Precast culvert solution for Wheatstone project

The decision to use precast rather than in-situ concrete culverts on the Wheatstone project in WA resulted in a number of econonmic advantages and efficiencies.Since late 2011, two liquefied natural gas (LNG) trains, with a combined capacity of 8.9 million tonnes a year, have been under construction as part of Western Australia’s momentous Wheatstone project.

Located at Ashburton North, the Chevron-operated Wheatstone project is one of Australia’s largest resource projects. The foundation of the project includes the two LNG trains as well as a domestic gas plant.

In 2014 National Precast Member and Western Australia precaster PERMAcast won the competitive tender to supply precast trench culvert units to Laing O’Rourke, which holds a major structural engineering and civil works contract for the project. The original plan called for the 1000-plus individual trench units to be cast in-situ.

The contractor recognised the potential benefits of using a precast alternative. “When you work up in the northwest of Western Australia, the cost of labour is so much higher than in other areas, and so is the cost of raw materials. You just don’t get the efficiency on site that you do with precast,” explains Alberto Ferraro, PERMAcast Managing Director.

The company proposed a precast solution, which was taken into consideration. “All the design work for the project was done in Houston, Texas and there was reluctance to change from the original in-situ design. However, Laing O’Rourke are quite an innovative company and they were receptive to looking at the precast option,” he says.

The company assisted the clients in formulating an alternative precast trench culvert design. The precaster’s in-house engineers developed manufacturing methodologies that enabled the trench culverts to be produced off-site. Three-dimensional modelling was used for all of the units to ensure the suitability of the culverts and that they fitted correctly on site.

Manufacturing the culverts also required some strong technical expertise, which the precaster strived to deliver.

“When we came up with a suitable design, we ended up making 1000 units of various sizes – no two units were the same,” says Mr. Ferraro. Variable falls meant that each trench culvert had to be cast not only with the required gradient for drainage, but also to accommodate variable heights at ground level. The average unit weighed 22 tonnes.

Mr. Ferraro says that the culverts, being a long-term and vital component of the overall project, required some innovative solutions to adhere to the crucial design specifications laid out by the client. A high quality S40 fly ash concrete mix was used throughout the construction and off-site fabrication period. A perlite aggregate – an exceptionally lightweight aggregate – was incorporated into the insulating concrete lining used in 560 lineal metres of the overall trench system. This was to cater for a potential emergency overflow of supercooled natural gas.

PERMAcast’s ISO 9001 quality-controlled factory was manufacturing up to eight units a day for a period of nearly 12 months, stocking in excess of 600 units in its storage yard. “One of our big advantages is that we have so much land – we can cater for large amounts of product storage,” adds Mr. Ferraro.

Mr. Ferraro  says the successful switch from in-situ to precast concrete trench culverts resulted in a number of benefits on the Wheatstone project. “I think using precast for this application was a very good option – it had both economical and program advantages,” he says. “No matter how big the job or how remote it is, using a precast solution also helps to minimise on-site hazards.” Of course, on a project of this magnitude, Mr. Ferraro says the standard of product had to be of the highest calibre, and a viable precast solution has helped deliver that. “Any precast that’s manufactured off site is an advantage for quality.”

This story has appeared in the Roads & Civil Works February/March 2016 edition – get your copy here today!

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