Printer-friendly versionHat Trench
While we are in support of the HSR project, we are sensitive to the potential impact of HSR in terms of noise, visual quality & aesthetics, community impacts, cost and long term flood hazards. In the spirit of creative civic imagination and to best address these issues we recommend that the CHSRA investigate an alternative, which we call a Hat Trench (Hybrid, Adaptive and Tiered Trench). A Hat Trench is a two-level tunnel with the lower tier dedicated to HSR and Baby Bullet, and the upper tier for local CalTrain and freight. The top of the upper tier, which is open to the air, may be fitted with various keystones (an arched cap). These keystones may be mass customized according to need (air exchange/security, street bridge, walking/bike bridge, light rail right-of-way, leasing of air-rights, stream crossing...).
A Little Civic Imagination
As we studied the various classic grading implementations, we found weaknesses in each alternative. An above-grade 4 track system, at-grade and a traditional below-grade trench would have the impact of dividing our community due to walls, fencing and trench gaps. A traditional tunneling alternative will most likely prove too costly. Furthermore, potential sea level rise across the next two centuries means adaptivity must be incorporated into any design.
From a requirements' perspective, the question is which design would be quiet, provide a low visual profile, allow for venting of diesel exhaust (freight, local CalTrain), require minimal or no community barriers (fencing, walls, wide trench), minimizes loss of private property via eminent domain and can be sealed at some future date to protect against flood? We support investigating a solution that combines a tunnel and a trench into a single unit, a Hat Trench.
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Trench (Reno) Tunnel (metro) Hat Trench
Keystones
The tunnel top may be open to allow for proper venting of exhaust. To maintain structural integrity this top is replaced with a structural arch/vent to act as a keystone. This vent keystone may then act as a structured backplane which may be then adapted to provide site specific requirements, such as road, bike and pedestrian crossings, right-of-way for light rail or as a foundation for buildings which are granted air-rights. Central fabrication and mass customization of keystones would allow for faster and more cost effective built-outs. The Hat (keystone) is kept to a length of less than 40 feet, thus allowing for intermodal transport (ship, truck, rail), thus allowing for diversification of suppliers, thus pushing the price per unit down. If sea levels rise, the upper tier may be caped with a sealed keystone, thus converting the HAT Trench into a dual tunnel design.
The Hat Trench provides a viable solution to many of the challenges for a grading strategy for the SJ-to-SF segment. We encourage CHSRA to invest design and engineering resources to properly investigate this option. We believe it may provide a viable cost effective alternative to more traditional grading strategies, protect the long term investment by providing an adaptive design, while meeting the needs of the local communities in terms of noise abatement and minimized visual impacts. Finally, a HAT Trench design unites rather than divides our communities.
Dimensions, Details and other Specifics
The diagram is a sketch, which leaves a lot to your imagination and your own invention. Should the walls be concave or straight? Is a central support needed on the lower tunnel? What are the specific dimensions? How can this be constructed using the current ROW (Right of Way)? What about the need for a shoofly during construction? What is the construction foot print? Below is a sample of some of the discussions needed to work through these issues.
Source: http://cahsr.blogspot.com/2009/05/how-does-redwood-city-define-hsr.html
Pros and Cons Summary
- Low Sound Profile: Sound from local CalTrain and freight are partially reflected back into the trench, with some sound being reflected skyward at a high angle.
- Low Visual Profile: As the Hat Trench is below grade, line-of-site impacts are small. A small footprint of the opening (keystone) will be seen when crossing over via a bridge.
- Narrow Profile: With two tracks stacked on top of each other the width is much less then the four track (~75 foot) alternative.
- Preserves existing homes and buildings.
- Minimizes need to cut down existing trees.
- Mitigates impacts to existing rail stations.
- Lowers Cost
- Decreased cost for acquisition of eminent domain rights.
- Ability to recoup some cost via sale of air-rights (right to build on top of a right-of-way.
- Lower cost build-outs, due to off-site central construction of keystones (top of the trench).
- Lower cost relative to a tunneling solution.
- Lower cost for bridges (road, bike, pedestrian) gives communities greater flexibility.
- Unites our Cities
- Increases the number of rail crossings.
- Replaces current above grade walls (think Belmont).
- Costs versus Alternatives: Construction cost differences against at-grade or above-grade still need to be determined.
- Some Added Costs:
- Cost to move underground utilities under current CalTrain tracks.
- Environmental issues surrounding any trenching operation.
- Adaptability: If sea levels rise, investment is secured, as future generations may cap the tunnel, forming a dual tunnel design, if necessary.
- Unites: Decreases division of cities by moats, gates or walls, while providing more opportunities to construct buildings and bridges across the current CalTrain right-of-way.
- Risks
- Construction Footprint: Although the final width of the HatTrench may mitigate impact to nearby buildings, during the build-out accommodations may be needed if CalTrain/freight continues service.
- For a comprehensive list of issues surrounding 'cut and cover' construction, please see: Table 2 here http://www.arup.com/_assets/_download/download235.pdf
What would the dimensions of the Hat Trench be? That's not too hard to figure out. It's driven by (a) vertical clearance requirements, and (b) tunnel cross section requirements, for aerodynamic reasons.
(a) for lower-story HSR, built to the shorter European UIC loading gauge, you need about 17 feet from top of rail to the overhead wire. Add another 4 feet of clearance from the high voltage wire to the tunnel ceiling. Under all that, assume about 6 feet of tunnel wall, track bed, and the track itself. Above all that, assume 5 feet of deck + track bed thickness to support the heavy trains above on the second story.
(b) To run trains in opposing directions at closing velocities of 250 mph, you need some aerodynamic buffer space both for vehicle stability and passenger comfort. Two-track tunnel sections would need about 80 to 100 m2 (800 to 1000 square feet) of open-air cross sectional area for 125 mph operation. That is probably a far more stringent constraint than (a) and may drive the height of the "downstairs" tunnel higher than I first described.
Then, above all that, add AAR Plate H or K clearance for freight trains (20 feet), and about another 10 feet of catenary poles and wires.
The depth of such a structure would be at least 50 feet from the top of a freight train (assumed level with the surrounding grade) to the bottom of the foundation. Wires and poles would stick up another 10 feet above ground.
Then, add the complication of ventilation and passenger evacuation. Then add the complication of periodically connecting the two levels so trains on the lower level can access stations (or do you double-deck stations too?)
In the end, the Hat Trench sounds an awful lot like this. But hey, kudos for trying to come up with solutions!