Design of Piled Foundations - · PDF fileObjectives To understand the empirical nature of pile design and the role of precedents (load tests and monitoring) To understand the role

Design of Piled Foundations

Sammy CheungSenior Geotechnical Engineer

Geotechnical Engineering OfficeCivil Engineering and Development Department

30 April 2011

Outline of Presentation

Vertical Load Horizontal Load Negative Skin Friction Pile Group Instrumented Pile Test Results

Objectives

To understand the empirical nature of pile design and the role of precedents (load tests and monitoring)

To understand the role of rational design approach and proper geotechnical input

To appreciate the interaction between pile construction and pile design

To appreciate what can go wrong with different piling techniques

General Perspective

Ground conditions in Hong Kong are complex and can pose major challenge to piling design and construction (e.g. corestone-bearing weathered profiles, karstic marble, deep and/or steeply inclined rock head)

Piling design in Hong Kong is always criticized for overly conservative design.

V

IV

III

II

V

III

Borehole B Borehole A Borehole log A

Simplified geology

Borehole log B

Simplified geology

Note : (1) Refer to Geoguide 3 (GCO, 1988) for classification of rock decomposition grade I to grade VI.

II

I I

VI VI Potential risk of using an overly simplified geological model(e.g. layered-model in corestone-bearing saprolites)

General Perspective

Pile Design in Hong Kong

Many Hong Kong-specific ‘deemed-to-satisfy’ rules are promulgated by the Authority

Rules were derived through experience but are applied without geological considerations and soil mechanics principles

Some rules are not conservative

Unnecessarily long piles may encounter major problems during construction (so could end up as being worse off!)

Pile Design in Hong Kong

Submissions for private and public housing projects Building (Construction) Regulations Code of Practice for Foundations, 2004 Practice Notes for AP/RSE/RGE Foundation Handbook (in preparation)

Submission for public projects GEO Publication No. 1/2006 Specifications (Arch SD) Engineer’s discretion on adopting standards for

private submission

Foundation Design for Private Projects

Buildings (Construction) Regulations

AP/RSE Notes Code of Practice for

Foundations (2004) deemed-to-satisfy rules more economic design may

be feasible by rational design methods

Relevant Practice Notes for Foundation Submissionfor Private and Public Housing Projects

Key PNs include: PNAP 66 (Acceptance criteria for pile testing) PNAP 161 (Scheduled Area for karstic marble) PNAP 227 (Structures On Grade on Newly Reclaimed

Land) PNAP 242 (Qualified supervision) PNAP 282 (Designated Area of Northshore Lantau) PNAP 289 (Ground-borne Vibrations Arising from

Pile Driving and Similar Operations)

Promote use of rational design First edition was published in

1996 Consolidate good design and

construction practice for pile foundations, with special reference to Hong Kong’s ground conditions

Foundation Design for Public Projects

GEO Publication No. 1/96

Updated experience cumulated in recent years

Piling data obtained from the instrumented piling load tests programme for the railway projects

Expanded scope to include shallow foundations and recent advances

Foundation Design for Public Projects

GEO Publication No. 1/2006

Other Useful References

Basic Facts about Piling

Varying ground conditions involve uncertainty and risk Completed works are buried; observations and supervision

during the installation process are important All forms of pile construction will affect the ground conditions -

the question is by how much Different piling techniques and workmanship will affect the

ground in different ways It is the behaviour of the ground after pile installation that

controls pile performance (pile soil interaction) In some cases, there may be time-dependent effects that

could influence the development of pile capacity in the long term

Pile Installation

Displacement piles–“hammering steel or concrete into the ground with sufficient energy to refusal"

Replacement piles–“dig a hole and fill with steel and concrete"

Sounds simple, but not so! Pile installation can affectpile material (damage), the ground (disturbance) andsurrounding facilities

Common pile types in Hong Kong

Pile Types Typical range of pile capacity (kN)

Geotechnical load carrying capacity

Displacement PilesDriven H-piles 2000 kN to 3500

kNShaft friction and end bearing

Driven prestressed precast concrete piles

1950 kN to 3500 kN

Jacked steel H-pile

2950 kN

Jacked steel H-pile – not that common

Common pile types in Hong KongPile Types Typical range of pile

capacity (kN)Geotechnical load carrying capacity

Replacement PilesSocketed H-piles 3500 kN to 5300 kN Shaft friction on rockAuger piles 1500 kN Shaft friction on soilMini-piles 1400 kN Shaft friction on rockMini-bored piles 2000 kN Shaft friction on rock

and end bearingBarrettes Up to 20,000 kN Shaft friction on soil

and end bearing Bored piles Up to 80,000 kN (3.8

m bell-out)Shaft friction on soil/rock and end bearing

Pile Design

Driven piles – piles usually driven to a set based on dynamicdriving formula to match the structural capacity (e.g. 0.3 fy forsteel H piles )

Bored piles & socketed H-piles – piles are usually designedas end-bearing and limited shaft friction on rock. If depth ofweathering is significant, the piles behave as ‘friction piles’instead.

Need to consider geotechnical capacity and structural capacity of piles

Effects of Pile Construction on Ground

• Displacement piles (driven piles) - akin to ‘cavity expansion’ problems, with the horizontal stresses increased and granular soils subject to densification and compaction

• Bored piles - stress relief effect due to hole formation; horizontal stresses in the ground reduced and ground is subject to loosening

Pile Design

DESIGN ISSUES

Ultimate foundation capacity & overall stabilityCyclic loading effects (wind, uplift)Overall settlementsDifferential settlementsStructural designEffects of external ground movements

Pile Design

Essential for a successful outcome

A key geotechnical output is the values of pile stiffness (axial, lateral, rotational) for each pile within the group

These can be incorporated into the structural analysis to obtain structural design actions and also to take account of structural stiffness for settlements and differential settlements

The stiffness values MUST take into account pile group interaction effects

Cooperation between Geotechnical Engineerand Structural Engineer

Deem-to-satisfy rules Simplified rules Code of Practice for Foundations (2004)

Rational design method Based on soil/rock mechanic principles Consider geotechnical capacity and settlement May require instrumented pile loading tests to confirm

design assumption More economical design can be achieved!

