Abstract:
In February of 2023, Schnabel was contracted to design and install temporary support of excavation to enable the construction of a new sewer lift station. The SOE system selected was an unreinforced circular secant shaft, and the geometry of the proposed lift station required two circular secant shafts constructed in a figure eight pattern. The figure eight configuration created complicated states of stress, which required three-dimensional FEM to verify the design. There were many construction challenges, such as piping sands and extremely cold temperatures.
In August of 2024, the contractor attempted to use trench boxes and dewatering to lay the sewer pipe and connect to the lift station; however, water and soil piped into the excavation around the trench box. The clean, poorly graded sand represented in the boring actually had a significant fines content, which prevented effective dewatering. An emergency intervention to improve the subgrade soil and cut off the water was needed to lay the sewer pipe. A soil-cement block installed using soil mixing was selected as the best solution, and trench boxes were used to provide lateral support. The performance criteria of the soil-cement were challenging; the soil-cement needed to cut off the water and improve the ground, and it needed to be weak enough that an excavator could dig out the center of the soil-cement block to lay sewer pipe. Premobilization laboratory mixing trials to inform the mix design were performed almost concurrently to the field soil mixing due to the tight project schedule, and the soil-cement performance criteria were met due to wet grab sampling and predictive strength equations to make mix design adjustments on the fly. The soil-cement performed as intended and finished on schedule. Injection grouting and permeation grouting were performed to seal between the soil-cement block and the secant shaft.