bearing piles and groups

7/31/2019 Bearing Piles and Groups

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Bearing Piles and GroupsOctober 2010

University of Bolton

School of Built Environment and Engineering

BEARING PILES AND GROUPS

CONTENTS:

1. Introduction2. Types of bearing pile3. Pile types in more detail.4. Load capacity of the pile shaft5. Load capacity of the soil.

a. Capacity by calculation from soil propertiesi. Ultimate capacity

1. General principles2. Piles in cohesive soils3. Piles in non-cohesive soils4. Time effects

ii. Working capacity, factors of safetyb. Capacity from driving formulae

6. Settlement of Piles7. Pile testing

a. Load testingb. Indirect testing methods

8. Tolerances, spacing, pile caps and ground beams9. Piles in tension10.Downdrag (negative skin friction)11.Laterally-loaded piles12.Capacity of pile groups.

NB: These notes do not cover retaining walls formed by sheet piles or diaphragm walls

1 Introduction

The function of a bearing pile is to transfer loads to lower levels of the ground which are

capable of sustaining the load, with an adequate factor of safety and without settling at the

working load by an amount detrimental to the supported structure

Bearing piles are used:

Where adequate bearing soil is at low depthWhere loading is uneven, thus making the use of a raft unadvisable

In shrinkable clay soils, where loads can be transferred to below the zone of shrinkage

Piles are normally used in compression. Sometimes piles have to carry tension, as shown in

the diagrams of a piled quay on the next page. Generally the lateral load capacity of piles is

much less than their capacity for tension or compression.


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Illustrations of Tension Piles used to Sustain Lateral Forces

2 Types of Bearing Pile

Plies can be classified:A By method of installation:

Bored piles

Driven piles.

B By the way that the soil is moved to make way for the pile:

Displacement Piles

Replacement PilesC By the way that they carry load :

Friction piles

End-bearing piles

Combined friction and end-bearing piles.D By material and installation

Steel piles

Pre-cast concrete piles

Cast-in-situ concrete piles

Timber piles.

3 Pile types in more Detail

Displacement Piles

The pile is driven, jacked or vibrated into the ground. The soil is displaced outwards but isnot actually removed:

Displacement Piles: Pre-formed

These are driven by heavy hammer blows until the required set is achieved. (ie number of

blows for 100mm penetration). Various devices (pile frames, hanging leaders, suspended

hammer guides) are used to keep the pile upright (or raked) and to align the hammer with the

top of the pile.

Pile hammers may be drop hammer, single-acting hammer or double acting hammer.

The first of these is a simple weight, the other two are specially designed machines, normally

hydraulically powered, that can deliver blows much faster than a drop hammer.

To prevent damage to the top of the pile, a cushion or helmet is used.

Ship Pull Quay

Pile inCompression

Pile inCompression Pile in

Tensionn

Ship ImpactQuay

Pile in

Compression

Pile in

Tension


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Water-jetting is sometimes used to aid pile penetration, and vibration is and alternative

driving method in granular soils. Where sufficient dead-weight or reaction is available, piles

can be jacked into place.

Driven Pre-Formed

Piles

Notes

Steel H piles H Small displacement

Tube piles O

Box piles

Pre-Cast Concrete Long Piles Made on site: heavily

reinforced or pre-

stressed to withstand

handling and driving

Modular jointed piles West Hardrive

Johnson Herkules

Timber Limited length

A special case is the screw pile, in which the helical head is both the boring device and theloadbearing element. These are mainly used in sand, and are found under Victorian seaside

piers.

Displacement Piles: Driven Cast-in-Place

A casing with a closed end is bottom driven into the ground to a set or a pre-determined

depth. The casing may be temporary or permanent.

Temporary casing. The casing is normally steel, and the closed end is a plug of dry

concrete or gravel which is driven out at the required depth. The empty casing can

then be inspected from the top. The pile is filled with concrete and the casing is

carefully withdrawn. The correct driving and extraction of the casing is a skilled job.

Permanent casing. The casing is usually a stack of pre-cast concrete tubes, sometimesa steel tube. The closed end is a steel shoe. After driving the empty casing can be

inspected from the top. The casing is then filled with concrete.

Replacement Piles can be divided into

Bored and cast-in-place piles

Drilled-in Tubular Piles.

Replacement Pile: Bored and Cast-in-Place

By Mechanical Auger. In stable ground, an unsupported hole can be drilled with amechanical auger. In suitable ground an enlarged base, or under-ream can be formed. A light

reinforcement cage, if required, is placed in the hole, followed by concrete.

In water-bearing soils support is required to the sides of the hole. This is provided by

temporary casings (often only for the top part of the PILE) or by bentonite slurry, or by some

combination of these.

If concrete is placed under water or under bentonite slurry, it must be fed to the bottom using

a tremie pipe. Thus delivers the fresh concrete below the surface of the concrete that is

already poured, so the concrete never falls free through the water. Care is needed to lift the

tremie pipe so that it remains within the concrete at all times, and failure to achieve correctpouring may result in necking or waisting of the pile.


