Micropiles Save Drilled Shafts

Allen Cadden, P.E.

Problem

• Settlement of up to 25 to 50 mm

• Cracks in supported floors

• Work stopped before last 2 floors were constructed

Subsurface ConditionsNew Fill

Alluvial Soils

Residual Soils

Partially Weathered Rock

Diabase Rock10-40 ft 3-12 m

Instrumented Load Test

Strain Gauge Configuration

14.4 ft (4.4 m)

10 ft (3 m)

Segment 1

2

3

4

10.9 ft

13.9 ft

16.9 ft

23.4 ft

7 inch, N800.5 inch wall

178 mm, 12.5 mm wall56 kg/mm2 Steel

Strain Gauge

Load Test ResultsCyclic Load Data

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 40 80 120 160 200 240 280 320 360Load (tons)

Se

ttle

me

nt

(in

)

1070 kN 2140 kN 3210 kN

9 mm

1070 kN

2140 kN

7.4 m

Average Shear Stress vs. Cycle @ 300 ton (2700 kN)(assumes nonmoving pile tip)

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8

Cycle

Sh

ear

stre

ss (

tsf)

(kg

/cm

^2)

Segment 1

Segment 2

Segment 3

Segment 4

1150 kPa, 165 psi

Connection to Drilled Shafts

Caisson

High Load, Large Shaft

Connection to Drilled ShaftsLow Load, Small Shaft

Micropile Caps for Caisson Repair

Conclusions• Micropiles fixed the problem

• Loads well in excess of 300 tons can be carried by a 7 inch pile

• The load transfer in a rock socket is not constant under cyclic loading