Thor packs a mighty punch

Anderson Drilling’s management, including Sr. Project Manager Mike Kennedy, Director of Business Development Dennis Poland, San Diego area General Superintendent Charlee Bixby and Vice President of Operations Kelly Hawes, got together with Atlas Copco Vista Store personnel Tom Liebl, Area Sales Representative, Service Manager Chris Woods, Ken McClanahan, geotechnical Sales Specialist, and DTH Center Business Development Manager Al Waltry. From the point of discovery through the bid process, the team looked at the project knowing if the job was successful both would benefit.

Anderson Drilling’s scope of work included the installation of 37 foundation piles to support two new bridge structures over the San Luis Rey River and Ostrich Creek near San Diego, California. The drilled piles varied in size from 60-inch to 108-inch in diameter and ranged from 55 – 80 feet in depth.   Each pile was designed with rock sockets that ranged from 48-inch to 96-inch in diameter up to 20 feet in length. The granitic rock of southern California is known for high strength and with compressive strengths nearing 25,000 psi the rock on this project was no exception. The critical issue facing Anderson Drilling was reducing the drill time within the rock socket zones.   The construction process was complicated by loose overburden materials and high groundwater conditions above the hard rock that required the installation of permanent steel casing and use of “wet-hole” construction methods (blind drilling with polymer drill fluids) for pile installation.

Atlas Copco and Anderson Drilling worked together to obtain a scalable solution that included multiple cluster drills to chew through the hard rock found in the rock socket zones. As the team contemplated the optimal size for the cluster hammer we considered several things like, the frequency of a particular rock socket diameter, tool weight, tool support equipment requirements, and especially consideration of our existing rock tool capabilities that included a wide range of double wall air or water calculating coring tools and other specialized tooling like mini carbide roller cutters and disc type cutters. Ultimately, we determined that a 48-inch tool was the optimal size allowing us to excavate all of the abutment shaft (21 total) rock sockets to their final design diameter, as well as, provide a sizable ‘pilot hole’ at each of the larger 72-inch to 96-inch rock sockets required at the bent pile locations. The pilot holes would be reamed out (enlarged) to the ultimate design diameter utilizing our repertoire of in-house rock tools. Dating back to the original founders of Anderson Drilling nearly everything in Anderson’s fleet is christened with a name.  Through the set up process in our MFG plant, plant manager, Rick Watson and his team of dedicated employees came up with a name for the 48-inch cluster drill…. It was affectionately called ‘Thor’ after the Norse hammer-wielding god of thunder.

“A number of things came together to make this the perfect situation for us to utilize this cluster drill,” Sr. Project Manager Mike Kennedy points out some of the key detail that were taken into consideration for the project. “The project is located just 10 minutes away from Atlas Copco’s Vista store. This would mean nearly immediate support in the event it was needed. To try something new requires purchasing equipment, which is hard to justify in a tough economic environment. But most importantly a new process would require a steep learning curve until the crew was efficient at the new drilling method,” explained Kennedy.

Poland agreed, “[In the past] we would always run a cost analysis between using existing tools and getting a cluster drill, and there had not been a big advantage to purchase a complete set up until this project.” As mentioned earlier, the driving factors for deciding on the 48-inch drill was the number of holes on this project that required a final diameter of 48” in the 25,000 psi of the rock, as well as, consideration of working under water and removal of full diameter rock cores, time consuming endeavors that tipped the scale in favor of trying something new. 

Speaking for the Anderson team, Kennedy said, “After five years of on and off renting of this type of tooling we started talking about purchasing a cluster drill.  It was the working relationship we had developed over the years along with communication and trust that gave us the confidence to go with Atlas Copco. We have watched others try this with [competitive] drills, but we wanted to go with someone we could trust,” said Kennedy.

The project can be described in two parts. The first includes 20 bore holes for the 1,000 foot bridge over the San Luis Rey River requiring four bridge abutment holes and 16 bent holes that run parallel, supporting the length of the bridge. The second part includes a total of 17 abutments shafts that support the bridge spanning over the 50-foot wide Ostrich Creek. The subsurface is comprised of sand and unconsolidated rock overburden in the river beds to approximately 60 feet in depth, with a weathered rock transition into solid granite rock. To meet the earthquake regulations for the state of California the piles have been engineered to provide bridge foundation support capable of withstanding an 8.0 magnitude earthquake event. “Really it’s not the depth of the hole that engineers are looking for; the driving factor is the skin friction of the caisson in competent ground. The support value and shear strength comes from the rock socket at the bottom of the caisson,” said Kennedy.

