Published in the May 2006 American Longwall Magazine

Keystone Mining Services, a leader in the development and application of finite element analysis (FEA) techniques, has used these techniques extensively and successfully for both primary and supplemental bolt design.

Over the years, the performance of the software has continued to expand, and now a three-dimensional modeling capability is routinely used to predict stress related to a retreating longwall panel.

Dr John Stankus developed the use of FEA as a methodology in 1995 as part of his Ph.D. dissertation. Stankus and his colleague Dr Song Guo developed the modeling system, which was patented to major bolting manufacturer Jennmar.

With modeling as a key capability, Keystone was formed to provide ground control engineering consulting services. The Jennmar subsidiary would also oversee the parent company’s research and development efforts.

“Prior to the advent of high-speed computers, if you tried to do the kind of stress analysis made possible by FE techniques manually you’d be there for months,” Stankus said.

“Now we have three-dimensional models that contain up to 30,000 elements.”

The methodology provides analysis of the stress on individual mine openings.

Starting at the mine surface, a ‘coarse’ model is initially generated looking at overburden and rock type. Boundary conditions are then developed, which are taken down to the mine opening where a more detailed sub-modeling process allows finer meshing of the elements.

Incorporating data about the medium roof rock characteristics of the individual mine allows designers to ascertain the appropriate bolt – in terms of length and tension – for the conditions.

“The program allows adjustments to be made to the length and tension of the bolt till you get the Optimum Beaming Effect (OBE), taking into account the in situ stress,” he said.

“Basically we aim to introduce the shortest bolt possible with the right amount of tension required to eliminate bed separation above and within the bolted horizon,” Stankus said.

Typically in situ conditions will be monitored to validate and prove the modeling.

Over time, the modeling package has been refined to include other factors such as coal seam elevations or changes in surface topography. This allows geomechanical engineers to very accurately predict the stress a mine is going to incur and take this down to the individual mine entry, and come up with right bolting length, bolting pattern and the right amount of tension (if any).

Three-dimensional modeling techniques are used in longwall applications. These techniques are needed to needed to design roof support around a retreating longwall panel because of the multiple complex interactions that occur between in situ stresses and mining-induced abutment pressures.

“Using this approach, we try to design a primary support plan that minimizes the amount of supplemental support needed when the longwall comes back,” Stankus said.

The push to minimize, even eliminate, tailgate support is becoming a hot topic in the US, Stankus said.

He said this has been a topic of discussion for some time but is gaining momentum with the price of wood increasing and timber consistency varying. Also, some of the recent fires in the US are driving a move to eliminate the use of wood and move to virtually ‘cribless’ tailgates.

“With some of the cable-bolting systems and our ability to use our computer modeling to accurately predict stresses in gate entries, we can do things with cable systems that mean you don’t need standing support,” he said.

Benefits include improved ventilation circuits and less manhandling in the tailgate.

Keystone is also using its modeling capability to accurately reproduce the stresses that can occur if cut-through entries are developed in the middle of a longwall panel.

This practice is becoming increasingly common as longwall panels become longer and wider and have greater ventilation requirements.

Armed with knowledge about what stress will come to bear on the entry configuration as the longwall approaches and mines through, allows better bolting choices to be made.

Another development will occur as room and pillar mines go deeper. Currently, these operations use minimal support but bolt systems might need to have higher capacity if mining goes deeper, Stankus said.

Another development Jennmar has pioneered is the application of engineered polymers to roof bolting systems.

“Everyone’s looking to reduce steel costs related to roof control so we’ve doing some things with our Buddy-Bar system, which has a polymer coating on a piece of rebar. You increase the anchorage capacity, reduce the amount of steel you have in the hole and reduce the amount of resin.”

Like many other bolt manufacturers, Jennmar is also exploring self-drilling bolts but exactly how far this work has progressed is being kept under wraps.

“The concept of a self-drilling bolt has been around since roof bolting started in the ’60s. There are a lot of good ideas out there but the expense would make them prohibitive,” Stankus said.

In other news, Jennmar recently began making its own resin at its Cresson bolt plant in Pennsylvania, under the brand name J-LOK. The company said with all the different types of bolts it was developing, it didn’t want to depend on external suppliers for resin supply.