Micropiles

Micropiles are small-diameter deep foundation elements that bypass poor bearing soils and transfer loads into a competent bearing stratum. They can be installed through almost any type of subsurface condition from sands to clay to bouldery soils to rock. Individual micropile capacities can be as large as 400+ tons. Micropiles are capable of resisting both lateral and axial loads. They are particularly advantageous for underpinning of existing structures since small, maneuverable micropile drill rigs can readily handle low headroom/restricted access situations that preclude other conventional methods. In most instances, facility operations can continue uninterrupted during the micropile program.

Installation

High strength steel casing (generally 4 to 12 inches in diameter) is drilled to design depth. A reinforcing element, typically a single steel threadbar, is placed in the casing extending the full length of the micropile. Cement grout is tremied in place by gravity methods. Often following tremie grouting, micropiles are grouted under pressure as the casing is extracted from the bottom of the bond zone to its final elevation.  These are called pressure grouted or “Type B” micropiles and are typically used in sands or slightly compressible soils. The intent of pressure grouting is to achieve a larger bond zone surface area for a higher bond capacity.

On rare occasions, micropiles may undergo further grouting, called post grouting, after the primary cement-grout has started to set. Micropile contractors refer to these piles as “Type C” or “Type D” micropiles.

Applications

  • Retrofit of existing structures to withstand increased loading
  • Underpinning of foundations adjacent to planned excavation
  • Structural settlement control
  • Seismic retrofit

Advantages

  • High capacities achievable
  • Low noise and vibration installation
  • Able to penetrate subsurface obstructions
  • Can be installed in low headroom limited access and within building interiors
  • Minimal disturbance to ongoing facility operations

Read about the installation of 80-ton micropiles in tight conditions 40 feet below street level and adjacent to an active subway.