Understanding Load Transfer Behaviour (and the geotechnical design) of Rock Socketed Bored Piles

Dr. Gary Chapman, Principal, Golder Associates

Piling and Deep Foundations 2010

Outline

December 6, 2010 2

� Geotechnical Design

� Rock socket behaviour -base and shaft

� Socket Design Methods

� load capacity

� settlement performance

� Required geotechnical design inputs

� Construction issues and costs associated with various design methodologies

� Specifications for rock socketed piles

� Testing and compliance issues

Field Tests for Base Resistance

Zhang and Einstein (1998)

• Embedment > 3

pile diameters.

• Pile diameters

from 0.3 to 1.9 m

•Rock strengths

0.5 to 30 MPa

•q b = 3.0 to 6.6 x

(UCS) 0.5 0

2

4

6

8

10

12

0 5 10 15

settlement / diameter (%)

qb / U

CS

fragmented

Large displacements are required to mobilize base

resistance

Field Tests in Melbourne Siltstone

Melbourne Siltstone tests by Williams, 1988

Diameters from 0.1m to1 m

Solid points are for

> 10 % of diameter displacement

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25

Embedment/footing dia.

qb

/ U

CS

Stanley Avenue - UCS = 0.4 to 0.7 MPa

Middleborough Road - UCS = 1.1 to 2.7 MPa

West Gate, Eastern Freeway - UCS = 4 to 8 MPa,

extremely fractured

qb > 5 UCS for surface footings

qb > 10 UCS for piles with embedment > 5 dia.

May be lower for extremely fractured rock

Ultimate not achieved for embedment > 2 dia.

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

Piles in Clay (after Kulhawy & Phoon, 1993)

Piles in Rock (after Kulhawy & Phoon, 1993)

Effective upper limit

Effective lower limit

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

Piles in Clay (after Kulhawy & Phoon, 1993)

Piles in Rock (after Kulhawy & Phoon, 1993)

Effective upper limit

Effective lower limit

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

Piles in Clay (after Kulhawy & Phoon, 1993)

Piles in Rock (after Kulhawy & Phoon, 1993)

Effective upper limit

Effective lower limitα

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

Piles in Clay (after Kulhawy & Phoon, 1993)

Piles in Rock (after Kulhawy & Phoon, 1993)

Effective upper limit

Effective lower limit

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

Piles in Clay (after Kulhawy & Phoon, 1993)

Piles in Rock (after Kulhawy & Phoon, 1993)

Effective upper limit

Effective lower limit

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

0.01 2 3 4 0.1 2 3 4 1 2 3 4 10 2 3 4 100 2

Unconfined Compressive Strength (MPa)

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

3

44

0.01

2

3

44

0.1

2

3

44

1

Ad

he

sio

n F

acto

r

Piles in Clay (after Kulhawy & Phoon, 1993)

Piles in Rock (after Kulhawy & Phoon, 1993)

Effective upper limit

Effective lower limitα

Shaft Resistance Test Data

• Log – Log plot

• Clear correlation with UCS and Adhesion factor

• aranges from0.02 to 1.0 x UCS

• Order of magnitude scatter in data

• For UCS < 5 MPa

α is greater than 0.1

Factors other than UCS are at play

Rock Socket Behaviour

base/passive resistance (nonbase/passive resistance (non--linear)linear)

> 10% of diameter

frictional resistance (frictional resistance (elastoelasto--plastic)plastic)

1% of diameter

Re

sis

tan

ce

(fr

icti

on

al, b

as

e/p

ass

ive

)

Displacement

Displacement at ultimate >10 % of dia.

Serviceability requirement : usually less than 1% of pile dia.

Bearing capacity is unlikely to control design of socketed piles

Settlement at Serviceability Rules !!!!

