Ch 6 Piles-modified

8/12/2019 Ch 6 Piles-modified

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Chapter 5

Piles

6th

Semester

Compiled by

Dr. Irshad Ahmad

Department of Civil Engineering

N-W.F.P. University of Engineering & Technology

Peshawar

Peshawar, N-W.F.P., Pakistan, 2008

Irshad 2008


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Table of Contents

Table of Contents ................................................................................................................................... iiChapter 5 foundation settlement ............................................................................................................ 1

5.1 Piles .............................................................................................................................................. 15.2 Uses of Piles ................................................................................................................................. 15.3 Classification of Piles .................................................................................................................. 2

5.3.1 Classification according to the mechanism of load transfer ................................................. 25.3.2 Classification of piles according to their method of installation ........................................... 45.3.3 Classification of Piles according to Materials ....................................................................... 5

5.4 Load Capacity of Piles ................................................................................................................. 75.5 Driven Piles .................................................................................................................................. 7

5.5.1 Dynamic Pile Formulas ......................................................................................................... 75.5.2 PILE DRIVING EQUIPMENT ............................................................................................ 85.5.3 HAMMER SELECTION ...................................................................................................... 9

5.6 STATIC PILE FORMULAS ..................................................................................................... 115.6.1 COHESIONLESS SOILS ................................................................................................... 12

5.7 COHESIVE SOILS .................................................................................................................... 155.7.1 Driven Piles ......................................................................................................................... 155.7.2 Bored Piles .......................................................................................................................... 15

5.8 FACTOR OF SAFETY .............................................................................................................. 185.9 NEGATIVE SKIN FRICTION .................................................................................................. 185.10 PILE GROUP........................................................................................................................... 19

5.10.1 Load Distribution in Pile Group........................................................................................ 205.10.2 Efficiency of Pile Group ................................................................................................... 205.10.3 Pile Group in Cohesionless Soils ...................................................................................... 205.10.4 Pile Group in Cohesive Soils ............................................................................................ 21

5.11 Settlement of Pile Group .......................................................................................................... 225.12 Pile Load Test .......................................................................................................................... 27

5.12.1 Ultimate Load ................................................................................................................... 305.12.2 Disadvantages ................................................................................................................... 31


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CHAPTER 5

FOUNDATION SETTLEMENT

5.1 Piles

For piles the length to width (diameter) ratio i.e. Lp/d 4 , where Lp is the pile length and d ispile diameter.

The basic situation for a pile foundation is where soft soil exists near the ground surface whichunderlain by rock formation e.g.

Figure 5-1 Pile foundation resting on hard stratum underlying a soft soil layer

5.2 Uses of Piles

Piles are commonly used for the following purposes (Figure 5-2).

To carry superstructure loads into or through a soil stratum. Both vertical and lateral loads may beinvolved.

To resist uplift or overturning forces such as for basement mats below the W.T. or to support thetower legs subjected to overturning from lateral loads such as wind.

To compact loose, cohesionless deposits through a combination of pile volume displacement anddriving vibration, thus increasing their bearing capacity.

Building

Soft Soil

Piles

Firm Soil


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To carry the foundation through the depth of scour to provide safety in the event the soil is erodedaway.

To stiffen the soil beneath machine foundations to control both amplitudes of vibration and thenatural frequency of the system.

In offshore construction to transmit the loads above the water surface through the water and intothe underlying soil. This case is one in which partially embedded piling is subjected to vertical

(and buckling) as well as lateral loads.

5.3 Classification of Piles

5.3.1 Classification according to the mechanism of load transfer

End/Point Beari ng Piles

Figure 5-2 (a) Tension pile to resist overturning movements in tall buildings (b) Shear pile to

resist horizontal forces or movements Friction pile (c) raking piles in harbor and river

(a) (b) (c)tension com ression

Wind


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If a bedrock or rocklike material is present at a site within a reasonable depth, piles can be extended

to the rock surface (figure 5-3(a)). In this case, the ultimate bearing capacity of the pile depends

entirely of the underlying material; thus the piles are called end or point bearing piles. In most of

these cases the necessary length of the pile can be fairly well established.

Instead of bedrock, if a fairly compact and hard stratum of soil is encountered at a reasonable depth,

piles can be extended a few meters into the hard stratum.

Fri ction Piles (fi gure 5-3 b)

When no layer of rock or rocklike material is present at a reasonable depth at a site, point/end bearing

piles become very long and uneconomical. For this type of subsoil condition, piles are driven through

the softer material to specified depth. These types of piles are called friction piles because the load on

the pile is resisted mainly by skin/friction resistance along the side of the pile (pile shaft). Pure

friction piles tend to be quite long, since the load-carrying capacity is a function of the shaft area in

contact with the soil.

