The growth of the technology in the U.S. can be attributed to the innovative spirit of the construction industry. Micropiles started to be used as individual load-carrying elements, or what are known today as Case I micropiles. This was made possible by innovations in drilling techniques and materials. The change in micropile loading mechanism fueled a demand for more efficient installation methods and materials, as well as a need for better quality assurance and design In 1997, an international organization of micropile specialists was established. This group, the International Workshop on Micropiles (IWM), supported the transfer of micropile technology to Japan shortly after the Hanshin earthquake in 1995. The International Society for Micropiles (ISM) evolved from this initial technology transfer workshop, and has continued to produce regular workshops and documentation of worldwide micropile advances and research. construction. They were incorporated into the International Building Code (IBC) in 2006 and the AASHTO LRFD Bridge Design Specifications in 2008, which allowed their use for bridges, highways and buildings. The technology is a valid and economical alternative to drilled shafts in sites with difficult or tight access. Some of the most extensive micropile applications in recent history have been for construction of new structures in karst sites, new bridges across deep valleys filled with colluvium, and slope stabilization. Micropiles have revolutionized slope Hollow bar micropiles were installed to increase the capacity of existing bridges on the New Jersey Turnpike (Schnabel Engineering, PKF Mark III, Inc.) procedures. Thus, a new discipline emerged that continues to grow from interaction between contractors, material suppliers, equipment developers, engineers, academia, and state and federal agencies. As the industry began to embrace this technology, testing programs and calibrated design methods yielded new information that provided confidence in the use of higher-capacity micropiles. In parallel, an expansion of literature was taking place as the deep foundations industry began to promote the use of micropiles through technical papers, presentations, and short courses. The Federal Highway Association (FHWA) sponsored the deve lopment of standardized design and specification guidelines, which serve as reference material across the U.S. and in many parts of the world. 62 • DEEP FOUNDATIONS • NOV/DEC 2013 As the use of micro- pi l e s be c ame mor e widespread, their cost decreased. It became common to use 80 ksi steel pipe for the micropile drill casing and for permanent reinforcing, providing increased compressive capaci ty and higher resistance to bending. Fortunately, steel casing used for a micropile need not meet the strict standards for tolerance mandated for pipe in the oil industry; therefore, secondary N80 casing is also a cost-effective solution. As the popularity of micropiles grew, so did their uses. By the early 2000s, micropiles were being used for new Hollow bars are drilled with a sacrificial bit, and grout is pumped through the bar as it is advanced, providing temporary support against hole instability. stabilization primarily through acces- sibility. While drilled shafts may be the most economic option, the drill rig and crane equipment needed for installing and handling the reinforcing cages is large and often not conducive to working on or near an unstable slope. Access for concrete delivery and pumping across the site may be a concern as well. The use of micropiles offers the ability to navigate a relatively steep slope using a track rig or detached masthead on a platform suspended from a crane. A grout mixing and pumping station may be set up on site near the micropile installation and moved around the site with relative ease. There have also been significant advances in the development of design procedures for micropile-based slope stabilization. Numerical, finite difference software for the analysis of deep foundations subjected to lateral loading has been improved to include a “soil movement option.” This option allows the designer to input any conceivable soil displacement profile without the need to pre- determine the location and magnitude of loads acting on the pile. Dramatic equipment i n n o v a t i o n s h a v e improved the ability to drill through difficult geological conditions in areas of very limited access. These improvements are a result of enhanced mechanical efficiency of the drills and drill tooling, including new families of duplex drilling systems. Another area of
