Definitions: What is a Micropile?

You may or may not be a stickler for definitions but I think that if you’re going to get into an in depth discussion about micropiles (as this site will do), it is important to pin down exactly what a micropile is (or is not). With the rapid rise of the micropile market in the 1990’s, different foundation products were jumping onto the micropile bandwagon while not really fitting the industry definition of what a micropile is.

Let’s take a look at the definitions put forth for micropiles from some of the foundational guides and specifications written on the subject. When you examine these, they share a number of similarities and have been modified and reused over time. I have ordered these publications from earliest to most recent.

FHWA-RD-96-016 (Drilled and Grouted Micropiles: State of Practice Review – Volume 1)
“A small diameter (less than 300mm [less than 250mm in france]), replacement, drilled pile composed of placed or injected grout, and having some form of steel reinforcement to resist a high proportion of the design load.”

FHWA-SA-97-070 (Micropile Design and Construction Guidelines – Implementation Manual)
“A micropile is a small-diameter (typically less than 300mm), drilled and grouted replacement pile that is typically reinforced.”

Deep Foundations Institute (DFI) Guide Specification
“A small diameter, bored, cast-in-place pile, in which most of the applied load is resisted by the steel reinforcement.”

FHWA-NHI-05-039 NHI Course No. 132078 (Micropile Design and Construction – Reference Manual)
“A micropile is a small-diameter (typically less than 300mm (12 inches)), drilled and grouted non-displacement pile that is typically reinforced.”

AASHTO LRFD Design Specification
“A small-diameter (typically less than 300mm (12 in.)), drilled and grouted non-displacement pile that is typically reinforced.”

AASHTO LRFD Construction Specification (latest draft)
“The term “micropiles” as used in this section means a small (4″ to 12″) diameter bored, cast-in-place composite pile, in which the applied loads are resisted by steel reinforcement, cement grout and frictional grout-to-ground bond.”

2006 International Building Code
“Micropiles are 12-inch (305mm) or less bored, grouted-in-place piles incorporating steel pipe (casing) and/or steel reinforcement.”

Taking this collection of definitions as a whole and combining them with what is typical in my experience, a better way of defining Micropiles might be to say that micropiles are piles which have the following characteristics:

  • Small diameter (<12″)
  • Drilled and grouted
  • Replacement (non-displacement) pile
  • Incorporating steel pipe casing and/or central steel core reinforcement
  • Typically reinforced; majority of load resisted by steel casing and/or central steel core reinforcement

The term “replacement” pile means that the pile is installed by removal and replacement of material as opposed to displacement of existing material. A “displacement” pile is one that installed by displacing existing material. A driven pile is an example of a “displacement” pile. In a sense, saying that a micropile is a drilled pile and also saying it is a “replacement” pile is redundant because all drilled piles would be replacement piles.