In cohesionless soils, such as sands of medium to low density, friction piles are often used to increase

the density and thus the shear strength.

Fri ction cum end bearing piles

In the majority of cases, however, the load-carrying capacity is dependent on both end-bearing and

shaft friction (figure 5-3 c).


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5.3.2 Classification of piles according to their method of installation (figure 5-4)

Dr iven or displacement pi les

They are usually preformed before being driven, jacked, screwed or hammered into ground. This

category consists of driven piles of steel or precast concrete and piles formed by driving tubes or

shells which are fitted with a driving shoe. The tubes or shells which are filled with concrete after

driving. Also included in this category are piles formed by placing concrete as the driven piles are

withdrawn.

Bored or Replacement pi les

They require a hole to be first bored into which the pile is then formed usually of reinforced concrete.

The shaft (bore) may be cased or uncased depending upon type of soil.

Soft

ground

hard

Soft to

firm

Soft to

firm

Firm to

hard

Soft

Figure 5-3(a) End bearing pile (b) Friction pile (c) friction cum end bearing pile

(a) (b) (c)


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5.3.3 Classification of Piles according to Materials

Timber piles

Timber piles are made of tree trunks driven with small end as a point Maximum length: 35 m; optimum length: 9 20m Max load for usual conditions: 450 kN; optimum load range = 80240 kN Disadvantages: difficult to splice, vulnerable to damage in hard driving, vulnerable to decay

unless treated with preservatives (If timber is below permanent W.T. it will apparently lastfor ever), if subjected to alternate wetting & drying, the useful life will be short, partly

embedded piles or piles above W.T. are susceptible to damage from wood borers and other

insects unless treated.

Advantages: comparatively low initial cost, permanently submerged piles are resistant todecay, easy to handle, best suited for friction piles in granular material.

(a) (b) (c) (d) (e) (f)

Figure 5-4 Principal types of pile: (a) precast RC pile (b) steel H pile (c) shell

pile (d) concrete pile cast as driven tube withdrawn (e) bored pile (cast in

situ), (f) under-reamed bored pile (cast in situ)


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Steel Piles

Max length: practically unlimited, optimum length: 1250 m load for usual conditions = maximum allowable stress x-section area, Optimum load range = 3501050 kN The members are usually rolled HP shapes/pipe piles. Wide flange beams & I beams

proportioned to withstand the hard driving stress to which the pile may be subjected, In HP

pile the flange thickness= web thickness, pipe piles are either welded or seamless steel pipes,

which may be driven either open ended or closed end. Closed end piles are usually filled with

concrete after driving. Open end piles may be filled but this is not often necessary.

Advantages: easy to splice, high capacity, small displacement, able to penetrate through lightobstructions, best suited for end bearing on rock, reduce allowable capacity for corrosivelocations or provide corrosion protection.

Disadvantages: Vulnerable to corrosion, HP section may be damaged/deflected by majorobstruction

Concrete Pi les

Concrete piles may be precast, prestressed, cast in place, or of composite construction. Precast concrete pilesmay be made using ordinary reinforcement or they may be prestressed.

Pecast piles using ordinary reinforcement are designed to resist bending stresses duringpicking up & transport to the site & bending moments from lateral loads and to provide

sufficient resistance to vertical loads and any tension forces developed during driving.

Prestressed piles are formed by tensioning high strength steel prestress cables, and castingthe concrete about the cable. When the concrete hardens, the prestress cables are cut, with the

tension force in the cables now producing compressive stress in the concrete pile. It is

common to higher-strength concrete (35 to 55 MPa) in prestressed piles because of the large

initial compressive stresses from prestressing. Prestressing the pile tends to counteract any

tension stresses during either handling or driving.

Max length: 1015 m for precast, 2030 m for prestressed Optimum length: 1012 m for precast, 1825m prestressed Loads for usual conditions 900 for precast, 8500 kN for prestressed Optimum load range: 3503500 kN Disadvantages: difficult to handle unless prestressed, high initial cost, considerable

displacement, prestressed piles are difficult to splice.


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Advantages: high load capacities, corrosion resistance can be attained, hard driving possible Remarks: cylinder piles in particular are suited for bending resistance. Cast in place concrete piles are formed by drilling a hole in the ground & filling it with

concrete. The hole may be drilled or formed by driving a shell or casing into the ground.

Disadvantages of Concrete piles: Concrete piles are considered permanent, however, certainsoil (usually organic) contain materials that may form acids that can damage the concrete.

Salt water may also adversely react with the concrete unless special precautions are taken

when the mix proportions are designed. Additionally, concrete piles used for marine

structures may undergo abrasion from wave action and floating debris in the water. Alternate

freezing & thawing can cause concrete damage in any exposed situation.

