When foundation installation options are limited by vibration and access constraints, Moretrench can draw on a wide range of specialty technologies, expertise and experience to develop a project-specific solution that meets the engineering and site requirements effectively, on time and within budget.
Such was the case for an extension to an operational laser eye surgical center. With the pile line just four to six inches off the existing building wall and given the proximity to sensitive surgical equipment, the solution needed to be vibration-free. Helical, standard micropiles and augercast piles were considered and were technically viable deep foundations for the 21-kip design load requirement and the installation restrictions. However, drilling the new piles in place would require penetrating existing footings and concern was raised that the existence of loose soils beneath these footings could cause undermining during pile construction.
With all of the project considerations in mind, Moretrench, in close consultation with architect Gordon & Associates, structural engineer Amore Engineering Inc. and geotechnical engineer Universal Engineering Sciences, developed an alternative micropile foundation system utilizing all-thread Geo-Drill hollow bars. With this technique, the pile is formed from the top down in one operation by the high-pressure injection of cementitious grout through the hollow bar, which also serves as the drill string. The grout exits the bar through ports at the end of the sacrificial drill bit. Continuous staged drilling and grouting causes the grout to permeate into the looser adjacent soils and into any voids in addition to forming a competent reinforced grout column.
Installation and Verification Monitoring
Prior to production work, a sacrificial test pile was installed adjacent to the work area and compression tested to 200% of the 21-kip design load. Test results showed a maximum settlement of 0.225 inch at 21 tons, with almost complete rebound.
Given the proximity of the pile locations to the existing wall, Moretrench used a modified rock drill rig fitted with a low-profile rotary motor and mast to install the production piles to a depth of 20 feet below ground surface. During pile installation, wet grout samples were retrieved and laboratory-tested to confirm the design 28-day grout strength of 5000 psi. Pressure gauge monitoring confirmed that the injection pressure required to construct the design column diameter of seven inches was maintained throughout drilling and grouting operations.
At the top of each completed pile, Moretrench installed a coupler and bearing plate assembly to effect connection between the piles and new footings. All 21 piles were successfully completed in three working days, allowing general contractor J. McLauchlin & Company to begin steel erection one week ahead of schedule.