Mirafi® PET Reinforcement Bridging Soft Coal Fines, Tailings Storage Closure

The challenge

TenCate Mirafi® PET reinforcement geotextiles are widely used worldwide in civil ground improvement works and the closures of tailings storage facilities (TSF) that need to be capped prior to cover material being placed.

A typical example of this is the use of Mirafi® PET reinforcement in capping of sections of soft coal tailings in a TSF impoundment in the Hunter Valley coal mining region of New South Wales, Australia. The areas to be capped comprised three sections with differing coal tailings strength conditions.

Tailing dams are used to store saturated mining by-products which makes for a very soft subgrade. This makes it challenging for construction machinery or people to traverse the site. Each of the three zones required geotextiles meeting specific long term design strengths (LTDS) whilst the plan area of each section required materials be supplied in customized lengths to bridge across the entire area to be capped.

The tailings surface comprised a crust 300 mm – 500 mm thick. The tailings below the crust towards the centre areas of each zone comprised soft tailings with undrained shear strengths of < 10 kPa. This required the geotextile reinforcement to be deployed along with the use of only light tracked equipment.

The design

For Zone 1, geotextile reinforcement with a long term design strength (LTDS) of 145 kN/m was specified. Zone 2, which had a stiffer subgrade, required the geotextile reinforcement to have a LTDS of 70 kN/m, while Zone 3, the stiffest of all three locations, required the geotextile reinforcement to have a LTDS of 50 kN/m.

Three different grades of Mirafi® PET - 200kN/m, 300kN/m and 600kN/m were approved for use. The geotextile reinforcement was to be laid in two layers perpendicular to each other and be securely anchored around the edges. The rolls were supplied in varying lengths according to the detailed layout plan developed by the contractor.

Zone 2, the largest of the three zones, was the first to be capped, with Mirafi® PET300 supplied in lengths ranging from 130 m - 200 m. This was followed by the installation of Mirafi® PET 600 over Zone 1, the softest zone. Finally, Mirafi® PET 200 was installed over Zone 3, the firmer of the three zones.

The construction

All materials manufactured and supplied to the project were tested and certified in accordance with the design requirements. Each roll was also individually marked to indicate the zone and layout section it was required to be laid to facilitate correct placement on site. Deployment over the soft tailings was undertaken using a light tracked Bobcat with a laydown dispenser to facilitate easy unrolling.

Following installation, the Mirafi® PET reinforcement was covered by an initial 400 mm layer of mixed spoil laid as finger berms progressively penetrating out towards the centre of each Zone and then being infilled. This method of filling tensions the geotextile and enables faster closure of the unfilled areas between the finger berms. Once capped, these layers of mixed spoil will be covered with layers of site dumped rock to a final level designed to facilitate natural vegetation growth.

The performance

Every soft site closure brings with it a series of unique challenges and this project is a case in point with three different locations requiring specific solutions. Invariably, in projects such as these, the areas to be covered are large and subgrades are often soft and unstable, or even under water.

TenCate can supply specification compliant, custom lengths, designed to bridge the entire area without seaming. Also, specially fabricated fabric panels can be deployed across large areas in a single pull.