TABLE OF CONTENTS

I.  Introduction

II.  Location and Ownership of the Underground Coal Mines

III.  Geological Setting of the Coalfields

IV.  Coal Seam Characteristics and Depositional History

V.  Methods of Mining

VI.  Mine Visits

VII.  Properties of the Coal Seams and Surrounding Strata

VIII.  Standardization of Geological Assessment

IX.  Conclusions

References

Appendices

Appendix 1: Physical and Mechanical Properties of Rocks Found in the Comox, Sydney, and Smoky River Coalfields

Appendix 2: Lithology Logs of Boreholes used to Determine RMR of Selected Roof Strata in the Comox, Sydney, and Smoky River Coalfields

LIST OF FIGURES

Figure 1: Location of the Sydney, Comox, and Smoky River Coalfields

Figure2: Location of the Sydney Coalfield

Figure 3: Distribution of Major Coal Seams within the Coal-bearing Formations in the Sydney, Comox, and Smoky River Coalfields

Figure 4: Contrasting Structural Style of the Three Coalfields

Figure 5: Location of the Comox Coalfied

Figure 6: Location of Smoky River Coalfield

Figure 7: Typical Profiles of the Coal Seams

Figure 8: Typical Longwall Retreat Setup in Sydney Coalfield

Figure 9: Typical Room and Pillar Retreat Mining Setup at Quinsam and Smoky River

Figure 10: Geographic Location of Phalen and Prince Collieries

Figure 11: Plan View of Phalen Colliery Workings

Figure 12: Debris Pile, and Roof Supports Damaged by Sandstone-Gas Outburst

Figure 13: Roof Fall on Longwall Face at Phalen Colliery

Figure 14: Cutter Roof Along Rib Side of Longwall Main Gate

Figure 15: Prince Colliery

Figure 16: Roadway Closure Resulting from Driving Heading at an Unfavorable Orientation to Direction of Principle Horizontal Stress

Figure 17: Improved Roadway Conditions in Heading Driven at an Orientation Favorable to Direction of Principle Horizontal Stress

Figure 18: Reduction in Coal Seam Height Cuased by Sandstone-Filled Erosional Channel

Figure 19: Plan View of No. 2 North Mine Workings

Figure 20: Small Displacement Thrust Fault in No. 2 Mine At Quinsam

Figure 21: Soft, Plastic Nature of the Main Stone Band in No. 1 Coal Seam-No. 2 North Mine

Figure 22: Photo Showing the Strong Ribs of No. 1 Seam at 2 North Mine

Figure 23: Plan View of No. 4 South Mine Workings

Figure 24: Sandstone Partings in No. 3 Seam at No. 4 South Mine. The Strong Nature of the Coal Seam is Evident by the Competency of the Pillar

Figure 25: Example of the Competent Nature of the Sandstone Overlying No. 3 Seam at No. 4 South Mine

Figure 26: Plan View of the No. 5B-4 Mine Workings

Figure 27: Typical Polished, Slickensided Roof Associated with Thrust Faulting and Shear Zones at No. 5B-4 Mine

Figure 28: Typical Thrust Fault Found in No. 3 Main Development Entry

Figure 29: Spectacular Folding Characteristic of Smoky River Coalfield

Figure 30: Highway Roadcut Showing Small Crop Pit, and Thrust Fault Passing from Coal Seam into Roof Strata

Table 1: Basic Coal Mine Statistics

Table 2: Geological Age and Nomenclature of the Coalfields

Table 3: General Characteristics of the Coal Seams

Table 4: Mining Methods Used to Extract Coal

Table 5: Some Descriptions of Sandstone in Core or Outcrop

Table 6: Comparison of RMR's Performed on Cored Boreholes in the Comox, Sydney, and Smoky River Coalfields

The geology of three major Canadian coalfields, and the five remaining underground coal mines that extract their seams were examined in this report. It describes the distinctiveness of these coalfields, which is the legacy of their own particular depositional environments and tectonic histories. It also describes how mining methods have evolved, and adapted to the different geological environments in which they are found. The longwall mining method has been very successful in the relatively undisturbed strata of the Sydney Coalfield, while the retreat room and pillar method has provided the flexibility needed to mine the tectonically troubled Smoky River and Comox Coalfields. All five mining operations recognize the importance of geological assessments in the design of their mining operations.
Because of this importance, standardization of these assessments has been investigated. It is the author's opinion that standardization should not be based on formal adherence to rigid procedures. It should be based on a few basic principles that keep one goal in mind; the design of safe, effective, and efficient mining operations. In this way, considerable leeway is left for initiatives that recognize the distinctiveness of the individual coalfields. These principles are as follows.

