Marysville & Binney Pumping Plants

At DHC we specialize in excavation shoring design for large underground water infrastructure. We love working with a contractor to come up with a solution that allows them to efficiently construct a new structure while also ensuring that all workers are safe during the process. So when we were contacted by Granite Construction to design (2) large pumping plants, we jumped at the opportunity.

The scope consisted of designing braced sheet pile shoring pits that were ~100’-long x 35’-wide x 27’ to 32’-deep and ~115’ x 35’-wide x 31’ to 36’-deep. In addition, each system had to retain about 15’ of groundwater. There were a few things about these shoring designs that created some unique challenges.

First, the bracing system could not interfere with the new structure. Thankfully, most of the new structure was about 10’ below grade, so we could fit the upper level of bracing above it and space the struts around to avoid the parts that would rise to the surface. The hope was to avoid any bracing below so that it wouldn’t interfere with the construction of the structure. If it weren’t for the next challenge that would have been the case.

The second challenge was that about 1/3 of the length of the pits needed to go ~5’-deeper than the rest of the pit. This, coupled with the groundwater, was creating the need for additional bracing, but the budget didn’t allow for there to be a full second level of bracing. Waler systems generally rely on a full ring of bracing so that opposing sides counteract the loading from each other. Instead, we needed to design a 3-sided bracing system. This meant that we needed to transfer the lateral force coming from one side of the bracing to the perpendicular walls and then to the soil through friction. This resulted in a substantial amount of welding of the bracing to the vertical sheet piles and then confirming that the sheet piles were strong enough to spread load vertically to activate enough resisting soil to act as a shear wall. After a lot of back and forth, we were able to get the concept to work.

Another reason this design was atypical was that the contractor was not going to be able to operate the dewatering system inside the excavation once they began to construct the permanent structure inside the shoring system. This was because the groundwater was being feed by a large underground source which would result in pulling a lot of water out of the ground, and options for where to put that water were limited. We all decided the best way to handle this was to design a concrete seal slab at the base of the excavation to seal off the water, similar to what is typically implemented for a cofferdam.

A secondary benefit was that we could also design the slab as a brace level which would allow for removal of the lower 3-sided level of bracing. If this was going to work, the slab would need to be designed for both the vertical uplift pressure from the water and the horizontal axial pressure from the sheet pile walls. The difficult part was that because the base of excavation varied, it wouldn’t just be one large slab. The solution, similar to the second brace level, was to treat them as (2) 3-sided systems that would need to pass loads to the sheet pile walls and then to the soils. After a lot of trial and error with the concrete strength and thickness, we were able to get the system to work. Although this design was not simple, the end result was rewarding as the teams worked together to finalize the solution.