Grossmont Hospital Sewer Main Upgrading


Underground Construction

Ledcor, of San Diego, was contracted to rehabilitate and replace more than 600 feet of sewer main that serviced Grossmont Hospital's main campus, along with Grossmont Medical Terrace and the Grossmont Medical Arts Building. The existing sewer main descended in elevation 113 feet down a canyon and discharged into the Alvarado Trunk Sewer Main on the opposite side of Metropolitan Transit's dual rail trolley line. 

The Alvarado Trunk is installed below the Alvarado Creek, a wetland protected by the California Department of Fish and Game and Army Corps of Engineers. Environmental permitting and compliance to strict tolerances were essential in maintaining the stability of the wetland and reconstruction of the Alvarado Creek channel.

The new sewer main was installed under the MTS dual rail line by means of directional bore (micro tunneling). A 36-inch-diameter steel casing was installed to shield the bore and provide accessibility for hand mining if unforeseen obstruction or refusal from bedrock would be encountered. 60;

Design and installation for the bore location was challenged by overhead utilities, multiple underground utilities, and afluctuating ground water table. Underground utilities consisted of a 42-inch water transmission main; two additional low pressure water mains; and a 3-inch high pressure, natural gas service main. The underground utilities conflicted not only with bore alignment, but also with slope percentages required to meet the fixed invert elevation of the existing Alvarado Trunk discharge manhole. Design could not be coordinated around the existing underground utilities. 

There were two options: Relocate underground utilities, or intercept the original sewer alignment and continue into the existing Alvarado Trunk. 

Ledcor Construction intercepted the existing ductile iron, concrete-encased sewer main and followed the existing alignment into the receiving pit. The extensive local and remote sewer bypass systems maintained uninterrupted service to all the facilities on Grossmont Hospital's campus. 60;60;60;60;60;60;60;60;60;

The $775,000 three-month project is just finishing up. Scott Ludwig was the project engineer for San Diego-based Ledcor Construction.