Shani Wallis, TunnelTalk
Los Angeles, the principal city on the west coast of the United States, has developed with a strong dependence on the automobile. So strong has been the influence of the car, that among the jungle of up to 12-lane freeways that snake across the metropolis, there is very little evidence of an effective transport system for the general public other than their private carriages. But this is to change. After years of unproductive talk, positive action on the provision of an integrated public transportation system has begun including the start of work on an underground mass rapid transit that is planned eventually to link the central downtown business district with the residential and retailing areas of the Wilshire corridor, North Hollywood, Santa Monica and Downey.
- Los Angeles County covers about 4,070 miles2 and has a population of about 7.4 million living mainly in more than 80 cities of the county. The Los Angeles metropolitan area, spilling over into neighbouring Ventura and Range Counties, is about 4,000 miles2 throughout which the main method of transportation at present is by far the private car. This is despite the fact that the Southern California Rapid Transit District (SCRTD), catering for a population of about 10 million in a 2,300 miles2 area, operates the nation's largest all-bus public transit system.
- Yet in spite of the area's impressive network of freeways, the ever-increasing number of private cars is strangling the system, causing intolerable congestion, particularly at peak times. It is exasperating to see an 8-lane freeway in the downtown area completely full and almost at a standstill in both directions for as far as you can see at 5pm every weekday!
- The need for a modern, efficient mass rapid transit system has long been recognized but covering such a vast area adequately has taken years of painstaking study by SCRTD. This culminated in a public transport masterplan integrating bus, heavy rail, light rail, subway or metro, and tramway services into a 150-mile network which is now being implemented (Fig 1).
- Metro Rail, as the high-speed mass transit railway is called, is the backbone of the masterplan. Estimated to cost $3,800 million, in 1987 prices, the system will extend for some 30 miles or 40 miles and have a capacity of more than 350,000 passengers/day. At present, buses carry about 200,000 commuters/day in Metro Rail's designated area.
- LA's Metro is based largely on the design of the Baltimore and Washington DC metro systems. There will be no eating or smoking throughout the system, barrier free ticketing, one-man operated trains with a third rail electric power pickup, and a sophisticated security system.
- Mostly underground
Of the possible construction options, tunneling is about twice as costly as elevated structures and about four times the cost of building at grade. Although both cheaper options are plausible in many areas, local community objections have forced the system underground. Metro Rail will be in tunnel for almost its entire length.
Fig 1. Transit development planned for Los Angeles County
- The 18.5ft i.d. parallel running tunnels will be approximately 20ft apart and on average about 80ft deep although this will increase to about 600ft to 700ft under the Hollywood Hills. Wherever possible, tunnels will run under streets with a minimum cover equal to one tunnel diameter and a minimum curve of 1,000ft radius. There are no mined stations envisaged since this is much more expensive than the chosen cut-and-cover option and would require the stations to be deeper than the proposed 80ft to 90ft maximum.
- Although there is hard rock under the Hollywood Hills, the geology of the area consists mainly of dense alluvial deposits and consolidated sandstone and siltstone of the La Puente and Fernando Formations with little water (Fig 2). In most areas the natural ground water table is well below tunnel level.
- Los Angeles is, however, a known seismically active center and an area with underlying oil and gas deposits both of which influence the 100-year design life of the system.
- The effect of the major earthquake on the Mexico City Metro in 1985 has shown that the principal concern in the event of an earthquake is not the underground stations or the running tunnels but buildings adjacent to stations. However, the metro system is being designed to withstand earthquakes of 7.5 on the Richter scale. Tunneling avoids known or potential seismic faults and the specified 12in thick full in-situ concrete lining in the tunnels is reinforced to a greater degree than would otherwise be necessary.
- A more urgent consideration is the high probability of methane gas. A minimum 50ft forward probe is specified during tunneling to warn of gas pockets.
- The design also specifies a full 100mm thick high density polyethelene (HDPE) between the primary support and the final lining in both stations and tunnels to prevent gas infiltration. Effective waterproofing is a secondary advantage of the membrane should any water seepage occur.
