Andy Bateman, Engineering Editor

The new overpass has dual roles. Visually, it complements the existing Gateway Park with a design that includes curved retaining walls at each abutment, embedded stainless steel plaques, sculpted abutments and pier caps as well as integral deck light standards and architectural parapets. Func-tionally, it will carry six lanes of traffic on Ellerslie Road over no less than fourteen lanes of traffic when the local road network is completed. A future section of Anthony Henday Drive, intersecting with Calgary Trail just north of here, will ultimately connect with an already completed section of the new parkway on the west side of the city.

The estimated overall cost of the Calgary Trail/Ellerslie Road Interchange Project is $44 million, shared by the Province of Alberta (75 per cent) and The City of Edmonton (25 per cent). The project was scheduled for completion by August 2001, in time for the 8th IAAF (International Amateur Athletics Federation) World Championships in Athletics which were held in Edmonton August 3-12.

Edmonton-based Kiewit Management Ltd. began work in early April 2000 under a $9 million contract that includes construction of the interchange’s overpass, associated ramps, backfill and subgrade preparation as well as a separate single span bridge over nearby Black Mud Creek.

The overpass deck is a cast-in-place posttensioned box girder design, 130 m long and 26 m wide to accommodate the six lanes of Ellerslie Road as well as a sidewalk. The design of the four access ramps incorporates a total of 500 lineal metres of retaining wall that facilitates steeper side slopes as well as providing positive separation between traffic on the ramps and the main highway beneath. Concrete volumes in these structures totalled some 10 000 m 3 , while earthmoving volumes included 47 000 m 3 of excavation for the ramps and 16 000 m 3 of excavation to the overpass structure. Fill quantities included 44 000 m 3 in the ramps, 16 000 m 3 in the structure and 21 000 m 3 of topsoil.

Kiewit employed the raft slab method where the deck is constructed at grade on compacted backfill material, instead of the typical method of constructing the deck on falsework. The backfill was then excavated to reveal the finished structure. According to Kiewit project manager Gordon Baglier, the raft slab method is a relatively recent innovation, having being in use for some 10 years. Recent applications include a bridge job on Ontario’s Highway 416 near Ottawa as well as projects in British Columbia and western United States.

The main advantages of this construction method are a safer working environment and the improved productivity that results from simplified materials handling.  Construction crews are working at grade level instead of at a height, working with mobile equipment such as cranes and concrete pumps at the same level.

The method also has advantages from a design perspective. In softer ground conditions, it reduces ground pressures and the likelihood of settlement, since the weight of the deck is spread over a large area compared to the point loads of a falsework frame. Existing soils at this location included medium to high plastic, stiff to hard clays in generally dry conditions. Settlement  is minimised here by the clay compaction process and the relatively short duration of loading, while the continuous raft slab also helps to control surface water.

The job was done in two phases, with 55 per cent in Phase 1 and 45 per cent in Phase 2, both being completed without disruption to traffic on the existing six-lane Calgary Trail.

Phase 1 work included the excavation of the existing soil in order to construct the east abutment as well as piers two and three of the overpass. During this phase, care had to be taken to protect existing underground services that include a 203 mm dia. high-pressure propane line. It was first covered with 250 mm of natural material, followed by 700 mm of 0.5 MPa fillcrete (low strength con-crete) and a geogrid mesh. These were all capped with between 300 mm and 1000 mm of clay subgrade, depending on location.

Following construction, the abutments and piers were wrapped with polyethylene to protect the concrete surface from soil staining. The area was then backfilled with clay, placed and compacted in 203 mm lifts, with each lift compacted to 98 per cent of its Proctor density, or about 150 mm after compaction. At the same time, the moisture content of the fill clay was monitored and preconditioned if necessary to ensure that the clay was within 2 per cent of its optimum moisture content. The subgrade under the highway and ramps was compacted to 95 per cent density, apart from the top 300 mm where compaction continued until 100 per cent density was achieved. Compaction was done with 10-tonne padfoot ride-on compactors.

Backfilling continued to just beneath the pier caps. A temporary concrete working slab, some 75 mm thick, was then placed around each pier cap, carefully levelled to the soffit (underside) elevation of each pier cap in readiness for the cap forms.  After the pier caps had been cast, struck and wrapped, clay backfilling continued to the top of each pier cap. The whole backfill area underneath the deck then received an 203 mm gravel layer and a temporary 75 mm concrete working raft slab, set at the elevation of the soffit of the deck’s box girders after allowing 5 mm for the small amount settlement expected. The working slabs for both the deck and pier caps were greased to prevent bonding with the structural concrete. After the deck was completed, the undermining of the backfilled clay was carried out under a separate contract. During this process, the 75 mm raft slabs fell away with the ex-cavated material to reveal the finished structure.

