An Alternative Foundation Solution State Route 22, Section A02 – Lewistown Bypass

This paper details the Value Engineering efforts to design and install micropiles as an alternative to the as-designed driven and pre-drilled H-piles situated over karst terrain. A total of 295, 178 mm (7-in) diameter micropiles with an axial compression capacity of 890 kN (200-kips) were installed for the bridge structures to replace 511 driven and pre-drilled H-piles with an axial compression capacity of 445 kN (100-kips). In karst areas where cutter and pinnacle formations must be penetrated, micropiles are readily advanced through these formations and socketed into competent bedrock. Innovative grout mix design and construction techniques minimize grout loss within cavernous material to develop a competent bond zone. Seven compression load tests were conducted to verify the design bond stress values. This paper also presents bond stress values from micropile load tests conducted on similar projects situated over karst in an effort to assess the current state of practice.