Design of Piled Foundations

Sammy Cheung Senior Geotechnical Engineer GEO, CEDD

20 April 2013 OUTLINE OF PRESENTATION

 Vertical Load

 Horizontal Load

 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 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 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 conservative design

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

Simplified Borehole log Borehole B Borehole A Borehole log Simplified geology geology B A

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

IV

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 Authority

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

 Some rules are not conservative and are not based on soil mechanics principles

 Unnecessarily long piles may encounter major problems during construction (so cou ld end up as bibeing worse off!) Common Pile Design in Hong Kong

 Submissions for private and housing projects

 Building (Construction) Regulations

 Code of Practice for Foundations, 2004

 Pract ice Notes for AP/RSE/RGE

 Submission for public projects

 GEO Publication No. 1/2006

 Specifications (Arch SD)

 EiEngineer ’s didiiscretion on ad opti ng stand ddards for pr ivate submission FOUNDATION DESIGN FOR PRIVATE PROJECTS

 Buildings (Construction) Regulations  AP/RSE Notes  Code of Practice for Foundations (()2004)  deemed-to-satisfy rules  more economic desiggyn may be feasible by rational design method Relevant PNAP for Foundation Submission for Private Projects

 Key PNs include:

 APP-18 (PNAP 66) (Accep tance cr iter ia for p ile tes ting )

 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  First edition was published in 1996  Consolidate good design and construction practice for pile foundations,,p with special reference to Hong Kong’s ground conditions

GEO Publication No. 1/96 Foundation Design for Public Projects

 Updated experience cumulated in recent years  Piling data obtained from the instrumented piling load tests programme for the rail projects  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

 comppgacting, loosenin g the soils

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

 Varying ground conditions involve uncertainty and risk – opportunity

 Comp ltdleted wor ks are bidburied; observa tions an d superv iiision diduring 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 Geotechnical load capacity (kN) carrying capacity Displacement Piles Driven H-piles 2000 kN to 3500 kN Shaft friction and end Driven prestressed 1950 kN to 3500 kN bearing precast concrete piles Jacked Steel H Pile 2950 kN COMMON PILE TYPES IN HONG KONG Pile Types Typical range of pile Geotechnical load capacity (kN) carrying capacity Replacement Piles Socketed H-piles 3500 kN to 5300 kN Shaft friction on rock Auger piles 1500 kN Shaft friction on soil Mini -piles 1400 kN Sha ft fr ic tion on roc k 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 m bell-out) and end bearing TRADITIONAL PILE DESIGN IN HONG KONG

 Need to cons ider geo tec hn ica l capac ity an d s truc tura l capac ity o f piles

 Driven piles – piles usually driven to a set based on dynamic driving

formula 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-bearing and limited shaft friction on rock. If depth of weathering is significant, the piles behave as ‘friction piles’ instead. PILE INSTALLATION

• Displacement piles –“hammeri ng st eel or concre te in to the groun d w ith sufficient energy to refusal" • Replacement piles –“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 EFFECTS OF PILE CONSTRUCTION ON GROUND

•Disppp(p)lacement piles (driven piles) - akin to ‘cavityyp expansion’ problems, with the horizontal stresses increased and granular soils subjjpect 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 PILE DESIGN

 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! RATIONAL PILE DESIGN APPROACH

 An alternative to use of default values [e.g. presumed bearing pressure, zero shfhaft fiifriction ]

 Adequate ground investigation to assist in formulation of appropriate ground model

 Characterization of ground ppproperties b y means of a pppppropriate 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)

P 

P = Qs + QB  

Soil type 1    

 Qs = shaft capacity

SlSoil 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

Qs = s x As

s = Ultimate shear stress in each soil stratum

As =Surfflhfhlface area of pile shaft in each soil stratum FACTORS AFFECTING SHAFT FRICTION

FACTOR AFFECTING SHAFT FRICTION v   r      r      θ     Changes of radial effective stress affects the skin friction  Displacement piles – increases in radial stress 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 construction h is the change of horizontal stress after pile construction

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 dense soil), or reduce (due to compression of a loose soil) SHAFT FRICTION IN GRANULAR SOILS

Two common design approaches as follows: MthdMethod 1 : Effec tive stress me tho d _ ’ 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 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

