What impact did the earthquakes have on the bridges?
In general the primary cause of damage to bridges was from the lateral spreading of the river banks supporting the bridge abutments. Liquefaction and lateral spreading, caused by the earthquakes, applied pressure to the abutments, often forcing them to rotate relative to the bridge deck. Land settlement also caused damage to bridge approaches and adjacent roads.
Basic bridge structure:
Abutment- An abutment is a structure located at the ends of a bridge, where the bridge deck adjoins the approaching road.
Piers- These are the intermediate supports, under the bridge, between the abutments, on which the bridge deck rests.
Piles-Bridge piles provide support to the abutments and piers. Piles are drilled down through softer soils until they reach stronger soil or rock.
Bridge deck- This is the surface of the bridge on which the road and/or footpath is laid. It is supported by the bridge abutments and piers.
Bridge approach- The section of road that connects the road onto the bridge.
Are the bridges safe to use now?
The bridges around Christchurch have been checked by engineers to make sure they are safe for traffic and pedestrians after the earthquakes.
Where a bridge is determined to be unsafe it is closed. Temporary works were also undertaken on some bridges to allow them to remain open. Some bridges may have a weight restriction or speed restriction so that it may continue to be used safely (e.g. the current 30km/h restriction on the Bridge Street Bridge). A weight restriction limits the size of vehicles that can use the bridge, usually preventing heavy or large vehicles crossing the bridge. A speed restriction limits the impact of vehicles going onto the bridge (e.g. the faster the car is travelling, the greater the impact on the bridge).
The bridges around the city are also checked after each significant earthquake to make sure they remain safe to use.
How many bridges require repair across the city?
There are approximately 225 bridge structures (including road and foot bridges, and larger culverts) in the city. Of these about 140 require some level of repair from minor works, such as repairing cracks, through to major repair work.
How are bridge repairs prioritised?
Bridge repairs are prioritised based on four factors:
- The level of earthquake damage,
- The traffic volume (the busiest bridges have priority),
- Weight restrictions,
- Road bridges versus footbridges (footbridges have a lower priority).
Those bridges requiring significant repairs are undertaken as individual projects (e.g. Bridge Street Bridge). Bridges with minor damage are grouped into work packages.
Will the bridges be replaced or repaired?
Each of the bridges is subject to a robust investigation and design process. The decision on whether a bridge is repaired or replaced is based on the level of damage to both the bridge, and the surrounding land. Bridges are significant pieces of infrastructure, and most can be safely repaired rather than being replaced. A bridge repair is often quicker, with less traffic disruption, environmental impact, and is more cost effective than a replacement.
How is SCIRT making bridges more resilient?
- Settlement slabs can be built into the abutments to reduce the ‘ski jump’ effect if there is any further land settlement.
- Piles may need to be drilled deep into the embankments in order to reach stable ground.
- In many cases the rigid pipes that run under the bridges (carrying other services) caused damage to the bridge approaches. These pipes can be replaced with flexible PVC (polyvinyl chloride) or PE (polyethylene) pipes.
- For some bridges there will need to be ground improvements in order to protect the piles on either end from lateral spreading and liquefaction. Possible design solutions include injecting the surrounding soil with grout to reduce the impact of future liquifaction.
What will the bridges look like when they are repaired?
SCIRT is repairing bridges to their pre-earthquake levels of service. Modern materials and construction methods will be used to improve the resiliency of the bridges, and bring them up to current design standards, where required. This means that the appearance of the bridges will not vary greatly. Where we do need to rebuild or replace sections of the bridges they will be of a similar design.
Will the bridge need to be closed during the repair?
We will keep the bridges open as much as possible during construction to minimise traffic disruption.
However, there may be times during the repair, such as when the bridge deck is jacked up, when the bridge must be temporarily closed to traffic for both technical and safety reasons. We will notify the community in advance of any planned bridge closures.
How will this work impact on traffic?
As with all of our rebuild work, safety is our number one priority. After ensuring the safety of our work crew and the general public, we will look for a setup that keeps traffic flowing and causes the least disruption to the neighbouring community. In order to work safely on the bridges some will have to be reduced to one-lane of traffic. There may need to be temporary detours, traffic lights, and other traffic management in place. We will notify you before the work begins, and keep you up to date throughout the construction process.
SCIRT has undertaken traffic modelling to investigate the impacts of restricting the traffic flow on bridges while they are repaired. This is done to make sure the wider road network has enough capacity to keep operating during the repair of the bridges. For example the modelling has shown that the wider road network will cope with reducing the Bridge Street Bridge to a one way west, although the network is likely to be busier in the evening peak period, particularly at Pages Road.
While SCIRT does try to minimise its impact on the community and motorists, the vast majority of our city’s horizontal infrastructure are located in the road reserve, which means rebuilding these networks will have an impact on roads and traffic. Over the next four or five years there will be hundreds of detours throughout the city while these vital rebuild works are carried out. These detours and traffic changes will affect thousands of people throughout the city, and are an unavoidable, realistic, part of the city’s rebuild.