and

Mr.

Penner, the Secretary. He also

intro-duced Mr. Eden, Secretary of the Associate Committee on Soil

and Snow Mechanics, and ﾥｾＮ Crawford, Head of the Soil

Mechanics Section of the Division of Building Research, who

had recently returned from a year in Europe. Finally, he

mentioned that Miss Gloria Zuana, who had served for some years as Assistant Secretary to the Associate Committee and who had assisted greatly in the arrangements for all recent Soil Mechanics conferences, was taking up other duties in the Division of Building Research and this was to be the last conference she would attend in her official capacity.

Reminding those present that the purpose of the conference was to stimulate discussion in the field of soil

mechanics research, Mr. Legget expressed the hope that the

program arranged would help to serve this end. A new method

of preparing a record of the procee1ings was to be followed.

All those wishing to have their cor ｾｲｩ｢ｵｴｩｯｮｳ to the

before the end of December. If this were done, it was hoped that the proceedings would be available before the end of

February. The Associate Committee welcomed the publication

of papers presented to the conferences in other media.

Mr.

Legget mentioned the recent appearance of two papers

presented at the Tenth Conference, one by

Dr.

Meyerhof in

Geotechnique, and the other by Dean Hardy in the first issue of the Transactions of the Engineering Institute of Canada.

Some Aspects of Pile Foundationsl by

I.F. Morrison and S.R. Sinclair Introduction

Because of space limitations this paper has been restricted to include only piles, driven or placed into the

ground vertically, which support vertical loads. Excluded,

therefore, are topics such as sheet-piling and piles placed for the purpose of ground modification for soil drainage or

for increased density state. In addition, a number of types

of pile have been omitted from the discussion because the basic principles of pile behaviour are essentially the same for all.

The use of pile foundations in Canada has generally

increased in the last several years for several reasons. For

example, despite the use of lighter materials and construction, loads on foundations have increased with the increase in the size of buildings and bridges, so that in many instances, pile foundations have now become advisable and perhaps

neces-sary. In addition, building sites originally avoided because

of poor shallow foundation conditions are now being occupied

due to increased land values. Moreover, there has been, in

the first half of this century, considerable improvement and development not only in pile-driving equipment and methods of placing but in the materials used for the various types of pile.

Since the first Canadian Conference on Soil Mechanics some ten years ago, very little conference time has been

devoted to a discussion on pile foundations; while the

importance of the subject is recognized there have been many

other subjects of equal or greater importance. The subject

of piles is an old one and has often been thoroughly discussed

elsewhere. In fact, the authors can introduce nothing new;

that is, nothing that has not already been written on the

subject. They have, however, sought to reiterate and

re-emphasize certain aspects of pile behaviour against a back-ground of the basic principles of soil mechanics which are now sufficiently well established both in theory and practice to be used with confidence.

The basic requirements of any building or ｢ｲｩ､ｾ･

foundation are:

1. To maintain the total vertical displacement at an

accep-table amount.

2. To restrict the differential displacements to values

consistent with freedom from structural damage.

3.

To place the supporting element at a depth below the

ground surface to be free from the influence of volu-metric changes of the soil that might be detrimental to the structure.

4.

To ensure that the supporting soil will be safe from erosion

and internal change of supporting value, as in the thawing of permafrost or drastic reduction of the strength of

cohesive soils due to increased moisture content, or other changes that may come about with time.

Pile foundations can often be used to satisfy these requirements and afford the best and most economical method of attaining them.

1. The Purpose of a Pile Foundation

The primary purpose of a pile foundation is to reduce the amount of settlement of a structure as compared to that

which would occur if piles were not used. This is accomplished

essentially by by-passing a depth of compressible soil which

would be the seat of the major portion of the ultimate

settle-ment of the structure. By embedding piles in this mass of soil,

the settlement is reduced in that region to the compression of the piles themselves; when it is recalled that the modulus of elasticity of the material of which the piles are composed 1s many times larger than that of the soil in which they are embedded, it will be recognized that a very substantial reduc-tion in settlement may be accomplished by the use of them. The use of piles, however, does not always lead to reduced

settlement.

The bearing capacity of a pile foundation, in the same

sense as of the bearing capacity of a spread footing, is of minor importance because of the relatively large depth to

which the piles extend. There have been few cases of the

structural failure of a pile foundation as a whole.

The bearing capacity of a single pile should be defined as that load, on the load-settlement curve, at which a

con-siderably increased rate of settlement becomes evident. It is

comparable to the yield point in the testing of materials and sUbject to the same uncertainty of accurate determination.

On the other hand, there is no definite relationshlp that can be established between the bearing capacity of a closely spaced group of piles and that of a single pile.

In a preliminary study, the acceptable settlement for a structure should be established, and also some value placed on the differential settlement the structure can

satisfactorily withstand. Some structures are much more

flexible than others depending on the materials, type of structure, its dimensions such as column-spacing, etc. The engineer does not usually have much choice in these

matters and must take them into account as they are presented. When he does have a choice, considerable benefit may result from adapations of the type of structure to the existent

foundation conditions. More thinking needs to be done in

the preliminary planning stage regarding the relationship between the type of structure and the type of foundation on

which it is to rest. This applies especially to the choice

between determinate and indeterminate structures.

2. Classification of Foundation Piles

It seems hardly necessary to describe a pile but a brief classification may be of some benefit in avoiding any

misinterpretation. Often there is a confusion in current

literature regarding definitions applying to pile foundations. Perhaps the best glossary is that published as Appendix A; definitions applying to Pile Foundation and Pile Structures

in the A.S.C.E. Manual No. 27 on that subject. For our

purposes here, however, we shall discriminate, under "bearing pile", two extreme cases:

(a) Friction, or Floating, Piles; which resist being

pushed into the ground by the reactive mantle friction between the cylindrical surface of the pile and the

soil in which it is embedded.

(b) Point Bearing Piles; which resist being pushed into the ground by the reaction pressure at the point of the pile.

Piles may also be classified according to the materials

of which they are made. The three materials in common use

are, of course, wood, concrete and structural steel. Each has

its advantages and disadvantages.