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

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 Coefficient, b Driven small displacement Saprolites 0.1 – 0.4 piles Loose to medium dense sand 0.1 – 0.5 Driven large displacement Saprolites 0.8 – 1.2 piles Loose to medium dense sand 0.2 – 1.5 Bored piles & barrettes Saprolites 0.1 – 0.6 Loose to medium dense sand 0.2 – 0.6 Shaft grouted bored Saprolites 0.2 – 1.2 piles/barrettes 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 1.5 – 2.0 x SPT, max 160 kPa displacement piles Driven large 4.5 x SPT, max 250 kPa displacement piles 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 YES - No Data - - No Data - using grab NO 121.2 x SPT, max 200kPa 10 x SPT, max 2000kPa Barrettes formed YES 2.5x SPT, max 200kPa using 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 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. 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 cak)ke) – 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 capacity theory

qb = Nq · v

(()b) Emp irical correlation with SPT

qb = fb · Nb

(c) Presumptive bearing pressure

qb = presumptive bearing pressure Relationship between Nq and ' (Poulos & Davis, 1980)

1000 For driven piles, ’ + 40 f' = 1 q 2 ty Factor, N ty Factor, ii 100 For bored piles, ' = '1 –3 ring Capac aa where f'1 is the angle Be of shearing resistance prior to installation. 10 25 30 35 40 45 Angle of Shearing Resistance' (°) Ultimate Bearing Capacity of Piles in Granular Soils Based on SPT N

0.6 Pile Length ≥ 15 Coarse sand Base diameter apacity CC 0.4 Value

b Fine sand d Bearing T N nn

SP Normally consolidated silt 0.2 Coarse sand

Ultimate E Fine sand 0.0 0 5 10 15 20 Depth in bearing stratum Driven piles Base diameter Bored piles Ultimate Bearing Capacity of Piles in Granular Soils Based on SPT N

1.0

Loose sand 0.75 r

Factor, f Factor, 0.5 nn Medium dense sand Reductio 0.25 Dense sand

000.0 0 0.5 1.0 1.52.0 2.5

Base Diameter (m) Load Transfer Mechanism and Mobilization of Load-Settlement Curve

Ultimate Qs typically develops in a stiff manner, at a pile settlement of only abou t 0.5% to 1% p ile diame ter

Total Base d aa Pile Lo

Shaft

Pile settlement

Ultima te QB typi call y devel ops at a pil e settl ement of @ 10% (c lay ) to 20% (sand) pile diameter Mobilisation Factors for Deriving Allowable Bearing Capacity

Qb Qs Allowable Load Carrying Capacity, Qa =+ fb fs

Mobilisation Factor for Mobilisation Factor for Material Sha ft Resistance, fs EdEnd-bibearing RitResistance, fb Granular Soils 1.5 3 – 5

 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 sha ft resis tance is high end-bearing resistance Recommended Global Safety Factors for Pile Design

Minimum Global Factor of Safety Method of Determining against Shear Failure of the Ground Pile Capacity Compression Tension Lateral Theoretical or semi-empirical 3.0 3.0 3.0 methods not verified by loading tests on preliminary piles Theoretical or semi-empirical 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: (a) ultimate capacity for bearing on and bond with the ground divided by suitable factor of safety, (b) structural capacity of the pile material divided by suitable factor of safety (e.g. permissible structural stresses or sufficient margin against buckling in slender piles), and ()(c) thevalue at whic h dfdeforma tion can be tltolera tdted by thestttructure Allowable Structural Stresses Building (Construction) Regulations

 The concrete stresses in cast-in-place concrete foundations at working load shall not exceed 80% of the appropriate limit design stress of concrete where groundwater is likely to be encountered during concreting KEY NON-GEOTECHNICAL FACTORS AFFECTING BEHAVIOUR OF BORED PILES

 Rate of concrete pour

 Fluidit y of concre te

 Time of pile bore being left open prior to concreting (- generally better to minimise the ‘waiting time’ to avoid excessive soil relaxation ) Distribution of Wet Concrete Pressure

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

10

15 2 hr (m) hh 20 2 hr Dept 25

30 4 hr 4 hr 35 40 Set = 6 hr Set = 6 hr 45 0 50 100 150 0 50 100 150 200 250 300 Concrete Pressure (kPa) Concrete Pressure (kPa) 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 Swelling of granitic saprolite due to stress relaxation