The intricate curved shapes of the pier caps presented an interesting challenge to the Kiewit team, who had to devise a practical method of forming and casting these in place.   Their ingenious three–stage solution necessitated some off-site experimental work and resulted in a steel and concrete composite form in the required  shape, bolted together in four sections to facilitate handling and stripping.

As a first step, a form was made containing expanded polystyrene foam, cut into numerous vertical strips to form the prototype of the required shape. The prototype was then enclosed in a box form and new concrete poured into the void space between the prototype and the form. After curing, the form was struck to reveal concrete in the shape of a negative of each pier cap. To construct each pier cap, the working slab was first greased to prevent bonding between the slab and the concrete of the pier cap. The cap form was then assembled around the pier, deck rebar added and the cap poured.

Elsewhere on site, effective forming solutions included a form liner and fibreglass wall ties. Zen Drain form liner was originally developed to help produce a durable concrete surface in demanding applications such as water treatment plants. Kiewit utilised this liner when forming the long retaining walls and other exposed concrete areas.   The result was a smooth surface that required minimal point and patch work before application of an acrylic pigmented sealer.

The fibreglass wall ties were popular with forming crews, combining high tensile strength with easy length adjustment. They were especially useful in situations where the width of the form changes gradually, such as the web (vertical section) of a bridge girder, where the width of the web varied according to its position in the span. The alternative here would have been multiple sizes of the conventional threaded steel bar.   The fibreglass ties have a rated safe working load (SWL) of 3400 kg. They were cut to length with a saw. After threading through the forms, the desired width was set by a sliding collar.

Concrete mix properties for locations such as foundations, piers and retaining walls included Type 50 Sulphate Resistant Cement with minimum cement contents ranging from 330 to 340 kg/m 3 and 25 – 5 mm coarse aggregate. Minimum 28-day compressive strength requirements ranged from 25 to 30 MPa. Locations such as cast-in-place girder soffits, webs and diaphragms utilised Type 10 cement with minimum cement contents between 350 and 360 kg/m 3 , and 20-5 mm coarse aggregate. Here a minimum 28-day compressive strength of 35 MPa was required.

All these mixes had a specified air content of 6.0 per cent with a tolerance of one per cent each way.  High performance concrete was used in the bridge deck, abutments and approach slabs (mix HPC1) as well as roadway barriers, sidewalk barriers and medians (mix HPC2). Mix properties for both of these included Type 10SF cement with 8 per cent silica fume, minimum cement contents of 340 kg/m 3 and 20 mm – 5mm coarse aggregate.

Performance requirements for mix HPC1 included a 45 MPa minimum 28-day compressive strength as well as a satisfactory result in a test that measures the concrete’s permeability (Rapid Chloride Permeability Test) and, therefore, its expected durability. Mix HPC2 has a specified strength of 40 MPa and was not subject to the permeability test.

The placement and curing of the High performance concrete was subject to strict controls. Placement operations could only commence if the anticipated air temperature during the pour was expected to exceed 22ÁC or if windy conditions were expected during the pour. This was due to the fact that the combined effects of air temperature, humidity, concrete temperature and wind may result in surface moisture evaporation that exceeds a specified limit. In addition, the temperature of the concrete during discharge into the forms had to be maintained between 10ÁC and 18ÁC, by the addition of ice if necessary for cooling. HPC used for bridge decks was required to be in place,with full wet curing and protection in place and operational before 10 a.m.

After placement and before wet curing, special precautions were taken to protect the HPC surfaces from the effects of wind and sun, including the immediate application of fogging, evaporation reducers or special curing compounds.

Pavement construction work associated with the interchange was carried out by Everall Construction, a Division of E Construction Ltd., whose work also included soil stabilization where necessary, laying of granular materials and asphalt paving.

The consulting engineer on the project was Reid Crowther & Partners Ltd., with Stantec Inc. also playing a significant role.  Major subcontractors included Lafarge Canada Inc., A&H Reinforcing, Double Star Drilling, Coram Construction and EBA En-gineering.

What is Silica Fume?