Design of Piled Foundations - · PDF fileSome rules are not conservative and are not based on soil mechanics principles ... APP-18 (PNAP 66) (A t it ... Based on soil/rock mechanic

Design of Piled Foundations

Sammy CheungSenior Geotechnical Engineer

GEO, CEDD GEO, CEDD

20 April 2013

OUTLINE OF PRESENTATION

Vertical Load Horizontal Load

Pile Group Pile Group Negative Skin Friction Instrumented Pile Test Results

Objectives

To appreciate the interaction between pile construction and pile design

To appreciate what can go wrong with different piling To appreciate what can go wrong with different piling techniques

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 inputproper geotechnical input

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

Short pile scandals in Hong Kong (magic tape, etc.)

Borehole B Borehole A Borehole log Simplified geologyBorehole log Simplified Borehole B Borehole A Borehole log A

Simplified geologyBorehole log B

Simplified geology

VI VIPotential risk of using an overly simplified

V

V

overly simplified geological model(e.g. layered-model in

IV

corestone-bearing saprolites)

III

III

II

II

I I

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

Common Pile Design in Hong Kong

Many Hong Kong-specific ‘deemed-to-satisfy’ rules are stipulated by the Authorityy

Rules were derived through experience & have been applied without geological considerationsgeological considerations

Some rules are not conservative and are not based on soil mechanics principlesprinciples

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

Common Pile Design in Hong Kong

Submissions for private and housing projects Building (Construction) Regulations Building (Construction) Regulations Code of Practice for Foundations, 2004

P i N f AP/RSE/RGE Practice Notes for AP/RSE/RGE

Submission for public projects GEO Publication No. 1/2006 GEO Publication No. 1/2006 Specifications (Arch SD)

E i ’ di i d i d d f i 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 g y

feasible by rational design method

Relevant PNAP for Foundation Submission for Private Projects

Key PNs include:APP 18 (PNAP 66) (A t it i f il t ti ) APP-18 (PNAP 66) (Acceptance criteria for pile testing)

APP-61 (PNAP 161) (Scheduled Area for karstic marble) APP-103 (PNAP 227) (Structures On Grade on Newly Reclaimed

Land) APP-16 (PNAP225) Ground Investigation Works in Scheduoled

Areas – Approval and Consent APP-134 (PNAP 283) (Designated Area of Northshore Lantau) APP-137 (PNAP 289) (Ground-borne Vibrations Arising from Pile

Driving and Similar Operations)

Foundation Design for Public Projects

Promote use of rational design Promote use of rational design First edition was published in 1996 Consolidate good design and Consolidate good design and

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

GEO Publication No. 1/96

Foundation Design for Public Projects

Updated experience cumulated in t recent years

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

expanded scope to include shallow expanded scope to include shallow foundations and recent advances

GEO Publication No. 1/2006

Other Useful References

INTRINSIC PROBLEMS ABOUT PILING DESIGN

The piling process changes the ground behaviour, for good or worse compacting, loosening the soilsp g g

It is the behaviour of the ground after pile installation that controls pile performance (pile soil interaction)performance (pile soil interaction)

Varying ground conditions involve uncertainty and risk – opportunity C l t d k b i d b ti d i i d i th Completed works are buried; observations and supervision during the installation process are important

In some cases, there may be time-dependent effects that could influence the development of pile capacity in the long term

COMMON PILE TYPES IN HONG KONG

Pile Types Typical range of pile i (kN)

Geotechnical load i icapacity (kN) carrying capacity

Displacement Piles

Driven H-piles 2000 kN to 3500 kN Shaft friction and end bearingDriven prestressed 1950 kN to 3500 kN gDriven prestressed

precast concrete piles1950 kN to 3500 kN

Jacked Steel H Pile 2950 kNJacked Steel H Pile 2950 kN

COMMON PILE TYPES IN HONG KONG

Pile Types Typical range of pile capacity (kN)

Geotechnical load carrying capacity

Replacement Piles

Socketed H-piles 3500 kN to 5300 kN Shaft friction on rockSocketed H piles 3500 kN to 5300 kN Shaft friction on rock

Auger piles 1500 kN Shaft friction on soil

Mi i il 1400 kN Sh ft f i ti kMini-piles 1400 kN Shaft friction on rock

Mini-bored piles 2000 kN Shaft friction on rock and end bearing

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

Bored piles Up to 80,000 kN (3.8 Shaft friction on soil/rock Bored piles Up to 80,000 kN (3.8 m bell-out)

Shaft friction on soil/rock and end bearing

TRADITIONAL PILE DESIGN IN HONG KONG

N d t id t h i l it d t t l it f Need to consider geotechnical capacity and structural capacity of piles

Driven piles – piles usually driven to a set based on dynamic drivingformula to match the structural capacity (e.g. 0.3 fy for steel H piles )

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

PILE INSTALLATION

• Displacement piles“h i t l t i t th d ith –“hammering steel or concrete into the ground with

sufficient energy to refusal"• Replacement piles

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

Sounds simple, but not so! Pile installation can affect pile material (damage), the ground (disturbance) & surrounding facilities(damage), the ground (disturbance) & surrounding facilities

EFFECTS OF PILE CONSTRUCTION ON GROUND

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

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

PILE DESIGNPILE DESIGN

PILE DESIGN

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

Rational design method 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! More economical design can be achieved!