* Important to ensure sufficient excess slurry head within pile bore DILATANCY EFFECTS IN A DENSE SOIL WITH A ROUGH PILE/SOIL INTERFACE CHANGES IN EFFECTIVE HORIZONTAL STRESSES DUE TO DILATANCY EFFECTS DURING SHEARING

r E   ’ = h r 1 +   r = dilatancy (change in radius) r relative to the pile radius 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

 Adopt a longer setting time for concrete – Wet concrete will exert an outward fluid pressure against the drill shaft (minimise stress relief)

– 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-cleansinggp of pile base (dela y concreting)

 Shaft grouting – grout pressure increase horizontal stress – improve strengthfh of inter face materia lhl hence s hffhaft friction SHAFT GROUTING PROCEDURE

1 – Crack fresh concrete cover using double packer and 2 – Carry out shaft grouting for each manchette from bottom to water within 24 hours of cas ting concre te. top. Water cracking must be carried out for all grouting pipes in Target Grout Intake used so far in Hong Kong is 35 l/m2  Area the barrette (even the spare ones). covered by each manchette or refusal pressure (around 50 bars), 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 Typical Grout Mix for 1 m3 for the manchette immediately above, below or on its side is increased if necessary. Cement: 1000kg Bentocryl 86: 1.5 litres Grouting for all pipes to be used in one barrette can be carried out Water: 666 litres Daracem 100: 4 litres simultaneously.

Bentonite: 15 kg Local Instrumented Test Data for Bored Piles

 =0.6  =0.5  =0.4 250 C3

t

ff  =03= 0.3 200

B2 P14 (kPa) verage Sha xx 150 AA  = 0.2

ma P1 B3 P21-2

 B11 B7T B10 B4 P20 100 B7C P23 B9 P19 stance, Mobilised P9 B5 P7 P15 P2  = 0.1 P6 Resi P22 P4 B6C C2 B1 P13 50 B8C P21-1 P11 C1 P18 P5 Maximum P17 B8T P10 P8 P12 B6T 0 0 100 200 300 400 500 600 700

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

Figure A1 of GEO Publication 1/2006 Instrumented Test Data for Bored Piles

/N = 2.5 /N = 1.5 250

C3 /N = 1.0 200 Pa) rage Shaft B11 kk

ee P14

( B2 150 max

 P21-2 B3 P1 B7T B10 /N = B4

ance, B7C obilised Av tt 100 005.5 B5 MM P7 P20 P19 P23 B9 P9 P2 P15 Resis P6 P22 C2 P16 B1 B6C

aximum 50 P4 B8C P21-1 P5 MM P11 P13 B8T P18 P17 C1 P10 P12 P8 B6T 0 0 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 pile tests (some unexpectedly low results have been measured for bored piles under bentonite. Thus, load tests are important to confirm design parameters and workmanship for friction bored piles).

  values from ldload tttests tdtend to be tdtowards the lower bdbound of tha t expected for bored piles in granular materials (possibly due to low horizontal stresses in weathered rocks, i.e. low Ko value) SOME OBSERVATIONS

 The  method and the SPT method for pile design are not necessarily consistent in that they may give different predictions

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

 It is important to make reference to the results of previous instrumented pile load tests in similar ground conditions for the respective pile construction methods [role of precedents + design by load tests] Deem-to-satisfied Rules PRESUMED ALLOWABLE BEARING PRESSURE

 Code of Practice for Foundations by Buildings Department (2004)

Presumed Category Description of Rock Pressure (kPa) Rock (granitic and vol)lcanic) : 2 Highly decomposed, moderately weak to weak rock of 1,000 material weathering grade IV or V or better, with SPT N value of  200 PRESUMED ALLOWABLE BEARING PRESSURE Presumed Category Description of Rock Pressure (kPa) RkRock (iti(graniticand voli)lcanic) : 1(a) Fresh strong to very strong rock of material weathering 10,000 grade I, with 100% total core recovery and no weathered joints, and minimum uniaxial compressive strength of rock

material (σc) not less than 75 MPa (equivalent point load index strength PLI50 not less than 3 MPa). 1(b) Fresh to slightly decomposed strong rock of material 7,500 weather ing grade II or bttbetter, with a ttltotal core recovery of more than 95% of the grade and minimum uniaxial

compressive strength of rock material (σc) not less than 50 MPa (equivalent point load index strength PLI50 not less than 2 MPa). PRESUMED ALLOWABLE BEARING PRESSURE Presumed Category Description of Rock Pressure (kPa) 1(c ) Slightl y tomodtlderately dddecomposed modtlderatelystrong 5, 000 rock of material weathering grade III or better, with a total core recovery of more than 85% of the grade and minimum uniaxial compressive strength of rock material

