The North Hollywood Project is presently under construction as the latest section of the LA Metro. It will add a further 10km and five stations to the metro when it opens in May 2000, making a total of 28km of underground line. For a city of 16m people the metro is still very small, however, even when this latest extension is in operation.
Los Angeles became a city of significance at the turn of the century. Public transport was first provided by the Pacific Electric Railway, having at its peak 1000 miles of track which operated from the 1920s to the ’50s. As the car gained in popularity and the city limits increased, the tramway was closed by the National City Lines, operated by General Motors and Firestone.
Traffic conditions in and around LA became notorious in the ’70s and ’80s owing to the permanent smog that hung over the city. A metro system was planned by people who had enough vision to see that it would relieve the burgeoning network of roads. Construction started in 1986 and the system now extends for 42 miles, with approximately 11 miles underground.
The metro system consists of the Green Line and the Red Line to downtown LA via Hollywood Boulevard. The North Hollywood extension runs under the Santa Monica mountains from Hollywood Boulevard through Universal City. Further extensions have been planned subject to funds being made available. Three stations and 5 miles of tunnel have to be constructed for a budget of $1.3bn. This equates to $262.4m/mile, which compares favourably with $421.1m/mile for the Jubilee Line Extension ($38.9m/mile for a light rail system).
The LA Metro Authority has a large number of members, including 13 politicians. There are also many major third parties, like utility companies, highway authorities and local authorities, which have an input into the construction programme and method of working and which must be satisfied that the construction work will have the minimum effect possible on their particular interests.
Construction is being managed by a JV of three partners with a budget of $60m for 41¼2 years which employs nine subconsultants. The JV is responsible for delivering the project to programme and budget with respect to safety, quality and protection of the environment, which is now taken very seriously. The contractors all signed up to a partnering philosophy after they had won the contract by competitive tender. Although there was initial distrust of the worth of partnering, it is now working very well.
A ‘quality system’ is employed similar to that used in the UK. Disputes are resolved by employing Review Boards which have a fixed time to decide on the validity and payment of compensation events.
Two examples of stations were described:
Starter tunnels
These were driven by hand mining methods to provide a launch area for the TBMs. The tunnels are 7.3m x 7.3m x 100m long. 150 x 150mm arch ribs were employed in conjunction with 3.6m long spiles and 4.5m long rock dowels. Elephants feet were fabricated into the ribs but this did not prevent settlement of up to 100mm under the Hollywood Freeway, caused by a combination of dewatering of the station ground and the tunnel construction. Some ground treatment was employed in the form of sodium silicate permeation grouting in the overlying alluvium. Grouting was undertaken in three stages to a maximum pressure of 0.45kg/cm²/metre of overburden.
Bored tunnels
Two 20 year old Robbins machines were refurbished for the main drives. The diameter was changed and the drive was altered from electric to hydraulic. New heads were fitted with 40 discs and a main beam supported by rear legs. The shields had a 1.2m stroke using two-part grippers which were located on each side of the arch ribs. The tunnels were finished with a delay of six months, mainly due to the poor ground conditions. Squeezing ground at the Topanga area caused delays, as did the ‘pillar failure’, which was due to inadequate tunnel separation.
The primary rib support system was designed to support the rock to a height of 1.6d, where d is the tunnel diameter. In practice, early on in the drive where pillar failure occurred, the actual ground load was 1.6D, where D is the overall width of the two tunnels including the separating pillar. The support system was therefore increased to resist the ground loads and this support was retained throughout the remainder of the drives.
Where the face was hard in the invert and soft at the crown, foaming polyurethene grout was successfully used to form an arch above the tunnel. The cutter life was good, and 265m³/cutter was achieved. Supplementary grouting to reduce the water intake into the tunnel was used when the flow rate was greater than 760 litres/min. TBM performance has seen a maximum of 40m in 24h, 139m in the best week. The overall average was 52m/week and 60m/week for the respective drives. The final in-situ lining of nominal 300mm thickness to an i.d. of 5.4m has taken a further year to complete.
Cross passages
Cross passages are spaced at 228m centres and are formed to a D shaped section with an initial layer of 150mm of shotcrete. They are completed with, first, a full geotextile drainage layer and waterproofing membrane and then a final layer of shotcrete. In areas of soft rock, blasting was not used in order to reduce vibration effects on the residential area above the tunnels. In the granodiorite, which was below a non-residential area, blasting was carried out. Cross passage excavation employing heading and bench was carried out concurrently with the main tunnel drive.
Special seismic section
Tunnelling went through the Hollywood Fault which last moved 12 000 years ago; any future movement is predicted to be as much as 2m. The solution was to build a larger tunnel through this section to allow for future realignment. A 7.3m diameter tunnel was driven through the fault region using a conventional sprayed concrete lining with lattice girders, constructed in heading and bench. In soft zones, dowels and spiles were used.
Track level rooms
Underground track level rooms were required to accommodate the signalling, ventilation, fire and power equipment. The locations for the rooms were chosen to be in the good rock where blasting was allowed. Emulsion was employed with XL initiation and up to 12 delays to maintain vibration levels to 0.25- 0.75mm/s ppv. The average consumption of explosive was 1.3kg/m³. Heading excavation was performed in three stages: centre and two side slashes followed by bench. Sprayed concrete lining was followed by a full geotextile drainage layer and HDPE waterproof lining with a 600mm thick in-situ lining.
Trackwork
The contract for the trackwork is $20m. Rail laying is finished and was progressed at an average rate of 250/week, including installation of crossovers. Rail is equivalent to UIC54. It is supplied in 24m lengths and is flashbutt welded at site to 220m lengths, which are transported into the tunnel for fixing on to plinth support and then thermit welded to form continuous track. Track is placed on resilient pads and, where required, floating track has been used in two areas. In the seismic section, a duo-block has been installed in weak concrete to allow for breakup and subsequent repair.
Conclusion
The project has employed traditional support systems and hand-mining methods in combination with mechanised TBM drives. Despite an eight months’ delay to the tunnelling, the opening date has not been put back and, by reprogramming the works, progress is now well advanced. At the time of reporting, all the track has been laid and the M&E systems are going well. Trial running is programmed for February 2000 and the opening date is May 2000. Further tunnels are planned and are required to reduce the use of the motor car and the resultant pollution levels.
Related Files
Cross Section
LA Metro Route
