DALLAS TX:HOT tunnel to be mined

“TxDOT is likely to recommend mined tunnels,” McGregor says. “Operations costs are the same. We’ll save two years. There will be less staging, less worry about utilities, fewer complications in building through the foundations of interchanges with. It will have fewer impacts. I think it’s an all-round better deal for everybody to mine it.”

The northern part of Dallas is good tunneling country. This has been demonstrated already in mined tunnels under the Addison municipal airport constructed by the North Texas Turnpike Authority, experience with drainage tunnels and a station for the DART passenger rail. No elaborate tunnel boring machines are needed and probably no blasting. The tunnel medium is limestone named Austin chalk. Tunnels can be excavated with a roadheader (which has a grinder headed boom) and haul trucks. Rockbolts plus shotcrete will bear the loads with a concrete liner acting largely to contain water and provide a neat appearance.

Costs

The mined tunnels will be in the section straddling the Dallas North Tollway between Preston and Midway roads. Construction cost for minimal length tunneling is estimated to be $160m for tunnels of 2,400m (1.5mi) and (3,000m) 1.9mi. If both tunnels are taken outboard of Preston and Midway they would each be 3,350m (2.1mi) and $189m. (Cost is about $10m/lane-km $16m/lane-mile). An earlier scheme omitted the breakdown shoulder, but MacGregor says the cost premium for the shoulder was $30m, and he thinks it provides a “greater comfort level,” and is worth the extra cost. At the deepest point under the Dallas North Tollway interchange the managed lanes would be about 25m to 30m (90' to 100') below the surface.

Each tunnel will provide 3x3.65m (12') travel lanes and a right hand breakdown shoulder of 3m (10') with a 0.6m (2') lefthand offset. There will be narrow emergency and staff walkways on either side, and a U-turn area in the middle for emergency and official vehicles. Jetfans will ventilate the tunnels by pushing the air in the direction of the traffic. The basic shape will be a stretched horseshoe.

The cost provides for an ITS system that avoids in-pavement devices. It will include full video surveillance and overhead radar devices to measure traffic speed, numbers, roadway occupancy and vehicle classification, and variable message signs. Some emergency vehicles specially assigned to the tunnel will be located at an operations center being combined with a TXDOT center for the Central Exwy (US-75).

Tolling will be by transponders with camera backup. MacGregor says he is hopeful that by the time the facility opens there will be a workable system of vehicle occupant counting so HOV eligibility can be assessed automatically. Various infrared systems are being tested in Georgia and Texas.

In sections of the project there will be managed lanes in structure – a combination of trenched but unlidded, trenched and half-lidded with unmanaged lanes cantilevered over, and some sections of fully lidded cut-&-cover doubledeck. In the approaches to the I-35E and US-75 interchanges the unmanaged lanes will be atop the free lanes in the center and in most of the rest of the western section the managed lanes will be on the outside under the frontage roads. A total of 4500m (2.8mi) will be in structure.

20 lanes

The freeway is presently 2x5-lanes with the inner pair of lanes HOV2. Plans are to enlarge the basic roadway in major sections to 20-lanes: 3F/4/3T/3T/4/3F, 3F/5/2T/2T/5/3F, and 3F/5/2RT/5/3F with up to 14 mainline lanes grade-separated and 6 frontage (F) road lanes. The mainline lanes are 2x5 and at the ends 2x4 unmanaged lanes with various configurations toll-managed (T) 2x3, 2x2 and 2-lanes reversible toll (RT) lanes.

The toll-managed lanes will be managed with tolls (and other eligibility criteria) to assure free flow traffic and efficient throughput, as compared with the unmanaged lanes which will be the normal free for all. A long period of ‘public involvement’ with local officials, interest groups and the public led to the principle that a mix of managed and unrestricted lanes would be used throughout the corridor.

The precise form of tolling has yet to be determined but TxDOT has submitted a proposed set of studies to the FHWA for support under the value-pricing program. Sometimes the project team has referred to the toll-managed lanes as HOT lanes, but MacGregor says there is no commitment to standard HOT lanes in which high occupancy vehicles are always free. A presentation he gives says merely that the managed lanes should encourage HOV usage in peak periods and that tolls for road use could vary with the level of congestion, the time of day or the vehicle type.

MacGregor says it may be possible to use other criteria of eligibility for the managed lanes. He prefers the general term ‘managed’ lanes since he sees pricing as just one management tool. Important service vehicles might get some rights to their use.

Along most of the length of the LBJ there is little directionality of traffic – the far eastern end being an exception. The western end traffic is heavily oriented to Dallas-Forth Worth Airport. Peak hours on the LBJ are already heavily spread, and travel times vary considerably from day to day. Traffic is in the high 200ks/day in many sections with the present 10-lanes.

Big IC at US-75

About equal in cost to the tunnels (at $210m) is the LBJ/US-75 interchange which will provide special ramps for the managed lanes both directions on both roadways. To the south on US-75., or Central Expressway as it is often known, they will end in some kind of park-&-ride at the DART rail line, but will continue north some distance on US-75. The 635/75 IC is going into detailed design and construction is set to begin spring 01 for a 6.5 year construction period.

The whole project forms something of a partial belt route at least in the east. Looking counter-clockwise it will go from US-80 at 3 o’clock over I-30 at 2:30 and US-75 at 1 o’clock over the Dallas North Tollway at a bit before 12 and I-35E at 10:30. The western section is more or less east-west and the whole is some 12km or 7 miles closer in to the center of Dallas than the President George Bush Turnpike – now actively under construction to the north.

Cost of the whole LBJ Fwy/I-635 project over its 34km (21mi) is estimated at $1.5b.The study process began in April 1987, ran into some opposition and fizzled. A new team with a new approach began work in 1994. By 1998 there was agreement on the concept of a mix of free and managed lanes. There have been preliminary toll and revenue studies and reports on the tunnel options. TxDOT endorsement and environmental assessments are being sought this year. MacGregor says that detailed design and contracting will take 2 years.

Construction and staging of traffic through the works will be greatly simplified if the main tunnels can be opened at the same time as the big 635/75 IC, scheduled for late 2008. So MacGregor is working to get the main tunnels under way quickly enough to be on the same finish date as the IC. (Contact Matt MacGregor LBJ Project Office 972 437 0101 mmacgre@dot.state.tx.us)

Rail again

Rail enthusiasts want to build a DART rail line in the LBJ corridor. The structured and tunneled roadways are being positioned to leave an underground right-of-way clear for such a rail line.

MacGregor says his opinion is that it will be more effective to make use of the managed lanes for bus-based transit. The LBJ will be linked to a whole network of other managed lanes being designed by the North Central Texas Council of Governments. Buses on these managed lanes should be able to provide good travel times, direct routes, and at the trip ends they offer greater convenience than station-based rail with its need for shuttles.

Cost arithmetic: If the LBJ reconstruction is providing 435 lane-km (275 lane-mi) at $1.5b cost is $3.5m/lane-km or $5.5m/lane-mi. These big ’arted Texans ‘ll throw in their frontage roads for free.