Bi-County Water Tunnel

OWNER: Washington Suburban Sanitary Commission

ENGINEER: Black & Veatch

SOUTHLAND HOLDINGS TEAM: Oscar Renda Contracting/Southland Contracting Joint Venture


This project received the 2016 American Society of Civil Engineers Project of the Year Award.

The Washington Suburban Sanitary Commission needed to transmit clean drinking water from the Potomac Water Filtration Plant to Montgomery and Prince George’s counties.

With only three shafts to serve a length in excess of five miles, typical tunneling activities – providing ventilation, handling muck, carrier pipe install and grouting, and addressing equipment wear and tear – were unusually difficult. In addition, the project passed through Maryland Piedmont bedrock with un-compressive strengths of up to 30,000 psi beneath environmentally sensitive parklands and highly developed suburban areas.

The three construction access shafts were excavated by drill and blast methods. The main construction shaft at Connecticut Avenue, 35 ft. in diameter by 164 ft. deep, was supported by liner plates in the overburden and by rock bolts and shotcrete in rock. The 25 ft. diameter Tuckerman shaft was 125 ft. deep and supported by secant piles in the overburden. The Stoneybrook shaft is 16 ft. in diameter and 150 ft. deep. The tunnel was excavated with a Robbins 10 ft. Hard Rock TBM.

With careful planning and swift execution, we overcame this $113.8-million project’s demanding challenges and bored a 10 ft. diameter deep hard rock tunnel 200-300 ft. below ground. The 28,000 LF tunnel is lined with 84″ welded steel carrier pipe and grouted in place.