Northern Busway - Bridge Structures Linking Communities, New Zealand

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1 SYNOPSIS Northern Busway - Bridge Structures Linking Communities, New Zealand Ted Polley, Kevin Crawford, Beca Infrastructure Ltd This project provides a new two lane busway located adjacent to Auckland s Northern Motorway. The structures include; a challenging underpass bridge at Northcote, constructed on one of North Shore s busiest urban roads, and several Super Tee girder interchange bridges at Onewa Interchange crossing the motorway. Each structure provides a new link to communities within North Shore City The Northern Busway Project, the subject of several environmental awards, is a comprehensive shift towards providing an efficient public transport system reducing Auckland s carbon footprint and providing expanded linkages between communities on either side of the Northern Motorway The most technically challenging structure was the underpass bridge constructed at Northcote Road supporting onramps and offramps to the Northern Motorway. The bridge was asymmetrical and skewed with extensive flares, requiring a combination of two bridge forms - prestressed Double Hollow Core and post tensioned in situ flares joined together providing a challenge for designers. A comprehensive construction staging sequence was necessary for no lane reductions during the peak periods of one of North Shore s busiest roads resulting in complex analysis of differential creep effects between the sections of bridge constructed in stages. The Northcote bridge also had to avoid significant services such as; the International Telecommunication Cables (ITC) which carried seven figure costs per day for temporary disruption to this service, two 600mm diameter watermains servicing the North Shore and an existing 800mm diameter sewer line crossing at a high skew relative to the alignment. The Onewa Interchange Super Tee structures are constructed on tight radius alignments with the piers on high skews requiring particular care with detailing of the pier heads. 1.0 INTRODUCTION This paper addresses the cross corridor bridge structures that link communities on either side of the existing Northern Motorway and its new partner, the Northern Busway. In particular the Interchanges at Northcote and Onewa are featured. The unique engineering challenges for each of these interchanges are described, presenting, first the problem, and then outlining the innovative solution. 1

2 2.0 PROJECT DESCRIPTION The Northern Busway project is a NZ$180m project involving a 8.7km busway. This includes a 6.3 km long dedicated two way busway from Constellation Drive in the north through the reconfigured Tristram, Northcote and Esmonde interchanges. A further 2.5 km length of one way busway runs southbound to the Onewa interchange. The project includes 5 bus stations for the North Shore to Auckland City route and the stations also act as hubs for cross suburb travel. Beca Infrastructure Ltd in consortium with Opus International Consultants for clients NZ Transport Agency (NZTA) (formerly Transit New Zealand) and North Shore City Council carried out the concept, detail design and construction management of the busway and the interchange bridge structures from 2004 to the completion in late

3 3.0 LINKAGE BETWEEN COMMUNITIES Transportation corridors invariably involve bridge projects crossing the corridor providing both key road and utility links between communities. The bridges of this project expand existing traffic capacity. Northcote also provides protection to two 600 mm trunk watermains and through its substructure elements, also protects an 800mm trunk sewer line and the International Telecom Cable (ITC). The latter service is a key strategic element of New Zealand s communication links to overseas. The Northern Busway Project also involved the routing of a new 240 kv Transpower cable ducting in the Busway designation, hence future proofing North Shore s power requirements. At Northcote this new ducting crossed above the sewer and ITC services with minimum cover. This had impacts on the available bus clearance at Northcote Underpass immediately to the north. The Onewa Interchange provides new north and south bound bridges replacing the existing two lane bridge which had considerable road safety issues of opposing traffic on a tight alignment. The provision of a westbound Esmonde road flyover adjacent to the existing eastbound provides the first westbound cross city link. This bridge and the Onewa southbound onramp account for 50% of the southbound motorway morning peak vehicles with the Onewa Interchange accounting for some 24,000 vehicles per day. Other associated projects for North Shore City at Onewa Road, widened the existing Onewa road bridge with a new bus lane and replaced the footpath with an award winning urban design of a shared use pedestrian cycleway providing a key link in North Shore city s cycleway network. 4.0 URBAN DESIGN NZTA and North Shore City provided the opportunity for a competitive urban design concept amongst consultant teams and Beca provided the urban design components to the project south of Northcote Interchange and in particular to the Onewa Interchange which provides the gateway to North Shore City. The concept chosen for the Onewa Interchange involved a theme that reflected that the site was a seasonal Maori fishing site. In consultation with local iwi, Beca Architects concept of a fish scale and basket weave pattern was imprinted onto the bridge barriers with fish hook elements at the pier locations. 3