Wooden piles may be of several different species. Some woods stand driving better than others but a more

important property is their susceptibility to rot. All

woods will rot under favourable conditions. One condition

necessary to fungus growth is a supply of oxygen and it is well known that wood totally immersed in water will last

indefinitely, provided that marine borers are not presento The significance of this is that for untreated wooden piles, the permanent ground water elevation should always be above the tops of the piles i.e., higher in elevation than the bottoms of the footings in which the tops of the piles are

embedded. It is necessary to ensure that there will be no

possible subsequent drainage of the soil. If the piles are

not to be completely and continuously immersed in water, then wooden piles should be treated with a suitable preservative. The preservative acts as a poison to the food supply of the

fungus; it is not a case of waterproofing the wood. Cases

have been observed in which wooden piles have rotted at the top because the soil has settled enough to leave a free space beneath the footings from which any water had been drained. Immediately after cut-off, the tops of wooden piles should be given two applications of preservative to prevent rotting.

Wooden piles generally, should be straight throughout their length and a limitation should be set in respect of twisted grain.

Precast concrete piles are now replacing wooden piles in many structures but unless larger bearing values are permitted, they are likely to lead to increased cost over wooden piles except in cases where durability is of paramount

importance. Near the ground surface, however, soils containing

alkali salts may cause the disintegration of the concrete so that durability is not always assured unless special cements

are used. In this regard, however , the permanence of the

alkali-resistant properties of special cementG has not yet been fully established.

These piles, being much heavier than wooden piles,

cost more to handle and to drive. Suitable reinforcement

should always be provided to withstand handling and driving. When driving in soft ground, the compression wave imparted by the hammer to the head of the pile returns from the point as a tension \-lave that may break the pile unless properly

re-inforced. The excellent handbook issued by The Portland

Cement Association (1) gives details about driving, handling and reinforcing precast concrete piles.

steel piles should be straight and handled with

reasonable care, especially in cold weather. The ends should

be inspected for lamination which may arise from insufficient

cropping of the ingot. This condition is not easily detected

but may be revealed in the H-type by means of a sharp blow on

the web at right angles to it. The soil should be eXQmined

for its corrosive action because some soils are much more

potent in this respect than others. For example, the soil in

the City of Calgary has produced practically no corrosion on steel pipes in place for 30 years; whereas at Magrath, Alberta,

Again, piles may be classified according to the

method of placing. They are usually driven into the ground

to the required depth, but with cast-in-place concrete piles, a hole is made in the ground either by driving a hollow tube

or by excavation. Distinction should be made between these

two methods because in the first the soil is often disturbed by the compaction, whereas this effect is generally absent

in the second. The disturbance may result in densification

of loose sands and therefore be beneficial. However, in the

case of sensitive cohesive soils, the disturbance may be a disadvantage because the soil may lose appreciable strength.

For all three materials, cut-off is easily effected except for precast concrete piles which must be cut off under

water. This problem does not arise with cast-in-place piles.

The loss is from 2 to

4

feet with wooden piles but precast

concrete and steel piles should have less waste if the ｬ･ｲｾｴｨｳ

have been properly determined before driving starts.

3.

The Mechanics of a Single Pile

It is necessary to understand in detail the reciprocal action between the pile and the soil in which it is embedded. The large difference that exists between the elastic modulus of the pile and that of the soil should be recognized at the

outset. The one is, for a wooden pile, from 60 to 8000 times

the other, for dense sand and gravel to soft clay respectively. This leads immediately to the theory that a pile may, with

sufficient accuracy, be considered a rigid body as far as the general mechanics of static pile behaviour is concerned.

We are of course primarily interested in the resistance offered by the ground to static-load penetration by the pile and to which the settlement of the foundation is directly

related. Such resistance consists mechanically of a friction

force along the mantle of the pile and a direct compressive

force at the toe of the pile. The total of these makes up

the resistance to penetration.

For a single pile, we may express this by means of the simple equation of static equilibrium,

Q

=

F + P

in which Q is the downward load on the pile and F and P

are the upward reaction forces due to friction and point

resistance respectively. So far, little is known about these

two forces. With friction, the distribution along the pile

will depend on the characteristics 0f the several soil strata

through which the pile extends. ｍＨｾ･ｯｶ･ｲＬ it will vary with

bearing piles. On the other hand, although some point resis-tance is always expected, it can vary from an extremely small

amount to a value larger than Q when F is negative.

With friction piles, the passive character of friction

must be considered. The frictional or adhesive force along

the surface of the pile depends, in saturated soils, on the

effective pressure in the soil skeleton. This, according to

the basic principles of soil mechanics, varies with time and, in saturated cohesive soils, may eventually increase several

fold. This is 'due to the transfer of the normal pressure from

the water phase to the solid phase of the soil. This explains

why a pile which, when driven into such a soil, shows a con-siderable penetration but is almost impossible to drive

several days later. At best, an average value for the friction

force per square foot C8ill only be assumed, dependent on the

type or types of soil stratum, and the value of F found by

the simple process of multiplication by the mantle area. Because the ground at the pile point may be soft in friction

piles, the value of P will not bo large and, from a practical

point of view, may be neglected in the equation. In this way

we may arrive at a rough estimate of the carrying capacity of

a single friction pile. This is the process contemplated in

the National Building Code where safe average values for the

friction force are set ､ｯｾｭＮ

Nevertheless, the only really accurate way to determine the safe load on a friction pile is to carry out an adequate loading test.

Turning now to point-bearing piles, the ｯ｢ｶｾｯｵｳ question

arises, "how nmch of the total resistance can be accounted

for by friction?" First, considering the pile as a rigid body,

if the point bears on, or in an unyielding stratum so that there can be no movement of it due to a force equal to, or

somewhat larger than Q, there will be no friction developed

because none is required to maintain static equilibrium.

Friction between two bodies comes into play only if there can

be a relative movement. In this case, therefore, the whole

load Q is accounted for by the point re.sistance. So far,

the pile has been considered as a rigid body. However, due

to its elasticity, the pile ｷｾｹ shorten as the load Q is

increased and appreciable relative .movement between the pile

and the soil may occur especially at the upper portion. For

example, a typical 40-foot wooden pile stressed to an average value of 500 psi will shorten 0.15 inches (i.e., the shortening

varies uniformly from 0 at the toe to 0.15 inches at the top)

and some friction might develop as a resistance to this shortening.

