Canadian Consulting Engineer

Feature

Myra Canyon Trestles

When lightning started a huge forest fire in the Okanagan Mountain Provincial Park in British Columbia in August 2003, it ravaged 26,000 hectares of land and 270 homes. As it raged on its destructive ...





When lightning started a huge forest fire in the Okanagan Mountain Provincial Park in British Columbia in August 2003, it ravaged 26,000 hectares of land and 270 homes. As it raged on its destructive path, the fire also destroyed 12 historic wooden trestles that were part of the Kettle Valley Railway in the Myra Canyon south of Kelowna.

The railway was originally built at the turn of the 20th century by the Canadian Pacific Railway to haul silver and other minerals out of the mines in this region of B.C.’s southern interior. After the railway was decommissioned in the 1970s, the right-of-way was obtained by the province and incorporated into the Trans Canada Trail. It has become a popular spot for hikers and cyclists, and by 2002 the number of tourists visiting the area annually reached 50,000.

Designing authentic wooden trestles

After learning that the fire had devastated 12 of the 16 trestles, the public overwhelmingly began to call for their reconstruction. The Myra Canyon Trestle Restoration Society had existed for over a decade and fortuitously, just prior to the fire, the Kettle Valley Railway had been designated as a National Historic Site. The federal and provincial governments cooperated to provide funding for reconstructing the structures.

Earth Tech Canada in Kelowna was contracted by the Ministry of the Environment as prime consultant to design authentic wooden trestles and to inspect the reconstruction efforts.

It may have been more economical simply to rebuild the bridges using conventional modern structure types, but it was agreed that the replacement structures would be historically accurate timber trestles.

The Canadian Pacific Railway archives in Montreal and the Chief Engineer’s office in Calgary provided background information and record drawings of the destroyed structures. However, the individual site conditions needed to be investigated to verify the requirements for the rebuilding. The trestles are in remote and difficult mountainous terrain and the sites vary considerably throughout the canyon. Several trestles sit on steep, rocky slopes and use rock anchors and concrete foundations. Others are situated on relatively even slopes with timber mudsills on loose granular material.

The tallest of the destroyed bridges, for example, was Trestle 4. It crosses a small canyon and was mostly supported on timber piles driven into the most unstable soils found along the line. The trestle is 130 ft. (40 m) high and over 500 ft. (152 m) long. Most of the piles had burned completely down to ground level. Trestle 5 is the smallest of the structures to be rebuilt. It measures 10 ft. (3 m) high and 75 ft. (23 m) long.

The first step of the field investigations was to determine the horizontal and vertical alignment of the trestles by locating the remains of the existing foundations. A survey crew was sent out and the data was compiled into a three-dimensional survey model that provided the basis for the geometry of the replacement trestles. Geotechnical investigations were required to determine if the existing foundation locations would support the new structures.

Douglas Fir and traditional preservative

It was not cost effective to build exact replicas since the trestles would never again carry the heavy load of a train. Consequently, two design models were adopted that maintained the historic “look” of the original trestles while offering significant cost saving features. Trestles that are directly across the canyon from each other or adjacent to other trestles are designed with their original 15-ft. spans. Single trestles and those that are not easily viewed from the existing trestles have 30-ft. spans that require fewer foundations and posts.

The timber used is B.C.-grown Douglas Fir, which remains the choice for railway and highway structures today. It is readily available, has good mechanical properties and a long service life. The wood is treated with a traditional railway preservative: creosote. Creosote is cost-effective, readily available and safe for the environment.

The new trestles are designed for a fully loaded concrete truck (250 kN) and other construction vehicles to pass over. Maintenance vehicles are expected to be the heaviest loads that will cross the trestles after they are constructed. The new design also had to meet the more stringent wind load criteria in today’s Canadian Highway Bridge Design Code, which meant the new structures needed more substantial foundations.

Restricted access high in the mountains

Access to Myra Canyon is restricted and only two trestles can be constructed at one time — one at each end of the canyon. All the materials have to be trucked in over rough forestry roads that are very difficult to navigate with 13.5-m trailers and 45-ton payloads. Once the trucks have climbed the 850-metre elevation from Kelowna, they continue along the railway grade to the contractor’s staging area near the former structure. The trestle bents and decks are prefabricated at the staging area and then erected by various means, including a high-line crane system in one instance, and a tracked crane with a moveable platform in another.

Five timber trestles were constructed in 2005 and contractors continue to work throughout the winters in order that the remaining seven timber trestles and two steel bridges can be completed by 2008. The total budget is approximately $15 million.

While the project required the engineers in some respects to take a step back in engineering and construction practices, it also required the use of advanced technologies, such as state-of-the-art survey instruments, static and dynamic 3-D computer analysis and CAD software for producing drawings. The project’s ultimate achievement, however, is to preserve the heritage value of the Myra Canyon and honour the engineering feats of the railway builders at the turn of the 20th century.

Client: B.C. Ministry of Environment, Environmental Stewardship Division

Prime consultant: Earth Tech Canada, Kelowna, B.C. (Kevin J Witzke, AScT, Kurt Weber, P.Eng., Laszlo Dunaszegi, P.Eng., Eric de Fleuriot, P.Eng.)

Engineering subconsultant, timber railway structures: Klett Consulting Services, Calgary (James M Klett, John F Unsworth, P.Eng., Barry Lidstone)

Geotechnical subconsultant: Golder Associates, Kelowna (Glen Rutherford, P.Eng.)

Contract management: Katim Enterprises

2005 Contractors: Surespan Construction, Seismic 2000 Construction