development has been the technological advancement in micropile materials. Of recent note is the increased availability and use of hollow bars and injection drilling for micropiles. 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. Using this technique, the bar is drilled and injected with grout during installation, making it a one-step process that is ideal when working in submerged sands and other challenging conditions. The rod sections are typically fully threaded and can be cut and coupled at any point along the length, allowing them to be used in areas of restricted access or limited headroom. Although hollow bars may carry a higher material cost, their increased rate of installation has made them attractive to contractors by giving them commercial advantages related to equipment size and labor requirements in many situations. Challenges Although we continue to learn more about micropile behavior and performance under different loading conditions, the industry still faces challenges. There is an increased need for support of lateral loading in com- bination with tension and compression. Because micropiles cannot take significant lateral loads individually, they are used in battered groups if lateral loads are sizeable. More efficient methods for design of battered micropile groups are necessary. As slender elements, micropiles are often under scrutiny for buckling potential, although this has been proven unlikely. As the loading demands on micropiles grow, it is important that we advance our knowledge on the behavior of the connection between the micropiles and the structure. Some limited research has been performed on bearing plate performance and on the capacity of connections between micropiles and existing footings; however, much work remains to be done as there are no widely accepted guidelines for design, and typical design procedures may not be appropriate. The industry also faces the challenge of improper application of the technology. In the past, micropile design was performed by a few experts, by hand, who would Micropiles provided the foundation for the Crystal Bridges Museum of American Art in Arkansas (Schnabel Engineering, Foundation Specialties, Inc.) generally weigh the potential consequences of each design decision. With the wealth of literature on micropiles, more and more inexperienced engineers are tasked with designing micropiles for very complex pro- jects. The same concern exists for installers, as inexperienced contractors are called upon to meet the growing demand for micropile construction and may lack significant knowledge and proper equipment. There is a current trend toward com- moditization of micropiles. In the past, it was common to assume that the design engineer would be present full time during installation of the micropiles to log details of drilling and grouting, and supervise load tests. Now, it is more and more difficult to educate owners on the need for appropriate Quality Control. This is because many owners are unaware of the delicate relat ionship between instal lat ion parameters and performance of micropiles. This may be the greatest challenge that will be faced by micropile technology. Future Advancements DFI has been performing a great service to the construction business through education of engineers and contractors in all aspects of the foundations industry. This is especially true for micropiles. The next step is to extend this education beyond the confines of the engineering community to owners and developers. This will help enhance the understanding of the value of micropiles as well as the challenges associated with constructing these systems that must be evaluated by experienced personnel during installation. Perhaps the design aspects most open for advancement in micropile technology are design of connections to existing structures, and efficient design of micropile groups supporting large lateral loads. Another area for advancement is more standardized quality control procedures, which would include requirements for observation and measurement of parameters, allowing owners to better understand what is being installed. Structurally, a possible advancement would be the use of higher-capacity steel, which is currently limited by strain compatibility concerns. Through the work of organizations such as the International Society for Micropiles (ISM), International Association of Foun- dation Drilling (ADSC) and the Deep Foun- dations Institute (DFI), and with the support of the Federal Highway Administration (FHWA) and universities, micropile technology will continue to advance and grow to meet even more challenges. Special thanks to Jesús Gómez and Allen Cadden from Schnabel Engineering for their help in preparing this article. DEEP FOUNDATIONS • NOV/DEC 2013 • 63
