Micropile construction commenced with a detailed review and understanding of the micropile construction specification and project drawings. Dimensions of permanent materials including casing, all-thread-bar are common for all types of micropiles. For pressure grouted micropiles, the minimum bond length is also required.
Pressure grouted micropiles are typically drilled into unconsolidated materials such as soil, sand or broken rock, to develop a bond zone which is incorporated into the micropile design. The injection of grout under pressure allows bonding to the subsurface strata to add load bearing capacity.
Pressure grouted micropiles are typically drilled into more challenging subsurface conditions. This type of installation allows the maximization of the potential benefits of the under laying soil formational 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 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 provide the foundation elements for the renovation and expansion of an existing wastewater treatment plant with many pile locations installed in limited work space areas. The subsurface conditions lead to the use of pressure grouted micropiles with a site specific design.
Piles installed at this vertical tolerance
Phases of project
Clearance from existing structure
(134) 9.625” pressure grouted micropiles were installed, many in close proximity to the existing structure, to a 0.30” vertical tolerance at all locations. (1) verification test was performed in tension to verify design and construction methods.
Micropiles, in general, provide the design engineers and builders access to the best available technologies when it comes to deep foundation solutions. For this project, the design engineers were faced with the challenge of high structural loading and installation in close proximity to the existing structure and, as always, were looking for the best economical value in their approach to the work. Pressure grouted micropiles were selected to maximize the bonding to the under laying formation to accept the heavy compressive loads and to counter any uplift forces.
The pressure grouted 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 pier, including the structure, 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 formation. Pressure grouted 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.