Micropiles, also called minipiles, are small-diameter piles that are constructed by drilling a hole, typically 2" to 8" in diameter, deep into the soil, inserting a high-strength threaded bar or pipe, and filling the hole with high strength grout.
Primarily, micropiles are designed around the skin friction developed between the grout/soil interface, but can also incorporate some end-bearing capacity when drilled into competent strata or socketed into rock. Soil conditions and capacity requirements dictate hole size and depth as well as reinforcing type.
Applications for Micropiles
Micropiles are suitable for a range of conditions throughout the southeast, including:
- Deep foundations in difficult soil conditions
- Temporary or permanent structural underpinning
- Supporting heavy loads in congested areas
- Mitigating foundation settlement
- Excavation support in confined spaces
Drilling and Installing Micropiles
Many times, especially in clay soils, an “open hole” drilling method can be used. This simply means that the soil is stiff enough to remain open during the entire operation of drilling, bar placement and grouting. Grouting is always performed from the bottom up using a tremie tube.
It is sometimes necessary to install temporary or permanent casing through the soft overburden to keep the soils from collapsing during pile installation. This drastically slows down the installation process and increases the cost. Alternatively, Foundation Repair Services can install “hollow bar” injection anchors when dealing with soft collapsing soils.
These piles are constructed by injecting grout through a hollow steel bar and bit during the drilling process. The grout serves to stabilize the hole as well as flushing the cuttings out during drilling. The hollow bar and bit serve as the drill string during installation, and they serve as the permanent reinforcing in the pile after completion.
This drilling method has several advantages: speed of installation and the ability to achieve high capacities with relatively small piles.