The shotcrete is reinforced. I don't mean the rollout mesh, but I mean, the shotcrete mixture itself is reinforced by means of these small wires. The wires here join together the way staples are attached in your stapling machine, but when they're agitated in the mixer, they come apart into separate wires, and they're mixed throughout the applied shotcrete. I also have a short video here showing the shotcrete application.
Here, the concrete truck is delivering concrete, placing it in that hopper, where it would be pumped under pressure and then sprayed onto the face of the excavation. The man doing the spraying is called the nozzle man. This is a very skilled role. If he just sprayed the exposed earth, the earth would probably just fall away.
You typically apply 4 inches, but you cannot do that all at once, because it simply won't stand in a thickness of 4 inches. In this application, I would say he's basically putting down an inch or two In an initial pass, and then he's going to come back and apply the final pass.
You can see this is applied in lifts, perhaps about three feet at a time, so you cannot really leave too much of this exposed earth. 3 feet looks like about the limit. Again, that would depend on the material. This is very dry material, very little cohesion. You can only go down, I would say about 3 feet, before you have to apply the shotcrete and the soil nails.
Now he's reached the end and he's going to build up the thickness of the shotcrete and apply the full four inch thickness. In this instance, because it's support of excavation wall, you don't care much about the finish, so he can leave the shotcrete just as it is. In some instances, you can actually follow him with the trowel and even off the shotcrete if for whatever reason you need a smoother finish. After the shotcrete is hardened, he will install the next row of soil nails.