the static loading test bengt h. fellenius
TRANSCRIPT
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1
Bengt H. Fellenius
The Static Loading Test
Performance, Instrumentation, Interpretation
2
A routine static loading test provides the load-movement of the pile head...
and the pile capacity?
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The Offset Limit Method Davisson (1972)
OFFSET (inches) = 0.15 + b/120
OFFSET (mm) = 4 + b/120
b = pile diameter
LL
EAQ
Q
LTom Davisson determined this definition as the one that fitted best to the capacities he intuitively determined from a FWHA data base of loading tests on driven piles. The definition does not mean or prove the the pile diameter has anything to do with the interpretation of capacity from a load-movement curve.
4
The Decourt Extrapolation Decourt (1999)
0 100 200 300 400 5000
500,000
1,000,000
1,500,000
2,000,000
LOAD (kips)
LOAD
/MVM
NT
-- Q
/s (
inch
/kip
s)
Ult.Res = 474 kips
Linear Regression Line
0.0 0.5 1.0 1.5 2.00
100
200
300
400
500
MOVEMENT (inches)
LOAD
(ki
ps)
Q
1
2
C
CQu
C1 = Slope
C2 = Y-intercept
Q
Ru = 470 kips – 230 tons
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5
Other methods are:
The Load at Maximum Curvature
Mazurkiewicz Extrapolation
Chin-Kondner Extrapolation
DeBeer double-log intersection
Fuller-Hoy Curve Slope
The Creep Method
Yield limit in a cyclic test
For details, see Fellenius (1975, 1980)
6
CHIN and DECOURT 235
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A rational, upper-limit definition to use for “capacity” is the load that caused a 30-mm pile toe movement. Indeed, bringing the toe movement into the definition is the point. A “capacity” deduced from the movement of the pile head in a static loading test on a single pile has little relevance to the structure to be supported by the pile. The relevance is even less when considering pile group response.
0
1,000
2,000
3,000
4,000
0 10 20 30 40 50 60 70 80
LO
AD
(kN
)
MOVEMENT (mm)
OFFSET LIMIT LOAD 2,000 kN
Shaft
Head
Toe
CompressionLOAD = 3,100 kN for 30 mm toe movement
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0
500
1,000
1,500
2,000
2,500
0 5 10 15 20 25 30 35 40
MOVEMENT (mm)
LOA
D (K
N)
Offset-LimitLine
Definition of capacity (ultimate resistance) is only needed when the actual value is not obvious from the load-
movement curve. However, the below test result is rare.
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What really do we learn from unloading/reloading and what does unloading/reloading do to the gage records?
10
Result on a test on a 2.5 m diameter, 80 m long bored pile
Does unloading/reloading add anything of value to a test?
0
5
10
15
20
25
30
0 25 50 75 100 125 150 175 200
MOVEMENT (mm)
LO
AD
(M
N)
Acceptance Criterion
0
5
10
15
20
25
30
0 25 50 75 100 125 150 175 200
MOVEMENT (mm)
LO
AD
(M
N)
Acceptance CriterionRepeat test
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Plotting the repeat test in proper sequence
0
5
10
15
20
25
30
0 25 50 75 100 125 150 175 200
MOVEMENT (mm)
LOA
D (
MN
)
Acceptance Criterion Repeat test plotted in sequence of testing
The Testing Schedule
0
50
100
150
200
250
300
0 6 12 18 24 30 36 42 48 54 60 66 72
TIME (hours)
"PE
RC
EN
T"
A much superior test schedule. It presents a large number of values (≈20 increments), has no destructive unloading/reload cycles, and has constant load-hold duration. Such tests can be used in analysis for load distribution and settlement and will provide value to a project, as opposed to the long-duration, unloading/reloading, variable load-hold duration, which is a next to useless test.
Plan for 200 %, but make use of the opportunity to go higher if this becomes feasible
The schedule in blue is typical for many standards. However, it is costly, time-consuming, and, most important, it is diminishes or eliminates reliable analysis of the test results.
XXXXX
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0
200
400
600
800
1,000
0 1 2 3 4 5 6 7
MOVEMENT (mm)
LOA
D (K
N) ACTIVE
PASSIVE
Caputo and Viggiani (1984) with data from Lee and Xiao (2001).
Load-Movement curves from static loading tests on two “ACTIVE” piles (one at a time) and one “PASSIVE”. . Diameter, b = 400 mm; Depths = 8.0 m and 8.6 m. The “PASSIVE” piles are 1.2 m and 1.6 m away from the “ACTIVE” (c/c = 3b and 4b).
0
400
800
1,200
1,600
2,000
0 4 8 12 16 20 24
MOVEMENT (mm)
LOA
D (K
N) ACTIVE
PASSIVE
Load-Movement curves from static loading tests on one pile (“ACTIVE”). Diameter (b) = 500 mm; Depth = 20.6 m. “PASSIVE” pile is 3.5 m away (c/c = 7b).
Pile Interaction
CASE 1 CASE 2
14
0
200
400
600
800
0 2 4 6 8 10
PILE HEAD MOVEMENT (mm)
LOA
D P
ER
PIL
E (
KN
) Single PileAverage of 4 Piles
Average of 9 Piles
O’Neill et al. (1982)
Group Effect
20 m
Single Pile
9-pile Group
4-pile Group
Comparing tests on single pile, a 4-pile group, and a 9-pile group
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15 Data from Phung, D.L (1993)
2.30
m
800
mm
800 mm
60 mm
#1
#2 #3
#4 #5
Loading tests on a single pile and a group of 5 piles in loose, clean sand at Gråby, Sweden.
Pile #1was driven first and tested as a single pile. Piles #2 - #5 were then driven, whereafter the full group was tested with pile cap not in contact with the ground.
16 Data from Phung, D.L (1993)
0
2
4
6
8
10
12
14
16
0 5 10 15 20 25 30 35 40 45 50
MOVEMENT OF PILE HEAD (mm)
LOA
D/P
ILE
(K
N) Average
Single
#2
#5
#4
#3
#1
#1
2,3 m
340 mm
#2
#3 #4
#5
800 mm
First test: Pile #1 as a single pile. Second test (after driving Piles #2 - #5): Testing all five piles with measuring the load on each pile separately.
#1
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17 Data from Phung, D.L (1993)
0
2
4
6
8
10
12
14
16
0 10 20 30 40 50 60 70 80
MOVEMENT OF PILE HEAD (mm)
LOA
D/P
ILE
(K
N) Pile #1 reloaded as
part of the group after Piles #2 - #5 were driven
#1
#1
Pile #1: Effect of compaction caused by driving Piles #2 through #5 and then testing #1 again
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Instrumentation
and
Interpretation
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T e l l t a l e s • A telltale measures shortening of a pile and must never be arranged to
measure movement.