Pile Design

Rational Pile Design Approach

An alternative to use of default values Adequate ground investigation to assist in formulation of

appropriate ground model Characterization of ground properties by means of appropriate

insitu and laboratory tests Proper geotechnical + engineering geological input Design analysis to be based on principles of mechanics,

and/or an established empirical correlations Pile testing programme to verify design assumptions

Design of Axially Loaded Pile (Geotechnical Capacity)

Piles found on soil

P = Qs + QB

Qs = shaft capacity

QB = base capacity

P

Soil type 1

Soil type 2

Ultimate Pile Shaft Capacity

Qs = s x As

s = Ultimate shaft friction in each soil stratum

As = Surface area of pile shaft in each soil stratum

Shaft Friction in Granular Soils

Two common design approaches as follows:

Method 1 : Effective stress method

s = Ks . v’ . tan [c’ is usually taken as zero]

_

The above may be simplified to:

s = . v’

_[ method, where = Ks x tan ]

Method 2 : Correlation with SPT N values

s = fs . N_

[SPT method]

where N is the average uncorrected SPT N values before pile construction

Ko is the earth pressure coefficient at rest (viz. before pile construction) and is usually taken as (1 - sin ’) for weathered rocks.

Suggested Ks Values for Method 1

Pile Type Ks/Ko

Large Displacement Piles 1 to 2

Small Displacement Piles 0.75 to 1.25

Bored Piles 0.7 to 1.0

'

s is interface friction’ is effective angle of friction

Pile Shaft Interface Friction Angle, s

Note - roughness of pile/ground interface is important, but difficult to quantify in practice

Pile/Soil Interface s/’Steel/sand 0.5 to 0.9

Cast-in-place concrete/sand

1.0

Precast concrete/sand 0.8 to 1.0

Typical Values in saprolites and sands for Method 1 based on back analysis of local instrumented pile loading tests

Type of Piles Type of Soils Shaft Resistance Coefficient,

Driven small displacement piles

SaprolitesLoose to medium dense sand

0.1 – 0.40.1 – 0.5

Driven large displacement piles


0.8 – 1.20.2 – 1.5

Bored piles & barrettes


0.1 – 0.60.2 – 0.6

Shaft grouted bored piles/barrettes

Saprolites 0.2 – 1.2

Noted: Only limited data for loose to medium dense sand

Design Parameters for Friction Piles- Method 2 (SPT correlation)

s = fs . N

For bored piles/barrettes in granitic saprolites :fs typically ranges from 0.8 to 1.4 [often taken to be 1.0 for preliminary design]

Pile types Ultimate Shaft Friction

Driven small displacement piles

1.5 – 2.0 x SPT, max 160 kPa

Driven large displacement piles

4.5 x SPT, max 250 kPa


Pile types Shaft grouting?

Ultimate Shaft Friction Ultimate End Bearing

Barrettes formed using grab

YES - No Data - - No Data -

NO 1.2 x SPT, max 200kPa 10 x SPT, max 2000kPa

Barrettes formed using cutter

YES 2.5x SPT, max 200kPa

NO 0.8 x SPT, max 200kPa

Bored piles YES 2.1 x SPT, max 200kPa

NO 0.8 x SPT, max 200kPa

Friction parameters previously accepted by BD :

The design method involving correlations with SPT results is empirical in nature

Level of confidence is not high particularly where the scatter in SPT N values is large.

Where possible, include a loading test on preliminary pile to confirm the design assumption.


rθ

v

r

Factor Affecting Shaft Friction

Pile Shaft

Changes of radial effective stress affects the skin friction Displacement piles – increases in

radial stress Replacement piles – decrease in

radial stress

= (ho + h ) tan = (hf) tan

ho is the locked-in effective horizontal stress after pileconstruction

h is the change of horizontal stress after pile constructionhf is the effective horizontal stress at failure and will be

affected by: interface dilation/compression under constant stiffness

condition during pile loading which can increase (due todilation of a dense soil), or reduce (due to compressionof a loose soil)

Factor Affecting Shaft Friction

Factors Affecting Shaft Friction of Bored Piles

Reduction in confining stress in bored piles– Stress relief– Arching effect– Loosening of soil due to poor construction control

Reduction in friction angle– Presence of weak materials at pile/soil interface (e.g.

bentonite filter cake)– Loosened/disturbed soil

Key Non-Geotechnical Factors Affecting Behaviour of Bored Piles

Rate of concrete pour

Fluidity of concrete

Time of pile bore being left open prior to concreting (- generally better to minimize the ‘wait time’ to avoid excessive relaxation)

Note: Faster concreting process will help to achieve higher wet concrete pressure, which would help to achieve higher locked-in horizontal stresses in the ground

Distribution of Wet Concrete Pressure

0 50 100 150 0 50 100 150 300250200

0

10

20

30

40

45

5

15

25

35

Concrete Pressure (kPa) Concrete Pressure (kPa)

Dept

h (m

) 2 hr

4 hr

Set = 6 hr

2 hr

4 hr

Set = 6 hr

Rise = 8 m/hr Rise = 12 m/hr

Swelling of granitic saprolite due to stress

relaxation

* Important to ensure sufficient excess slurry head within pile bore

2.4

1.6

0.8

0.020016012080400

Horizontal Effective Stress (kPa)

Radi

al st

rain

(%)

anisotropic

3 ’

1 ’

decreasing

constant

Swelling Effect due to Stress Relaxation

Good Practice for Enhancing Shaft Friction in Bored Piles

Sink casing in advance of excavation– to prevent loosening of soil/stress relief

Maintain a high hydraulic head inside temporary casing

Adopt a longer setting time for concrete– Wet concrete will exert an outward fluid pressure

against the drill shaft (minimize stress relief)– Horizontal stress h that can be restored after

excavation may be controlled by concrete pressure

Avoid delay in construction to minimize potential of stress relief– minimize delay in concreting after excavation– avoid unnecessarily over-cleansing of pile base

(delay concreting) Shaft grouting

– grout pressure increase horizontal stress– improve strength of interface material hence shaft

friction

Good Practice for Enhancing Shaft Friction in Bored Piles

QB= qb x Ab

Ultimate End-bearing Capacity

qb = Ultimate end bearing pressure

Ab = Bearing area of pile base

Ultimate Bearing Capacity of Piles in Granular Soils

qb = Nq · v

qb = fb · Nb

qb = presumptive bearing pressure

(b) Empirical correlation with SPT

(a) Classical bearing capacity theory

(c) Presumptive bearing pressure

Relationship between Nq and '(Poulos & Davis, 1980)

For driven piles,

' =

For bored piles, ' = '1 – 3 where '1 is the angle of shearing resistance prior to installation.10

100

1000

25 30 35 40 45Angle of Shearing Resistance, ' (°)

