A new pump station 50 feet from Lake Tahoe was required as part of a water distribution system for Tahoe City Public Utility District. The pump station dimensions were 28 feet by 12 feet in plan and 24 feet in depth. Previous attempts to excavate the ground using conventional shoring techniques while keeping within the restrictive dewatering requirements were unsuccessful.

Ground conditions at the site were highly variable. They consisted of sand and gravel over clay seams and very large rock fragments. The proximity to the lake combined with highly permeable soil layers would result in very high dewatering pumping rates – beyond that allowed by the Lake Tahoe water conservation district. Installation of any shoring type would be inhibited by the presence of large rock materials within the sand, silt and clay

SoilFreeze was retained by the general contractor to design and construct a continuous, structural frozen soil wall around the site to retain soil and groundwater. To prevent groundwater inflow from below the excavation, we installed a series within the patented zone freeze pipes (ZFP) in the middle of the site to create an impermeable frozen soil barrier below the excavation base.

The frozen soil perimeter wall was successfully created using our conventional full freeze pipe (FFP). The bottom frozen soil plug was successfully created using our ZFP. Together, the freeze system performed very well to retain the soil and groundwater around the excavation perimeter and to prevent upward flow of groundwater through the excavation base. The general contractor was able to complete the pump station construction with no dewatering.