There is a science to safe bridge demolition methods and our work on the Chinaman Creek bridge highlights many of the challenges that must be considered in determining how to demolish a bridge over water.
Located in tropical North Queensland, the Chinaman Creek bridge initially appeared to be an assignment that could be undertaken with standard bridge demolition methods. Beyond the preliminary analysis, however, the project presented multiple unique and demanding challenges that drew on our best problem solving talent.
For a start, Chinaman Creek bridge was located in Cairns; a city considered to be at the heart of the Great Barrier Reef Marine Park. Naturally, this meant strict environmental conditions were imposed on the project.
Another element of the local environment was the native saltwater crocodiles. It just so happened that Chinaman Creek is their native habitat, adding another dimension to safety considerations.
The challenges didn’t stop there. After the project was underway, we discovered alkali silica reaction, or ASR, had caused significant damage to the bridge structures. Consequently, we were forced to re-evaluate the bridge demolition methods planned for the project.
Add to these complexities a significant volume of nightworks, strong tidal and current movements and severe tropical storms resulting in 1,036mm of rain over the course of the project and the outcomes – safe delivery, zero incidents, and a 97 percent recycling of demolition waste – are even more remarkable.
Read on to learn how we went about a project to demolish a bridge over water.
How to demolish a bridge over water
If the Chinaman Creek bridge is any kind of measure, the first step in understanding how to demolish a bridge over water is to expect the unexpected.
While we were fully aware of the site’s environmental obligations, it did require that we adopt some innovative solutions to complete the project within the constraints that were inherent in them because the bridge needed to be removed in its entirety down to riverbed level. This meant removing a total of 68 prestressed concrete bridge deck units (14 metres and 20 metres in length; relieving slabs; abutments; and three piers comprised of headstocks and sixteen 500mm piles with pile encasements.
After assessing the options for bridge demolition methods, we decided on a number of demolition techniques to safely remove the bridge:
Span 3 bridge deck units were removed by using dual lifts with 250T and 200T mobile cranes established on the abutments. To eliminate any hazards with live loads over the operating highway, the dual lifts were conducted during nightworks. Remaining bridge deck units were removed individually with the 250T or 200T mobile crane.
Working with the low tides, a number of piers were mechanically demolished with excavators and the remaining piers were cut, released and lifted with the 250T mobile crane.
All bridge components were mechanically processed onsite to maximise recycling on the project.
Working with the environment
We had three major priorities in terms of health and safety with the project. Ensure the safety of our people, eliminate any impact on the sensitive mangrove environment, and avoid disturbing the crocodiles’ habitat.
Fortunately, we achieved all three with a little innovation and ingenuity.
Because the demolition required significant amounts of concrete saw cutting to release the prestressed concrete decks and to separate piers, and the fact that the bridge decks units contained polystyrene cores which need to be contained, we implemented a comprehensive environmental catchment system. In doing so, we were able to eliminate any slurry, polystyrene or debris entering the waterway.
We further controlled the slurry at the point of generation with dust suppression and wet vacuuming. As a secondary measure, the team installed two custom Envirosmart “floating blankets”. The floating blankets were made up of five layers, custom designed to capture and retain any debris:
Base layer – Heavy duty needle punched polypropylene geotextile.
Two layers of industrial-grade plastic air bubble flotation layers
Fourth layer of netting made from strong HDPE fibre
Top layer – Heavy duty needle punched polypropylene geotextile.
The floating blankets were used for the duration of the project and successfully eliminated all environmental impacts on the waterway and mangroves.
Working with saltwater crocodiles
Mention working in a saltwater crocodile habitat and most people’s eyes widen in awe, and with good reason.
Estuarine (saltwater) crocodiles have a reputation for being deadly, but they are a protected species in Queensland under the Nature Conservation Act 1992. Commonwealth law also provides for their protection, as does international law articulated under the Convention on the International Trade of Endangered Species.
Due to the project’s location, the risk of human–crocodile interaction was high due to the significant number of people living in the area and the high likelihood of crocodiles entering the area from surrounding crocodile habitat.
Crocodiles are opportunistic, ambush predators that can feed on any sized animal they can overpower, including humans. They are highly territorial, particularly during breeding seasons, and will attack anything in their habitat that they perceive is a threat. As a result, crocodiles can, and do attack humans. Queensland Government records indicate that between 1985 and 2016, 17 non-fatal attacks and eight fatal attacks by estuarine crocodiles have occurred in Queensland in the wild.
To complete the demolition of the bridge, it was necessary to enter the waterway to install environmental control measures and remove the piles at riverbed level by administering underwater saw cuts. To allow these cuts to be made, the team designed and fabricated a custom dive enclosure that took into account the need to fit around the large concrete piers in different configurations. This obstacle was overcome by adopting a modular design which was assembled onsite to suit the conditions.
The crocodile dive enclosure was crane lifted into position, ensuring no crocodiles were inadvertently entrapped within the enclosure during positioning. Divers entered the murky water via the top access hatch and administered the underwater saw cuts to safely separate the piles for removal from the waterway.
Needless to say, the project team breathed easier once this aspect of the project was completed.
Alkali silica reaction in concrete: The impact on bridge demolition methods
After mobilising to site and commencing preliminary works, an investigation of the bridge concrete revealed significant structural damage on the bridge deck units and piles as a result of Alkali silica reaction (ASR).
ASR is a chemical reaction that occurs between aggregates and alkalis to form an expansive gel. It leads to exposure of the reinforcement to oxidation and allows moisture to seep in.
No waterproofing or sealing had occurred during the life of the bridge and consequently, moisture had penetrated the substructure, resulting in structural damage, which meant the overall condition of the bridge structure was extremely poor.
The ASR damage presented the risk of complete or partial concrete break up or collapse during the crane lift operations which would have resulted in a catastrophic environmental and safety incident. This led to a complete re-evaluation of the bridge demolition methods that had been considered previously, including:
Increasing the size of the cranes
Revising the positioning of the cranes
Increasing the quantity of saw cuts
Removing the headstock and piles in smaller sections.
These changes required comprehensive secondary investigations. Among these were engineered crane pads, lift studies, ground bearing pressures and engineering sign off. Working collaboratively with the client, the new procedures were adopted with minimal delay and the demolition was completed without incident.
Meeting our recycling targets
Increasingly, we strive to minimise the quantity of recyclable materials entering landfill and contribute to a circular economy. We track our recycling rate as a key performance indicator and set stringent targets at commencement of each project.
During project planning, we identified an opportunity for selected bridge deck units to be donated and repurposed in the local community. All remaining demolition waste was mechanically processed, including capturing and removing deck unit Styrofoam cores. Waste was processed and sorted onsite to ensure the maximum recycling rate was obtained.
Based on the effective waste management strategies adopted, 97 percent of demolition waste was recycled or repurposed with only minimal waste entering landfill for disposal.
On the world stage…again
The project team didn’t set out to profile their efforts internationally, but that’s exactly what happened with the Chinaman Creek bridge demolition. Our submission for the World Demolition Summit 2021 has seen the project shortlisted and we are hopeful that the team will be recognised for their professionalism, commitment to excellence, and remarkable outcomes.
DEMEX is a specialist in bridge demolition methods for implementing how to demolish a bridge over water, highways, roads, and pedestrian thoroughfares. Bringing world leading innovation, teams, and technology to demolition projects for bridges, commercial and industrial structures, including structures at heights, on mine sites, energy utilities, and renewables, we approach every project with a commitment to performance excellence in safety, environment, and our people. For information on efficient, safe, and cost effective ways to a complete bridge demolition, contact us directly on email@example.com.