Bear

ing C

apac

ity Fa

ctor

, Nq ’1 + 40

2

0.6

0.4

0.2

0.00 5 10 15 20

Coarse sand

Fine sand

Normally consolidated siltCoarse sand

Fine sand

Driven piles

Bored piles

Depth in bearing stratumBase diameter

Ultim

ate E

nd B

earin

g Cap

acity

SPT N

bVa

lue

Pile LengthBase diameter ≥ 15

Ultimate Bearing Capacity of Piles in Granular SoilsBased on SPT N

Base Diameter (m)

Redu

ctio

n Fa

ctor

, fr

Ultimate Bearing Capacity of Piles in Granular SoilsBased on SPT N

1.0

0.5

0.25

0.00 0.5 1.0 1.5 2.5

Loose sand

Medium dense sand

Dense sand

2.0

0.75

Max

imum

Mob

ilise

dAv

erag

e Sha

ft Re

sista

nce,

max

(kPa

)

=0.6 =0.5 =0.4

= 0.3

= 0.2

= 0.1

P23

C1B8C

P11

P9P15

P7P19

P6

P14

B5

C2

P21‐2

P20

P5

P10 P8 P12P17

B6C

B4

B7C

B2

P21‐1

P4P13

P1

P2

B3

P22

P18

B1

B6T

C3

B7T

B8T

0

50

100

150

200

250

0 100 200 300 400 500 600 700

Mean Vertical Effective Stress, 'v (kPa)

B9

B11

B10

Figure A1 of GEO Publication 1/2006

Local Instrumented Test Data for Bored Piles

/N = 1.0

/N = 0.5

Max

imum

Mob

ilise

dAv

erag

e Sha

ft Re

sista

nce, m

ax(k

Pa)

/N = 2.5 /N = 1.5

C1

B8CP11

P16

P9P15

P7 P19

P6

P14

B5

C2

P21-2

P20

P5

P10P8 P12P17

B6C

B4 B7C

B2

P21-1 P4P13

P1

P2

B3

P22

P18

B1

B6T

C3

B7T

B8T

P23

0

50

100

150

200

250

0 50 100 150 200

Mean SPT N Value

B11

B10

B9

Figure A2 of GEO Publication 1/2006

Local Instrumented Test Data for Bored Piles

Some Observations

Significant scatter in the pile performance based on localinstrumented pile tests. Large variability recorded in the samesite.

Some unexpectedly low results have been measured for boredpiles under bentonite. Thus, load tests are important toconfirm design parameters and workmanship for friction boredpiles).

β values from load tests tend to be towards the lower bound ofthat expected for bored piles in granular materials (possiblydue to low horizontal stresses in weathered rocks, i.e. low Kovalue)

The method and the SPT method for pile design are notnecessarily consistent in that they may give different predictions

As a pragmatic approach, it is probably best to use bothmethods to assist in decision-making regarding pile designcapacity

It is important to make reference to the results of previousinstrumented pile load tests in similar ground conditions for therespective pile construction methods [role of precedents +design by load tests]

Some Observations

Ultimate Qs typically develops in a stiff manner, at a pile settlement of only about 0.5% to 1% pile diameter

Ultimate QB typically develops at a pile settlement of @ 10% (clay) to 20% (sand) pile diameter

Load Transfer Mechanism and Mobilizationof Load-Settlement Curve

Pile settlement

Pile

Load

Total

Base

Shaft

Material Mobilisation Factor for Shaft Resistance, fs

Mobilisation Factor for End-bearing Resistance, fb

Granular Soils 1.5 3 – 5

Mobilization Factors for Deriving Allowable Bearing Capacity

Mobilisation factors for end-bearing resistance depend very much on construction. Recommended minimum factors assume: good workmanship no 'soft' toe based on available local instrumented loading tests on friction

piles in granitic saprolites. Lower mobilisation factors when the ratio

shaft resistance end-bearing resistance

is high

Allowable Load Carrying Capacity, QaQbfb

Qsfs

= +

Method of DeterminingPile Capacity

Minimum Global Factor of Safetyagainst Shear Failure of the Ground

Compression Tension LateralTheoretical or semi-empirical methods not verified by loading tests on preliminary piles

3.0 3.0 3.0

Theoretical or semi-empirical methods verified by a sufficient number of loading tests on preliminary piles

2.0 2.0 2.0

Overall Global Factor of Safety

Code of Practice for Foundations by Buildings Department (2004)

Category Description of Rock Presumed Pressure (kPa)

2Rock (granitic and volcanic) :Highly decomposed, moderately weak to weakrock of material weathering grade IV or V orbetter, with SPT N value of 200

1,000

Presumed Allowable Bearing Pressure


1(a)Rock (granitic and volcanic) :Fresh strong to very strong rock of materialweathering grade I, with 100% total corerecovery and no weathered joints, and minimumuniaxial compressive strength of rock material(σc) not less than 75 MPa (equivalent point loadindex strength PLI50 not less than 3 MPa).

10,000

1(b) Fresh to slightly decomposed strong rock ofmaterial weathering grade II or better, with atotal core recovery of more than 95% of thegrade and minimum uniaxial compressivestrength of rock material (σc) not less than 50MPa (equivalent point load index strength PLI50not less than 2 MPa).

7,500




1(c) Slightly to moderately decomposed moderatelystrong rock of material weathering grade III orbetter, with a total core recovery of more than85% of the grade and minimum uniaxialcompressive strength of rock material (σc) notless than 25 MPa (equivalent point load indexstrength PLI50 not less than 1 MPa).

5,000

1(d) Moderately decomposed, moderately strong tomoderately weak rock of material weatheringgrade better than IV, with a total core recoveryof more than 50% of the grade.

3,000

Based on simple material classification Intended for foundations on horizontal ground with negligible

lateral loads & structures not unduly sensitive to settlement(i.e. routine problems)

Minimum socket length = 0.5 m for categories 1(a) & 1(b),and = 0.3 m for categories 1(c) & 1(d)

Total core recovery = % ratio of rock recovered (whether solidintact with no full diameter, or non-intact) to 1.5 m length ofcore run + should be proved to at least 5 m into the specifiedrock category

Self weight of pile - no need to further consider in calculationof bearing stresses


Use of Total Core Recovery (TCR) as sole means of determining founding level + presumptive bearing value in rock is experience-based and tends to be conservative

TCR can be affected by effectiveness of drilling technique in retrieving the rock cores What are the requirements of the 15% of material?

