Unique Horizontal Drain Installation under a Building Using Pilot Tube Guided Auger Bore Methods

摘要

This paper describes design and construction considerations for the pilot tube guided auger bore installation of one 10-inch diameter drain line and four 6-inch diameter horizontal drains under a critical FAA Air Traffic Control Facility with no interior access for construction. Key considerations included high water table in granular soils, limited work area around the building, and coordination around (and under) a critical infrastructure facility that operates continuously and cannot shut down. The facility was built on wetlands in the 1960s and expanded in the 70’s and 80’s with deeper subbasements and a larger 400’ by 400’ footprint. This facility has reoccurring problems with flooding in the basement areas, which houses critical equipment/electrical components. The original construction included waterproof membranes, sub-slab drain tile, and perimeter drains to allow dewatering under the basement structure. In the 1990s water ingress was observed in the basement and a groundwater management system of 7 vertical dewatering wells was installed around the perimeter of the facility. These wells worked effectively until 2010 when groundwater was again observed in areas of the basement. The existing vertical wells were subsequently upgraded with larger pumps/stainless steel screens and an additional vertical well was installed to mitigate water infiltration into the facility. These measures worked until 2014 when the wells were found to be heavily fouled with iron-fixing bacteria which resulted in lost efficiency. After significant study, a plan was developed that will provide redundancy of the system and groundwater protection for the building. This plan included installation of four horizontal wells under the existing building to augment the 8 vertical dewatering wells and installation of a 10-inch diameter drain line under the building to tie to a new pump station. Design and construction challenges are discussed in this paper.