completing the shoring wall

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Feb 262007

Work has proceeded over the last two weeks to finish the final 6 feet of shoring wall. The crew installed drain matting and wire mesh to support the wall material, while carefully removed the 4×4 shoring timbers and the fallen rock from the two cave-ins. Finally, they brought in the last loads of shotcrete and sprayed it into place.

Here is a closer look at some of the work. You can see the big cave-in on the right. The crew is wiring the sheets of mesh together, working from the left to the right. As they reached the cave-in, they dug out enough material to let them complete the mesh across the front. You can also see that there is 1 to 2 feet of space behind the drain mat and the mesh; they removed a lot more material than they should have when they excavated the final lift.


Of course, the space behind the wall needed to be filled in from the top. The best material for this is called “3/4 crush”, or crushed rock that is about 3/4” in size, which provides good drainage. This truck is delivering about 12 yards of rock, in preparation for filling the caved-in areas behind the wall — from the top.


Here is the final result. You can see the change in color at the edge between the last two lifts of the wall, and also where they sprayed shotcrete from different truck loads. As it turned out, they indeed had to use an enormous amount of concrete to fill in the over-excavated areas.

The shoring wall is finally done. They’ve removed more than 190 truck loads of dirt and rock from our land, and added dozens of mixers full of shotcrete and several truckloads of gravel. We now have a huge hole in the hillside, surrounded by concrete. It’s eerily quiet, and standing in the middle, where our house will be, I think this structure should be generating megawatts of power or something

a little problem

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Feb 112007

It started raining Thursday night, and we’ve had a setback. Part of the rock behind the shoring wall has collapsed down below the wall, before we had a chance to get shotcrete over the excavated area.

The drilling and soil nail placement was completed on Thursday, but a nice big Pacific storm moved in. It rained for about 18 hours, and by a stroke of luck, one of the subcontractors visited the site early Friday morning and saw the collapse beginning. I came out and watched as a couple of yards of material came down and contributed to the pile. Quite a rumble. And heartbreaking to watch.

This is what it looked like this morning (Saturday), after the rain stopped. The rock on this side (the right, or south side) is very fractured, and Bob Settgast, our geotechnical engineer calls it “melange”. It breaks apart easily, and since we’ve excavated away the soil and rock around it, it’s been starting to fall apart. The shelf in the wall above this area is open to the weather, and our contractor did not put any weatherproofing (like plastic sheeting) over the exposed surface. The water rained right in, and apparently contributed to the mess. Here is the top side of the cave-in, the unprotected surface.


Because the rock is no longer packed around the soil nails, this area of the wall is at risk of falling apart. The soil nails are not capable of holding the wall up, and this shotcrete does not contain any structural steel. So the subcontractor brought in 4×4 lumber, and put wooden shores up under the wall to help keep it up…just in case.


By the way, this shows just how much over-excavation is happening. This whole area under the wall has to be filled in with concrete and rock!