A notch above

A westbound BNSF Intermodal train approaches the Cascade Tunnel East Portal.

The “who’s who” of projects, in civil engineering, according to the American Society of Civil Engineers, includes iconic projects such as the Eiffel Tower, Golden Gate Bridge, Machu Picchu, Panama Canal, and the Statue of Liberty. More technically known as the Historic Civil Engineering Landmarks, the 260-plus honorees have developed, advanced, and transformed the United States, the world, and the profession of civil engineering. Not all of the landmarks have achieved the same level of public fame, but all have equally compelling stories. One such story is of the Stevens Pass Railroad Tunnels in Washington State, one of which is the Cascade Tunnel, the longest railroad tunnel in North America.

The Stevens Pass area in the Cascade Mountains provides the gorgeous yet treacherous story backdrop. In 1890, John F. Stevens discovered Stevens Pass. Stevens than managed ensuing rail development, ascending to chief engineer of the Great Northern Railroad in 1895 and president of the ASCE in 1927. 

The act of crossing mountains has always been a complex engineering challenge, and this particular endeavor spanned three phases:

Phase 1: 1892 — This phase comprised construction of an interim system of switchbacks to negotiate the steep mountains. (A switchback is a portion of railroad track that has multiple sharp turns or changes in direction, used when traversing an elevation by use of a straight track alignment is too steep for rail vehicle operations.) 

Phase 2: 1897 to 1900 — This phase comprised construction of the first Cascade Tunnel through the mountains and the supporting switchbacks, forming a more direct and efficient route. The route was 9 miles shorter than the previous route. The 2.6-mile tunnel and switchbacks reduced the risk of train slippage, problems in uphill climbing, and problems in downhill braking. The tunnel and switchbacks have grades of up to 2.2 percent or 1.3-degree angle inclines, which is the standard maximum on mountain railroads and a significant improvement on the interim switchback grades of up to 4 percent or 2.3-degree angle inclines. 

Cascade Tunnel East Portal

The tunnel also reduced operating expenses in adverse weather conditions. The area is prone to severe winter snowfall and avalanches, which obstructed uncovered switchback tracks. When relying on switchbacks, maintaining train service required maintaining expensive snow sheds to protect switchback tracks. The tunnel reduced reliance on switchbacks, which reduced the number of snow sheds and the associated maintenance.

The tunnel, however, posed a dangerous health hazard by enclosing the poisonous fumes from the coal-powered trains of the time. The tunnel was electrified in 1909 to avoid this issue and the coal powered locomotives were replaced with electric ones. 

Phase 3: 1925 to 1929 — This phase comprised construction of the current Cascade Tunnel through the mountains, circumventing the first Cascade Tunnel and switchbacks and forming the most direct and efficient route. The route is 8.7 miles shorter than the previous route. Until 1989, the current 7.8-mile Cascade Tunnel was the longest tunnel in the Western Hemisphere and is still the longest railroad tunnel in North America. The tunnel has a grade of 1.6 percent or a 0.9-degree angle incline, which further reduces the risk of train slippage, problems in uphill climbing, and problems in downhill braking. The tunnel more or less eliminates reliance on switchbacks, which further reduces operating expenses in adverse weather conditions. 

Modernizing a historical landmark

The robust design, engineering, and construction of the current Cascade Tunnel established it as a main cargo link to and from the Port of Seattle, operated by Burlington Northern Santa Fe Corp. (BNSF Railway). The beauty of the surrounding Stevens Pass area and the Cascade Mountains established the Cascade Tunnel as a landmark on Amtrak’s Empire Builder scenic route.

The hearty tunnel has also aged gracefully, as it has only required routine maintenance and accommodations to evolve with the rail industry. In 1956, a new ventilation system was installed to allow diesel locomotives to pull straight through the tunnel. In 1984, a crown-cutting project created notches in the tunnel crown to allow for the additional height of double-stack container cars. 

Project professionals set up a mobile LiDAR GPS observation control point at the East Portal.

For modern day projects in the Cascade Tunnel, a major engineering challenge is to obtain access to the tunnel for surveying, engineering, and construction work. BNSF Railway and Amtrak depend on the tunnel as a main cargo and scenic link for the Port of Seattle. The heavy traffic restricts any work on the tunnel to one or two hours at a time. 

