Wet Well Installation (emphasis on the “wet”!)
In a previous post, there was an introduction to what a pump station is and the preparation for the wet well installation for that pump station. It is the receiver of all of the project’s gravity sewer, which means it is the deepest part of the sewer system. The earlier post showed the well point system that was set up to prepare for this deep hole that must be dug. This post will show the installation of the wet well and the challenges that came with it.
Digging a hole that goes below sea level can be challenging enough. Unfortunately, there were some other factors that added to the difficulty for this particular job. One factor was the weather. We just had not had a lot of sunshine to help dry things out and had more wet weather on the way with 2 hurricanes looming off the coast. In addition, the location of the wet well is on the edge of the “limits of disturbance”. Part of the construction permit that is required by one of the state agencies is that you have to establish limits of disturbance, which a lot of times is where the silt fence is located. The earth cannot be disturbed beyond that line. In this case, the limit of disturbance was determined by the required 100’ buffer in this area, so it was as far as it could be anyways. What made it difficult is that it was close enough to the wet well location that it did not allow for the proper benching on that side of the hole. Instead, it was more like a shear wall of dirt, which can be dangerous because of caving in, but also makes the digging very difficult because of constantly having to deal with the dirt sliding down into the hole.
We set up a Timelapse video of the process of setting the wet well to help show the process from beginning to end. In the Timelapse, you can see the difficulty the contractor encountered throughout the week. Things had to be done multiple times dues to the wetness and the walls sliding in. It was a rough week for the contractor, but in the end, they got it set correctly and were able to start backfilling. The time lapse below had to be broken into 2 videos due to limits on file size, but know the second is a continuation of the first.
In the video, you’ll see them first digging down as far as they can and then setting the trench box to allow them to prepare the base where the bottom of the wet well will sit and to provide a safe place to work. You’ll then see the crane lifting and setting the different pieces of the wet well. They then had a hard time getting their trench box out due to more rain and dirt sliding in from around the wet well. The video ends prior to the full trench box coming out and the remaining sections placed on top, but we have pictures showing everything in place.
After letting it sit a week and getting some good sunshine, the top piece was set and it was properly backfilled and compacted. The contractor then began running the sewer line out from the wet well to the first manhole, which is almost just as deep as the wet well. The sewer line is continued out from manhole to manhole, going from the deepest point outward. So far, this week, the contractor has gotten in approximately 4 manholes and about 400’ of sewer pipe out from the wet well location. Rock is put around the entire length of the sewer pipe to help prevent the pipe from settling. A trench box is continued to be used with this deep sewer and can just be dragged as they dig the trench and lay the pipe.
Coming up this week, multiple crews will be on site and they will continue to lay sewer. In addition, while the contractor was waiting for things to dry out a little after setting the wet well, some water pipe was laid. Water is typically done after sewer because it is only 4’ deep and therefore goes on top of everything. Thankfully, there is a pretty decent length of water pipe that doesn’t have anything underneath it, so they were able to go ahead and knock this section out last week while things were wet.
