design of piled foundations - · pdf fileobjectives to understand the empirical nature of pile...
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Design of Piled Foundations
Sammy CheungSenior Geotechnical Engineer
Geotechnical Engineering OfficeCivil Engineering and Development Department
30 April 2011
Outline of Presentation
Vertical Load Horizontal Load Negative Skin Friction Pile Group Instrumented Pile Test Results
Objectives
To understand the empirical nature of pile design and the role of precedents (load tests and monitoring)
To understand the role of rational design approach and proper geotechnical input
To appreciate the interaction between pile construction and pile design
To appreciate what can go wrong with different piling techniques
General Perspective
Ground conditions in Hong Kong are complex and can pose major challenge to piling design and construction (e.g. corestone-bearing weathered profiles, karstic marble, deep and/or steeply inclined rock head)
Piling design in Hong Kong is always criticized for overly conservative design.
V
IV
III
II
V
III
Borehole B Borehole A Borehole log A
Simplified geology
Borehole log B
Simplified geology
Note : (1) Refer to Geoguide 3 (GCO, 1988) for classification of rock decomposition grade I to grade VI.
II
I I
VI VI Potential risk of using an overly simplified geological model(e.g. layered-model in corestone-bearing saprolites)
General Perspective
Pile Design in Hong Kong
Many Hong Kong-specific ‘deemed-to-satisfy’ rules are promulgated by the Authority
Rules were derived through experience but are applied without geological considerations and soil mechanics principles
Some rules are not conservative
Unnecessarily long piles may encounter major problems during construction (so could end up as being worse off!)
Pile Design in Hong Kong
Submissions for private and public housing projects Building (Construction) Regulations Code of Practice for Foundations, 2004 Practice Notes for AP/RSE/RGE Foundation Handbook (in preparation)
Submission for public projects GEO Publication No. 1/2006 Specifications (Arch SD) Engineer’s discretion on adopting standards for
private submission
Foundation Design for Private Projects
Buildings (Construction) Regulations
AP/RSE Notes Code of Practice for
Foundations (2004) deemed-to-satisfy rules more economic design may
be feasible by rational design methods
Relevant Practice Notes for Foundation Submissionfor Private and Public Housing Projects
Key PNs include: PNAP 66 (Acceptance criteria for pile testing) PNAP 161 (Scheduled Area for karstic marble) PNAP 227 (Structures On Grade on Newly Reclaimed
Land) PNAP 242 (Qualified supervision) PNAP 282 (Designated Area of Northshore Lantau) PNAP 289 (Ground-borne Vibrations Arising from
Pile Driving and Similar Operations)
Promote use of rational design First edition was published in
1996 Consolidate good design and
construction practice for pile foundations, with special reference to Hong Kong’s ground conditions
Foundation Design for Public Projects
GEO Publication No. 1/96
Updated experience cumulated in recent years
Piling data obtained from the instrumented piling load tests programme for the railway projects
Expanded scope to include shallow foundations and recent advances
Foundation Design for Public Projects
GEO Publication No. 1/2006
Other Useful References
Basic Facts about Piling
Varying ground conditions involve uncertainty and risk Completed works are buried; observations and supervision
during the installation process are important All forms of pile construction will affect the ground conditions -
the question is by how much Different piling techniques and workmanship will affect the
ground in different ways It is the behaviour of the ground after pile installation that
controls pile performance (pile soil interaction) In some cases, there may be time-dependent effects that
could influence the development of pile capacity in the long term
Pile Installation
Displacement piles–“hammering steel or concrete into the ground with sufficient energy to refusal"
Replacement piles–“dig a hole and fill with steel and concrete"
Sounds simple, but not so! Pile installation can affectpile material (damage), the ground (disturbance) andsurrounding facilities
Common pile types in Hong Kong
Pile Types Typical range of pile capacity (kN)
Geotechnical load carrying capacity
Displacement PilesDriven H-piles 2000 kN to 3500
kNShaft friction and end bearing
Driven prestressed precast concrete piles
1950 kN to 3500 kN
Jacked steel H-pile
2950 kN
Jacked steel H-pile – not that common
Common pile types in Hong KongPile Types Typical range of pile
capacity (kN)Geotechnical load carrying capacity
Replacement PilesSocketed H-piles 3500 kN to 5300 kN Shaft friction on rockAuger piles 1500 kN Shaft friction on soilMini-piles 1400 kN Shaft friction on rockMini-bored piles 2000 kN Shaft friction on rock
and end bearingBarrettes Up to 20,000 kN Shaft friction on soil
and end bearing Bored piles Up to 80,000 kN (3.8
m bell-out)Shaft friction on soil/rock and end bearing
Pile Design
Driven piles – piles usually driven to a set based on dynamicdriving formula to match the structural capacity (e.g. 0.3 fy forsteel H piles )
Bored piles & socketed H-piles – piles are usually designedas end-bearing and limited shaft friction on rock. If depth ofweathering is significant, the piles behave as ‘friction piles’instead.
Need to consider geotechnical capacity and structural capacity of piles
Effects of Pile Construction on Ground
• Displacement piles (driven piles) - akin to ‘cavity expansion’ problems, with the horizontal stresses increased and granular soils subject to densification and compaction
• Bored piles - stress relief effect due to hole formation; horizontal stresses in the ground reduced and ground is subject to loosening
Pile Design
DESIGN ISSUES
Ultimate foundation capacity & overall stabilityCyclic loading effects (wind, uplift)Overall settlementsDifferential settlementsStructural designEffects of external ground movements
Pile Design
Essential for a successful outcome
A key geotechnical output is the values of pile stiffness (axial, lateral, rotational) for each pile within the group
These can be incorporated into the structural analysis to obtain structural design actions and also to take account of structural stiffness for settlements and differential settlements
The stiffness values MUST take into account pile group interaction effects
Cooperation between Geotechnical Engineerand Structural Engineer
Deem-to-satisfy rules Simplified rules Code of Practice for Foundations (2004)
Rational design method Based on soil/rock mechanic principles Consider geotechnical capacity and settlement May require instrumented pile loading tests to confirm
design assumption More economical design can be achieved!
