The Metropolitan Water District (MWD) is in the process of relining a major water feeder, and that process is currently focused on Bixby Rd in Bixby Knolls. There is a 78″ water main running under Bixby Rd that connects the Palos Verde Reservoir and the Diemer Water Plant in the Chino Hills. This pipe, referred to as the Second Water Feeder, was constructed in 1967, and 28 miles of it were constructed from pre-stressed concrete pipe (PCCP) and need to be relined with steel pipe for increased water security. You can find more information here.
Yesterday I saw a truck carrying two segments of the new pipe to the excavation on Bixby at Pine. The pipes are fabricated from tubes that have a diameter smaller than the interior of the existing PCCP. Spacers are welded to the outside so that the diameter of the pipe plus the spacer is the same as the PCCP interior diameter.
They are cut lengthwise and then compressed like a rolled up piece of paper. In the picture below you can see the overlap of the pipe on itself down in the bottom corner.
The pipe is secured in this highly compressed configuration by rectangular cleats that are welded on the inside of the pipe as you can see below.
Down in the current excavation they have a nifty machine that can pick up a section of pipe, carry it down the pipe to butt up against the last section set, and then automatically cut the interior cleats. The steel pipe expands with a terrific clang to its uncompressed diameter inside the PCCP. Welders then secure the connection to the previous section and the lengthwise split.
Eventually, concrete is forced into the gap between the pipes to further secure it, and the interior is lined with some kind of substance to protect the steel from the water.
It is a pretty neat process! A few years ago I observed while they were lining the section that ran below the LA River. I think at that point they were working on a 3000 ft segment, so they laid a lot of pipe originating at Carson and Via Oro Ave. You can find updated photos and information at this Metropolitan Water District site.