Research at Northwestern University: End-bearing Micropiles in Dolomite

Outline

Introduction Test section details Axial load test results Axial load distributions Design implications Conclusions

Participants:

TCDI-Hayward Baker, Lincolnshire, ILVulcan Quarry, McCook, ILNorthwestern University

ObjectiveTo evaluate the axial load

transfer characteristics of micropiles embedded in dolomite so that rational design procedures can be developed

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

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

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

Vulcan Quarry, McCook, IL.

On site preparation of micropiles

Micropile 1

Micropile 2

Micropile 3

Rock Conditions

Top - 2Top - 2BottomBottom

BottomBottom

Top - 3Top - 3

BottomBottom

Top -4Top -4

BottomBottom

Top - 1Top - 1

Load test frame

transfer girder

transfer beams

Reaction anchor

test pile

hydraulic jack

Axial load test results

-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

-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

-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

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

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

Axial load distributions

Determining moduli for composite pile – Fellenius (1989) method

Data

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

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

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

?

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

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

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)

  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

 

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

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.

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

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