The hole story

In 1952 Brian Mercer, a mill owner’s son from Blackburn invented plastic netting, the sort of thing we use every day. As with all good ideas, there were soon many imitators.

But Mercer was an innovator. In the 1980s he developed his plastic netting idea into Tensar, a geogrid used to strengthen groundworks. For the uninitiated, think of huge nets with square holes. Of course, others copied this too. At the turn of the century, Tensar’s market share had been eaten away by cheaper overseas products.

‘We were under pressure from increased competition from all over the world,’ says group marketing manager Tim Oliver. ‘At the same time, there was pressure from the market. Customers were saying: “You saved us 30% last year. What can you do for us this year?”’

So in the tradition of Brian Mercer, Tensar reinvented its product. This year it is unveiling TriAx, a triangular geogrid. The principle is that triangular ‘holes’, rather than square ones, offer multi-directional strength, rather than bi-directional.

It took a fresh pair of eyes to see a route away from the square grid of Tensar. It was Anthony Walsh, who joined the firm in 2001 from the automotive industry to head up its R&D operations, who asked, simply: ‘Why square?’

Geogrids work by trapping aggregate in their holes to form a composite layer of grid and the interlocking aggregate. Walsh suggested that triangular holes would give better interlock, meaning less aggregate for the same stiffness, which would mean less cost and less environmental impact from activities such as quarrying and transportation. Once Walsh had made his observation, the logic of a triangular form did seem to make sense.

That was the easy bit. Three years of development followed. On the wall of Walsh’s laboratory at Tensar’s Blackburn plant is an array of different triangular grids. But these are but a fraction of the 400 samples made and tested.

Changes and improvements were made through 18 months of trafficking tests carried out at Nottingham University and the Transport Research Laboratory. These determined the optimum aperture size, rib depth and the shape of the rib cross-section. The changes were incremental, sample by sample. ‘It was very time-consuming, very expensive, very painstaking,’ says Oliver.

But the hard work has paid off. Laboratory testing shows that TriAx can cope with loads in any direction, in contrast with biaxial geogrids (see diagram, left). This is relevant because although many geogrid applications are for roads – so movement only occurs in one direction – they have to be designed to cope with the construction traffic, which can cross a site at any angle.

Tensar reckons that where using a biaxial geogrid leads to aggregate savings of around £2.50 per square metre, TriAx shaves off an additional 50p.

Tensar hopes to win back market share with its new product and is pricing it at around 5% less than its comparable biaxial product. ‘The market place does not expect to be paying for the improvements it demands,’ comments Oliver.

Since it requires a different manufacturing process, TriAx will initially come from the company’s plant in China. The Blackburn factory will be converted to the new process in 2008.

For Walsh the research and development has been an emotional rollercoaster. A true engineer, he describes the highs and lows he has been through in terms of ‘sinusoidal waves’, although he concedes the waves are now levelling out.

So what comes next? ‘It’s a tough one,’ says Walsh. ‘In your heart of hearts you think there may be only one of these in a lifetime.’ cm