Micropiles are a small-diameter - typically less than 12 inches (305 mm)- high-capacity drilled and grouted replacement pile, reinforced with a threaded bar and steel casing. They are constructed by drilling a borehole through overburdened material and into a bearing stra­tum, placing reinforcement and grouting. They are capable of resisting axial tension and compression with applied lateral load.

Micropiles have a cased upper section, composed of steel tubes, and an uncased lower bond section, which develops friction with the surrounding bearing stratum. The cased section interacts with the surrounding soil or rock to provide lateral capacity to the foundation . The piles also are reinforced with a high-capacity threaded steel bar, which extends from the top of the pile through the lower bond section, transferring axial force through friction with the grout and ground. The pile group works in tension and compression to resist overturning.

Prior to foundation construction, two rounds of value engineering were completed, effectively decreasing the number of micropiles and total distur­bance area of the concrete caps. Re­ducing the required number of piles and amount of concrete significantly benefitted the project schedule. The first round of value engineering looked at altering the original conceptual foundation design, which included larger­diameter micropiles using a bolted steel pile cap. By changing the cap material from steel to concrete, the fixity between the piles and cap was improved, and the field fit-up associated with the bolted steel cap was eliminated. The revision to the cap design led to a reduction in micropile diameter.

The second round of value engineering was a joint effort between Crux and Burns & McDonnell to use site-specific directional structure loading. By using directional loading components and individual load combinations, the founda­tion design team was able to reduce the size of the pile cap, decreasing the total permanent disturbance area.

The specialty drill rigs provided by Crux were lightweight and componentized, lending themselves well to a helicopter-supported project. Beyond access constraints, additional foun­dation challenges included limited geotechnical data, drilling spoils containment with the presence of standing surface wa­ter, concrete placement within a protected habitat and install­ing heavily loaded structures in deep, soft soils. The tubular steel-pole design loads were an approximately 20,000 kip-ft (27,000 kN-m) overturning moment with 600 kips (2,669 kN) axial and 160 kips (711 kN) shear, and geotechnical reports suggested rock at depths ranging from 80 ft to 120 ft (24 m to 37 m). The ambiguity in depth to rock and rock quality prior to installation necessitated designs be developed for both type A (rock) and type B (soil) micropiles. Crux employed a patent-pending field characterization method to adapt the pile design to actual geotechnical conditions at the time of installation.

Geological characterization was completed dur­ing the drilling of the first pile at each foundation location, effectively determining the pile type, quantity and depth for that structure. The ability to characterize materials and em­ploy predetermined solutions removed much of the risk as­sociated with limited geotechnical data. The project schedule would not have allowed for delays associated with unexpected changes. To reduce construction impacts to the wetland, a unique closed-cell cofferdam setup was employed at each site. These setups provided a stable platform for equipment in soft soils and groundwater, contained the drill cuttings and fluids from entering the wetland, and acted as a form for concrete place­ment during micropile cap construction. The cofferdams were constructed using helicopter-portable mini cranes and small excavators set onto local areas of crane matting.

The use of this equipment significantly reduced helicopter hours, posi­tively contributing toward schedule compliance and control of and overall project costs. A rotating drill carriage and micro­pile drill were set on the cofferdam and used to install micro­piles in an array of vertical and battered piles. Following micropile installation, the drill and drill car­riage were removed from the site to allow for tying of rebar form construction. A preconstructed anchor-bolt cage was flown to the site and supported by the cofferdam. Concrete was cast from crane-type concrete buckets. An accelerated high-strength concrete mix was chosen to ensure sufficient strength prior to form removal. Poles could be set less than one week following placement.

The use of micropile foundations and helicopter-portable tubular steel poles allowed the project team to meet an aggres­sive construction schedule within the Troy Meadows wetland. Foundation work was completed ahead of schedule, and all seven monopoles were erected by Kh&m and its helicopter provider in just three-and-a-half days. The innovative design and construction methods used in this project allowed for work to be completed with mini­mal impacts to the protected habitat. Accessing foundation sites by helicopter eliminated 108,000 sq ft (10,034 sq m).