Swinerton Management & Consulting (SMC) has been leading a challenging project on the San Mateo Community College District – Skyline College (College) campus: the construction of a 1.2-million-gallon water tank on a hillside project site less than 1 acre. Additionally, the project is divided in two, between the project site and the laydown area, by the perimeter road of an active campus. SMC and the general contractor and they have been working very hard to keep this project moving along.
The project really slowed during the placement of the temporary shoring when it was discovered that the entire upper hillside is a large and very hard boulder. This boulder, so we learned after meeting with a regional geologist, is called cemented sandstone. Google cemented sandstone and you’ll see that it is “sedimentary rock composed of quartz compressed and cemented together.” In this case it became cemented via a hot vapor that flowed through cracks a long, long, long time ago.
Also, in this case, as the shoring was for the retaining of the existing 800,000-gallon water tank, we had no choice but to bore through the rock to the 30 feet required for the shoring placement. The team planned for five days of boring. Instead, the boring took 17 days. It was slow going, we hit slow speeds of 1 foot and hour with occasional burst to 3 feet an hour.
On about day 12, the team was grinding along with their 24” bit, when they lifted the rig and discovered that one rock had been cored to a perfect 24-inch by 4-foot cylinder. The core came up in perfect shape. It didn’t fracture or break! We were all excited. Don’t judge us, when you’ve been drilling for hours on end, we’ll take excitement where we can.
This beautiful rock weighs about 800 pounds, and is dark, almost black, with white grain. But what should we do with it? It’s way too large to take home and turn into a fountain or yard art and too pretty to crack up. In construction on a college campus, I am always looking for ways to create a teaching moment, so I called the College’s Geology department and explained our unique find. Professor Linda Hand came over, looked, and she too was super excited to see the rock and the formation.
We agreed that if I could find a location suitable to facilities and near the Geology department and deliver the rock (all for free), then she could use the rock as an example rock in her Geology classes. A few weeks later, we were able to keep our commitment to the College and education excellence. Today, the rock has a new home.