(and is dominated by shaft resistance)

(Serviceability

limit)

Factors affecting shaft friction

•Initial normal stress (concrete placement)

•Pile diameter

•Socket roughness

0

500

1000

1500

2000

0 5 10 15

Shaft displacement (mm)

30

100

900

1200

300

Annotations denote initial normal stress in kPa

600

Increasing normal stress

Sh

aft

res

ista

nce

(k

Pa

)

0

200

400

600

800

0 5 10 15

Displacement (mm)

0.35

0.6

0.9

1.5

2.0

Annotations denote diameter of shaft in (m)

Increasing diameter

Sh

aft

res

ista

nce

(k

Pa

)

0

200

400

600

800

1000

0 5 10 15

Displacement (mm)

2.5

5.0

7.5

10.012.5

15.0 17.5

Annotations denote mean absolute

asperity angle in degrees

Sh

aft

res

ista

nce

(k

Pa

)

Increasing roughness

Increasing roughness

Clean sockets

We know that shaft resistance

Shaft resistance of a clean socket:

� increases with increased rock (intact)strength

� increases with increased rock (mass) modulus

� increases with increased initial normal stress (e.g. grouting pressure & expansive concretes)

� increases with increased interface roughness

� decreases with increased pile diameter

Why is it so?

Because of Socket Dilation

.... .

..

. .

.... .

..

. .

.... .

..

... .

(b) Pile after displacement

.

.

...

.

. .

Pile shaft

.

. ..

.

.... .

..

.

.

.... .

..

.. .

. .

..

..

Socket diameter D+DD

ShearShear forceforce

Rough wall rock socket

Pile andsocket

diameter D

Normal force

Normal force

(a) Pile before displacement

Pile diameter D

Increased normal force

Vertical displacement of pile

K∆σ∆σ∆σ∆σ

=∆∆∆∆

n

r

E=

m

(1+ ).ννννm r

=∆∆∆∆

∆σ∆σ∆σ∆σnrEm

(1+ ) ννννm r

Em = rock mass Young’s modulus

ννννm = rock mass Poisson’s ratio

Increase in normal stress

r = D/2 = radius of socket

r

∆r

∆∆∆∆σσσσn = change in normal stress

K = normal stiffness

∆∆∆∆r = dilation of socket

Constant Normal Stiffness

It is the interplay between interface roughness, pile diameter and rock mass stiffness that

defines shaft resistance in a clean socket

What Role does Rock Strength Play?

Friction angle of interface (residual friction angle) and intact (not mass) strength of asperities control interface behaviour

Sliding and shearing of

roughness asperities

Shearing

direction

There is a clear correlation between UCS and E and Interface behaviour

What does a traditional designer do ?

What does the client get ?

A (usually) safe and over-designed foundation but

at a potentially higher cost.

A (usually) safe and over-designed foundation but

at a potentially higher cost.

Adopts lower bound design parameters to account

for variability and risks (ground and construction).

Adopts lower bound design parameters to account

for variability and risks (ground and construction).

Design Methodology

6/12/2010

Starting Point for Socket Design

� Determine ultimate load for pile

� Select trial diameter of pile considering

� Concrete strength available

� Ductility – additional confining steel if > 60 MPa

� Out of position bending moments

� Ability to clean base effectively

� Usually most economic to make shaft work as

hard as possible

� Proceed with socket design to satisfy both

settlement (controls) and ultimate capacity

AS 1170 Design Loadings

� Load combinations

� 1.35 G or 1.2 G + 1.5Q or

� 1.2G + Wu +yc.Q yc = 0.4 – 0.6

� G + Equ +yc.Q

� Determine maximum design action effect (Ed)

� Ultimate wind = 1.5 x working wind

� Ultimate Eq = 1.4 x working Eq

� Design pile/s for Fg. Rug > Ed

� Select Fg from Pile Code AS 2159G = dead load, Q = live load, Wu = ultimate wind, Equ = ultimate

earthquake

Design Inputs and Process

December 6, 2010 15

� Given service settlement limit and SLS & ULS load

� We then need rock modulus and rock UCS values

over the proposed socket length

� Calculate geotechnical strength reduction factor Fg

considering:

