SH20 Mt Roskill Extension Project

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1 SH20 Mt Roskill Extension Project Brent England, Associate Structural Engineer, URS New Zealand Limited Brent England is a structural engineer with 15 years experience primarily in bridge design. Brent has worked for URS New Zealand for 10 years. Recent projects include the Newmarket Viaduct replacement, Alpurt B2 Orewa to Puhoi extension and the SH20 Mt Roskill extension. Contacts: Ph Fx brent_england@urscorp.com SH20 Mt Roskill Extension Project B.England Page 1 of 15

2 SH20 Mt Roskill Extension Project Brent England, Associate Structural Engineer, URS New Zealand Limited SYNOPSIS SH20 Mount Roskill Extension is a key part of the new Western Ring Route around Auckland. The Mt Roskill section of the Ring Route is a $186m motorway extension, 4 km long and provides for 4 traffic lanes, a dedicated bus shoulder lane in each direction, a cycleway and a future rail line. The route includes 5 road bridges, 2 pedestrian bridges and several large anchored retaining walls up to 10m in height. The road bridges employ teeroff girders and DHC beams for the superstructure. The substructures comprise reinforced concrete piers and foundations. The pedestrian bridges are up to 170m long and comprise cable stayed main spans, precast concrete beams and columns, and reinforced concrete piled foundations. The longest bridge is the Hayr Road Motorway Underpass at 158m long. It has been constructed without expansion joints. Deletion of the expansion joints was considered beneficial as they typically require frequent inspections and ongoing maintenance. The two cable stayed footbridges provide for pedestrian and cycle traffic. The cable stayed system was selected primarily for its minimal structural depth but also to create a landmark structure on the new motorway. 1.0 INTRODUCTION Design of the SH20 Mt Roskill Extension project was carried out by URS New Zealand limited and completed in At the time of writing this paper the project is under construction and due to open April The project includes 5 road bridges, 2 pedestrian bridges and several large retaining walls. This paper describes these structures and discusses some of the design issues encountered. URS New Zealand was responsible for all the structural, geotechnical and civil design aspects of the project. Maunsell carried out ground investigations and provided the geotechnical parameters. Boffa Miskell and Jasmax provided urban design expertise particularly focusing on the aesthetics of the road bridge piers and parapets, the pedestrian bridges and the retaining wall facing panels. 2.0 ROAD BRIDGES The 4km extension of the SH20 Motorway through Mt Roskill extends the existing motorway from Hillsborough Road to Maioro Street. Four new bridges have been constructed over the new motorway and one short span bridge over Oakley Creek on the motorway. The four bridges over the new motorway (typically described as motorway underpass bridges below) are located at Hillsborough Road, Hayr Road, Dominion Road and May Road. SH20 Mt Roskill Extension Project B.England Page 2 of 15

3 2.1 Motorway Underpass Bridges at Hillsborough Road, Hayr Road, Dominion Road and May Road The underpass bridges have used consistent structural solutions were feasible for ease of construction and to provide consistent aesthetic flow along the route. The edge parapets, teeroff girders, diaphragm beams, elastomeric bearings and piers are common to all four underpass bridges Motorway Underpass Bridge Superstructure The bridge superstructure comprises 1000mm deep teeroff girders with an insitu topping of minimum 165mm depth. Span lengths vary from approximately 19m to 24.8m between pier centrelines. The bridges at Hillsborough Road, Hayr Road, Dominion Road and May Road have 2, 7, 2 and 3 spans respectively. A typical cross section through the May Road Motorway Underpass superstructure is shown in Figure 1 below. The longest bridge at Hayr Road is 158m long. The scheme design for Hayr Road Motorway Underpass was 197m however during the detailed design phase it was confirmed that the approach embankments could be extended allowing the two end spans to be removed. Bridge widths vary with between 6 and 13 Teeroff girders used per span. In total there are 118 teeroff girders used in these bridges. Fig 1: May Road Motorway Underpass Cross Section A 2m wide reinforced concrete footpath is provided each side of each of the bridges. The edge parapets are reinforced concrete 820mm high with a VCB front face profile. The parapets were designed as precast units, typically in 3m lengths, cast integrally with the adjacent footpath. The outside face of the parapets has a tapered top portion and a negative stripe to improve aesthetics as shown in the diagram below. A steel handrail is provided above the concrete parapet extending to 1100mm above the footpath level. The handrail mirrors the top portion of the pedestrian bridge handrails (refer Figure 10). The edge barriers are designed to withstand TL-3 performance level loads as defined in the TNZ Bridge Manual. The teeroff girders span simply supported between elastomeric bearings at the piers and abutments. The deck linkage slab over the piers is debonded from the girders over a 1.425m length with a 12mm layer of Expandafoam to avoid inducing large forces in the slab due to deck rotation. The deck spans were modelled using 2D finite element grillages with STRAP computer software. STRAP allows for simple and quick analysis with moving vehicle loads (HN-HO-72) applied at varying positions within each design load lane. The beam and finite element grids are quick to set up and modify for the varying SH20 Mt Roskill Extension Project B.England Page 3 of 15