RATIONAL PILE DESIGN APPROACH

An alternative to use of default values [e.g. presumed bearing pressure, h f f i i ]zero shaft friction]

Adequate ground investigation to assist in formulation of appropriate ground model

Characterization of ground properties by means of appropriate insitu and p p y pp plaboratory tests

Proper geotechnical + engineering geological input Proper geotechnical engineering geological input Design analysis to be based on principles of mechanics, and/or an

established empirical correlationsestablished empirical correlations Pile testing programme to verify design assumptions

Design of Axially Loaded Pile (Geotechnical Capacity)

P = Qs + QB

P

s B

Soil type 1

Qs = shaft capacityS l 2

Soil type 2

QB = base capacity

DESIGN OF AXIALLY LOADED PILE (STRUCTURAL CAPACITY)

Structural strength of piles to be determined in accordance with appropriate limitations of design stresses

Permissible stresses given in Code of Practice for Structural Use of Concrete & Code of Practice for Structural Use of Steel

For bored piles, reduce concrete strength by 20% where groundwater is likely to be encountered during concreting, or where concrete is placed underwater

Ultimate Pile Shaft Capacity

Q = x AQs s x As

= Ultimate shear stress in each soil stratums = Ultimate shear stress in each soil stratum

f f l h f h lAs = Surface area of pile shaft in each soil stratum

FACTORS AFFECTING SHAFT FRICTIONFACTORS AFFECTING SHAFT FRICTION

FACTOR AFFECTING SHAFT FRICTION

v

r

rθ

θ

Changes of radial effective stress affects the

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

Pile Shaft

Displacement piles – increases in radial stress

Replacement piles – decrease in radial Pile Shaft Replacement piles – decrease in radial stress

Factor Affecting Shaft Friction

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

ho is the locked-in effective horizontal stress after pile constructionho ph is the change of horizontal stress after pile construction

is the effective horizontal stress at failure and will be affected by:hf 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 to dilation of a denseduring pile loading which can increase (due to dilation of a densesoil), or reduce (due to compression of a loose soil)

SHAFT FRICTION IN GRANULAR SOILS

Two common design approaches as follows:

M th d 1 Eff ti t th dMethod 1 : Effective stress method

= K ’ tan [c’ is usually taken as zero]_

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

Suggested Ks Values for Method 1

Pile Type Ks/Kos o

Large Displacement Piles 1 to 2

Small Displacement Piles 0.75 to 1.25p

Bored Piles 0.7 to 1.0

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

Pile Shaft Interface Friction Angle, s

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

s is interface friction’ is effective angle of friction is effective angle of friction

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

TYPICAL VALUES IN SAPROLITES AND SANDS FOR METHOD 1

Type of Piles Type of Soils Shaft Resistance Type of Piles Type of Soils Shaft Resistance Coefficient, b

Driven small displacement Saprolites 0.1 – 0.4ppiles

pLoose to medium dense sand 0.1 – 0.5

Driven large displacement Saprolites 0.8 – 1.2ppiles

pLoose to medium dense sand 0.2 – 1.5

Bored piles & barrettes Saprolites 0.1 – 0.6Loose to medium dense sand 0.2 – 0.6

Shaft grouted bored Saprolites 0.2 – 1.2piles/barrettes

Noted: Only limited data for loose to medium dense sand

DESIGN PARAMETERS FOR FRICTION PILESMETHOD 2 (SPT CORRELATION)- METHOD 2 (SPT CORRELATION)

= f Ns fs . N

For bored piles/barrettes in granitic saprolites :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]preliminary design]

Pile types Ultimate Shaft FrictionPile types Ultimate Shaft Friction

Driven small 1.5 – 2.0 x SPT, max 160 kPadisplacement piles

Driven large 4.5 x SPT, max 250 kPaDriven large displacement piles

4.5 x SPT, max 250 kPa

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

Friction parameters previously accepted by BD :

Pile types Shaft grouting? Ultimate Shaft Friction Ultimate End Bearing

Barrettes formed using grab

YES - No Data - - No Data -

NO 1 2 SPT 200kP 10 SPT NO 1.2 x SPT, max 200kPa 10 x SPT, max 2000kPa

Barrettes formed YES 2.5x SPT, max 200kPausing cutter

NO 0.8 x SPT, max 200kPa

Bored piles YES 2.1 x SPT, max 200kPa

NO 0.8 x SPT, max 200kPa

DESIGN PARAMETERS FOR FRICTION PILES- METHOD 2 (SPT CORRELATION)

The design method involving correlations with SPT results is empirical

( )

The design method involving correlations with SPT results is empirical in natureLevel of confidence is not high particularly where the scatter in SPT N 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 Where possible, include a loading test on preliminary pile to confirm the design assumption.

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

k )cake)– Loosened/disturbed soil

Loss of Confining Stress due to Arching Effect

ULTIMATE END-BEARING CAPACITY

QB= qb x Ab

qb = Ultimate end bearing stress

Ab = Bearing area of pile base

ULTIMATE BEARING CAPACITY OF PILES IN GRANULAR SOILS

(a) Classical bearing capacit theor

qb = Nq · v

(a) Classical bearing capacity theory

q q

(b) Empirical correlation with SPT

qb = fb · Nb

( ) p

(c) Presumptive bearing pressure

qb = presumptive bearing pressure

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

1000

For driven piles,

f' =q

’1 + 402

For bored piles, ' = 100ity Fa

ctor, N 2

For bored piles, '1 – 3

where f'1 is the angle

100

aring

Capa

ci

where f 1 is the angle of shearing resistance prior to installation.

