Combigrid Reinforces Container Hardstand in Gladstone

The Project

In 2010, Northern Stevedoring Services (NSS) committed to the opening of a new stevedoring operation in Gladstone at the Port Auckland wharf. A site was selected to utilise the existing adjacent rail corridor allowing NSS to offer a complete port logistics service to the Gladstone region.

As part of the development, NSS needed a hardstand container terminal for the storage and distribution of containers. This hardstand needed to be capable of withstanding the stacking of containers in many vertical multiples, and supporting the loads of the container stackers and long-reach equipment to be used on the hardstand.

Design challenges

NSS commissioned a suitable design for the hardstand providing a 25 year design life for the facility. The rubber-tyred container handling equipment would weigh up to 40 tonnes when loaded, and all of this was to be built over a relatively weak subgrade of CBR 2.5% strength.

The engineers completed a design option for the client, both with and without geogrid reinforcement. The preferred geogrid considered in the design option was the Combigrid® geogrid supplied and supported by Global Synthetics.

Due to the high Secant Modulus of >1.1MN/m afforded by Combigrid®, the pavement could be designed using conventional techniques, but with a theoretically improved CBR of 4.75% (an improvement of some 1.9 times).

The secant stiffness of Combigrid® at typical low operating strains of 2% is significantly higher (a factor 2.5 times higher) than geogrids manufactured in other ways such as multi-directional extruded polypropylene geogrids and flexible coated woven polyester geogrids.

Our solution

The design was completed using the adopted 4.75% CBR (with Combigrid® present), with assumptions made on the number of container movements, the proportion of 20ft to 40ft containers and average container weights, ultimately showing a 400mm reduction of base course layer thickness could be achieved.

The substantial time and cost savings based on this thickness reduction were clearly apparent and Combigrid® was ultimately selected for use.

A local contractor was awarded the construction contract, and their team was very pleased with how easy Combigrid® composite geotextile/geogrid was to use. Being a composite product, the Combigrid® did not require an additional layer of geotextile to be laid for separation purposes.

This project was successfully completed in April 2011.