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By Continuous Flight Auger. The CFA is drilled into the ground to the correct depth. A

cement-sand mortar is then pumped down the hollow stem of the auger to fill the void as the

auger is slowly withdrawn while still rotating. The sides of the bore are first supported by the

auger and the soil, then by the mortar as it is pumped in. If required, a cage of reinforcement

can be pushed into the top of the fluid mortar (max length about 12m)..

By Percussion Rig. For small and medium diameter piles. A conventional cable percussionrig (similar to those used for site investigation borings) is used. Appropriate granular soil

shells or clay cutters are used, with a casing. The hole is filled with concrete and the casing is

withdrawn. Although this is a labour-intensive technique, it can be used in cramped

conditions and where headroom is limited.

Replacement Piles: Drilled-in Tubular Piles or Caission Piles

These have a robust permanent casing, normally steel, which is pushed or drilled into the

ground. The soil inside the casing is removed by grabbing, augering or other methods. Large

machines can install 1m diameter casings, but the technique can be expanded to almost any

size with complete bridge foundations being formed in a single caisson that is driven down by

deadweight. Classically large caissons were kept dry by compressed air as men worked toexcavate the soil inside, but the hazards of compressed-air working make this technique

unacceptable today.

Note that modern piling machines are versatile, and can drive many different types of pile.

A Categorisation of Pile Types

Permanent Casing Temporary Casing

Driven Cast-in-Situ Concrete

Normal

Reinforced

Concrete

Pre-Stressed

Concrete

Concrete Timber

Box Tune H Section Screw

Steel

Pre-Formed

Displacement

Unsupported

Permanent Casing

Casing Drilling Mud

or Water

Temporary Support

Supported

Replacement CFA

Pile Types

Type Materials Advantages Disadvantages

Pre-Formed

Displacement

Piles

Steel

Timber

Pre-Cast

Concrete

Inspected for quality

and soundness before

driving.

Not liable to squeezing

or necking.

Construction notaffected by

groundwater.

Can be left protruding(useful for marine

applications)

Can withstand high

bending and tensile

stresses.

Can be driven in longlengths

Unjointed types cannot

easily be varied in length.

May break or bend duringdriving.

Uneconomic if the design

is governed by driving

stresses rather than

working stresses,

Noise and vibrationduring driving.

Displacement of soil may

damage adjacent

installations.

Cannot be driven in low

headroom.


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Driven Cast-

in-Place Piles

Concrete Length easily adjusted

Groundwater can be

excluded by a driven

closed end.

Enlarged base

possible.Design governed by

working conditions.

Noise and vibration

reduced by internal

hammer.

Where temporary tubes

are used, necking is

possible.

Concrete cannot be

inspected after

installation.Length may be limited if

tubes are to be extracted.

Displacement of soil maydamage adjacent

installations.

Noise and vibrationduring driving.

Bored and

Cast-in-Place

Piles

Concrete Length easily adjusted

Removed soil can beinspected for

comparison with

design data.

Very large bases can

be formed in

favourable ground.

Drilling tools can

break up boulders and

other obstructions.

Pile is designed to

working stresses.

Very long lengthspossible.

Little noise and

vibration duringconstruction.

No ground heave.

Pile liable to squeezing

and necking in soft

ground.

Special techniquesneeded for concreting in

water-bearing ground.

Concrete cannot beinspected after

installation.

Enlarged bases cannot beformed in collapsible

soil.

Cannot easily be

extended above ground.Boring may cause loss of

ground and settlement at

adjacent structures.

Choice of Pile

Materials

Timber Cheap and easy tohandle

Durable below water

table.

Decays above water table

Limited lengths

Unsuitable for heavy

loads

Concrete Suits a range of pile

types

Resistant to aggressive

conditions

Resistant to reasonablyhard driving.

Pre-cast piles can be

damaged by very hard

driving.

Cast-in-place piles cansuffer from necking or

from poor concrete

quality.

Steel High strength

Easy to handle

Can be driven hard.

Little grounddisplacement

Expensive

May require protection

from corrosion.


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4 Load Capacity of the Pile Shaft

The axial load capacity of the pile shaft is not usually critical. Although the pile is a slender

column, buckling will be prevented by the upper layers of soil unless these are very weak (cu

< 20 kN/m2)

Working stresses (calculated as axial force/area) on piles should not exceed:

Concrete piles: fcu/4 fcu is the cube strength of the concrete, typically 35 N/mm2

for

cast-in-place piles or 45 N/mm2

for pre-cast piles.

Steel piles py/3 py is the yield strength of the steel, typically 275 N/mm2

Driven piles have to resist installation damage, and so pre-cast concrete piles may need to be

stronger than is required for the permanent load.

5 Load Capacity of the Soil:

5a Capacity by Calculation from Soil Properties

5(a)(i) Ultimate Load Capacity

5(a)(i)(1) General Principles

The Ultimate Load Capacity of a pile is:

Shaft Resistance plus Base Resistance

Qu = Qs + Qb where Qu = ultimate total resistance, kNQs = ultimate shaft resistance, kNQb = ultimate base resistance, kN

Qs = sAs where s = ultimate skin friction or cohesion, kN/m2

( s will vary down the pile.)