Bea

1025 30 35 40 45

Angle of Shearing Resistance' (°)

Ultimate Bearing Capacity of Piles in Granular SoilsBased on SPT N

0.6

Coarse sandCa

pacit

y Pile LengthBase diameter

≥ 15

0.4

Fine sand

nd Be

aring

CT N

bVa

lue

0.2Normally consolidated silt

Coarse sand

Fine sandUltim

ate E

n SP

0.00 5 10 15 20

Fine sand

Driven piles

Bored piles

Depth in bearing stratumBase diameter

Bored piles

Ultimate Bearing Capacity of Piles in Granular SoilsBased on SPT N

1.0

Loose sand0.75

n Fac

tor, f

r

0.5 Medium dense sand

Redu

ction

0.25 Dense sand

0 0

Base Diameter (m)

0.0

0 0.5 1.0 1.5 2.52.0

Base Diameter (m)

Load Transfer Mechanism and Mobilizationof Load-Settlement Curve

Ultimate Qs typically develops in a stiff manner, at a pile settlement of only b t 0 5% t 1% il di t

of Load Settlement Curve

about 0.5% to 1% pile diameter

Total

ad

Total

Base

Pile L

oa

Shaft

Ulti t Q t i ll d l t il ttl t f @ 10% ( l ) t 20% Pile settlement

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

Mobilisation Factors for Deriving Allowable Bearing Capacity

Allowable Load Carrying Capacity, QaQb

fb

Qs

fs= +

MaterialMobilisation Factor for

Sh f R fMobilisation Factor for

E d b i R i t f

b s

Shaft Resistance, fs End-bearing Resistance, fb

Granular Soils 1.5 3 – 5

Mobilisation factors for end-bearing resistance depend very much on construction. Recommended minimum factors assume: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

h ft i t shaft resistance end-bearing resistance

is high

Recommended Global Safety Factors for Pile Design

Method of DeterminingMinimum Global Factor of Safety

Method of DeterminingPile Capacity

against Shear Failure of the GroundCompression Tension Lateral

Theoretical or semi-empirical methods not verified by loading tests on preliminar piles

3.0 3.0 3.0

tests on preliminary pilesTheoretical or semi-empirical methods verified by a sufficient

2.0 2.0 2.0

methods verified by a sufficient number of loading tests on preliminary piles

Design Requirements

The allowable pile working load must not exceed: The allowable pile working load must not exceed:(a) ultimate capacity for bearing on and bond with the ground divided by

suitable factor of safetysuitable factor of safety,(b) structural capacity of the pile material divided by suitable factor of

safety (e g permissible structural stresses or sufficient marginsafety (e.g. permissible structural stresses or sufficient marginagainst buckling in slender piles), and

( ) th l t hi h d f ti b t l t d b th t t(c) the value at which deformation can be tolerated by the structure

Allowable Structural StressesBuilding (Construction) RegulationsBuilding (Construction) Regulations

The concrete stresses in cast-in-place concrete foundations The concrete stresses in cast in place concrete foundationsat working load shall not exceed 80% of the appropriatelimit design stress of concrete where groundwater is likelylimit design stress of concrete where groundwater is likelyto be encountered during concreting

KEY NON-GEOTECHNICAL FACTORS AFFECTING BEHAVIOUR OF BORED PILESPILES

Rate of concrete pour Rate of concrete pour

Fl idit f t Fluidity of concrete

Time of pile bore being left open prior to concreting (-generally better to minimise the ‘waiting time’ to avoid

l l )excessive soil relaxation)

Distribution of Wet Concrete Pressure00

5Rise = 8 m/hr Rise = 12 m/hr

10

15

h (m)

2 hr

20

25

Dept

h

2 hr

30

35

4 hr

4 hr

0 50 100 150 0 50 100 150 300250200

40

45

Set = 6 hr Set = 6 hr

Note: Faster concreting process will help to achieve higher wet concrete pressure, which

0 50 100 150 0 50 100 150 300250200Concrete Pressure (kPa) Concrete Pressure (kPa)

would help to achieve higher locked-in horizontal stresses in the ground

Swelling of granitic saprolite due to stressdue to stress

relaxation

* Important to ensure sufficient excess slurry head sufficient excess slurry head within pile bore

DILATANCY EFFECTS IN A DENSE SOIL WITH A ROUGH PILE/SOIL INTERFACEPILE/SOIL INTERFACE

CHANGES IN EFFECTIVE HORIZONTAL STRESSES DUE TO DILATANCY EFFECTS DURING SHEARINGDURING SHEARING

h

’ = rr

E1 +

dilatancy (change in radius)

r 1

r= dilatancy (change in radius)

relative to the pile radius rr

E = Young’s modulus = Poisson’s ratio

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 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 – Wet concrete will exert an outward fluid pressure against the drill shaft (minimise stress relief)

– Horizontal stress that can be restored after excavation may – Horizontal stress h that can be restored after excavation may be controlled by concrete pressure

GOOD PRACTICE FOR ENHANCING SHAFT FRICTION IN BORED PILES

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

concreting) Shaft grouting Shaft grouting

– grout pressure increase horizontal stressh f f l h h f f– improve strength of interface material hence shaft friction

SHAFT GROUTING PROCEDURE

1 – Crack fresh concrete cover using double packer and t ithi 24 h f ti t

2 – Carry out shaft grouting for each manchette from bottom to twater within 24 hours of casting concrete.

Water cracking must be carried out for all grouting pipes in the barrette (even the spare ones).

top.

Target Grout Intake used so far in Hong Kong is 35 l/m2 Area covered by each manchette or refusal pressure (around 50 bars), whichever occurs first The overall minimum average intake of 25 whichever occurs first. The overall minimum average intake of 25 l/m2 over the whole frictional area.If intake cannot achieved on some manchettes, the target intake for the manchette immediately above, below or on its side is Typical Grout Mix for 1 m3

increased if necessary.Grouting for all pipes to be used in one barrette can be carried out simultaneously.

