What Is a Micropile?
A micropile is a small-diameter pile that is drilled, reinforced with steel and grouted, and carries load through the bond between grout and ground.
The FHWA reference manual defines it as a small-diameter (typically less than 300 mm) drilled and grouted non-displacement pile that is typically reinforced. A crew drills a borehole, places the steel reinforcement and grouts the hole. The rig is the type that installs ground anchors, and it can drill at any angle below the horizontal.
How a micropile carries load
Steel elements resist most or all of the load. They can occupy as much as half of the drillhole cross section. The grout transfers the load by friction from the steel to the ground in the bond zone, as a ground anchor does. The pile is thin, so its end bearing is generally neglected. The grout type and the ground type mainly set the bond strength.
Grouting types
FHWA classes micropiles by how the crew places the grout.
- Type A. Grout goes in under gravity head only.
- Type B. Neat cement grout goes in under pressure while the crew withdraws the temporary casing. Pressures are typically 0.5 to 1 MPa.
- Type C. Grout goes in under gravity, then once more through a sleeved pipe at 1 MPa or more. FHWA notes that this type appears to be used only in France.
- Type D. Grout goes in as for Type A or B. After it hardens, more grout goes in through a sleeved pipe at 2 to 8 MPa, if needed several times at the same level.
Uses
FHWA names two application areas:
- Structural support: new foundations, underpinning, seismic retrofit and support for earth retaining structures.
- In-situ reinforcement: slope stabilization, earth retention, ground strengthening and settlement reduction.
A micropile can suit a site with restricted access, a need to work close to existing structures, difficult ground, or limits on vibration and noise.
Load testing
Test methods follow the standards for conventional piles: ASTM D1143 for compression, ASTM D3689 for tension and ASTM D3966 for lateral load. The maximum verification test load is usually 2.0 to 2.5 times the design axial load.
Frequently Asked Questions
- How big is a micropile?
- The FHWA reference manual describes a micropile as typically less than 300 mm (12 in.) in diameter. It gives this as a typical size, not as a hard limit.
- Is a micropile a driven pile?
- No. A micropile is a drilled, non-displacement pile. The manual says the installation causes minimal vibration and noise, so micropiles are often used to underpin existing structures.
- How is a micropile load tested?
- With static load tests. Verification tests on pre-production piles check the design assumptions for the bond zone and the installation method. Proof tests on selected production piles check that the pile carries its design load.
- What is a root pile?
- A root pile (palo radice) is the Italian forerunner of the micropile. It is a small-diameter, drilled, cast-in-place, lightly reinforced, grouted pile that an Italian contractor developed in the early 1950s to underpin historic buildings.
References
- fhwa-nhi-05-039
- Micropile Design and Construction (Reference Manual for NHI Course 132078), FHWA NHI-05-039 U.S. Federal Highway Administration
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