• Let toe movement be the pile head movement minus the pile shortening. • For a single telltale, the shortening divided by the distance between
the pile head and the telltale toe is the average strain over that length.
• For two telltales, the distance to use is that between the telltale tips.
• The strain times the cross section area of the pile times the pile material E-modulus is the average load in the pile.
• To plot a load distribution, where should the load value be plotted? Midway of the length or above or below?
20
Load distribution for constant unit shaft resistance Unit shaft resistance (constant with depth)
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Linearly increasing unit shaft resistance and its load distribution
DE
PT
H
LOAD IN THE PILE
DE
PT
H
SHAFT RESISTANCE
DE
PT
H
UNIT RESISTANCE
x = h/√2
h/2
A1
A2
x = h/√2
h
ah
ah2/2
ah2/200
00 "0"
ah
TELLTALE 0 Shaft Resistance
Resistance at TTL FootTelltale Foot
Linearly increasing unit shaft resistance 2
5.02
22 ahax
2
hx ===> 21 AA ===>
x
22
• Today, telltales are not used for determining strain (load) in a pile because using strain gages is a more assured, more accurate, and cheaper means of instrumentation.
• However, it is good policy to include a toe-telltale to measure toe movement. If arranged to measure shortening of the pile, it can also be used as an approximate back-up for the average load in the pile.
• The use of vibrating-wire strain gages (sometimes, electrical
resistance gages) is a well-established, accurate, and reliable means for determining loads imposed in the test pile.
• It is very unwise to cut corners by field-attaching single strain gages to the re-bar cage. Always install factory assembled “sister bar” gages.
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Rebar Strain Meter — “Sister Bar”
Reinforcing Rebar
Rebar Strain Meter
Wire Tie
Instrument Cable
or Strand
Wire Tie
Tied to Reinforcing Rebar Tied to Reinforcing Rings
Reinforcing Rebaror Strand
(2 places)
Rebar Strain Meter
Instrument Cables
(3 places, 120° apart)
Hayes 2002
Three bars?!
Reinforcing Rebar
Rebar Strain Meter
Wire Tie
Instrument Cable
or Strand
Wire Tie
Tied to Reinforcing Rebar Tied to Reinforcing Rings
Reinforcing Rebaror Strand
(2 places)
Rebar Strain Meter
Instrument Cables
(3 places, 120° apart)
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0
2
4
6
8
10
12
14
16
18
20
0 50 100 150 200STRAIN (µε)
LO
AD
(M
N)
Level 1A+1C
Level 1B+1D
Level 1 avg
Load-strain of individual gages and of averages
0
2
4
6
8
10
12
14
16
18
0 100 200 300 400 500 600 700
STRAIN (µε)
LO
AD
(M
N)
LEVEL 1 D CA B
A&C B&D
The curves are well together and no bending is discernible
Both pair of curves indicate bending; averages are very close; essentially the same for the two pairs
PILE 1 PILE 2
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If one gage “dies”, the data of surviving single gage should be discarded.
It must not be combined with the data of another intact pair. Data from two surviving single gages must not be combined.
0
2
4
6
8
10
12
14
16
18
20
0 100 200 300 400 500 600 700
STRAIN (µε)
LO
AD
(M
N)
A&C+D
Means: A&C, B&D, AND A&B&C&D
A&C+BD B+CA+D
LEVEL 1
Error when including the single third gage, when either Gage B or Gage D data are discarded due to damage.
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Hanifah, A.A. and Lee S.K. (2006)
Glostrext Retrievable Extensometer (Geokon 1300 & A9)
Anchor arrangement display Anchors installed
Geokon borehole extensometer
28
Gage for measuring displacement, i.e., distance change between upper and lower extensometers. Accuracy is about 0.02mm/5m corresponding to about 5 µε.
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29
That the shape of a pile sometimes can be quite different from the straight-sided cylinder can be noticed in a retaining wall built as a pile-in-pile wall
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0
5
10
15
20
25
0.00 0.50 1.00 1.50 2.00 2.50
DIAMETER RATIO AND AREA RATIO
DE
PT
H (
m)
Gage Depth
DiameterRatio
Area Ratio
O-cell
Nominal Ratio
Determining actual shape of the bored hole before concreting
Bidirectional cell
Data from Loadtest Inc.
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31
We have got the strain. How do we get the load?
• Load is stress times area
• Stress is Modulus (E) times strain
• The modulus is the key
E
32
For a concrete pile or a concrete-filled bored pile, the modulus to use is the combined modulus of concrete,
reinforcement, and steel casing
cs
ccsscomb AA
AEAEE
Ecomb = combined modulus Es = modulus for steel As = area of steel Ec = modulus for concrete Ac = area of concrete
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• The modulus of steel is 200 GPa (207 GPa for those weak at heart)
• The modulus of concrete is. . . . ?
Hard to answer. There is a sort of relation to the cylinder strength and the modulus usually appears as a value around 30 GPa, or perhaps 20 GPa or so, perhaps more.
This is not good enough answer but being vague is not necessary.
The modulus can be determined from the strain measurements.
Calculate first the change of strain for a change of load and plot the values against the strain.
Values are known
tE
34
0 200 400 600 8000
10
20
30
40
50
60
70
80
90
100
MICROSTRAIN
TAN
GE
NT
MO
DU
LUS
(G
Pa)
Level 1
Level 2
Level 3
Level 4
Level 5
Best Fit Line
Example of “Tangent Modulus Plot”
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bad
dEt
ba
2
2
sE
Which can be integrated to:
But stress is also a function of secant modulus and strain:
Combined, we get a useful relation:
baEs 5.0
In the stress range of the static loading test, modulus of concrete is not constant, but a more or less linear relation to the strain
and Q = A Es ε
36
0 200 400 600 8000
10
20
30
40
50
60
70
80
90
100
MICROSTRAIN
TAN
GE
NT
MO
DU
LUS
(G
Pa)
Level 1
Level 2
Level 3
Level 4
Level 5
Best Fit Line
Example of “Tangent Modulus Plot”
Intercept is
”b”
Slope is “a”
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Note, just because a strain-gage has registered some strain
values during a test does not guarantee that the data are useful.
Strains unrelated to force can develop due to variations in the pile material and temperature and amount to as much as about 50±
microstrain. Therefore, the test must be designed to achieve
strains due to imposed force of ideally about 500 microstrain and beyond. If the imposed strains are smaller, the relative errors and
imprecision will be large, and interpretation of the test data
becomes uncertain, causing the investment in instrumentation to be less than meaningful. The test should engage the pile material
up to at least half the strength. Preferably, aim for reaching close
to the strength.