Pile Design
Rational Pile Design Approach
An alternative to use of default values Adequate ground investigation to assist in formulation of
appropriate ground model Characterization of ground properties by means of appropriate
insitu and laboratory tests Proper geotechnical + engineering geological input Design analysis to be based on principles of mechanics,
and/or an established empirical correlations Pile testing programme to verify design assumptions
Design of Axially Loaded Pile (Geotechnical Capacity)
Piles found on soil
P = Qs + QB
Qs = shaft capacity
QB = base capacity
P
Soil type 1
Soil type 2
Ultimate Pile Shaft Capacity
Qs = s x As
s = Ultimate shaft friction in each soil stratum
As = Surface area of pile shaft in each soil stratum
Shaft Friction in Granular Soils
Two common design approaches as follows:
Method 1 : Effective stress method
s = Ks . v’ . tan [c’ is usually taken as zero]
_
The above may be simplified to:
s = . v’
_[ method, where = Ks x tan ]
Method 2 : Correlation with SPT N values
s = fs . N_
[SPT method]
where N is the average uncorrected SPT N values before pile construction
Ko is the earth pressure coefficient at rest (viz. before pile construction) and is usually taken as (1 - sin ’) for weathered rocks.
Suggested Ks Values for Method 1
Pile Type Ks/Ko
Large Displacement Piles 1 to 2
Small Displacement Piles 0.75 to 1.25
Bored Piles 0.7 to 1.0
'
s is interface friction’ is effective angle of friction
Pile Shaft Interface Friction Angle, s
Note - roughness of pile/ground interface is important, but difficult to quantify in practice
Pile/Soil Interface s/’Steel/sand 0.5 to 0.9
Cast-in-place concrete/sand
1.0
Precast concrete/sand 0.8 to 1.0
Typical Values in saprolites and sands for Method 1 based on back analysis of local instrumented pile loading tests
Type of Piles Type of Soils Shaft Resistance Coefficient,
Driven small displacement piles
SaprolitesLoose to medium dense sand
0.1 – 0.40.1 – 0.5
Driven large displacement piles
SaprolitesLoose to medium dense sand
0.8 – 1.20.2 – 1.5
Bored piles & barrettes
SaprolitesLoose to medium dense sand
0.1 – 0.60.2 – 0.6
Shaft grouted bored piles/barrettes
Saprolites 0.2 – 1.2
Noted: Only limited data for loose to medium dense sand
Design Parameters for Friction Piles- Method 2 (SPT correlation)
s = fs . N
For bored piles/barrettes in granitic saprolites :fs typically ranges from 0.8 to 1.4 [often taken to be 1.0 for preliminary design]
Pile types Ultimate Shaft Friction
Driven small displacement piles
1.5 – 2.0 x SPT, max 160 kPa
Driven large displacement piles
4.5 x SPT, max 250 kPa
Design Parameters for Friction Piles- Method 2 (SPT correlation)
Pile types Shaft grouting?
Ultimate Shaft Friction Ultimate End Bearing
Barrettes formed using grab
YES - No Data - - No Data -
NO 1.2 x SPT, max 200kPa 10 x SPT, max 2000kPa
Barrettes formed using cutter
YES 2.5x SPT, max 200kPa
NO 0.8 x SPT, max 200kPa
Bored piles YES 2.1 x SPT, max 200kPa
NO 0.8 x SPT, max 200kPa
Friction parameters previously accepted by BD :
The design method involving correlations with SPT results is empirical in nature
Level of confidence is not high particularly where the scatter in SPT N values is large.
Where possible, include a loading test on preliminary pile to confirm the design assumption.
Design Parameters for Friction Piles- Method 2 (SPT correlation)
rθ
v
r
Factor Affecting Shaft Friction
Pile Shaft
Changes of radial effective stress affects the skin friction Displacement piles – increases in
radial stress Replacement piles – decrease in
radial stress
= (ho + h ) tan = (hf) tan
ho is the locked-in effective horizontal stress after pileconstruction
h is the change of horizontal stress after pile constructionhf is the effective horizontal stress at failure and will be
affected by: interface dilation/compression under constant stiffness
condition during pile loading which can increase (due todilation of a dense soil), or reduce (due to compressionof a loose soil)
Factor Affecting Shaft Friction
Factors Affecting Shaft Friction of Bored Piles
Reduction in confining stress in bored piles– Stress relief– Arching effect– Loosening of soil due to poor construction control
Reduction in friction angle– Presence of weak materials at pile/soil interface (e.g.
bentonite filter cake)– Loosened/disturbed soil
Key Non-Geotechnical Factors Affecting Behaviour of Bored Piles
Rate of concrete pour
Fluidity of concrete
Time of pile bore being left open prior to concreting (- generally better to minimize the ‘wait time’ to avoid excessive relaxation)
Note: Faster concreting process will help to achieve higher wet concrete pressure, which would help to achieve higher locked-in horizontal stresses in the ground
Distribution of Wet Concrete Pressure
0 50 100 150 0 50 100 150 300250200
0
10
20
30
40
45
5
15
25
35
Concrete Pressure (kPa) Concrete Pressure (kPa)
Dept
h (m
) 2 hr
4 hr
Set = 6 hr
2 hr
4 hr
Set = 6 hr
Rise = 8 m/hr Rise = 12 m/hr
Swelling of granitic saprolite due to stress
relaxation
* Important to ensure sufficient excess slurry head within pile bore
2.4
1.6
0.8
0.020016012080400
Horizontal Effective Stress (kPa)
Radi
al st
rain
(%)
anisotropic
3 ’
1 ’
decreasing
constant
Swelling Effect due to Stress Relaxation
Good Practice for Enhancing Shaft Friction in Bored Piles
Sink casing in advance of excavation– to prevent loosening of soil/stress relief
Maintain a high hydraulic head inside temporary casing
Adopt a longer setting time for concrete– Wet concrete will exert an outward fluid pressure
against the drill shaft (minimize stress relief)– Horizontal stress h that can be restored after
excavation may be controlled by concrete pressure
Avoid delay in construction to minimize potential of stress relief– minimize delay in concreting after excavation– avoid unnecessarily over-cleansing of pile base
(delay concreting) Shaft grouting
– grout pressure increase horizontal stress– improve strength of interface material hence shaft
friction
Good Practice for Enhancing Shaft Friction in Bored Piles
QB= qb x Ab
Ultimate End-bearing Capacity
qb = Ultimate end bearing pressure
Ab = Bearing area of pile base
Ultimate Bearing Capacity of Piles in Granular Soils
qb = Nq · v
qb = fb · Nb
qb = presumptive bearing pressure
(b) Empirical correlation with SPT
(a) Classical bearing capacity theory
(c) Presumptive bearing pressure
