Specialty Drilling Equipment UMA used one of its specialty Casagrande C7 XP-2 Crawler Drilling Rigs for all micropile drilling. It worked in tandem with an excavator with an EZ Spot UR attachment, which fed casing sections to the rig. A positive displacement pump supplied water to the C7 during drilling to flush the casing sections down to depth. The benefit of such a pump was that it supplied a constant flow and pressure of fluid throughout the process. UMA’s goal was to use the longest sections of steel possible. The C7 is equipped with a 30-foot stroke so the team was able to install a 30-foot section for the first piece, followed by two 25-foot sections. UMA used its specialty Casagrande C7 XP-2 Crawler Drilling Rig to drill all micropiles to 70 feet through a formation called the Cooper Marl.
“Seashells, organic matter, and sand layered on top of each other, and compacted more and more over millions of years, and formed this re- ally dense sand layer that is commonly referred to as Cooper Marl,” explains UMA President Brian DeSpain. “It’s interesting when you drill in it because this mixture of material from millions of years ago comes up out of the ground.” The bond zone for the micropiles fell within the Cooper Marl layer. UMA drilled the piles to an average depth of 70 feet, although it fluctu- ated because the layer was 50 feet down on one side of the site and 25 feet in the other section. The Cooper Marl acts like a degradated rock during drilling so the hole will stay open. The wet sandy overburden layer above; however, col- lapses into the hole. This required UMA to tap OCI Division of Global Drilling Supply in Brookville, Pa. to make a special drill bit it has used several times since 2011. OCI also supplied all casing for the project. UMA worked closely with OCI on the size of the bit. The length that the cutting teeth projected from the casing was critical to ensure that the outside diameter of the bit was larger than that of the casing. This dimension was critical to ensure that drill cuttings returned to the sur- face effectively; the casing did not get locked into the sand by friction; and the grout could fully encapsulate the casing. “When you drill below the water table, the pressure required to over- come the hydrostatic head pressure increases by approximately one atmosphere every 33 feet,” DeSpain explains. “You want to be able to push fluid out of the bit, but you don't want fluid or cuttings to come back in the bit. This bit only lets the water go out, which allows us to flush the cuttings from around the outside of the pipe back up to the surface.” Fessler & Bowman had covered the entire site with a mud mat before UMA fired up the drilling rig. This was basically a concrete slab to serve as a stable working platform on this muddy site. UMA utilized a Penndrill cement silo to reduce labor and increase production. Positioning it above the grout plant allowed cement to be loaded directly from truck to silo without breaking open and dumping each bag.
UMA utilized a Penndrill cement silo to reduce labor and increase produc- tion. Positioning it above the grout plant allowed ce- ment to be loaded directly from truck to silo without breaking open and dump- ing each bag. Drilling through Cooper Marl After penetrating a layer of black organic, silty sand, UMA was drilling through Cooper Marl, which is a soil formation prevalent in South Caro- lina’s Lowcountry Region along the coast. The con- tractor is no stranger to drilling in this material.
An excavator with EZ Spot UR attachment fed casing sections to the Casagrande C7 during micropile installation.
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march 2021
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