Observations From The Instrumentation of a

Micropiled and Tied Back Reticulated Grid

Walter E. Vanderpool, P.E.Senior Geotechnical Engineer

Terracon Consultants, Inc.

Site: The C2K Theatre at the Palazzo370 kN wall load at the crest of a 15.3 m excavationTheatre construction concurrent with excavation and tower construction

Site excavation began 9/1/04Theatre construction began 11/25/04Secant pile installation began 12/6/04Underpinning installed 12/11/04Secant piling complete 12/16/04Theatre structure frame complete 3/14/05

• Reticulated group plan• VW gauge locations• Micropile locations

• Tieback locations• Foundation shafts for

cantilevered tower

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Easting (m)

Nor

thin

g (m

)

Secant Piles MicropilesBonded Tie Back VW Strain GaugeInclinometer PiezometerTower shafts

N

605

610

615

620

625

630

635

15 20 25 30 35 40 45 50 55 60

Viewing WestEl

evat

ion

(m M

SL)

Secant Piles

VW Strain Gauge

Inclinometer

Bonded Tie Back

Mat

Micropiles

Piezometer

Northing (m)

SG 616.2 m

FF 631.5 m

Caliche Layers

• Reticulated group profile• VW gauge locations• Micropile locations• Tieback locations

• Reticulated tieback intersections

• Cemented beds• Piezometer location

• Relative top-drive hammer drilling resistance• Measured by rate of advance• Subjectively interpreted relative to geotechnical logging

Micropile, Relative Drilling Resistance, 1=very soft/loose, 3=very stiff/very dense and partially cemented,

5=strongly cemented

612

614

616

618

620

622

624

626

628

630

632

0 1 2 3 4 5

Drilling Resistance Rating

Elev

atio

n (m

MSL

)

Relative Drilling Resistance, count=103

629.4 m MSL

Flush grout injection drillingVW strain gaugeInsertion by fiberglass rod

Inclinometer at P1-015NSPiezometerat P1-013SCemented layer at approximately elevation 626.5 m to 625 m MSL

Notch at tower cantileverTheatre wall 36.3 m - wide by

32 m - high by 457mm - thick

Carbonate precipitate at tie-backs

Tower shaft group at cantilever

• Micropile strain history• VW strain gauge data record

• Installation 12/04 through completion 1/06

-1000

-750

-500

-250

0

250

500

750

1000

1250

1500

1750

2000

2250

12/4/2004 0:00 1/29/2005 0:00 3/26/2005 0:00 5/21/2005 0:00 7/16/2005 0:00 9/10/2005 0:00 11/5/2005 0:00 12/31/20050:00Date & Time

Gau

ge M

icro

stra

in

M4,17Curing

M6, 6

M4, 10

M8, 2

M2, 8

M4, 17

M1, 11

MatFillPlaced

Theatre Construction

M1,5

• Initial tension during curing and excavation to subgrade12/11/04 – 12/18/04

• Mat placement 12/24

• Theatre construction 1/05 – 3/05

• Fill placement 3/14/05 – 3/20/05

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

12/9/2004 0:00 12/23/2004 0:00 1/6/2005 0:00 1/20/2005 0:00 2/3/2005 0:00 2/17/2005 0:00 3/3/2005 0:00 3/17/2005 0:00 3/31/2005 0:00 4/14/2005 0:00

Date & Time

Gau

ge M

icro

stra

in

M6, 6

M4,17

M1, 5

M1, 11

M2, 8M8, 2

M4, 17Grout Curing(see Figure 9)

Mat Fill Placed

Mat Placed

Wall Construction

(see Figure 10)

• Forming and concrete placement load/unload at interior location on mat

-72

-71

-70

-69

-68

-67

-66

2/9/2005 18:00 2/10/2005 6:00 2/10/2005 18:00 2/11/2005 6:00 2/11/2005 18:00 2/12/2005 6:00

Date & Time

Gau

ge M

icro

stra

in

M4, 17

• Tieback stressing effects• 15 Tiebacks at Level -1 stressed

90

95

100

105

110

5/19/2005 5:00 5/19/2005 7:00 5/19/2005 9:00 5/19/2005 11:00 5/19/2005 13:00 5/19/2005 15:00Date & Time