Relationship between Nq and '(Poulos & Davis, 1980)
For driven piles,
' =
For bored piles, ' = '1 – 3 where '1 is the angle of shearing resistance prior to installation.10
100
1000
25 30 35 40 45Angle of Shearing Resistance, ' (°)
Bear
ing C
apac
ity Fa
ctor
, Nq ’1 + 40
2
0.6
0.4
0.2
0.00 5 10 15 20
Coarse sand
Fine sand
Normally consolidated siltCoarse sand
Fine sand
Driven piles
Bored piles
Depth in bearing stratumBase diameter
Ultim
ate E
nd B
earin
g Cap
acity
SPT N
bVa
lue
Pile LengthBase diameter ≥ 15
Ultimate Bearing Capacity of Piles in Granular SoilsBased on SPT N
Base Diameter (m)
Redu
ctio
n Fa
ctor
, fr
Ultimate Bearing Capacity of Piles in Granular SoilsBased on SPT N
1.0
0.5
0.25
0.00 0.5 1.0 1.5 2.5
Loose sand
Medium dense sand
Dense sand
2.0
0.75
Max
imum
Mob
ilise
dAv
erag
e Sha
ft Re
sista
nce,
max
(kPa
)
=0.6 =0.5 =0.4
= 0.3
= 0.2
= 0.1
P23
C1B8C
P11
P9P15
P7P19
P6
P14
B5
C2
P21‐2
P20
P5
P10 P8 P12P17
B6C
B4
B7C
B2
P21‐1
P4P13
P1
P2
B3
P22
P18
B1
B6T
C3
B7T
B8T
0
50
100
150
200
250
0 100 200 300 400 500 600 700
Mean Vertical Effective Stress, 'v (kPa)
B9
B11
B10
Figure A1 of GEO Publication 1/2006
Local Instrumented Test Data for Bored Piles
/N = 1.0
/N = 0.5
Max
imum
Mob
ilise
dAv
erag
e Sha
ft Re
sista
nce, m
ax(k
Pa)
/N = 2.5 /N = 1.5
C1
B8CP11
P16
P9P15
P7 P19
P6
P14
B5
C2
P21-2
P20
P5
P10P8 P12P17
B6C
B4 B7C
B2
P21-1 P4P13
P1
P2
B3
P22
P18
B1
B6T
C3
B7T
B8T
P23
0
50
100
150
200
250
0 50 100 150 200
Mean SPT N Value
B11
B10
B9
Figure A2 of GEO Publication 1/2006
Local Instrumented Test Data for Bored Piles
Some Observations
Significant scatter in the pile performance based on localinstrumented pile tests. Large variability recorded in the samesite.
Some unexpectedly low results have been measured for boredpiles under bentonite. Thus, load tests are important toconfirm design parameters and workmanship for friction boredpiles).
β values from load tests tend to be towards the lower bound ofthat expected for bored piles in granular materials (possiblydue to low horizontal stresses in weathered rocks, i.e. low Kovalue)
The method and the SPT method for pile design are notnecessarily consistent in that they may give different predictions
As a pragmatic approach, it is probably best to use bothmethods to assist in decision-making regarding pile designcapacity
It is important to make reference to the results of previousinstrumented pile load tests in similar ground conditions for therespective pile construction methods [role of precedents +design by load tests]
Some Observations
Ultimate Qs typically develops in a stiff manner, at a pile settlement of only about 0.5% to 1% pile diameter
Ultimate QB typically develops at a pile settlement of @ 10% (clay) to 20% (sand) pile diameter
Load Transfer Mechanism and Mobilizationof Load-Settlement Curve
Pile settlement
Pile
Load
Total
Base
Shaft
Material Mobilisation Factor for Shaft Resistance, fs
Mobilisation Factor for End-bearing Resistance, fb
Granular Soils 1.5 3 – 5
Mobilization Factors for Deriving Allowable Bearing Capacity
Mobilisation factors for end-bearing resistance depend very much on construction. Recommended minimum factors assume: good workmanship no 'soft' toe based on available local instrumented loading tests on friction
piles in granitic saprolites. Lower mobilisation factors when the ratio
shaft resistance end-bearing resistance
is high
Allowable Load Carrying Capacity, QaQbfb
Qsfs
= +
Method of DeterminingPile Capacity
Minimum Global Factor of Safetyagainst Shear Failure of the Ground
Compression Tension LateralTheoretical or semi-empirical methods not verified by loading tests on preliminary piles
3.0 3.0 3.0
Theoretical or semi-empirical methods verified by a sufficient number of loading tests on preliminary piles
2.0 2.0 2.0
Overall Global Factor of Safety
Code of Practice for Foundations by Buildings Department (2004)
Category Description of Rock Presumed Pressure (kPa)
2Rock (granitic and volcanic) :Highly decomposed, moderately weak to weakrock of material weathering grade IV or V orbetter, with SPT N value of 200
1,000
Presumed Allowable Bearing Pressure
Category Description of Rock Presumed Pressure (kPa)
1(a)Rock (granitic and volcanic) :Fresh strong to very strong rock of materialweathering grade I, with 100% total corerecovery and no weathered joints, and minimumuniaxial compressive strength of rock material(σc) not less than 75 MPa (equivalent point loadindex strength PLI50 not less than 3 MPa).
10,000
1(b) Fresh to slightly decomposed strong rock ofmaterial weathering grade II or better, with atotal core recovery of more than 95% of thegrade and minimum uniaxial compressivestrength of rock material (σc) not less than 50MPa (equivalent point load index strength PLI50not less than 2 MPa).
7,500
Presumed Allowable Bearing Pressure
Presumed Allowable Bearing Pressure
Category Description of Rock Presumed Pressure (kPa)
1(c) Slightly to moderately decomposed moderatelystrong rock of material weathering grade III orbetter, with a total core recovery of more than85% of the grade and minimum uniaxialcompressive strength of rock material (σc) notless than 25 MPa (equivalent point load indexstrength PLI50 not less than 1 MPa).
5,000
1(d) Moderately decomposed, moderately strong tomoderately weak rock of material weatheringgrade better than IV, with a total core recoveryof more than 50% of the grade.
3,000
Based on simple material classification Intended for foundations on horizontal ground with negligible
lateral loads & structures not unduly sensitive to settlement(i.e. routine problems)
Minimum socket length = 0.5 m for categories 1(a) & 1(b),and = 0.3 m for categories 1(c) & 1(d)
Total core recovery = % ratio of rock recovered (whether solidintact with no full diameter, or non-intact) to 1.5 m length ofcore run + should be proved to at least 5 m into the specifiedrock category
Self weight of pile - no need to further consider in calculationof bearing stresses
Presumed Allowable Bearing Pressure
Use of Total Core Recovery (TCR) as sole means of determining founding level + presumptive bearing value in rock is experience-based and tends to be conservative
TCR can be affected by effectiveness of drilling technique in retrieving the rock cores What are the requirements of the 15% of material?
No account taken directly of discontinuity spacing, aperture, persistence and infill, strength properties etc.
