Little Patuxent Parallel Interceptor Sewer

George, Miles & Buhr, LLC (GMB) conducted a study of the Little Patuxent Interceptor Sewer.  The general scope of the project was to prepare a study and develop a preliminary design for augmenting the capacity of the Little Patuxent Interceptor Sewer.  In addition to the study and preliminary design phase services, the scope included final design, bidding, and selected construction phase services.

In an earlier study, performed in the 1980’s, it was indicated that parallel interceptor sewers would be required along selected reaches of the existing Little Patuxent Interceptor that extends from U.S. Route 1 upstream to Broken Land Parkway.  The earlier study contemplated that the existing Little Patuxent Interceptor Sewer, which generally varies in size from 36-inches to 30-inches, would be paralleled along several reaches.  The previous study generally assumed that the improvements would include approximately 900 feet of 24‑inch, 1,300 feet of 18‑inch, and 5,600 feet of 15-inch sewer to meet projected wastewater flow requirements. The Little Patuxent Sewershed consists of three drainage areas: Patuxent Sub-Sewershed; Middle Patuxent Sub-Sewershed, and the Route 108 Sub-Sewershed. This established design flows for the interceptor sewers located in the 49-square mile sewershed. A hydraulic model was built and was used to assist in the analysis of the existing sewer system. GMB’s flow metering program consisted of 19 flow meters and 1 rain gauge which were operated and maintained for four (4) months.  In addition, GMB had access to data from an additional 5 flow meters and 5 rain gauges for use in determining flows.  GMB analyzed the flow meter data to determine wastewater flows and their patterns.  Flow was classified into three components (sewage flow, infiltration, and inflow) for use in the flow model.

GMB and our design team provided the following services as a part of this sewer design effort:

• Hydraulic modeling analysis of the existing and proposed systems to determine the system’s capacity and weaknesses at ultimate build-out of residential, commercial, and industrial development within the Little Patuxent Sewershed.
• Subsurface Investigations including Utility Test Holes, Soil Borings, Auger Probes, Sampling and Laboratory Testing. A Seismic Refraction Survey was also be undertaken as part of this work.
• Detailed site investigations including determination of the most viable construction access routes.
• Assembly, review and analysis of pertinent available record data.
• Topographic surveys along the project corridor and construction access routes and final ground-over survey.
• Coordination with the Wetland and Forest Stand Consultant.
• Preparation of Construction Documents including detailed Contract Plans and Specifications.

• Preparation of Construction Cost Estimates.
• Provided recommendations regarding construction materials.
• Special utility design and structural design.
• Development of Property Mosaics and Property Plats.
• Preparation of permit applications and documentation.
• QA/QC Reviews.

• Bidding Phase Services.
• Preparation and implementation of a Public Outreach program.

We were provided with a GIS skeleton in ARC View by the County which included nearly 2,000 features of the Little Patuxent Interceptor and its branches 12-inches and larger.  These features required additional contract drawing research to populate the data required for hydraulic modeling in XP-SWMM.  Upon completion of this research, we will have incorporated 4,000 contract drawings into the GIS database.  The GIS data was also used to identify sewersheds and determine the population and flow data.

George, Miles & Buhr, LLC (GMB) provided design, bidding and construction phase services for the Little Patuxent Parallel Interceptor Sewer. This project involved preparation of bid-ready plans and specifications for five (5) separate contracts to construct approximately 21,900 linear feet of new 30-inch diameter through 42-inch diameter parallel interceptor sewer. The sewer originates at the tie-in to the existing Little Patuxent Interceptor just north of the Little Patuxent Water Reclamation Plant, and runs north / upstream and generally parallel to the existing Little Patuxent Interceptor to just south of the Maryland Route 32 crossing at Guilford Road. The sewer alignment included several river crossings and traversed heavily wooded areas, rock outcroppings, wetland areas and historic preservation areas. The design was a follow-up to the sewershed service area assessment, sewer system evaluation survey and preliminary alignment study prepared by GMB, to determine existing and future flows and establish the best alternative to augment the existing capacity of the Little Patuxent Interceptor Sewer to meet projected wastewater flow requirements.