Challenges in Dewatering for Underground Construction in Flowing Artesian Conditions: A Case Study

An underground 1.9-mi-long (3.1 km) sanitary sewer line was designed and constructed in the greater Seattle area. Once completed, substantial groundwater seepage was observed within a 600-ft-long (183 m) segment of the pipeline. The repair to arrest the seepage cost over $1 million and resulted in a six-month service delay. Geotechnical studies identified groundwater conditions consisting of shallow and deep aquifers separated by a confining unit. The construction contract specified using a combination of trenchless and open trench construction methods. Anticipating flowing artesian conditions in the seepage area during construction, a dewatering system was installed. The trenchless installation in the seepage area used an H-pile shoring system for the receiving pit. Once the dewatering wells were decommissioned, seepage was observed from the newly decommissioned wells and shoring system. This case study provides relevant lessons learned and recommendations for future project planning, design, and construction.