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The secant stiffness approach can only be applied to the gage level immediately below the pile head (must be uninfluenced by shaft resistance), provided the strains are uninfluenced by residual load.
Field Testing and Foundation Report, Interstate H-1, Keehi Interchange, Hawaii, Project I-H1-1(85), PBHA 1979.
y = -0.0014x + 4.082
0
1
2
3
4
5
0 500 1,000 1,500 2,000
STRAIN (µε)
TA
NG
EN
T S
TIF
FN
ES
S,
∆Q
/∆ε (
GN
)
TANGENT STIFFNESS, ∆Q/∆ε
y = -0.0007x + 4.0553
0
1
2
3
4
5
0 500 1,000 1,500 2,000
STRAIN (µε)
SE
CA
NT
S
TIF
FN
ES
S, Q
/ε (
GN
)
Secant DataSecant from Tangent DataTrend Line
SECANT STIFFNESS, Q/ε
Strain-gage instrumented, 16.5-inch octagonal prestressed concrete pile driven to 60 m depth through coral clay and sand. Modulus relations as obtained from uppermost gage (1.5 m below head, i.e., 3.6b).
The tangent stiffness approach can be applied to all gage levels. The differentiation eliminates influence of past shear forces and residual load. Non-equal load increment duration will adversely affect the results.
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Unlike steel, the modulus of concrete varies and depends on curing, proportioning, mineral, etc. and it is strain dependent. However, the cross sectional area of steel in an instrumented steel pile is sometimes not that well known.
y = -0.0013x + 46.791
30
35
40
45
50
55
60
0 100 200 300 400 500 600
STRAIN, με
SE
CA
NT
ST
IFF
NE
SS
, EA
(G
N)
30
35
40
45
50
55
60
0 100 200 300 400 500 600
STRAIN, με
TA
NG
EN
T S
TIF
FN
ES
S, E
A (
GN
)
EAsecant (GN) = 46.5 from tangent stiffness
EAsecant (GN) = 46.8 - 0.001µε from secant stiffness
y = 0.000x + 46.451
TANGENT STIFFNESS, EA = ∆Q/∆εSECANT STIFFNESS, EA = Q/ε
(Data from Bradshaw et al. 2012)
Pile stiffness for a 1.83 m diameter steel pile; open-toe pipe pile. Strain-gage pair placed 1.8 m below the pile head.
y = -0.0013x + 46.791
30
35
40
45
50
55
60
0 100 200 300 400 500 600
STRAIN, με
SE
CA
NT
ST
IFF
NE
SS
, EA
(G
N)
30
35
40
45
50
55
60
0 100 200 300 400 500 600
STRAIN, με
TA
NG
EN
T S
TIF
FN
ES
S, E
A (
GN
)
EAsecant (GN) = 46.5 from tangent stiffness
EAsecant (GN) = 46.8 - 0.001µε from secant stiffness
y = 0.000x + 46.451
TANGENT STIFFNESS, EA = ∆Q/∆εSECANT STIFFNESS, EA = Q/ε
40
Pile stiffness (Q/ε and ΔQ/Δε versus ε) for a 600 mm diameter concreted pipe pile. The gage level was 1.6 m (3.2b) below the pile head
Data from Fellenius et al. 2003
0
5
10
15
0 50 100 150 200 250 300
STRAIN, µε
SE
CA
NT
ST
IFF
NE
SS
, EA
(G
N)
0
5
10
15
0 50 100 150 200 250 300
STRAIN, µε
TA
NG
EN
T S
TIF
FN
ES
S, E
A (
GN
)
y = -0.003x + 7.41
y = -0.004x + 7.21
EAsecant (GN) = 7.2 - 0.002µε from tangent stiffness
E Asecant (GN) = 7.4 - 0.003µε from secant stiffness
TANGENT STIFFNESS, EA = ∆Q/∆εSECANT STIFFNESS, EA = Q/ε
For "calibrating" uppermost gage level, the secant method appears to be the better one to use, right?
0
5
10
15
0 50 100 150 200 250 300
STRAIN, µε
SE
CA
NT
ST
IFF
NE
SS
, EA
(G
N)
0
5
10
15
0 50 100 150 200 250 300
STRAIN, µε
TA
NG
EN
T S
TIF
FN
ES
S, E
A (
GN
)
y = -0.003x + 7.41
y = -0.004x + 7.21
EAsecant (GN) = 7.2 - 0.002µε from tangent stiffness
E Asecant (GN) = 7.4 - 0.003µε from secant stiffness
TANGENT STIFFNESS, EA = ∆Q/∆εSECANT STIFFNESS, EA = Q/ε
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y = -0.0053x + 11.231
0
10
20
30
40
50
0 100 200 300 400 500
STRAIN (µε)
SE
CA
NT
ST
IFF
NE
SS
, EA
(G
N)
y = -0.0055x + 9.9950
10
20
30
40
50
0 100 200 300 400 500
STRAIN (µε)
TA
NG
EN
T S
TIF
FN
ES
S, E
A (
GN
)
EAsecant (GN) = 10.0 - 0.003µε from tangent stiffness
E Asecant (GN) = 11.2 - 0.005µε from secant stiffness
Secant stiffness after adding 20µε to each strain value
Secant stiffness from tangent stiffness
TANGENT STIFFNESS, EA = ∆Q/∆εSECANT STIFFNESS, EA = ∆Q/∆ε
Pile stiffness for a 600-mm diameter prestressed pile. The gage level was 1.5 m (2.5b) below pile the head.
Data from CH2M Hill 1995
Or this case? Here, that initial "hyperbolic" trend can be removed by adding a mere 20 µε to the strain data, "correcting the zero" reading, it seems.
42 42
But, is the “no-load” situation really the
reading taken at the beginning of the test? What is the true “zero-reading” to use?
The strain-gage measurement is supposed to be the change of strain due to the applied load relative the “no-load”
situation (i.e., when no external load acts at the gage location).
Determining load from strain-gage measurements in the pile
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43 43
• We often assume – somewhat optimistically or naively – that the reading before the start of the test represents the “no-load” condition.
• However, at the time of the start of the loading test, loads do exist in the pile and they are often large.
• For a grouted pipe pile or a concrete cylinder pile, these loads are to a part the effect of the temperature generated during the curing of the grout.
• Then, the re-consolidation (set-up) of the soil after the driving or construction of the pile will impose additional loads on the pile.
