Bentley’s rail and transit solution enables rapid 3D modelling, analysis and design while minimizing impact on business operations
The Cascade Tunnel, at 7.8 miles, is the longest railroad tunnel in the United States. Built between 1925 and 1929 near Stevens Pass in Washington State, this single-track tunnel was designed to reduce the route length for trains by 8.7 miles and eliminate 21 miles of 2 per cent or steeper grade. In 1989, railroads operator BNSF affected a tunnel clearance programme by notching the tunnel’s roofline to allow double stack intermodal trains to operate through it. Normal track structure movement caused trains to strike the underside of the tunnel making it necessary to revisit tunnel clearance. Given its high utilization, surveying of the tunnel using conventional manual methods was impossible.
Identifying Tunnel Impact Points
BNSF has been monitoring sporadic clearance events or strikes for a number of years. However, the frequency and severity of strikes caused BNSF to take a much closer look at the source of the problem. Management presumed that problems were the result of poor horizontal track alignment with notches added to the very top of the tunnel in 1989 as part of the clearance programme. In order to fix the problem, however, they needed to identify exactly where these impact points were and make necessary changes to the tunnel clearance.
To do this, the company hired J.L. Patterson & Associates (JLP). “We needed detailed knowledge about the interior of the tunnel,” explained Marc Canas, Vice President, JLP. “But we couldn’t use conventional tunnel survey methods because of the heavy traffic and time constraints. Engineering and surveying crews are limited to no more than 1- to 2-hour access windows at any given time. With these access constraints, it would have taken us weeks using conventional survey methods to model the interior of the tunnel and determine the clearance optimization plans.” Furthermore, BNSF Railways wanted to minimize cost and project scope. Its early estimate of the project indicated that fixing the problem would require notching the tunnel for 177 cubic yards – a project that would cost approximately $10 million, require several months of construction and seriously impact operations.
Capturing Data While Minimizing Business Disruption
JLP partnered with a mapping sub-consultant that used LiDAR technology and Bentley Pointools and MicroStation to analyze the existing alignment of the track inside the tunnel and model the tunnel’s interior surface. JLP used Bentley’s point-cloud solutions to quickly survey the entire tunnel, analyzing the data to determine the required modifications before making the necessary structural changes to the tunnel. “We equipped a high rail vehicle with a global positioning system (GPS) and an inertial measurement unit (IMU),” explained Canas. “As the vehicle traversed the tunnel, it captured point-cloud data needed to create a 3D model of the tunnel’s internal surface, the precise centre line of track, and the precise location of the existing notches in the tunnel.” The team established horizontal and vertical survey controls with targets placed at each portal, as well as a control target along the tunnel at about 1,500-feet spacing. The control points and inertial navigation data were used to determine the smoothest, best estimate trajectory of the 3D route of the mobile mapping system.
Creating a 3D Model to Determine Optimal Solution
Analyzing several million data points captured using Bentley software – more than 2 gigabytes of point-cloud data – JLP developed the digital 3D model, analyzed it, and prepared proposed alignment modifications that would minimize clearance conflicts while maintaining the alignment with intolerances for tangent track. JLP also identified tunnel soffit modifications in the areas of conflict between the clearance envelope and the crown of the tunnel that would remain once the track alignment adjustments were made.
“Bentley software allowed us to model the tunnel without affecting operations in this busy corridor,” commented Canas. Using this 3D model, JLP developed proposals for:
- Best-fit track alignment to match the existing notch location
- Additional notching required within the tunnel to line up with the existing track alignment
- Best-fit track alignment with tangents and small angle breaks, while minimizing proposed notching
Because JLP had such a detailed and complete 3D model of the tunnel interior and track, proposals were very accurate and exact. “The tunnel was analyzed using the desired clearance envelope, the track cross level was accounted for, and then we calculated the area of cut with no track realignment,” explained Canas. “We determined we needed to make 177 cubic yards of cut to meet the clearance envelope parameters.” However, by using the powerful regression tools within Bentley Rail Track, designers were able to develop a track alignment with tangents and small angle breaks that enabled the 25-miles-per-hour operational speed to be maintained while significantly reducing cut.
Realizing the Beneﬁts
The geometric results of the proposed track realignment yielded fantastic results. “We were able to minimize corrections to the tunnel clearance envelope to just 8.3 cubic yards of excavation, saving a tremendous amount of time, maintenance funding and operational disruption,” explained Canas. “Bentley software was pivotal in saving BNSF Railways tremendous time, money and effort by reducing the amount of cut, improving operations and minimizing disruptions to this very busy line in the system. Using Bentley’s innovative solutions, we found a better way to address the problem – an approach that would cost about $1 million, about 10 per cent of original estimates and cut construction time to weeks instead of months.
“The Cascade tunnel project is unique because it’s the first time that software has been used to minimize work in the field,” concluded Canas. “The proposed solutions prior to our involvement included very expensive propositions, such as excavating inside a busy tunnel and realigning the tunnel itself. But we collected accurate information from inside a 7.8-mile-long tunnel using LiDAR technology and powerful tools from Bentley – and were able to design and clear the tunnel for container traffic at a fraction, specifically, one-tenth of the cost.”