It’s not every day you see a helicopter flying over the Potomac River carrying a 14-ton drill rig and workers suspended midair to work on DC Water’s sewer lines. The typical equipment used on sewer lines includes vacuum trucks, backhoes, excavators, and trenchers, but when you’re working in the middle of a river only a helicopter will do.

Workers began drilling in the Potomac this week at Great Falls, to remove rock core samples as much as 50-feet deep at five different boring locations. It’s part of the planned rehabilitation of the Potomac Interceptor, a 54-mile sanitary sewer system that runs from the Dulles area in Virginia, across the river to Maryland, and down to DC Water’s advanced wastewater treatment plant.  

At Great Falls, the sewer line cuts through the river, 3000-feet across and 78-inches in diameter and is beginning to show its age. It’s 60-years old and nearing the end of its useful life. The rock samples will provide valuable information to geologists and engineers on how best to rehab this section of pipe, which could include repairs to the existing line and building another pipe alongside for redundancy.  

The project, still in the early stage of planning, is part of DC Water’s 10-year, $7.7 billion Capital Improvement Plan to upgrade the water and sewer system, parts of which are more than a century old. The Potomac Interceptor is a key infrastructure asset, handling 60 million gallons of wastewater daily. Ensuring its long-term reliability is essential in maintaining the region’s wastewater infrastructure.  

During the week, a Sikorsky S-64E helicopter – similar to the ones used to fight wildfires – is transporting a drill rig, and a second MD-500 helicopter is flying workers to the site using a man basket suspended from the aircraft. At the end of each day the drill and workers will be airlifted out and return to a new location on five consecutive days.

 The drill weighs 14 tons and reaches heights of 31-feet tall. It is specially designed with a hollow rod and diamond tip to cut through the rock and remove a core sample. Some of the rock in this area was formed more than a billion years ago and varies in composition and type, including metagreywacke, quartz-schist, and biotite-schist. 

Based on the samples, geologists will be able to determine the composition of the rock below the river, its stability, and suitability for the work ahead. 

Great Falls’ proximity to the nation’s capital posed additional complications because the area is within restricted airspace. This required additional time and planning to get the clearance needed to fly such a large rig into an area sensitive to security concerns.  

Throughout the project, the surrounding parks, including the C&O Canal and Great Falls Park, remain open to the public.  

For more information visit our Potomac Interceptor Project page at dcwater.com/potomacinterceptor, or check out our Fact Sheet on the Potomac River Crossing.