4 configurations of span length and skew angle - up to 33 degree skew at the Hayr Road Motorway Underpass. The design and modelling of the girders accounts for the loads applied at each stage of construction teeroff girders alone, composite action with the insitu deck and second stage composite action with the reinforced concrete footpath. The number of prestress strand required varies with up 44 x 12.7mm diameter low relaxation superstrand required in the longest span edge girders at May Road Motorway Underpass. The design provisions in the Concrete Structures Standard NZS 3101:1995, for fully prestressed sections were used. The deck design for the spans was based on the TNZ Bridge Manual empirical design provisions however additional analysis was carried out at the edge cantilevers and linkage slabs over the piers; additional longitudinal deck reinforcement was provided at these locations. Insitu reinforced concrete diaphragms are provided between the teeroff girders at all support locations. Reidbar couplers were cast into the teeroff girder end diaphragms to provide for continuity of the diaphragm through the girders. Reinforced concrete shear keys are provided at the piers and abutments to transfer lateral loads to the substructure based on elastic response to seismic loads. May Road Motorway Underpass has a large turning flare at the NW corner of the bridge. The flare intersects the edge girder at approximately mid span and curves in plan as shown in Figure 2. The flare was included in the finite element span analysis. Propping of the flare slab and edge teeroff girder were required during construction of the flare. Fig 2: May Road Motorway Underpass - Plan Motorway Underpass Bridge Substructure The substructure for the bridges varies although similar solutions are used at each location. The piers comprise thin (360mm thick) reinforced concrete blade piers as shown in Figure 3. The pier thickness is minimised to improve the aesthetic appearance of the bridges and minimise the width of the motorway carriageway. SH20 Mt Roskill Extension Project B.England Page 4 of 15

5 Further aesthetic enhancement is provided by applying horizontal negative stripes to the pier wall and the leading and trailing outside edges of the piers taper in plan and elevation as indicated below. The pier walls of the bridges adjacent to the motorway were designed to withstand TL-5 performance level barrier impact loads. Fig 3: Motorway Underpass Blade Pier Elevations Pier founding conditions vary at the bridge locations from competent basalt at shallow depth to basalt or sandstone at up to 20m depth. Hence a combination of shallow pad footings and piled foundations were used. Some piles at the Hayr Road Motorway Underpass are sleeved through a basalt layer and socketed into the founding sandstone beyond. The basalt layer was unsuitable for founding due to fractures within the basalt and the potential for settlement in the alluvium layer located between the basalt and sandstone. The piles at the sleeve locations used a temporary 1050mm diameter steel casing to allow for placement of a weak cement/bentonite layer between the permanent pile casing and the basalt. The abutments vary at the bridges with either pad or piled foundations supporting a capping beam. All the bridges are designed without expansion joints at the abutments. This is standard practice for the short span bridges however the Hayr Road Underpass is 158m long and is believed to be the longest integral abutment bridge in New Zealand. This is discussed further below. The Hillsborough Road Motorway Underpass abutment support is provided by the retaining walls either side of the motorway carriageway at this location. The retaining wall (RW49) comprises 900mm diameter piles at 1800mm centres with a SH20 Mt Roskill Extension Project B.England Page 5 of 15

6 reinforced concrete capping beam and Dywidag ground anchors at 1800mm centres. A 5m long settlement slab is provided at each abutment. The Dominion Road Motorway Underpass has 2400mm wide reinforced concrete pad foundations founded on a soil nailed retaining wall as shown in Figure 4. Fig 4: Dominion Road Motorway Underpass Section The May Road Motorway Underpass abutments comprise 2400mm wide reinforced concrete pad foundations and are founded on the MSE embankments as shown in Figure 5. SH20 Mt Roskill Extension Project B.England Page 6 of 15