From a practical standpoint, however, the soil near the top of a pile is usually capable of producing only a low

friction value. Moreover, it is possible that even here the friction value will disappear with time on account of the

plastic flow of the soil. In driving onto a hard stratum,

the pile is likely stressed to higher values than those that

will be produced ultimately by the static load, Q. Also,

the pile vibrates both longitudinally and laterally, making it difficult to visualize what the final condition will be. For example, one of the writers, on a recent job, observed that due to the lateral vibrations of the pile and the compressible character of the damp silt through which the

piles were driven, there was an appreciable separation between the pile and the soil which appeared to extend "downward from

the ground surface by several feet. The piles were thus

literally standing in a hole slightly larger than the pile. In this case there could be no friction.

Some cohesive soils that are saturated and sensitive

to remolding have been ｫｮｯｾｭ to undergo consolidation due to

their intrinsic weight, after being disturbed by the driving

action. This remolding effect will produce a relative

movement between the pile and the soil. There will be a

､ｯｾｭｷ｡ｲ､ drag on the pile i.e., a negative value of F will

be present so that P will be larger than Q. The ground

surface below the footing sinks leaving an air space. Untreated wooden piles have been known to rot at the tops

due to this. Moreover, since there will often be a slight

settlement of the soil below the footing, one cannot count on any support from such soil in the case of point-bearing piles.

However, if there is a subsequent swelling of the soil in which the piles are embedded, some of the load may then be carried by soil pressure under the footing itself thus relieving the piles of a portion of the total load.

In placing cast-in-place concrete piles, a very dry

concrete shou11 be used and ｲｾ･､ into place if friction

between the s, 11 and the pile is to be depended upon. On the

other hand, if the piles can reach a solid stratum, sufficient to support the load by end bearing, then the concrete can be of a more workable mix.

In view of the previous discussion, it seems best to

neglect the value of F in the case of point-bearing piles,

thus assuming P equal to Q. This raises the question of

allowable value for p. It may be based: (a) on the bearing

value of the stratum on or in which the point rests; or (b) on the strength of the material of which the pile is made, which-ever is the lesser.

The safe strength of the material of which the pile is made can be determined within reasonable limits so that the second method of design presents no undue difficulties.

However, the determination of the bearing value of the hard

supporting stratum may be difficult. It is best established

by actual physical tests.

4.

Pile Groups

Until now only single piles have been considered. With most types of pile foundations, however, it is necessary

to consider the action of a group of piles. Here also, there

must be discrimination between friction and point-bearing

piles. The latter normally rest on relatively stiff and highly

incompressible material so that settlement due to consolidation of the underlying soil or rock usually presents no problem, whether a single pile or a group of piles is being considered.

Thus, such piles in a group may usually be spaced as closely as driving conditions permit without a shearing failure

occurring in the soil or rock supporting the\piles. However,

if there is a stratum of highly compressible 'soil below the hard stratum even at considerable depth, and the plan area of the structure is large, then the piles may be quite ineffective

in reducing the settlement. The overlapping pressure bulbs

should be ｣ｯｮｳｩ､ｾｲ･､ and an estimate made of the pressure

transmitted to the soft stratum.

Friction piles must be considered in a different manner. It is now understood through theory and observation that there is no definite relationship between the settlement of a single pile due to consolidation of the underlying soil and that of the same pile when it forms one of a group of piles, each

subjected to the same load. The pressure bulb surrounding a

single pile is such that only a relatively shallow depth of soil below the pile tip is stressed sufficiently to cause consolidation.

The force developed by a friction pile spreads outward

into the soil over a gradually expanding horizontal area. It

is the vertical compression of the soil due to the stress on it supplied by the pile, that results in the settlement, i.e., the sinking of the pile and the compression of the soil are

conjunctive. If the mantle friction-force is assumed uniform

throughout the length of the pile, the approximate angle of

spread f'rom the top at 100 _{to the vertical may be taken. The}

variation of stress on the soil over a horizontal circular area at the point of the single pile is of the familiar

bell-shaped form. The pressure bulbs of the single piles will

overlap for piles spaced close together and, when the ratio of length to spacing is large, the resultant pressure distribu-tion on the soil over a plane at the elevadistribu-tion of the points

will be approximately uniform. Thus, there is developed a

of the group as a whole will conform to the consolidation of

the soil, below and in the vicinity of the pile points. Thus,

the settlement of a pile group due to consolidation may be

many times that of a single pile due to the same cause. It

is therefore important that the corresponding settlement of the friction pile group is analyzed to insure its being within

allowable limits. There is no exact method for carrying out

such an analysis because the distriblltion of stress throughout the soil mass can only be estimated in a qualitative manner.

One empirical method of estimating the settlement of the pile group due to consolidation of the underlying soil is to consider a horizontal area, slightly larger than the pile group, as being loaded uniformly with the superimposed load. Such settlement would then be computed by means of the usual theory of consolidation starting from the elevation of the

pile points. An alternative method assumes the load is

dis-tributed over the area of the pile group at an elevation

one-third up from the bottom of the piles. It is obvious that

the superimposed load should be the increase in effective load on the area in excess of that which it originally

sustained in its normal state. No frictional forces should

be assumed along the perimeter of the pile group. ｜ｾｾ･ｲ･ an

individual pile group is one of a larger group, further con-sideration is necessary because the settlement may then be influenced by the general settlement of the whole area.

When the pile group covers a large plan area unless the piles are very long, they may have little effect in

reducing the settlement either in magnitude or pattern except as they eliminate the settlement which would be contributed

by

a

very compressible upper stratum by bypassing such a

stratum.

The action of groups of friction piles must also be considered for the shearing resistance of the group as

compared to the shearing resistance of an individual pile. As outlined previously, a reciprocal action is developed

among the piles of a group. Thus, a loading test on a single

pile carried to failure will not indicate the load-carrying capacity of the group.

One method of dealing with this problem in an isolated group is to reduce the allolvable load per pile, the reduction depending upon the size of the group and the pile spacing.

Empirical formulas have been devised for this purpose. One

is given in the National Building Code based on load bearing tests by F.M. Masters (2), where the ultimate bearing capacity of several groups of piles was compared to that of single

piles. It should be pointed out that such so-called

'efficiency' formulas do not take into account the ratio of the length of pile to the pile spacing.