� Construction process and controls

� Basic Fg factor and testing benefit factor

� Then adopt a trial shaft diameter and socket length

� Estimate pile ULS capacity

� Estimate pile head service settlement

Available Design Methods

� Code based allowable strength methods

� Strength based α methods

� Williams / Vicroads non linear elastic method

� Pells - Elastic design method

� Pells - Rowe & Armitage side slip methods

� RATZ – and other load transfer methods

� Golder GARSP method & Rocket - Monash University program

6/12/2010

Strength Based Methods

� Q ultimate = Ultimate shaft + Ultimate base

� Q allowable = Qult/FOS or

� Q allowable = allowable shaft + allowable base

� Prescriptive methods such as Q base = 1.5 UCS or RQD correlations for base and shaft resistance.

� Building Code values

6/12/2010

But in Strength Based Design -

� Load will be shared between base and shaft

according to pile & rock shaft and toe stiffness

� In rough sockets (grooves>1-4mm @ 50 -200mm)

shaft displacement is elasto - plastic

� Peak shaft is mobilized well before peak base

� Allowable side and base resistances are notadditive

� Settlement is not considered and is (hopefully)

allowed for by use of suitable Factors of Safety

6/12/2010

Williams-Vicroads Method – 1980’s

� Method was developed for Westgate Freeway

� Used for settlement sensitive structural design of

elevated Westgate Freeway in Melbourne

� Large diameter bored piles into Silurian rock at

around 30 m depth

� Design uses a Factor of Safety on Settlement

� Allows for non linear elasto-plastic socket

behaviour

� Proven with static load tests on sockets

6/12/2010

Williams Method -Design steps

� Select pile diameter (structural or construction

related)

� Determine shaft and base modulus and UCS

� Select trial socket length (L)

Design load Ql and

allowable settlement

Pile properties :

