The challenge: 17 new bridges, a severely seismic area with the liquefaction potential, and a requirement to be aesthetically pleasing, sensitive to the natural environment, fit for purpose, and to maintain social and cultural values.
It’s in one of New Zealand‘s most severe seismic regions, with two faults within a 30km radius. Sand dunes and inter-dune peat deposits can be found along the stretch of highway, as can a roads and waterways, and a mixture of rural and urban environments.
Traversing this challenging environment is the new MacKays to Peka Peka Expressway – a four-lane highway to provide safer and more reliable journeys for Kāpiti Coast residents, as well as improve connectivity to the world’s southernmost capital city of Wellington. Throughout the 18-km of new expressway are 17 multi-span and single-span bridges.
Reflecting the natural surroundings and landscapes, the designers and constructors used technology and innovation to create structures of elegance, strength and durability. Through collaboration, they balanced ground improvement, structural design, construction, performance, risks and cost.
The approach: Accelerated Bridge Construction techniques, innovation, technology and collaboration between designers and constructors.
Accelerated Bridge Construction (ABC) uses innovative planning, design, materials and construction methods, to help reduce the time and cost involved in building, replacing or rehabilitating bridges. But with no guidelines available for ABC in seismically active areas, the design team and constructors had to develop some smart technologies for the highly seismic zone.
No one technique was used; instead, across the majority of the project’s multispan bridges, you’ll find the team have used a variety of ABC construction techniques to help find the right balance. They include:
- Standardisation of bridge components for efficient and cost effective design
The confinement of, and restricting of splicing, to longitudinal reinforcement within the potential plastic hinge zone, usually severely limits the use of standardisation in high seismic areas. However, through careful planning the team were able to repeatedly use a number of components including piers and columns, a large diameter bored pile foundation system, pre-cast inverted-T reinforced concrete crosshead beams, abutments and facia panels. The latter, when combined with the architecturally-shaped bridge piers, provide the bridges with a common appearance to the superstructure and substructure throughout the project. The bridges look like a family, so to speak.
- Foundations systems for efficient, quality and speedy recovery
Providing lots of space and natural light to the pedestrians and traffic underneath the bridges, are hammerhead/portal frame piers. They are supported by a monopile system of 3.0m diameter piles, while 2.1m diameter bored piles support the portal frame pier columns. The pier core and pile reinforcement cages are circular to enable the pier reinforcement to plunge into the pile top concrete. The system is flexible to accommodate the pile construction tolerances.
- Prefabrication of bridge components or complex and heavy reinforcement cages
Pre-cast beams in bridge construction are common. In this instance, the Alliance team maximised the prefabrication of bridge components for efficient, cost effective and quality construction. Super T/Single Hollow Core beams, cross-head beams, fascia panels and steel cages are just a few of the prefabricated components uses.
- Elimination to improve construction efficiency
Eliminating an element, operation or dependency of elements on each other can reduce the total number of elements/operations in the construction sequence; thus, making bridge construction more efficient and improving the quality of work at the site.
Take the pile caps – by using bridge piers supported on large mono-piles the team were able to eliminate the pile caps and reduce the loads applied to the piles from laterally spreading ground in liquefaction. They also eliminated the need for concreting the pile-column interface as a second stage, saving a whopping 210 days in preparation and 175 days in fixing steel at height, off the time to construct these elements; improving safety by removing confined space work and reducing work at height.
- Connection of different bridge elements for safe and durable construction
In highly seismic areas, lapped/coupled reinforcement connections are not allowed within the plastic hinge zones. The lack of widely accepted, well-developed, and proven ABC connection details in high seismic zone was a challenge. But by considering the connection details during design, the team were able to save time and effort during construction.
Examples include the architecturally designed piers – an elongated hexagon at the lower end, ascending into a rectangular shape at the top; allowing the pier main cage to be circular to provide flexibility in accommodating the pile construction tolerance. A post-tensioned connection which attaches the precast crosshead beam to the cast-in-situ pier - reducing the joint reinforcement congestion and increasing the speed of construction. And linkage reinforcements, to restrain the bridge span to the pier in the event of an earthquake.
- Installation of bridge components in a safe and cost-effective manner
No ordinary crane was used to put the bridge and cross-head beams, and facia and facing panels into place. Instead, a modern high capacity 400 tonne crane was transported from South Africa. Capable of travelling over the haul road and being set up in 2-3 hours, it enabled the heavy pre-cast elements on the two largest bridges - the 180m long Waikanae River Bridge and the 125m long Te Moana Road Overpass Bridge – to be installed at the same time.
Ngarara Road Bridge, was constructed slightly differently - using the top-down construction method with an innovative technique for installing the facing panel underneath the bridge. This low cost and efficient method, only required a 20 tonne excavator, a bulldozer and some recycled steel from the yard to install the precast panels under the bridge.
The result: The ability to push boundaries for more efficient, cost-effective construction and a safer working environment.
The high seismicity, soil conditions and urban environment presented a unique challenges, especially when the combined exposure of the bridges to inertia loading as well as embankment movement, presented a scenario where standard design approaches were inadequate.
Through collaboration and innovation, the team developed appropriate direct displacement based design methods, which would allow the structures to be designed to more realistic levels of performance. The developed approach provided the client with greater confidence in how their structures will perform under a specific design event.
Under normal circumstances, the confinement of longitudinal reinforcement, restriction on use of lapped or coupler connections and splices to the longitudinal reinforcement within the potential plastic hinge zones severely limits the implementation of ABC techniques in high seismic areas. But by pushing hard to find solutions in design and construction of M2PP bridges, we made it happen. Adopting a number of ABC techniques improved construction efficiency, made a safer working environment and provided structures that will serve the Kāpiti Coast community for many years to come.
This article was originally presented as a white paper at the 2017 Austroads Bridge Conference. The full white paper - Use of Accelerated Bridge Construction Techniques in a High Seismic Area – was written by Jamil Khan (Technical Director) and Geoff Brown (Senior Technical Director) from Beca, and Tim Pervan (Site Engineer) and Matt Zame (Construction Manager) from Fletcher Construction.
Images are courtesy of the M2PP Alliance, with credit and copyright attributed to Mark Coote.