Coring

With all the construction going on around town, you’ll see lots of drill rigs taking geotechnical cores. This one was at work at 2330 Webster, where the Webster Alexan development will go.

Just a few days earlier, a rig was collecting cores in the parking lot at 20th and Telegraph, slated to become one of two residential towers.

Crews like this are testing the ground to firm up the construction plans. The weight of these buildings requires a foundation that won’t sink, buckle or deform under the expected loads over the building’s life, including earthquake loads. At the Telegraph site two holes were bored, at opposite ends of the lot.

It takes a couple of workers to run the rig and a geologist to log the hole. They look the same — vests, boots, hardhats — except the geologist carries a clipboard and isn’t quite as muddy. The geologist on this job was a young guy, crouched in the sun and processing sediment plugs that looked like this.

It’s nice, clean marine clay from the lower part of the hole. I refrained from nibbling on a piece to gauge its silt content. It was real firm, not sticky. I’d put a house on it, no problem.

The geologist was poking at the plugs with a pocket tool and keeping them properly organized. He told me the hole was around 90 feet deep, with this stuff at the bottom. The top 20 feet was sand and gravel, then about 30 feet of clay, then some more sand and gravel and finally this clay. It’s a common pattern around the Bay, reflecting the changes in sea level over the last few hundred thousand years.

The crew was finished in less than a day, and they tidied up nicely afterward.

The U.S. Geological Survey’s Open-File Report 2014-1127, “Geologic Logs of Geotechnical Cores from the Subsurface Sacramento-San Joaquin Delta, California,” will give an idea of what core logging involves when it’s done right. What seems like painstaking drudgery is essential for building safely, and geologists can get called into court to vouch for the accuracy of their core logs.

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