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Micropiles (Minipiles)

Micropiles, also known as minipiles, (and less commonly as pin piles, needle piles and root piles) are deep foundation elements constructed using high-strength, small-diameter steel casing and/or threaded bar. Capacities vary depending on the micropile size and subsurface profile. Allowable micropile capacities in excess of 1,000 tons have been achieved.

The micropile casing generally has a diameter in the range of 3 to 10 inches. Typically, the casing is advanced to the design depth using a drilling technique. Reinforcing steel in the form of an all-thread bar is typically inserted into the micropile casing. High-strength cement grout is then pumped into the casing. The casing may extend to the full depth or terminate above the bond zone with the reinforcing bar extending to the full depth. The finished micropile (minipile) resists compressive, uplift/tension and lateral loads and is typically load tested in accordance with ASTM D 1143 (compressive), ASTM D 3689 (uplift/tension), and ASTM D 3966 (lateral). The technique has been used to support most types of structures. Hayward Baker’s micropile drill rigs allow installation in restricted access, low headroom interiors, permitting facility upgrades with minimal disruption to normal operations. (Click on “view all images” at the left to see typical applications).

Hayward Baker has the capability of combining our micropile technology with one or more of our other ground modification techniques to meet unique or complex project requirements cost-effectively and efficiently. Lines of micropiles spanned by wooden lagging can be ideal where excavation walls are required in low headroom and other confined areas. Post-grouting within the bond length can increase frictional forces with surrounding soils, thus achieving greater capacity. Micropiles can serve to “stitch” the soil together, within predicted shear zones to enhance mass stability. In liquefiable profiles micropiles can transfer loads to competent bearing strata to conform to seismic design requirements. Underpinning of foundations adjacent to planned excavations is another micropile application.

For planned foundations in areas with multiple underground utilities, the cost of a cast-in-place piling system may often be substantially increased by the expense of utility re-routing, creation of adequate access, and sometimes even a shutdown of facility operations. With Hayward Baker’s micropiles (minipiles), these complications are rarely an issue. Small diameter micropiles can be installed while avoiding existing utilities. Hayward Baker works closely with engineers, contractors, and owners to help to ensure that micropile installation minimizes impact on facility operations. Moreover, micropiles greatly alleviate the quality assurance concerns associated with cast-in-place piling in weak soils.

Contact your local Hayward Baker office for more information.

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