By: Schaun Valdovinos
The Sea-to-Sky Highway is consistently cited as one of the most scenic drives in North America.
As part of Highway 99 that stretches between West Vancouver and Whistler, B.C., Canada, this 60-mile drive provides expansive views of snow-capped mountains rising out of the deep blue waters of Howe Sound. Bald eagles can be seen soaring overhead, while seals swim in the sea below. The highway itself is a tourist destination, hugging the cliffs as it snakes along the Coast Mountains.
In addition to being one of the most scenic routes in British Columbia, the Sea-to-Sky Highway also was one of the most dangerous. Two-lane traffic would wind around the mountainsides with nothing preventing cars from entering the opposite lane, leading to devastating head-on collisions. In 2005, the provincial government decided to upgrade the highway as part of their infrastructure improvements in preparation for the 2010 Winter Olympic Games. The design of the new roadway emphasizes safety and mobility of the public, while minimizing maintenance costs over the 25-year design-build-finance-operate contract period. The project was awarded to a consortium including Hatch Mott MacDonald (HMM) as design lead, design-build contractor Peter Kiewit Sons Co. Ltd., and S2S Transportation Group, a concession company created by Macquarie Group with overall responsibility for the highway during the 25-year term.
Because of heavy year-round traffic volumes, the design-build agreement required minimal closures of traffic lanes and the rail tracks below the highway during the four-year construction period. Widening the corridor from a two-lane highway to three- and four-lane configurations with a central median barrier required a careful traffic-management program and design solutions that maintained traffic flow during construction.
The steep landscape with a sensitive environment necessitated unique structural and geotechnical strategies from the design team and contractor. To support the additional lanes of upgraded highway, a total of 40 bridge structures and 100 retaining walls were designed to overcome site constraints.
To manage the design and construction of the highway improvements, the contractor subdivided the route into four design-build segments. From the outset of the project, Segment 2 was flagged as the most challenging in the corridor with its steep terrain. Along this stretch, nearly 6 miles of additional roadway are supported on new walls and bridges designed to resist seismic loads.
The highway design was modified throughout the design phase to best fit the new curvilinear roadway through the terrain. Slope cuts necessary to accommodate the highway alignment became rock quarries for retaining-wall backfill. By balancing these material quantities, trucking distances and fuel consumption were minimized.
Around every corner the team faced the challenge of supporting walls and bridges on steep downslope terrain. The design depended heavily on survey data that could only be obtained once pioneering was carried out by the contractor to expose the variable ground conditions.
“The design team could not simply prepare design drawings and hand them over to Kiewit to build,” stated Tony Purdon, HMM design manager for the project.
Clever nickname
To simplify construction of several simple span bridges crossing over steep side-slope terrain, a unique bridge archetype nicknamed “suspended fill” was conceived through a collaborative effort by the design-build team. This bridge type uses two-stage precast panels spanning up to 16½ ft between a median wall and steel edge beam. The panels are made composite with the steel girder through a concrete overlay. Using precast panels enabled rapid construction by eliminating the need for soffit formwork.
The composite panels are buried by 12 in. of crushed base course and 5 in. of asphalt to create a seamless extension of the at-grade road surface onto the bridge. With a conventional girder bridge, a longitudinal joint is necessary at the interface between the bridge deck and the at-grade lanes to accommodate vertical deflections of the deck. However, a longitudinal joint is not acceptable within travel lanes because it presents a risk to motorcycles. With the edge-beam concept, joint deflections are eliminated, and the panels merely rotate about their bearing support along the median wall. Considering serviceability of the road surface, the depth of the edge girder was increased beyond strength requirements to minimize this rotation.
Carving effort into rock
It was necessary to make construction rapid and economical to enable the colossal highway project to be completed in time for the Winter Olympics. Multispan bridge superstructures were primarily constructed from prestressed concrete I-girders and precast deck panels. For the composite deck, high-performance concrete was cast over the top of the precast panels and sealed by a waterproof membrane prior to applying an asphalt overlay. The high-performance concrete, along with eliminating deck joints and including approach slabs at each abutment, results in a better riding surface and will reduce maintenance costs during the 75-year design life.
At a modest 330 ft in overall length, the multispan structures might seem simple feats. In fact, an array of innovative features allow them to efficiently span the steep, mountainous topography. Some of these features are described below.