As = area of shaft = dL(d = shaft diameter, L = shaft length)

Qb = qbAb where qb = end bearing resistance kN/m2

Ab = base cross sectional area = D2

/4(D = base diameter)

Typical amounts of movement to develop the full resistances are:

1% to 2% of diameter for skin friction

10% to 20% of diameter for end bearing.

It is common to apply different factors of safety to the two resistances.

In layered soil, the skin friction resistance is the sum of the resistances in the various layers.

Skin friction is ignored for the depth of any under-ream.


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d Shaft diameter

Symbol indicates adding

The cohesion between the pile and the clay may be less than cu

(See Note 1). The factor takes account of this.

Ways of finding :

1. Simple Values for bored pilesSoft and firm clays = 1.0 (Vesic 1977)

London claylong piles = 0.45 (Skempton 1959)

London clayshort piles = 0.30 ( .. )

Underreamed piles = 0.30 (Tomlinson 1995)

2. Tomlinson (driven piles)

See Barnes Figures 10.3 (short piles) or 10.4 (long piles).

3. Weltman and Healy (driven piles in boulder clays)

See Barnes Figure 10.5 for predictions of based on cu

4. Semple and Rigden (driven piles)

For values based on the soil strength ratio cu/ vand thelength/diameter ratio L/d, see Barnes Figure 10.7. Note that

Semple's pF corresponds to the used here.

See Note 2 on maximum adhesion values.

cu See above. See also Note 2.

L Length of pile shaft. Divide the pile shaft into suitable lengths

and calculate average friction over each length.

Note 1Softening of the clay may be caused by

disturbance during piling.

swelling in the unconfined pile bore,

groundwater,

water from in-situ concrete

Note 2 - Maximum Adhesion Values

Suggested maximum values for adhesion cu are

Most clays 100 kN/m2

(Skempton, Vesic)

Glacial Clays 70 kN/m2

(Weltman and Healy)


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Under-Reamed Piles. The diameter D used forcalculating the base resistance is greater than

the diameter d of the shaft. Skin friction on the

under-ream is ignored: (see diagram).

Design Charts for Piles in Cohesive Soils

Shaft

resistance

ignored

Shaft resistance

used

UnderReamed Pile

Base resistance used

D

d


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Design Charts for Piles in Cohesive Soils


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5(a)(i)(3) Piles in Non-Cohesive Soils

If friction did not vary with depth

Qs = fs dL fs = ultimate skin friction of sand (kN/m2)

d = pile diameter (m)L = length of pile in sand (m)

Assuming:

A vertical effective stress = 'v kN/m2

A coefficient Ks, so that horizontal stress = 'vK s

A coefficient of friction between the pile and the soil = tan( )

Then the ultimate skin friction fs = 'vKs tan( )

The vertical stress, and so the friction, will increase with depth, so we replace the formula for

Qs by

Qs = ( 'vKs tan( ) dL), where we add the resistances for the different depths.

Normally d will be constant, so we can write

Qs = d ( 'vKs tan( )L),

Now add the base resistance to get

Now we have to determine values for all the bits of the formula for Qu.

Parameter Commentary

Ab Ab = D2/4, where D = base diameter

q0 Vertical effective pressure in the soil beside the base of thepile.(but see Note 1 on Critical Depth)

Nq Bearing capacity factor (see Note 2). Values from Beresantzev

are shown in Barnes Figure 10.8. The value of ' after

installation should be used: see Note 3.

d Shaft diameterSymbol indicates adding

'v Effective Vertical Stress (but see Note 1 on Critical Depth)

Ks Coefficient of Horizontal Effective Stress

tan( ) Coefficient of friction between the pile and the soil.

Kstan( As an alternative to determining Ks and tan( ) separately, Poulos

has published a direct correlation between ' and Kstan( ). SeeBarnesFigure 10.12

L Length of pile shaft. Divide the pile shaft into suitable lengthsand calculate average friction over each length.

Ultimate Load Capacity of a Single Pile in Sand

Qu = Abq0Nq + d ( 'vKs tan( )L)


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Note 1 - Critical Depth.

The equations for Qb and Qs suggest that skin friction and bearing capacity

increase without limit as the depth increases. Field tests show that both reach

peak values at a "Critical Depth" zc of between 10 and 20 diameters. Barnes

Figure 10.10 suggests values of Critical Depth zc based on the value of ' afterinstallation. See note 3.

Note 2 - Bearing Capacity Factor.

The Nq values used here are equivalent to Terzaghi's (Bearing Capacity

Factor) (Depth Factor)

Note 3: Values of before and after installation.

The initial value of the soil can be estimated from site data (SPT or qc tests)

using Barnes Figure 10.11. Note that the SPT or qc values should be sampled

to below the pile base. will change as the pile is installed. Values of after

installation can be estimated from this table (Poulos 1980)

Value of after installation( I = value before installation)

Bored Piles Driven Piles

For finding Nq I - 3 ( I + 40 )/2

For finding zc 0.75 I + 10

For finding K0tan( ) I

Design Charts for Piles in Non-Cohesive Soils


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Design Charts for Piles in Non-Cohesive Soils


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5(a)(i)(4) Time Effects

Driving piles into soft clays increases the pore-water pressure and so reduces the effective

stress. As the excess pore water pressure dissipates, the bearing capacity of the pile increases.