No account taken directly of discontinuity spacing, aperture, persistence and infill, strength properties etc.

Can we find Grade I rock with no weather joints? Category 1(d) rock should be “grade IV” material instead of

“better than grade IV”


Uniaxial compressive strengthof intact rock (MPa)

Prov

en b

earin

g pre

ssur

e (M

Pa)

P10-2OP15O

P14P7-2O

P2CP13-2O

P9-3O

P9-1P1C

P3C

P11-2O

P11-1

(2.5)(1.2)

(64)

(86)

(15.5)

(2)

(12.6)(13.6)

(3) (7.5)

(11.3)(?)

0

5

10

15

20

25

30

0 25 50 75 100 125 150 175 200

Code of Practice for FoundationsCategory 1(a)

Category 1(b)

Category 1(c)

settlement at pile base (mm)

P9 founded on granodiorite. UCS of rock ~ 15 MPa

pile load predominately taken by shaft resistance

Comparison of Allowable Bearing Pressure with Results obtained from Local Instrumented Pile Loading Tests

Rock Sockets

Calculation of Rock Socket Length

• General equation :R = Acontact fs L

• Check which scenario is more critical : (a) failure between rock and cement grout and (b) failure between steel and cement grout. Take the longer of the calculated socket length.

Design of Rock Sockets

Rock socket friction depends on: – wall roughness– tendency for pile dilation during displacement upon

loading under constant normal stiffness condition (dilatancy component may possibly reduce if load beyond the peak shear stress, depending on nature of material)

– strength and stiffness of concrete relative to that of the rock

For piles socketed in rock of categories 1(a) to 1(d), the total capacity may be taken as the sum of the bond resistance of the socket length corresponding to not more than 2 x pile diameters or 6 m (whichever is shorter) plus the presumptive bearing value

Not evident from results of instrumented pile loading tests The minimum socket depths stipulated in the presumed

bearing pressures should be ignored in bond calculation.

Recommendations in Code of Practice for Foundations (2004)


Presumptive Design Parametersin BD’s Code of Practice for Foundations

Category Rock Mass Weathering

Minimum Embedment (m)

Allowable ShaftFriction (kPa)

1a Grade I or better 0.5 7001b Grade II or better 0.5 7001c Grade III or better 0.3 7001d Grade IV or better 0.3 300

Note: Use of rock socket bond in conjunction with the end bearing component is more rational than assuming end bearing only and will help avoiding the need to use bell-outs in some cases (also, presence of soil seams below pile base will be less of a problem)

10

Uniaxial Compressive Strength of Rock, q (Mpa)

Mob

ilize

d Sh

aft R

esist

ance

in

Rock

, (k

Pa)

P16

C1

P10-1

P10-2O

P9-1

P7-2O

P8

P7-1

P3T

P3C

P2T

P1T

P1C

100

1000

10000

1 100 1000

s = 0.2 c0.5


Most of the results were not fully mobilized

Load-carrying capacity of bored piles socketed in rock (based on available data): pile shaft resistance and end-bearing resistance can be

added together settlement of pile base < 1% of pile diameter at working

loads socketed length / pile diameter ratio < 3 (GEO Publication

No. 1/2006) otherwise, pile loading tests need to be carried out to

confirm the design

Note : Load transfer in a rock socket is a function of the slenderness ratio of the rock socket & the relative pile/rock stiffness


Design of Driven Piles

Design of Driven Piles (Hong Kong practice)

Design is rarely based on soil mechanic principles

Load carrying capacity of pile is based on structural capacity.

Drive to set as calculated from dynamic pile driving formula

Estimates of required pile depth is usually based on rules ofthumb (e.g. by relating to SPT N values - typically drive to adepth with N value of 100 for large displacement concretepiles, or a depth with N value of 180 to 200 for H-piles)

Design of Steel H-piles

For Grade 55C steel H piles, allowable load is taken as 30% yield stress (fy, which is a function of the steel grade and thickness) x As [e.g. fy for 305x305x223 pile = 430 MPa]

Pile driving formula (Hiley) used and final set criteria (typically, 25mm/10 blows to 50 mm/10 blows if not in rock)

Dynamic load tests + static load tests are used

Hiley Pile Driving Formula -(commonly used in Hong Kong)

R = W H S + 1

2 (C1 + C2 + C3)X

where = (W + e2p)(W + P) = efficiency of hammer

blow

Based on energy consideration

Hiley Pile Driving Formula -(commonly used in Hong Kong)

E’ = W H = effective energy impacted to pile (allowing for hammer efficiency, )

S = permanent set (i.e. pile penetration for the last blow) c1 = temporary compression of pile head (elastic)c2 + c3 = temporary compression of pile and ground (elastic)

W = weight of hammer

P = weight of pile

e = coefficient of restitution between hammer and pile cushion

H = drop distance of hammer

Pile Length 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 FINAL SET (mm) PER 10 BLOWS

15 -- -- -- -- -- -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- 16 -- -- -- -- -- -- -- -- -- -- -- 48 43 38 33 28 -- -- -- -- -- 17 -- -- -- -- -- -- -- -- -- -- 50 45 40 35 30 -- -- -- -- -- -- 18 -- -- -- -- -- -- -- -- -- -- 47 42 37 32 27 -- -- -- -- -- -- 19 -- -- -- -- -- -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- 20 -- -- -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- 21 -- -- -- -- -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- 22 -- -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- 23 -- -- -- -- -- -- -- 48 43 38 33 28 -- -- -- -- -- -- -- -- -- 24 -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- -- 25 -- -- -- -- -- -- 48 43 38 33 28 -- -- -- -- -- -- -- -- -- -- 26 -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- -- -- 27 -- -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- -- -- -- 28 -- -- -- -- -- 47 42 37 32 27 -- -- -- -- -- -- -- -- -- -- -- 29 -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- -- -- -- -- 30 -- -- -- -- 47 42 37 32 27 -- -- -- -- -- -- -- -- -- -- -- -- 31 -- -- -- -- 45 40 35 30 25 -- -- -- -- -- -- -- -- -- -- -- -- 32 -- -- -- 48 43 38 33 28 -- -- -- -- -- -- -- -- -- -- -- -- -- 33 -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- -- -- -- -- -- 34 -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 35 -- -- 47 42 37 32 27 -- -- -- -- -- -- -- -- -- -- -- -- -- --

Typical Final Set Table (mm) per 10 Blows

Temporary Compression, Cp + Cq (mm)