7 Fig 5: May Road Motorway Underpass Section The analysis and design of the substructure was relatively simple for the Hillsborough Road, Dominion Road and May Road Motorway Underpasses. The seismic loading was based on an elastic response to seismic loads (µ=1) in the transverse direction and locked-in (zero period) seismic loads in the longitudinal direction as prescribed in the TNZ Bridge Manual. Seismic loads utilised the results of a site specific seismic study however a lower bound of 2/3 the TNZ Bridge Manual seismic load level was applied. The Hayr Road Motorway Underpass abutments comprise 3 x 900mm diameter reinforced concrete piles and a reinforced concrete capping beam. The capping beam supports the superstructure and a 5000mm long settlement slab as shown in Figure 6. At 158m long Hayr Road Motorway Underpass is exceptionally long for an integral abutment bridge. TNZ Bridge manual allows concrete bridges up to 70m long to have integral abutments but longer bridges require special study. At Hayr Road the special study considered the shortening effects on the abutment piles and soilstructure interaction of the retained soils. A literature search was also undertaken to assess the likely long-term performance of such a long integral abutment bridge. Deletion of the expansion joints was considered beneficial as they typically require frequent inspections, ongoing maintenance and they are usually noisy and affect ride quality. However integral abutment bridges are not without maintenance. The ratcheting effect of thermal expansion and contraction cycles of the bridge are SH20 Mt Roskill Extension Project B.England Page 7 of 15

8 expected to cause the abutment fills to settle and some cracking of the asphalt at the ends of the bridge. These negative effects have been mitigated to some extent by a reinforcing grid in the asphalt and two layers of heavy geogrid connecting the abutments to the approach fills. The geogrid length and strength was selected to distribute the imposed displacement (as the bridge superstructure shortens) over an adequate length of embankment and minimise crack widths in the asphaltic surfacing. The geogrid grade was specified with adequate strength to resist the shear friction mobilised on the underside of the reinforced layer of approach fill. The geogrid fabric extends into the diaphragm beam, wraps around a reinforcing bar and is cast integrally with the diaphragm. Fig 6: Hayr Road Motorway Underpass Abutment Section Construction of the Hayr Road bridge was completed in October A visual inspection in January 2009 showed no defects or cracking in the asphaltic concrete surfacing adjacent to the abutments. The Hayr Road Motorway Underpass utilised Larsa2000 computer software with 3D stick models and response spectrum seismic loading. The structure is designed for a limited ductile response to seismic loads (µ=3) although the potential plastic hinge regions in the piles comply with the detailing requirements for full ductility (µ=6) as specified in the Concrete Structures Standard NZS 3101: Oakley Creek Bridge The motorway bridge over Oakley Creek comprises a single span precast DHC (double hollow core) superstructure. Standard 700mm deep x 1144mm wide DHC units are used as shown in Figure 7. The edge parapets comprise F-shape TL-4 reinforced concrete insitu barriers. The DHC units are stressed transversely at the ends, ¼ span and mid span points. SH20 Mt Roskill Extension Project B.England Page 8 of 15

9 Fig 7: Oakley Creek Bridge Typical Section The substructure comprises 900mm diameter reinforced concrete piles with a reinforced concrete capping beam. The piles are sleeved through a thin basalt layer and socketed into the sandstone below. Reinforced concrete settlement slabs 7m long were used to minimise road distortion in the settlement prone ground adjacent the bridge. 3.0 PEDESTRIAN BRIDGES There are two pedestrian bridges located at Keith Hay Park and Ernie Pinches Street. Figures 8a & 8b show the plan and elevation of Keith Hay Park footbridge. The pedestrian bridges were designed with cable stayed main spans. The cable stayed solution minimises the depth of the slab and provides an aesthetically pleasing visual landmark. The cable stayed spans are up to 40m long. The cable stay system allows for a slab depth of 350mm. The reduced structural depth minimises the visual bulk of the elevation and shortens the bridge ramps (at 1:12 gradient for disabled access) compared to a long span deck option. Fig 8a: Keith Hay Park Footbridge Plan SH20 Mt Roskill Extension Project B.England Page 9 of 15