Stiff clays are cracked and heaved by the driving, and vibrations form an enlarged hole which

fills with groundwater. Some strengthening occurs as the disturbed soil stiffens.

The diagram below suggests that it may take between one day and a year for the bearing

capacity to stabilise.

Diagram from Vesic/Tomlinson

Gain of Carrying Capacity with Time: Driven Piles in Soft to Stiff Clay(after Vesic, figure from Tomlinson 1995)


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5(a)(ii) Working Load Capacity, Factors of Safety

According to the Code BS8004,

The main function of bearing piles is to transfer the load to lower levels of round which

are capable of sustaining the load with an adequate factor of safety and without settling

at working load by an amount detrimental to the structure they support

It suggests

Working load for single pile = Ultimate Bearing Capacity FoS

where FoS is normally 2 to 3

The ultimate bearing capacity may be taken to be that load applied to the head of the pile

which causes the head of the pile to settle 10% of the pile diameter

Tomlinson (1987) suggested that for piles up to 600mm dia, if an overall FoS of 2.5 isadopted, then the settlement of the pile under working load is unlikely to exceed 10mm.

In the case of large bored piles, particularly ones with enlarge bases, it is advisable to take

into account the different resistance/settlement relationships of the shaft and base when

calculating the working load by applying different load factors to the calculated ultimate

resistances of the shaft and base.

Burland et al (1966) suggested that, for bored piles in London Clay, the allowable working

load, Qa, is the smaller of :

Qa = Qult = Qs + Qb or Qa = Qs + Qb

2 2 3

where Qs = Ultimate shaft resistance

Qb = Ultimate base resistance

The first expression tends to govern design for straight sided piles, the second for large

underreamed piles.

Where there is less certainty about ground conditions, loads and pile construction effects,

higher factors of safety should be applied.

EC7 also gives separate factors for base and shaft resistance:

EC7: Partial Safety Factors b, for Base

Resistances for Shaft

Resistancet for Total

Resistance

Driven Piles 1.3 1.3 1.3

Bored Piles 1.6 1.3 1.5

CFA Piles 1.45 1.3 1.4

5(b) Capacity from Driving FormulaeAn accepted in-situ test for soil is the Standard Penetration Test (SPT) which measures the

number of blows needed to drive a standard cone 300mm. By analogy, it is suggested that thecapacity of a driven pile can be determined by its set, the number of blows needed to drive it

through a certain distance. In effect, the installation of the pile IS the site investigation.


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For design purposes, the relationship is stated the other way round:

For a driven pile of ...... diameter, the safe working load will be ........ kN if it is

driven to a set of ...... blows per 100mm

Intuitively, there must be some rationality behind this approach. A pile which is hard to driveis likely have a higher bearing capacity than a pile which is easy to drive. For piles which are

driven through a weak layer of soil to become endbearing on a stronger stratum at greater

depth, the approach makes more sense than trying to predict pile lengths from a limited site

investigation. However the approach has drawbacks. First we look at two common driving

formulae.

The Sanders Driving Formula.

This is based on the idea simple idea that the energy in the hammer blow is absorbed by the

pile moving through the soil.

AssumeThe required ultimate load capacity of the pile is U kN

The weight of the hammer is W kN

The height of drop of the hammer is h metres.

Then each hammer blow delivers W h kNm of energy.

When the pile has sufficient ultimate capacity, each blow will push the pile S = Wh/U metres

into the soil. The formula U = Wh/S was published by Sanders in about 1850.

Example: Required safe working load on pile = 30 tonnes

Required factor of safety = 2.5

3 tonne hammer, W = 30kN

200mm drop, h = 0.2m

So Required ultimate load capacity 30 x 2.5 = 75 tonnes , U =750 kN

Set per blow = 30 0.2/750 = 0.008m.

Set for 10 blows = 80mm

The Hiley Driving Formula.

The Hiley Formula takes a slightly more sophisticated approach and assumes:1. Only a certain percentage (usually 50% to 70%) of the hammer energy is usefully

delivered to the pile. Say this efficiency is n (n = 0.5 to 0.7)2. Some of the energy causes elastic deformation of the soil and pile. This elastic

deformation is called the temporary compression and about half is recovered after the

blow. Say this temporary compression is c metres.

From this we get the Hiley Driving Formula, also known as the Danish Formula:

U = nWh/(S + c/2)or S = nWh/Uc/2

In the example, taking n = 0.6 (60% efficient) and c = 0.005m (5mm)

Set per blow = nWh/Uc/2 = 0.6 30 0.2/7500.005/2 = 0.0023m.Set for 10 blows = 2.3mm


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There are no reliable ways of estimating n and c. Terzaghi and Peck state that no

satisfactory relation exists between the capacity of the piles as determined by load tests and

as calculated from the formula, and that for 2% of piles installed using the formula the

factor of safety may be as low as 1.2 or as high as 30.