44
B. Load and resistance in DA
for the maximum test load
Example from Gregersen et al., 1973
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200 250 300
LOAD (KN)
DE
PT
H (
m)
Pile DA
Pile BC, Tapered
0
2
4
6
8
10
12
14
16
18
0 100 200 300 400 500 600
LOAD (KN)
DE
PT
H (
m) True
Residual
True minus Residual
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200 250 300
LOAD (KN)
DE
PT
H (
m)
Pile DA
Pile BC, Tapered
0
2
4
6
8
10
12
14
16
18
0 100 200 300 400 500 600
LOAD (KN)
DE
PT
H (
m) True
Residual
True minus Residual
A. Distribution of residual load in DA and BC
before start of the loading test
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Hysteresis loop for shaft resistance mobilized in a static loading test
ABOVE THE NEUTRAL PLANE When no residual load is present in the pile at the start of the test, the starting point of the load-movement response is the origin, O, and in a subsequent static loading test, the shaft shear is mobilized along Path O-B. Residual load develops as negative skin friction along Path D-A (plotting from “D” instead of from “O”). Then, in a static loading test, the shaft shear is mobilized along Path A-O-B. However, if presence of residual load is not recognized, the path will be thought to be along A-B -- practically the same. If Point B represents fully mobilized shaft resistance, then, the assumption of no residual load in the pile will indicate a "false" resistance that is twice as large as the "true" resistance.
SH
AFT
RE
SIS
TAN
CE
MOVEMENT
+
-
+rsB
A
O
-rs
FALSE
TRUE
C
D
Above the neutral plane
SH
AFT
RE
SIS
TAN
CE
MOVEMENT
X
O
TRUE
Y
Z
+Below the neutral plane
FALSE
Residual load affecting shaft resistance determined from a static loading test
BELOW THE NEUTRAL PLANE When no residual load is present in the pile at the start of the test, the starting point of the load-movement response is the origin, O, and in a subsequent static loading test, the shaft shear is mobilized along Path O-Z-X. Residual load develops as positive shaft resistance along Path O-Z. Then, in a static loading test, additional shaft resistance is mobilized along Path Z-X. However, if presence of residual load is not recognized, the origin will be thought to be O, not Z. If Point X represents fully mobilized shaft resistance, then, the assumption of no residual load in the pile will indicate a "false" resistance that could be only half the "true" resistance.
“Continuation” loop for shaft resistance mobilized in a static loading test
46
Similar to the below the N.P. for the shaft, if residual load develops, it will be along Path O-Z. Then, in a static loading test, the toe resistance (additional) is mobilized along Path Z-X and on to Y and beyond.
Pile toe response in unloading and reloading in a static loading test
When the pile construction has involved an unloading, say, at Point X, per the Path X-I-II, the reloading in the static loading test will be along Path I-II-X and on to Y. Unloading of toe load can occur for driven piles and jacked-in piles, but is not usually expected to occur for bored piles (drilled shafts). However, it has been observed in such piles, in particular for test piles which have had additional piles constructed around them and for full displacement piles. The toe resistance be underestimated and the break in the reloading curve at Point X can easily be mistaken for a failure load and be so stated.
SH
AFT
RE
SIS
TAN
CE
MOVEMENT
X
O
TRUE
Y
Z
+Below the neutral plane
FALSE
TO
E
RE
SIS
TAN
CE
MOVEMENT
O
TRUE
RESIDUALTOE LOAD
FALSE
X
II
Y
Z
FALSE III
I
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Method for evaluating the residual load distribution
0
2
4
6
8
10
12
14
16
0 500 1,000 1,500 2,000
RESISTANCE (KN)
DE
PTH
(m
)
Measured Shaft ResistanceDivided by 2
Residual Load
Measured Load
True Resistance
ExtrapolatedTrue Resistance
Shaft Resistance
48 48
Immediately before the test, all gages must be checked and "Zero Readings" must be taken.
Answer to the question in the graph:
No, there's always residual load in a test pile.
0
5
10
15
20
25
30
35
40
45
50
55
60
-300 -200 -100 0 100 200 300 400
DE
PT
H (
m)
STRAIN (µε)
Shinho Pile
Zero Readings. Does it mean zero loads?
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49 49
Gages were read after they had been installed in the pile ( = “zero” condition) and then 9 days later (= green line) after the pile had been concreted and most of the hydration effect had developed.
0
5
10
15
20
25
30
35
40
45
50
55
60
-300 -200 -100 0 100 200 300 400
DE
PT
H (
m)
STRAIN (µε)
"Zero" conditionBefore grouting
9 days laterafter installing gages (main hydraution has developed)
Change(tension)
50 50
Strains measured during the following additional 209-day wait-period.
0
5
10
15
20
25
30
35
40
45
50
55
60
-300 -200 -100 0 100 200 300 400
STRAIN (µε)
DE
PTH
(m
)
9d
15d
23d
30d
39d
49d
59d
82d
99d
122d
218d Day ofTest
At an E- modulus of 30 GPa, this strain change corresponds to a load change of 3,200 KN
4/18/2015
26
51 51
Concrete hydration temperature measured in a grouted concrete cylinder pile
0
10
20
30
40
50
60
70
0 24 48 72 96 120 144 168 192 216 240
HOURS AFTER GROUTING
TEM
PE
RA
TUR
E (
°C)
Temperature at various depths in the grout of a 0.4 m center hole in a 56 m long, 0.6 m diameter, cylinder pile.
Pusan Case
52 52
-400
-300
-200
-100
0
100
0 5 10 15 20 25 30 35
DAY AFTER GROUTING
CH
AN
GE
OF
ST
RA
IN (µ
ε)
Rebar ShorteningSlight Recovery of Shortening
Change of strain measured in a 74.5 m long, 2.6 m diameter bored pile
Change of strain during the hydration of the grout in the Golden Ears Bridge test pile
4/18/2015
27
53 53
0
10
20
30
40
50
60
70
80
90
100
0 40 80 120 160 200 240
Time (h)
TEM
PTA
TUR
E (
ºC)
SWNE
April 25,
2006
Temperature During Curing of a 2m Lab Specimen
54 54
Strain History During Curing of a 2 m Long Full-width Lab Specimen First 5 days after placing concrete
Pusan Case
-300
-250
-200
-150
-100
-50
0
50
0 1 2 3 4 5
Days after Grouting
CH
AN
GE
OF
ST
RA
IN (µ
ε)
PHC Piece
RECOVERY OF SHORTENING (LENGTHENING)INCREASING SHORTENING
OF REBAR
A
Incr
eas
ing
For
ce in
the r
ebar
COMPRESSION
4/18/2015
28
55 55
-200
-150
-100
-50
0
50
100
0 100 200 300 400 500
Time (days)
ST
RA
IN (µ
ε)
SNEW
Pile piece submerged
INCREASING TENSION
Concrete Cylinder Piece
Strain History during 400+ days including letting it swell from absorption of water
Incr
eas
ing
For
ce in
the r
ebar
56 56
The strain gages themselves are not are
temperature sensitive, but the records may be!