Composite pil es

In general, a composite pile is made up of two or more sections of different materials ordifferent pile types. The upper portion could be cased cast-in-place concrete combined with alower portion of timber, steel H or concrete filled steel pipe pile. These piles have limited

application and are employed under special conditions.

5.4 Load Capacity of Piles

Three general methods are available to establish load capacity:

(1) Static Analysis (2) Dynamic Analysis (3) Load Testing (4) Correlation with field tests (SPT,CPT etc)

Dynamic formulae are used for driven piles. Static formulae are used both for bored and driven piles.

Load testing is the most reliable method to determine the load capacity of the pile in the field. They

should be performed on all piling projects. However, they are considerably more expensive than the

other methods used to determine pile capacity, and economic consideration sometimes preclude their

use on projects. Field tests like SPT, CPT are also used to correlate to load carrying capacity

particularly for cohesionless soils.

5.5 Driven Piles

5.5.1 Dynamic Pile Formulas

Piles are usually forced into the ground by a pile driver or pile hammer. In medieval times piles were

driven by men manually swinging hammer, which consists of a weight raised by ropes or cables and

allowed to drop freely striking the top of the pile. After the drop hammer came the single acting

hammer, double acting hammer, differential acting hammer, diesel pile hammer, vibratory driver.

Dynamic pile formulas are widely used to determine the static capacity of the driven pile. These

formulas are derived starting with the relation

Energy Input = Energy Used + Energy Lost


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The Energy used equals the driving resistance (Pu) the pile movement (s).

Energy lost is due to friction, heat, hammer rebound, vibration and elastic compression of the pile, the

pacing assembly, and the soil.

5.5.2 PILE DRIVING EQUIPMENT

Piles are installed by a special pile driving device know as a pile hammer. The hammer may be

suspended from the boom of a crawler crane, supported on a large frame called a pile driver or carried

on a barge for construction in water. In all cases, the hammer is guided between two parallel steel

members called leads. The leads may be adjusted at various angles for driving vertical and batter

piles.

Several types of hammers are in use and each of which are different sizes. The hammer types are:

Drop hammer

The drop hammer consists of a heavy ram in between the leads. The ram is lifted up to a certainheight and released to drop on the pile. This type is slow and therefore not in common use. It is used

in the cases where only a small number of piles are driven.

Single-acting hammer

In single acting hammer a heavy ram is lifted up by steam or compressed air but dropped by its own

weight. The energy of a single acting hammer is equal to the weight of the ram times the height of

fall.

Double-acting hammer

The double-acting hammer employs steam or air for lifting the ram and for accelerating thedownward stroke. The energy of a double-acting hammer is equal to the (weight of the ram + mean

effective pressure x the effective area of ram) x times the height of fall.

Di esel hammer

The diesel hammer is a small, light weight and highly mobile. They use gasoline for fuel. To start the

operation, the ram is raised, and the fuel is injected. As the ram is released, the ram falls and

compresses air and fuel. The air and fuel becomes hot because of the compression and the air-fuel

mixture is ignited. The resulting explosion (1) advances the pile and (2) lifts the ram. If the pile

advance is very great as in soft soils, the ram is not lifted by the explosion sufficiently to ignite the

air-fuel mixture on the next cycle, requiring that the ram be again manually lifted.

Vibratory hammer

The principle of the vibratory driver is two counter-rotating eccentric weights. The driving unit

vibrates at high frequency and provides two vertical impulses-one up and one down. The downward

pulse acts with the pile weight to increase the apparent gravity force. These hammers have reduced

driving vibrations, reduced noise, and great speed of penetration.


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5.5.3 HAMMER SELECTION

Generally the size of hammer is more important factor than type of hammer.

A heavy pile should be driven by a heavy hammer delivering large energy. Preferably the weight of

the hammer should be at least on-half the total weight of the pile, and the deriving energy should be atleast one foot-pound for each pound of pile weight.

Each type of hammer has its use under suitable conditions. The advantages and disadvantages of each

type are summarized below:

Single-acting hammer

They are advantageous when driving heavy piles in compact or hard soils; the heavy ram striking at

low velocity produces least damage due to impact. The disadvantages are low driving speed and

large headroom requirement.

Double-acting hammer

They are generally used to drive piles of light or moderate weight in soils of average resistance

against driving. This type of hammer can drive piles at fast speed, requires less headroom and can be

used to extract piles by turning them [i.e. the double-acting hammer] upside down.

Diesel hammer

They are similar in application as double-acting hammers, but driving may become difficult in

extremely soft ground.

Vibratory hammer

They have fairly good results in silty and clayey deposits. They are used in heavy clays or soils with

appreciable numbers of boulders. These hammers have reduced driving vibrations, reduced noise, and

great speed of penetration.


