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The Walterdale Bridge: DYWIDAG Micropiles stabilize Edmonton’s new Landmark

The Walterdale Bridge: DYWIDAG Micropiles stabilize Edmonton’s new Landmark

In Edmonton, Alberta, Canada, the new Walterdale Bridge is currently being built. When completed, the arch bridge with two 56m high arches will replace an old bridge structure built in 1913. The Walterdale Bridge crosses the North Saskatchewan River with a 230m long span. With three lanes instead of the previous two lanes and additional lanes for bikes and pedestrians,it will ensure a better link to the center of the city.

The arch loads are transferred to deep foundation thrust blocks at both river banks, and the abutments are stabilized using micropiles. The piling subcontractor, R.S. Foundation Systems Ltd. (RSFS), was contracted to design, supply, install and test the micropiles. After a competitive bidding process, DSI Canada was selected by RSFS to supply the micropiles.

The four foundations thrust block bases are each supported by 44 triple corrosion protected, galvanized, 75mm Ø DYWIDAG Micropiles in lengths of approx. 18m; in total, 3,168m of DYWIDAG Micropiles with a total weight of 114t were installed.

Prior to the installation of the 176 DYWIDAG Micropiles, 2 demonstration anchors were installed on each side of the river. The test anchors differed from the production micropiles in that bare bar was utilized, a free stress length was installed through the overburden soils and the pile was installed vertically.

The test consisted of cyclically loading each pile between compression and tension loads, to a maximum of 2,175kN in compression and 1,131kN in tension. RSFS was requested to conduct the tests through approx. 12-15m (40-50ft) of overburden soils. The largest challenge to satisfying this request was to isolate the pile section through the overburden, allowing it to move while maintaining the structural rigidity and capacity of the test piles required to resist the large compression loads. This was accomplished by installing a highly reinforced pile section complete with a free stressing length through the overburden.

Each test took approximately 35 hours to set up, and 14 hours to perform. In addition, each test was conducted in sub-freezing outside temperatures of down to -28°C. Despite all difficulties, the tests were very successful and proved the load-bearing capacity of the installed micropiles.

In order to permit the installation of the DYWIDAG Micropiles in the foundation, berms were temporarily constructed on the river bed. Behind these berms, intermediate levels were excavated on the dewatered, approx. 14-15m² large area in the river bed. It was only from these berms situated approx. 13-17m below grade that the 18m long DYWIDAG Micropiles could be installed in the foundation of the abutments.

Although the majority of ground encountered while drilling was clay bedrock, the formation was also full of fractures, coal and bentonite seams, causing both artesianing water and positive gas pressure. In some instances, several cycles of consolidation grouting and re-drilling was required. Each micropile was installed approximately 50° from horizontal in a 200mm Ø and 19m deep hole. The DYWIDAG Micropiles were first installed with 50 MPa primary grout and post-grouted through 7 Post Grout Valves. Each pile was designed to provide a long term service load of 1,131kN in both compression and tension and an ultimate load of 1,600kN and 2,830kN in tension and compression.

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