- The probe hole in conjunction with magnetometer surveys will also detect any abandoned oil well casings from the oil exploration days in the area. Another unusual natural feature is possible hydrogen sulphide contamination of soil and ground water which, if and when encountered, must be specially treated and disposed of.
Fig 2. The geology of the central area consists mainly of the alluvia! deposits and consolidated sandstone and siltstone
Who will pay?
On seeking federal approval and funding for the scheme in 1970, SCRTD was granted permission on the proviso that at least 50% of the total cost be contributed locally. The majority of this would come from the taxpayers based on an increase in sales tax. Referenda for a 1% increase in LA County failed in 1974 and 1976. A 0.5% increase, known as Proposition A, was passed in 1980 with 54% of the vote.
- This sales tax is collected by the State of California, and the Los Angeles County Transportation Commission (LACTC) in turn allocates state budget funds to the various projects of the masterplan as required. With this money LACTC has, among other things, started construction of the $690 million, 25-mile long, 21-station light rail line from Long Beach to Los Angleles which will link with Metro Rail in the downtown area (Fig 1). Work has also started on the 18-mile Century Line, a light rail line running down the meridian of the Century Freeway from the City of Downey to the Los Angeles International Airport, LAX. This line is expected to be in operation by 1993 and intersects with the LA-Long Beach light rail line south of the city center (Fig 1).
- Construction of Metro Rail had to wait until promised federal funds were received from UMTA, the Urban Mass Transportation Administration, which allocates the federal transportation budget among the many nationwide petitioners.
- It was not until 1986 that repeated requests for these funds met with success. But only $1,250 million of the original request for $3,800 million for an initial 18 mile section from LA city center to North Hollywood was approved. This limited the first construction phase to about a quarter of the original plan and although Congressional approval was granted for subsequent funding for the balance of the 18 mile initial phase, deciding just where to start was a political and administrative problem. This was resolved by identifying a 'minimum operable segment' (MOS) and starting there. Work on the Los Angeles Metro has therefore started in the central area on what is known as MOS-1, a 4.4-mile section from the central maintenance yard near Union Station to Wilshire/Alvarado Station as illustrated in Fig 3.
- To design the system, Metro Rail has engaged Metro Rail Transit Consultants (MRTC) comprising Daniel, Mann, Johnson & Mendenhall and Parsons Brinckerhoff Quade & Douglas (concerned mainly with the underground construction design); Kaiser Engineers (responsible for sub-systems); and Harry Weese & Assoc (architect). Converse Consultants and Lindball-Richter Assoc were retained for soil and seismic analysis.
- Construction management for MOS-1 was awarded to the Ralph M Parsons/Dillingham Construction/Deleuw Cather & Co JV (PDCD) in 1986.
With Federal funds in hand, contract documents were prepared and contractors were invited to prepare their bids for the eight main tunnel and station contracts that constitute MOS-1. There were no restrictions on the invitations to bid and several international construction companies, particularly Japanese contractors, have shown keen interest.
- The contracts are a combination of lump sum, unit cost contracts with no in-built contingency allowances. Any change from contract conditions must be notified by the contractor within 30 days if it is to be considered by the client. Full bonding is required from each contractor before its bid is valid.
- The contracts also contain the 'Buy America' clause whereby materials for permanent fixtures such as concrete, steel and HDPE membrane must be bought from US suppliers if available. This does not apply to tunneling machines and equipment as these are temporary.
- Contractors must also make 'good faith efforts' to comply with the Disadvantaged Business Enterprise (DBE) and Women's Business Enterprise (WBE) clauses if their bid is to be eligible for contract award. These stipulate that 10% of the construction crew be from ethnic minority groups and another 4% be women.
- As there were several large hotels adjacent to the construction sites, work was limited by contract to two 10h shifts/day, five days/week with a 4h maintenance shift on Saturdays.
