Case story

Mirafi® PET - Basal Reinforced Embankments on Soft Soil.

Project Data

ProjectHigh Strength Polyester Woven Geotextile as Basal Reinforcement
LocationCape Preston, WA, Australia
ClientCITIC Pacific Mining Management Pty Ltd
Products usedMirafi® PET

Overview

The Sino Iron Project was a world class, large scale magnetite iron ore project located at Cape Preston, 100km south west of Karratha, in Western Australia’s Pilbara region. This iron ore project was the largest planned magnetite project in Australia with an estimated 2 billion tonnes of identified magnetite ore. Mine development and infrastructure costs were estimated at USD 3.5 billion.

The project had an extremely tight time schedule, with construction beginning in mid-2008 and was due for completion at the end of 2010. A crucial component of the overall project was the construction of a 30km long services corridor connecting the port to the mine site.

Part of this services corridor consisted of a 2km long causeway constructed through a river estuary. The foundation conditions within the river estuary consisted estuarine mud which was a very soft to firm silty clay of medium to high plasticity. Investigation by in-situ cone penetration and vane shear tests resulted in a design undrained shear strength of 6kPa from the surface to 1.5m depth, increasing thereafter at 7kPa to a maximum of 20 kPa. This layer was unable to support a 7m high rockfill embankment that needed to be constructed with the required service loading.

The causeway embankment ranged in height from 1m to 7m, with a crest width of 32m. The causeway embankment had to be designed to allow for heavy vehicle loadings from 240 tonne haul trucks as well as the safe transportation of 1400 tonne giant grinding mills for the mining operations.

Sollutions

To construct the causeway a number of design and construction options were evaluated. These ranged from stage construction to soft soil replacement. The option must meet the stringent environmental and safety requirements of the project. Most important, the option must meet the tight time construction schedule.

3 layers of Mirafi PET 800-50 geotextile reinforcement were chosen as the basal reinforcement for its ability to meet all of the requirements. Mirafi PET 800-50 geotextile reinforcement is a woven polyester geotextile with a tensile strength of 800 kN/m at 10% strain. The high tenacity polyester yarns have high tensile modulus and have an excellent resistance to creep.

installation

The Mirafi PET 800-50 geotextile reinforcement was placed directly on the surface of the soft estuarine mud with the rolls of geotextile been laid perpendicular to the direction of the causeway embankment. No geotextile joins were allowed in the direction across the full width of the embankment. The first mine waste rockfill was placed on top of the geotextile reinforcement, spread out and compacted to construct an initial fill platform of 0.5m thickness. On top of this fill platform a second geotextile reinforcement layer was placed and then a 0.3m thick fill layer placed on top. Finally, a third geotextile layer was placed and then the embankment was constructed to its completed grade alignment.

performance

The use of basal reinforcement had enabled the causeway embankment to be constructed quickly, directly on the estuarine mud foundation, without soil replacement. Consequently, environmental impact had been reduced to a minimum. Further, the services corridor had been completed on schedule. The settlement of the embankment was within the predicted value of 250mm.

Basal Reinforcement on Soft Soil