The formulas are unreliable because in all piles except those driven to bedrock, and especially

those in soft cohesive soils, strength increases after driving. Piles in permeable soil shouldnot be tested until 2 to 3 days after driving; in impermeable soils a delay of a month may be

required.

For friction piles in soft clay, the driving resistance is almost constant with depth and

application of a driving formula would suggest that the ultimate load capacity does not

increase with depth. In fact the ultimate load capacity increases almost in direct proportion to

the depth, so the driving formulae are of no use. To quote Terzaghi and Peck again, in

Shanghi and New Orleans .. no experienced engineer would even consider using a pile

formula.

6 Settlement of Piles

As noted above, ultimate skin friction resistance may be developed at settlements of 1% to

2% of pile diameter, and ultimate end-bearing resistance at 10% to 20% of pile base diameter.

As the working load is likely to be 1/2 to 1/3 of the ultimate load, these figures do not assist in

predicting working load settlements.

If the soil behaviour is assumed elastic, then settlement can be predicted using this equation

from Tomlinson (equation 7.15)

P

BB

B

PS

BS I

E

B

A

W

EA

LW

W

)1(42

2

Load carried by shaft WS

Load carried by base WB

The first term gives the elastic shortening of the pile:

Pile Length L

Pile Shaft Area As

Pile Elastic Modulus EP

The second term gives the settlement at the base:

Area of pile base AB (equal to AS unless the pile is under-reamed)

Pile base diameter B

Soil Elastic Modulus EB ) beneath base

Soil Poisson's Ratio )

Influence factor IP (related to L/B)

For most piles and soils ( between 0 and 0.25, L/B >5), the equation can simplified to:

BB

B

PS

B

S

E

B

A

W

EA

LWW

22


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More sophisticated methods are available which take account of shaft slip before base failure.

Similarly the effect of layered soils can be included.

The settlement of a pile in a group is likely to be greater than the settlement of an isolated pile

carrying the same load. See later notes.

7 Pile Testing

Once a pile is installed it can be tested, directly or indirectly, to determine its load capacity.

Direct load tests are much more reliable than indirect tests, but much more expensive.

There should be a reasonable delay between installation and testing, to allow the soil strength

to develop. Suitable delays are:

Piles in granular soil: 2 days

Piles in silt or clay: 1 month

7(a) Direct Load Testing

The two main types of test are:i) Constant Rate of Penetration Test (CRP)ii) Maintained Load Test (MLT)Both these tests are normally carried out on preliminary test piles, and sometimes on a

proportion (perhaps two per hundred) of the working piles.

Constant Rate Penetration Test

The compressive force is increased to cause the pile to penetrate the soil at a constant rate

until failure occurs. The ultimate load is therefore determined and the factor of safety with

respect to the design working load calculated. Typical penetration rates are

0.75 mm/min for piles in clay, and 1.5 mm/min for piles in sand. (BS8004)

The "failure load" for the pile is the lesser of:

the load at which settlement increases without significant increase of load

the load which produces a settlement of pile diameter/10

The pile will pass the test if its failure load is at least the specified value.

This test does not give a good indication of the settlement of the pile under long-term serviceload.

Maintained Load Test

Results are obtained by measuring deflections of the pile as loads are added and relieved.

A typical loading sequence for a "proof load" test is:

0, 0.5WL, 0, 0.75WL, 0, 1.0WL, 0, 1.25WL, 0, 1.5WL, 0.

Typical Maintained Load tests are shown in this diagram.


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In both cases it is important that the deflection gauge is supported on an independent frame,

not on the main beam, so that the gauge gives true readings of pile penetration.

The criteria for passing the test may be specified as one or more of:

maximum penetration at working load.

maximum penetration at 150% of working load.

maximum permanent penetration after the load is removed.

In determining these criteria, the specifying engineer will take account of the effect of

settlement on the structure that will be built on the piles.

This test does not necessarily determine the ultimate load capacity of the pile.

Determining the Design Bearing Resistance of a Pile from Load Tests.

A "proof load" test confirms the safety of a pile that has already been designed, but is not

itself a design tool. If the test is continued until "failure" (penetration = 10% of pile

diameter), then the result can be used to determine a Design Bearing Resistance for the pile.

EC7 defines an allowance factor to convert failure loads from tests to characteristic failureloads.

Characteristic Bearing Resistance Load = Test Failure Load /

Values of depend on the number of tests carried out:

EC7: Number of tests 1 2 More then 2

value to be used on the average test strength 1.5 1.35 1.3

to be used on the minimum test strength 1.5 1.25 1.1

Further partial safety factors ( factors) are used to derive the Design Ultimate Bearing

Resistance:

Design Ultimate Bearing Resistance = Characteristic Bearing Resistance /

Test

Pile

Tension

Pile

Tension

Pile

Kentledge Test Tension Pile Test

Test

Pile

JackLoad Cell Deflection

Gauge

Beam

Kentled e

JackLoad Cell

Deflection

Gauge

Beam


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If the skin friction and end bearing components can be determined separately, either by

purpose-designed load tests or by calculation, then different factors can be used for each. If

it is not possible to distinguish between the two, then a single factor is used.