Cement: 1000kg Bentocryl 86: 1.5 litres

Water: 666 litres Daracem 100: 4 litres

Bentonite: 15 kg

Local Instrumented Test Data for Bored Pilesft

=0.6 =0.5 =0.4

= 0 3

C3250

Aver

age S

haf

x(kP

a)

= 0.3

P14B2

150

200

Mob

ilise

dA

stan

ce,

max = 0.2

P23P9 P7

P19B5

P21-2

P20B4

B7C

P1 B3B7T

100

150

B9

B11

B10

Max

imum

Re

si

= 0.1

C1B8C

P11

P15P7

P6

B5

C2

P5

P10 P8 P12P17

B6CP21-1

P4P13

P2P22

P18

B1

B8T

50

P10 P8 P12B6T

00 100 200 300 400 500 600 700

Mean Vertical Effective Stress, 'v (kPa), v ( )

Figure A1 of GEO Publication 1/2006

Instrumented Test Data for Bored Piles

/N =

/N = 2.5 /N = 1.5

C3

250

1.0

erag

e Sha

ft kP

a)

P14B2

200

B11

/N = 0.5M

obili

sed

Ave

tanc

e, m

ax(k P14

P21-2

B4 B7C

B2

P1 B3

B7T

100

150

B10

0 5

Max

imum

MRe

sist

B8C

P16

P9P15

P7 P19

P6

B5

C2

P20

P5

B6CP21-1 P4

P13

P2

P22 B1

P23

50

100

B9

M

C1

P11P5

P10P8 P12P17

P13P18

B6T

B8T

00 50 100 150 200

Mean SPT N Value

Figure A2 of GEO Publication 1/2006

SOME OBSERVATIONS

Significant scatter in the pile performance based on local instrumented piletests (some unexpectedly low results have been measured for bored pilestests (some unexpectedly low results have been measured for bored pilesunder bentonite. Thus, load tests are important to confirm designparameters and workmanship for friction bored piles)parameters and workmanship for friction bored piles).

l f l d t t t d t b t d th l b d f th t values from load tests tend to be towards the lower bound of thatexpected for bored piles in granular materials (possibly due to lowhorizontal stresses in weathered rocks i e low K value)horizontal stresses in weathered rocks, i.e. low Ko value)

SOME OBSERVATIONS

The method and the SPT method for pile design are not necessarilyconsistent in that they may give different predictionsconsistent in that they may give different predictions

As a pragmatic approach, it is probably best to use both methods to assist indecision-making regarding pile design capacitydecision-making regarding pile design capacity

It is important to make reference to the results of previous instrumented pileload tests in similar ground conditions for the respective pile constructionload tests in similar ground conditions for the respective pile constructionmethods [role of precedents + design by load tests]

Deem-to-satisfied RulesDeem to satisfied Rules

PRESUMED ALLOWABLE BEARING PRESSURE

Code of Practice for Foundations by Buildings Department (2004)

Category Description of RockPresumed

Pressure (kPa)k ( d l )

2Rock (granitic and volcanic) :Highly decomposed, moderately weak to weak rock ofmaterial weathering grade IV or V or better with SPT N

1,000material weathering grade IV or V or better, with SPT Nvalue of 200



Pressure (kPa)R k ( iti d l i )

1(a)Rock (granitic and volcanic) :Fresh strong to very strong rock of material weatheringgrade I, with 100% total core recovery and no weathered

10,000grade I, with 100% total core recovery and no weatheredjoints, and minimum uniaxial compressive strength of rockmaterial (σc) not less than 75 MPa (equivalent point loadindex strength PLI50 not less than 3 MPa).

1(b) Fresh to slightly decomposed strong rock of materialth i d II b tt ith t t l f

7,500weathering grade II or better, with a total core recovery ofmore than 95% of the grade and minimum uniaxialcompressive strength of rock material (σc) not less than 50compressive strength of rock material (σc) not less than 50MPa (equivalent point load index strength PLI50 not lessthan 2 MPa).



Pressure (kPa)1( ) Sli htl t d t l d d d t l t 5 0001(c) Slightly to moderately decomposed moderately strong

rock of material weathering grade III or better, with atotal core recovery of more than 85% of the grade and

5,000

total core recovery of more than 85% of the grade andminimum uniaxial compressive strength of rock material(σc) not less than 25 MPa (equivalent point load indexstrength PLI50 not less than 1 MPa).

1(d) Moderately decomposed, moderately strong tod t l k k f t i l th i d

3,000moderately weak rock of material weathering gradebetter than IV, with a total core recovery of more than50% of the grade.50% of the grade.


Based on simple material classificationI t d d f f d ti h i t l d ith li ibl l t l l d 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 Minimum socket length = 0.5 m for categories 1(a) & 1(b), and = 0.3 m forcategories 1(c) & 1(d)Total core recovery = % ratio of rock recovered (whether solid intact with Total core recovery = % ratio of rock recovered (whether solid intact withno full diameter, or non-intact) to 1.5 m length of core run + should beproved to at least 5 m into the specified rock categoryproved to at least 5 m into the specified rock category

Self weight of pile - no need to further consider in calculation of bearingstressesstresses


Use of Total Core Recovery (TCR) as sole means of determining founding level y ( ) g g+ 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

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


P10-2O (13.6)30

sure

(MPa

) P15OP14

P7-2O

P2CP13-2O

P11-2O

P11-1(15 5)

(2)

(12.6)(3) (7.5)

(11.3)(?) 20

25settlement at pile base (mm)

n bea

ring p

ress

P9-3O

P9-1 (64)

(86)

(15.5)

10

15

Code of Practice for FoundationsCategory 1(a)

Prov

en P1C

P3C(2.5)(1.2)

5

10Category 1(b)

Category 1(c) pile load predominately taken by shaft resistance

Uniaxial compressive strengthf i k (MP )

00 25 50 75 100 125 150 175 200

P9 founded on granodiorite. UCS f k 15 MP of intact rock (MPa)of rock ~ 15 MPa

Key Points to Remember

Geotechnical and engineering geological input - very important for proper pile g g g g p y p p p pdesign