(σc) not less than 25 MPa (equivalent point load index strength PLI50 not less than 1 MPa). 1(d) Moderately decomposed, moderately strong to 3,000 modtlderatelyweak rock of matilterial weather ing grade better than IV, with a total core recovery of more than 50% of the grade. PRESUMED ALLOWABLE BEARING PRESSURE

 Based on simple material classification

 ItIntend dded for fdtifoundations on hihorizon tltal ground with neglig ible ltlateral ldloads & 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 solid intact with no full diameter, or non-intact) to 1.5 m length of core run + should be proved to at least 5 m into the specified rock category

 Self weight of pile - no need to further consider in calculation of bearing stresses PRESUMED ALLOWABLE BEARING PRESSURE

 Use of Total Core Recoveryy( (TCR ) as sole means of determinin g foundin g 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

 No account taken directly of discontinuity spacing, aperture, persistence and infill,,gpp strength properties etc. PRESUMED ALLOWABLE BEARING PRESSURE

30 P10-2O (13.6) (12.6) settlement at P7-2O 25 P15O P14 (3) pile base (mm) (7.5) P11-2O (2) P13-2O P2C 20 P11-1 (?) (11.3) ure (MPa) ure ss (15. 5)

15 P9-3O (86)

P9-1 (64) Category 1(a) Code of Practice for Foundations bearing pres nn 10 P1C Category 1(b) (1.2) (2.5)

Prove P3C 5 Category 1(c) pile load predominately taken by shaft resistance 0 0 25 50 75 100 125 150 175 200

P9 founded on granodiorite. UCS Uniaxial compressive strength of rock ~ 15 MPa of intact roc k (MPa ) Key Points to Remember

 Geotechnical and engggggpineering geological input - veryyp important for pppproper 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

 UdlUnduly conservative didesign -can make matters worse by making cons truc tion process difficult + prone to problems

 Appreciate problems of different processes + compatibility of design assumptions & construction techniques is key Rock 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 rock Design of Rock Sockets

10000 )) P10-2O (kPa P7-2O  P10-1 P1T P7-1 ock, P1C RR

P16 P3C P8 P3T

istance in P2T ss 1000 C1 P9-1 d Shaft Re ee

0.5 s = 0.2 c Mobilis

100 110 100 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 corresponding to not more t han 2 x p ile diameters or 6 m (whic hever 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 Parameters in BDBDs’s Code of Practice for Foundations

Category Rock Mass Weathering Minimum Allowable Shaft Embedment (m) Friction (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 conjjgpunction with the end bearing component is more rational than assuming end bearing only and will help avoid the need to use bell-outs in some cases ((,palso, presence of soil seams below pile base will be less of a problem) 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 Note : Loadfd transfer in a roc kkfk socket is a function o fhldf the slenderness ratio o fhf 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?

 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 MPa to 300 MPa )(re f. Li et a l, 2000) – important to use appropr iate stiffness for settlement calculations Design 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 reealatin gto SPT N val ues -typi call y dderive to a depth with N vaauelue of 50 to 100 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/m – 305 x 305 x 180 kg/m – 305 x 305 x 223 kg/m

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

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

 Dynamic load tests + static load tests are used

 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 = 030.3 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, SingaporeSingapore Driven Piles Founded on Rock

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

 Typical hard driving criterion for final set, e.g.

− <10 mm per 10 blows with 16-tonne drop hammer

− But is hard drivinggg doing more harm than good? Hiley Pile Driving Formula - (commonly used in Hong Kong)

Based on energy consideration

W H R = X h S + 1 2 (C1 + C2 + C3) (W + e2p) where h = = efficiency of hammer blow (W + P) 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 Table for Final Set (mm) per 10 Blows

Temporary Compression, Cp + Cq (mm)

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

Sample Final Set Calculation by Hiley Formula

TYPE OF PILE 305 x 305 x 180kg/m Grade 55C

ULTIMATE PILE LOAD Ru 5916 kN (2 x Design Working Load) HAMMER MODEL Drop Hammer (8 ton) WEIGHT OF RAM, W 80 kN COEFFICIENT OF RESTITUTION, r0.32

