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Micropiles: A Solution for Structural Settlement Issues in Alluvial and Residual Soils

This study by Allen Cadden, P.E. addresses significant settlement problems (up to 50 mm) and structural cracks observed in supported floors of a multi-story building. Work was halted before completing the last two floors due to problematic subsurface conditions, including alluvial and residual soils, as well as partially weathered rock. An instrumented load test demonstrated that micropiles effectively resolved the settlement issues, with load capacities exceeding 300 tons for a 7-inch pile. The findings highlight the importance of connecting micropile caps to drilled shafts for both high and low load scenarios.

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Micropiles: A Solution for Structural Settlement Issues in Alluvial and Residual Soils

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

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

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

  4. Instrumented Load Test

  5. 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 Strain Gauge 10.9 ft 2 13.9 ft 3 14.4 ft (4.4 m) 16.9 ft 4 23.4 ft

  6. 9 mm 1070 kN 2140 kN 3210 kN Load Test Results

  7. 1070 kN 2140 kN 7.4 m

  8. Average Shear Stress vs. Cycle @ 300 ton (2700 kN)(assumes nonmoving pile tip) 1150 kPa, 165 psi

  9. Caisson Connection to Drilled Shafts High Load, Large Shaft

  10. Connection to Drilled Shafts Low Load, Small Shaft

  11. Micropile Caps for Caisson Repair

  12. 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

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