Close supervision of critical activities by experienced supervisors - vitally 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 Very difficult and costly to rectify pile defects later - must try to get things right first timeU d l ti d i k tt b ki t ti 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

Rock SocketsRock Sockets

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 rockg

Design of Rock Sockets

)10000

Rock

, (kP

a)

P10-1

P10-2O

P7-2O

P7-1P1T

P1C

sista

nce i

n R

P16 P8

P3T

P3C

P2T

P1C

1000

ed Sh

aft R

es

C1 P9-11000

Mobil

ise

s = 0.2 c0.5

10100

1 100 1000

Uniaxial Compressive Strength of Rock, q (Mpa)


Recommendations in Code of Practice for Foundations

• 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

di h 2 il di 6 ( hi h i corresponding to not more than 2 x pile diameters or 6 m (whichever is shorter) plus the presumptive bearing value

• The minimum socket depths stipulated in the presumed bearing pressures should be ignored in bond calculation.

Presumptive Design Parametersin BD’s Code of Practice for Foundationsin BD s Code of Practice for Foundations

Category Rock Mass Weathering Minimum Embedment (m)

Allowable ShaftFriction (kPa)

1a Grade I or better 0.5 700

1b Grade II or better 0.5 700

1c Grade III or better 0.3 700

1d Grade IV or better 0.3 300

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

of a problem)

CALCULATION OF ROCK SOCKET LENGTH

• General equation :

R A f LR = 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.

d f k k f f h l d f h


Note : Load transfer in a rock socket is a function of the slenderness ratio of the rock socket & the relative pile/rock stiffness (based on numerical analysis)

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 (BD CoP stipulates L/D

ratio up to 2 or 6 m length, whichever is less, for shaft resistance calculation)

otherwise, pile loading tests need to be carried out to confirm the design

SOIL SEAMS/SEDIMENTS BELOW BASE OF BORED PILES ON ROCK -PROBLEM OR NOT?PROBLEM OR NOT?

Presumed bearing pressure of, for example, 5MPa – corresponds to 85% TCR, therefore not all needs to be rock!

With rock sockets, the confinement at base is substantially increased – this will give rise to an effective increase in the strength

Increase in 3, due to confinement - approximately follows a constant (1 /3) stress path, which has very high constrained secant modulus (about 200 MP 300 MP )( f Li l 2000) i i 200 MPa to 300 MPa )(ref. Li et al, 2000) – important to use appropriate stiffness for settlement calculations

Design of Driven PilesDesign of Driven Piles

Design of Driven Piles (Hong Kong practice)

Working load = allowable material compressive stress x cross-sectional area Drive to set as calculated from dynamic pile driving formula Estimates of required pile depth is usually made based on rules of thumb (e.g.

by relating to SPT N values - typically drive to a depth with N value of 50 toby e a g o S a ues yp ca y d e o a dep a ue o 50 o100 for concrete piles, or a depth with N value of 180 to 200 for H-piles)

Design of Steel H-piles

Typical H-sections

– 305 x 305 x 110 kg/mg/– 305 x 305 x 180 kg/m

305 x 305 x 223 kg/m– 305 x 305 x 223 kg/m

For Grade 55C steel H piles, allowable load is taken as 30% yield stress (fy, p , % y ( y,which is a function of the steel grade and thickness) x As [e.g. fy for 305x305x180 pile = 430 MPa]p ]

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

Dynamic load tests + static load tests are usedy

The final set table is developed using a factor of safety of 2

Driven Piles Founded on Rock

Grade 55C steel sections with yield stress, fy, of 425 MPa, allowable stress = 0 3 f (129 MPa)0.3 fy (129 MPa)

Very high stresses on rock why okay? 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) 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 SingaporeFoundation Practice, Singapore

Driven Piles Founded on Rock

A suitable pile point (stiffener) may be used at the pile toe to prevent sliding i li d k fon 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? g g g

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

Based on energy consideration

W H

gy

R = W H S + 1

2 (C1 + C2 + C3)X h

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

Hiley Pile Driving Formula -(commonly used in Hong Kong)(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)c1 temporary compression of pile head (elastic)c2 + c3 = temporary compression of pile and ground (elastic)

W = weight of hammer

P = weight of pileP weight of pile

e = coefficient of restitution between hammer and pile cushion

H = drop distance of hammer

Table for Final Set (mm) per 10 Blows

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Temporary Compression, Cp + Cq (mm)

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 -- -- -- -- -- --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 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 -- -- -- -- -- -- -- -- -- -- 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 -- -- -- -- -- -- -- -- -- -- -- -- -- --

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)uHAMMER MODEL Drop Hammer (8 ton)WEIGHT OF RAM, W 80 kNCOEFFICIENT OF RESTITUTION, r 0.32,TEMPORARY HELMET COMPRESSION, Cc 2.5 mmWEIGHT OF PILE HELMET, Wd 3 kNHEIGHT OF DROP H 2 8 mHEIGHT OF DROP, H 2.8 mENERGY EFFICIENCY, 0.8ENERGY OUTPUT PER BLOW, E 224 kN-mEFFECTIVE ENERGY E' = E 179 kNEFFECTIVE ENERGY, E = E x 179 kN-m

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

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

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

Problems with Hiley Formula

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

Hammers do not always operate at their rated efficiency and can be highlyi blvariable

Energy absorption property of cushions can vary with timell b d f d d l h h d l Past experience generally based on use of drop or diesel hammers; hydraulic

hammers presents a problem with the empirical factors, therefore a drophammer is used to check final sethammer is used to check final set