- LA trams of yesteryear
Trams, known as Red Cars, used to travel the streets of LA
- LA residents were not always so dependent on the private car to get from A to B. In the late 1800s the Pacific Electric Company began the introduction of an extensive tram system which was electrified in 1925. In 1940, Pacific Electric Co operated some 1,100 miles of electric tramways throughout the Los Angeles metropolitan district running both above and below ground. It was in the 1950s that the people of Los Angeles became so attracted to their private automobiles that numbers taking the trams declined, making it an uneconomic system. The tramways began to be dismantled and the last tram left the streets of LA in 1957.
- A thorough site investigation costing some $5 million was carried out for the underground works and these documents were made available to all bidders. Contract documents give each bidder a large degree of freedom in choosing his own tunneling method although drill+blast is prohibited in this urban area. To date, contractors have chosen open-faced tunneling shields for which Metro Rail has specified that overcut is limited to 3/8in, that the shield length is 0.9m of the diameter unless articulated, and that the machine is capable of fully breasting the face. Tunnel lining is a contractor's choice between either precast concrete segments or ribs and lagging followed by an in-situ concrete lining. Specified tolerance on line and level is 4in on tunnel diameter.
- There is a Value Engineering Cost Proposal clause in the contract whereby contractors can put forward more economic methods than those proposed by the Engineer. If such a VECP is accepted, the cost savings are shared equally by client and contractor.
- The ground conditions along the tunnel line are generally very competent with a good stand up time. Good enough many say for an NATM approach, but no VE proposal for the NATM was submitted despite the fact that the NATM can reduce construction bid cost substantially. James Crawley, director of engineering for Metro Rail, believes this is because US contractors are not prepared to take what they see to be the risks involved in using the method under major buildings. "We may see VECPs for the NATM for future contracts," he said, "more likely for those tunnels under the Hollywood Hills where the rock is stronger, the overburden is greater and the number of buildings over the tunnel line fewer." It was also stated that shotcrete had been eliminated as temporary tunnel support because of long-term seismic considerations.
- Primary lining of the twin tube running tunnels is followed by the specified HDPE membrane and a reinforced in-situ concrete final lining. Most contractors have chosen railed muck hauling systems with muck disposal subcontracted out.
- The five stations on MOS-1 are sub-surface constructed by cut-and-cover. Each contractor is again free to choose its method of excavation support and most have chosen soldier piles and timber lagging. Diaphragm walls, another possibility, are more expensive and not necessary in the prevailing ground conditions.
- Of these five stations, three are on city street right of way and two, Union Station and Wilshire/Alvarado, are on private property. In the case of Union Station the private owners, three major railway companies, have become actively involved in the planning and design of the stations and will contribute financially to the construction in return for the obvious benefits they will receive as a result. "Wherever possible Metro Rail will encourage private business to participate and contribute to the cost of the system," said Sam Louis, acting director of construction management for Metro Rail. To date, some $10 million from private enterprise is at work in MOS-1 with much more currently being negotiated on future phases," he added.
Fig 3. Construction begins with the Minimum Operable Segment of five underground stations and 4.4 miles of running tunnels
- Of the $400 million allocated for construction of MOS-1, $250 million to $300 million worth of contracts have been let. These include six of the eight main tunnel and station contracts, the main particulars of which are listed in Table 1.
- As has been the trend over recent years, competition for the construction work for the LA Metro has been fierce. As many as ten bids have been received for the same contract, many submitted by groups of contractors and JVs. Of the bids received for the six major construction contracts let so far, only a few have been over the Engineer's Estimate. The winning bids, usually the lowest, have been well below the Engineer's Estimate. Notice to proceed has generally been granted within four weeks of the bid opening date. Like Singapore, Los Angeles may well acquire its metro system at a substantially lower price than originally budgeted for even though the construction methods are more expensive than those used on other systems (Fig 4).
- The maximum distance between two stations on MOS-1 is 5,000ft and since each station has a ventilation facility, no intermediate vent shafts are required. Any intermediate shafts sunk by the contractor to facilitate construction must be backfilled.