Can we find Grade I rock with no weather joints? Category 1(d) rock should be “grade IV” material instead of
“better than grade IV”
Presumed Allowable Bearing Pressure
Uniaxial compressive strengthof intact rock (MPa)
Prov
en b
earin
g pre
ssur
e (M
Pa)
P10-2OP15O
P14P7-2O
P2CP13-2O
P9-3O
P9-1P1C
P3C
P11-2O
P11-1
(2.5)(1.2)
(64)
(86)
(15.5)
(2)
(12.6)(13.6)
(3) (7.5)
(11.3)(?)
0
5
10
15
20
25
30
0 25 50 75 100 125 150 175 200
Code of Practice for FoundationsCategory 1(a)
Category 1(b)
Category 1(c)
settlement at pile base (mm)
P9 founded on granodiorite. UCS of rock ~ 15 MPa
pile load predominately taken by shaft resistance
Comparison of Allowable Bearing Pressure with Results obtained from Local Instrumented Pile Loading Tests
Rock Sockets
Calculation of Rock Socket Length
• General equation :R = Acontact fs L
• Check which scenario is more critical : (a) failure between rock and cement grout and (b) failure between steel and cement grout. Take the longer of the calculated socket length.
Design of Rock Sockets
Rock socket friction depends on: – wall roughness– tendency for pile dilation during displacement upon
loading under constant normal stiffness condition (dilatancy component may possibly reduce if load beyond the peak shear stress, depending on nature of material)
– strength and stiffness of concrete relative to that of the rock
For piles socketed in rock of categories 1(a) to 1(d), the total capacity may be taken as the sum of the bond resistance of the socket length corresponding to not more than 2 x pile diameters or 6 m (whichever is shorter) plus the presumptive bearing value
Not evident from results of instrumented pile loading tests The minimum socket depths stipulated in the presumed
bearing pressures should be ignored in bond calculation.
Recommendations in Code of Practice for Foundations (2004)
Design of Rock Sockets
Presumptive Design Parametersin BD’s Code of Practice for Foundations
Category Rock Mass Weathering
Minimum Embedment (m)
Allowable ShaftFriction (kPa)
1a Grade I or better 0.5 7001b Grade II or better 0.5 7001c Grade III or better 0.3 7001d Grade IV or better 0.3 300
Note: Use of rock socket bond in conjunction with the end bearing component is more rational than assuming end bearing only and will help avoiding the need to use bell-outs in some cases (also, presence of soil seams below pile base will be less of a problem)
10
Uniaxial Compressive Strength of Rock, q (Mpa)
Mob
ilize
d Sh
aft R
esist
ance
in
Rock
, (k
Pa)
P16
C1
P10-1
P10-2O
P9-1
P7-2O
P8
P7-1
P3T
P3C
P2T
P1T
P1C
100
1000
10000
1 100 1000
s = 0.2 c0.5
Design of Rock Sockets
Most of the results were not fully mobilized
Load-carrying capacity of bored piles socketed in rock (based on available data): pile shaft resistance and end-bearing resistance can be
added together settlement of pile base < 1% of pile diameter at working
loads socketed length / pile diameter ratio < 3 (GEO Publication
No. 1/2006) otherwise, pile loading tests need to be carried out to
confirm the design
Note : Load transfer in a rock socket is a function of the slenderness ratio of the rock socket & the relative pile/rock stiffness
Design of Rock Sockets
Design of Driven Piles
Design of Driven Piles (Hong Kong practice)
Design is rarely based on soil mechanic principles
Load carrying capacity of pile is based on structural capacity.
Drive to set as calculated from dynamic pile driving formula
Estimates of required pile depth is usually based on rules ofthumb (e.g. by relating to SPT N values - typically drive to adepth with N value of 100 for large displacement concretepiles, or a depth with N value of 180 to 200 for H-piles)
Design of Steel H-piles
For Grade 55C steel H piles, allowable load is taken as 30% yield stress (fy, which is a function of the steel grade and thickness) x As [e.g. fy for 305x305x223 pile = 430 MPa]
Pile driving formula (Hiley) used and final set criteria (typically, 25mm/10 blows to 50 mm/10 blows if not in rock)
Dynamic load tests + static load tests are used
Hiley Pile Driving Formula -(commonly used in Hong Kong)
R = W H S + 1
2 (C1 + C2 + C3)X
where = (W + e2p)(W + P) = efficiency of hammer
blow
Based on energy consideration
Hiley Pile Driving Formula -(commonly used in Hong Kong)
E’ = W H = effective energy impacted to pile (allowing for hammer efficiency, )
S = permanent set (i.e. pile penetration for the last blow) c1 = temporary compression of pile head (elastic)c2 + c3 = temporary compression of pile and ground (elastic)
W = weight of hammer
P = weight of pile
e = coefficient of restitution between hammer and pile cushion
H = drop distance of hammer
Pile Length 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 FINAL SET (mm) PER 10 BLOWS
15 -- -- -- -- -- -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- 16 -- -- -- -- -- -- -- -- -- -- -- 48 43 38 33 28 -- -- -- -- -- 17 -- -- -- -- -- -- -- -- -- -- 50 45 40 35 30 -- -- -- -- -- -- 18 -- -- -- -- -- -- -- -- -- -- 47 42 37 32 27 -- -- -- -- -- -- 19 -- -- -- -- -- -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- 20 -- -- -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- 21 -- -- -- -- -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- 22 -- -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- 23 -- -- -- -- -- -- -- 48 43 38 33 28 -- -- -- -- -- -- -- -- -- 24 -- -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- -- 25 -- -- -- -- -- -- 48 43 38 33 28 -- -- -- -- -- -- -- -- -- -- 26 -- -- -- -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- -- -- 27 -- -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- -- -- -- 28 -- -- -- -- -- 47 42 37 32 27 -- -- -- -- -- -- -- -- -- -- -- 29 -- -- -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- -- -- -- -- 30 -- -- -- -- 47 42 37 32 27 -- -- -- -- -- -- -- -- -- -- -- -- 31 -- -- -- -- 45 40 35 30 25 -- -- -- -- -- -- -- -- -- -- -- -- 32 -- -- -- 48 43 38 33 28 -- -- -- -- -- -- -- -- -- -- -- -- -- 33 -- -- -- 46 41 36 31 26 -- -- -- -- -- -- -- -- -- -- -- -- -- 34 -- -- 49 44 39 34 29 -- -- -- -- -- -- -- -- -- -- -- -- -- -- 35 -- -- 47 42 37 32 27 -- -- -- -- -- -- -- -- -- -- -- -- -- --
Typical Final Set Table (mm) per 10 Blows
Temporary Compression, Cp + Cq (mm)
Latest BD practice : Allow 100 mm per 10 blows but set 50 mm instead
Sample Final Set Calculation by Hiley Formula
TYPE OF PILE 305 x 305 x 180kg/m Grade 55CULTIMATE PILE LOAD Ru 5916 kN (2 x Design Working Load)HAMMER MODEL Drop Hammer (8 ton)WEIGHT OF RAM, W 80 kNCOEFFICIENT OF RESTITUTION, r 0.32TEMPORARY HELMET COMPRESSION, Cc 2.5 mmWEIGHT OF PILE HELMET, Wd 3 kNHEIGHT OF DROP, H 2.8 mENERGY EFFICIENCY, 0.8ENERGY OUTPUT PER BLOW, E 224 kN-mEFFECTIVE ENERGY, E' = E x 179 kN-m
Pile Length, L (m) = 25 mEffective Pile Weight, P = Wp + Wd = 25 x 1.8 + 3 = 48.0 kN
For Cp + Cq = 30 mmC = Cc + (Cp + Cq) = 33 mm
S = 3.8 mm / BlowS = 38 mm / 10 Blows
Problems with Hiley Formula
Basic assumptions on rigid body collision and conservationof energy is considered problematic.