Pile Hammers

Previous extensive use of diesel hammers was effectively banned since 19971997

Drop hammers (typical efficiency assumed in private sector = 0.7 to 0.8) -normally site measurements (by PDA) required if proposed energy coefficient normally site measurements (by PDA) required if proposed energy coefficient is >0.8Hydraulic hammers (not accepted by BD for final set) typical efficiency = 0 9 Hydraulic hammers (not accepted by BD for final set); typical efficiency = 0.9 or higherHKCA studies on hydraulic hammers in 1995 and 2004 respectively 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 settingproductivity), but a drop hammer is used for final setting

Recent Work on Design of Driven Piles

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

measure the driving energyC l ( hS 2003) f d f h CAPWAP analysis (ArchSD, 2003) to find parameters for matching the pile capacity as determined by Hiley Formula (combination of and e as ‘correction factors’)

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

R =EMX

where EMX is the actual energy transfer to pile head

R =[s + ½ (cp + cq)]

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 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)

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) not > 50% of yield stress of the combined axial and flexural stress) not 50% of yield stress of the steel H pile (contribution by grout ignored), because no need to consider driving stressesg

Pre-bored Steel H-piles

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

Under Compression : allowable grout/steel bond <600 kPa (x reduction factor 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 providednominal shear studs are provided

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

Design of Mini-piles

Assessment of structural capacity (BD allows consideration of steel bars only. p y ( yOverseas 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%) –p ( )presumed allowable rock/grout bond strength up to 700 kPa for compression (see CoP)

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)

Negative Skin FrictionNegative Skin Friction

93

Negative Skin Friction (Downdrag)

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

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

ttl t d ti f t d t d t i d i isettlement, reduction of water pressure due to dewatering and increase ineffective stress, dissipation of excess pore water pressure (and hencesettlement) in soft clay induced by pile drivingsettlement) in soft clay induced by pile driving

94


P Pile shortening

N ti ki f i ti

Negative skin friction(Soil drags down pile) Soil type 1

Neutral plane

Positive skin friction

S l 2

pNo relative movement

Positive skin friction(Pile settles relative

to the ground)

Soil type 2

to the ground)

Ground settlement

QB = base capacity


NSF = Ks V’ tan p LNSF = V’ p L

Soil Type

Soft Clay 0.20 - 0.25

Silt 0.25 - 0.35

Sand 0.35 - 0.50

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

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

(BD s CoP on Foundations)

(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 :(b) Pile structural integrity check :

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

(c) Settlement check :

l d ( ) h ld b fSettlement under (D + L + NSF) should be satisfactory

Means to Reduce NSF

Driven piles - bitumen coating or asphalt coating, plastic sheet, “Yellow p g p g, p ,Jacket”, etc. (Note - need to carefully review effectiveness and potential for damage to such protective layers during pile driving into competent ground)

Permanent casing for bored piles Sacrificial protection piles around the structure foundation Ground improvement techniques to strengthen/stiffen the soft soilsp q g /

98

Design of Lateral Load Capacity of Piles

The lateral load capacity of a pile may be limited in three ways :The lateral load capacity of a pile may be limited in three ways :

(a) shear capacity of the soil,(b) structural (i.e. bending moment and shear) capacity of the pile

section, and(c) excessive deformation of the pile.(c) excessive deformation of the pile.

l i l l il i f fi d h d d f h d il i


ultimate lateral soil resistance for fixed-head and free-head piles in granular soils are put forward by Broms

e1

HH

L L

Centre ofrotation

Free-head Fixed-head

(a) Short Vertical Pile under Horizontal Load

H


ultimate lateral soil resistance for fixed-head and free-head piles in granular soils are put forward by Broms

e1HH e11

F t

Fracture

1

L LFracture

Free-head Fixed-head

(b) Long Vertical Pile under Horizontal Load


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

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

E I5

T = √Ep Ip

nh

5

where nh is the constant of horizontal subgrade reaction


Pile Type Soil ModulusPile Type Soil Modulus

Linearly increasing Constant

Short (rigid) piles L ≤ 2T L ≤ 2R

Long (flexible) L ≥ 4T L ≥ 3.5Rpiles

Design of Lateral Load Capacity of Piles0

1

0

1

= 2 = 2

2

3

2

3L

z

dM

M

L

z

dH

H

2

3 3

-1 0 1 2 3 -1 0 1 2 3

44 dM = Fd dH = Fd 4, 5 & 10

4

5 & 10

Deflection Coefficient, Fd for Applied Moment M Deflection Coefficient, Fd for Applied Lateral Load, H

0

1

0

1= 2

d pp d pp

2

3

2

3L

z

M

L

z

H

= 2

3

4

3

44

0 0.2 0.4 0.6 0.8 1.00 0.2 0.4 0.6 0.8

MM

MM = FM (M)MH = FM (HT)

LMH

4

5 10

4

5 10

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


00

11

2

3

2

3

z

H

z

M

= 2 = 2

3 3

4

3

4

V = F (H)

L

VH

VM = Fv ()

L

VM

10 10 5

4

5

4

3

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

VH = Fv (H)10 5

Shear Coefficient, Fv for Applied Moment M Shear Coefficient, Fv for Applied Lateral Load, H