EC7: Partial Safety Factors b, for Base

Resistances for Shaft

Resistancet for Total

Resistance

Driven Piles 1.3 1.3 1.3Bored Piles 1.6 1.3 1.5

CFA Piles 1.45 1.3 1.4

7(b) Indirect Testing Methods, Integrity TestsRefer to CIRIA Report 144 - `Integrity Testing in Piling Practice` (1997) Turner MJ

Testing techniques can be classified into `direct` and `indirect` techniques.

Direct Examination Techniques:

Visual examinationduring or after installationDrilling, boring or probingalongside, or into the pile

Indirect Examination Techniques:

Internalutilising drillholes or pre-formed ducts within the pile, and includes sonic

logging and nuclear techniques

Externalfrom top or side of exposed pile, and includes integrity and resistivity

techniques

Remotealongside the pile where access to the pile head is not available, and includes

parallel seismic techniques

The following charts, taken from the CIRIA 144, give a useful overview of the above

techniques, and the suitability of the techniques to detect particular pile construction defects.

CIRIA 144 Figures 12. and 13.


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CIRIA 144 Figure 1.3


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CIRIA 144 Figures 1.4, 1.5 and 1.6


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CIRIA 144 Table 1.1


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Low Strain Integrity Tests

`Sonic Echo` Testing

The most commonly used integrity test is the `sonic echo` test, a form of low strain integrity

test. The pile head is struck by a light, hand-held hammer and a shock wave propagates down

the pile at a constant velocityif the pile is homogeneous. The wave will be reflected at anychange of impedance, and the greater the change of impedance, the greater the proportion of

the wave that will be reflected. Changes of impedance may be caused by:

the pile toe

inclusions within the pile

cracks or pile joints

dimensional changes

variations in concrete quality

overlapping of reinforcement

(in unusually heavily reinforced piles)

variations in soil stiffness

A typical signal response is shown below

The length L to the point of

reflection is given by:

L = ct/2 where c is the velocity of propagation of the wave through the pile

(typically 4000m/s in sound concrete)

t is the total time for the wave to travel to the point of reflection andreturn to the pile head

CIRIA 144

CIRIA 144


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8 Tolerances, Pile Spacing, Pile Caps and Ground Beams

Tolerances

It is difficult to locate piles precisely. The contractor will be given a specification which states

acceptable tolerances.

eg. The pile must be placed within 75mm of the location shown on the drawings and bewithin a slope of 1/100 from vertical.

If the top of the pile is not at ground level, the tolerance will be affected by both limits. eg. If

the top of the pile is to be 1m below ground level, the tolerance at the top of the pile will be

75 + 1000/100 = 85 mm.

The structure must be designed to be safe wherever the piles are within these tolerances. If the

piles are found to be outside these tolerances, the Contractor will be obliged to pay for

changes.

The structure supported by the pile (usually a pile cap or ground beam) must be large enough

so that the pile will always be under the structure so long as its position is within the giventolerance. On small works, the structure is designed to extend 75mm beyond the nominal

edge of the piles, on larger work this is increased to 150mm.

Pile Spacing.

A pile in a group will have a lower load capacity than an isolated pile. To reduce interference

between piles, the centre-to-centre distances should be at least:

Friction Piles: 3 x pile diameter

End-bearing Piles 2 x pile diameter.

For groups of more than 4 piles the capacity may be reduced unless the pile spacing is much

larger: see later notes.

Columns on Single Piles

Single piles should be designed for the axial load (N) plus a bending

moment due to tolerances (M = N*e). The lateral stability of this

arrangement is suspect, and so it is seldom used except on very

large piles.

Multi-Pile Groups

It is good practice to support each column on at least two and preferablythree piles. This evens out the effects of badly-positioned piles, gives

some protection against a single weak pile, and removes from the pile

any bending moment due to eccentricity of the load.

The piles will be connected by a pile cap which carries the column. The

pile cap is usually quite rigid (thickness = approx half of the pile

spacing), so differences between the settlement of the piles is evened

out.

To support loadbearing walls, it is more convenient to use a ground

beam, which spans between the piles and carries the wall. Groundbeams are often also used to support the ground floor slab.

N

A Column putting

an Eccentric Load

onto a Single Pile

e

A Square Column on a

Square Pile Cap

supported by four Piles

A Small Building

supported on Nine Piles

and Ground Beams


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Stability

Pile caps or connected ground beams give adequate stability to the top of the pile (ie prevent

the top of the pile from moving sideways).

If the piles are required to resist significant lateral loads (eg wind load in high buildings,

mooring forces in quaysides) then the piles should be specifically designed for this or raked

piles should be used. See later notes on the design of piles for lateral load.

9 Piles in Tension

Piles in tension are used to resist uplift or overturning, and raking tension piles are used to

resist horizontal loads. Clearly piles cannot resist tension by end bearing (though an under-

ream can be used to develop more strength) so the tension force must be carried by skin

friction. Tomlinson suggests that skin friction capacity in tension may be only 50% of the

compression capacity. In addition:

Short-term load (eg wind loads) will be sustained more easily than long-term sustained

loads.

