The research is part of ongoing research into roof mapping and drilling being undertaken by the West Virginia University Department of Mining Engineering. Recent research was undertaken to model realistically, for the first time, tensioned and fully grouted roof bolting using finite element modelling techniques.

While much research has been done on this area it has proven difficult to apply in practical bolting design because of complicated interactions between the bolts and host rock. The modelling, using powerful computer technology, took into account the physical dimension of bolts, bedding planes and in-situ stresses.

For the past 20 years US coal mines have consumed approximately 100 million pieces of roof bolts annually but a lack of roof bolting design criteria means roof bolting design is still largely based on practical experience, according to research leader Syd Peng, WVU’s Mining Engineering department chairman.

Six types of roof bolts are currently used for reinforcing entries but there are only two basic types: tensioned bolts (20% of bolts used) and fully grouted resin bolts (80%).

Because both types involve different loading mechanisms it is important to understand factors that affect their stability.

Based on the analysis of the mechanisms of both bolting systems and failure modes of the bolted strata, roof bolting design criteria and programs for modern roof bolting systems have been developed.

Realistic modelling of the entry excavation sequence, roof bolting components, bolt installation procedure and pre-tension allowed researchers to develop some basic design guidelines based on the geological condition in the Pittsburgh seam.

To obtain how the overburden depth, horizontal stress and roof type affect the tensioned roof bolting design, 36 models with overburden depths of 400ft, 800ft and 1200 ft; major horizontal-to-vertical stress ratios of one, two and three; and different roof types were run using the geological condition of the Pittsburgh seam.

With the model built, it will allow design engineers to design a suitable tensioned roof bolting approach. Information related to depth of cover, entry width, and other relevant data is inputted, then the database searches for the closest case. A bolt length, spacing, amount of pre-tension and bolt diameter is then provided.

The program developed can be used for tensioned bolting design to determine the bolt length, optimum pre-tension, bolt diameter, and bolt spacing.

The results of the numerical modelling make the following broad recommendations for tensioned roof bolting design:

Tensioned bolts are suitable for a weak roof under a low stress level but are not suitable for a roof with large deformation after bolt installation. If yielding occurs only around entry corners, possible failures would occur around the entry corners. In this case, side bolts should be installed within 2-3ft from the entry corners to prevent cutter failure.

If the roof yielding develops over the entry, both the entry corners and centre of the entry may fail. Here, emphasis should be on the entry corners because both the horizontal and shear stresses are concentrated on these areas.

The bolt length should increase as the overburden depth and horizontal stress level increase.

If the roof is subjected to high horizontal stress and the immediate roof has multiple bedding planes, a large diameter bolt should be considered to prevent either bolt yielding or bolt breaking.

With passive fully grouted bolts, failure can take place in the bolt, steel bolt axial or shear failure, or at the grout/rock interface failure.

According to the analysis results, the fully grouted bolt is mainly subjected to axial loads, shear loads, and bending moments.

For each roof type about 45 models were conducted with a total of 135 finite element models for three roof types.

The horizontal stress is more important for controlling roof stability than the vertical stress because the vertical stress is born by the pillar while the roof must bear nearly all of the horizontal stress. In the US the horizontal stress is larger than the vertical stress with the ratio of horizontal-to-vertical stress ranging from one to three; and the maximum horizontal stress is about 40% greater than the minimum one.

The tensioned bolt is more effective when the immediate roof layers are weak but the horizontal stress level is low.

Recommendations for the fully grouted bolting design are as follows:

Longer bolts should be installed for a weak immediate roof as well as for high overburden depth and high in-situ horizontal stress.

Loading capacity is a controlling factor in successful rock reinforcement design and can be controlled by the bolt diameter. Fully grouted bolts are most suitable for supporting highly stressed roof strata with weak roof.

Peng said industry response to the concept was positive so far with a few companies eager to trial the new modelling approach. He said Foundation Coal's Emerald Mine would be the first operation to test the bolting modelling in March this year on a renovated R&D bolter owned by JH Fletcher & Co.