Micropiles are a type of small diameter drilled pile typically used in situations requiring relatively high axial loads and less than favorable access.
Micropiles are extremely useful in limited access spaces, such as: inside buildings, plants, and factories, for slopes too steep for standard foundation rigs, and in areas with limited headroom. They are also useful when building on bedrock. Micropiles can be used to underpin existing structures.
How it's done
Chris-Hill's preferred micropile type is a hollow bar micropile. These consist of high strength, continuously threaded hollow steel bars, neat cement grout, and occasionally top casings for resistance to shear. The steel hollow-bar is first advanced into the earth using specialty drill rigs (hydraulic drifters). Drilling spoils (cuttings) are evacuated upward thru the annular space created by the larger drill bit and by the pressurization of either air, water, or weak "drilling grout" through the hollow bar and out of the holes in the drill bit at the bottom of the pile. As the micropile is advanced, 10 foot long hollow bars are coupled together to create a drill string. When the desired depth is achieved, 4000 psi enriched neat cement grout is pumped under high pressure to remove all remaining cuttings and fill the annular space. The drill stem is left in the micropile as reinforcing for the pile. The threads on the hollow bar allow for customizable and convenient connections to existing or future structures.
Micropiles are the preferred foundation choice in many situations. They eliminate issues with access and can be installed in any type of soil/rock with limited vibration. Pile capacity is dependent on skin friction of the grout with the soil, meaning you do not have to achieve "end bearing" i.e. have the bottom of the pile resting on ultra hard soil or rock. Since a hollow bar micropile incorporates the drilling bar into the pile, installation is a one way trip. Installation of cased micropiles typically calls for multiple trips down the hole. Choosing hollow bar micropiles lowers cost and improves schedule, and still results in piling that can achieve up to 200 kips.
Chris-Hill's crews have over 20 years of experience in the successful installation of hollow bar and other micropile types. We have an extensive reference base of owners and general contractors from past projects across the midsouth.