Two Successful Installations of Large Diameter Rigid-Design Concrete Pressure Pipelines in Very Soft Soils

The first pipeline installation is the 12-mile long 96-inch diameter Renton prestressed concrete cylinder pipe (PCCP) forcemain for the Municipality of Metropolitan Seattle in Washington State (Metro). One of the critical design criterions for the external loading on the forcemain pipe was the assumption that the pipe had to be designed to carry the full external load without any contribution from the backfill side support (zero side support) since the forcemain, which was generally paralleling the Duwamish River, went through very soft soils, ground water, and unstable soils at many locations. The forcemain was designed for earth covers of 10 ft to 35 ft. and internal pressures from 125 to 325 feet of head. The schedule for completion of the Renton forcemain required Metro to purchase all the pipeline material through a pipe supply contract and also award five installation contracts to five general contractors for the nine separate installation contracts. It was also a major challenge for the pipe manufacturer to mobilize a plant and keep up with pipe and fittings delivery to the five installation contractors. Metro's engineer conducted many feasibility studies for different pipe materials which led to the selection of rigid-wall pipe alternatives only, due to the very soft soil field conditions. The selection of rigid-wall PCCP amongst the pipeline material alternatives will be discussed and presented. The design and manufacturing challenges will also be discussed. The second pipeline installation is the 16 miles of 78-inch, 90-inch and 96-inch diameter Los Vaqueros and Old River reinforced concrete cylinder pipelines (RCCP) for the Contra Costa County Water District in Northern California. These pipelines generally went through soft soils with poor side supporting strength. The pipelines were designed for earth covers of 8 ft. to 40 ft. and internal pressures from 50 psi to 175 psi. Both flexible steel pipe and rigid-design concrete pressure pipe were considered for the Los Vaqueros and Old River pipelines. The rigid-pipe design assumed low side supporting strength and the flexible steel pipe design mandated wider trenches and higher compaction levels than the rigid pipe design to offset the poor side support strength of the native soils. The RCCP option was selected by all the bidders. The design and manufacturing challenges will be discussed and presented.