DRILLED MICROPILES TECHNICAL SUMMARY Back to Drilled Micropiles Main Page 2 of 3 Back 1   2   3 Next APPLICATIONS Structural Foundation Support Structural Underpinning Soil/Slope Stabilization Excavation Support ADVANTAGES High bearing capacity Measure 6 to 12 inches in diameter Consistent capacities of 20 to 100 tons Special applications of up to 200 tons Installation in areas presenting difficult access and/or low headroom Minimal disturbance/vibration to adjacent structures Capability to penetrate subsurface obstructions DESIGN PROCESS Drilled micropiles are designed to resist the required compression, tension, and lateral loads applied. This is accomplished by analyzing the structural and geotechnical capacities of the pile. The structural capacity of the pile is governed by the grout and reinforcing steel strengths. The typical 4,000 psi grout compressive strength and 60,000 psi steel strengths are reduced by appropriate design safety factors to calculate allowable design stresses. The geotechnical capacity of the pile is typically developed in friction between the grout and the in situ soils. Published values and those derived from local experience are typically used in the initial design of the bond length for the pile. On most projects, these values are then proven with the field load test. Drilled micropiles are typically installed with rotary drilling techniques, using an external water flush with flush joint casing. The piles are drilled to their design depth, including a bond zone into load bearing soils. Upon completion of advancement of the drill string to the design depth, the grouting process commences. The casing is tremie grouted then pressure grouted though the drill head until the grout return is observed about the annulus between the casing and the existing ground. The result is a grouted-in-place micropile with reinforcing, able to carry loads from 10 to 200 tons.