Displacement
Forc
e
Cq
Cq S
S
Assume elasto-plastic soil and no damping effect considered
Problems with Hiley Formula
Rates effects and set-up effects not accounted for (assumedstatic capacity = dynamic capacity)
Hammers do not always operate at their rated efficiency andcan be highly variable
Energy absorption property of cushions can vary with timeand based on assumed values. For long pile, only a portionof the pile length is mobilized by the hammer blow.
Use of hydraulic hammers is not accepted by the BuildingAuthority. Drop hammer is used to take final set.
Pile Hammers Previous extensive use of diesel hammers was effectively
banned since 1997 Drop hammers (typical efficiency assumed in private sector =
0.7 to 0.8) - normally site measurements (by PDA) required if proposed energy coefficient is > 0.8
Hydraulic hammers (not accepted by BD for final set); typical efficiency = 0.9 or higher
HKCA studies on hydraulic hammers in 1995 and 2004 respectively
In Hong Kong, it is common to use hydraulic hammers for pile driving (higher productivity), but a drop hammer is used for final setting
Proposed improvement to Hiley Formula :− Energy approach (HKCA, 2004) using Pile Driving
Analyzer to measure the driving energy CAPWAP analysis (ArchSD, 2003) to find parameters for
matching the pile capacity as determined by Hiley Formula (combination of and e as ‘correction factors’)
Recent Work on Design of Driven Piles
Recent Work on Design of Driven Piles
Proposed Pile Driving Formula for Hydraulic Hammers by HKCA (2004)
where EMX is the actual energy transfer to pile head
Pile driving system not taken as part of pile-soil system, therefore Cc is not considered and subsumed in EMX, which is determined by CAPWAP
Final set table to be prepared based on average EMX (done during trial piling & use simple statistical methods to determine average EMX
cp = elastic compression of pile & cq = quake (elastic compression of ground)
R =[s + ½ (cp + cq)]
EMX
Driven Piles Founded on Rock
A suitable pile point (stiffener) may be used at the pile toe to prevent sliding on an inclined rock surface
Typical hard driving criterion for final set, e.g.− <10 mm per 10 blows with 16-tonne drop hammer− But is hard driving doing more harm than good?
Driven Piles Founded on Rock
Grade 55C steel sections with yield stress, fy, of 425 MPa, allowable stress = 0.3 fy (129 MPa)
Very high stresses on rock - why okay?
Rocks upon which driven piles are founded will be are subject to high confining pressure and hence can develop very high bearing capacity (also possible soil plug formation and local yielding leading to a larger base area) - see paper by Li & Lam (2001) - Proc. 5th International Conf. on Deep Foundation Practice, Singapore
Design of Prebored Piles
Pre-bored Steel H-piles
Prebore (using temporary casing as necessary), place H-section into bore and grout up [acts as a friction pile]
Compression loading - maximum allowable axial working stress (or combined axial and flexural stress) < 0.5 fy
Rock/grout bond limited 700 kPa in compression (or 350 kPa for permanent tension) for Category 1(c) or better rock in Code of Practice for Foundations
Under Compression : allowable grout/steel bond <600 kPa (x reduction factor of 0.8 when grouting under water). Under Tension : same assumptions if nominal shear studs are provided
Use shear studs to ensure proper bonding at grout/steel interface
If rock socket is subject to lateral load, need to check for additional stresses
Pre-bored Steel H-piles
Design of Mini-piles
Assessment of structural capacity (BD allows consideration of steel bars only. Overseas practice generally allow to account for load taken by grout also)
Mini-piles socketed in rock (Grade III or better with TCR of min. 85%) – presumed allowable rock/grout bond strength up to 700 kPa for compression (re. CoP for Foundations) Loading test results gave higher bond between rock/grout
May need to check buckling capacity for slender piles with substantial length embedded in soft/weak ground
Working load controlled by permissible structural stresses (typical maximum load capacity @1300 kN)
Raking mini-piles are usually used to resist lateral load
Negative Skin Friction
Negative Skin Friction (Downdrag)
Caused by ground settlement relative to the pile Need to understand site history and consolidation parameters
to assess potential for NSF NSF may arise due to surcharge or recent filling inducing
consolidation settlement, reduction of water pressure due todewatering and increase in effective stress, dissipation ofexcess pore water pressure (and hence settlement) in soft clayinduced by pile driving
Positive skin friction
Negative skin friction(Soil drags down pile)
(Pile settles relativeto the ground)
QB = base capacity
P
Soil type 1
Soil type 2
Negative Skin Friction (Downdrag)
Neutral plane
No relative movement
Ground settlement
Pile shortening
NSF = V’ D L
Soil Type
Soft Clay
Silt
Sand
0.20 - 0.25
0.25 - 0.35
0.35 - 0.50
NSF = Ks V’ tan D L
Negative Skin Friction (Downdrag)
Estimation of Negative Skin Friction
(a) Ground bearing capacity check (exclude NSF) :
Pc D + L (where Pc is the allowable ground bearing capacity & D and L are the dead load and live load)
(b) Pile structural integrity check :
Ps D + L + NSF (where Ps is the structural strength of the pile)
(c) Settlement check :
Settlement under (D + L + NSF) should be satisfactory
Design Checks for Negative Skin Friction(BD’s CoP on Foundations)
More efforts are required to drag the entire pile group including the pile cap. The pile-pile cap-soil interaction helps to reduce negative skin friction on a pile group.
The magnitude of free field soil movement for pile group is reduced especially for inner piles.
Group Effect for Negative Skin Friction
Group interaction effects are beneficial as negative skinfriction on individual piles will be reduced (up to 30%reported).
