Reaching Greater CPT Depths

Responsible for completing all soil mechanic and bridge foundation drilling, design, and testing, members of the Nebraska Department of Transportation team in Lincoln are adept at all applications – hollow stem, Shelby tube, mud rotary, coring, continuous sampling with direct push, and Cone Penetration Testing (CPT). So when it came time to replace one of their conventional rigs, they wanted something as versatile as they are. Upon delivery of their Geoprobe^® 3230DT, they quickly learned its operation and headed into the field.

“We can go into places throughout Nebraska to replace bridges and drill for slope failures and complete direct push until we hit rock and then core samples to 100 feet. It works really well,” Nikolas Glennie, engineer, said. “Without that capability, in the past we hadn’t been able to seal off the hole, which creates a big mess with hollow stem auger. Now we make less mess, it’s lots easier, and we’re quicker to get projects done.”

In addition to the mess, the Nebraska DOT had also been dissatisfied with depths their old rig could push CPT.

“We can take the head of the 3230DT closer to the ground, so there’s not a lot of rod in the air compared to a traditional rig,” Glennie said. “We’re able to do more CPT pushing through sand layers and denser materials, gathering more data than before. With a traditional rig, we’d start bending rods in stiffer materials rather than going deeper.”

The ability to anchor the 3230DT, achieving greater CPT depths, has led to the development of a new procedure integrating the CPT data into the bridge piling design.

“Before we would do four to six mud rotary SPT borings. Now we can, if the site allows, do one or two mud rotary and four to six CPT in half the amount of time,” Glennie said.

The Nebraska DOT has also found themselves developing a preference for direct push, valuing the solid profile of the soil materials that, when compared side-by-side to Shelby tubes, appear to be nearly identical.

“The plastic liners seem to provide a more realistic sample than the Shelby tubes because they aren’t compressed,” Glennie said. “They provide a nice, clean, continuous geological profile rather than guessing where layers meet in the ground.”

The team also values the safety features built into the 3230DT, including the presence bar and the ability to move the control panel.

“The operator is not only able to monitor what is happening all around the project site, but is also able to stand away from the rotary drive,” Glennie said.

The ability to safely, quickly, and easily switch from percussion direct push to core drilling or to hollow stem auger over CPT has saved the office significant time. Now there is no downtime when switching over different drilling methods and they’re completing a lot more CPT work.

“We’ve had a lot of flooding the past year with 27 bridges needing work or replacement. We’ve gone out to most of them with the 3230DT,” Glennie said. “We’re able to get to other projects faster. We can get ahead of schedule versus being on or behind schedule.”