Latest BD practice : Allow 100 mm per 10 blows but set 50 mm instead

Sample Final Set Calculation by Hiley Formula

TYPE OF PILE 305 x 305 x 180kg/m Grade 55CULTIMATE PILE LOAD Ru 5916 kN (2 x Design Working Load)HAMMER MODEL Drop Hammer (8 ton)WEIGHT OF RAM, W 80 kNCOEFFICIENT OF RESTITUTION, r 0.32TEMPORARY HELMET COMPRESSION, Cc 2.5 mmWEIGHT OF PILE HELMET, Wd 3 kNHEIGHT OF DROP, H 2.8 mENERGY EFFICIENCY, 0.8ENERGY OUTPUT PER BLOW, E 224 kN-mEFFECTIVE ENERGY, E' = E x 179 kN-m

Pile Length, L (m) = 25 mEffective Pile Weight, P = Wp + Wd = 25 x 1.8 + 3 = 48.0 kN

For Cp + Cq = 30 mmC = Cc + (Cp + Cq) = 33 mm

S = 3.8 mm / BlowS = 38 mm / 10 Blows

Problems with Hiley Formula

Basic assumptions on rigid body collision and conservationof energy is considered problematic.

Displacement

Forc

e

Cq

Cq S

S

Assume elasto-plastic soil and no damping effect considered

Problems with Hiley Formula

Rates effects and set-up effects not accounted for (assumedstatic capacity = dynamic capacity)

Hammers do not always operate at their rated efficiency andcan be highly variable

Energy absorption property of cushions can vary with timeand based on assumed values. For long pile, only a portionof the pile length is mobilized by the hammer blow.

Use of hydraulic hammers is not accepted by the BuildingAuthority. Drop hammer is used to take final set.

Pile Hammers Previous extensive use of diesel hammers was effectively

banned since 1997 Drop hammers (typical efficiency assumed in private sector =

0.7 to 0.8) - normally site measurements (by PDA) required if proposed energy coefficient is > 0.8

Hydraulic hammers (not accepted by BD for final set); typical efficiency = 0.9 or higher

HKCA studies on hydraulic hammers in 1995 and 2004 respectively

In Hong Kong, it is common to use hydraulic hammers for pile driving (higher productivity), but a drop hammer is used for final setting

Proposed improvement to Hiley Formula :− Energy approach (HKCA, 2004) using Pile Driving

Analyzer to measure the driving energy CAPWAP analysis (ArchSD, 2003) to find parameters for

matching the pile capacity as determined by Hiley Formula (combination of and e as ‘correction factors’)

Recent Work on Design of Driven Piles

Recent Work on Design of Driven Piles

Proposed Pile Driving Formula for Hydraulic Hammers by HKCA (2004)

where EMX is the actual energy transfer to pile head

Pile driving system not taken as part of pile-soil system, therefore Cc is not considered and subsumed in EMX, which is determined by CAPWAP

Final set table to be prepared based on average EMX (done during trial piling & use simple statistical methods to determine average EMX

cp = elastic compression of pile & cq = quake (elastic compression of ground)

R =[s + ½ (cp + cq)]

EMX

Driven Piles Founded on Rock

A suitable pile point (stiffener) may be used at the pile toe to prevent sliding on an inclined rock surface

Typical hard driving criterion for final set, e.g.− <10 mm per 10 blows with 16-tonne drop hammer− But is hard driving doing more harm than good?

Driven Piles Founded on Rock

Grade 55C steel sections with yield stress, fy, of 425 MPa, allowable stress = 0.3 fy (129 MPa)

Very high stresses on rock - why okay?

Rocks upon which driven piles are founded will be are subject to high confining pressure and hence can develop very high bearing capacity (also possible soil plug formation and local yielding leading to a larger base area) - see paper by Li & Lam (2001) - Proc. 5th International Conf. on Deep Foundation Practice, Singapore

Design of Prebored Piles

Pre-bored Steel H-piles

Prebore (using temporary casing as necessary), place H-section into bore and grout up [acts as a friction pile]

Compression loading - maximum allowable axial working stress (or combined axial and flexural stress) < 0.5 fy

Rock/grout bond limited 700 kPa in compression (or 350 kPa for permanent tension) for Category 1(c) or better rock in Code of Practice for Foundations

Under Compression : allowable grout/steel bond <600 kPa (x reduction factor of 0.8 when grouting under water). Under Tension : same assumptions if nominal shear studs are provided

Use shear studs to ensure proper bonding at grout/steel interface

If rock socket is subject to lateral load, need to check for additional stresses

Pre-bored Steel H-piles

Design of Mini-piles

Assessment of structural capacity (BD allows consideration of steel bars only. Overseas practice generally allow to account for load taken by grout also)

Mini-piles socketed in rock (Grade III or better with TCR of min. 85%) – presumed allowable rock/grout bond strength up to 700 kPa for compression (re. CoP for Foundations) Loading test results gave higher bond between rock/grout

May need to check buckling capacity for slender piles with substantial length embedded in soft/weak ground

Working load controlled by permissible structural stresses (typical maximum load capacity @1300 kN)

Raking mini-piles are usually used to resist lateral load

Negative Skin Friction

Negative Skin Friction (Downdrag)

Caused by ground settlement relative to the pile Need to understand site history and consolidation parameters

to assess potential for NSF NSF may arise due to surcharge or recent filling inducing

consolidation settlement, reduction of water pressure due todewatering and increase in effective stress, dissipation ofexcess pore water pressure (and hence settlement) in soft clayinduced by pile driving

Positive skin friction

Negative skin friction(Soil drags down pile)

(Pile settles relativeto the ground)

QB = base capacity

P

Soil type 1

Soil type 2


Neutral plane

No relative movement

Ground settlement

Pile shortening

NSF = V’ D L

Soil Type

Soft Clay

Silt

Sand

0.20 - 0.25

0.25 - 0.35

0.35 - 0.50

NSF = Ks V’ tan D L


Estimation of Negative Skin Friction

(a) Ground bearing capacity check (exclude NSF) :

Pc D + L (where Pc is the allowable ground bearing capacity & D and L are the dead load and live load)

(b) Pile structural integrity check :

Ps D + L + NSF (where Ps is the structural strength of the pile)

(c) Settlement check :

Settlement under (D + L + NSF) should be satisfactory

Design Checks for Negative Skin Friction(BD’s CoP on Foundations)

More efforts are required to drag the entire pile group including the pile cap. The pile-pile cap-soil interaction helps to reduce negative skin friction on a pile group.