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5.6 STATIC PILE FORMULAS

The ultimate load which can be carried by a pile is equal to the sum of the base resistance and theshaft resistance (figure 5-5).

Pu+ Wp= Abqb(gross)+ Asqs

Pu is the ultimate load that can be carried at top of pile, qbultimate (gross) bearing capacity at base

level, Ab= base area of pile, qs=ultimate shearing/skin resistance per unit area, As= perimeter area of

pile, and Wp = weight of the pile.

Subtracting Ws from both sides of the equation. Where Ws is effective soil weight replaced/displaced

due to pile volume. Ws = LAb where is the effective weight of soil, and L is pile length.

Pu+ (WpWs)= Abqb(gross)+ Asqs-Ws

Pu+ (WpWs) = Abqb(gross)+ Asqs - LAb

Pu+ (WpWs) = (qb(gross)- L)Ab+ Asqs

Pu+ (WpWs) = (qb(gross)- o)Ab+ Asqs

Where o=L is effective vertical stress at pile base

Pu+(WpWs) = Abqb+ Asqs

Pu= Abqb+ Asqs(WpWs)

Pu= Abqb+ AsqsW

Pu is the ultimate load that can be carried at top of pile, qb ultimate (net) bearing capacity at base

level, Ab= base area of pile, qs=ultimate shearing/skin resistance per unit area, As= perimeter area of

pile, W= WpWs = weight of the pile effective weight of soil replaced. In most cases WpWsand hence W0.

Pu= Abqb+ Asqs

However in the case of under-reamed piles (figure 5-4 f) the reduction in pressure on the soil at base

level due to the removal of soil is greater than the subsequent increase in pressure due to the weight of

the pile and hence use equation-1 (i.e. do not assume that WpWs)


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5.6.1 COHESIONLESS SOILS

End beari ng Resistance (qb)

The ultimate B.C. and settlement of a pile depends mainly on the relative density of sand. However, ifa pile is driven into sand the relative density adjoining the pile is increased by compaction due to soil

displacement (except in dense sands, which may be loosened). The soil characteristics governing

ultimate bearing capacity and settlement, therefore, are different from the original characteristics prior

to driving. This fact, in addition to the heterogeneous nature of sand deposits, makes the prediction of

pile behavior by analytical methods extremely difficult.

The ultimate (net) B.C. at base level can be expressed as

qb= cNc+ oNq+ B N-o[ois subtracted to get net value of qb]

Where ois the effective overburden pressure at base level of pile.

qb= cN

c+

o(N

q-1) + B N

c=0 for sand and 1/2BN term can be neglected because the B (width/diameter of pile) is smallcompared to the length of pile. so

qb= o(Nq-1) oNq (Nq-1) Nq [because reduction of Nq by 1 is a substantialrefinement not justified by estimated soil parameters]. Nqis a B.C. factor (figure 5-6)

qs

qb

WP

Figure 5-5 Free body diagram of a pile

Pu


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Figure 5-6: Values of Nq for pile formulae (Meyerhof, 1976)

Fri ction/Shaft Resistance (qs):

The average value of skin resistance (qs) over the length of pile embedded in sand can be expressed as

tan''cqs

Where'' osK and c=0

tan'oss Kq

Ksa coefficient of earth pressure dependent largely on the relative density of the soil, ' = averageeffective pressure in the layer perpendicular to qs (i.e. horizontal).

'

o = average effective vertical

overburden pressure in the layer, and =angle of friction between the pile and the soil.

Table 5-1 Ksand values

Pile TypeKs

Loose Sand Dense Sand

Concrete 3/4 1.0 2.0

Steel 20 0.5 1.0

Wood 2/3 1.5 4.0

Bored or Jetted

pile0.5


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Variation of qband qswith depth

The above equations for qband qsindicate a linear increase with depth of qband qs. However, tests on

full scale and model piles have shown that these equations are valid up to certain depth called critical

depth (zc). Below this depth zc(=15d to 20d, conservatively take zc= 15 d), base (qb) and shaft/skin(qs) resistances do not develop linearly [i.e. become constant]. This is because the vertical effective

stress adjacent to the pile is not necessarily equal to the effective overburden pressure (away from the

effect of pile) but reaches a limiting value at critical depth zc(figure 5-7).

qband qsfr om SPT Test

Ultimate base resistance qb

Due to the critical depth limitation and to the difficulty of obtaining values of the required parameters,

the above equations are difficult to apply in practice. It is preferable to use empirical correlations

The following empirical correlations have been proposed by Meyerhof for driven piles in sand.

qb= (40N55)Lb/B 400N55 (kN/m2)

N= is the value of standard penetration resistance in the vicinity of the pile base. Use any applicable

SPT N corrections discussed in earlier chapter-3.