Distribution of negative friction among piles is not thesame (centre piles has the least negative skin friction dueto shielding and the most severe interaction among piles inthe group).
There are a variety of recommended methods for thedesign of pile groups against negative skin friction!
Group Effect for Negative Skin Friction
Means to Reduce NSF
Driven piles - bitumen coating or asphalt coating, plasticsheet, “Yellow Jacket”, etc. (Note - need to carefully revieweffectiveness and potential for damage to such protectivelayers during pile driving into competent ground)
Permanent casing for bored piles
Sacrificial protection piles around the structure foundation
Ground improvement techniques to strengthen/stiffen thesoft soils
Design of Lateral Load
The lateral load capacity of a pile may be limited in three ways :Shear capacity of the soil,Structural (i.e. bending moment and shear) capacity of the pile section, andExcessive deformation of the pile.
Design methods by Broms (1980) and Reese & Matlock (1960)Computer programs for pile groups, e.g. PIGLET, ALP, etc.
Design of Lateral Loads
For fixed-head short piles in granularHu = 1.5 D L2 Kp s‘
0.2
0.40.60.81.01.52.03.0
e1/L = 0
0 5 10 15 20
200
160
120
80
40
0
Fixed-head
Free-head
Pile Embedment Ratio, L/D
L
Fixed-head Deflection
3Ds'LKp
Mmax
L
e1
Free-head Deflection
3Ds'LKp Mmax
PL
Hu
Soil Reaction
Bending Moment
Soil Reaction
Bending Moment
Hu
0.5 D L3 Kp s’
e1+LHu =
For free-head short piles in granular soils
Ultimate lateral soil resistance for piles in granular soils(Broms )
(b) Long Vertical Pile under Horizontal Load
Ultimate lateral soil resistance for piles in granular soils(Broms )
e1
Soil Reaction
Free-head Deflection
Fixed-head Deflection
Bending Moment
Soil Reaction
Bending Moment
Mmax Mmax
Mu
f*f*
H
H
3s'f*Kp
41 2 8 16 32
Mu
D4 s’ Kp
Fixed-head
Free-head
e1/D =01
10
100
1000
H u
D3 s
’ Kp
Ultimate lateral soil resistance for piles in granular soils(Broms )
Fixed-head long piles in granular soilsMmax = H (e1 + 0.67f*)
HD s’ Kp
f* = 0.82 √
Mmax = 0.5 H (e1 + 0.67f*)
Free-head long piles in granular soilsMmax = H (e1 + 0.67f*)
(1) For constant soil modulus with depth (e.g. stiff overconsolidated clay), pile stiffness factor R = (in units of length) where EpIp is the bending stiffness of the pile, D is the width of the pile, kh is the coefficient of horizontal subgrade reaction.
(2) For soil modulus increases linearly with depth (e.g. normally consolidated clay & granular soils), pile stiffness factor,
T = √where nh is the constant of horizontal subgrade reaction
Ep Ip
nh
5
Ultimate lateral soil resistance for piles in granular soils(Broms )
Pile Type Soil ModulusLinearly increasing
Constant
Short (rigid) piles
L ≤ 2T L ≤ 2R
Long (flexible) piles
L ≥ 4T L ≥ 3.5R
Design of Lateral Load - Pile Stiffness
Consistency Loose(N = 4-10)
Medium Dense (N =11-30)
Dense(N =31-50)
nh for dry or moist sand
(MN/m3)2.2 6.6 17.6
nh for submerged sand
(MN/m3)1.3 4.4 10.7
Design of Lateral Load - Horizontal Subgrade Reaction
0 0.2 0.4 0.6 0.8
0
1
2
3
4
MH = FM (HT)
L
z
MH
= 2
3
4
5 10
Deflection Coefficient, Fd for Applied Moment M-1 0 1 2 3
0
1
2
3
4
L
z
H
H = F
= 2
3
4
5 & 10
Deflection Coefficient, Fd for Applied Lateral Load, H
Moment Coefficient, FM for Applied Moment M Moment Coefficient, FM for Applied Lateral Load, H
0
1
2
3
4
0 0.2 0.4 0.6 0.8 1.0
L
z
MM
MM = FM (M)
= 2
3
4
5 10
-1 0 1 2 3
0
1
2
3
4
L
z
M
M = F
= 2
4, 5 & 10
3
Lateral Soil Resistance for Piles in Granular Soils(Reese & Matlock )
0
1
2
3
4
0
1
2
3
4
-0.8 -0.6 -0.4 -0.2 0 -0.8 -0.4 0 0.4 0.8
VH = Fv (H)
L
z
VH
H
VM = Fv ()
L
z
VM
M
= 2 = 2
10 10 5
3
4
5
4
3
Shear Coefficient, Fv for Applied Moment M
Shear Coefficient, Fv for Applied Lateral Load, H
Lateral soil resistance for piles in granular soils(Reese & Matlock )
Pile Group
= Ultimate capacity of a pile groupSum of ultimate capacity of individual piles
Design of Pile Group
Pile Group Efficiency Factor
For piles founded on soil:Group factor for pile capacity ≥ 0.85Higher group factor can be used with justification by soil mechanic principlesNeed to consider separately the pile group settlement
For piles founded on rock or driven to refusal:No group factor is required
Surface of assumed failure block
Block Failure
End-bearing resistance
Shaft resistance
W'
Design of Pile Group
1 2 3 4 5 6 70.5
1.0
1.5
2.0
2.5
3.0Shaft efficiency
4-pile group9-pile group
4-pile group
4-pile group
9-pile group Total efficiencywith pile capTotal efficiency
Base efficiency(average of tests)
Pile Spacing/Pile Diameter
Grou
p Ef
ficie
ncy F
acto
r
Model Tests on Groups of Instrumented Driven Piles
in Granular Soils
Design of Pile Group
X
Y
Z
P
MX
My
xi
yi
Rigid cap
Pile
Design of Pile Group
Pai = Pnp
+ My*xi
Ix +
Mx*yi
Iy
Mx* = Mx -
MyIxy
Ix
1 - Ixy2
IxIy
and My* = My -
MxIxy
Iy
1 - Ixy2
IxIy
Based on rigid cap assumption Rotation of principle axes
where pile arrangement is not symmetrical
Pile groups subjected to vertical load and moments in both horizontal directions
Realistic soil profiles Non-linear soil-pile behaviour Different pile types within group Raft/cap flexibility incorporated Structure stiffness incorporated
Some desirable features for undertaking analysis by commercial software
Design of Pile Group
ANALYSISSome Programs (commercially available)