The magnitude of free field soil movement for pile group is reduced especially for inner piles.

Group Effect for Negative Skin Friction

Group interaction effects are beneficial as negative skinfriction on individual piles will be reduced (up to 30%reported).

Distribution of negative friction among piles is not thesame (centre piles has the least negative skin friction dueto shielding and the most severe interaction among piles inthe group).

There are a variety of recommended methods for thedesign of pile groups against negative skin friction!

Group Effect for Negative Skin Friction

Means to Reduce NSF

Driven piles - bitumen coating or asphalt coating, plasticsheet, “Yellow Jacket”, etc. (Note - need to carefully revieweffectiveness and potential for damage to such protectivelayers during pile driving into competent ground)

Permanent casing for bored piles

Sacrificial protection piles around the structure foundation

Ground improvement techniques to strengthen/stiffen thesoft soils

Design of Lateral Load

The lateral load capacity of a pile may be limited in three ways :Shear capacity of the soil,Structural (i.e. bending moment and shear) capacity of the pile section, andExcessive deformation of the pile.

Design methods by Broms (1980) and Reese & Matlock (1960)Computer programs for pile groups, e.g. PIGLET, ALP, etc.

Design of Lateral Loads

For fixed-head short piles in granularHu = 1.5 D L2 Kp s‘

0.2

0.40.60.81.01.52.03.0

e1/L = 0

0 5 10 15 20

200

160

120

80

40

0

Fixed-head

Free-head

Pile Embedment Ratio, L/D

L

Fixed-head Deflection

3Ds'LKp

Mmax

L

e1

Free-head Deflection

3Ds'LKp Mmax

PL

Hu

Soil Reaction

Bending Moment

Soil Reaction

Bending Moment

Hu

0.5 D L3 Kp s’

e1+LHu =

For free-head short piles in granular soils

Ultimate lateral soil resistance for piles in granular soils(Broms )

(b) Long Vertical Pile under Horizontal Load


e1

Soil Reaction

Free-head Deflection

Fixed-head Deflection

Bending Moment

Soil Reaction

Bending Moment

Mmax Mmax

Mu

f*f*

H

H

3s'f*Kp

41 2 8 16 32

Mu

D4 s’ Kp

Fixed-head

Free-head

e1/D =01

10

100

1000

H u

D3 s

’ Kp


Fixed-head long piles in granular soilsMmax = H (e1 + 0.67f*)

HD s’ Kp

f* = 0.82 √

Mmax = 0.5 H (e1 + 0.67f*)

Free-head long piles in granular soilsMmax = H (e1 + 0.67f*)

(1) For constant soil modulus with depth (e.g. stiff overconsolidated clay), pile stiffness factor R = (in units of length) where EpIp is the bending stiffness of the pile, D is the width of the pile, kh is the coefficient of horizontal subgrade reaction.

(2) For soil modulus increases linearly with depth (e.g. normally consolidated clay & granular soils), pile stiffness factor,

T = √where nh is the constant of horizontal subgrade reaction

Ep Ip

nh

5


Pile Type Soil ModulusLinearly increasing

Constant

Short (rigid) piles

L ≤ 2T L ≤ 2R

Long (flexible) piles

L ≥ 4T L ≥ 3.5R

Design of Lateral Load - Pile Stiffness

Consistency Loose(N = 4-10)

Medium Dense (N =11-30)

Dense(N =31-50)

nh for dry or moist sand

(MN/m3)2.2 6.6 17.6

nh for submerged sand

(MN/m3)1.3 4.4 10.7

Design of Lateral Load - Horizontal Subgrade Reaction

0 0.2 0.4 0.6 0.8

0

1

2

3

4

MH = FM (HT)

L

z

MH

= 2

3

4

5 10

Deflection Coefficient, Fd for Applied Moment M-1 0 1 2 3

0

1

2

3

4

L

z

H

H = F

= 2

3

4

5 & 10

Deflection Coefficient, Fd for Applied Lateral Load, H

Moment Coefficient, FM for Applied Moment M Moment Coefficient, FM for Applied Lateral Load, H

0

1

2

3

4

0 0.2 0.4 0.6 0.8 1.0

L

z

MM

MM = FM (M)

= 2

3

4

5 10

-1 0 1 2 3

0

1

2

3

4

L

z

M

M = F

= 2

4, 5 & 10

3

Lateral Soil Resistance for Piles in Granular Soils(Reese & Matlock )

0

1

2

3

4

0

1

2

3

4

-0.8 -0.6 -0.4 -0.2 0 -0.8 -0.4 0 0.4 0.8

VH = Fv (H)

L

z

VH

H

VM = Fv ()

L

z

VM

M

= 2 = 2

10 10 5

3

4

5

4

3

Shear Coefficient, Fv for Applied Moment M

Shear Coefficient, Fv for Applied Lateral Load, H

Lateral soil resistance for piles in granular soils(Reese & Matlock )

Pile Group

= Ultimate capacity of a pile groupSum of ultimate capacity of individual piles

Design of Pile Group

Pile Group Efficiency Factor

For piles founded on soil:Group factor for pile capacity ≥ 0.85Higher group factor can be used with justification by soil mechanic principlesNeed to consider separately the pile group settlement

For piles founded on rock or driven to refusal:No group factor is required

Surface of assumed failure block

Block Failure

End-bearing resistance

Shaft resistance

W'


1 2 3 4 5 6 70.5

1.0

1.5

2.0

2.5

3.0Shaft efficiency

4-pile group9-pile group

4-pile group

4-pile group

9-pile group Total efficiencywith pile capTotal efficiency

Base efficiency(average of tests)

Pile Spacing/Pile Diameter

Grou

p Ef

ficie

ncy F

acto

r

Model Tests on Groups of Instrumented Driven Piles

in Granular Soils


X

Y

Z

P

MX

My

xi

yi

Rigid cap

Pile


Pai = Pnp

+ My*xi

Ix +

Mx*yi

Iy

Mx* = Mx -

MyIxy

Ix

1 - Ixy2

IxIy

and My* = My -

MxIxy

Iy

1 - Ixy2

IxIy

Based on rigid cap assumption Rotation of principle axes

where pile arrangement is not symmetrical

Pile groups subjected to vertical load and moments in both horizontal directions

Realistic soil profiles Non-linear soil-pile behaviour Different pile types within group Raft/cap flexibility incorporated Structure stiffness incorporated