4 The bridge piers were designed as V shaped whales tails rising from the ground with curve of the tail on a straight concrete element provided by a flared stainless steel cladding element and contrast provided by chases in the pier concrete surface. Three dimensional modelling was carried out to assess the visual aesthetics of the bridge as seen by motorway traffic. The landscaping for the bridge approaches as a future stage is intended to be planted to represent a fish head and tail at either end of the bridge. 5.0 STRUCTURE DESCRIPTION 5.1 Northcote Underpass The busway alignment passes underneath Northcote Road, one of North Shore s busiest roads. Northcote Road passes over the motorway via an overbridge and has on and offramps on both sides. The busway alignment is typically immediately adjacent to the motorway in order to remain within the motorway designation. At Northcote, the busway alignment went through the existing onramp and offramp locations. This requires the on and offramp to shift towards the motorway. To avoid having the Busway meeting Northcote Road in a signalised intersection, a lowered Busway alignment and a top down constructed underpass structure with retained approaches was the preferred option. The underpass structure comprises of two 750mm diameter bored pile walls supporting a deck formed of both precast DHC units and cast insitu post tensioned flared regions. The deck provides the restraint at the top of the bored pile walls. The walls extend a short distance beyond the bridge before changing to mechanically stabilised earth (MSE) retaining walls. The MSE walls retain the relocated motorway on and offramp approach embankments. Refer to figure 1 for a plan and cross section. 4

5 N Figure 1 Plan and Section of Northcote Underpass The bored piles at the bridge location function both as retaining walls and as vertically load carrying members. The founding Waitemata sandstone was approximately 40m below the bridge soffit. Only every forth pile was extended to socket into the competent sandstone. The remaining piles were keyed into a thin layer of volcanic tuff which provided the lateral key for retaining the soil above. Refer to figure 2 for a long section of the underpass. 5

6 Figure 2 Northcote Underpass Long Section The highly trafficked road combined with several significant services made this structure the most complex of all the bridges and required every form of civil engineering discipline to be involved. The engineering challenges are discussed in Section Onewa Interchange Bridges The upgraded Onewa Interchange comprises of a new two lane southbound onramp and a new single lane southbound offramp bridge. These bridges replace the existing two lane bridge which carried on and offramp traffic. Refer to figure 3 for a plan of the two bridges. The new bridges are on tight 200m radius alignments. The span arrangement for both bridges has a central pier in the motorway median and the adjacent piers are located outside the clearance envelope required for the future third harbour crossing. The allowance is for five lanes in each direction. The onramp bridge second lane is a priority lane for buses and other HCVs. The carriageway width is 9m. The offramp bridge is a single lane, having a carriageway width of 7m. 6

7 N Figure 3 Plan of Onewa Interchange Bridges Showing Skewed Piers The onramp bridge superstructure comprises of 1200mm deep super tee girders with a 200mm thick insitu deck. The offramp has longer spans and uses 1500mm deep super tee girders. The tight alignments required the external super tee flange cantilevers to vary from 900mm down to the minimum of 100mm. The median and shoulder piers are parallel to the motorway so the spans are minimised. However, this results in skewed piers relative to the superstructure due to the bridge curving in plan. This is further complicated by a carriageway crossfall of 6%. The bridge piers are constructed in a V shape with additional bolted on curved flares to represent whale s tails. Combined with the fish scale patterns on the barrier, these features represent the local iwi s historical fishing tradition in this area The abutments and piers are supported by bored piles socketed into competent Waitemata group sandstone. The piles are grooved within the rock socket to develop the required axial resistance by skin friction. The approach embankments comprise of up to 6m high fill located on compressible marine sediments. Wick drains were used to accelerate the settlements but still required a consolidation period of 18 months for most of the settlement to occur. The 7