An alternative method is to consider the ultimate bearing capacity of the group as made up of two parts: the friction around the perimeter of the group for the depth of the piles and; the ultimate end-bearing capacity of the group as if a footing of an area equal to the enclosed area of the

group were placed at the elevation of the pile tips. This

method ｡ｳｳｾｾ･ｳ that the piles and enclosed soil must act as

a unit so that any shearing failure must consist of the whole mass punching into the underlying soil.

A third method considers the pile group to be equivalent

to a footing at the elevation of the pile tips with a ｳｯｭ･ｬｾｨ｡ｴ

larger area than that enclosed by the piles. Various methods

have been used to estimate the size of such an area.

Regardless of the methods used, it is necessary to analyze a friction pile group to be certain that it has a suitable factor of safety against shear failure and also that there will not be excessive settlement of the group due to consolidation of the underlying soil.

5.

Pile Driving Formulas

The subject of the determination of the static bearing capacity of a single pile by means of an empirical formula

based on dynamic loading has intrigued many engineers. Some

of these formulas are simple and give a quick, easy answer.

However, the answer may be entirely ｾｾｯｮｧＮ It is not proposed

here to discuss this subject which has been set forth in great detail elsewhere.

It is the writers' opinion that the only satisfactory application of a pile-driving formula would be as a check on the bearing capacity when driving into sando

6. Design of Pile Foundations

In the design of a foundation, one must consider the problem as a whole and decide if piles will be beneficial. This important decision must often be made before any piles are driven but it should not be final until a thorough explora-tion of the site has been made.

The first step is to secure accurate information about

soil characteristics over the entire site. Soil strata vary

in a most unexpected and surprising manner both horizontally and vertically, and even where uniformity appears reasonably

certain. The necessity of a thorough sctl survey carried out

by competent engineers is essential. Pl)per methods in the

hands of those capable of correct interpretation of the results

cables and all other underground structures should be accurately located well in advance of the planning stage.

The second step is to select suitable methods for obtaining the characteristics of the various soil strata. These depend on such things as ground water level and the

general geological character of the soil deposit. The work

may be carried out by one of a number of direct or semi-direct methods of exploration from which samples may be

obtained. Records of indirect methods such as penetration

tests may give valuable information when properly interpreted. The important point is, however, that whatever method or

methods are selected, the exploration should be thorough and

extensive. It should be carried out as early as possible,

sometimes even before the site is finally chosen. If properly

done, it will usually save many times its cost.

If very hard and impenetrable strata are not encoun-tered at a lesser depth, the depth to which investigations

of this sort should be carried is at least ｬｾ times the width

of the structure. The water table should always be located

and information obtained, if possible as to its seasonal variation.

In the case of non-cohesive soils, care should be

taken to determine the density state of the material. This

is not easy and sand in the loose state may not be recognized

by those inexperienced in observation of soil behaviour. In

the fluvio-glacial deposits in the Edmonton district pockets of loose sand, some of which contain water, are sometimes

encountered. Piles that encounter hard driving at one place

may be driven several feet deeper a few feet away due to

these conditions. One case of a thick, very hard gravel

deposit was underlain by such a 5tratum about

5

feet thick.

It was necessary to excavate with a soil auger to a depth

just above the loose sand. The piles were then driven the

remaining distance.

Samples of cohesive soil are generally not difficult to obtain and should be tested in a properly equipped

laboratory. The water content and the coefficient of

con-solidation together with the pressure voids ratio curve, should be obtained for all the strata that lie at or below

the contemplated level of the pile points. Similar

informa-tion should be obtained for the soil in the remolded state in which the piles will be embedded, especially in the case of point-bearing piles.

On the basis of information thus obtained, the design of the pile foundation as a whole can be carried out.

It is often advisable, of course, to drive a few test piles and conduct loading tests, especially on pile groups. However, this is a costly and time-consuming procedure and can be justified only for fairly large jobs so that more often the engineer must base his design on the soil characteristics as determined from samples.

Something should also be said regarding the loads which

finally must be supported by the pile foundation. The dead

loads, including the weight of the footings are quite easily

｣ｯｮｾｵｴ･､Ｎ Live loads are much more difficult to estimate and

the time factor is important. Non-cohesive soils compress

quickly and may continue to compress a little due to vibrations, but this process soon ends and the amount of compression is

small, except with a change from the very loose to the dense

stateo For non-cohesive soils, therefore, the total load

must be considered in the design of a pile foundation, when allowing for both the possibility of a shear failure and settlement from consolidation.

Although the cohesive soils consolidate slowly, the

amount may be quite large. For this reason it is not necessary

to account for loads due to wind, impact and snow or for other short-term live loads in pile foundations on water saturated clays when settlement due to consolidation is under

considera-tion. They simply do not act long enough to produce an

appreciable effect in transferring the load from the water

phase to the solid phase of the soil. However, the total

load should be considered in any analysis based on shearing

strength of the soil. Good judgment is required to estimate

live floor loads, for the period when the loads will be on

the floors. Warehouse loads are of long duration and should

be applied in full value to the foundations. On the other

hand, a powerhouse floor must be designed for a large load

which may be applied only for a short time during the construc-tion period and never again.

The spacing of piles should not be less than

2i

to

3

feet and not greater than 6 to 7 feet in order to keep the

cost of the footing down. A minimum edge distance of

18

inches

is usually satisfactory except for very large piles. When

friction piles are used, it is advisable to have a larger spacing than for point-bearing piles, keeping in mind the practical limits stated above.

7.

Driving Phenomena

There is no better way to learn about pile driving

than actually being on the job. Piles driven into non-cohesive

soil encounter hard driving in a high density state and a water jet or excavation may be required to penetrate such a

stratum. When the loose state is encountered, however, a consolidation takes place during driving which results in a depression of the surface of the ground around the piles

and in the vicinity. In this case, the ground should not be

excavated to the level of the bottoms of the footings before

driving. Also, the groups of piles should be driven by

starting at the perimeter of the area and driving successively

towards the centre. This requirement should be stated clearly

in the specifications since it often requires the cut-off operation immediately after each pile is driven in order to leave room to manoeuver the pile-driving equipment.