To reach the desired depth and diameter the hole needs to be completed in a series of steps. The cluster drill isn’t utilized until solid granite bedrock is reached. Bedrock contact is achieved by installing a series of casings through the overburden soils   Each new casing installed is slightly smaller in diameter and longer than the previous casing providing a “telescoping” effect. The overburden material within the each casing is drilled out using an auger tool under a polymer fluid head used to control bottom heave. (see illustration).

The final piece of casing installed was a 51-inch (ID) segmental pipe.  This pipe is “seated” into the bedrock and provides an airtight annulus for the cluster drill. The segmental pipe also assisted in maintaining the drilled shaft alignment.

In the hole photographed, the segmental pipe is 63 feet long extending nearly 60 feet below ground surface. The rock socket will be nearly 20 feet deeper into the bedrock. The cluster drill is being used to drill a pilot hole for our core barrel tool that will follow resulting in a final rock socket diameter of 96-inch.

Anderson Drilling has been drilling foundations for years with large diameter core barrels and other rock tools. The process is slow and laborious, but effective. As the large core barrel with cutter teeth rotates, it forms a “curf” or groove leaving a rock core in the center. To cut this core could take days depending on the depth, quality and hardness of the rock. After the initial coring effort, the process of removing and disposing of the core is another major task that requires time, special resources and skills. Fractured or broken rock cores may be augered out however, large solid cores may require heavy crane lifting capabilities.

The cluster drill approaches the rock in a different way, essentially pulverizing the rock mass into sand and gravel sized pieces making cutting removals relatively easy and efficient. During drilling the cuttings rise above the drill tool and settle in what is called a Calyx basket. The driller monitors the tool advancement by markings on the drill string. In this case, when the tool advanced approximately 4 feet it was time to retrieve the tool to dump the cuttings. A forklift at ground surface assisted raising the basket allowing the cuttings to fall away from the tool.

There is something to be said about the right combination of “parts” to make for an efficient operation. Anderson Drilling paired the cluster drill with one of their newest heavy drill units, a SoilMec SR 100 (a 100 Ton Class machine) and an experienced operator in Ronnie Nourse.   Ronnie has been drilling for Anderson for 15 years and is a top operator of the SR 100. “This is Anderson’s baby,” he said, referring to the drill rig with pride. He says its standard depth capacity is 134 feet, with the ability to drill to 205 with an optional feed system. As for drilling with Atlas Copco’s cluster drill Nourse said, “This is one bad-ass 4x4!”

He likes the fact it drills fast, and when he’s finished with the 4-foot lift, the rock goes with it. It takes 45 to 50 minutes to drill the 4-foot depth and another 20 minutes to remove the cuttings and get back in the hole. Using a core barrel, Nourse said, “It would take much longer and, then the core has to be removed and hopefully it breaks off clean, not always so easy when you’re underwater.”

Once the outer casings and segmental guide casing is in place, Nourse has a special cutter that gives the bottom of the hole a flat surface. Then once the hole is drilled with the cluster drill, the segmental guide casing is removed and the ream process begins to the desired 72-inch or 96-inch diameter. To complete a hole will take approximately a week and a half.

Drilling with the cluster drill makes the job work very smooth. When the cluster drill was brought on site Atlas Copco’s Chris Woods stayed with the project for the first few days. Nourse said, “Chris assisted us on site to make sure we had it right correct down pressure rotation speed etc. Once we got the sweet spot for rotation, it has gone great.” As an example, when they had it set at 2 RPM the hammers were regrinding the rock. Nourse found that 4 RPM with the 150 psi air worked well. The 50,000 pound weight of the drill stem and Kelly bar is all the pressure needed on the bit.

Anderson is using three Atlas Copco XAS 1600 CD6 compressors on the job. Each of the seven Atlas Copco Secoroc CDS 8-inch hammers, with the 10-inch bit, requires 527 cfm. Together the units put out the required 4800 cfm for the job.

Oiler John Baker is responsible for the ground work on the job. Quite visibly the hardest working guy on the job, Baker never rests. When he is not monitoring the drilling fluid pumping to maintain the proper consistency in the hole, he is watching the hammer fluids or compressor output.

The compressors have run perfectly and quiet. Because they are running so quiet, he has put yellow tape at the air vent to make sure they are working. “They run so much quieter than other compressors,” he said. They run at 1650 RPM and 150 psi with element temperatures at 204 degrees F. He has little to do with them during the day other than checking and topping off fluids in the morning and monitoring output. “Atlas Copco is taking care of service with the rental agreement. I just get to use them.”

Anderson has found success with the cluster drill and would like to get a bigger one in the future. A 48-by 72-inch to pair with “Thor” tops the crew’s wish list, but they would also like a 60-by 92-inch pair of cluster drills. For right now, however, the 48-inch drill is the right compromise and a great beginning for what is looking like a great future between Anderson Drilling and Atlas Copco.