B = width or diameter of pile point

Lb= the length of pile embedded in sand

W.T.

d

oc

d

oc

zc=15 dzc=15d

Figure 5-7 Effective vertical stress distribution diagram adjacent to pile


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Skin f ri ction resistance

Skin friction resistance is Nqs 2 [kN/m2]

WhereN

is the average value of standard penetration resistance over the embedded length of the pilewithin the sand stratum.

The values of qsshould be halved in the case of small displacement piles such as steel H piles. For

bored piles the values of qband qsare approximately 1/3 and 1/2, respectively, of the corresponding

values for driven piles.

5.7 COHESIVE SOILS

5.7.1 Driven Piles

In the case of driven piles, the clay adjacent to the pile is displaced both laterally and vertically.

Upward displacement of the clay results in heaving of the ground surface around the pile and cancause a reduction in the bearing capacity of adjacent piles already installed. The clay in the disturbed

zone around the pile is completely remoulded during driving. The excess porewater pressure set up by

the driving stresses dissipates within a few months as the disturbed zone is relatively narrow (of the

order B): in general, dissipation is virtually complete before significant structural load is applied to

the pile. Dissipation is accompanied by an increase in skin friction. Thus the skin friction at the end of

dissipation is normally appropriate in design.

5.7.2 Bored Piles

In the case of bored piles, a thin layer of clay (of the order of 25 mm) immediately adjoining the

shaft will be remoulded during boring. In addition, a gradual softening of the clay will take place

adjacent to the shaft due to stress release, pore water seeping from the surrounding clay towards the

shaft. Water can also be absorbed from wet concrete when it comes in contact with the clay.

Softening is accompanied by a reduction in shear strength and a reduction in skin friction.

Construction of a bored pile, therefore, should be completed as quickly as possible. Limited

reconsolidation of the remoulded and softened clay takes place after installation of the pile.

Base Resistance

The relavent shear strength for the determination of the base resistance of a pile in clay is the

undrained strength at base level. The ultimate bearing capacity is expressed as

qb= cuNc+ oNq+ B N-o [ois subtracted to get net value of qb]

1/2BNterm can be neglected because the B (width/diameter of pile) is small compared to the lengthof pile and for u=0, Nq=1, we get

qb= cuNc where Nc= 9, and cu is undrained shear strength at pile base

Skin Resistance


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Total Stress (Undrained Conditions u=0)

Total stress analyses are relevant, for example, for a short-term loading conditions such as from wind

or earthquake loads. These loads are so rapidly applied that excess pore water pressure is developed

in the clay- the clay is under undrained condition. In such cases-total stress parameters are used.

)tan( uas cq [where cais average adhesion]

0)tan( u as u=0

Hence uas ccq is a coefficient depending on type of clay, the method of installation, and the

pile material. The appropriate value of is obtained from load tests. Values of range from 0.3 to 1.

uc is the average undrained shear strength. One difficulty with this approach is that there is usually a

considerable scatter in the plot of undrained shear strength against depth and it may be difficult to

define the value ofu

c . See figure 5-8 for different values of .

Effective Stress (drained conditions)

An alternative approach is to express skin friction in terms of effective stress. The zone of soil

disturbance around the pile is relatively thin, therefore dissipation of the positive or negative excess

pore water pressure set up during installation should virtually be complete by the time the structural

load is applied. In principle, therefore, an effective stress approach has more justification than one

based on total stress. In terms of effective stress the skin friction can be expressed as

)'tan(' ' oss Kcq

)'tan(' oss Kq [c=0 for saturated clay under drained conditions]

Where Ks is the average coefficient of earth pressure and 'o is the average effective overburden

pressure adjacent to the pile shaft. Failure is assumed to take place in the remoulded soil close to the

pile shaft, therefore the angle of friction between the pile and the soil is represented by the angle of

shearing resistance in terms of effective stress () for the remoulded clay: the cohesion intercept forremoulded clay will be zero. The above equation can also be written as

'

osq [where )'tan( sK ]

Approximate value of can be deduced by making assumptions regarding the value of Ks, especiallyin the case of normally consolidated clays. However, the coefficient is generally obtained empirically

from the results of load test carried out a few months after installation. Correlations with loading tests

have shown that for soft clays falls within a narrow range of values (0.25 to 0.4), irrespective of theclay type.


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Figure 5-8 Relationship between the adhesion factor and undrained shear strength su


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5.8 FACTOR OF SAFETY

The base resistance requires a larger deformation for full mobilization than the shaft resistance,

therefore different values of load factor may be appropriate for the two components, the higher factor

being applied to the base resistance.

In the case of large-diameter bored piles, including underreamed piles, the shaft resistance may be

fully mobilized at working load and it is advisable to ensure a load factor of 3 for base resistance,

with a factor of 1 for shaft resistance, in addition to the specified over all load factor (generally 2) for

the pile.