When piles are driven into a cohesive soil such as clay, especially when it is water saturated, there is no compaction of the soil but merely a displacement of a volume

equal to the volume of the piles. This results in lateral

and upward displacement of the ground. In this case, pile

driving should start at the centre of the area because if the outer piles are driven first they are often displaced laterally

and may ultimately be found outside the footing plan. In

staking out, the stakes should be set only a little in advance of the driving operations as they will be displaced if laid

out all at one time. Vertical displacement of the piles may

take place, making resetting necessary. This requires that

the inspector keep a record of the elevation of every pile after it is driven and that this elevation be checked after

the neighboring piles are driven. For such soils, the ground

surface rises between the piles by extrusion and must be excavated to the elevation of the bottoms of the footings. The amount of this excavation will be roughly equal to the

volume of the piles driven. When such excavation is necessary

a generous clear distance betvTeen piles should be planned for

to give room for mechanical equipment. The extruded soil is

broken up and not difficult to handle.

8.

Canadian Specifications

At present our National Building Code specifies the

C.S.A. Specification A56-l942 for round wooden piles. For

straightness, this requires that a chord extending from the centre of the butt to the centre of the tip shall not pass

outside the body of the pile. ｃｯｮｾｵｴ｡ｴｩｯｮ for the combined

stress in ccraprcssion and bending shows a five-fold increase over the average stress when the line of action of the end loads, assumed to be acting at the centres of the butt and tip respectively, passes just tangent to the surface at

mid-length of the pile. It seems advisable to modify this rule

for straightness to include the length as a factor in

deter-mining the limit of crookedness of a pile. It is suggested

from the centre of the cross-section of the pile to the chord where the largest deviation occurs, shall not be more than DL ,

128

where D is the average diameter of the pile in inches at that section and L the length of the pile in feet. TillIS, for a 26-foot pile, the deviation would be 0.2D or, if D ｾ 12 inches, 2.4 inches. Thf:? maximum compressive fibre stress will be given by f =

hp

[1 + ｾ ] and amounts in this case to

11DT

16

2.6 times the average stress. A point-bearing pile of mountain fir 12 inches average diameter, loaded to 25 tons will have an average compressive stress of Ａｾｏ psi and for a crooked pile

in the numerical example above, a maximum of 1150 psi. This seems a reasonable value but, as the specifications now stand, the maximum value could be 2220 psi. It is our opinion that

this is too high considering the grade of wood and the conditions, including possible slight damage due to handling and driving,

to which the pile may be subjected. On the other hand, it is to be expected that, for very long piles, the suggestion might lead to unfair elimination of timbers that would just meet the present specification. It will be noted that for 64-foot

piles the above specification could be allowed to stand. In such cases, however, the allowable average stress should be reduced.

As regards twisted grain, such piles appear to be more susceptible to splitting with hard driving than those with straight grain. The present specifications permit one full turn of the helix in 40 feeto It is our opinion that this

should be reduced to one full turn in 80 feet.

It is the authors' opinion that article 4 02.2.4.7 of the National Building Code should be clarified as to the methods of determining the allowance-bearing capacity of soil or rock supporting end-bearing piles. A person reading the Code might interpret the allowable values to be given only by Table 4.201. Obviously the alloHable bearing values for soil and rock given in Table 4.2.1 are too highly conservative to be of practical value for use under most circumstances.

References

1. Portland Cement Association. Concrete piles. Design, manufacture, driving. Chicago, Ill. 1949, 80p. 2. Masters, F.M. Timber f'r-Lc t Lon pile foundations.

Transactions A.S.C.E., vol. 108, paper No. 2174, 1943. pp 115-140.

Section

3

Introductory Remarks to the Panel Discussion on the Soil Bearing Capacity of Piles

by

D.F. Coates, Panel Moderator

ｹｾＮ Coates introduced the panel discussion on the

SolI Bearing Capacity of Piles by pointing out that due to the lack of scientific knowledge on the sUbject, millions of dollars are wasted every year on pile foundations: structures are placed on piles that need not be; other structures should

be on piles and are not. Many structures on piles either have

an excessively high,(i.e., uneconomic) safety factor or they may have a dangerously low safety factor that sometimes leads to failure.

It was hoped the panel discussion would provide two services: that it would help to transmit the knowledge of the panel to the members of the conference; and that it would point to aspects of the subject on which research and

inves-tigation should be concentrated. It was thought that the

conference time would be used to best advantage by limiting the discussion to theories and controlled empirical data applicable to the sUbject.

Section

4

(a)

"/hat is Pile Failure - \'lith Rospoct to the Soil

by

G.Go Meyerhof

Since the main function of piles is to transmit loads safely to Lower- levels of the ground, pile failure Hith respect to the supporting soil generally may be considered whenever

that function has not been fulfilled adequately. The d.esign

of piled foundations has hitherto been largely of an empirical nature in which the results of pile-driving formulas are taken as an indication of the allowable bearing capacity of piles

and the adequacy of piles in supporting the loads. Recent

work has shown, however, that this procedure is at best a check of pile-driving conditions; it is only a rough guide to the relative penetration resistance of piles in cohesionless soils and is quite unreliable in cohesive soils.

vlhile carefully conducted loading tests are at present the best means of ascertaining the bearing capacity of single piles in the field, it is now possible to estimate the allowable loads approximately from soil mechanics' principles as in other

types of foundations. Thus, rational pile design requires a

site exploration with soil tests to ascertain the nature and probable behaviour of the ground during and after construction of the works and to determine the most suitable layout of the

foundation. In addition, it is necessary to ensure the

stability of the foundation with an adequate margin of safety and to keep the settlement and any lateral movements of the piles within limits that can be tolerated by the structure. To avoid pile failure (with respect to the soil) the pile loads must be within the ultimate bearing capacity of the

soil. They are often limited to lie between third and

one-half this value in order that variations in loading, soil, and

foundation conditions are adequately covered. Moreover, it

has been found that in coh8sionless soils the bearing capacity of piles is due mainly to the point resistance, so that for a pile group, the sum of the bearing capacities of the

individual piles can be used. On the other hand, in cohesive

soils the bearing capacity is due largely to skin friction so that for a pile group the bearing capacity is usually much less than the sum of the individual pile values since it

