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LA story Third phase of the LA Metro extension looks set to produce some good news on a scheme known for churning out bad. James Macneil reports.

When work began on the North Hollywood extension of the Los Angeles Metro three years ago, there was immense pressure on the joint construction managers, JMA joint venture and Parsons/ Dillingham joint venture. Not only did they have to construct 7.5km of dual-bore tunnel through both soft and hard ground and build three stations - one directly beneath Hollywood Boulevard - they also had to ensure that the project was completed on time, on budget and without a whiff of corruption.

The previous phases of the project - that aims to put a modern public transport system among the perpetual congestion of LA's freeways - were beset by technical problems, including a tunnel collapse, and allegations of financial mishandling. Parsons/ Dillingham had been involved in the earlier parts, and JMA - made up of Hatch Mott MacDonald (the US arm of Mott MacDonald), Jacobs Engineering and ACG Engineering - was brought in as joint construction manager to aid the recovery of the project.

Bolstering the team seems to have worked. The third phase of the project looks set to produce some good news on a scheme known for churning out bad. In late June, tunnelling was completed on the $1.3bn project, and Alastair Biggart, chief executive of the JMA joint venture, predicts that the project will hit its 17 May 2000 opening date. 'And,' he adds, 'even more amazing for LA, we are going to end up under budget.'

The JMA joint venture's slice of the work involved driving the metro line underground from Hollywood to the San Fernando Valley. The 3,840m long, 5.4m finished internal diameter twin tunnels begin from where tunnels constructed in the previous phase of the scheme were terminated, and at the point where ground conditions change from alluvial to rock. They head north beneath the Santa Monica Mountains at depths of up to 275m to connect to Universal City Station. From there, the Parsons/Dillingham joint venture has taken the tunnels on to the North Hollywood terminus. JMA's construction management package also includes two stations, Hollywood Highland and Universal City, and the lining of tunnels bored by a previous contractor at the Hollywood Highland station.

JMA's section passed through an array of ground conditions in driving from Universal City towards Hollywood Highlands. The tunnellers first encountered Topanga Shales, and then passed through slightly harder sandstone and basalt, and basalt breccia or leached basalts which also were not particularly hard. Tunnellers then passed through more weak sandstone materials before hitting a granodiorite, which is about 10 times stronger than elsewhere.

This combination of variable strength materials meant that the tunnel boring machines had to be able to excavate in both soft and hard materials. Biggart says this is unusual. 'The face of the TBM was well plated,' he says. 'Normally, on a rock cutting machine, the face would be more open because the ground is usually assumed to be self-supporting.'

The two TBMs were made by Robbins. Both machines were second-hand, but were substantially refurbished. 'As far as the working parts were concerned, they were new machines,' says Biggart. 'They changed the diameter, they changed the type of drive from direct electric to hydraulic, they changed all the motors. All that was left were the substantial metal parts that weren't affected by the change in diameter.'

The excavated rock was supported by 150mm thick steel ribs, built inside the machine as it moved forward. A permanent insitu concrete lining incorporating the track bed and an escape walkway was installed after the TBM had passed. To stop methane gas penetration, tunnels were lined with heavy duty polyethylene sheet, with steel reinforcement laid over this, and a secondary 300mm thick insitu concrete lining formed.

The tunnels are designed to establish a hydraulic gradient within the ground, and residents of the mountains above were concerned about this draining their natural springs. 'That's exactly what happened, and we expected that to happen,' says Biggart. As a temporary solution, the springs have been connected to the local water mains. But Biggart expects that the springs will begin to recover once the tunnel has its concrete lining, and return to normal within two to three years.

Blasting for the cross tunnels also concerned the residents, but Biggart says that there have yet to be any complaints. 'Conventional wisdom says that vibrations should be limited to 50mm/s. The specification limited that further to 12mm/s, and then we further limited it to less than 1mm/s. That is the lower limit of human perception.'

Both stations are cut and cover construction. The Hollywood Highland site was complicated, as it had to be constructed directly below Hollywood Boulevard. This was done by placing soldier piles at 1.5m centres along either side of the road, working from just one lane. These soldiers were grouted in place, and capped with beams on either side of the road. During a subsequent three week operation, the 210m length of the station beneath the road was excavated to a depth of 3m, spanning beams were placed between the pile caps, and the road surface reinstated on concrete planks on the top. The contractors were then able to work undisturbed beneath the road while traffic ran smoothly above.

In contrast, the Universal City Station was on a vacant site. This made construction simpler, as it could be serviced by cranes sitting on the side of the box, whereas at Hollywood Highland the contractor had to use runway beams and hoists to handle materials.

Both stations are designed to comply with seismic codes. This meant that the joints in the station boxes are much stronger. 'A lot more of the bars in the boxes are hooked than if you didn't have to allow for earthquakes,' says Biggart.

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