Halcrow’s expertise has helped the Metropolitan Water District of Southern California to increase water-supply reliability in the region while overcoming major technical hurdles that could have brought the project to a standstill.
The company has designed tunnel lining segments for two tunnels bored through the San Bernadino Mountains, which will eventually become part of the $1.2 billion Inland Feeder project to transfer and store water from the north of the state to the south. When complete, the scheme will be able to deliver an additional 3 million cubic yards of water a day through 44 miles of new tunnels and buried pipelines.
The 16ft diameter Arrowhead East and West Tunnels, with their combined length of eight miles, are a key feature of this massive project, but at the start of tunnelling, the scheme hit problems when high pressure groundwater got into one of the tunnel excavations. The client appointed a new tunneling contractor, Shea-Kenny, to complete the job, and new tunnel boring machines were brought in that could excavate through the hard rock, but also be converted to earth pressure balance machines if water pressures got too high.
Halcrow was appointed by the tunnel lining subcontractor, Traylor-Shea-Ghazi to design a bolted and gasketed segmental lining for the tunnels that could cope with the high ground and water pressures on the job. It was anticipated that water pressures of up to 900ft would develop above the tunnel crown during the excavation through granitic and gneissic rocks.
The segmental linings were constructed directly behind the tunnel boring machines, and are the main support for the tunnel until steel and concrete pipes are constructed inside to form the permanent lining. Halcrow’s design utilises two different concrete strengths to suit the varying loads within the tunnel drives, and the segments are reinforced, with particular attention given to the radial joints where the stresses are at their highest.
The concrete lining consists of five segments plus a key, all with a structural thickness of 330mm. Straight "spear" bolts go through both radial and circumferential joints. In areas of high water flow, a grout bag was incorporated on the outside of the lining, to form an external seal between the lining and the rock.
As well as designing the lining, Halcrow also developed and witnessed test programmes for the gaskets and the radial joints. The gaskets were tested for external water pressure, compression loads and long term relaxation, and joint tests at the University of Illinois proved that the joint strength exceeding original design expectations. This allowed some of the reinforcing steel to be removed from the joints for a less congested and more economical segment design.