The Balboa Terrace Trunk Sewer is unique in that it is only one of a handful of tunneling projects designed and built by the City of San Diego.

This project consisted of 2700 feet of open cut trenching and 1650 feet of tunneling for a 21 inch PVC SDR26 and 20 inch of CCFRPM (fiberglass pipe) trunk sewer.

The 21 inch PVC was installed from 12 to 23 feet in depth within roadways and as close as 15 feet from three story condominiums. This trenching was accomplished with conventional and slide rail shoring. One section of the 20 inch fiberglass pipe was installed in a conventional 760 feet x 48 inch jack and bore casing and the remaining 890 feet was installed in a 72 inch rib and lag tunnel. Both the casing and rib and lag tunnels were required due to the proximity of buildings, infrastructure and depths ranging from 20 to 60 feet. 

What sets the tunneling project apart from most other tunnels is that a 300 feet length was built with a 500 feet radius, which required the 72 feet tunnel boring machine to deflect as it progressed. Another unique feature of this project was 52 feet deep structure at the intersection of two tunnel headings. This intersection also doubled as an access point for the construction of the tunnels, so a concrete secant pile system was selected that produced a solid, maintenance free, water tight shoring. The secant shaft was constructed with 30 inch diameter, cast in place piles, that were drilled to a depth of 57 linear feet. The the interior was excavated with a combination of excavator and mini excavator/breaker resulting in a 23 inch inside diameter and 52 feet deep concrete compression ring. This access shaft remained in use for 7 months and facilitated construction of the tunnels and the cast in place structure that was built within it.

Another important aspect of this project is it was required to be built with three different headings. Because the tunnels are notoriously time consuming, two tunnels approximately 1000 linear feet apart were in progress, while the open cut section between them was built in advance. This normally out of sequence operation saved at least 5 months and the expedited construction, resulted in on time delivery of the new trunk sewer.