10 Fig 8b: Keith Hay Park Footbridge Part Elevation The deck slab comprises reinforced concrete precast units with insitu concrete infills at the piers. Span lengths are typically 10.2m for the pier spans. The cable stayed spans have cable supports at approximately 9.4m centres. The deck slab width is 4.2m and 3.2m at Keith Hay Park (refer Figure 9) and Ernie Pinches footbridges respectively. Fig 9: Keith Hay Park Footbridge Section The edge handrails comprise 1400mm high combined pedestrian-cycle rail as shown in Figure 10. The handrail has profiled (curved and tapered) steel plate posts, a steel tube top cycle rail at 1400mm height, a stainless steel tube handrail at 900mm height and stainless steel infill wire strands at varying centres. URS had obtained prior approval for this type of balustrade from the Consenting Authority (Auckland City Council) prior to detailed design commencing. We have not been able to obtain approval since despite the balustrade meeting the requirements of the Building Code i.e unscalable. SH20 Mt Roskill Extension Project B.England Page 10 of 15

11 Fig 10: Footbridge Handrail Section VSL 36mm diameter stressbar were used for the cable stays. The cables splay in plan and elevation with splay angles varying from 3 to 10 degrees in plan and 30 to 60 degrees in elevation. The anchorage detail at the underside of the deck is shown in Figure 11 below. A 3 coat paint system was applied to provide corrosion protection to the cables. Fig 11: Footbridge Cable Anchorage Detail SH20 Mt Roskill Extension Project B.England Page 11 of 15

12 The structural steel pylons supporting the cable stays comprise twin welded plate 600mm x 600mm I sections that transition to a diamond shape cross section at the top connection point to the cable stays as shown in Figure 12. A zinc metal spray coating of 150 microns with seal coat corrosion protection system was specified. Fig 12: Footbridge - Pylon Elevations The reinforced concrete Y-shape piers were detailed as precast elements as shown in Figure 13. The piers are founded on 900mm diameter reinforced concrete piles typically. SH20 Mt Roskill Extension Project B.England Page 12 of 15

13 Fig 13: Footbridge Pier Elevation Design of the pedestrian bridges utilised Larsa2000 computer software analysis. A 3D model of the structure was analysed for the gravity and lateral loads. The structure is designed for elastic response to seismic loads (µ=1). The structure was checked for deck vibration and dynamic wind response in accordance with BS5400. Various broken cable scenarios were considered to avoid progressive collapse should a cable break. 4.0 RETAINING WALLS There are 63 retaining walls with approximately 12 different types of retaining wall utilised including timber pole, keystone, soil nail, precast concrete cantilever, reinforced concrete pile with and without ground anchors (refer Figure 14) and MSE retaining walls. SH20 Mt Roskill Extension Project B.England Page 13 of 15

14 Fig 14: Anchored Pile Retaining Wall - Section The tallest retaining wall comprises an anchored concrete pile wall up to 10m high and is surcharged with an 8m high soil nailed slope above as shown in Figure mm diameter reinforced concrete piles with prestressed ground anchors (up to 18 x 12.7mm diameter strand anchors) at 1800mm centres were used. Wallap and Slope computer software were used to analyse the retaining walls. The concept design had full height retaining walls with two levels of anchors. Significant savings were realised in the solution adopted during the detailed design phase of the project. Fig 15: Combined Soil Nail / Anchored Pile Retaining Wall - Section SH20 Mt Roskill Extension Project B.England Page 14 of 15

15 The precast facing panels on the piled retaining walls at and adjacent to Hillsborough Road are very dominant features of the landscape. They were given a decorative finish pattern as shown in Figure 16 to enhance the aesthetic appeal of the route. 5.0 CONCLUSIONS Fig 16: Retaining Wall Precast Panel Decorative Pattern The SH20 Mt Roskill Extension project is a key part of the Western Ring Route around Auckland. The route includes 5 road bridges, 2 cable stayed pedestrian bridges and several large anchored retaining walls. The variable ground conditions and geometric constraints provided design challenges. Innovative design solutions were developed during the detailed design phase of the project including deletion of the expansion joints at the 158m long Hayr Road Motorway Underpass abutments and the combined soil nail slope and anchored pile retaining wall up to 18m high. The aesthetics of the structures were carefully considered and the solutions adopted, such as the decorative retaining wall facing panels, slender blade bridge piers, cable stayed pedestrian bridge main spans and profiled handrail detailing, are consistent along the route and enhance the visual appeal of this section of Motorway. ACKNOWLEDGEMENTS URS thanks NZTA for awarding us this project and for giving us permission to write this paper. The author would like to thank the many staff at URS who worked diligently on this project to create a successful design solution to the SH20 Mt Roskill Extension with particular reference to the Design Team Leader Peter Lipscombe for his guidance and overview. SH20 Mt Roskill Extension Project B.England Page 15 of 15