The first downslope structure to be designed in Segment 2 was named eleven90. The span configuration was based on field visits prior to felling the trees that concealed the slopes. In the original 50% design, the bridge was to be a symmetric structure with two 130-ft spans using British Columbia Type 5 girders. Once excavation took place, unfavorable rock fractures were discovered at the site of the original south abutment.
Upon learning of this adverse condition, the design team carried out a detailed cost comparison for three options: an abutment on a bed of minipiles, geofoam in place of backfill (to lighten the surcharge load) and an angled strut supporting a 33-ft jump span. The latter proved to be most economical and favorable from a geotechnical perspective.
The angled strut was kicked into the rock face to support the girders of the main span and jump span. This solution directs the reaction deep into sound rock and off the adverse rock mass. The lean of the pier is resisted by high-strength rock anchors installed in the upper end of the cap beam. The performance of the system is analogous to an arch bridge thrusting against rock at its spring point. Load is delivered across the dipping rock planes to act as a stabilizing force.
As a way of resolving the large longitudinal earthquake load, the central median wall separating the at-grade lanes from the bridge was detailed as a seismic shear wall. The wall was made integral with the main pier and doweled into the rock to give the wall sufficient interface shear resistance.
At the three-span structure, twelve31, the overall bridge length was reduced through the construction of toe walls below the north and south bridge abutments beside the railroad tracks. These enabled the construction of engineered fill slopes to support terraced MSE walls terminating at each end of the bridge.
M Creek Bluffs Bridge was built on the most challenging site along the highway, requiring high levels of ingenuity. By combining the suspended fill concept with a conventional girder span, this bridge is an extremely economical solution despite its difficult setting. M Creek Bluffs Bridge used the longest precast panels on the project, the deepest girders and had the longest span at 164 ft.
With a set of railroad tracks at the base of the cliffs, using a conventional column at the first pier was not an option. Therefore, an angled strut was chosen because of its slope-stabilizing benefits. The rock at this site has very steep fracture planes requiring the strut to be at a 45° angle. This directs the load perpendicular to the fracture planes and stabilizes the overall rock mass. The steep lean creates a very high-tension force in the cap beam that is resisted by post-tensioned rock anchors embedded deeply into sound rock. A free stressing length was used for the 60-ft-long anchors to ensure the resistance was developed completely within bedrock.
Carrying people, Olympic torch
The Stawamus Chief Pedestrian Overpass is 125 ft long and crosses over the upgraded Sea-to-Sky Highway. It is located at the entrance of the Stawamus Chief Provincial Park and offers visitors unrestricted access from the parking lot to hiking trails, camping and rock-climbing routes.
The footbridge was designed as a gateway structure to the town of Squamish, the Provincial Park, and as a symbol of the 2010 Olympic legacy. Its elegant form is harmonious with the site and utilizes two unbraced, splayed arches with a vertically curved deck.
This bridge was included as part of the design-build package, and its form was authored by the design team that also carried out the detailed design. The arch form uses minimal steel and supports a thin concrete deck with radiating stainless steel hanger rods. It was important to create an economical and constructable solution, because the budget for this bridge was constrained. Stockbridge-type dampers were installed internally near the crown of each arch to mitigate wind vibrations.
The bridge has received much acclaim as it gracefully connects the natural rock outcrops on each side of the road.
Sea-to-Sky Highway has been a very successful design-build-finance-operate project. By the design team working closely with the contractor every step of the way, design work was able to quickly respond to site constraints. Rework was minimized as the terrain was excavated for the bridges and walls based on 50% general arrangement drawings. This made it possible to adjust the final design to suit the specific requirements of each location.
The suspended fill bridge archetype proved very efficient and greatly simplified construction while enabling traffic to flow unimpeded during the four years of construction. In locations that required conventional bridge spans, the design team thought creatively and used the girder reaction forces for rock stabilization. Stawamus Chief Pedestrian Bridge capped off construction activities to give the scenic Sea-to-Sky Highway a gateway structure.
“We were fortunate to have some great people on this project. On the design side, we had some dedicated, creative, hardworking professionals who rose to the technical and schedule challenges. On the constructor’s side, we had people who saw opportunities where others would see problems,” said Purdon.
About The Author: Valdovinos is with Hatch Mott MacDonald, Vancouver, B.C., Canada.
