Research at Northwestern University: End-bearing Micropiles in Dolomite
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Transcript of Research at Northwestern University: End-bearing Micropiles in Dolomite
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Research at Northwestern University: End-bearing Micropiles in Dolomite
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Outline
Introduction Test section details Axial load test results Axial load distributions Design implications Conclusions
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Participants:
TCDI-Hayward Baker, Lincolnshire, ILVulcan Quarry, McCook, ILNorthwestern University
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ObjectiveTo evaluate the axial load
transfer characteristics of micropiles embedded in dolomite so that rational design procedures can be developed
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Overview: Axial load tests in Vulcan Quarry Four test piles with lengths of 0.6, 1.2, 1.8 and 2.4 m Piles consist of 178-mm-diameter, 13 mm wall
thickness, 550 MPa steel casings filled with 38 MPa grout. Roller bit is welded to bottom.
Axial load distribution determined by vibrating wire strain gages on steel, embedment gages in grout and telltale readings
Two piles were extracted to examine grout-steel and grout-rock interfaces
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Installation procedures
Test piles ~ 1 m1. Hole cored2. Pile assembled
and placed in hole3. Pile grouted under
low pressure
Production piles ~ 30 m long
1. Assembled pile with roller bit attached used to drill hole
2. Left in place and grouted under high pressure
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Allowable stress design:Pallow = α f 'c Agrout + β fy x A steel
Method fy max (MPa) α β Allowable Load (kN)
AASHTO (Service load design) 550 0.4 0.47 2000
Chicago Building Code 200 0.4 0.4(1) 800
Massachusetts Building Code 410 .33(3) 0.4(2) 1400
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Vulcan Quarry, McCook, IL.
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On site preparation of micropiles
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Micropile 1
Micropile 2
Micropile 3
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Rock Conditions
Top - 2Top - 2BottomBottom
BottomBottom
Top - 3Top - 3
BottomBottom
Top -4Top -4
BottomBottom
Top - 1Top - 1
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Load test frame
transfer girder
transfer beams
Reaction anchor
test pile
hydraulic jack
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Axial load test results
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-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
00 1000 2000 3000 4000 5000
Load (kN)
Def
lect
ion
(mm
)
Failure(2nd Loading)
Tip
Head
Elastic line
Axial Load vs. Deflection of Micropile 1
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-40
-35
-30
-25
-20
-15
-10
-5
00 500 1000 1500 2000 2500 3000 3500 4000
Load (kN)
Def
lect
ion
(mm
)
Tip
Head
Elastic line
Failure (1st Load)
Failure (2nd Load)
Axial Load vs. Deflection for Micropile 2
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-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
00 1000 2000 3000 4000 5000
Load (kN)
Def
lect
ion
(mm
)Tip
Head
Elastic line
Failure
Axial Load vs. Deflection for Micropile 3
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Figure 5 - Axial load-deflection curve of micropile 4
-15
-12
-9
-6
-3
00 500 1000 1500 2000 2500
Load (kN)
Def
lect
ion
(mm
)
Head
-15
-12
-9
-6
-3
00 500 1000 1500 2000 2500
Elastic line
Tip
?
?
Failure
Axial Load vs. Deflection for Micropile 4
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Summary of load test results
Pile 1 failed at 2000 KN and 4000 KN on second loading, cumulative tip movement = 10 mm (RQD = 22)
Pile 2 failed 800 KN on first loading and 2000 KN on second loading, cumulative tip movement =25 mm (RQD = 0)
Pile 3 did not fail at 4450 KN, tip movement = 2 mm (RQD = 87)
Pile 4 with soft bottom exhibited a plunging failure at 2000 KN
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Axial load distributions
Determining moduli for composite pile – Fellenius (1989) method
Data
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0 500 1000 1500 2000M icrostra in
0500
10001500200025003000350040004500
Load
(kN
)
0 500 1000 1500 2000M icrostra in
0
50
100
150
200
250
Tang
ent M
odul
us (G
Pa)
-0.54 m-0.54 m-1.15 m-1.15 m-1.5 m-1.5 m
B=78 G PaA=-0.013 G Pa/
BAEs 5.0
Strain Gage Data from Micropile 3
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0 2000 4000Load (kN )
-2
-1 .5
-1
-0 .5
0
Em
bedd
ed D
epth
(m)
M icrop ile 3
0 2000 4000Load (kN )
-2
-1.5
-1
-0.5
0
M icropile 1
3115
4450
35604003
Axial Load Distributions for Micropiles 1 and 3
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0 500 1000 1500Load (kN )
-1 .5
-1 .25
-1
-0 .75
-0 .5
-0 .25
0
0.25
0.5
M icrop ile 2 (F irst Loading)
0 1000 2000 3000Load (kN )
-1 .5
-1.25
-1
-0.75
-0 .5
-0.25
0
0.25
0.5
Em
bedd
ed D
epth
(m)
M icrop ile 2 (Second Load ing)
Axial Load Distribution of Micropile 2
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?
