Micropiles Save Drilled Shafts Allen Cadden, P.E.

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

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

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

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.

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

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.

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

Average Shear Stress vs Average Shear Stress vs. Cycle @ 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

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.

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.

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

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