the story

in the sEDIMENT

Reading the soil
Everything beneath the earth’s surface affects everything built on it.

It’s a fact many of us rarely think about. Geology directly impacts the quality and longevity of every office building, neighborhood street, rail viaduct, parking lot, water tower, and flood-control dam.

The characteristics of soil, clay, sand, and gravel reveal a story that informs the work of our geotechnical engineers and hydrogeologists. But the story remains out of sight, buried by millennia.

Until our drillers go get it.

drilling Down

The ground yielded easily to a massive auger that twists with up to 30,000 foot-pounds of torque. After sinking the auger only 5 feet, the senior driller at the controls brought it to a halt.

Next, a cable running through the rig’s tall derrick hoisted a metal cylinder from inside the auger. An assistant driller carried the tube – called a continuous sampler -- over to a series of tables lined end-to-end and covered with white paper. He opened the tube to reveal a 5-foot continuous core sample of sediment roughly the diameter of a soda can.

Because drillers usually complete their work before project construction begins, their role is frequently overlooked. But at this sediment classification workshop southeast of Lincoln, Nebraska, our geotechnical drillers took center stage.

Olsson co-sponsored the workshop with Midwest GeoSciences Group, a firm based in Carmel, Indiana, that offers professional development opportunities for geologists, hydrogeologists, and civil engineers. The workshop took place on the property of Glacial Till Vineyard and was attended by geosciences professionals with the U.S. Department of Agriculture and the University of Nebraska-Lincoln.

For the next hour or so, our drilling crew sunk additional augers down the same bore hole to retrieve the next sediment sample. They uncased each sample in succession until the drill hit bedrock. By then, a roughly 70-foot-long column of sediment ran down the center of the tables.

Dan Kelleher, president and co-founder of Midwest GeoSciences Group, carefully split the core sample lengthwise. He wanted to better reveal a column of sediments that hadn’t been exposed to sunlight for hundreds of thousands of years.

the workshop

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With the drilling completed, the workshop participants took a closer look at the core sample.

They moved slowly, bending down to peer at subtle gradients in color and texture. Sometimes they pinched a bit of dirt to roll in their fingers or palms, a method for gauging clay content. Occasionally, they looked up at the surrounding topography to gather additional clues.

They took notes and engaged in conversations interrupted by frequent laughter. The more experienced professionals shared hard-earned lessons with younger colleagues. A spirited collegiality emerged among those who share this highly specialized knowledge.

Despite all that’s been mapped and studied of our geology, the chance to examine a continuous core sample always yields valuable information.

For example, the workshop participants knew that the winery sits amid the dissected till plains of eastern Nebraska, a region shaped by glaciers more than 500,000 years ago. But the sample recovered by our drillers contained glacial till exhibiting secondary weathering, which differed from the underlying till. In other words, the discovery could very well affect engineering design considerations for a project built at the site.

“Taking the mystery out of the subsurface requires us to understand subsurface relationships,” said Dan Kelleher, who is a hydrogeologist. “And that part is not easy. Every site is different. Every project’s different. And being able to decipher it and put that puzzle together really takes academic training, it takes background, it takes practice, and it takes coaching.”

Safe and Sound from Field to Lab

Samples can be gathered to whatever depth is required up to 250 feet. Our drilling teams also can collect samples of bedrock and conduct electronic cone penetration tests, which do not involve removing samples.

Drillers and their assistants work in just about every weather condition except for electrical storms. They perform physically demanding work that requires constant problem solving. They must maintain vigilance for overhead powerlines, underground utility service lines, and other potential hazards, said Adam Snook, senior leader for our Drilling team.

“Safety is the number one most important thing,” Adam said. “We all want everyone to go home at the end of the day the same way we came to work. So, that’s paramount to our success.”

Had this been a project site instead of a workshop, our drillers would have catalogued and loaded each sample for analysis in one of our laboratories. The subsequent test results would help our engineers design projects that endure.

“They need to know how the sediments are going to behave,” Adam said. “Everywhere you go, every road you drive on, every building you walk into that’s a commercial structure, it likely had some geotechnical analysis done.”