Principle 1:

Principle 2:

Principle 3:

Principle 4:

Principle 5:

Principle 6:

The author wishes to thank CANMET for the opportunity to carry out this project. The support offered by Gary Bonnell is especially acknowledged. Thanks is also offered to the Quinsam Coal Corporation, and Smoky River Coal Ltd. (especially Peter Cain) for the hospitality extended while on their mining properties. The Cape Breton Development Corporation is acknowledged for providing the time and facilities to write this report. A final thanks is extended to Dan Payne, who suggested this project, and acted as a most capable and attentive guide on my trip to the western Canadian coalfields.

In the spring of 1999, Natural Resources Canada approved a research project entitled "Standardization of Geological Assessment at Underground Coal Mines in Canada". CANMET’s "Underground Coal Mining Safety Research Collaboration Committee" administered the project. The objective of the study is to standardize geological assessments at the five remaining underground coal mines in Canada. The goal of the project is to contribute, in some positive way, to the on-going effort to insure that Canadian coal mines are safe places to work, and that coal resource recovery is optimized by effective and efficient mining operations. The study process involved a review of geological information supplied by the mine operators, and a visit to each of the mine sites. This report contains the authors personal observations and comments derived from this examination.

The five underground coal mines operating during the time of this project are owned by mining companies located in three distinct geographical areas of Canada. (See Figure 1 and Table 1)

MINE NAME	MINE OWNER	LOCATION	PROVINCE	YEAR OPENED
North	Quinsam Coal Corp	Campbell River	British Columbia	1990
4 South	Quinsam Coal Corp	Campbell River	British Columbia	1998
5B-4	Smoky River Coal Limited	Grande Cache	Alberta	1998
Prince Colliery	Cape Breton Development Corp	Point Aconi	Nova Scotia	1976
Phalen Colliery	Cape Breton Development Corp	New Waterford	Nova Scotia	1984

Table 1: Basic Coal Mine Statistics

The mines are located in three different coalfields, each with a unique geological setting. The sedimentary nomenclature and age of the coalfields is summarized in Table 2. It is apparent that the coals were deposited in environments separated not only in space, but also by vast intervals of time. One of the interesting aspects of this study is an examination of the extent, if any, that these factors has been reflected in conditions experienced during coal mining.

COALFIELD	GROUP	FORMATION	GEOLOGIC AGE	SIZE
Comox	Nanaimo	Comox	Upper Cretaceous (80,000,000 YRS)	1500 km2
Smoky River	Luscar	Gates	Lower Cretaceous (100,000,000 YRS)	550 km2
Sydney	Morien	South Bar	Carboniferous (300,000,000 YRS)	600 km2

Table 2: Geological Age and Nomenclature of the Coalfields.

The Sydney Coalfield is located in Nova Scotia along the eastern coastline of Cape Breton Island (See Figure 2). It consists of the on-land and near shore portion of a greater, largely unexplored sedimentary basin that extends almost to the coastline of the province of Newfoundland. The Coalfield parallels the Cape Breton coastline for a distance of about 50 kilometers. Economically mineable coal seams are exploitable for a distance of about 12 kilometers from shore.

The coal bearing strata are of Carboniferous age and occur within the Sydney Mines Formation of the Pictou Group (See Figure 3). Eleven major coal seams are present, six of which are considered mineable using today’s mining technology. The coals are generally ranked as high volatile "A" bituminous.

The coal seams are separated by relatively consistent intervals of rock strata composed of sandstone, siltstone, mudstone, and shale. The entire sequence bears the characteristics of deposition in a fluvial environment on a slowly subsiding coastal plain.

Tectonically, the Coalfield has not been appreciably disturbed. Some faulting has occurred, particularly along the margins of the basin, but mining operations for the most part, have been spared the effects of major faulting. The main structural features of the Coalfield are its folds, which plunge gently northeastward and fan out beneath the floor of the Atlantic Ocean. The gradients on the flanks range from 2 to 50 degrees, but generally do not exceed 10 degrees.

Pleistocene glaciation has deposited a veneer of glacial till on the erosional surface that may be as thick as 50 meters. The coastline is currently being attacked by the sea and vertical cliffs (up to 50 meters high) are frequently encountered. For this reason, many of the coal seams outcrop beneath the sea. A photo (Figure 4a) of the coastline gives the reader a general impression of the areas geological setting.

