High-capacity pressure-injected steel micro piles 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″. Specific expected capacities can exceed 200 tons.
Since the installation procedure causes minimal vibration and noise and can be performed in low headroom conditions, micro piles are often used to underpin existing structures. Specialized drilling equipment is usually required to install the micro piles 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 micro piles involve a one-step process. The borehole 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, the 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 usually 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 with the 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 like 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 the grout-to-ground interface.
Micro piles are most commonly used in two general application areas: (1) deep foundation application for structural support of new construction builds (2) retrofit or remedial foundation repair/support. Structural support includes new foundations, underpinning existing foundations, seismic retrofitting applications and foundation support for earth retaining structures. Micro piles are also used for slope stabilization and earth retention, ground strengthening; settlement reduction; and structural stability.
Micro piles have specific advantages compared to other support systems. In general, micro piles are feasible under the following project-specific constraints:
• project has restricted access or is located in a remote area;
• required support system needs to be in close pile proximity to existing structures;
• ground and drilling conditions are difficult due to karstic areas, uncontrolled fills, boulders, debris);
• vibration or noise needs to be minimized;
• hazardous or contaminated spoil material will be generated during construction, and
• adaptation of support system/underpinning bracket to the existing structure is required.
The drilling and grouting equipment used for micropile installation is relatively small and can be mobilized in restrictive areas conventional pile installation equipment would be prohibited. Micro piles can be installed near existing walls or foundations. Micropile installation is typically not as affected by overhead power lines or other obstructions as are conventional pile installation systems. The equipment can be mobilized up steep slopes and in remote locations. Also, drilling and grouting procedures associated with micropile installations do not cause damage adjacent existing structures or affect adjacent ground conditions when proper drilling and grouting procedures are utilized.
Micropiles can be installed in challenging, variable, or unpredictable geologic conditions such as ground with cobbles and boulders fills with buried utilities and miscellaneous debris, and irregular layers of incompetent or weak materials. Soft clays, running sands, and high groundwater that are not conducive to conventional drilled shaft systems cause minimal impacts on micropile installations. Micro piles are commonly used in Karstic limestone formations.
Micro piles can be installed when hazardous or contaminated soils will be encountered. Because of the small diameter, fewer spoils are produced during drilling than would be produced by conventionally drilled piles. Also, the flush grout/water can be controlled easily at the ground surface through containerization or lined surface pits. These factors considerably reduce the potential for surface contamination and handling/disposal costs. Micro piles can be installed in environmentally sensitive areas, including areas with fragile natural settings. The installation equipment is not as large or as heavy as conventional pile driving or shaft drilling equipment. It can be used in swampy areas or other wet or soft surface soils with minimal impacts on the environment. Portable drilling equipment is frequently used in areas of restricted access and low overhead obstructions.
Micro piles have proven to be a great solution in the deep foundation world. Our experienced and professional design, engineering, and installation teams at Thrasher can provide your next micropile application.