DEFPIG – pile groupsPIGLET – pile groupsREPUTE – pile groupsPLAXIS (2D & 3D)ABAQUSFLAC (2D & 3D)
2D analyses tend to: over-estimate settlements under-estimate differential settlements & raft moments over-estimate proportion of load carried by piles
SAFE Analysis
Design of Pile Group
Design of Pile GroupFlexible Cap Analysis
SAFE – Structural Analysis program─ Pile stiffness based on EA/L─ Length based on tentative founding level─ Need for performance review?─ Effect of structural walls/columns ignored?─ Interaction between piles ignored? Could be
detrimental for long piles
Scheduled Area No. 2 in the Northwest New Territories
Scheduled Area No. 4 in Ma On Shan reclamation area
Foundation Design in Marble Bearing Area
Designated Area in Northshore Lantau
Foundation Design in Marble Bearing Area
Tuen Mun Formation
Yuen Long Formation
Tin Shui Wai
Ma Tin
> 200 m
Long Ping
> 300 m
Interbeds of volcanic rocks including tuff-breecia, tuff & tuffite with clasts
of white marble, quartzite, metasiltstone etc,
clasts < 3 m
Massively bedded, white crystalline marble, locally dolomitic and siliceous
Grey to dark grey, finely crystalline marble intercalated and interbedded
with meta-sediment
Formation Member / Thickness Material Description Age
Uppe
rJu
rass
icCa
rbon
ifero
us
Dissolution
Limited
Main dissolution
Limited
Carbonate Rocks in Northwest New Territories
Ma On Shan Formation > 200 m
Grey to off-white, dolomite to calcite marble with thin interbeds of dark grey
to black meta-siltstone
Formation Member /Thickness Material Description Age
Carb
onife
rous
Vary
Carbonate Rocks in Ma On Shan
Dissolution
Pure Marble in Ma Tin Member
White, pure, crystalline marble
Impure Marble in Long Ping Member
Grey to dark grey, fine-grained dolomitic marble
Marble-clast bearing rock
Marble clast
Foundation Design
Foundation system
suitability of foundation types bored piles, driven steel H piles friction piles for lightly loaded building
founding levels of deep foundation sound marble (Class I or II) redundancy for driven piles
increase of stresses at marble surface
Ground investigation
Ground modelling Foundation design
Foundation construction
Review of construction
Monitoring of building
Foundation Design in Marble Bearing Area
Geotechnical Contents in Design Submission
Interpretation of ground conditions geological model karst geomorphology (GEO Report Nos. 28, 29, 32)
Foundation system founding levels of deep foundation increase of stresses at marble surface
Supplementary explanation on foundations on marble-bearing rock (TGN 26)
Foundation Design in Marble Bearing Area
Construction
driven piles pile driving record
bored piles pre-drilling investigation
Conclusion of construction performance review post-construction tests, e.g. CAPWAP, PDA, pile
loading tests PDA useful to identify broken piles and 12% ~ 28
% of piles were tested in some projects
Foundation Design in Marble Bearing Area
Monitoring
Building settlement monitoring building taller than 20 story high foundations on marble measurements undertaken by CEDD after building
occupied
Foundation Design in Marble Bearing Area
Core at least one full diameter
Core at least one full diameter
Core at least one full diameter
Length > 100 mm Length > 100 mmLength > 100 mm
RQD1RQD2 RQD3
Computation of Rock Quality DesignationFoundation Design in Marble Bearing Area
Computation of Marble Quality Designation
RQD3
Average RQD =
RQDi x i
L1
L2
L2 – L1
Leng
th >
100
m
mLe
ngth
> 1
00
mm
Leng
th >
10
0 m
m
RQD1
RQD22
1
3L2(mPD)
L1 (mPD)
Cavities or infill
Marble Rock Cover Recovery =
MR
i
L1
L2
L2 – L1
Foundation Design in Marble Bearing Area
Marble Mass Classes
Rock with widely spaced fractures and unaffected by dissolution
Rock slightly affected by dissolution, or slightly fractured but essentially unaffected by dissolution
Features
Fractured rock or rock moderately affected by dissolution
Very fractured rock or rock seriously affected by dissolution
Rock similar to Class IV marble except that cavities can be very large and continuous
I
II
Marble Class
III
IV
V
Very good
Good
MarbleClass
Fair
Poor
Very Poor
75 < MQD
50 < MQD ≤ 75
MQD Range(%)
25 < MQD ≤ 50
10 < MQD ≤ 25
MQD ≤ 10
Foundation Design in Marble Bearing Area
833790
833840
833890
821690 821740 821790 821840
No. of selected borehole: 6
Displayed depth: -10 mPD ~ -15 mPD
Section 1-1
Section 2-2
Section 3-3
Section 4-4
Section 5-5
Marble with overhang
Driven piles with preboring
Driven piles
Boreholes
Area with insufficient
boreholes to identify the karstic features
Example of Usage of Karst Geomorphology on Piling Design
Contour of good marble rock for foundation
Pile Testing
Static Pile Load Tests
Preliminary or Trial Piles (to check design and workmanship)vs. compliance tests on Working Piles
Specifications - define load-unload cycles, criteria forstabilisation and acceptance criteria (controversial!)
Automation of static load tests [see Chan et al (2004), Proc.Conf. On Foundation Practice in Hong Kong, Centre forResearch & Professional Development]
Concrete block
Hydraulic jack
GirderStiffeners
Steel cleat
Test pile
Universal beam
Reference beam
Dial gauge
1.3 m minimum or 3D whichever is
greater
Kentledge block
Load cell
Pile diameter, D
Compression Load Test Using Kentledge
Typical Set-up for a Compression Load Test Using Tension Piles
Girders (2 nos.)
