TOP-DOWN CONSTRUCTION OF THE NEW LRT OVERHEAD BRIDGE, KITCHENER, ONTARIO

摘要

A new Light Rail Transit (LRT) system is being constructed in Waterloo Region. The first phase of construction is scheduled for completion in 2017 and will connect Conestoga Mall in the City of Waterloo and Fairview Park Mall in the City of Kitchener, a distance of 19 km. The LRT route crosses the existing Conestoga Parkway (Highway 7/8) in Kitchener-Waterloo, requiring the construction of a new structure under the busy freeway. In 2014-15, the Ministry of Transportation, Ontario (MTO) administered the construction of the new single span bridge using a unique top-down construction technique. The highway was built during the late 1960s and early 1970s and Kitchener-Waterloo was by-passed by a new four-lane freeway now known as the Conestoga Parkway. To accommodate the existing Canadian National Rail (CNR) track which crossed the then new highway alignment, the highway was built-up on 7.5 m of fill. Recently the Conestoga Parkway was widened to six lanes between Courtland Avenue and Fischer-Hallman Road and part of the construction project included the construction of the new LRT Overhead bridge to accommodate two LRT tracks below the existing highway. The LRT crossing posed three significant challenges; first being the 7.5 m embankment, second being the requirement to maintain four lanes of traffic during construction and the third being the close proximity of the existing adjacent CNR Overhead bridge which was offset 22 m from the new LRT Overhead bridge (centerline to centerline). Traditional bottom-up construction techniques were considered but not preferred due to the extensive excavation and temporary roadway protection works required to construct the new rail crossing. Since the existing CNR bridge was not designed for unbalanced lateral earth loading, significant excavation on both sides of the CNR bridge would also be required. A top-down construction approach using secant pile abutments and a reinforced cast-in-place concrete deck slab was used to create a 12 m single-span, rigid frame bridge to accommodate two LRT tracks. This concept was similar to the Hespeler Road / CPR grade separation project, constructed in 2012, by the Region of Waterloo. A critical aspect of the LRT Overhead design was to minimize the required excavation and impacts to the existing adjacent CNR bridge with less than 11 m of soil between the two structures. The new LRT bridge design considered the future excavation and replacement of the CNR bridge. In order to maintain traffic on this busy highway, the new LRT structure was constructed in two stages. The first stage included the diversion of four lanes of traffic to the north half of the existing highway platform to permit the south half of the bridge to be constructed and the CNR bridge to be rehabilitated and widened. As previously discussed, the top-down construction approach was selected because it minimizes excavation; but some staged excavation of the existing embankment, adjacent to traffic, was required. To install the secant pile abutments and retaining walls, a level platform was prepared for the drill rigs, approximately two meters below grade which was the approximate top of caisson elevation. A temporary H-pile and lagging protection system was installed between construction stages to accommodate an approximate three meter excavation. The additional one meter excavation would be required following the installation of the secant caissons for the installation of the deck false work and construction of the concrete abutment caisson cap and deck. Two parallel secant pile abutment walls and retaining walls were constructed using a combination of unreinforced concrete filler caissons and reinforced concrete king caissons. For both types of caissons, steel liners were used to temporarily support the shaft walls to prevent collapsing of the augered holes. Where loose soil was present, the steel liner was inserted during the augering of the shaft. The liner was vibrated out immediately fol