Some desirable features for undertaking analysis by commercial software


ANALYSISSome Programs (commercially available)

DEFPIG – pile groupsPIGLET – pile groupsREPUTE – pile groupsPLAXIS (2D & 3D)ABAQUSFLAC (2D & 3D)

2D analyses tend to: over-estimate settlements under-estimate differential settlements & raft moments over-estimate proportion of load carried by piles

SAFE Analysis


Design of Pile GroupFlexible Cap Analysis

SAFE – Structural Analysis program─ Pile stiffness based on EA/L─ Length based on tentative founding level─ Need for performance review?─ Effect of structural walls/columns ignored?─ Interaction between piles ignored? Could be

detrimental for long piles

Scheduled Area No. 2 in the Northwest New Territories

Scheduled Area No. 4 in Ma On Shan reclamation area

Foundation Design in Marble Bearing Area

Designated Area in Northshore Lantau


Tuen Mun Formation

Yuen Long Formation

Tin Shui Wai

Ma Tin

> 200 m

Long Ping

> 300 m

Interbeds of volcanic rocks including tuff-breecia, tuff & tuffite with clasts

of white marble, quartzite, metasiltstone etc,

clasts < 3 m

Massively bedded, white crystalline marble, locally dolomitic and siliceous

Grey to dark grey, finely crystalline marble intercalated and interbedded

with meta-sediment

Formation Member / Thickness Material Description Age

Uppe

rJu

rass

icCa

rbon

ifero

us

Dissolution

Limited

Main dissolution

Limited

Carbonate Rocks in Northwest New Territories

Ma On Shan Formation > 200 m

Grey to off-white, dolomite to calcite marble with thin interbeds of dark grey

to black meta-siltstone

Formation Member /Thickness Material Description Age

Carb

onife

rous

Vary

Carbonate Rocks in Ma On Shan

Dissolution

Pure Marble in Ma Tin Member

White, pure, crystalline marble

Impure Marble in Long Ping Member

Grey to dark grey, fine-grained dolomitic marble

Marble-clast bearing rock

Marble clast

Foundation Design

Foundation system

suitability of foundation types bored piles, driven steel H piles friction piles for lightly loaded building

founding levels of deep foundation sound marble (Class I or II) redundancy for driven piles

increase of stresses at marble surface

Ground investigation

Ground modelling Foundation design

Foundation construction

Review of construction

Monitoring of building


Geotechnical Contents in Design Submission

Interpretation of ground conditions geological model karst geomorphology (GEO Report Nos. 28, 29, 32)

Foundation system founding levels of deep foundation increase of stresses at marble surface

Supplementary explanation on foundations on marble-bearing rock (TGN 26)


Construction

driven piles pile driving record

bored piles pre-drilling investigation

Conclusion of construction performance review post-construction tests, e.g. CAPWAP, PDA, pile

loading tests PDA useful to identify broken piles and 12% ~ 28

% of piles were tested in some projects


Monitoring

Building settlement monitoring building taller than 20 story high foundations on marble measurements undertaken by CEDD after building

occupied


Core at least one full diameter



Length > 100 mm Length > 100 mmLength > 100 mm

RQD1RQD2 RQD3

Computation of Rock Quality DesignationFoundation Design in Marble Bearing Area

Computation of Marble Quality Designation

RQD3

Average RQD =

RQDi x i

L1

L2

L2 – L1

Leng

th >

100

m

mLe

ngth

> 1

00

mm

Leng

th >

10

0 m

m

RQD1

RQD22

1

3L2(mPD)

L1 (mPD)

Cavities or infill

Marble Rock Cover Recovery =

MR

i

L1

L2

L2 – L1


Marble Mass Classes

Rock with widely spaced fractures and unaffected by dissolution

Rock slightly affected by dissolution, or slightly fractured but essentially unaffected by dissolution

Features

Fractured rock or rock moderately affected by dissolution

Very fractured rock or rock seriously affected by dissolution

Rock similar to Class IV marble except that cavities can be very large and continuous

I

II

Marble Class

III

IV

V

Very good

Good

MarbleClass

Fair

Poor

Very Poor

75 < MQD

50 < MQD ≤ 75

MQD Range(%)

25 < MQD ≤ 50

10 < MQD ≤ 25

MQD ≤ 10


833790

833840

833890

821690 821740 821790 821840

No. of selected borehole: 6

Displayed depth: -10 mPD ~ -15 mPD

Section 1-1

Section 2-2

Section 3-3

Section 4-4

Section 5-5

Marble with overhang

Driven piles with preboring

Driven piles

Boreholes

Area with insufficient

boreholes to identify the karstic features

Example of Usage of Karst Geomorphology on Piling Design

Contour of good marble rock for foundation

Pile Testing

Static Pile Load Tests

Preliminary or Trial Piles (to check design and workmanship)vs. compliance tests on Working Piles

Specifications - define load-unload cycles, criteria forstabilisation and acceptance criteria (controversial!)

Automation of static load tests [see Chan et al (2004), Proc.Conf. On Foundation Practice in Hong Kong, Centre forResearch & Professional Development]

Concrete block

Hydraulic jack

GirderStiffeners

Steel cleat

Test pile

Universal beam

Reference beam

Dial gauge

1.3 m minimum or 3D whichever is

greater

Kentledge block

Load cell

Pile diameter, D

Compression Load Test Using Kentledge

Typical Set-up for a Compression Load Test Using Tension Piles

Girders (2 nos.)

Test pile

Hydraulic jack

Dial gaugeLoad cell

Reference beam

Locking nut

Steel plate

Tension members

Reaction piles

Stiffeners

Minimum spacing

2m or 3 D whichever is greater

Pile diameter, D

Typical Set-up for Uplift (Tension) Load Test

Reaction beam

Hydraulic jack

Dial gauge

Clearance for pile movement

Reference beam

Minimum spacing

2m or 3 D whichever is greater

Locking nutSteel plates

Reaction pile or on crib pads

Stiffeners

Tension connectionSteel bearing plates

Pile diameter, D

Steel plate

Typical Set-up for Horizontal Load Test

Dial gauge

Reference beam

Test plates

Hydraulic jack

Test piles

Steel strut

Clear spacing and avoid connection between blinding layer

Pile cap

Hydraulic jack

Test pile

Clear spacing

Pile cap Dial gauge

Reference beamSteel strut

Deadman

(b) Deadman

(a) Reaction Piles

Pile cap

Test plate

(c) Weighted Platform

Hydraulic jack

Test pile

Clear spacing

Dial gauge

Reference beam

Platform

Weights

Pile cap

Test plate

Typical Set-up for Horizontal Load Test

Enable higher test load Test load ~ 30 MN Shaft resistance in uplift

direction

Osterberg load cell

O-cell

bored pile

rock mass

Osterberg Cell at pile toe (cast in and jack up

the pile column from below after concreting)

