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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
PRESUMED ALLOWABLE BEARING PRESSURE
Category Description of RockPresumed
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).
PRESUMED ALLOWABLE BEARING PRESSURE
Category Description of RockPresumed
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.
PRESUMED ALLOWABLE BEARING PRESSURE
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
PRESUMED ALLOWABLE BEARING PRESSURE
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
PRESUMED ALLOWABLE BEARING PRESSURE
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)
DESIGN OF ROCK SOCKETS
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
DESIGN OF ROCK SOCKETS
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
Negative Skin Friction (Downdrag)
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
Negative Skin Friction (Downdrag)
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
Design of Lateral Load Capacity of Piles
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
Design of Lateral Load Capacity of Piles
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
Design of Lateral Load Capacity of Piles
(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
Design of Lateral Load Capacity of Piles
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
Design of Lateral Load Capacity of Piles
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
Foundation Design in Marble Bearing 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
Foundation Design in Marble Bearing Area
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
Foundation Design in Marble Bearing Area
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
Foundation Design in Marble Bearing Area
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
Foundation Design in Marble Bearing Area
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
Foundation Design in Marble Bearing Area
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
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
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
Steel bearing pads Dial gauge
INSTRUMENTATION PILE LOADING TESTSpressure gauges
Strain gauge for measuring concrete modulus
Data logger
Reference beam
Telltale extensometer
Outer ring casing
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
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
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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
DYNAMIC PILE LOAD TEST
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DYNAMIC PILE LOAD TEST
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
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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
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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
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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