Understanding Load Transfer Behaviour and Rock Socketed Bored Piles DrGary Chapman
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Transcript of Understanding Load Transfer Behaviour and Rock Socketed Bored Piles DrGary Chapman
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)