Foundation Design in Marble Bearing Area

Scheduled Area No. 2 in the Northwest New Territories

Scheduled Area No. 4 in Ma On Shan reclamation area


Designated Area in Northshore Lantau

Carbonate Rocks in Northwest New Territories

FormationMember / Thickness

Material Description Age Dissolution

Tuen Mun F

Tin Shui Wai

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

bl t it t ilt t t pper

rass

ic

LimitedFormation

Tin Shui Wai marble, quartzite, metasiltstone etc, clasts < 3 m

Up Jur Limited

Yuen Long Formation

Ma Tin

> 200 mMassively bedded, white crystalline marble,

locally dolomitic and siliceous

erou

s Main dissolution

FormationLong Ping

> 300 m

Grey to dark grey, finely crystalline marble intercalated and interbedded with meta-

di

Carb

onif

Limited 300 m sediment

Carbonate Rocks in Ma On Shan

FormationMember /Thickness

Material Description Age Dissolution

Ma On Shan > 200 m

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

rous

VaryFormation

> 200 m with thin interbeds of dark grey to black meta-siltstone

Carb

on Vary

Pure Marble in Ma Tin Member

White pure crystalline marbleWhite, pure, crystalline marble

Impure Marble in Long Ping Member

Grey to dark grey fine grained dolomitic marbleGrey to dark grey, fine-grained dolomitic marble

Marble-clast bearing rock

Marble clastMarble clast

Foundation Design

Foundation systemy

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


d dGround investigation

Ground modelling

Foundation design

Foundation R i f i M i i f b ildi

constructionReview of construction Monitoring of building

FOUNDATION DESIGN IN MARBLE BEARING AREA

Geotechnical Contents in Design Submission

Interpretation of ground conditionsl i l d l geological model

karst geomorphology (GEO Report Nos. 28, 29, 32)

Foundation systemf di l l f d f d ti founding levels of deep foundation

increase of stresses at marble surface

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

FOUNDATION DESIGN IN MARBLE BEARING AREA

Construction

driven piles pile driving record pile driving record

bored piles pre-drilling investigation pre-drilling investigation

Conclusion of construction performance review performance review post-construction tests, e.g. CAPWAP, PDA, pile loading tests PDA useful to identify broken piles and 12% ~ 28 % of piles PDA useful to identify broken piles and 12% 28 % of piles

were tested in some projects


Monitoring

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

i doccupied

Computation of Rock Quality Designation

Foundation Design in Marble Bearing AreaComputation of Rock Quality Designation

Core at least one Core at least one Core at least one full diameterfull diameterfull diameter

Length > 100 mm Length > 100 mmLength > 100 mm

RQD1RQD2 RQD3

100 mm


Computation of Marble Quality h >

m

m

L1 (mPD)

Computation of Marble Quality Designation

Leng

t10

0 m

RQD11

RQDi x i

L1

00

1

Average RQD = i iL2

L2 – L1

Leng

th >

1m

m RQD22

L2 L1Cavities or infill

L1

RQD3

h >

100

m

m

3L2(mPD)

Marble Rock Cover Recovery =

MR

iL2

Leng

th mMR L2 – L1

M bl M Cl


Marble Mass Classes

FeaturesMarble ClassMarble MQD Range

Rock with widely spaced fractures and unaffected by dissolution

Features

I

Marble Class

Very good

Class

75 < MQD

(%)

dissolution

Rock slightly affected by dissolution, or slightly fractured but essentially unaffected by dissolution II Good 50 < MQD ≤ 75

Fractured rock or rock moderately affected by dissolution III Fair 25 < MQD ≤ 50

Very fractured rock or rock seriously affected by dissolution IV Poor 10 < MQD ≤ 25

Rock similar to Class IV marble except that cavities can be very large and continuous V Very Poor MQD ≤ 10

No. of selected borehole: 6

Displayed depth: -10 mPD ~ -15 mPDDriven piles with preboring

Driven pilesExample of Usage of Karst

M bl i h h

Driven piles

Boreholes

p gGeomorphology on Piling Design

833890

Marble with overhang

Contour of good marble rock for foundation

Section 1-1

Section 2-2

Section 3-3

Section 4-4

Section 5-5

833840

Area with insufficient Boreholes to identify the

833790821690 821740 821790 821840

ykarstic features


Attention!

No simple rule in a complex ground conditionp p g

Engineering judgement is important g g j g p

Pile TestingPile Testing

Static Pile Load Tests

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

Specifications define load unload cycles criteria for stabilisation and Specifications - define load-unload cycles, criteria for stabilisation andacceptance criteria (controversial!)A i f i l d [ Ch l (2004) P C f O Automation of static load tests [see Chan et al (2004), Proc. Conf. OnFoundation Practice in Hong Kong, Centre for Research & ProfessionalD l t]Development]

Compression Load Test Using Kentledge

Kentledge blockblock

Concrete

GirderStiffeners

Steel cleat

Universal beam

Dial gauge

Load cell

Concrete block

Hydraulic jackReference beam

Test pile

beam

1.3 m minimum or 3D whichever is greater Pile diameter,

D

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

Girders (2 nos.)Locking nut

Steel plate

Di l Load cell

Tension members

Stiffeners

Hydraulic jack

Dial gauge

Reference

Test pile

Hydraulic jackbeam

Reaction piles Minimum spacing

2m or 3 D whichever is

Pile diameter, D

greater

Typical Set-up for Uplift (Tension) Load Tests

Reaction beam

Locking nutSteel plates

Hydraulic jackTension connection

Steel bearing plates

Steel plate

Clearance for pile t

Reaction pileStiffenersDial gaugemovement

Reference beamMinim m spacing

or on crib pads

beamMinimum spacing

2m or 3 D whichever is greater

Pile diameter, D

Typical Set-up for Horizontal Load Test

Reference beamHydraulic jack

Steel strut

Dial gauge

Test l t

Clear spacing and avoid

Pile capPile cap

plates

Test piles

connection between blinding layer

(a) Reaction Piles

Reference beamSteel strut

Hydraulic jack

Pile cap Dial gauge

e e e ce ea

Test pile

Clear spacing Deadman Test plate

(b) Deadman

Typical Set-up for Horizontal Load Test

Weights

Hydraulic jack Reference beam

Dial gaugePlatformPile cap

( )

