The Potomac Interceptor (PI) sanitary sewer system near Washington DC plays a critical role in managing wastewater from several communities near Dulles Airport, transporting an average of 60 million gallons of wastewater daily.

From January 27 to February 2, 2025, 7NT conducted a geotechnical investigation in the Potomac River. Rock core samples were taken just below the Washington Aqueduct dam in Great Falls Park. The investigation required the transport of both workers and a drilling rig by helicopter.

7NT strategically executed all the field tasks. Every detail had to align, and timing was critical. This was not a routine investigation; the coordination between the team, the helicopter pilots, and the various locations on the Potomac River was challenging. 7NT also adhered to a very intense timeline, drilling precise 50 ft, 20-degree angle, boreholes and conducting packer tests with the integration of cameras for monitoring. For a project that could have easily stretched over the allotted time, 7NT completed the task while overcoming challenges using effective teamwork and communication.

The scope of work included two (2) land borings in addition to the five (5) river borings. The land borings were drilled to 85 feet each. Triple barrel NQ-size rock coring was performed. Field permeability tests were performed and open standpipe monitoring wells were installed. The wells consist of 2-inch diameter PVC pipe. 7NT also completed a laboratory testing program to determine the engineering properties of the site soils.

Lab tests included:

• Moisture content and Natural Dry Density
• Atterberg limits
• Grain size distribution, using sieve analysis and hydrometer analysis
• Specific gravity of soils
• Triaxial compression tests
• Consolidation tests
• Classification of soil using the unified soil classification
• Permeability tests
• Proctor compaction tests

Engineering