The vibrating wire and the rebar have almost the same temperature
coefficient. However, the coefficients of steel and concrete are slightly
different. This will influence the strains during the cooling of the grout.
More important, the rise of temperature in the grout could affect the zero
reading of the wire and its strain calibration. It is necessary to “heat-cycle”
(anneal) the gage before calibration. (Not done by all, but annealing is a
routine measure of Geokon, US manufacturer of vibrating wire gages).
4/18/2015
29
57 57
• Readings should be taken immediately before (and after) every event of the piling work and not just during the actual loading test
• The No-Load Readings will tell what happened to the gage before the start of the test and will be helpful in assessing the possibility of a shift in the reading value representing the no-load condition
• If the importance of the No-Load Readings is recognized, and if those readings are reviewed and evaluated, then, we are ready to consider the actual readings during the test
58 58
Good measurements do not guarantee good conclusions!
A good deal of good thinking is necessary, too
Results of static loading tests on a 40 m long, jacked-in,
instrumented steel pile in a saprolite soil
0
5
10
15
20
25
30
35
40
45
0 2,000 4,000 6,000 8,000 10,000
LOAD (KN)
DE
PTH
(m
)
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250
UNIT SHAFT SHEAR (KPa)
DE
PTH
(m
)
?
4/18/2015
30
59
0
5
10
15
20
25
30
35
40
45
0 2,000 4,000 6,000 8,000 10,000
LOAD (KN)
DE
PTH
(m
)
ß = 0.3
ß = 0.4
A more thoughtful analysis of the data
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250
UNIT SHAFT SHEAR (KPa)
DE
PTH
(m
)
ß = 0.3
ß = 0.4
The butler The differentiation did it
60
0
500
1,000
1,500
2,000
0 10 20 30 40 50
LO
AD
at P
ILE
HE
AD
(kN
)
PILE HEAD MOVEMENT (mm)
HEADTOE
300 mm diameter, 30 m long concrete pileE = 35 GPa; ß = 0.30; Strain-softening
SHORTENING
0
500
1,000
1,500
2,000
0 10 20 30 40 50
LO
AD
(kN
)
MOVEMENT (mm)
HEAD
SHAFTTOE
300 mm diameter, 30 m long concrete pileE = 35 GPa; ß = 0.30 at δ = 5 mm;
Strain-softening: Zhang with δ = 5 mm and a = 0.0125
SHORTENING
HEAD
TOE
t-z curve (shaft)Zhang function
with δ = 5 mm and a = 0.0125
0
250
500
750
1,000
0 5 10 15 20 25 30SEGMENT MOVEMENT AT MID-POINT (mm)S
EG
ME
NT
SH
AF
T R
ES
SIS
TA
NC
E A
T M
ID-P
OIN
T
(kN
)
lower
middle
upper
0 5 10 15 20 25 30
0 5 10 15 20 25 30
0 5 10 15 20 25 30
0 5 10 15 20 25 30
TOE
HEAD
TOE
MOVEMENT
A static loading test with a toe telltale to measure toe
movement—typical records
Now with instrumentation to separate shaft and toe
resistances
Shaft resistance load-movement curve (t-z function) —typical
4/18/2015
31
61
0
5
10
15
20
25
30
35
40
0 500 1,000 1,500 2,000 2,500 3,000
DE
PT
H (
m)
LOAD (kN)
FILLß = 0.30
SOFT CLAYß = 0.20
SILTß = 0.25
SANDß = 0.45
rt = 3 MPa
Profile
FILL with ß = 0.30 Ratio function
with δ = 5 mm and θ = 0.15
Shaft
CLAY with ß = 0.20 Hansen 80-% function
with δ = 5 mm and C1 = 0.0022
Shaft
SILT with ß = 0.20 Exponentional function
with b = 0.40
Shaft
SAND with ß = 0.45 Ratio function
with δ = 5 mm and θ = 0.20
Shaft
SAND with rt = 2 MPa
Ratio functionwith at δ = 5 mm and θ = 0.50
Toe
400 mm diameter, 38 m long, phictitious concrete pile
0
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
0 10 20 30 40 50 60 70
LO
AD
(kN
)
MOVEMENT (mm)
Head-down static loading test load-movements
Head
Shaft
Toe
Compression
The simulations are made using UniPile Version 5
62
Arcos Egenharia
E-cell pileat
Rio Negro Ponte Manaus – Iranduba
Brazil
The bi-directional test
Arcos Egenharia de Solos Bidirectional test at
Rio Negro Ponte Manaus—Iranduba, Brazil
4/18/2015
32
63 63
The difficulty associated with wanting to know the pile-toe load-movement response,
but only knowing the pile-head load-movement response, is overcome in the
bidirectional test, which incorporates one or more sacrificial hydraulic jacks placed at or
near the toe (base) of the pile to be tested (be it a driven pile, augercast pile, drilled-
shaft pile, precast pile, pipe pile, H-pile, or a barrette). Early bidirectional testing was
performed by Gibson and Devenny (1973), Horvath et al. (1983), and Amir (1983). About
the same time, an independent development took place in Brazil (Elisio 1983; 1986),
which led to an industrial production offered commercially by Arcos Egenharia de Solos
Ltda., Brazil, to the piling industry. In the 1980s, Dr. Jorj Osterberg also saw the need for
and use of a test employing a hydraulic jack arrangement placed at or near the pile toe
(Osterberg 1989) and established a US corporation called Loadtest Inc. to pursue the bi-
directional technique. On Dr. Osterberg's in 1988 learning about the existence and
availability of the Brazilian device, initially, the US and Brazilian companies collaborated.
Somewhat unmerited, outside Brazil, the bidirectional test is now called the “Osterberg
Cell test” or the “O-cell test” (Osterberg 1998). During the about 30 years of commercial
application, Loadtest Inc. has developed a practice of strain-gage instrumentation in
conjunction with the bidirectional test, which has vastly contributed to the knowledge
and state-of-the-art of pile response to load.
The bi-directional test
64 64
Schematics of the bidirectional test (Meyer and Schade 1995)
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 10 20 30 40 50 60 70 80
MO
VE
ME
NT
(m
m)
LOAD (tonne)
Test at Banco Europeu
Elisio (1983)
Pile PC-1June 3, 1981
Upward
Downward
11.0
m2.
0 mE
520 mm
Upward Load
THE CELL
Telltales
and
Grout Pipe
Pile Head
Downward Load
Typical Test Results (Data from Eliso 1983)
4/18/2015
33
65 65
Three Cells inside the reinforcing cage (My Thuan Bridge, Vietnam)
66 66
4/18/2015
34
67 67
The bidirectional cell can also be installed in a driven pile (after the driving). Here in a 600 mm cylinder pile with a 400 mm central void.