Test pile
Hydraulic jack
Dial gaugeLoad cell
Reference beam
Locking nut
Steel plate
Tension members
Reaction piles
Stiffeners
Minimum spacing
2m or 3 D whichever is greater
Pile diameter, D
Typical Set-up for Uplift (Tension) Load Test
Reaction beam
Hydraulic jack
Dial gauge
Clearance for pile movement
Reference beam
Minimum spacing
2m or 3 D whichever is greater
Locking nutSteel plates
Reaction pile or on crib pads
Stiffeners
Tension connectionSteel bearing plates
Pile diameter, D
Steel plate
Typical Set-up for Horizontal Load Test
Dial gauge
Reference beam
Test plates
Hydraulic jack
Test piles
Steel strut
Clear spacing and avoid connection between blinding layer
Pile cap
Hydraulic jack
Test pile
Clear spacing
Pile cap Dial gauge
Reference beamSteel strut
Deadman
(b) Deadman
(a) Reaction Piles
Pile cap
Test plate
(c) Weighted Platform
Hydraulic jack
Test pile
Clear spacing
Dial gauge
Reference beam
Platform
Weights
Pile cap
Test plate
Typical Set-up for Horizontal Load Test
Enable higher test load Test load ~ 30 MN Shaft resistance in uplift
direction
Osterberg load cell
O-cell
bored pile
rock mass
Osterberg Cell at pile toe (cast in and jack up
the pile column from below after concreting)
135
Specifications for Pile Load Test
Maintained Load Test
─ General Specification for Civil Engineering Works (HongKong Government)
─ BD’s Code of Practice for Foundations─ Architectural Services Department
─ Criteria similar to CoP for Foundations, but the rateof recovery of settlement and magnitude ofallowable residual settlement after removal of testload
─ Housing Department (now follows CoP for Foundations)─ No unified standard as yet in Hong Kong
Residual settlement
Loading
Applied load P2WL
Max. totalsettlement
Settlement duringmaintained load stage of pile load
test
Allowableresidual
settlement
Allowabletotal settlement
1
LAE
WL = working loadD = pile diameter
Pile Loading Test Acceptance Criteria (for small diameter piles)
*The consideration of residual settlement on unloading from twice design load not rational, particularly for long friction piles, & tends to
give a conservative assessment of pile capacity
= PL/AE+ D/120 + 4Davisson Criterion is based on quick
loading procedures!
Load Test on Piles Designed to Take Negative Skin Friction
Test load should allow for effects of NSF to examine adequacy of pile design
Should load to [2 P + 3NSF] assuming a factor of safety of 2, because 1 x NSF is acting against the applied load during load test
139
140
141
142
Instrumented Pile Load Tests
Purpose of pile instrumentation is to provide a betterunderstanding of the load transfer mechanism (i.e.mobilization of base capacity and shaft friction with piledisplacement)
Axial strains are usually measured (e.g. using strain gauges),which can be converted to stress and hence load at a givenlevel. The corresponding displacement can also be assessed,taking into account elastic compression of the pile shaft.
Given the pile load profile with depth, one can work out theshaft friction at different levels
Possible pile instrumentation :– Strain gauges (measure strain)– Fibre optics (measure strain)– extensometer (measure displacement)
Instrumented Pile Load Tests
Properly plan the pile loading test programme
– What parameters are being measured?– Will the installation method be used in production piles?– Is sufficient instruments allowed for redundancy?– Is loading test properly set up without unforeseen
interference?
Instrumented Pile Load Tests
Osterberg cell (Optional)
Hydraulic pump with pressure gauges
Strain gauges (at least two and preferably four gauges at each level). Quantity and number of gauges depend on the purpose of investigation and geology.
Telltale extensometer
attached to load cell
Expansion displacement
transducer
Cast-in-place large-diameter pile
Strain gauge for measuring concrete modulusData logger
Hydraulic supply line
Steel bearing plates
Rod extensometer
Reference beam
Steel bearing pads Dial gauge
Instrumentation Pile Loading Tests
Vibrating Wire Strain Gauge
147
Extensometers
148
= strain in steel or concrete [usual assumption of plain sections remain plain, therefore equal]
Ec = Young’s modulus of concreteEs = Young’s modulus of steelAc = cross sectional area of concreteAs = cross sectional area of steel
Shaft friction stress, fs, is given by:
fs = (P1 - P2) / Ashaft
where Ashaft = surface area of pile shaft between levels 1 and 2
P = pile loadP = (Ec Ac + Es As)
P1
P2
fs
Instrumented Pile Load Tests
Instrumented Pile Load Tests
Samples for measuring Young’s modulus of concrete Samples for measuring Young’s modulus of steel Strain correction for concrete Young’s modulus
/N = 1.0
/N = 0.5
C1
P11
P16
P9P15
P7 P19
P6
P14
B5
C2
P5
P10P8 P12P17
B4
B2
P4P13
P1
P2
B3
P18
B1
C3
B8C
P21-2
P20
B6C
B7C
P21-1
P22
B6T
B7T
B8T
P23
Mean SPT N Value
Mob
ilise
d Av
erag
e Sha
ft Re
sista
nce,
(k
Pa)
/ N = 5 / N = 3 t / N = 2 / N = 1.5 / N = 4
0
50
100
150
200
250
0 50 100 150 200
Mobilised average shaft resistance and SPT N values for replacement piles
Instrumented Pile Load Tests
/ N = 12
D15
D14
D13
D12
D11
D10
D7
D8
D9D6
D5
D3D4
D2
D1
0
50
100
150
200
250
0 10 20 30 40 50 60 70 80 90 100
Mob
ilise
d Av
erag
e Sha
ft Re
sista
nce,
(k
Pa)
Mean SPT N Value
/ N = 9 / N = 6 / N = 5 / N = 4 / N = 3
/ N = 2
τ / N = 1
/ N = 1.5
τ / N = 0.5
Mobilised average shaft resistance and SPT N values for displacement piles
large-diameter displacement piles
Instrumented Pile Load Tests
B2P2B4
B5
B3
B1
P1
B6
/ N = 5
0
100
200
300
400
500
0 50 100 150 200 250Mean SPT N Value
Mob
ilise
d Av
erag
e Sha
ft Re
sista
nce,
(k
Pa)
/ N = 4 / N = 3 / N = 2
/ N = 1.5
/ N = 1
/ N = 0.5
Mobilised average shaft resistance and SPT N values for replacement piles with shaft-grouting
Instrumented Pile Load Tests
Dynamic Pile Load Test
Measure the time history of force (using strain gauges) and acceleration (using accelerometers and integrate to get velocity) - e.g. Pile Driving Analyzer (PDA)
CASE method to determine ultimate pile capacity using a damping factor, Jc (typically 0.45 to 0.5 in Hong Kong) -primarily for end-bearing piles
PDA can determine the energy transfer ratio (hammer efficiency), soil resistance to driving (driveability study), dynamic pile stresses and pile integrity
Pile Driving Analyzer
Dynamic Pile Load Test
Strain gauge and accelerometers installed on steel piles
Dynamic Pile Load Test
157
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 resistance, dynamic soil response and predict the pile-settlement curve for the pile
CAPWAP parameters can be correlated with site-specific static load tests
Dynamic Pile Load Test
Key Points to Remember Geotechnical and engineering geological input - very
important for proper pile design Close supervision of critical activities by experienced
supervisors - vitally important Very difficult and costly to rectify pile defects later - must try
to get things right first time Unduly conservative design - can make matters worse by
making construction process difficult + prone to problems Appreciate problems of different processes + compatibility of
design assumptions & construction techniques is key Performance review and monitoring – important for
advancement of foundation design
QUESTIONS