Both poLr;.. resistance and skin friction depend on the properties of the soil, the ground-water conditions, the

lay-out of the foundations and the Hay in vrhich the piles are

installed. Since pile driving 1Tl8.y have a cons iderable effect

on the soil properties by compaction of cohesionless materials and remoulding or softening of cohesive soils, theoretical estimates using the bearing capacity theory and the results of soil tests or an extrapolation of penetration tests should

be checked by loading tests on selectod piles. Even then

careful interpretation of the rosults is required to determine the probable bearing capacity of the pile group.

vfuen individual piles are loaded to failure, the skin friction is mobilized before the point resistance reaches its

maximum value at the ultimate lord. Failupe of a friction

pile occurs therefore at a smaller settlement than tho..t of a bearing pile for vrhich the factor of safety on the ultimate

load would accordingly have to be greatorG Moreover, when a

pile group is loaded to failure, the outside piles ｣ｭｾｲｹ a

greater load than the piles near the centre of the group so

that failure of the outside piles would ｯ｣｣ｵｾ first. The

factor of safety on the ultimate load of a pile group therefore must be greater than that of single piles to ensure an

adequate margin of safe'cy against local failure.

If the piles extend fer enough beyond the depth at

l1hich the soil is subj ect to volume changes from cli:natic

influences, the a'l LowabLe p LLe loads are gover-ned by t he

settlement under structural loads within the upper limit of

bearing capacity mentioned above. The mQgnitudG, rate and

distribution of the probable settlement of a pilos foundation

can be estimated from consolidation theory and the ｲＰｾｵｬｴｳ of

Boil tests only where the piles are underlain by cohesive

soils. ｾｦｵ･ｲ･ piles are embeddod in such soils or in granular

materials, the chanse of soil properties by installation of the piles makes theoretical estimates difficult and, in the latter case, at present impossible.

Moreover, the extrapolation of loading tests on single piles is not entirely satisfactory because group action and

any long-term movements cannot be readily predicted. Thus

from these tests a failure of single piles Hould be indicated if the rate of settlement under a constant load continues undiminished or is increasing, but the present knowledge of

soil behaviour does not yet permit an estimate of the

corres-ponding state of failure for a pile group. Since the driving

of selected test piles also affects the ground in a different

manner from the driVing of a whoLe group 01" a number of groups,

it would appear that many mor-e field observations are required before the allowable pilo loads can be predicted with confidence.

On the assumption that soil conditions remain unchanged by pile driving, the settlement of a pile group increases with

the width of the group. A rough estimate of the allowable

settlement of a piled foundation can thus be obtained in a similar way to that of a spread foundation, and the allowable movements depend maiIlly on the soil type, foundation layout

and type of the structure. In any given case a check is

advisable from a study of settlement records and the structural behaviour of nearby similar buildings having essentially the

same foundation conditions.

Section

4

(b)

Cast-in-place Piles by

C.Vo Antenbring

History

This system was established in Winnipeg about 1930 for underpinning and was expanded into general use for new

construction at a later period. It is now widely used for

all types of buildings in this area.

Local conditions in Winnipeg favour the cast-in-place pile, where 30 to 60 feet of stiff clays lie over a glacial

till and rock. No boulders are found in this clay and there

is no water table to interfere with boring. Water-bearing

strata at the till or rock are sometimes found. In the

Winnipeg area the problem is to provide a cheap and reliable foundation for buildings to be constructed over clay beds, which are subject to large variations in volume due to moisture changes.

Method

Hand-turned augers were devised originally, and later these were modified and mechanical methods used to turn the

augers. These methods have now been replaced by power machinery

capable of boring holes from 16 inches to

96

inches 1n diameter

The bored hole normally stands open without sheeting

but liners can be provided where caving occurs. Holes 28

inches and larger can be cleaned off by hand at the bottom to obtain positive bearing at the desired strata.

Piles - Caissons

By local custom, piles are referred to as borings up

to 20 inches in diameter. The hole is drilled to refusal of

the auger, at or near the glacial till and as yet no satis-factory way has been developed for cleaning off the bottom.

Caissons are holes from 28 inches up that are enlarged on the bottom by hand to provide positive bearing on till or rock.

Carrying Capacity

A limited number of tests have been made and some standards have been set up by authorities such as the City

of Winnipeg's Building Inspection Department. For piles a

skin friction of 300#7sq ft on contact surface in moist clay

is used in computing the bearing capacity. In general the

holes are drilled to maximum possible depth and the top 5 or 6 feet from grade are not included in the length computation. No definite information exists on the performance of piles

in clusters. The minimum recommended spacing for piles is

4

feet. For caissons positive end bearing and values of 6

tons on glacial till and 30 tons per sq ft on rock are commonly used.

The result of this development is a satisfactory and

cheap means of support for buildings in this areao Lighter

buildings such as schools and apartments not more than two storys high are usually supported on concrete piles; heavier

buildings are supported on caissons. The cost of the complete

installation is approximately ｾｾＬＱＮＵＰ per foot for piles and

$9.00 per foot for a 28-inch diameter caisson.

At the outset there were many problems and difficulties, but with time and experience plus thousands of installations the following considerations have been answered:

1. Pile loads - end bearing or friction

2. Segregation of concrete

3 Verticality

It:

Column action

5. Steel reinforcing

6. Uplift

7.

Settlement under load

8. Types of failures 9. Contact with clay 10. Shrinkage of concrete 11. Clay mixed with concrete

Conclusions

For concrete piles the values given indicate a factor

of safety of at least 2. It is evident that the length of the

shaft limits the carrying capacity of piles to 20 to

25

tons.

With 4-ft minimum spacing it is soon necessary to use clusters

of piles with large pile caps on heavy buildings. 'l'his results

in a cost factor favouring caissons; for average conditions the cost of 4 piles plus the necessary pile caps equals the cost of one caisson.

C.aissons are more desirable because they are large enough to permit inspection at the bottom and cleaning off by hand on a satisfactory bearing strata.

Section

4

(c)

Reaction of Clays to the Driving of Friction Piles by

E.l. Rubinsky

The interaction between clays and a friction pile is a subject that has provoked many, often conflicting ideas. Vllch has been written on the subject by prominent engineers and much has been refuted since with the result that our

present day knowledge is very confused. Perhaps the best

way to illustrate the complexity of the problem is to list a few of the questions that face the designing engineer.