Modulus, diameterRock properties : Shaft and

Base E & UCS

Eb

L

6/12/2010

Williams Method

� Select pile diameter and socket length

� Calculate fictitious elastic load for design settlement

� Determine base and side components of elastic load

� Determine ultimate side resistance

� Calculate fse/fsu then fsp/fsu

� Calculate actual stress ratio

� Calculate actual side and base resistances

� Determine pile load

� Compare to design load and repeat until agreement

� Check overall capacity

6/12/2010

Elastic Load for a given settlement

Calculate a fictitious elastic load

Qe = δ x Es x D

/FOSr x I r

δ = design settlement

6/12/2010

Calculate elastic load distribution

Given L/D find Qbe / Qe

Calculate Qbe and Qse = Q-Qbe

Then calculate base and shaft

elastic stresses

6/12/2010

Peak Shaft Resistance

Peak shaft resistance

fsu =

� x � x UCS

� is related to UCS

� is related to jointing

of the rock mass

6/12/2010

Rock Mass Effects

Effect of

rock jointingon

shaft resistance

6/12/2010

Normalize Shaft Resistance

How to normalizeshaft load

settlement curve

6/12/2010

Elastic and Plastic shaft ratios

Given a value

fse / fsuthis curve will

yield a plastic stress ratio

fsp / fsu

6/12/2010

Given a value of

elastic stress ratio

fbe / fbl this graph will

yield a value for

plastic stress ratio

fbp / fbl

Calculate plastic stress ratio

6/12/2010

Relax Side Resistance

� Calculate peak side resistance using

fsu = � x � x UCS

� Calculate elastic stress ratio fse / fsu

� Calculate plastic stress ratio fsp / fsu

� Calculate actual side load Qs

� Actual stress ratio fs/fsu =fse/fsu – fsp/fsu

� We now have values of fse, fsp, fsu so we can

� Calculate fs, the actual shaft resistance

6/12/2010

End Bearing Calculation

� Determine

ultimate end

bearing

6/12/2010

Finish Design

� Total load = Qs + Qb

� Repeat until Total load ~ Design load

� Then check FOS for Capacity

peak shaft load = Qsu

Peak base resistance >= 5 x UCS

� FOS = (Qsu+Qbu)/ design load

6/12/2010

Actual pile load test

showing accuracy

of

Williams/Vicroads

method

Verification

6/12/2010

Pells - Elastic Method Design Inputs

� Socket diameter and length

� Socket shaft and base modulus values

� Average UCS for socket shaft and base

� Average roughness of socket walls

Documented in Hobart ANZ Geomechanics

Conference

6/12/2010

Design Step - 1

� Calculate peak side shear � av. peak using

� 0.45 x UCS sockets <R3 roughness

� 0.6 x UCS sockets R4 or more or

� � x � x UCS

6/12/2010

Determine Peak Side Shear

6/12/2010

Construction effects on side shear

6/12/2010

Design Step 2

� Calculate max socket length (Lmax) using peak side shear � av. peak

� Calculate Lmax/D

� Select appropriate design chart for Er/Ep and

Er/Eb

� Draw line on chart from L/d=0, 100% base to

Lmax/D 0% base

6/12/2010

Design Step 4

•Dotted line shows all

elastic solutions which

satisfy tav. Peak

•Select intersection on dotted line with relevant

Epile/Erock line

•Determine L/D and

% Pbase/Ptotal for intersection point

6/12/2010

Pells Elastic Method Chart

6/12/2010

Design Step - 5

� Calculate settlement d = swl x I�

/(Er x D) using

influence factor for revised L/D

� Er is the average factored shaft modulus

� Calculate base load at serviceability using % base

load for revised L/D, and check that this load is

within the elastic range for the base -

� For intact rock 2 - 4 times UCS

� For jointed rock 75 -125 % UCS

� Check that settlement is typically less than 1%

diameter (include shaft settlement if significant)

6/12/2010

Rowe & Armitage - Side Slip

� Draw Lmax/D line

� Calculate elastic base load

� Calculate % Pbe/Pbt

� Draw horizontal line on chart for % Pbe

� Intersection of 2 lines gives L/D and I�

� Calculate settlement d = SWL x I�

/(Er x D)

� Check ultimate geotechnical strength

6/12/2010

Side Slip Allowed

6/12/2010

RATZ & Load Transfer Computer Analysis

� Input Parameters

� Pile data

� Socket layer shear modulus

� Load transfer parameters – deflections to fully

mobilize base and shaft

� cyclic load parameters (if any)

� peak and residual skin frictions

� displacement to achieve residual shaft

� strain softening parameter

6/12/2010

Rocket Program Capabilities

� Can handle multi layered sockets

� Varying base properties and base debris

� Socket roughness

� Socket diameter effects

� Insitu stresses from concrete head

� Based on Melb mudstone (1-10 MPa UCS)

but applicable for UCS 1-100 MPa

� Requires detailed strength data

6/12/2010

Rocket Design Parameters

� Input data required

� Layers

� Pile properties

� Layer properties

� Layer stress conditions

� Layer geometry

� Pile base properties

� Load Transfer Parameters

� Input parameters

� depth

� Ep, L, diam

� Er, c’ ,F’

� insitu horiz stress

� thickness

� Eb, c’ , F’, debris

� segment length, height

Soft

overburden

Load from

structure

HW

MW

HW

Socket

Layer 1

Layer 2

Layer 3Base

displacement

Stress

Layer 2

Layer 1

Layer 3

Base

Calculates load

displacement

response for each

layer and the base

Load

displacement

Sum layers and

base to obtain full

pile response curve

Rocket Inner Workings

Is there a better way ?

