By: Lance Pepper
Four years of detours, traffic snarls, varying speed limits, and stop-and-start driving has come to an end for drivers in Edmonton, Alberta, Canada, with the completion of the northeast stretch of Anthony Henday Drive.
The northeast segment is the final leg of Edmonton’s transportation ring road, consisting of 5 miles of reconstructed, six-lane and eight-lane divided freeway. It also includes 11 miles of new green-field six- and eight-lane divided freeway, nine service interchanges, seven grade separations, 37 highway bridges, seven rail bridges, and twin river bridges over the North Saskatchewan River. Total lane-miles is slightly more than 117, and includes 58 overhead sign bridges and 45 cantilevered sign structures.
The project corridor includes more than 70 wetlands, directly or indirectly affected by the project footprint. It also contains a major crossing in the form of the North Saskatchewan River at an environmentally and geotechnically sensitive location.
Encircling the city of Edmonton, Anthony Henday Drive is a heavily traveled commuter and truck bypass route, with the southwest quadrant serving as a portion of the CANAMEX Corridor linking Canada to the U.S. and Mexico. It is one of the busiest roads in western Canada, carrying more than 105,000 vehicles per day. Construction on the overall project began in 1990. Work on the northeast segment of the road started in 2012 and was completed at a cost of $1.81 billion CDN, bringing the total cost of the project to $4 billion.
The Alberta Government used the public-private partnership (P3) model to build this segment of the Anthony Henday Drive from Manning Drive to Whitemud Drive (east). The P3 concession was awarded to FDAL, a joint venture of Flatiron, Dragados, AECON and LaFarge, with responsibility for designing, building, financing and operating the northeast segment. AECOM acted as lead designer, providing the overall design for the roads, bridges, drainage, utilities, street lighting, traffic signals, signage, environmental approvals and mitigation, and noise analysis based on projected traffic volumes.
Digging up answers
Much of the Yellowhead Trail/Anthony Henday interchange was located over an abandoned coalmine site for which there were limited records. An extensive geotechnical drilling program was undertaken to determine whether bridge sites on the periphery of the old mine were outside the limits of the mining operation. For many of the bridges over the mine site, a wide variety of bridge foundation solutions were developed depending on the underground conditions. Designs varied from raft foundations to deep piles with casings that penetrated through the mined areas into the bedrock below.
The pavement design solutions varied widely throughout the project. In the green-field areas, a special modeling program was used to evaluate numerous pavement design options, taking into account original capital costs as well as interim rehabilitations, and operation and maintenance costs to generate the lowest net present value (NPV).
For the existing Highway 16 and the Anthony Henday roadways within the project, thorough evaluations were undertaken to establish which portions could be reused and again provide the lowest NPV.
Falling weight deflectometer testing was performed to establish accurate subgrade modulus strengths. Recycling of milled asphalt into new asphalt mixes was utilized where feasible. Instead of using foamed in-place recycled pavement, deep asphalt overlays up to 2.2 in. were employed, as this was more cost-effective than constructing detours, excavating existing pavement structures, and encountering wet subgrades.
In locations where there were property constraints, detailed geotechnical evaluations were carried out to evaluate slope stability, and reinforced slopes were incorporated where necessary. Mechanically stabilized earth (MSE) walls with single- or two-stage systems were incorporated into the design to optimize bridge span lengths. Two-staged walls also were used where settlement was anticipated.
Because of the presence of voids at variable depths of the underlying coal seam, several bridges had to be designed to accommodate the potential collapse of the voids. Based on the cover depth and the overlying bedrock thickness, both sinkhole and ground subsidence failure modes were predicted. For each failure mode, several foundation options were investigated against cost, risk and constructability. In the end, a thick-spread footing capable of bridging a 5-m sinkhole was selected for final design and construction to address sinkhole failure mode. To eliminate differential settlement and high downdrag load associated with ground subsidence failure mode, driven piles through double casings supported by the bedrock underlying the coal seam were selected for design and construction.
Ducks and birds
The storm-water management system for the project involved integration of compensatory developed and constructed wetlands with storm-water management objectives and features. The integration of the two objectives was particularly challenging since the system required 15 storm-water ponds with inlet ditching, three North Saskatchewan River outfalls and related piping, and discharge to several wetlands and North Saskatchewan tributaries.
Although efforts were made to avoid disturbing naturally occurring wetlands, roadway construction necessarily resulted in the disturbance or loss of some wetland habitats. However, the Provincial Water Act allowed for the loss of wetlands to be compensated for by contributing financially to wetland restoration projects elsewhere or, preferably, by constructing wetlands within the project’s storm-water management system. The design-build joint venture provided wetland loss compensation locally wherever feasible as part of the roadway storm-water management system. When not possible, donations were made to Ducks Unlimited Canada to support its Cooking Lake initiative, a wetlands restoration effort at Alberta’s Blackfoot Recreation Area.
A significant challenge to the preservation of wetlands was the location of portions of the project corridor within the Bird Hazard Zone of the Federal Department of National Defense (DND). DND prohibits the creation of habitats for waterfowl such as ducks and geese as these are seen as potential hazards for aircraft bird strikes. To mitigate this concern, open water was limited and longer narrow channels were incorporated, offset by wider wet meadow areas in the floodplain of the facility.