68 Bidirectional cell attached to an H-beam inserted in a augercast pile after grouting.
4/18/2015
35
69 69 Inchon, Korea
70 Sao Paolo, Brazil
4/18/2015
36
71 71
Test on a 1,250 mm diameter,
40 m long, bored pile at US82 Bridge
across Mississippi
River installed into dense sand
-80
-60
-40
-20
0
20
40
60
80
100
120
0 2,000 4,000 6,000 8,000 10,000
LOAD (KN)
MO
VE
ME
NT
(m
m)
UPPER PLATE UPWARD MVMNT
LOWER PLATE DOWNWARD MVMNT
Weightof
Shaft
Residual Load
Shaft
Toe
Project by Loadtest Inc., Gainesville, Florida
72 72
The Equivalent Head-down Load-movement Curve
Measured upward and downward curves
With correction for the increased pile compression in the head-down test
Construction of the “Direct Equivalent Curve”
Reference: Appendix to regular reports by Fugro Loadtest Inc.
4/18/2015
37
73 73
From the upward and downward results, one can produce the equivalent head-down load-movement curve that one would have obtained in a routine “Head-Down Test”
“Head –down”
cnt.
74
Upward and downward curves fitted to measured curves
UniPile5 analysis using the t-z and q-z curves fitted to the load-movement curves at the gage levels in an effective-stress simulation of the test
-10
0
10
20
30
40
0 500 1,000 1,500 2,000
LOAD (kN)
MO
VE
ME
NT
(m
m)
Upward
Downward
TEST DATA FITTED
TO TESTDATA
Example 1 Example 2
-80
-60
-40
-20
0
20
40
60
80
100
1200 2,000 4,000 6,000 8,000 10,000
MO
VE
ME
NT
(m
m)
LOAD (kN)
UPWARD
DOWNWARD
Weight ofShaft
Residual Load
Simulated curve
Simulated curve
4/18/2015
38
75
0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
0 5 10 15 20 25 30 35 40 45
MOVEMENT (mm)
LO
AD
(K
N)
Fitted to the upward curve
Calculated as "true" head-down response using the t-z functions fitted to the test data
Measured upward curve
Example 2: The upward shaft response extracted and compared to the response of the shaft to an "Equivalent Head-down Test
The difference shown above between the upward BD cell-plate and the head-down load-movement curves is due to the fact that the upward cell engages the lower soil first, whereas the head-down test engages the upper soils first, which are less stiff than the lower soils.
Example 1 Example 2
0
2,000
4,000
6,000
8,000
10,000
0 25 50 75 100 125
LO
AD
(kN
)
MOVEMENT (mm)
Measured Upward
( )
Upward TestMeasured and simulated
As in a Head-down Test
Head-down combining upward and downward curves
Head-down combining upward and downward curves
76
The effect of residual load on a bidirectional test
The cell load includes the residual load whereas the load evaluated from the strain-gages does not.
0
5
10
15
20
25
30
35
0 500 1,000 1,500 2,000 2,500 3,000 3,500
LOAD (KN)
DE
PT
H (
m)
O-CELLAND PARTIAL RESIDUAL LOAD
Residual
Load
False
Resistance
True
Resistance
True Shaft
Resistance
Equivalent
Head-down
False Resistance
The O-cellThe Cell
4/18/2015
39
77
Similar to combining a compression test with a tension test. combining a bidirectional test and a head-down compression
test will help in determining the true resistance
0
5
10
15
20
25
30
35
0 500 1,000 1,500 2,000 2,500 3,000 3,500
LOAD (KN)
DE
PTH
(m
)
HEAD-DOWN AND PARTIAL RESIDUAL LOAD
Residual
Load
True
Resistance
False
Head-down
Residual and True
Toe Resistance
Transition
Zone
True Shaft
Resistance
False
Tension
Test
Not directly
useful below
this level
78
-15
-10
-5
0
5
10
15
20
25
0 100 200 300 400 500 600 700 800 900 1,000
MO
VE
ME
NT
(m
m)
LOAD (kN)
E
8.5
3.0
If unloaded after 10 min
ß = 0.4 to 2.5 m δ = 5 mm ϴ = 0.24 ß = 0.6 to 6.0 m δ = 5 mm ϴ = 0.24 ß = 0.9 to cell δ = 5 mm ϴ = 0.24
ß = 0.9 to toe δ = 5 mm ϴ = 0.30rt = 3,000 kPa δ = 30 mm ϴ = 0.90
PCE-02
-30
-25
-20
-15
-10
-5
0
5
10
15
0 100 200 300 400 500 600 700 800 900 1,000
MO
VE
ME
NT
(m
m)
LOAD (kN)PCE-07
E
7.2
4.3
ß = 0.5 to 2.5 m δ = 5 mm ϴ = 0.30ß = 0.7 to 6.0 m δ = 5 mm ϴ = 0.30 ß = 0.8 to cell δ = 5 mm ϴ = 0.30
ß = 0.6 to toe δ = 5 mm C1 = 0.0063 rt = 100 kPa δ = 30 mm C1 = 0.0070
A CASE HISTORY Bidirectional tests performed at a site in Brazil on two Omega Piles (Drilled Displacement Piles, DDP, also called Full Displacement Piles, FDP) both with 700 mm diameter and embedment 11.5 m. Pile PCE-02 was provided with a bidirectional cell level at 7.3 m depth and Pile PCE-07 at 8.5 m depth.
Acknowledgment: The bidirectional test are courtesy of Arcos Egenharia de Solos Ltda., Belo Horizonte, Brazil.
4/18/2015
40
79
0
500
1,000
1,500
2,000
2,500
3,000
0 5 10 15 20 25 30
LO
AD
(kN
)
MOVEMENT (mm)
PCE-02
PCE-07
Max downward movement
Max upwardmovement
Max upwardmovement
Max downwardmovement
Pile PCE-02Compression
PCE-02, Toe
PCE-07, Toe
PCE-02 and -07Shaft
0
2
4
6
8
10
12
0 400 800 1,200 1,600
DE
PT
H (
m)
LOAD (kN)
PCE-07
PCE-02
0
5
10
15
20
25
30
0 400 800 1,200 1,600
MO
VE
ME
NT
(m
m)
TOE LOAD (kN)
PCE-02
PCE-07
A
B
Equivalent Head-down Load-movements
Equivalent Head-down Load-distributions
After data reduction and processing
A conventional head-down test would not have provided the reason for the lower
“capacity” of Pile PCE-02
80 80
Pensacola, Florida
410 mm diameter, 22 m long, precast concrete pile driven into silty sand
4/18/2015
41
81 81
Pensacola, Florida, USA
-4
-3
-2
-1
0
1
2
3
4
0 500 1,000 1,500 2,000 2,500
LOAD (KN)M
OV
EM
EN
T (
mm
)
-10
0
10
20
30
40
50
60
0 500 1,000 1,500 2,000 2,500
LOAD (KN)
MO
VE
ME
NT
(m
m)
Bidirectional-cell test on a 16-inch, 72-ft, prestressed pile driven into sand.
