Micropiles are small diameter, bored cast-in-place piles, with most of the applied load being resisted by skin friction. They can also be designed to be end bearing piles. They are constructed by drilling a borehole, often using steel casing, then placing steel reinforcement and grouting the hole. Micropiles have a wide range of uses and are becoming a more mainstream method of supporting and reinforcing foundations, seismic retrofits, stabilization of slopes and even earth retention.
Micropiles are usually designed in small clusters or groups with each typically carrying an equal amount of load. These piles may also be designed with a batter to improve the lateral rigidity of the group. They can be designed to resist a combination of compression, tension and lateral forces.
Micropiles are an ideal pile for complex sites where low vibration or low noise levels are required, or where limited access such as low headroom and drilling is difficult. Other site conditions that make micropiles attractive are: obstructions, large cobbles or boulders, nearby sensitive structures, karst topography or high groundwater conditions. The unique characteristics of micropiles make them a perfect solution when other deep foundation methods are not suitable.
Key Benefits and Implementation of Micropiles
Micropile technology offers distinctive advantages that make it an ideal solution across numerous construction scenarios. The versatility and efficiency of micropiles provide contractors, engineers, and project owners with a foundation system that performs exceptionally well where conventional deep foundation methods face limitations.
The minimal vibration and noise characteristics of micropiles are particularly valuable in urban environments, allowing work to continue during normal business hours without disrupting nearby operations or residents. This makes micropiles an excellent solution for projects in densely populated areas adjacent to or inside sensitive facilities like hospitals and data centers.
Advantages:
- Creates a pile with a relatively high axial load capacity
- Works in compression and tension
- Provides lateral load resistance
- Smaller drilling rigs can be used, reducing mobilization costs
- Easily installed within confined spaces
- No harmful vibrations or noise to surrounding structures
- Minimal spoil generation on contaminated sites
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Applications:
- Structural Support of Directly Loaded Piles
- New Foundations
- Underpinning of Existing Structures
- Seismic Retrofitting
- Reinforcement for Slope Stabilization and Settlement Reduction
- Support of Excavation - Soldier Piles
- Permanent Retaining Walls
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Hollow Bars: Micropiles
Micropiles produced with hollow bars have been well proven and are widely accepted in private and public works in the United States.
In collapsable soils, the use of hollow bars to produce micropiles is ideal. Where open hole drilling may be limited in depth or temporary casing is required, hollow bars offer a single-step process of grouting and reinforcing. In addition, higher values of ground to grout bond have resulted from the use of this method. The smaller, and often less expensive equipment used with the hollow bar system, allows for lower project costs and makes it easier for less specialized general contractors to use. This system also excels in sites with limited access or low headroom.
Advantages:
- Works in compression and tension
- Potential for very high production rates
- Lightweight rotary percussive equipment can be used
- The most efficient form of micropile in confined spaces
- Hollow bars form smaller pile groups
- Grouting while drilling provides a potential for densification of in-situ soils
Threaded Micropile Systems
- Small diameter pile, usually less than 14” Ø (some exceptions), smaller than a drilled shaft – also known as minipiles or pin piles and can consist of casing, grout and threaded bar reinforcement
- Flush joint threaded steel casing is most often manufactured to ASTM A252 Gr 3 or ASTM A500C standards, both modified to 80 KSI minimum yield
- High-strength Grade 80 thread bar systems are available in various diameters, providing optimal tensile capacity and allowing for precise field adjustments in projects with variable soil conditions
- Typically installed with a small-bore drilling rig using a duplex drilling method with API style drill rod and drill bits or a down-hole hammer. Connects via a duplex flange adaptor and crossover/saver sub to attach the “starter” piece with welded carbide J-teeth or a Ring Bit, on the bottom, with adding Male x Female (Pin x Box) extension pieces, as needed
- Depending on the type of Micropile constructed, the capacity is developed through skin friction (the bond between grout and soil), bearing conditions and the strength of the casing/bar
- Steel casing may extend the full depth of the pile or end at the top of the bond zone
- Increase capacities through grout methods (pressure vs gravity), size, and strength of casing/bar
- Corrosion Protection
- Bare/Coated Bar Compression Piles
- Grout Thickness & High pH of Grout help passivate steel
- Encapsulated PTI Class I Tension Piles
- Corrugated Sheath provides a water/moisture barrier
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Deep Foundation Solutions with Micropiles
Micropiles have become a premier solution for deep foundation projects requiring high load-bearing capacity with minimal site disturbance. Their versatility in confined spaces and ability to perform across challenging subsurface conditions make them the foundation element of choice for projects with restricted access or complex geotechnical challenges.
Micropile solutions should be engineered precisely to your project's specific load and soil conditions. They deliver reliable performance and long-term structural stability whether used as primary foundation support or for reinforcing existing foundations.
Superior Load-Bearing Capacity
Despite their compact diameter (typically 5 ½ to 14 inches), micropiles deliver exceptional load-bearing performance. By transferring structural loads deep into competent soil or rock strata, these foundation elements effectively support substantial structures even in areas with weak or variable soil conditions.
Key benefits in load-bearing applications include:
- High axial and lateral load resistance
- Performance in both tension and compression applications
- Adaptability across variable soil conditions
Micropiles for Foundation Repair
Micropiles excel in foundation repair applications, providing stability to structures suffering from settlement or structural damage. Engineers can redistribute loads and restore structural integrity by installing micropiles beneath existing foundations, all without the extensive excavation traditional methods require.
Common repair applications include:
- Stabilizing structures experiencing settlement
- Repairing bridge foundations and infrastructure
- Reinforcing historical structures while preserving architectural integrity
Enhancing Soil Stabilization
A significant advantage of micropiles is their soil stabilization capability. They improve ground conditions by transferring loads from unstable soil layers to more competent strata deeper in the ground profile.
Micropiles deliver exceptional results in:
- Landslide prevention strategies
- Slope and embankment stabilization
- Reinforcement around retaining structures and excavation sites
Retrofitting Existing Structures with Micropiles
When existing structures require additional support due to increased loads, seismic requirements, or foundation issues, micropiles provide an efficient, cost-effective solution. Their low-vibration installation process makes them ideal for urban environments and projects involving sensitive structures.
Key retrofitting applications include:
- Strengthening bridge foundations and roadway supports
- Reinforcing aging infrastructure elements
- Supporting building expansions and structural renovations
FAQs
What are micropiles used for?
Micropiles provide structural support and soil stabilization in challenging ground conditions where traditional foundation systems prove impractical. Primary applications include foundation repair, retrofitting existing structures, slope stabilization, and supporting substantial loads in difficult site conditions.
How deep can micropiles go?
Micropile depths typically range from 20 to over 200 feet, depending on soil conditions, structural requirements, and load-bearing specifications. Their narrow diameter enables penetration through difficult ground formations and secure into bedrock when necessary.
How much weight can a micropile support?
Load-bearing capacity varies based on micropile design specifications, soil conditions, and installation methods. Micropiles commonly support loads ranging from 50 to over 1,000 kips (kilopounds), demonstrating their adaptability across various structural requirements.
What is the difference between micropiles and helical piles?
While both systems provide foundation support, their design and installation differ significantly. Micropiles utilize steel casing and reinforcement bar, installed through drilling and grouting methods, making them suitable for high-load applications and challenging ground conditions, including cobbles, boulders, and socketing into bedrock. Helical piles feature helical plates and are screwed into the ground, best suited for lighter loads and softer soil environments.