Improved health and safety in mining through helical drilling and rock bolt anchoring.
Hill JL III; Giraldo LB
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-007727, 2006 Aug; :1-66
Bolt installation characteristics near roof falls have been identified as contributing to failure. One documented and regularly occurring rock bolt failure mechanism is loss of grout shear bond to the rock wall of the bolt hole. Key contributors to the integrity of the grout interlocking with the rock mass are the diameter of the hole relative to the diameter of the bolt, resin versus cement type grouts, rock type and condition of the hole. Smooth bolt holes consistently produce a reduction in rock bolt load bearing capacity over rough walled holes. This is especially true in mines where the roof rock is weak. A new rock drilling technology, the Helical Drag Bit (HDB) developed by Raytheon UTD (RUTD) was used to develop the Helical Rock Bolt (HRB). HDB technology was used during the initial phase of the project to demonstrate that a helical thread cut in the internal walls of the bolt hole results in a higher anchorage capacity when a bolt is installed in that borehole. When a bolt is grouted into a hole that has been modified using the HDB, the grout fills the helical groove and provides a mechanical lock between the rock and the grout. This mechanical lock was found to produce stronger anchorage into the rock than roof bolts installed following standard procedures. Short encapsulation pull out tests have shown considerable improvement of pull out strength when hole geometry is modified using the HDB. The newly developed HRB incorporates the thread-cutting capabilities of the HDB into the roof bolt. The new bolt design was built, tested and refined to optimize performance. The resulting HRB has been extensively tested in the laboratory and in the field. Installation of the HRB is similar to the installation of standard fully grouted roof bolts. The only requirement is that insertion be made at a prescribed rate of advance and rotational speed to obtain particular thread geometry. This requirement was automated by making a minor modification to the roof bolting equipment. Short Encapsulation Pull tests showed that the HRB can produce up to 140% improvement in anchorage capacity when compared with standard fully grouted bolts. It was found that the anchorage improvement depended on the mechanical properties of the rock in which the bolt is installed. Other advantages provided by the HRB include; reduction of finger gloving (an installation problem that prevents the grout from curing properly), improvement of grout mixing, and reduction of corrosion potential since the body of the bolt remains centered in the borehole and is surrounded by the grout. Because of its unique characteristics, the HRB has the potential of reducing fatalities and severe injuries resulting from ground failures while allowing to maintain and even increase mine productivity. On a parallel development, HDB drilling was evaluated for its potential to interpret mechanical rock properties. The ability for the driller to interpret lithologic changes and make real-time decisions about bolt length and anchor selection would be of great benefit. In conventional drilling several drilling variables must be simultaneously monitored in order to interpret lithologic changes including thrust, rotational velocity, torque, and penetration rate. This study presents a new approach by using the HDB to interpret lithologic changes. The HDB employs spaced apart cutting members arranged in a helical pattern to form a helical thread within a rock bore. This geometry provides symmetry of forces such that the normal force on each cutter is balanced by the cutter on the opposite side of the bit. Every rotation of the HDB results in a prescribed advance into the rock and the cutting depth is defined only by the initial hole diameter. Laboratory testing of prototype helical cutters showed that once the bit geometry and depth of cut for each helical cutter were established, the measured force of cutting depended only on the type of rock being cut. This observation led to further theoretical, laboratory, and field investigations which confirmed this relationship. A mathematical model that directly correlates material unconfined compressive strength with the forces of cutting is presented. Results of laboratory experiments in sedimentary rocks of various strengths confirm the torque-unconfined compressive strength relationship for the rock types drilled with the HDB. Recent tests in a mine environment have subsequently allowed refinement of the original HDB design. The technique presented has the potential to quickly and accurately detect and interpret lithologic changes within the borehole giving the roof bolter operator real time information on the condition of the roof and allowing him to make decisions about bolt length and anchor selection thus maintaining safety in the workplace while reducing cost of roof bolting.
Roofing-industry; Rock-mechanics; Construction-industry; Failure-analysis; Mine-workers; Mining-industry; Accident-analysis; Accident-prevention
Final Grant Report
NTIS Accession No.
National Institute for Occupational Safety and Health
Raytheon Utd, Inc.