ｾｦｵ｡ｴ soil shear value should be used for determining

the capacity and size of the pile? Should it be based on the

undisturbed soil strength, remolded strength, or some

inter-mediate value? To what degree Hill the soil "set-up" after

driving a pile, i.e., does the soil have thixotropic properties Rnd will it consolidate, or is it liable to relax some time

after driving? Can the final penetration resistance be used

as a measure of pile capacity? What effect does the pile

shape have? How much heave should be expected? What lateral

movements may result and how will these affect existing

adjacent foundations? What effect will the displacement pile

have on the surrounding soil? Will its bearing capacity be

Heakened? Will there be "negative friction" on the piles due

The best way to approach these problems is perhaps by an analysis of the soils after driving and testing the piles,

but how often will the client pay for such ･ａｾ･ｲｩｭ･ｮｴｳ＿ The

engineer is expected to come up with the ｡ｮｳｾ･ｲｳ before

starting the field Hork, yet our knovrledge of this subject is so limited that even after field soil tests and laboratory tests we are not sure of predicting correctly the interaction

of the pile and the soil. As in most soil foundation problems,

case histories of ｳｩｭｩｬｭｾ conditions are perhaps our best

yardsticks. With the intention of adding to ｯｕｬｾ understanding

of a very complex subject, this paper describes briefly a research project undertaken mainly to study only one aspect of the problem - tho effect of pile driving on sensitive soils. It will not be possible to give a very complete picture at

this time, and only the general results of the investigations will be presented.

Studies were made of three distinct soil types found

in Eastern Canada. The soils ranged from a glacial silt to

a marine clay with the P.I.1S ranging from 10 to 25 to 43. In the three cases under study the same type of pile was used, a cast-in-place tapered, cased pile, driven with a

collapsible mandrel. All the piles were designed to carry

30-ton loads; they were driven to 2 to

3

blows per final inch

with a single acting steam hammer having a 5,000-lb ram and

a 3-ft drop. The lengths of the piles at the three sites

were between 27 and 35 feet. In all three cases the soil

formation was fairly uniform to depths of 50 to 100 feet, at which depth refusal was encountered.

The investigation procedure was generally as follows: First a pile was driven and load tested - this was possible in two of the cases only; actual pile capacity was thus established.

Borings were then made around each pile at distances varying from 2 inches to 12 feet from the perirreter of the

pile. Shelby tube samples were obtained at three levels

-,generally at the 10-, 15- and 20-foot ､･ｰｴｨｳｾ

The samples thus obtained Here examined for distortion; the unconfined compressive stir-engbb wa s then determined and

subsequently the common classification tests vrore run.

Before reviewing each of these cases singly, an interesting phenomenon should be mentioned that haG been observed sometimes during driving of this type of cased,

tapered, cast-in-place pile into sensitive soils ｾｬｩｴｨ a

collapsible mandrel. If the mandrel is 'Hit hdr-mrn immediately

after driving, the empty casing occasionally comes up uith it. This is prevented by leaving the heavy mandrel in the 8round

for varying periods before withdrawal. Job progress is

main-tained by using additional mandrels. During driving the soil

in the iu@ediate perifery of the pile becomes soft (almost a liquid), and in some cases, the pile acquires an elastic

bounce towards the completion of drivin*. After a lapse of

one half to one hour the soil seems to set-up" sufficiently

to hold the shell and thus the mandrel can be safely ｷｩｴｨ､ｲ｡ｾｮｌＮ

It is this phenomenon of sensitivity, and quick Hset-up" in particular, that prompted this study.

Case I - Fort William, Ontario. At this location a

study was made 01" one of the piles driven 27 years ago ..

The original mill, constructed in 1925, was supported

on over 2,500 thirty-ton piles. Records disclosed that the

piles averaged 22 feet in length. No records or references

were found of laboratory soil tests or pile load tests. It

was mentioned, however, that the niles were driven to 2 blows to the last inch and that on several occasions when driving was stopped over the lunch period a "considerable" number of blows were required to set the pile in motion again.

No load tests were possible at this site but a study

of the soil strengths was made. The soil at Fort William is

a slightly clayey varved inorganic silt (layers of clayey silt)

having a P.I. of 10 and a L.L. of 27. The liquidity index was

found to be 50 per cent and the peak sensitivity 10

5

to

30

It was found that the strength of the soil adjacent to the pile surpassed the strength of the soil 18 inches from

the pile. The water content was found lowest at the pile face

indicating that some consolidation must have taken place. Basing calculations on the regained strength of the soil and the surface area of the pile, this pile could conceivably

support an ultimate load of 111-0 tons, well in excess of the

30 tons for which it was designed.

Visible stratun disturbance was found most pronounced in the uppermost samples where volurae displacement was greatest ..

MaximuJn distortion occurred wi thin 8 inches of the pile and

at 18 inches almost no disturbance would be discerned.

Case II - Shawinigan Falls, Quebec. At this location

a study was made in a medium plastic clay. This soil has a

P.I. of about

25

and a L.L. of

45.

Its peak sensitivity is

between 7 and 15 and its liquidity index is approximately 130. It also has strong thixotropic properties.

Five and a half days after drLvl ug, the pile was tested

from the load test results that pile f'ad Lur-e wouLd have occurred between 105 and 115 tons.

From the laboratoI'Y tests it was found that

48

hours

after driving the strength of the soil at the pile was about

78

per cent of the undisturbed unconfined compressive strength

of the soil, prior to drivin3. Using this value in

calcula-tions, the theoretical ultimate capacity of the test pile is 109 tons which agrees with the actual results of the load test.

A study was also made on a large laboratory sample of

the thixotropic properties of this soil. The cohesive and

adhesive strengths were determined at various time intervals

up to 50 days after complete remolding. To ensure pure

thixo-tropic action, the water content was not per-mdt t ed to change.

The shear strength, as determined by vane tosto, increased from 20 lb per square foot one minute after remolding to over

200 lb per square foot 50 days after remolding. The adhesive

f'or-c e s b etnrcen soil and a smooth varnished surface as

de'cel"-mined by miniature "pull-outH _{tests, increased from}

₈

_{lb to}

ｯｾｬｹ

80

lb per square foot during the ｳｾｭ｣ time interval thus

indicating the importance of surface treatment of the pile to

prevent interface failure and to mobilize the full ｳｨ･ｯｾ

strength of the soil.