Test pile

Clear spacing Test plate

(c) Weighted Platform

Osterberg load cell

Enable higher test load bored

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

pile

Shaft resistance in uplift directionrock

O-cellmass

Hydraulic pump with

Steel bearing pads Dial gauge

INSTRUMENTATION PILE LOADING TESTSpressure gauges

Strain gauge for measuring concrete modulus

Data logger

Reference beam

Telltale extensometer

attached to load cell

Cast-in-place large-diameter pile

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.g g gy

Rod extensometer

Hydraulic pump with




Data logger

Reference beam





Hydraulic supply line

Rod extensometer

Expansion displacement

Steel bearing plates

Osterberg cell (Optional)

displacement transducer

OSTERBERG Cell at pile toe (cast in and jack up the pile

column from below after concreting)

133

SPECIFICATIONS FOR PILE LOAD TEST

General Specification for Civil Engineering Works (Hong Kong General Specification for Civil Engineering Works (Hong KongGovernment) and corresponding Guidance Notes

Architectural Services Department Architectural Services Department PNAP 66 and BD’s Code of Practice for Foundations Housing Department (previous one superseded now adopt criteria in Housing Department (previous one superseded, now adopt criteria in

CoP) No unified standard as yet in Hong Kong No unified standard as yet in Hong Kong

PILE LOADING TEST ACCEPTANCE CRITERIA (FOR SMALL DIAMETER PILES)

Residual settlement

Applied load P2WLAllowable

residualsettlement

SMALL DIAMETER PILES)

Residual settlement

Loading

D/120 + 4

Max. totalsettlement

Settlement duringmaintained load stage

of pile load test

Allowabletotal settlement L

AE= PL/AE+ D/120 + 4 of pile load test1

WL = working loadD = pile diameter

= PL/AE+ D/120 + 4

D pile diameter

*The consideration of residual settlement on unloading from twice design load not rational,

= PL/AE+ D/50Allowable

total settlement

g g ,particularly for long friction piles, & tends to give a conservative assessment of pile capacity

135

LOAD TEST ON PILES DESIGNED TO TAKE NEGATIVE SKIN FRICTION

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

137

138

139

140

Instrumented Pile Load Tests

Purpose of pile instrumentation is to provide a betterunderstanding of the load transfer mechanism (i.e.mobilisation of base capacity and shaft friction with piledi l t)displacement)

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

INSTRUMENTED PILE LOAD TESTS

Given the pile load profile with depth, one can work out the shaft frictionat different levels

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

• Place the instruments carefully with full understanding of what isbeing measured.

142

Hydraulic pump with




Data logger

Reference beam


Outer ring casing




Rod extensometer

VIBRATING WIRE STRAIN GAUGE

144

EXTENSOMETERS

145

P = ( Ec x Ac + Es As)

= t i i t l t [ l ti f l i ti i P = pile load = strain in steel or concrete [usual assumption of plain sections remain plain, therefore equal]Ec = Yo ng’s mod l s of concrete (adj st for different stress ratio)Ec = Young’s modulus of concrete (adjust for different stress ratio)Es = Young’s modulus of steelAc = cross sectional area of concreteAc = cross sectional area of concreteAs = cross sectional area of steel

Shear stress, fs, is given by:

f = (P P / Afs = (P1 - P2) / Ashaft

where Ashaft = surface area of pile shaft shaftbetween levels 1 and 2 146

DYNAMIC PILE LOAD TEST

Measure the time history of force (using strain gauges) and acceleration ( i l t d i t t t t l it ) Pil D i i (using accelerometers and integrate to get velocity) - e.g. Pile Driving Analyser (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

148

Involve signal matching to get a good enough fit by adjusting the input values of Involve signal matching to get a good enough fit by adjusting the input values of the pile-ground model

Dynamic Pile Load Test

Strain gauge and accelerometers installed on steel piles


151


High-strain tests (stresses generated by pile driving hammer) CAPWAP analysis can be carried out to determine the distribution of soil 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 for the pile

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

Note : pile capacity may not be fully mobilised in dynamic load tests because of limited pile movement

152

PILE INTEGRITY TESTS

Quality control - serve as comparative and screening tests Augment other tests and control measures Retrospective investigation (after pile construction) Indirect testing (need expert interpretation) Indirect testing (need expert interpretation) Checking pile integrity but not the bearing capacity

TYPES OF PILE INTEGRITY TESTS

Sonic logging (also known as sonic coring) Sonic logging (also known as sonic coring) Pile integrity test (PIT)

frequency-based (or impedance) tests time-based (or echo) tests( )

Dynamic pile load tests

SONIC LOGGING

Acoustic principles - measure propagation time of sonic transmission between itt & i b i t b t i ilemitter & receiver probes in tubes cast in pile

Used in bored piles & barrettes Check for presence of defects in concrete Tests can’t tell you the nature of defectsy No depth limitation due to damping effects

Need pre selection of piles (okay if all!) Need pre-selection of piles (okay if all!) Sudden increase in sonic wave travel time suggests local area of lower quality

concrete

156

157

TYPICAL TUBE LAYOUTS FOR SONIC LOGGING

(a)

With 3 t b (3 th )

(b)

With 4 t b (6 th )With 3 tubes (3 paths) With 4 tubes (6 paths)

WAYS OF CONDUCTING SONIC LOGGING TESTS

159

PILE INTEGRITY TESTS

Acoustic anomalies may not correspond to structural defects Cannot identify definitely whether defects will affect pile behaviour

under loading or long-term performance Interpretation of test results needs expert input and possibly

subjective in not so straightforward casesj g

Documents

Design of Piled Foundations - · PDF fileSome rules are not conservative and are not based on soil mechanics principles ... APP-18 (PNAP 66) (A t it ... Based on soil/rock mechanic