1. Micropiles Save Drilled Shafts
Allen Cadden, P.E.

2. 7-Story office building, cast in place concrete structure founded on drilled shafts. Diameters ranged from about 3-6 ft.

3. Problem Settlement of up to 25 to 50 mm Cracks in supported floors
Work stopped before last 2 floors were constructed
Overview of conditions

4. Subsurface Conditions
New Fill
Alluvial Soils
Residual Soils
Generalized Geology
Partially Weathered Rock
10-40 ft
3-12 m
Diabase Rock

5. Instrumented Load Test
Micropiles were used to enhance the stability of the shafts and limit further settlement. A test pile was installed using 7 inch N80 casing with 0.5 inch wall thickness. Casing extended full depth, and was grouted in place into at least 10 ft of diabase rock. Four geokon sister bar strain gages were attached to a one inch thing wall metal pipe.

6. Strain Gauge Configuration
7 inch, N80
0.5 inch wall
178 mm,
12.5 mm wall
56 kg/mm2 Steel
10 ft (3 m)
Segment 1
Generalized layout of Test Pile
Strain Gauge
10.9 ft 2
13.9 ft 3
14.4 ft (4.4 m)
16.9 ft 4
23.4 ft

7. Load Test Results 1070 kN 2140 kN 3210 kN 9 mm
Basic load test results. First set of cycles used to increase load to twice the design load. Resulting gross movement was about 0.35 inch.

8. 1070 kN
Strain gage measurements for the first set of cycles to twice the design load.
2140 kN
7.4 m

9. Average Shear Stress vs
Average Shear Stress vs. 300 ton (2700 kN) (assumes nonmoving pile tip)
1150 kPa, 165 psi
Following completion of the initial test (Cycle 0 on the plot) to twice the design load, the pile was reloaded after about 24 hours.
During the second loading test, the pile was loaded to about 300 tons through seven cycles (Cycle Nos. 1-7). Cycles generally consisted of going directly from an alignment load to the full load without intermediate stopping points.
Resulting shear stress was calculated from intervals between strain gages

10. Connection to Drilled Shafts
High Load, Large Shaft
Caisson
Connection of the piles to the existing shafts required two methods. Where the loads required the use of a seven inch pile with an allowable (working) load of about 150 tons, the piles required a new pile cap to be constructed around the shaft and connected to the shaft through the use of dowels.

11. Connection to Drilled Shafts
Low Load, Small Shaft
Where loads were lighter, and shafts were long enough, a single all thread bar was used and grouted into the rock for capacities on the order of 50 tons. These piles were installed through the center of the existing drilled shafts. Load transfer from the pile to the building was achieved through bond with the shaft concrete.

12. Micropile Caps for Caisson Repair
Views of the pile and the cap construction

13. 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