TEMPORARY HELMET COMPRESSION, Cc 2.5 mm WEIGHT OF PILE HELMET, Wd 3 kN HEIGHT OF DROP, H 282.8 m ENERGY EFFICIENCY,  0.8 ENERGY OUTPUT PER BLOW, E 224 kN-m EFFECTIVE ENERGY, E' = E x  179 kN-m

Pile Length, L (m) = 25 m

Effective Pile Weight, P = Wp + Wd = 25 x 1.8 + 3 = 48.0 kN For Cp + Cq = 30 mm C = Cc + (Cp + Cq) = 33 mm S = 3.8 mm / Blow S = 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 highly varibliable  Energy absorption property of cushions can vary with time  Past experience generally bdbased on use of drop or dldiesel hammers; hdhydrau lic hammers presents a problem with the empirical factors, therefore a drop hammer is used to check 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

Recent Work on Design of Driven Piles

 Proposed improvement to Hiley Formula :

− Energy approach (HKCA, 2004) using Pile Driving Analyzer to measure the driving energy

 CAPWAP ana lysis (Arc hSD, 2003) to find parameters for match ing 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)

EMX 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 & 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 steel H pile (contribution by grout ignored), because no need to consider driving stresses Pre-bored Steel H-piles

 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 of 0.8 when grouting under water). Under Tension : same assumptions if nominal 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 cappy(acity (BD allows consideration of steel bars onl y. Overseas practice generally allow to account for load taken by grout also)

 Mini-pp(iles socketed in rock (Grade III or better with TCR of min. 85% ) – 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 Friction

93 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 settlemen t,redtiduction of water pressure due to dtidewatering and increase in effective stress, dissipation of excess pore water pressure (and hence settlement) in soft clay induced by pile driving

94 Negative Skin Friction (Downdrag)

P  Pile shortening 

 NtiNegative skin fitifriction

Soil type 1 

 (Soil drags down pile) Neutral plane 

 No relative movement  Positive skin friction

SlSoil type 2 

 (Pile settles relative

 to the ground)

 Ground  settlement

 QB = base capacity Negative Skin Friction (Downdrag)

NSF =  Ks V’ tan  p  L

NSF =   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(BDs’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 :

Ps  D + L + NSF (where Ps is the structural strength of the pile) (c) Settlement check : Settld()hldbflement under (D + L + NSF) should be satisfactory Means to Reduce NSF

 Driven piles - bitumen coatinggp or asphalt coatin g,pg, plastic sheet , “Yellow 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 imppqg/rovement techniques to strengthen/stiffen the soft soils

98 Design of Lateral Load Capacity of Piles

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. Design of Lateral Load Capacity of Piles ulti mate lateral soil resi stance for fixe d-hdhead and free-hdhead piles in granular soils are put forward by Broms

H  H e1

L L Centreof rotation 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

H  H  e1 e1

Fracture

L FtFracture L

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 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 & granu lar soils), pile stiffness factor,

5 Ep Ip T = √ nh

where nh is the constant of horizontal subgrade reaction Design of Lateral Load Capacity of Piles

Pile Type Soil Modulus Linearly increasing Constant Short (rigid) piles L ≤ 2T L ≤ 2R Long (flexible) L ≥ 4T L ≥ 3.5R piles Design of Lateral Load Capacity of Piles

0 0

1 1 = 2 = 2 M H 2 2  z z 3 3 3 L 3 L d M dH

4 dM = Fd 4 4 dH = Fd 4, 5 & 10 5 & 10 -10123 -1 0 1 2 3

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

0 0

1 = 2 1 = 2 2 M 2 3 H z 3  3 z L 3 4 L MM 4 MH 4 5 4 10 M = F (M) M M 5 0 0.2 0.4 0.6 0.8 1.0 10 MH = FM (HT) 0 0.2 0.4 0.6 0.8

Moment Coefficient, FM for Applied Moment M Moment Coefficient, FM for Applied Lateral Load, H Design of Lateral Load Capacity of Piles

0 0

1 1

= 2 2 2 = 2 M H  z 3 z 3 3 3 L L

VM 4 V 4 H 4 4 10 VM = Fv () 5 10 5 VH = Fv (H)

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

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 Scheduled Area No. 4 in Ma On Shan Northwest New Territories reclamation area Foundation Design in Marble Bearing Area

Designated Area in Northshore Lantau Carbonate Rocks in Northwest New Territories