� Serviceability based design process for bored

piles socketed into weathered rock

� Allows final design in ‘real time’ during logging

of the sockets

� Developed in house using state of the art

software package ROCKET

� Extensive experience in Melbourne

The Golder Approach : GARSP Golder Associates’ ROCKET field Socket Procedure

Stage 1 : Site Investigation

� sufficient boreholes to assess variation across

site and with depth

� insitu testing

� pressuremeter tests every 2m to 3m

� laboratory testing

� moisture contents at 1m intervals

� UCS tests at pressuremeter test locations

� point load index tests (for stronger rocks)

� CNS direct shear tests

� (keep core moist and tests ASAP)

� preliminary sizing -

for costing (increase

socket lengths by

10% to allow for

variations in the field)

� final design done in

“real-time” during

logging of sockets 0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16

Socket Length (m)

All

ow

ab

le S

ocket

Lo

ad

(M

N)

Upstream end

Downstream end socket diameter

= 1.8m

1.2m

1.5m

1.8m

1.2m

1.2m

0.9m

For estimating purposes

only. Actual socket lengths

to be assessed based on

ground conditions at pile

locations

Stage 2 Preliminary Sizing

Stage 3 : Pre Construction

Preparation

1. ROCKET analyses

2. Logging Sheets

3. Factors of Safety

4. Field Staff Briefing

settlement : 1.5 or 2

ultimate load : 2.0 shaft only

2.5 shaft and base

FRESHWATER PLACE : ROCK SOCKET DESIGN SHEETDeveloped specifically for ground conditions at Freshwater Place.

Not to be used for any other site.

Disp Load (MN/m)

(mm) HW HW-MW MW MW-SW SW<RL-35m SW>RL-35m

Base (MN) 14.2 26.9 33.4 66.3 114.6 114.6

Max All. Shaft 1.15 1.64 2.65 5.05 5.92 10.40

Residual 1.44 2.05 3.32 6.31 7.40 13.01

0 0.00 0.00 0.00 0.00 0.00 0.00

0.5 0.08 0.19 0.35 0.50 0.47 1.12

1 0.16 0.36 0.67 0.95 0.90 2.14

1.5 0.25 0.53 0.95 1.37 1.31 3.06

2 0.33 0.68 1.19 1.75 1.68 3.89

2.5 0.42 0.82 1.41 2.10 2.03 4.64

3 0.51 0.95 1.60 2.42 2.36 5.31

3.2 0.54 1.00 1.67 2.54 2.49 5.56

3.4 0.57 1.05 1.73 2.66 2.61 5.80

3.6 0.61 1.09 1.79 2.77 2.73 6.02

Stage 4 : Construction

1. Socket Logging: Golder Associates’ Geotechnical

Engineer on site - to optimise socket lengths, confirm design assumptions (insist on good construction practices),

keep the piling contractors honest and control risk

2. Roughening : To obtain minimum roughness levels

(design assumption)

3. Cleaning : To obtain clean sockets (design assumption)

4. Moisture Contents : To confirm logging

5. ROCKET check : To confirm pile performance

6. Certification : pile sign-off

Site

GARSP – In summary

� State of the art technology

� Optimises socket dimensions

� Controls risk (e.g. dykes)

� Design considers construction practice

� Promotes good construction practice

� Requires detailed site investigation

� Net gain = Confidence + Savings

6/12/2010

Conclusions for Socket Design

� Need socket UCS and Modulus for rational design

� Consider using pressuremeter tests to get modulus

data

� Pells elastic or Armitage side slip method is easy

and quick to use

� For complicated sockets and good data consider

using Rocket/GARSP

� For down drag and cyclic loads consider using a

load transfer program such as RATZ

6/12/2010

SITE INVESTIGATION INPUTS

� Intact rock modulus

� Drained rock mass modulus (Es, Eb)

� Rock unconfined compressive strength (qus qub)

� Residual friction angle (F’)

� Intact cohesion and friction angle (c’ F’)

� Socket roughness (segment length & height)