0 800 1600 2400Load (kN)
-2.25
-2
-1.75
-1 .5
-1.25
-1
-0.75
-0 .5
-0.25
0
0.25E
mbe
dded
Dep
th (m
)
M icropile 4
??
Axial Load Distribution for Micropile 4
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0 -4 -8 -12 -16 -20A xia l H ead D eflection (m m )
-1000
0
1000
2000
3000
4000
5000
6000
Uni
t Sid
e R
esis
tanc
e (k
Pa)
M icropile 1 (0 .0-0 .24 m )M icropile 2 (0 .0-0 .33 m )M icropile 2 (0 .33-0.68 m )M icropile 3 (0 .0-0 .54 m )M icropile 3 (0 .54-1.15 m )M icropile 3 (1 .15-1.5 m )M icropile 4 (0 .0-1 .15 m )M icropile 4 (1 .15-1.76 m )M icropile 4 (1 .76-2.11 m )
Mobilized Side Resistance vs Axial Head Deflection
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Summary of load transfer data
No load transfer in upper 1 m – due to low confinement and poor rock quality
Critical interface was steel/grout; verified from visual observations of extracted piles
Shorter piles (1 and 2) were end-bearing; capacity a function of RQD
Pile 4 with soft bottom had an average unit side resistance approximately equal to that of a smooth bar pulled from concrete (3500 kPa)
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Computed Observed
No. Allowable structural load (kN)
Davisson allowable
load with FS = 2 (kN)
Allowable load for 13 mm movement
(kN)(1) (2) (3)
1 1630 880 1380 2000 3800
2 1560 800 1320 400 1200
3 1560 800 1320 >2225 >4450
4 1560 800 1320 1000 not applicable
(1) – AASHTO(2) – Chicago Building Code(3) – Massachusetts Building Code
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Example: Production pile
Typical length in Chicago: 25 to 30 m When pile tip moves 2 mm under 4450
KN (like pile 3), design for movements For 27.5 m long pile:
– 12.5 mm deformation – 1350 KN capacity– 25 mm deformation – 2600 KN capacity– Both greater than 800 KN based on
Chicago code
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Conclusions Stresses in piles were in excess of those specified in
codes without detrimental effects on performance Steel-grout interface governed axial load transfer
behavior along side No side resistance mobilized in top 1 m of test piles due
to low stresses and grout pressures and poor quality rock
Due to relatively high compressibility, allowable axial loads of full-scale piles, founded on competent rock, are determined more rationally from allowable deformation considerations, rather than code-specified allowable stresses.
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Total Movement vs Load
-3.00
-2.80
-2.60
-2.40
-2.20
-2.00
-1.80
-1.60
-1.40
-1.20
-1.00
-0.80
-0.60
-0.40
-0.20
0.000 100 200 300 400 500 600 700 800
Load (kip)
Def
lect
ion
(in)
Adaptive Reuse of Soldier FieldMicropile Load Test by Hayward Baker
Total Top of MP Deflection vs. Load5.5" OD x 0.415" Wall
DL = 150 KipsLength of Pile = 103'
ASTM D1143 Quick Load TestTest Date 2/23/02
Theoretical ElasticDeflection
Top of Pile Deflection
Tip Movement
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Total Movement vs Load
-3.00
-2.80
-2.60
-2.40
-2.20
-2.00
-1.80
-1.60
-1.40
-1.20
-1.00
-0.80
-0.60
-0.40
-0.20
0.000 200 400 600 800 1000 1200 1400
Load (kip)
Def
lect
ion
(in)
Adaptive Reuse of Soldier FieldMicropile Load Test by Hayward Baker
Total Top of MP Deflection vs. Load9.625 OD x 0.545 Wall
DL = 400 KipsLength of Pile = 103'
ASTM D1143 Quick Load TestTest Date 2/21/02
Theoretical ElasticDeflection
Top of Pile Deflection
Tip Movement per Tell Tale