Cell
After the push test, the pile toe is located higher up than when the test started!
cnt.
Data courtesy of Loadtest Inc.
82
Los Angeles Coliseum, 1994
The Northridge earthquake in Los Angeles, California, in January 1994 was a "strong" moment magnitude of 6.7 with one of the highest ground acceleration ever recorded in an urban area in North America.
The piles had been designed using the usual design approach with adequate factors of safety to guard against the unknowns. Moreover, the acceptable maximum movement was more stringent than usual.
The earthquake caused an estimated $20 billion in property damage. Amongst the severely damaged buildings was the Los Angeles Memorial Coliseum, which repairs and reconstruction cost about $93 million. The remediation work included construction of twenty-eight, 1,300 mm diameter, about 30 m long, bored piles, each with a working load of almost 9,000 KN (2,000 kips), founded in a sand and gravel deposit.
It was imperative that all construction work was finished in six months (September 1994, the start of the football season). However, after constructing the first two piles, which took six weeks, it became obvious that constructing the remaining twenty-six piles would take much longer than six months. Drilling deeper than 20 m was particularly time-consuming. The design was therefore changed to about 18 m length, combined with equipping every pile with a bidirectional cell at the pile toe. Note, the cell was now used as a construction tool.
4/18/2015
42
83
Los Angeles Coliseum, 1994
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
0 2,000 4,000 6,000 8,000 10,000
LOAD (KN)
MO
VE
ME
NT
(m
m)
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
0 2,000 4,000 6,000 8,000 10,000
LOAD (KN)
MO
VE
ME
NT
(m
m)
Schmertmann (2009, 2012), Fellenius (2011)
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
0 2,000 4,000 6,000 8,000 10,000
LOAD (KN)
MO
VE
ME
NT
(m
m)
THE BIDIRECTIONAL-CELL AS A CONSTRUCTION TOOL
TESTS ON EVERY PILE
ONE OF THE PILES
THREE OF THE PILES
ALL PILES
cnt.
84
FIRST STAGE LOADING; "VIRGIN" TESTS WITH LINE SHOWING AVERAGE SLOPE
RELOADING STAGE. DOWNWARD DATA ONLY
The first stage loading is of interest in the context of general evaluation of test results and applying them to design. The second is where the bidirectional cell was used as a preloading tool, resulting in a significant increase of toe stiffness, much in the way of the Expander base.
0
2,000
4,000
6,000
8,000
10,000
0 25 50 75 100 125 150
MOVEMENT (mm)
LOA
D (
KN
)
Downward, First Loading
A
0
2,000
4,000
6,000
8,000
10,000
0 25 50 75 100 125 150
MOVEMENT (mm)
LOA
D (
KN
)
Downward, Second Loading
B
cnt.
4/18/2015
43
85
0 2,000 4,000 6,000 8,000
Rea
ltiv
e Fr
eque
ncy
Mean
2σ
4σ
σ = 1,695µ = 3 , 7 3 1
NORMAL DISTRIBUTION of TOE LOAD AT 25 mm
0
Rea
ltiv
e Fr
eque
ncy
Mean = 71 MPa
2σ
4σ
σ = 26µ = 7 1
NORMAL DISTRIBUTION OF STIFFNESS BETWEEN 25 mm AND 50 mm
STIFFNESS (MPa)
Variation of toe load for 25 mm movement
Slope of load-movement curve (i.e., stiffness)
E = 70 MPa = 10 ksi
= 1.4 ksf
m = 700 (j=1)
0
3,000
6,000
9,000
20 30 40 50 60MOVEMENT (mm)
LOA
D (
KN
)
cnt.
86 86
Bidirectional Tests on a 1.4 m diameter bored pile in North-
West Calgary constructed in silty
glacial clay till
A study of Toe and Shaft Resistance Response to Loading and correlation to CPTU
calculation of capacity
4/18/2015
44
87 87
0
5
10
15
20
25
30
0 10 20 30Cone Stress, qt (MPa)
DE
PTH
(m
)0
5
10
15
20
25
30
0 200 400 600 8001,00
0
Sleeve Friction, fs (KPa)
DE
PTH
(m
)
0
5
10
15
20
25
30
-100 0 100 200 300 400 500
Pore Pressure (KPa)
DE
PTH
(m
)
0
5
10
15
20
25
30
0 1 2 3 4 5
Friction Ratio, fR (%)
DE
PTH
(m
)
PROFILE
The upper 8 m will be removed for basement
uneutral
14 m net pile length
GW
Cone Penetration Test with Location of Test Pile
cnt.
88
Pile Profile and Cell Location Cell Load-movement Up and Down
cnt.
-30
-20
-10
0
10
20
30
40
50
60
70
80
0 1,000 2,000 3,000 4,000 5,000 6,000
MO
VE
ME
NT
(m
m)
LOAD (kN)
ELEV.(m)
#5
#4
#3
#2
#1
Pile Toe Elevation
4/18/2015
45
89 89
Load Distribution cnt.
0
5
10
15
20
25
0 5 10 15 20
LOAD (MN)
DE
PTH
(m
)
O-cellCell Level
0
5
10
15
20
25
0 5 10 15 20
LOAD (MN)
DE
PTH
(m
)
ß = 0.75
ß = 0.35
0
5
10
15
20
25
0 5 10 15 20D
EP
TH
(m
)
LOAD (MN)
ß = 0.65
Residual
90 90
0
5
10
15
20
25
0 5 10 15 20
LOAD (MN)
DE
PTH
(m
)
Schmertmann
E-F
LCPC withoutmax. limitsTEST
LCPC withmax. limits
Load Measured Distribution Compared to Distributions Calculated from the CPTU Soundings
cnt.