A pull-out test made soon after installation may be a poor guide to long-term strength.Despite this, designs should be confirmed by pull-out tests (which are much cheaper than

similar tests on compression piles).

As with compression piles, the tension capacity of a pile in a group may be less than the

capacity of a similar isolated pile.

Tension piles are also known as ground anchors.

10 Downdrag (Negative Skin Friction)

Applies when piles are constructed through recently placed unconsolidated fill, or soils that

may consolidate after the pile is placed.. In addition to the working load on the head, a

downdrag force is transmitted by `negative skin friction`. If the fill is placed over

compressible material, negative skin friction may increase through consolidation.

Tomlinson (1995) states that, for mobilisation of maximum negative skin friction, the soil (or

fill) must move downwards relative to the pile by around 1% x pile dia. The unit negative skin

friction force at any depth can be estimated from the equation

Fsneg = 0 where 0 =effective overburden pressure

= factor = 0.3 for piles up to 15m long

= 0.2 for piles up to 40m long= 0.1 for piles up to 60m long

(Tomlinson 1995)

The negative skin friction becomes and additional load on the pile, so

FoS = Ultimate bearing capacity

Working Load + neg. skin friction

In extreme cases (long piles through unconsolidated fill) it may be worth sleeving the piles,

surrounding them with a soft asphalt coating that will limit the downdrag. This is expensive,

and it is difficult to be sure that the coating will survive the driving process, so it is moreusual to take account of the extra load in the design of the loadbearing part of the pile.


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11 Laterally-Loaded Piles

If it is necessary to use piles to resist lateral loads, they must be designed for the shear forces

and bending moments that will develop. Laterally loaded piles should be checked for strength

under factored loads and for serviceability under working loads.

Reference: Design of laterally-loadedpiles CIRIA Report 103,1984.

12 Capacity of Pile Groups.

Bearing piles are seldom used alone because of overturning and eccentricity effects. Instead

groups of piles are often used, with thick reinforced pile caps to spread loads between the

piles.

Centre-to-centre distances s for piles are normally taken as three diameters for friction pilesand two diameters (or two pile base diameters if under-reamed) for end-bearing piles.

If the pile spacing is more than necessary, a large and expensive pile cap will be needed. Ifthe piles are too close, the soil between the piles will be disturbed and the load-bearing

capacity reduced.

The stressed zone around a single pile is much smaller than the stressed zone around and

beneath a pile group. As a result:

Group capacity is not greatly dependant on installation method.

A compressible layer beneath a pile group may produce more settlement than it would

beneath a single pile.

It follows that:

The capacity of a group of N piles may not be N times the capacity of one pile.

Tests on a single pile may not adequately predict the performance of a pile group.

Model piles in clay tested by Whittaker (1957) showed that

Under a rigid pile cap, the piles do not all carry the same load. Outer piles carry more

load than inner piles (this effect is reversed in sand). See figure 10.15 (Barnes, 1995)

If the efficiency of a pile in a pile group is defined as:

= average load per pile at failure of groupfailure load of a single isolated pile

the efficiency decreases as the pile spacing decreases. See figure 10.16 (Barnes 1995)


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Ultimate CapacityThe results above apply only to Whitakers experiments. For a generally-applicable rule:

Pile Group Capacity is the lesser of:1. the sum of the failure loads of the individual piles

Pult = n(Qs + Qb)2. the bearing capacity (including side friction) of a block of soil defined by the

perimeter of the pile group.Pult = cuNcscdcBgLg + 2(Bg + Lg)Lc

If the bottom of the pile cap is in contact with the supporting soil, then the first of these can be

increased by the failure capacity of this contact surface.

1. The sum of the failure loads of the individual piles plus the failure bearingcapacity of the reminder of the pile cap.

Pult = n(Qs + Qb) + cuNcscdc(BcLcnAp)

End

Figure 10.15 Figure 10.16

Individual Pile Failure

Pile Cap

Block Failure

Pile Cap


29/36


Laterally-Loaded Piles

Ultimate lateral resistance

Near the surface, passive pressures may be developed.

At depth, local flow of soil around the pile limits the lateral resistance.

Broms simplification for piles in clay

For a pile with a diameter D, ignore the passive pressure down to a depth of 1.5D.

Then take a limiting lateral pressure of 9cu.

LH

9(cu/ m)D

1.5D

D

L

e

Ha

Ha

LH

A

B

Providing the pile is strong enough, the whole pile will rotate about the point A.

Normally the pile is not strong enough and the horizontal force is limited by the moment

capacity of the pile at B.

Example

Calculate the ultimate moment capacity required for a long pile of 600 mm diameter if alateral force of 100 kN (short-term) is to be applied 1 m above ground level.

Assume the clay has cu = 100 kN/m2.

Partial safety factor on loads = 1.6

Partial safety factor on cu = 1.6

Design load = 1.6*100 = 160 kN

Ignore top 1.5*0.6 = 0.9 m.