Case III - Varennes, Quebec. This was a study of a

frictionpfle driven into a soZt gl"ey marine clay of high

plasticity. The clay is faintly varved and extremely sensitive

to remolding (peak sensitivity of 8 to 15). Its P.lq is about

43

and its L.L. is about

65.

The liquidity index is 90 per

cent in the upper

30

feet of the stratum. The soil does not

appear to have as quick a thixotropic gain in strength as the clay from Shawinigan.

A load test was performed six days after driving from which it is estimated that the ultimate capacity of the pile

is around 76 tons.

This year, five years after the load test I'ms per-f'or-mcd ,

borings were made in the ｩｭｾ･､ｩ｡ｴ･ vicinity of the pile and

Shelby tube samples were obtained at various depths as in the

previous cases. It was found that the soil strength was

greater adjacent to the pile than at a distance of 3 feat

from it. An estimate of the ultimate pile capacity based on

the undisturbed, unconfined compressive strength of the soil

is

64

tons, a figure close to the actual load test result of

Conclusions and Evaluation of Results

The interaction of sensitive soils and friction piles

has been studied at three locations. The soils chosen varied

in plasticity from an almost cohesionless silt to an almost

pure clay. Conclusions should not be formulated from a few

isolated cases concerning the reaction of general soil types to pile driving; the results of this study emphasize this point and perhaps indicate that many more investigations

should be made before we condemn a sensitive soilo

In all the cases studied it was found that the full undisturbed strength of the soil had been regained in the

immediate vicinity of the pile sometime after driving. It

was also shown that ultimate pile capacities could have been estimated for these three soils on the basis of pile surface areas and shear strengths determined prior to pile driving.

The visible remolding effect of the piles seemed to be confined within the distance of one diameter of the pile.

It was found in the small-scale tests that the adhesive forces between a smooth surface and the soil were

lower than the cohesive forces within the soil. In order to

mobilize the full cohesive strength of the clay for load transfer from pile to soil and to prevent interface failure between pile and soil, the smoothness of the pile surface and the pile shape should not be disregarded.

Apparently a soil with a high liquidity index is not

necessarily incapable of carrying friction piles. Sensitivity

should not be used indiscriminately as a lneasure of a soil's

unreliability to carry a pile. The recovery of full undisturbed

strength in the soil surrounding the pile seems to indicate that even in extremely sensitive soils the damage done in the vicinity of the pile by remolding during driving, is not

necessarily permanent; it is conceivable that piles driven through a sensitive clay do not necessarily reduce the bearing capacity of the surrounding clay.

Perhaps to emphasize the lack of understanding of this very complex subject a final questioning note should be

sounded. What part does the volume displaced play in all

this? What effect does the taper of the pile have? Would

we get the same results on a straight-sided pile? How can

Section

4

(d)

Considerations of the Ultimate Bearing Capacity of Deep Concreted Footings Installed in Granular Soils

by

L.A. Fraikin

All buildings are supported either on piles supported by friction - OR - on foundation units carrying the weight of the structure down to strata capable of supporting the load.

In the first instance, the piles supported by friction will be driven usually to a penetration determined by a

formula (Engineering News formula), the coefficients of which have been found by studying the results of hundreds of

driving and loading tests of friction piles allover the world and in many types of soil.

In the second instance, except for "Hit and tube

piles resting directly on rock and acting more like laterally supported columns - how do we determine the ultimate bearing capacity of end-bearing foundation units, like caissons, piers or footings?

When computing the ultimate bearing capacity of these end-bearing units, the scientist has to assume that they will rest on soils having well known characteristics.

He has then established a general formula for the ultimate bearing capacity of these units,

If the depth to be reached is substantial, and

particularly if it is far below the water table, it becomes extremely difficult to assume that the formula can be

applied without a doubt.

If rock is reached (the ideal condition) it may still be very difficult to pour concrete on a clean, dry, horizontal

surface.

If Ithard panlt _{or clay is reached there is great danger}

of remoulding the clay when excavating, and even if this

oper ｾｴｯｮ is done with great care the swelling of the clay

When working in fine sands and silts, the problems

increase in size. It becomes almost impossible to assume that

footings can be poured in the dry unless the water table is lowered by one of the well-known methods - all time-consuming

and expensi ve.

All this does not mean that the formula must be

discarded. However, it forces the engineer to guess when he

has to estimate the amount of disturbance of the bearing strata. He has to talce safety coefficients of such magnitude that a

man of experience will find the same answer without knowing the theory of soil mechanics.

In granular soils particularly, the engineer is forced

to limit the influenc e of the depth factor ("6 D_{f Nq) which is}

so important for such soils.

There is no doubt, however, that this formula could be ｡ｰｰｬｩｾｔｕｬｬＮｙｩｦ a footing could be placea_1. miracurously,

In tile middle of a mass of sand of given characteristics

without ｡ＺｩｳｴｵｲｾＩｩｮｧ surrounding conditions.

If a large boulder in the mass of sand could be reached by a needle and this needle could be loaded, the ultimate

bearing capacity of this boulder would be as given by the previously

mentioned f'or-mu l.a and full advantage could be taken of the

depth factor.

Also, it is well-known that in all structures a sudden discontinuity in the material transmitting stress is always a source of trouble and, if failure occurs, it is always at such points.

If the boulder was loaded, the sudden change in density and shear resistance that occurs when the stresses transmitted

by ｴｾ･ needle pass from the boulder to the sand surrounding it

would certainly be a source of trouble.

If this boulder was formed to be resistant at the

point of application of the load, and this resistance decreased gradually in all directions until it reached the resistance of the mass of sand surrounding it, there would be no sudden change in resistance and the full ultimate bearing capacity given by the fornmla would be further assured.

This kind of boulder can be built. The theorL of its

ultimate bearing ｣｡ｰ｡｣ｩｾｹ is being estaDlished by studying

results of hundred of tests on such boulders. How are they made?

If a given depth can be reached in granular material with a closed-end pipe of any diameter, and if concrete is