A micropile is a high capacity small diameter pile that can be installed through difficult soil conditions and in limited access. Micropiles are typically installed by advancing casing pipe to rock, drilling a rock socket, then grouting reinforcing steel in the rock socket.
In spite of their small diameter, less than 12″, micropiles capacities typically range from 100 kips to 400 kips.
Is my project a good fit for micropiles?
Advantages of Micropiles
Micropiles are constructed by advancing casing through the overburden soil to the top of rock. The casing is threaded together in sections as the piles are advanced. Once the top or rock is reached, a hole is drilled in rock with a bit that fits inside the casing. The rock socket is an open hole in rock that is typically 5″ to 8″ diameter depending on the casing size and pile capacity needed. Rock sockets are 5′ to 20′ or more in length depending on the quality of the rock and the pile capacity. Once the casing has been advanced to rock, the inner drill rod, down hole hammer, and bit are extracted leaving the cased hole and open rock socket. Steel reinforcement is then lowered through the casing and into the rock socket. The hole is then tremie grouted displacing any ground water. The grout bonds the reinforcing steel to the rock socket, provides corrosion protection, and assists the steel in carrying compression load.
Learn more about QC of Micropiles
The majority of the compression load is carried by steel with a contribution from the grout. Micropile design is governed by two main bodies, the FHWA and International Building codes. The structural capacity is determined by combining the allowable contribution of the steel and grout.
FHWA NHI-05-039 December 2005
0.47*Asteel*Fy-steel + 0.40*Agrout*fc-grout = Compression Capacity
0.40*Asteel*Fy-steel + 0.30*Agrout*fc-grout = Compression Capacity
Note that Fy-steel is limited to a maximum of 87 ksi.
FHWA Micropile Manual
IBC 2015 Chapter 18 Deep Foundations
Guide to Drafting a Specification for Micropiles
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