Spray-on liners need performance goal posts
While thin spray-on liners have been approached quite cautiously so far by Australia’s hard-rock miners, a researcher recently recruited to the University of New South Wales hopes to expand research done here on these polymer-based products.
Among other uses such as sealants and open-pit slope stabilisers, thin spray-on liners are a mode of surface support in underground mines.
Once the thin spray-on liner mixture is prepared, it is typically applied at a thickness of 3-5 mm. Liners fall into the category of membrane technology, alongside shotcrete/fibrecrete and geomembranes.
Dr Serkan Saydam, who joined UNSW’s School of Mining Engineering in August as senior lecturer, says he would like to establish standard tests for thin spray-on liners, which are also known as TSLs.
“There is no standard testing procedure that is accepted and implemented by all parties,” he said.
“There is an urgent need for the development of standard tests for TSL applications.” This was because their use will potentially grow in the near future, he said.
Saydam, who obtained his undergraduate and postgraduate mining engineering qualifications at Turkey’s Dokuz Eylul University, told Australian Mining he believed that if standard testing methods were developed and used for TSL products, the uptake of this technology would probably increase. In turn, the risk of rockfall injuries at Australian underground mines would likely fall.
He is not alone with this thought experiment. This train of thinking was taken by at least one other researcher in recent years.
In a 2002 paper, Australian Centre for Geomechanics director Professor Yves Potvin said statistics suggested current approaches to controlling small pieces of freshly exposed rock near the active face can be improved.
He cited a 2001 ACG study showing over 90% of rockfall injuries involved rocks weighing less than one tonne.
Four-fifths of these injuries occurred within metres of the active face.
Potvin said stepping up the use of more traditional methods of surface support – such as mesh and shotcrete – could increase costs and reduce productivity through time penalties.
“Thin spray-on liners may develop the attributes to address this specific problem,” Potvin said in his paper.
“The installation of remote and rapid spraying techniques has the potential to minimise interference with the mining cycle.”
UNSW’s Saydam said some South African underground mines were installing TSLs and after a one-hour curing period, bolts were being put in. It was his understanding that under this bolted TSL support, drillers could work safely.
“A TSL application in no way replaces conventional support like roofbolts, but should be considered as complementary, as it provides total coverage, which promotes block interlock,” he said. Interlocking prevented shear movement and restricted block rotation.
TSL products that penetrate rock cracks and joints increased the frictional strength of those partings, he said. They typically penetrated more easily than shotcrete thanks to viscosity and particle size.
Among other geotechnical benefits was the use of TSLs as an intermediate layer to improve shotcrete-to-rock bonding.
There were also wider implications for TSLs used as sealants to reduce rock weathering or to prevent the entry of groundwater.
The possible non-geotechnical benefits of TSLs were their remote and safe application, improved logistics and lowered costs.
If he were asked who the leader was in TSL use, he would name South Africa, he said.
TSLs were particularly useful in South African gold mines because they were considerably faster to install than shotcrete and they were easier to spray near the tunnel face area during development.
TSLs were used as initial support in South Africa because it was typically difficult to install shotcrete in confined conditions.
He knew papers had been published on some mines that had already replaced shotcrete with TSL technology in a few of their tunnels.
Saydam told Australian Mining that currently more than 50 mines worldwide were using TSLs for rock support. The figure was increasing steadily, with the greatest interest coming from North America, Australia and South Africa. The first TSL tests began in Canada in the late 1980s.
“It’s a growing sector, but I think Australia is a little bit cautious about it.”
While his knowledge about the Australian experience with liners was currently limited, his impression generally was that “people at mines are reluctant to change from tried and trusted methods”. He said he couldn’t blame them, because once mines had made their plans, they were usually disinclined to change them – especially geotechnical plans. This was due to the difficulties involved.
TSLs were about rapid installation of support, Saydam said.
“You can’t beat it. You don’t need to put a plant in underground to produce shotcrete and you don’t need to handle large quantities of steel mesh.”
Nevertheless, he believed a research contribution was needed to investigate the potential instability mechanisms that might be controllable with TSLs.
“This technology is still not very well known or understood,” he said. “Most of the mines are conducting relatively small-scale investigations to evaluate the support performance of different liners in a particular environment.”
According to Saydam, standard performance tests were not the only need, because TSL design standards were not clearly available yet either.
“Manufacturers have been testing their products in a laboratory environment. And based on these test results they continually change their product properties.”
He asserts there is also “no reliable correlation” yet between lab and field results, surface or underground.
A mining engineer with more than 13 years of mining research experience – including a year spent studying TSLs at Johannesburg’s University of the Witwatersrand plus three years of TSL-related project work with De Beers – Saydam believes he is the right one for the job of helping establish standard performance tests. He aims to better understand the mechanical interactions between liners and the rock they support.
The spraying stage of TSL installation may be another part of the reason why relatively few Australian companies have introduced liners into their surface support scheme. Spraying involved the “human factor” – perhaps one of the most important factors in testing TSLs, Saydam said. Liner performance is related to the installation experience and technique of the person at the nozzle.
“Lots of companies are failing with their in-situ tests because of inexperienced nozzlemen.” Application was easy, but miners installing the liners needed to be trained well.
The TSL standardising work he hoped to do included bending tests, shear tests, adhesion tests, pull-off tests of layered TSL and shotcrete, and FLAC3D, Phase3D and particle flow code modelling.
Saydam also wanted to improve the doubled-sided shear strength test he developed and which he said was already being used by one of the TSL manufacturers with a presence in Australia.
An environmental control room was also on the wish list, according to Saydam. It would involve an underground-like environment in the laboratory, where tests could be done.
Time-dependent tests were also needed to compare liners with other support types.
“TSLs haven’t been used for more than a few years in routine mining applications,” Saydam said. “Long-term performance of TSLs is as yet unknown.” He had tests such as creep tests in mind.
13-Nov-2006