Member / Formation Material Description Age Dissolution Thickness

Interbeds of volcanic rocks including tuff- Tuen Mun breecia, tuff & tuffite with clasts of white per assic rr Tin Shui Wai pp Limited

marble, quartz ite, metas ilts tone etc, U Formation Ju clasts < 3 m

Ma Tin Massively bedded, white crystalline marble, Yuen Long > 200 m locally dolomitic and siliceous Main dissolution

Formation erous Long Ping Grey to dark grey, finely crystalline marble intercalated and interbedded with meta- Carbonif > 300 m Limited sediment Carbonate Rocks in Ma On Shan

Member / Formation Material Description Age Dissolution Thickness

Grey to off-white, dolomite to calcite marble Ma On Shan > 200 m with thin interbeds of dark grey to black meta- iferous Vary Formation siltstone Carbon 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 Foundation Design in Marble Bearing Area

Ground Ground Foundation design investigation modelling

Foundation Moni tori ng of bbildiuilding RiReview of construct ion construction FOUNDATION DESIGN IN MARBLE BEARING AREA

Geotechnical Contents in Design Submission

 Interpretation of ground conditions  geolillogical modldel  karst geomorphology (GEO Report Nos. 28, 29, 32)

 Foundation system  fdifounding lllevels of deep fdtifoundation  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

 Buildinggg settlement monitoring  building taller than 20 story high  foundations on marble  measurements undertaken by CEDD after building occupidied Foundation Design in Marble Bearing Area Computation of Rock Quality Designation

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

RQD RQD RQD1 2 3 Length > Length > 100 mm Length > 100 mm 100 mm Foundation Design in Marble Bearing Area

L1 (mPD) m h > Computation of Marble Quality mm 100 Designation Lengt

RQD1 1

L1

 RQDi x  i 00 L2 RQD mm 2

Average RQD = 2 Length > 1 L2 – L1 Cavities or infill

L1 Marble Rock   i L2 Cover Recovery = 3 L (mPD) RQD3 m 2 > 100 hh MR mm L2 –L1 Lengt Foundation Design in Marble Bearing Area MblMarble Mass Classes

Marble MQD Range Marble Class Features Class (%)

Rock with widely spaced fractures and unaffected by I Very good 75 < MQD dissolution

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

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

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

Rock similar to Class IV marble except that cavities can V Very Poor MQD ≤ 10 be very large and continuous Driven piles with preboring Displayed depth: -10 mPD ~ -15 mPD ExamNo. of selectedppgle borehole:of Usa 6 ge of Karst Driven piles

Geomorphology on Piling Design Boreholes

MblMarble wihith over hang 833890 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 karstic features

833790 821690 821740 821790 821840 Foundation Design in Marble Bearing Area

Attention!

No simpppgle rule in a complex ground condition

Engggjgpineering judgement is important 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 for stabilisation and acceptance criteria (controversial!)

 AiAutomation of static ldload tests [see Chan et al (2004), Proc. CfConf. On Foundation Practice in Hong Kong, Centre for Research & Professional DlDevelopment] Compression Load Test Using Kentledge

Kentledge block

Universal beam Stiffeners Girder Load cell Steel cleat Dial gauge Concrete block

Reference beam Hydraulic jack

Test pile 1.3 m minimum or 3D Pile diameter, whichever is greater D Typical Set-up for a Compression Load Test Using Tension Piles

Girders (2 nos.) Locking nut Steel plate

Stiffeners Tension Load cell members Dial gauge

Reference Hydraulic jack beam Test pile

Minimum spacing Pile diameter, Reaction piles 2m or 3 D whichever is D greater Typical Set-up for Uplift (Tension) Load Tests

Locking nut Steel plates

Reaction beam

Hydraulic jack Steel plate Tension connection Steel bearing plates

Clearance for pile Stiffeners Reaction pile movement Dial gauge or on crib pads

Reference beamMinimu m spacing Pile diameter, D 2m or 3 D whichever is greater Typical Set-up for Horizontal Load Test

Reference beam Steel strut Hydraulic jack

Pile cap Dial gauge Pile cap

Clear spacing Test and avoid pltlates connection Test piles between blinding layer

(a) Reaction Piles

Steel strut Reeeeferen ce b eam Hydraulic jack

Pile cap Dial gauge

Clear spacing Deadman Test plate

Test pile

(b) Deadman Typical Set-up for Horizontal Load Test

Weights

Hydraulic jack Reference beam

Pile cap Platform Dial gauge

Test plate Clear spacing

Test pile

()(c) Weighted Platform Osterberg load cell

bored pile  Enable higher test load  Test load ~ 30 MN    Shaft resistance in uplift       direction    rock  mass O-cell INSTRUMENTATION PILE LOADING TESTS Steel bearing pads Dial gauge Hydraulic pump with pressure gauges Reference beam Strain gauge for measuring concrete modulus Data logger

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 investiggggyation and geology.