5.9 NEGATIVE SKIN FRICTION

When piles are driven through a layer of fill material which slowly compacts or consolidates due to

its own weight, or if the layers underlying the fill consolidate under the weight of the fill, a downward

drag is imposed in the pile shaft (figure 5-9).

The skin friction between the pile and soil therefore acts in a downward direction. The force due to

this downward or negative skin friction is thus carried by the pile instead of helping to support the

external load on the pile.

Negative skin friction increases gradually as consolidation of the clay layer proceeds, the effective

overburden pressure gradually increasing as the excess pore water pressure dissipates.

qsN

qb

qsN

qs

Fill: consolidating

under own weight

Soft clay: consolidating

under weight of fill

Firm or Hard bearing

layer

Figure 5-9 Negative skin friction

Pu


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Pu+ qsNAs = qbAb+ qsAs where qsN is negative skin friction (downward), AsN is thecorresponding surface area of pile, qsis the skin friction (upward) and Asis the corresponding surface

areas of pile, Abis the end bearing area of pile.

To calculate negative skin friction equation

'

oNsq can be used. In normally consolidated clays,a value of =0.25 represents a reasonable upper limit to negative skin friction for preliminary design

purposes.

It should be noted that there will be a reduction in effective overburden pressure adjacent to the pile

in the bearing stratum due to the transfer of part of the overlying soil weight to the pile: if the bearing

stratum is sand, this will result in a reduction in bearing capacity above the critical depth.

5.10 PILE GROUP

Rarely is the foundation likely to consist of a single pile. Generally, there will be a minimum of two

of three piles under a foundation element or footing to allow for misalignments and other inadvertent

eccentricities.

The group of piles is installed fairly close together (typically 2B-4B where B is the width or diameter

of a single pile) and joined by a slab, known as the Pile Cap, cast on top of the piles.

The cap is usually in contact with the soil in which case part of the structural load is carried directly

on the soil immediately below the surface. The group of piles in this case is called piled foundation. If

the cap is clear of the ground surface, the piles in the group are referred to as free-standing (figure 5-

10).

Pile CapPile Cap

(a) (b)

Figure 5-10 (a) A group of free-standing piles (b) A group of piled foundation


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5.10.1 Load Distribution in Pile Group

It is generally assumed that the load distribution between the piles in an axially loaded group is

uniform.

However experimental evidence indicates that for a group in sand the piles at the center of the groupcarry greater loads than those on the perimeter.

In clay, on the other hand, the piles on the perimeter of the group carry greater loads than on those at

the center.

5.10.2 Efficiency of Pile Group

In general the ultimate load which can be supported by a group of N piles is not equal to N times the

ultimate load of a single isolated pile of the same dimensions in the same soil. Where N is the number

of piles in a group. So

[in general] Ultimate load of Pile Group N Ultimate load of a single pile

The ratio of the average load per pile in a group at failure to the ultimate load for a single pile is

defined as the efficiency of the group ().

Average load per pile in a group at failure = Ultimate group load / N

= ( Ultimate group load ] / ( N Ultimate Individual load )

5.10.3 Pile Group in Cohesionless Soils

Dr iven Piles

The driving of a group of piles into loose sand or medium-dense sand causes compaction of the sand

between the piles, provided that the spacing is less than about 8B: consequently the efficiency of thegroup is greater than unity. The maximum efficiency is reached at a spacing of 2 to 3 diameters and

generally ranges between 1.3 to 2. It is recommended that in this case the design value of =1 betaken.

In the case of piles driven into dense sand, the group efficiency is less than unity due to loosening of

the sand and the overlapping of zones of shear (figure 5-11).

Bored Pil es

However, for a group of bored piles the efficiency may be as low as 2/3 because the sand between the

piles is not compacted during installation but the zones of shear of adjacent piles will overlap.


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Figure 5-11 Stress surrounding a friction pile and the summing effects of a pile group

5.10.4 Pile Group in Cohesive Soils

A closely spaced group of piles (spacing = 2B to 3B) in clay may fail as a unit, with shear failure

taking place around the perimeter of the group and below the area covered by the piles and theenclosed soil. This is referred to as Block Failure. (figure 5-12)

The ultimate load in the case of a pile group which fails as a block is given by

usgbbgug cAqAP

Four piles contributing

to this stress zone

Three piles contributing to

this stress zone

Two piles contributing to

this stress zone


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Abg = base are of the group = Bg Lg; Asg= perimeter area of the group = 2D(Bg + Lg); cu =undrained shear strength at depth D

uc = average undrained shear strength between 0 and D below the ground.

