Sliplining A Brick Sewer — Circa 1896

Staff | September 28, 2010

In 1986, the United States Congress established The United States Institute of Peace (USIP) and provided $100 million for construction of a permanent headquarters facility in Washington, D.C. It will be located at the northwest corner of the National Mall, facing the Lincoln Memorial and adjacent to the Korean War and Vietnam Veterans memorials.

Construction of the facility is scheduled for completion by the first quarter of 2010. However, there is a great deal of work to be done prior to groundbreaking. This includes shoring the existing infrastructure. The "United States Institute of Peace Sewer Rehabilitation Project" will ensure structural integrity to the existing brick sewer that is located under the proposed building site.

The District of Columbia's Water and Sewer Authority, DCWASA, operates 1,800 miles of sewers and provides retail water and wastewater services to its customers in the district. DCWASA is charged with ensuring reliability of the area's infrastructure. A brick sewer that was reportedly built in 1896 has been serving the area. Although it was in fair condition, with only a few repairs required in recent years along its entire length, planners decided that it needed permanent preservation to support future construction.

They also decided that slipline rehabilitation was necessary to ensure the sewer would not interfere with the future USIP facility. HOBAS centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe was the only product that met all of the project requirements. The new CCFRPM sewer is structurally sound, leak free and provides adequate capacity.

Randolph Rostas, senior project manager with the engineering firm of Metcalf & Eddy (M&E), Washington, D.C., was design engineer for the project. Project manager was Naveen Krishnamurthy from their office in Baltimore, Md. M&E engineers evaluated the existing site conditions as well as the building proposals. The approximate ground profile now exists 40 feet above the top of the existing sewer. Depending on which of the final designs for the USIP building is adopted, the proposed lowest level of the slab could be just 5 feet above the existing sewer. Regardless of the final design, the pipe to be installed had to be structurally sound, grouted in place and capable of handling the final loads.

The existing brick sewer was sliplined with 360 linear feet of 69-inch-diameter HOBAS CCFRPM pipe. The actual inside diameter of the original brick sewer varied from location to location but was generally 73 inches to 75 inches. M&E thoroughly evaluated the host pipe conditions in order to maximize the diameter of the sliplining pipe. The radial clearance calculated between the HOBAS pipe outside dimension and the brick host pipe inside dimension ranged between 0.25 inch and 1.25 inches.

HOBAS CCFRPM's efficient cross-section provides high strength with a thin wall. HOBAS also offers many pipe diameter choices for this type of application, including the 69-inch nominal diameter used on this project. The very tight fit possible with the flush bell spigot pipe connectors also contributed to maximum flow recovery.

General contractor for the USIP project, Clark Construction Group, LLC, Bethesda, Md., brought Hall Contracting, Charlotte, N.C., onboard as the installation subcontractor for the sliplining operation.

The capacity of the existing sewer in the area that was relined ranged from 103 million gallons per day (mgd) to 119 mgd, while the capacity after rehabilitation was calculated to be 101 mgd. Flow monitoring after rehabilitation on previously installed HOBAS projects showed a Manning's value of 0.009 to about 0.011. Manning's values in this range are commonly utilized to predict capacity after sliplining.

"On this particular project, we designed the sewer utilizing a predicted Manning's value of 0.011 for the HOBAS pipe," stated Rostas.

K. Michael Hall, CEO, commented on the installation, "It appeared from our 'post TV run' that the flow depths had indeed been lowered by some 10 percent to 20 percent.

"The sliplining went very well," said Hall, "and after the pipe insertion, we grouted the annular space ourselves with a lightweight grout and this also went very well. The inspection ports revealed that the grouting filled the annulus completely. Three ports were installed at the 12 o'clock position and spaced evenly along the 330-foot run. Upon inspection, the ports were solid with grout. We removed the valves and replaced the tapped holes with 1.5-inch PVC plugs."