High-capacity pressure-injected steel micropiles are small diameter piles with diameters ranging from 2-1⁄2” to under 12″, with a typical or common range of 3-1⁄2” to 6″ used in Kansas City foundation repair.
Since the installation procedure causes minimal vibration and noise and can be performed in conditions of low headroom, micropiles are often used to underpin existing structures. Specialized drilling equipment is often required to install the micropiles from within existing basement facilities. Thrasher Foundation Repair specializes in such applications and has specialty equipment specifically designed for confined space, limited access, or low headroom applications, along with multiple specialized engineered brackets to transfer loads from the structures to the pile.
Pressure-grouted micropiles involve a one-step process. The bore hole is drilled using a sacrificial drill bit connected to a high-strength hollow bar drill stem, drilling until the desired depth is achieved. Next, a cementitious grout is injected under pressure through the hollow bar and out the drill bit through a series of holes in the drill bit. The penetration through varying and mixed soil strata (such as clay, sand, rock, etc.) is made possible by a combination of the drill bits, percussion of the drill bit from the drill motor, and displacement of the soil or rock via water, grout, or air injection.
Because the diameters of the piles are generally small, the end value is generally ignored. Capacities are derived through the high-strength hollow bar that is left in place and reinforces the surrounding grout body, and from the high grout-to-ground bond values (friction) along grout-to-ground interface from the pile length. The grout transfers the load through friction from the reinforcement to the ground in the micropile bond zone in a manner similar to that of ground anchors. The high-pressure grout injection (300-1,200 psi) displaces soil along the pile shaft length, often creating a series of irregular bulges and irregular hole diameters, thereby significantly increasing the surface or bond area of the grout-to-ground interface.