qb=cuNc where Nc=5.14(1+0.2Bg/Lg)[1 + (0.053D/Bg)]9

)(2 gguggucug LBDcLBcNP

Design Ul timate Load

Piled Foundation

The ultimate load should be taken as the lesser of the

Block Failure value (2) The sum of the individual pile values

Free Standing Group of Piles

The ultimate load should be taken as the lesser of the

(1) Block Failure value (2) 2/3 of the sum of the individual pile values

5.11 Settlement of Pile Group

The settlement of pile group is always greater than the settlement of a corresponding single pile, as a

result of the overlapping of the individual zones of influence of the piles in the group. The bulbs of

pressure of a single pile and a pile group (with piles of the same length as the single pile) are of the

form illustrated in figure 5-13. The settlement ratio of a group is defined as the ratio of the settlement

of the group to the settlement of a single pile when both are carrying the same proportion of theirultimate load.

5.11.1 Settlement of pile group in clay

In order to estimate settlement for a pile group, it is assumed that the total load is carried by an

Equivalent raft located at a depth of 2L/3 where L is the length of piles (figure 5-14). It may be

assumed as shown in figure 5-14 that the load is spread from the perimeter of the pile group at a slope

of 1 horizontal to 4 vertical to allow for that part of the load transferred to the soil by skin friction.

The vertical stress increment at any depth below the equivalent raft may be estimated by assuming in

turn that the total load is spread to the underlying soil at a slope of 1 horizontal to 2 vertical.

It should be appreciated that settlement is normally the limiting design criterion for pile groups inboth sands and clays.


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Pile CapPile Cap

(a) A group of free-standing piles (b) A group of piled foundation

Lg

Bg

DD

Lg

Bg

D

Figure 5-12Block failure of pile group in clay

(c ) Dimensions of Failure block


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Figure 5-13 Bulbs of pressure for a single and a pile group

Figure 5-14 Equivalent raft concept

5.11.2 Settlement of pile group in sand


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Solution


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5.12 Pile Load Test

The loading of a test pile enables the ultimate load to be determined directly and provides a means of

assessing the accuracy of predicted values.

Tests may also be carried out in which loading is stopped when the proposed working load has been

exceeded by a specified percentage.


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Figure 5-15 shows a schematic diagram of the pile load test arrangement for testing in axial

compression in the field. The load is applied to the pile by a hydraulic jack. The load is applied in

suitable increments, allowing sufficient time between increments for settlement to be substantially

complete. According to ASTM D1143, the test pile is loaded in eight equal increments up to a

maximum load, usually twice the predetermined working (allowable) load. Unloading stages arenormally included in the test program. This testing procedure is Maintained load test (or Controlled

load test).

In constant rate of penetration (CRP) test the pile is jacked into the soil at a constant speed, the

load applied in order to maintain the penetration being continuously measured. Suitable rates of

penetration for tests in sands and clays are 1.5 mm/min and 0.75 mm/min respectively.

Another type of pile load test is cyclic loading, in which an increment load is repeatedly applied and

removed.

Driven piles in clays should not be tested for at least a month after installation to allow most of the

increase in skin friction (a result of dissipation of the excess pore water pressure due to the driving

stresses) to take place. Load tests on piles in sand can be carried out immediately after the piles aredriven.


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5.12.1 Ultimate Load

Figure 5-15 shows load settlement diagram obtained from fried loading and unloading. For any load

Q, the pile settlement can be calculated as follows. When Q=Q1,

net settlement, snet(1)= st(1)se(1)

When Q=Q2

net settlement, snet(2)= st(2)se(2)

and so on.

Where snet= net settlement

se= elastic settlement of the pile itself


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st= total settlement

These values of Q can be plotted in a graph against the corresponding net settlement, s netas shown in

figure 5-15 (c). The ultimate load of the pile can be determined from this graph. Pile settlement may

increase with load to a certain point, beyond which the load settlement curve becomes vertical. The

load corresponding to the point where Q-snet becomes vertical is the ultimate load, Qu, for the pile. It

is shown by curve 1 of the figure 5-15 (c ).

In many cases, the latter stage of the load-settlement curve is almost linear, showing large degree of

settlement for a small increment of load; it is shown by curve 2 in figure 5-15 ( c). The ultimate load

for such cases is determined from the point of the curve where this steep linear portion starts.

5.12.2 Disadvantages

The performance of single pile does not correspond to actual conditions of performance underneath

the structure within the entire group of piles. (2) The loading test must be performed at the actual

construction site and under real conditions of the blueprint conditions which are often difficult to

fulfill and to execute. (3) This method of test requires specially heavy, sturdy equipment and

platforms, precise settlement measuring devices, large quantities of dead load, or powerful hydraulic

jacks. (4) The aforementioned conditions and factors make this kind of pile bearing capacity test very

expensive.