Gau

ge M

icro

stra

in

M2, 8

• Inclinometer data record• Weekly record 4/05 to 8/05 and 2/06

• Hourly data by IPI system 8/05 – 12/05 not included

• Piezometer data record 12/04 – 1/06

• Head rise during tieback grouting• Head drop during drilled shaft

construction

Secant Pile P1- 013,S

608

610

612

614

616

618

620

622

624

626

628

630

632

634

11/9/20040:00

1/4/20050:00

3/1/20050:00

4/26/20050:00

6/21/20050:00

8/16/20050:00

10/11/20050:00

12/6/20050:00

Date and Time

Gro

undw

ater

hea

d (m

)

Head (m MSL)

O. G. 630.3 m

S. G. 616.2 m

F. F. 631.5 m

Tieback Grouting(see Figure 14)

Shaft Drilling

• Tieback grout placement at Level – 2

Secant Pile P1- 013, S

615.0

615.5

616.0

616.5

617.0

617.5

618.0

618.5

619.0

619.5

620.0

7/1/2005 11:00 7/1/2005 12:00 7/1/2005 13:00 7/1/2005 14:00 7/1/2005 15:00 7/1/2005 16:00 7/1/2005 17:00

Date and Time

Gro

undw

ater

hea

d (m

MSL

)

Head (m MSL)

Elastic Analyses vs. Observations

• Boussinesq elastic analyses by superposition• Instrumented micropile locations and depths

-250

-200

-150

-100

-50

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Depth (D/B) - D/B 0=629.3 m MSL, D/B 2=609.1 m MSL

Vert

ical

Str

ess

(kPa

)

M1,5 M2,8 M4,17 M8,2

M6,6 M1,11

Embedded straingauge depths (D/B)

14.8 m

2.7 m 2.7 m

36.3 m

4.9 m

B=10.1 m

30.8 m

Excavation subgrade 616.2 m MSL

Secant pile tip elevation610.1 m MSL

PLAN (sketch)

RowM1

RowM8

Line 1 Line 22

M6,6

M4,17

M8,2

M1,17

M5,1

M2,8

Ground/Micropile Load Sharing Pre-Excavation, April 1, 2004

17.1 %5.62-33.4-30.260.80.58M6,6

11.8 %5.64-49.4-32.940.50.37M4,17

10.9 %5.64-43.3-26.550.80.60M4,10

47.0 %2.70-50.9-64.710.50.47M8,2

16.6 %4.18-72.0-50.000.50.30M2,20

28.1 %3.70-67.3-69.930.50.30M2,14

5.6 %3.70-67.4-14.020.50.30M2,8

73.2 %2.02-34.0-50.321.10.66N2,2

50.4 %1.96-43.2-42.620.80.57M5,1

50.9 %1.62-58.7-55.050.50.30M1,17

88.3 %1.62-36.7-52.411.10.66M1,11

37.6 %1.62-67.5-41.080.50.30M1,5

Stress Ratio %

Tributary Area (m^2)

Boussinesq Stress (kPa)

4/1/05 Load (kN)

D/BD/LLocation

37.6% to 88.3% at perimeter5.6% to 73.2% at 1st interior row10.9% to 11.8% near middle of mat (row 4)17.1% at back edge of mat (row 6)Prior to stressing

Ground/Micropile Load Sharing Post Construction January 31, 2006

139.5 %*5.62-33.4261.760.80.58M6,6

11.4 %5.64-49.4-31.710.50.37M4,17

83.0 %*5.64-43.3201.220.80.60M4,10

184 % *2.70-50.9252.840.50.47M8,2

29.0 %4.18-72.0-87.140.50.30M2,20

79.6 %3.70-67.3-198.270.50.30M2,14

6.7 %3.70-67.4-16.600.50.30M2,8

86.12.02-34.0-59.431.10.66N2,2

66.4 %1.96-43.2-56.180.80.57M5,1

26.6 %1.62-58.7-25.220.50.30M1,17

115.5 %1.62-36.7-68.561.10.66M1,11

20.8 %1.62-67.5-22.770.50.30M1,5

Stress Ratio %

Tributary Area (m^2)

Boussinesq Stress (kPa)

1/31/06 Load (kN)

D/BD/LLocation

After tieback stressing20.8% to 115.5% at perimeter6.7% to 184% (tension) at 1st

interior row11.4% to 83.0% (tension) near middle of mat (row 4)139.5% (tension) at back of mat (row 6)

CONCLUSIONS

The micropiles accumulated 10 to 90 percent of the load as it was applied in proportion to the relative stiffness between the soils and the micropiles.

Instrumented micropiles responded with strains consistent with their location within the group and the forces applied.

The micropiles provided confinement and resistance to axial deformation during tieback stressing.

Tie back stressing caused micropiles to act in tension at the tieback bond zone.