Published in the December 2004 American Longwall Magazine
Since a longwall tailgate usually experiences more severe roof and rib loading than a headgate entry during retreat mining, adequate support is important.
A paper titled Pumpable cribs – a new deformable concrete support system in longwall tailgates, presented at Aachen International Mining Symposia 2003 by Joe Bower, HeiTech Corporation and others, described the ideal support as providing an adjustable, active load to the mine roof and floor which would be stiff initially to assist in load-controlled roof activity. It would also provide a designed adjustable peak load and remain stable against relative horizontal movement of the roof and floor.
The only support technology that provides this degree of flexibility is a longwall shield but the necessary abandonment of tailgate supports makes this option too expensive.
Until recently US mine operators took a very conservative approach to tailgate support, using conventional wooden cribs. But rising costs of timber and general low availability of adequate quality has led to a growth in alternatives.
In the US there is a trend toward prop type systems, but the stability in high deformation environments with lateral roof to floor movements remains a concern for some operators.
Strata Products pioneered the development of engineered wood crib supports with the Link-N-Lock and Hercules crib. Now the company has released the Sand Prop – a compact, high capacity prop. It can be used for bleeder support, cross cut and intersection stabilization, and longwall moves.
Designed for rapid installation, the Sand Prop provides non-yielding standing support with a 60-ton capacity. The Sand Prop can be reused by removing the wedges and recharging the sand into the upper unit.
It has been successfully introduced into six US longwall mines and will be launched in Australia early next year.
Strata Group president Rory Paton-Ash said the Propsetter product – a high capacity yielding timber post - offered the lowest cost alternative to tailgate support. He said the product was used extensively in tailgate support in the US, and also on longwall moves and as a bleeder support product.
“We recently introduced a cluster of Propsetters – three strapped together which gives us a multiplier effect on the Propsetter – ie. three times the strength with more stability,” Paton-Ash said.
The major advantage that the Cluster offers is efficiency of handling and transportation, particularly underground where the number of supply trips required to keep a tailgate supplied can be reduced.
“Two large mines have moved from test areas to full application of the Cluster Prop and four mines in the eastern US are in the advanced stage of testing with at least two of these mines having committed to convert their tailgates during the next panel installation,” he said.
Jennmar’s answer to standing support innovation is the “Little John” Extreme Mine Prop. The Little John can be configured to be a stiff or yielding system and is available in a 60t capacity. The height of the adjustable post is achieved by extending the inner pipe of the telescoping pipe arrangement.
Tests at NIOSH’s (National Institute for Occupational Safety and Health) Safety Structures Testing Laboratory which involved collapsing the prop in a vertical displacement showed 5 and 6in yield displacement.
Underground roof support specialists, Heintzmann Corporation have released the South African developed Omni 150. Described as a sacrificial support prop, the 150mm diameter, 50-ton prop can potentially be used for bleeder/tailgate support or shield recovery.
Tested extensively in South Africa and developed by Elbroc, the manufacturer said the Omni 150 offered superb energy absorption values and was quick to install.
Heintzmann also represent the German developed HeiTech pumpable crib. Since its introduction to the US market in 1998, over 50,000 pumpable supports had been installed. These are filled in place in the mine entry, eliminating the material handling bottlenecks of many other systems. They also eliminate the need for a secondary material to establish roof contact.
The roof supports are constructed in 24in or 30in diameter bags which are hung from the mine roof and pumped full of cementitious material. The two-component grout has been successfully pumped distances of 15,000ft from a surface borehole to underground locations within the longwall tailgate and bleeder entries.
These cribs have been used extensively at Emerald, Cumberland and Shoal Creek mines, and according to the Aachen paper, offered several advantages. These included better roof control, quick installation, less material handling and improved safety.
When initially introduced the crib was relatively expensive but efforts by RG American Coal, HeiTech and NIOSH have reduced costs by 35%.
“The current roof control cost for using pumpable cribs is about the same as that for using wood cribs and less than that for using fiber cribs,” the paper said.
According to HeiTech northern region manager Joe Bower, 45-50 HeiTech cribs can be installed during an eight-hour shift.
Not strictly on the topic of tailgate support, but certainly worth mentioning, is a new generation of hydraulic prestressing devices developed for the hard rock sector and being evaluated in US longwall mines.
The prestressing units (PSU) provide prestressing to a range of roof support products, from cans to props and cable bolts.
Trials to date have been carried out primarily in longwall tailgate installations and shield recovery, mostly on the Can support and Pencil Prop.
The PSUs consist of a thin-walled steel shell which is machine welded and can be inflated with water. The cells are capable of withstanding pressures from 1000-6000psi.
The product was developed by New Concept Mining in the late 1990s in South Africa for the hard rock mining sector, to provide active loading to a wide variety of support systems previously only used as passive supports.
Recently trials of the PSUs have begun in the coal industry.
The units are currently available in the US through Heintzmann.
