Micropile construction commenced with a detailed review and understanding of the micropile construction specification and project drawings. Dimensions of permanent materials including casing and all-thread-bar are common for all types of micropiles. For rock bonded micropiles, the minimum embedment into rock referred to as the bond length is also required.
Rock bonded micropiles are the most common form of micropiles realizing the benefit of the underlying subsurface rock to add load bearing capacity.
Rock bonded micropiles maximize the potential benefit of the under laying rock formation characteristics. The design engineer reviews all available geotechnical information including core borings and unconfined compressive strength of the formation in developing the micropile design calculation. These design calculations provide the theoretical minimum rock socket bond length. Once mobilization to site occurs, the theoretical design parameters are validated in a pre-production testing program prior to commencing production.
The challenge was to construct a state-of-the-art, 8,500 seat arena and parking deck encompassing an entire city block in the heart of an urban area. The underlying complex subsurface conditions lead to the use of compaction grouting, and the utilization of three different types of micropile designs.
Miles of drilling
Installed (1705) high capacity micropiles to a 3” tolerance at all locations. (451) were installed with a diameter of 5.5” with #18 reinforced bar, (517) 7” with #20 bar and (737) 9.625” with #24 bar. The 5.5” and 7” piles utilized a 10’ rock socket whereas the 9.625” piles were installed with a 13’ socket. (2) compression tests, (9) tension tests and (2) lateral tests were performed to verify design and construction methods.
Micropiles, in general, provide the design engineers and builders with access to the best available technologies when it comes to deep foundation. For this project, the design engineers were faced with the complex geological conditions with high structural loading and, as always, were looking for the best economical value in their approach to the work. Rock bonded micropiles were selected to maximize the benefits of the under laying rock formation to accept the heavy compressive loads and to counter any uplift forces.
The opportunity was provided to install and test on the actual site location prior to the installation of production piles. The entire location was mapped out, several sites were pinpointed, drilled and test piles installed. This approach enabled the utilization of multiple bond zones over the various locations for the tests. A registered professional engineer administered the tests and the subsequent results led to the determination of the optimal “site specific” bond zone. This was found to be substantially less than the original design. Armed with this data, the new design protocol was implemented and the updated procedure led to several hundred thousand dollars of cost savings on the project. The rock bonded micropiles were installed in small clusters or groupings and were then tied together at the surface with rebar and concrete in the pile cap. These caps were then spanned by concrete and steel beams called grade beams. As viewed from the end user, the structural loading of the parking garage, including the structure and the parked vehicles, is transferred from the concrete floor(s) through beams and columns down onto the grade beams and then down onto the pile caps and ultimately down through the micropiles into the under laying rock formation. Rock bonded micropile technology provided the best value on this project because the design engineers had the ability to vary the fundamental design elements that make up the pile. The designers calculated the diameter, length of the bond zone and the specifics of the materials that made up the pile to formulate the precise performance necessary to achieve the design criteria. They also varied the number of piles that made up each of the individual pile caps. Application specific designs ensure the proper foundation element is developed to maximize the benefits and contribution of the actual subsurface characteristics.