8 settlements were in the order of 2m and applied significant negative skin friction to the piles. 6.0 CONSTRUCTION STAGING 6.1 Northcote Tunnel The brief was to construct a five lane bridge on a four lane road with an AADT of 38,000 vehicles. The adjacent Northcote Bridge restricted the lateral width that the temporary lanes could shift during the construction stages. The new bridge had to be constructed using the top down method. Other constraints included Watercare s main trunk water line that could only be shut down for 6 hour periods at night and only between August and October low demand period. The traffic staging dictated the bridge form with only a small section of the bridge able to be constructed at one time. This constraint affected the design and construction activities from boring the piles to erecting the deck. Once the first section of bridge was completed, it was immediately opened to traffic so a new work front could be opened. Refer to figure 5 for the construction staging sequence located at the end of this paper. Precast Double Hollow Core girders were the preferred option where they could be used. The girders were designed so they could support traffic loading without any load distribution. This provided the Contractor the ability to open a section of bridge prior to stressing the precast units transversely. The new Busway alignment was located underneath the existing on and offramp approach embankments. A temporary new onramp deviation had to be constructed and a temporary lateral shift of the new offramp was required before any other construction activities occurred. 6.2 Onewa Bridges Constructing the median piers was constrained by the existing motorway underpass bridge. There was insufficient room to create a construction zone in the median and shift the northbound traffic to the west because of the existing underpass shoulder pier. The solution was to split the northbound carriageway around the pier, two lanes on the eastern side of the pier and a single lane on the western side. To create the room, the existing spill through underpass abutment embankment had to be excavated to a vertical face and retained by a soil nail wall. 7.0 TECHNICAL CHALLENGES 7.1 Northcote Tunnel Reflective Cracking The construction staging required the design to detail the DHC units to carry traffic without load distribution. The DHC units were stressed together in the final configuration to reduce the likelihood of longitudinal reflective cracking forming in the pavement. The transverse stressing had to be joined in several stages due to the construction sequence. The issue was how the transverse stressing could be joined 8

9 between two DHC units already placed together. The solution was to use transverse stressbars and couplers instead of the standard transverse strand system. The DHC units had pockets cast into the unit to provide the access needed to couple the stressbars at the interface the between two DHC units. Differential Creep Between Different Bridge Forms The complex construction staging discussed in the earlier section resulted in different bridge forms being used on this structure. The alignment required significant flares at the western end of the bridge. The only cost effective structural form was to construct this section cast insitu with post tensioning tendons. The rest of the bridge was formed using precast DHC sections. The different bridge structural forms had different bending stiffnesses and creep deflections. Prestressed DHC units are not dead load balanced and will tend to creep upwards over time. If no allowance of this effect were considered, the DHC end units would begin to carry some of the insitu flared slab dead load as the DHC attempted to deflect higher than the slab. The method to overcome this effect was to tune the insitu slab prestress, increasing the number of strands until the same creep deflection occurred in both bridge forms. This resulted in several additional strands in the insitu section than would not have been required if this bridge deck was formed entirely with an insitu slab. International Telecommunication Cables (ITC) The Busway alignment crossed over the nationally strategic telecommunication cables which carry communications between New Zealand and Australia. These cables are located in ducts placed in the ground before the Northern Motorway was constructed. Disruption of these communication lines would cost in the order of 11 million dollars per day. The Busway alignment was lifted to provide 800mm cover to the top of the ITC. This resulted in a vertical clearance of 4.5m from busway alignment to the lowest point of the Northcote Bridge soffit level. The 800mm was not sufficient depth for the brittle ITC ducts to remain undamaged when subjected to cyclic HCV loading without protection. The solution was to pile and bridge over the ITC at busway level. The slab had to be as thin as possible so the Transpower ducts buried below the busway pavement could pass longitudinally under the slab while traversing above the ITC. It was considered too risky to divert the Transpower ducts below the ITC. Refer to figure 4 for a cross section of the protection system. 9