Resistance = 9*(100/1.6)*0.6 = 337 kN/m


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Depth to resist lateral force = 160/337

= 0.475 m


31/36


160 kN

337 kN/m

0.9 m

0.6 m

1.0 m

160 kN0.475 m

Depth to point of maximum moment

= 1.0 + 0.9 + 0.475 = 2.375 m

Maximum moment

= 160*2.375 - 160*0.475/2

= 342 kNm

Note: Serviceability should also be checked because the passive pressures develop only withsubstantial movements.


32/36


Example to show the implications of design to EC7.

In the following case, design the length of pile required

i) to BS 8004, with an overall FoS of 2.5ii) to EC7, to the ULS, assuming case C to be critical.Details :

Pile dia 450 mm

Dead load 550 kN

Variable load 300 kN

Ground conditions :

05m Clay Cu = 90 kN/m2

530m Clay Cu = 120 kN/m2

Assume the top metre of the clay does not support load

Adhesion factor = 0.45, bearing capacity factor Nc = 9

Revision Sheet Pile Design

1. a) A closed end steel tubular pile, 0.6m dia, is driven into stiff clay with a penetrationof 35m. The undrained shear strength of the clay is 130 kN/m

2and the submerged unit

weight is 13 kN/m3. Assuming a bearing capacity factor, Nc, of 9, determine the

allowable pile working load assuming an overall factor of safety of 2.5.

(2414 kN)

Relevant charts and expressions are given on Fig. Q6(a) and (b)

(9 marks)

With reference to Figs Q6(a) and (b), explain why

(i) the peak adhesion factor p reduces as the soil strength ratio increases(ii) the length factor F reduces as the length/diameter ratio increases

(5 marks)

2. A 750 mm dia bored pile is to support a dead load of 900 kN and a variable load of300 kN. Ground conditions involve two layers of clay. The upper layer is 8m thick

and has an undrained shear strength of 50 kN/m2. The lower layer is of considerable

thickness and has an undrained shear strength of 120 kN/m2.

The top metre of the shaft does not support any load.

The adhesion factor is 1.0 for the upper clay layer and 0.5 for the lower clay.

The bearing capacity factor, Nc, is 9

Determine the required pile length :

(i) in accordance with BS 8004 assuming factors of safety of 1.5 and 3.0 areapplied to the shaft load and base load respectively (8 marks)


33/36


(ii) in accordance with Eurocode 7, to the ultimate limit state, assuming case C tobe critical. Relevant clauses and tables from EC 7 are provided

(8 marks)

(i) 13.2 m(ii) 17.2m


34/36


3. A precast concrete pile, 0.4m dia., is to be driven through a deposit of stiff clay 6mthick and into a thick deposit of dense sand. The water table lies at 2m below ground

level. Properties of the soils and relevant parameters are as follows :

CLAY

Undrained shear strength increases from 90 kN/m2

at ground level to 126 kN/m2

at thebase of the deposit.

Bulk unit weight b = 21 kN/m3

Adhesion factor = 0.35

SAND

Angle of friction = 37

Bulk unit weight b = 19 kN/m3

kstan = 1.5

For a working load of 1600 kN, determine the length of pile required assuming:

(i) no adhesion occurs over the top metre of the pile(ii) the factor of safety on adhesion in the clay is 2.0(iii) the factor of safety on skin friction in the sand is 2.5 and on the base it is 3.0(iv) there is a critical depth within the sand only, and measured below the top of the

sand (Use the Meyerhof curve for determination)

Design charts, tables and expressions are given in Fig Q5

(15 marks)

(13.5m)


35/36


Elastic Pile Settlement Example:

12m long concrete pile, diameter 450mm

Working load = 600 kN

Load is carried 65% by skin friction and 35% by end bearing

E value of soil beneath base is estimated to 250 MN/m2

E value for concrete is estimated to be 10,000 MN/m2

Using equation:

BB

B

PS

B

S

E

B

A

W

EA

LW

W

22

WS = 0.65 600 = 390 kN, WB = 0.35 600 = 210 kN

AS = AB = 0.452/4 = 0.16 m

2

mmorm 4,004.00012.0003.010250

45.0

16.02

210

10000,1016.0

122102

39033


36/36

Example of Pile Capacity from Static Test: to EC7

Three CFA piles tested. Estimate that 80% of capacity is in shaft friction and 20% in end

bearing. Use the EC7 method to determine the Design Ultimate Bearing Capacity a pile

similar to those tested.

Static Load Test Results:Pile Number 1 2 3

Test Load at Failure 145 kN 120 kN 115 kN

Calculation:

Average test strength = 127 kN

Minimum strength = 115 kN

From the table of values

Characteristic Bearing Resistance is the lesser of:

127/1.3 = 98 kN (adopt this)115/1.1 = 105 kN

Estimate of shaft friction resistance is 0.8 98 = 78 kN

Estimate of end-bearing resistance is 0.2 98 = 20 kN

Ultimate Design Bearing Resistance = 78/1.45 + 20/1.3 = 69 kN

bearing piles and groups

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