135

Specifications for Pile Load Test

Maintained Load Test

─ General Specification for Civil Engineering Works (HongKong Government)

─ BD’s Code of Practice for Foundations─ Architectural Services Department

─ Criteria similar to CoP for Foundations, but the rateof recovery of settlement and magnitude ofallowable residual settlement after removal of testload

─ Housing Department (now follows CoP for Foundations)─ No unified standard as yet in Hong Kong

Residual settlement

Loading

Applied load P2WL

Max. totalsettlement

Settlement duringmaintained load stage of pile load

test

Allowableresidual

settlement

Allowabletotal settlement

1

LAE

WL = working loadD = pile diameter

Pile Loading Test Acceptance Criteria (for small diameter piles)

*The consideration of residual settlement on unloading from twice design load not rational, particularly for long friction piles, & tends to

give a conservative assessment of pile capacity

= PL/AE+ D/120 + 4Davisson Criterion is based on quick

loading procedures!

Load Test on Piles Designed to Take Negative Skin Friction

Test load should allow for effects of NSF to examine adequacy of pile design

Should load to [2 P + 3NSF] assuming a factor of safety of 2, because 1 x NSF is acting against the applied load during load test

139

140

141

142

Instrumented Pile Load Tests

Purpose of pile instrumentation is to provide a betterunderstanding of the load transfer mechanism (i.e.mobilization of base capacity and shaft friction with piledisplacement)

Axial strains are usually measured (e.g. using strain gauges),which can be converted to stress and hence load at a givenlevel. The corresponding displacement can also be assessed,taking into account elastic compression of the pile shaft.

Given the pile load profile with depth, one can work out theshaft friction at different levels

Possible pile instrumentation :– Strain gauges (measure strain)– Fibre optics (measure strain)– extensometer (measure displacement)


Properly plan the pile loading test programme

– What parameters are being measured?– Will the installation method be used in production piles?– Is sufficient instruments allowed for redundancy?– Is loading test properly set up without unforeseen

interference?


Osterberg cell (Optional)

Hydraulic pump with pressure gauges

Strain gauges (at least two and preferably four gauges at each level). Quantity and number of gauges depend on the purpose of investigation and geology.

Telltale extensometer

attached to load cell

Expansion displacement

transducer

Cast-in-place large-diameter pile

Strain gauge for measuring concrete modulusData logger

Hydraulic supply line

Steel bearing plates

Rod extensometer

Reference beam

Steel bearing pads Dial gauge

Instrumentation Pile Loading Tests

Vibrating Wire Strain Gauge

147

Extensometers

148

= strain in steel or concrete [usual assumption of plain sections remain plain, therefore equal]

Ec = Young’s modulus of concreteEs = Young’s modulus of steelAc = cross sectional area of concreteAs = cross sectional area of steel

Shaft friction stress, fs, is given by:

fs = (P1 - P2) / Ashaft

where Ashaft = surface area of pile shaft between levels 1 and 2

P = pile loadP = (Ec Ac + Es As)

P1

P2

fs



Samples for measuring Young’s modulus of concrete Samples for measuring Young’s modulus of steel Strain correction for concrete Young’s modulus

/N = 1.0

/N = 0.5

C1

P11

P16

P9P15

P7 P19

P6

P14

B5

C2

P5

P10P8 P12P17

B4

B2

P4P13

P1

P2

B3

P18

B1

C3

B8C

P21-2

P20

B6C

B7C

P21-1

P22

B6T

B7T

B8T

P23

Mean SPT N Value

Mob

ilise

d Av

erag

e Sha

ft Re

sista

nce,

(k

Pa)

/ N = 5 / N = 3 t / N = 2 / N = 1.5 / N = 4

0

50

100

150

200

250

0 50 100 150 200

Mobilised average shaft resistance and SPT N values for replacement piles


/ N = 12

D15

D14

D13

D12

D11

D10

D7

D8

D9D6

D5

D3D4

D2

D1

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100

Mob

ilise

d Av

erag

e Sha

ft Re

sista

nce,

(k

Pa)

Mean SPT N Value

/ N = 9 / N = 6 / N = 5 / N = 4 / N = 3

/ N = 2

τ / N = 1

/ N = 1.5

τ / N = 0.5

Mobilised average shaft resistance and SPT N values for displacement piles

large-diameter displacement piles


B2P2B4

B5

B3

B1

P1

B6

/ N = 5

0

100

200

300

400

500

0 50 100 150 200 250Mean SPT N Value

Mob

ilise

d Av

erag

e Sha

ft Re

sista

nce,

(k

Pa)

/ N = 4 / N = 3 / N = 2

/ N = 1.5

/ N = 1

/ N = 0.5

Mobilised average shaft resistance and SPT N values for replacement piles with shaft-grouting


Dynamic Pile Load Test

Measure the time history of force (using strain gauges) and acceleration (using accelerometers and integrate to get velocity) - e.g. Pile Driving Analyzer (PDA)

CASE method to determine ultimate pile capacity using a damping factor, Jc (typically 0.45 to 0.5 in Hong Kong) -primarily for end-bearing piles

PDA can determine the energy transfer ratio (hammer efficiency), soil resistance to driving (driveability study), dynamic pile stresses and pile integrity

Pile Driving Analyzer


Strain gauge and accelerometers installed on steel piles


157


High-strain tests (stresses generated by pile driving hammer)

CAPWAP analysis can be carried out to determine the distribution of soil resistance, dynamic soil response and predict the pile-settlement curve for the pile

CAPWAP parameters can be correlated with site-specific static load tests


Key Points to Remember Geotechnical and engineering geological input - very

important for proper pile design Close supervision of critical activities by experienced

supervisors - vitally important Very difficult and costly to rectify pile defects later - must try

to get things right first time Unduly conservative design - can make matters worse by

making construction process difficult + prone to problems Appreciate problems of different processes + compatibility of

design assumptions & construction techniques is key Performance review and monitoring – important for

advancement of foundation design

QUESTIONS

Documents

Design of Piled Foundations - · PDF fileObjectives To understand the empirical nature of pile design and the role of precedents (load tests and monitoring) To understand the role