Over 2,000 rock bags holding over 8,000 tonnes of stone are being used in the construction of our cruise berth, the first time this effective engineering solution will be used in New Zealand.

LPC Infrastructure Manager Mike Simmers says over 1,000 rock bags have already been successfully placed underwater to provide scour protection to the seawall slope.

The large cruise vessels that will be calling at Lyttelton have substantial bow thrusters. This means they are extremely manoeuvrable but they place a significant amount of pressure on the seawall slope beneath the cruise berth.

“Initially it was determined we would need to place a significant amount of rock – and extremely large rock – as the outer layer to maintain the stability of the seawall.”

Mike says this would have been very technically challenging and require large rock pieces to be transported by truck to the construction site, which is costly and not an environmentally friendly solution.

“Then our designers recommended using rock bags, which have been used in Japan for over 20 years and in many other parts of the world.

Rock bags have been used to provide seawall protection in Chiba, Japan, and also after major typhoon events on the Tokyo coastline.

LPC undertook due diligence on the rock bags, including commissioning scale trials at the University of New South Wales, and Mike says all results were extremely positive.

The rock bags are made from 100% recycled polyester (PET). This is very similar material to geotextile fabric, which is used in most construction and infrastructure projects, and accelerated testing confirms they have a 50-year plus lifespan with 100% of strength remaining.

“Overall, the rock bags are an effective solution for this project and can be maintained by our team long term – we’re really pleased with the results.”

A total of 2,058 recycled rock bags filled with 8,232 tonnes of rock are being used to provide scour protection to the seawall beneath the cruise berth. Pictured here are the rock bags in a live trial at LPC.