� Load transfer function

Estimating Rock UCS and Modulus

December 6, 2010 55

� Ideally we will have lots of boreholes to below socket depth

with UCS tests and pressuremeter tests - a Platinum Class

investigation

� Or UCS and some UCS with modulus measurement and/or

pressuremeter – a Platinum/Gold Class investigation

� Or point load index tests and hopefully moisture contents

over socket length (if in Melbourne where we have good correlations between E and UCS and mc in Silurian rock) –

a Silver Class investigation

� Or bore holes and coring with visual strength & weathering

assessment only often not over the full depth of socket – a

Bronze class investigation

UCS Estimation

December 6, 2010 56

� From direct tests

� Inferred from Point load tests with some UCS correlations . But UCS can vary from as low as 5 times Is50 to as much as 30

Intact Rock Strength and Modulus

� Point Load Strength Index� Quick and inexpensive

� Large scatter

� Tensile test (?)

� Axial vs diametrical

� Failure mode

� No reliable relationship with UCS

� Unconfined Compressive Strength� Strength

� Failure mode

� Preparation, saturation, test rate

� Modulus

� End platen measurement - compliance effects, soft rocks only, max. tangent modulus

� local measurement

� Drained or undrained

� Triaxial tests

� Softer rocks Multi-stage ?

� Drained or undrained ?

� Moisture Content Correlations/Empirical Correlations

0.01

0.10

1.00

10.00

0 2 4 6 8 10 12 14 16 18 20

Moisture Content (%)

Poin

t L

oad

In

dex

- I

s(5

0)

(M

Pa)

Siltstone

0.01

0.10

1.00

10.00

0 2 4 6 8 10 12 14 16 18 20

Moisture Content (%)

Poin

t L

oad

In

dex

- I

s(5

0)

(M

Pa)

Siltstone

0.1

1

10

100

0.01 0.10 1.00 10.00 100.00

Is(50) (MPa)

UC

S (

MP

a)

5 Is(50)

25 Is(50)

Melbourne

Siltstone

6/12/2010

Modulus – moisture content correlations

� Correlation of

modulus with in situ

moisture content is

possible for

sedimentary rocks

e.g. Melbourne

Mudstone

6/12/2010

• In the absence of

insitu pressuremeter

of UCS test data

•strength can be

correlated to:

•moisture content

•RQD

•core logs

Intact Rock Strength Correlations

6/12/2010

Rock Modulus vs. UCS

E rock ~= 350. UCS

� Serviceability is usually

critical (not ULS)

� Shaft resistance usually

dominates settlement

� Construction processes

are critical

� shaft integrity

� how rough and clean is

the shaft?

� base cleanliness

Side

resistance

Base resistance

Debris

Construction Issues

Overburden

Rock

Possible Construction Options

December 6, 2010 62

� Longer Shaft and not a so clean base

� Roughen sides of sockets to increase shaft resist.

� Allow for a reduced % of base area cleaned

� Consider additional geotechnical investigation with

UCS and pressuremeter tests to refine design

� A 30 m borehole with pressuremeter testing

would roughly equate to about 15 to 20 m of rock

socket

Rock Socket Specifications

December 6, 2010 63

� Should be settlement based and state of the art

� No need for down hole inspections (OHS issues)

� Can design for the use of drilling fluids with

experienced contractors and appropriate on site

supervision

� Consider using the socket excavation as a design

tool – e.g. GARSP

� Should be aimed at producing durable & intact pile

shafts

� Allow the use of appropriate tremie concrete

Testing and Compliance Issues

December 6, 2010 64

� Specifications should consider integrity testing -

CHS and high/low strain PDA particularly for

heavily loaded piles close to structural capacity

� Consider high strain PDA or O cell tests for critical

designs, highly variable sites or where cost of

testing can be offset by potential savings in sockets

� Consider engagement of an independent

geotechnical engineer to log sockets and confirm

capacity and construction methodology compliance

� Consider settlement monitoring

Why Review Socket Designs?

� Consultants are generally conservative because they don’t know who will construct the piles

� Structural consultants often only quote allowable loads

� Rarely is settlement considered in detail

� Socket length is usually very expensive

� Often there is scope for alternative designs

6/12/2010

Thank You!!

Questions (?s)

and

Answers (!!!s)