In recent years, freight train cargo, especially double-stack container cars, have been making unintended and unwanted physical contact with the tunnel crown. In 2012, BNSF Railway started to contemplate improving clearance in the Cascade Tunnel, but obtaining access to the tunnel for the project was indeed the major engineering challenge. Surveying the track with conventional methods would take weeks, if restricted to one or two hours at a time. 

With this challenge in mind, BNSF contacted J.L. Patterson & Associates, Inc. (JLP) to develop an innovative approach to study and model the tunnel. Partnering with mapping subconsultant, Surveying and Mapping, Inc., JLP engineers created a 3D model of the existing 7.8-mile tunnel. Instead of using the conventional method of two separate processes of surveying and modeling, the project professionals created a new method that combined the surveying and initial modeling into one process. The new method combined conventional survey control and LiDAR analysis to simultaneously survey the track and collect the needed information to model the tunnel interior, effectively mapping a 3D representation of the entire 7.8-mile tunnel.

New technologies provide new solutions

LiDAR uses lasers to reflect light off of a target surface in pulses and light sensors to measure the distance to those target surfaces. The LiDAR equipment traveled through the entire 7.8 miles of tunnel to effectively map the track and the interior surfaces. The LiDAR equipment was mounted onto a high-rail vehicle able to traverse the track on a route that passes control targets every 1,500 feet. The setup provided a data density of up to 150 measurements per square foot, generating millions of 3D points per minute. 

In one short hour, the setup collected all of the data necessary to map a 3D representation of the entire Cascade Tunnel. The LiDAR setup was not only quick but also minimally disruptive, operable day or night, and operable even in a tunnel of Cascade’s length and depth, which would render conventional GPS equipment inoperable. 

With the point data, project professionals created the following workflow to determine and analyze the tunnel’s current condition:

1. Process data

2. Analyze data

3. Generate existing tunnel 3D surfaces

4. Analyze existing track alignment

5. Analyze existing notch alignment

6. Propose best fit track alignment

7. Propose additional notching

8. Conclude with best and most economical total project plan

Using Bentley software tools, project professionals mapped a digital 3D representation of the tunnel interior, located the exact centerline of the track alignment, and located the notches in the tunnel crown that accommodated the height of double-stack container cars. The track centerline revealed that the track alignment had slightly shifted from decades of service and maintenance, causing the track alignment to not perfectly line up with the notches in the tunnel crown. Furthermore, in one particular segment of track, one rail of the track was, in the worst case, a few inches higher than the other. As a result, trains, especially double-stack container cars, could rotate. This could contribute to the unintended and unwanted physical contact with the tunnel crown. With this existing condition information, JLP now reviewed the initial proposed solutions to either realign the track or re-notch the tunnel crown. 

The 3D representation of the tunnel helped predict the cost and operational impacts of both options in advance. Realigning the track proved to offer substantial projected cost and operational advantages compared with re-notching the tunnel crown, BNSF Railway’s original plan. If only re-notching along the current rail alignment was undertaken, it would have required removal of 177 cubic yards from the tunnel crown at an estimated cost of $10 million, and would have disrupted train operations. Realigning the track to a contractible alignment that roughly follows the existing notch only requires a projected removal of 8 cubic yards at an estimated cost of approximately $1 million, a substantial saving of 90 percent that would also minimally disrupt train operations. Project professionals decided to design the realignment of the entire track with 37 horizontal tangent segments throughout the tunnel to facilitate construction methods and available work windows. The track is essentially realigned to a best fit solution following the existing notches in the tunnel crown. 

Through the innovative use of LiDAR technology, the recent track realignment project in the Cascade Tunnel achieved substantial estimated cost and operational savings in surveying, engineering, and construction. The project ultimately achieved its purpose, which was to provide a constructible and efficient design, to eliminate the unintended and unwanted physical contact between trains and the tunnel crown. At the end of the project, JLP had analyzed more than 2 tetrabytes of point data and was able to model the tunnel without affecting operations in this busy corridor. 

At the 2013 Year in Infrastructure Conference in London, JLP received the prestigious Bentley Be Inspired Award for Innovative use of Point Cloud Processing and Management on the Cascade Tunnel.

Marc Cañas, GISP, is the vice president and chief operating officer of J.L. Patterson & Associates, Inc., a Southern California-based civil, trackwork, environmental, and structural engineering firm. He can be reached at macanas@jlpatterson.com. Nathan Ortega, P.E., is an assistant project manager at J.L. Patterson & Associates. He can be reached at nortega@jlpatterson.com.

Posted in Uncategorized | June 12th, 2014 by

Comments are closed.