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Concrete In Australia : September 2008
pile had approximately 70% of the geotechnical capacity required by the rail bridge. It was felt that the gravels could be loose and there were some minor clay layers which could severely affect the theoretical vertical pile capacity. Test piles Increasing the pile diameter meant trackworks and even longer construction windows. Deeper bored piles meant drilling below the watertable and very expensive liners. CFA (continuous fl ight auger) piles of the required diameter could not be constructed and the depth to rock was estimated as being over 100m. To resolve this impasse, it was decided to take the unusual step (in the design phase) of constructing a test pile and loading it statically and dynamically in order to get a better understanding of the performance of the founding soils under load. It was not practical to construct 1500mm-diameter test piles since an appropriate dynamic hammer, was not available in Australia and a special static load rig would need to be made. The pile testing was therefore undertaken on two 600mm-diameter bored piles founded at 15m depth in a clay layer in the gravels. In order to mimic excavation of the underpass, and hence removal of skin friction in the excavated depth, the top 8m of pile was sleeved. Both piles were dynamically loaded using an 8t hammer and analysed using PDA and CAPWAP analysis. One pile was then statically loaded. Pile vertical design The results of the pile testing were extremely encouraging and showed that the actual bearing capacity and skin friction were much higher than estimated. The gravels were not as loose as expected, and the skin friction in the clays, sands and gravels was higher than expected. Mobilising a piling rig to conduct large diameter test piling prior to construction was expensive and unusual. However, the results justified this approach and ensured that a D&C lump sum fee could be agreed, without excessive loading to allow for geotechnical risk. Pile lateral design Although provided primarily for vertical support of the bridges, the bored piles also retain the soils behind them. The fl oor of the underpass at the deepest point is approximately 6.5m below the bridge beam soffi t and nearly 8.2m below ground level. It is not practical to cantilever a pile this far, particularly without a propping base slab, so the bridge superstructure is used to prop the top of the piles. This is achieved by installing a thrust pad between the deck (as low as possible) and the backwall of the headstock. The resulting structural system was bored piles at 3.2m centres with a shotcrete arch between the piles and integral headstock and backwall. This propping arrangement is highly unusual for deck spans as high as the 28m eventually employed. Urban design initiatives Achieving a single span for the bridge decks was the single greatest urban design outcome for the project. Given the diffi cult geotechnical conditions, the cost of a twin span plank deck with central piles was found to be comparable to that of a much heavier single span super-tee alternative, even considering the secondary effects of lowering the vertical alignment. The clear span provides an open sense for all users and is considered to be particularly benefi cial for pedestrians, avoiding the perception of an enclosed tunnel. Feature lighting schemes have been designed to add visual interest to the structure and highlight key elements. Coloured parapet lighting illuminates the exterior faces of the fluted precast barriers. Downlights at the precast feature wall panels under the bridges highlight the patterned panels. The night view of the structure is spectacular. Other social outcomes Apart from the aesthetic benefi ts gained from employing good urban design on this project, as described above, considerable effort was expended to ensure that the new structure contributed to the urban renewal of the area. The old bridge was ugly and provided spaces that were unsafe for pedestrians at night. Homeless people were known to sleep under the approaches. The challenge was to provide an underpass that reversed the public’s perception of the area. Underpasses are generally considered more difficult than bridges to make open and provide a feeling of security for pedestrians. To this end, a CPTED (crime prevention through environmental design) consultant was engaged to provide progressive review and input to the design. Through this interaction, several modifi cations to the geometry and landscaping of the underpass were made. In particular, lines of sight and escape routes were considered. Potential hiding spots were eliminated. The challenge was balancing the conflicting needs to maximise access whilst minimising the chances of criminal activity (including muggings and graffiti). Equity of access was also a major consideration for the design. A DDA (disability discrimination act) consultant was also employed to progressively review our design and provide input. This resulted in at least one major change from the preliminary to the fi nal design. As would be normal for an underpass, stairs and DDA compliant ramps were provided for pedestrian and cyclist access to the underpass in the preliminary design. However, the DDA consultant questioned the equity of this arrangement, which required disabled users to travel much further than others in order to access the underpass. After extensive review and consultation with user groups, it was decided to take the unusual step of deleting all stair access. Thus all users are required to take a slightly longer route than would otherwise be necessary. The side benefits were a simplifi cation of the appearance and a cost saving. No complaints have been noted since the structure was opened to pedestrians and traffi c in January 2008. Acknowledgements The Department of Transport Energy and Infrastructure (DTEI) – Principal McConnell Dowell – Contractor. URS Australia – Geotechnical sub-consultant. QED – Traffic sub-consultant. Concrete in Australia Vol 34 No 3 53