WorldWide Drilling Resource

Get the Scoop on Micropiles Compiled by Bonnie Love, Editor, WorldWide Drilling Resource ® Whether you call them micropiles, minipiles, pin piles, needle piles, or root piles, this method of underpinning is often an affordable option for providing support in difficult ground conditions, such as karst, or in areas with restricted access. With micropiling, each pile consists of steel elements fused into the load-bearing soil or rock using a cement grout. The bearing layer is logged during drilling to make sure the capacity is acceptable. Micropiles can be used for a variety of projects from building large structures, to adding a basement to a home. Although it is a popular choice for underpinning existing buildings, it can also be used to strengthen bridges, retaining walls, towers, and other man-made structures, either as new foundations or remedial construc- tion. Typical micropiles usually measure between 6-12 inches in diameter, and can go as deep as 200 feet; however, they can be as small as 3-5 inches in diameter. This small size is one of the advantages of micropiles, as the equipment needed for installation is much smaller than the equipment needed for tra- ditional piling. Micropiles work by going through the weaker soil layers and transfering the building load to a stronger layer of soil or bedrock. For the most common type of micropiling, a hollow steel shaft is either driven or drilled into the ground. The casing is also advanced as the piles make their way into the bedrock. Once the drill rod is removed, the steel casing remains in place, a reinforcement load bar can then be lowered into the casing. At this point, cement grout is pressure fed into the micropile casing and rock socket to ensure bonding. In addition to using less concrete and steel than other pil- ing systems, micropiles also produce less noise and fewer vibrations. Depending on the type of project, there are a couple of different techniques for micropiling. Sectional Flight Auger (SFA) piling is generally used in areas with restricted access or low working height. Piles are formed using sectional lengths of hollow stem. SFA can be used in all unobstructed ground conditions including made-up ground and ground with high water tables. Augered Piling is used in cohesive soils where segmental augers are rotated into the ground and the soil being pene- trated is flighted to the surface by the augers. Where the ground is unstable, temporary casings can be installed in conjunction with the augers to progress the bore. When the desired pile length has been achieved, steel reinforcement is placed, and the pile bore is filled with either concrete or grout. This technique is particularly time efficient and cost effective where open boring is possible. Mini Bottom-Driven Piling is efficient and suitable for most ground conditions. A closed-ended thin-walled steel tube is driven in lengths of between about 6-20 feet using an internal drop hammer onto a dry concrete plug. The tubes are welded together as the installation proceeds until the pile is driven to a predetermined length. Once the required depth is reached, the tube is filled with concrete or grout and a single bar or cage is inserted. Micropiling has its advantages in many situations, especially those jobsites with confined spaces. It has also been used successfully for offshore wind farms. When owners wanted to add three new parking levels to their four-story parking garage, Keller North America used 240, 7-inch micropiles to support the additional building load. Photo courtesy of Keller North America. C&G More education opportunities during events can be found by clicking here online at: Education Connection Groundwater / Water Well by: Princeton Groundwater, Inc. Pollution & Hydrology Course May 17-21 ~ Tampa, FL phone: 813-964-0800 by: Texas Ground Water Association Continuing Education May 21 ~ Odessa, TX May 22 ~ San Angelo, TX May 29 ~ Schertz, TX phone: 512-472-7437 Irrigation by: Rain Bird Academy Training May 3-7 ~ Savannah, GA May 10-14 ~ Fairfax, VA May 11-13 ~ Bakersfield, CA May 17-21 ~ Denver, CO phone: 800-498-1942 7 APRIL 2021 WorldWide Drilling Resource ®