10 Figure 4 Cross Section of the ITC Protection Structure The Transpower ducts, typically arranged in two triangle, (termed trefoil) arrangement of three ducts, had to flatten out into two wider groups of three ducts in a single row to provide clearance. The ITC and the Transpower duct configuration determined the location of the piles which then dictated the slab thickness. A further consideration on the material under the slab was the fact that the existing onramp embankment was being relocated to create room for the Busway. The removal of six metres high fill resulted in the ground heaving as vertical pressure was reduced. However, the heave upwards was only a small portion of the settlement magnitude that the cables were subjected to when the original embankment fill was placed above the cables. The unloading of the adjacent embankment was instrumented and compared to heave predictions. The actual measured results were close to predicted. The bored pile wall retaining the onramp had to span across the ITC. A larger 900 diameter pile was constructed either side of the ITC. Micro piles were constructed between these two piles. As the busway was excavated down, three steel beams were welded to both 900 piles. The steel beams transferred the lateral reaction of the micro piles to the larger piles. The beams were then cast in concrete for durability reasons. North Shore 800mm Diameter Sewer Line Several metres below the Busway alignment lay an existing 800mm diameter sewer line that crossed the alignment on a 65 degree skew. The large skew angle caused the span between the retaining wall bored piles to be too large for the soil to arch 10

11 between the piles. Therefore the same method of using large piles and micro piles to bridge the sewer as used for crossing the ITC was used. Since the location was under the bridge, the span between the 900 diameter piles needed to be minimised. Therefore the pile wall kinked at this location to minimise the span. The precast panels in front of the wall hide this kink. A condition survey of the sewer line was carried out by a remote controlled camera. This survey also provided the actual sewer line alignment as there were no As Built records of this sewer line. Just beyond the bored pile wall, the sewer line changed direction and crossed the motorway. There was a concern that as the ground in front of the bored pile wall was excavated, the piles could move and load the sewer pipe, possibly damaging the pipe joints. The pile movements were monitored during the excavation and a fall back option of a temporary sewer pipe was included in the works. The inspection after the excavation was completed showed no movement at the pipe joints. North Shore Two 600mm Diameter Water Mains Two large 600mm diameter water mains travelled along Northcote Road in the existing median to service the entire eastern side of the North Shore. The existing water main alignment conflicted with the bridge deck and would have also projected into the busway s 4.5m vertical clearance envelope. The watermain alignment had to be shifted laterally to the centre of the new underpass and also lifted upwards to remain outside the clearance envelope. Due to the construction staging, the watermain alignment could not be shifted into its final position in one sequence and therefore required several shifts. A major constraint was that the watermain could only be shut down at night for a maximum of 6 hours and could only occur during the months of September to November. Lifting of the watermain split the bridge deck effectively into two sections with a gap in the middle for the watermains. A thin cast insitu slab was constructed above the watermains to act as the median kerb and also provide a loadpath for in plane horizontal seismic shears travelling through the deck. Unbalanced Soil Reactions From Retaining Walls The asymmetrical shape of the bridge in plan resulted in asymmetrical soil reactions loading the eastern side. The centroid of the eastern wall was eccentric to the centroid of the western wall. The eccentric reactions of the wall resulted in higher soil reactions towards the south eastern corner of the bridge. The deck transferred the eccentric soil reaction by in plan diaphragm action. The skewed DHC units required additional transverse stressing to ensure the diaphragm in plane shears could be transferred from one abutment to the other. Making the design more complicated was the watermain position which effectively severed the diaphragm into two sections. The insitu slab over the watermain provided a kinked load path for the diaphragm shears. 11

12 7.2 Onewa Bridges Highly Skewed Piers The piers were highly skewed relative to the bridge alignment. This resulted in pier movements occurring in several directions for both shortening effects and seismic loading. The piers needed to be flexible transversely to accommodate the bidirectional movement during the shortening effects from creep and shrinkage and temperature, yet stiff enough to prevent deflections becoming excessive under seismic loading. The transverse shear keys are located with a larger gap on the inside of the pier centreline. This prevents contact between the superstructure and the transverse shear key during SLS shortening effects as the alignment curvature and pier skew results in transverse movements. 8.0 SUMMARY Technically challenging structures were overcome by good teamwork and excellent coordination across many different engineering disciplines. Construction was completed in late The result is a world class busway that provides a new link between the North Shore and Auckland City 9.0 ACKNOWLEDGEMENTS Consent of the Clients, NZTA and North Shore City Council, to publish this paper is gratefully acknowledged. 12

13 Figure 5 - Construction Staging Sequence for Northcote Underpass 13