One of two Mitsubishi shields built in Japan to be used by the Shank-Obayashi JV on its two tunneling contracts
- As has been mentioned, the water table is mostly well below the tunnel level except in the Union Station area where it rises to within 20ft to 25ft of the surface. To facilitate open-cut station excavation, the water table has been lowered to between 60ft to 70ft below the ground surface, 4ft to 5ft below the station invert, using deep well vacuum pump dewatering wells. The total amount of dewatering expected is about 4,800gal/min. However, this is hydrogen sulphide contaminated water and must be carefully treated before it is discharged into the storm drain system and then into the Los Angeles River.
- Settlement is a potential threat to the many high rise and some historical buildings along the route of MOS-1 and is being monitored continuously.
- Public utility mains in the area, some of which are very old, also need protection.
- Construction work on MOS-1 started early in 1987 and is expected to take six years to complete, the section coming into operation in early 1993. Preliminary design work for MOS-2 which will extend the line from Wilshire/Alvarado across to North Hollywood is well under way but the start of construction must wait until the next allocation of Federal funds which is not expected until June 1988.
- There is also a problem with the alignment of MOS-2. Originally the line was to proceed down Wilshire Blvd making a right hand turn into Fairfax St and continue toward the Hollywood Hills to the North Hollywood terminus. However, in the summer of 1985, an accumulation of methane gas which exists naturally in the Wilshire/Fairfax area ignited. The explosion shattered windows and gas jets were seen flaming from cracks in the ground.
Fig 4. Capital cost comparison of LA Metro and other US transit systems
- This caused considerable local concern and after long debate in the US House of Representatives, the Federal Government put an injunction on the proposal to construct the metro through this area in tunnels for safety reasons. If the line was to continue on this route, it would have to be elevated which is publicly unacceptable. Additional studies are currently under way to resolve this alignment problem.
- Future extensions to the system, west to Santa Monica and southeast to Downey, are programmed for construction in the late 1990s with the entire Metro Rail system as it stands today being in operation by about the year 2005. Tunneling and underground excavation will play a significant role in the realization of the full system and over the construction period TunnelTalk will report on progress and the underground work involved.
|Table 1. List of contracts and contractors for MOS-1 of Metro Rail|
|Contract & description||Contractor||Bid price||EE*||Method||Start/Finish|
|A130 2,700ft approach from Union Station to yard||Yet to be bid||-||$45 million to $50 million||Open cut Dewatering required||June 1988 to June 1990|
|A135 980ft long||Bid opening in April 1988||-||$55 million to $60 million||Open cut Dewatering required||May 1988 to April 1991|
|A141 570ft long Civil center Station and 5,900ft of twin tube running tunnels from Union to 5th/Hill Stations||Tutor-Saliba/ J Groves JV||$61.5 million||$76.3 million||Open cut station Robbins shield manufactured in Oregon arrived in
Ribs and lagging support
|March 1987 to April 1990|
|A145 850ft long 5th/Hill Station||Guy F Atkinson||$38.7 million||$40.9 million||Open cut Piles and lagging||April 1987 to February 1990|
|A146 2,100ft of twin tube running tunnels from 5th/Hill to 7th/Flower Stations||Shank-Ohbayashi JV||$18.2 million||$20.5 million||Mitsubishi shield from Japan arrived November 1986. Ribs and lagging support initially with concrete segments subsequently||April 1987 to May 1989|
|A165 670ft long 7th/ Flower Station||Granite Construction Co||$42.7 million||$47.2 million||Open cut Piles and lagging||January 1988 to September 1990|
|A171 5,000ft of twin tube running tunnels from 7th/Flower to Wilshire/ Alvarado Station||Shank-Ohbayashi JV||$26.3 million||$38.5 million||Mitsubishi shield from Japan arrived in October 1987 Concrete segmental support||February 1987 to July 1989|
|A175 900ft long Wilshire/ Alvarado||Tutor/Saliba/ Perini JV||$23.4 million||$31.5 million||Open cut Piles and lagging||July 1987 to March 1990|
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