Now we’re rolling! On December 1st, excavation began in earnest. This picture, taken from the stairway on the side of the excavation, shows a small skiploader knocking rocks off the excavation face, and moving them into a pile behind. The big backhoe on the right is filling trucks that each carry about 14 cubic yards of material. A truck arrives about every 15 minutes, so on a good day up to 200 yards can be excavated and removed.
The smaller plastic pipe sticking out next to each tieback is used to pump the grout in to the hole. The larger pipe around the tieback protects it until it’s time to spray shotcrete against the excavation face. The end of each tieback and soil nail is threaded, because a metal plate will be bolted on after the wall is built, to secure the wall to each tieback. Also, these tiebacks will all be stress-tested to make sure they can withstand a 90,000-lb pull without coming out of the rock, and the test gear has to be bolted to each tieback. If a tieback fails during testing, and starts to come out, the plastic pipe can be used to pump more grout into the hole and re-cement the tieback.
The first lift is complete, and you can see the row of tiebacks and soil nails sticking out of the rock face. Tiebacks go about 40 feet deep, while soil nails are only about 12 to 16 feet deep.
Here is what tieback drilling looked like, when this row was installed a couple of days ago. Tiebacks in the foreground are already set into their holes, ready for the grout that cements them in place. You can also see a soil nail, which is shorter, sticking out on the left side of the picture. The drilling rig on treads is powered by compressed air, and the compressor and drill together make a ton of noise.
Our project manager was kind enough to send this picture of some of the kids in the neighborhood, inspecting the work!