4/18/2015
46
Analysis of the results of a bidirectional test on a 21 m long bored pile
A bidirectional test was performed on a 500-mm diameter, 21 m long, bored pile constructed through compact to dense sand by driving a steel-pipe to full depth, cleaning out the pipe, while keeping the pipe filled with betonite slurry, withdrawing the pipe, and, finally, tremie-replacing the slurry with concrete. The bidirectional cell (BDC) was attached to the reinforcing cage inserted into the fresh concrete. The BDC was placed at 15 m depth below the ground surface. The pile will be one a group of 16 piles (4 rows by 4 columns) installed at a 4-diameter center-to-center distance. Each pile is assigned a working load of 1,000 kN.
compact SAND
CLAY
compact SAND
dense SAND
The sand becomes very dense at about 35 m depth
91
0
5
10
15
20
25
0 5 10 15 20 25
DE
PT
H (
m)
Cone Stress, qt (MPa)
0
5
10
15
20
25
0 20 40 60 80 100
DE
PT
H (
m)
Sleeve Friction, fs (kPa)
0
5
10
15
20
25
0 50 100 150 200 250
DE
PT
H (
m)
Pore Pressure (kPa)
0
5
10
15
20
25
0.0 0.5 1.0 1.5 2.0
DE
PT
H (
m)
Friction Ratio, fR (%)
0
5
10
15
20
25
0 10 20 30 40 50
DE
PT
H (
m)
N (blows/0.3m)
compact SAND
CLAY
compact SAND
dense SAND
The soil profile determined by CPTU and SPT
92
0
5
10
15
20
25
0 5 10 15 20 25
DE
PT
H (
m)
Cone Stress, qt (MPa)
0
5
10
15
20
25
0 20 40 60 80 100
DE
PT
H (
m)
Sleeve Friction, fs (kPa)
0
5
10
15
20
25
0 50 100 150 200 250
DE
PT
H (
m)
Pore Pressure (kPa)
0
5
10
15
20
25
0.0 0.5 1.0 1.5 2.0
DE
PT
H (
m)
Friction Ratio, fR (%)
0
5
10
15
20
25
0 10 20 30 40 50
DE
PT
H (
m)
N (blows/0.3m)
compact SAND
CLAY
compact SAND
dense SAND
4/18/2015
47
The results of the bidirectional test
93
-50
-40
-30
-20
-10
0
10
20
30
40
50
0 200 400 600 800 1000 1200
MO
VE
ME
NT
(m
m)
LOAD (kN)
Pile HeadUPWARD
BDC DOWNWARD
15.0
m6.
0 m
Acknowledgment: The bidirectional test data are courtesy of Arcos Egenharia de Solos Ltda., Belo Horizonte, Brazil.
To fit a simulation of the test to the results, first input is the effective stress parameter (ß) that returns the maximum measured upward load (840 kN), which was measured at the maximum upward movement (35 mm). Then, “promising” t-z curves are tried until one is obtained that, for a specific coefficient returns a fit to the measured upward curve. Then, for the downward fit, t-z and q-z curves have to be tried until a fit of the downward load (840 kN) and the downward movement (40 mm) is obtained.
Usually for large movements, as in the example case, the t-z functions show a elastic-plastic response. However, for the example case , no such assumption fitted the results. In fact, the best fit was obtained with the Ratio Function for the entire length of the pile shaft.
94
t-z and q-z Functions
SAND ABOVE BDCRatio function
Exponent: θ = 0.55δult = 35 mm
SAND BELOW BDCRatio function
Exponent: θ = 0.25δult = 40 mm
TOE RESPONSERatio function
Exponent: θ = 0.40δult = 40 mm
CLAY (Typical only, not used in the
simulation) Exponential functionExponent: b = 0.70
4/18/2015
48
The final fit of simulated curves to the measured
95
-50
-40
-30
-20
-10
0
10
20
30
40
50
0 200 400 600 800 1,000 1,200
MO
VE
ME
NT
(m
m)
LOAD (kN)
Pile HeadUPWARD
BDC DOWNWARD
15.0
m6.
0 m
0
5
10
15
20
25
0 200 400 600 800 1,000
DE
PT
H (
m)
LOAD (kN)
BDC load
0
5
10
15
20
25
0 10 20 30 40 50
DE
PT
H (
m)
N (blows/0.3m)
compact SAND
CLAY
compact SAND
dense SAND
With water force
Less buoyant weight
UPWARD
DOWNWARD
The test pile was not instrumented. Had it been, the load distribution of the bidirectional test as determined from the gage records, would have served to further detail the evaluation results. Note the below adjustment of the BDC load for the buoyant weight (upward) of the pile and the added water force (downward).
The analysis results appear to suggest that the pile is affected by a filter cake along the shaft and probably also a reduced toe resistance due to debris having collected at the pile toe between final cleaning and the placing of the concrete.
96
4/18/2015
49
The final fit establishes the soil response and allows the equivalent head-down loading- test to be calculated
0
500
1,000
1,500
2,000
2,500
0 5 10 15 20 25 30 35 40 45 50
LOA
D (k
N)
MOVEMENT (mm)
EquivalentHead-Down test
HEAD
TOE
Pile head movement for 30 mm pile toe
movement
Pile head movement for 5 mm pile toe movement
97
When there is no obvious point on the pile-head load-movement curve, the “capacity” of the pile has to be determined by one definition or other—there are dozens of such around. The first author prefers to define it as the pile-head load that resulted in a 30-mm pile toe movement. As to what safe working load to assign to a test, it often fits quite well to the pile head load that resulted in a 5-mm toe movement. The most important aspect for a safe design is not the “capacity” found from the test data, but what the settlement of the structure supported by the pile(s) might be. How to calculate the settlement of a piled foundation is addressed a few slides down.
The final fit establishes also the equivalent head-down distributions of shaft resistance and equivalent head-down load distribution for the maximum load (and of any load in-between, for that matter). Load distributions have also been calculated from the SPT-indices using the Decourt, Meyerhof, and O’Neil-Reese methods, as well that from the Eslami-Fellenius CPTU-method.
98
0
5
10
15
20
25
0 500 1,000 1,500 2,000
DE
PT
H (
m)
LOAD (kN)
SPT-Meyerhof
Test
0
5
10
15
20
25
0 10 20 30 40 50
DE
PT
H (
m)
N (blows/0.3m)
compact SAND
CLAY
compact SAND
dense SAND
SPT-Decourt
SPT-Decourt
SPT-O'Neill
Test
CPTU-E-F
By fitting a UniPile simulation to the measured curves, we can determine all pertinent soil parameters, the applicable t-z and q-z functions, and the distribution of the equivalent head-down load-distribution. The results also enable making a comparison of the measured pile response to that calculated from the in-situ test methods.
However, capacity of the single pile is just one aspect of a piled foundation design. As mentioned, the key aspect is the foundation settlement.
Note, the analysis results suggest that the pile was more than usually affected by presence of a filter cake along the pile shaft and by some debris being present at the bottom of the shaft when the concrete was placed in the hole. An additional benefit of a UniPile analysis.