Wetland scientists from AECOM’s team worked collaboratively with design team drainage engineers and landscape architects to develop wetland designs that would meet the requirements of DND from the perspective of aircraft safety, while also meeting the compensation requirements of Alberta Environment.
In the designs
Road design: The Highway 16/Anthony Henday Drive system interchange involved the design of major directional ramps, rail crossings, major utilities and 12 bridges, all needing to be constructed while ensuring that the existing roadways remained operational to traffic. The project team optimized the roadway geometry within the boundaries of the project’s technical requirements to minimize encroachment outside the road right-of-way and/or maintained separation from utility right-of-ways by constructing reinforced fills. Reinforced fills were favored over MSE walls as a cost-savings measure and to create enhanced aesthetics, which was a project requirement.
To promote more barrier-free design, standard barrier protection was limited. The ultimate roadway subgrade was constructed so that ditches, backslopes and appurtenances were constructed and installed in a permanent location to avoid future throwaway costs. The pavement structure was not fully built, but limited to the surface area called for at this stage. When future traffic demands require it, the roadway surface can then be cost-effectively expanded. All interchanges were either staged or fully built to their ultimate configuration.
The roadway design along the green-field section north of the North Saskatchewan River up to the project limits differed significantly from the original functional plan, which showed the Anthony Henday profile going under two major railway crossings and one crossroad, amounting to 528 million gal of surplus cut. The design was adjusted to take Anthony Henday Drive over the crossings, thereby balancing earthworks. Crossing over the railway gave the joint venture the advantage of being able to better control its own construction schedule.
Pavement design: The pavement engineering undertaken for the project considered innovative use of existing materials into the new pavement structures, reducing the use of non-renewable aggregate resources. This provided the added benefit of lower greenhouse-gas emissions without sacrificing pavement quality and performance. Pavement design was optimized through the use of performance models developed specifically for the project, making use of historical pavement performance data from adjacent highways. The selection of materials was made with the intent of eliminating the potential for low temperature-induced cracking. This resulted in enhanced performance in terms of smoothness, reducing the need for future maintenance and rehabilitation activities as well as delays to the traveling public.
Bridge structure designs: Extensive effort was put into the planning and design of the bridge structures to accommodate an aggressive construction schedule, produce the most cost-effective design and minimize the impact of construction on existing traffic. In order to provide flexibility in the timing between earthworks and bridge construction, the structures were designed to allow the earthworks and bridges to be built independently, enabling the construction team to advance elements of construction as site conditions and labor-resource constraints permitted.
MSE walls were used in the majority of interchanges, reducing bridge span lengths and the overall bridge deck area by 15-20% as compared to those presented in the original functional planning study. Reducing the bridge span made it possible to utilize integral abutment designs for many of the bridges, eliminating the need for bridge bearings and expansion joints. The combination of these components reduced the overall superstructure depths, which, through effective multi-disciplinary planning and design with the roads team, allowed the road profile to be optimized, reducing the fill material required for adjacent bridge structures.
Environmental considerations
The AECOM team worked closely with the contractor to address critical environmental elements of the project. This included temporary berms that had to be constructed in the North Saskatchewan River in order to build the bridge, three river outfalls, two Goldbar Creek channel realignments, and 20 additional drainage structures. The environmental, design and construction team reacted quickly to the impact of natural events. For example, they managed to salvage the fish population in the North Saskatchewan River when high water levels resulted in the instream berm being overtopped by river flows. Corrective measures were put in place to restore and protect the north river bank in the area of the new river crossing bridge when significant bank erosion occurred due to elevated river levels.
A key environmental element of the project was achieved when wildlife movement was maintained and integrated along the North Saskatchewan River corridor. This safe wildlife passage was ensured while including the added complexity of pedestrian access from each side of the bridge using the new underslung pedestrian bridge; river bank erosion was protected through the placement of heavy rock rip rap; and a major storm outfall was installed on the north river bank.
Working with walls
A unique combination of cast-in-place concrete walls was designed and constructed at the north abutment of the North Saskatchewan River bridge. One wall was supported on a shallow footing and another was pile-supported. These retaining walls allowed for the pedestrian pathway while also maintaining a relatively steep north headslope, reducing excavation and bridge length. A row of slope stabilization piles also was installed further downslope of the retaining walls in order to allow for a steeper headslope while achieving the required minimum factor of slope safety.
The overpasses on the project incorporated vertical, mechanically stabilized earth walls with metal reinforcement at the bridge abutments. These walls are relatively heavy and the near-surface soils are relatively weak. As a result, an iterative process was implemented in which several options appropriate for the soil conditions at each site were developed by geotechnical designers.
The completion of the Northeast Anthony Henday Drive project affords the public many benefits. It makes daily commuting faster and easier while improving the movement of goods and services around the province. Environmentally, it results in fewer emissions produced from idling cars on busy roadways, as well as assuring safe passage for wildlife and protecting fish in the North Saskatchewan River.
About The Author: Pepper is an associate vice president of AECOM, Edmonton, Canada.
