By: Mikayla Sockwell, Word of Rock Drills & Sean Hibbits, Intech Anchoring Systems

A key component of growth, hands-on learning is a proven method to help ingrain the principles discussed in the classroom for real-life applications. However, as useful and necessary as it is, sometimes a controlled “school yard” education setting doesn’t match the pace and scope of a real world project. Painting a true picture of the extremes and nuance that can make a particular system viable and efficient can be nearly impossible in these controlled settings. This deeper understanding is something typically gained through experience alone, but knowledge of the tools and resources available can be invaluable in overcoming a difficult installation scenario. Back in early June, Intech Anchoring, WORD Rock Drills, and Chemgrout hosted a Hollow Bar School for several dozen individuals from varying specialty foundation contractors around the country. With the combined goal to educate potential customers on the equipment, tools, and additional resources available for future jobs, this school included classroom learning and hands-on training to establish the basic principles behind Hollow Bar Micropiles. The school began with a classroom experience which included presentations on the technical principles and installation techniques surrounding Hollow Bar Micropiles, after which the class went out into the field. “In-the-field” attendees performed hands-on installation of different types and sizes of Hollow Bar using a WORD excavator-attached rock drill and Chemgrout grout plant. This included going over the different equipment components in depth, and direct instruction on how to efficiently operate and handle the equipment.

One of the installation contractors in attendance came to the event to put a new tool in their tool belt—they had previous experience with other forms of deep foundation installation, but hoped to see an advantage in adding hollow bar. They were excited to see how simple and fast the installation of a Hollow Bar Element went, and how it could save a lot of man-hours compared to a traditional solid bar micropile when open hole drilling isn’t an option. Everything at the event went smoothly, and the steel was in the ground before everyone knew it.

Fast forward to July—only a few weeks after the Hollow Bar School—this contractor had their first Hollow Bar Micropile job in their project queue. They were contracted to install several large-diameter (76mm) hollow bar elements, including relatively large diameter grout columns (130mm), with a depth requirement of 15 feet of pile socketed into rock. This project also included a separate nonproduction load test element to verify the system, as installed. The project aimed to support a multi-leg communication tower, providing an increased load resistance in tension to withstand increased loads due to a renewed design and risk evaluation per IBC 2018. This resistance was accomplished via the design of a number of retrofit elements (one per leg) penetrating into the rock for engagement, with project-specific custom bracketry for connection the tower legs. The re-design resulted in relatively high additional net loading per leg (more than 150 kips service load in tension), and for that they required the thick section of the T76/45 hollow bar reinforcing—a nearly 3-inch wide hollow tube with 0.5-inch wall thickness—to transfer the loads from the legs down into competent rock. The area near the tower had existing outbuildings and structures that limited access, in addition to the overall site being relatively inaccessible with conventional equipment.

Faced with this application, the industry tends to recommend the robust all-in one machines most well-known for tackling large materials, such as the T76 bar used in this project. Drilling attachments, including the WORD excavator attachment used in this scenario, have not typically been considered when assessing equipment for this type of work because of the power needed to handle this bar and tackle a larger rock socket. However, this project presented the additional challenge of restricted access meaning that these larger all-in one machines were not an option. Where all-in-one units have the advantage in certain applications, attachment drills provide versatility among an equipment fleet because the customer’s excavator can serve a dual purpose: providing additional savings in equipment cost, through rental or purchase, and sourcing time. Additionally, these attachment units have the capacity to be operated by one crew member in the cab or by a having a second crew member on the ground. The ability to have a crew member at the hole operating the drill was vital to the success of this particular job. Understanding the capabilities and advantages of the excavator attachment on hand, the Intech and WORD team were confident that they could make a way for their installation contractor to be successful. The soil overburden was quite problematic from a geotechnical point of view. The residual soils were relatively variable in thickness and had some pockets of soil having potential for hole collapse with open-hole drilling. There was also a potential for lenses of shelf rock that needed to be penetrated. Based on the boring logs, the depth to competent bedrock was moderately shallow (< 30 feet), but some areas had a somewhat thicker overburden profile before getting to rock. The project site was relatively remote, with relatively encumbered adjacent site conditions—essentially a wooded hill-top that was leveled for construction of the tower and existing outbuildings and structure pads. The soil conditions at the ground surface were also very wet and soft, further restricting access to the drilling location. These conditions played a role in the equipment selected. The equipment selected for the job was a 20-ton excavator mounted with a WORD EXRH10 drilling attachment, including a steel mast system specifically designed to resist flexure during heavy drilling. This compact setup was able to nimbly maneuver and position the drilling equipment over the pile locations and around the existing site structures, and to limit the total amount of disturbance of the soft ground surface. With the excavator hydraulics attached to the drill and feed system, the contractor penetrated the residual soils with ease, hammering through thin lenses of gravels and rock with the top hammer. Once bedrock was encountered, a polymer additive was added to the drilling fluid as a coolant, and the drill hammered the bit into the rock—pulverizing the bedrock and creating a socket. Once the 15-foot rock socket was achieved, the structural grout was pumped through the system to displace the drilling fluid. After a few days of curing, the system was tested and deflected less than 0.25 inch under design load.

Equipped with the larger RH10x drilling head by Eurodrill, the heavier capacity of this head gave the machine the ability to handle the T76 bar to the required rock socket depth With the site located inside an electrical compound soil conditions socket depth. With the site located inside an electrical compound, soil conditions prevented the use of other types of drilling machines considered such as the all-in one units. This unique combination of requirements and access issues inspired the Intech team towards an innovative solution not commonly seen in our industry. The use of an excavator-attached unit provided the necessary maneuverability and capacity while remaining light enough to not excessively rut up the soils. The great success of this job type with the use of an attachment drilling unit has pushed the limits of drilling attachments beyond what has traditionally been accomplished, leaving the possibilities open for more future success using attachments in the future.