Rod extensometer       INSTRUMENTATION PILE LOADING TESTS Steel bearing pads Dial gauge Hydraulic pump with pressure gauges Reference beam Strain gauge for measuring concrete modulus Data logger

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 investiggggyation and geology.

Hydraulic supply line Rod extensometer  

 Steel bearing plates 

Expansion   displacement transducer

Osterberg cell (Optional) 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 Government) and corresponding Guidance Notes

 Architectural Services Department

 PNAP 66 and BD’s Code of Practice for Foundations

 Housing Department (previous one superseded, now adopt criteria in CoP)

 No unified standard as yet in Hong Kong PILE LOADING TEST ACCEPTANCE CRITERIA (FOR SMALL DIAMETER PILES) Allowable 2WL residual Applied load P settlement Residual settlement D/120 + 4

Loading Max. total settlement

Allowable Settlement during total settlement L maintained load stage AE = PL/AE+ D/120 + 4 of pile load test 1 WL = working load D = pile diameter Allowable total settlement = PL/AE+ D/50 *The consideration of residual settlement on unloadinggg from twice design load not rational , 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 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 better understanding of the load transfer mechanism (i.e. mobilisation of base capacity and shaft friction with pile displ acement)

 Axial strains are usually measured (e.g. using strain gauges), which can be converted to stress and hence load at a given lllevel. The corresponding displ acement 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 friction at different levels

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

• Place the instruments carefully with full understanding of what is being measured. 142 INSTRUMENTATION PILE LOADING TESTS Steel bearing pads Dial gauge Hydraulic pump with pressure gauges Reference beam Strain gauge for measuring concrete modulus Data logger

Telltale Outer ring casing 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 investiggggyation and geology.

Rod extensometer       VIBRATING WIRE STRAIN GAUGE

144 EXTENSOMETERS

145 P =  ( Ec x Ac + Es As) P = pile load  = stitrain in stlteel or concre te [usua l assump tion of plilain sec tions rema in plain, therefore equal] Ec = Young’s modu lu s of concrete (adju st for different stress ratio) Es = Young’s modulus of steel Ac = cross sectional area of concrete As = cross sectional area of steel

Shear stress, fs, is given by:

fs = (P1 - P2) / Ashaft

where Ashaft = surface area of pile shaft

between levels 1 and 2 146 DYNAMIC PILE LOAD TEST

 Measure the time history of force (using strain gauges) and acceleration (us ing acce lerome ters and iitntegra te to get velit)locity) -e.g. Pile Dr iv ing Analyser (PDA)

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

 PDA can determine the energy transfer ratio (hammer efficiency), soil resistance to driving (driveability study), dynamic pile stresses and pile integrity 148 Involve signal matching to get a good enough fit by adjusting the input values of the pile-ground model Dynamic Pile Load Test

Strain gauge and accelerometers installed on steel piles DYNAMIC PILE LOAD TEST

151 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

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

152 PILE INTEGRITY TESTS

 Quality control - serve as comparative and screening tests

 Augment other tests and control measures

 Retrospective investigation (after pile construction)

 Indirect testing (need expert interpretation)

 Checking pile integrity but not the bearing capacity TYPES OF PILE INTEGRITY TESTS

 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 emitter & recei ver prob es in tbtubes cas t in pile

 Used in bored piles & barrettes

 Check for presence of defects in concrete

 Tests can’t tell you the nature of defects

 No depth limitation due to damping effects

 Need pre-selection of piles (okay if all!)

 Sudden increase in sonic wave travel time suggests local area of lower quality concrete 156 157 TYPICAL TUBE LAYOUTS FOR SONIC LOGGING

(a) (b) With 3 tu bes (3 pa ths ) With 4 tu bes (6 pa ths ) WAYS OF CONDUCTING SONIC LOGGING TESTS

159

PILE INTEGRITY TESTS

 Acoustic anomalies may not correspond to structural defects

 Cannot identify definitely whether defects will affect pile behaviour under loading or long-term performance

 Interpretation of test results needs expert input and possibly subjective in not so straightforward cases