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Figure 5-15 (a) Schematic diagram of pile load test arrangement; (b) plot of load against total

settlement (c) plot of load against net settlement


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Figure 5-16 Schematic setup for applying vertical load to the test pile using a hydraulic jack

acting against an anchored reaction frame


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Problem 5-1Single Pile in Sand

A 12m long, 305mm square section pile is to be embedded in sand. Water table is encountered at 2m

depth below the ground surface. Sand has the following properties: =16.8 kN/m3above WT, sat=18kN/m3, =35. Angle of friction between soil and pile is taken to be =0.6, lateral earth pressure

coefficient Ks=1.4. Calculate the ultimate compressive load.

Solution:

End-bearing resistance:

qb= oNq (Nq=42 given)

qb= 54.742=2297.4 kPa

Friction resistance:

tan'oss Kq

First find average vertical effective stress along the pile length, it is equal to the area

under vertical effective stress distribution diagram divided by the length of pile.

2' kN/m46=4.575)-(1254.7+2)-(4.57554.7)/2+(33.6+2)/2(33.612/1 o

kPakKq oss 7.24)356.0tan(464.1tan'

Ultimate compressive load (Pu)

Pu = Abqb+ Asqs

Ab=0.3052= 0.093 m2, As= 40.30512= 14.64 m

2

Pu= 0.0932297.4 + 14.6424.7 = 213.6 + 361.6 = 575.2 kN

L=12m

zc=150.305=4.575 m

16.82=33.6 kPa

(18-9.81)(4.575-2)+33.6

=54.7 kPa

o=54.7 kPa

2m


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Example 5-2 Single Pil e Capacity in Sand using SPT

A precast concrete pile 450 mm square in section and 9 m long is to be driven into a river bed which

consists of a depth of sand. The standard penetration resistance (N) at the pile base is 24, and the

average value of N along the pile length is 13. Calculate the ultimate compressive and tensile loadcarrying capacity of the pile.

Solution

N = 24

13N

Lb= 9.0 m

Ultimate compressive load capacity = Abqb+ Asqs

Ultimate tensile load capacity = Asqs +Wp (weight of pile)

qb= (40N)Lb/B 400N (kN/m2)

qb = 40 24 9/ 0.45 =

Nqs 2 [kN/m2]

=2 13 = 26 kN/m2

Solve yourself

Problem 5-3, Single Pile in Clay

A 400mm, square section concrete pile is driven to an embedded depth of 12m in a cohesive soil,

which has the following properties, u=0, =20 kN/m3both above & below W.T., cu at 12m depth is85.4 kPa. The water table is at a depth of 3m. Assume =0.4. Calculate safe load capacity for the pileadopting a FOS of 3 for the base shear and factor of safety of 2.5 for skin resistance.

L=12m

203=60 kPa

(20-9.81)(12-3)+60=151.7 kPa

3m


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Solution

End bearing resistance

kPacNq cb 6.7684.8599c

Skin/friction resistance:

osq

Let us find average effective overburden pressure

kPao 9.86)]312(2/)607.151(2/)603[(12/1

kPaq os 7.349.864.0

Ultimate & Allowable Compressive loads:

ssbbu qAqAP

kNPu 7922.6668.1257.3412)4.04(6.786)4.04.0(

kNPa 3085.2

2.666

3

8.125

Problem 5-4Pil e Group in Clay

Determine the safe load capacity for a square group of 9 piles in cohesive soil. Safety factor 2.5

against block failure. =20 kN/m3, cu at the base 85.4 kPa, average undrained shear strength =60.2kPa, u=0. Pu for single pile is equal to 819 kN.

Given Data

No of piles = n = 9, =20 kN/m3, cu (base) = 85.4 kN/m2, cu (avg.) = 60.2 kPa

Pu(single pile) = 819 kN

Required

Safe Load Capacity of pile group = Pug

Solution

ssbbgpu qAqAP )(


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2/2.60 mkNcq us

cub Ncq

Find Ncby using the relation:-

9053.012.0114.5

gg

g

cB

D

L

BN

925.2

12053.01

25.2

25.22.0114.5

cN

945.9 cN

So take 9cN

kPaNcq cub 6.76894.85

20625.525.225.2 mAb

depthParameterAs

210812)25.225.2(2 mAs

kNP gpu 103922.601086.7680625.5)(

Ultimate load for piled foundation (Pile cap resting over group).(a) Base Failure Value=10392 kN(b) Such as of induced pile cap=8199=7371 kN

Minimum of (a) & (b) is selected for Pu(group)= 7371 kN

kNFOS

PP uallowable 4.2948

5.2

7371

12 m

2.25 m

2.25 m

Documents

Ch 6 Piles-modified