Underground coal mining has taken place in the Sydney Coalfield for more than 200 years. At the present time, two mines (Prince and Phalen Collieries) are open. Phalen Colliery is slated for closure in December 1999 while Prince will continue to produce for at least the near future. Both collieries extract coal beneath the floor of the Atlantic Ocean.

The Comox Coalfield is located in British Columbia on the eastern side of Vancouver Island (See Figure 5). It stretches southward from the city of Campbell River to Parksville, a distance of about 120 kilometers. It’s width averages about 13 kilometers. The Coalfield extends eastward, for an undetermined distance, under the Strait of Georgia.

The Coalfield contains three major coal seams (No. 1, No. 2, and No. 3) deposited during the Late Cretaceous age. Two of these seams (No. 2, and No. 3) are considered mineable using underground mining technology. The coals are found within the Comox Formation of the Nanaimo Group (See Figure 3) of sediments. The Comox Formation ranges from 150 to 200 meters in thickness, and unconformably overlies granitic and volcanic basement rocks of Triassic and Jurassic age. Its sediments consist of sandstone, mudstone, siltstone, conglomerate, and the economic coal seams. Their character suggests deposition in a fluvial environment in close proximity to an ancient seashore. The nearness of the irregular basement topography has exerted a significant influence on the distribution and thickness of the mineable coal seams. Coal rank ranges from high volatile "A" bituminous to anthracite. The anthracite occurs in the southern part of the Coalfield where coalification has been upgraded by Tertiary intrusives. In the northern part of the Coalfield, where mining is currently taking place, the coal is classed as high volatile bituminous. (Note: In the remainder of this chapter, discussion will be confined to the northern portion of the Coalfield, in the area of the Quinsam mines.)

The structural geology of the Comox area was established by tectonic events associated with Tertiary folding and faulting (Cathyl-Bickford, 1991). The Coalfield is an isolated segment of the Nanaimo Group rocks formed by post-Cretaceous block faulting and tilting (from G. C. Smith, 1989). Extensive normal faulting is the legacy of this tectonic activity in the Quinsam mines area. Two general groups of faults are recognized. The primary group trends east west while the secondary group trends north south. In the mining areas, faults can vary both direction and displacement (1 to 8 meters). Faulting has impacted mining operations by restricting the size and location of mineable coal blocks, by creating barren zones within the coal mining horizons, negatively impacting roof stability, and providing conduits for surface water to enter the mines. Folding has left the sediments with a gentle northeast dip of 6-10 degrees.

The area has been glaciated and deposits of glacial till ranging from 2-3 meters in thickness overlie the sedimentary bedrock; locally in troughs, till thickness can exceed 45 meters. The northern part of the Coalfield is characterized by low undulating hills adjacent to high ridges of basement rock to the south and west. Figure 4b, a photo taken at the portal of No. 2 North mine, gives a general impression of the geography of the area.

Coal has been mined in the northern portion of the Coalfield since 1979. Currently, underground mining is being conducted on the No. 1 and No. 3 seams at two sites (No. 2 North Mine and No. 4 South Mine) owned by Quinsam Coal Corp.

The Smoky River Coalfield is located in west-central Alberta about 20 kilometers north of the town of Grande Cache (See Figure 6). It is one of a number of coalfields that extend northwest for more than 800 kilometers within the Inner Foothills Belt of the Canadian Rocky Mountains. With a length of 29 kilometers, and a width of 19 kilometers, its area is about the same as the Sydney Coalfield.

The coal seams are contained within the Gates Formation of the Lower Cretaceous Luscar Group of sediments. The Grande Cache Member of the Gates Formation hosts the major coal seams, and most mining to date has occurred within this member. Seven major coal beds are present ranging in thickness from 0.8 m and 8 m (Smith, 1989). The general rank of the coals is low volatile.

The strata enclosing the coal seams are composed of mudstone, shale, siltstone, and sandstone. The character of the sediments suggests deposition in a low energy environment behind coastal shoreline deposits. The trough cross-bedded sandstones in fining upward sequences are indicative of the presence of fluvial channel sandstones (Langenburg, Kalkreuth, Wrightson, 1987).

Structurally, the Coalfield has been deformed by the Larimide Orogeny. The compressional forces associated with tectonism have produced a complex system of folding and faulting. The presence of intense folding and large thrust block structures has had a major influence on the location of areas suitable for underground mining. No. 5B mine is located near the top of a prominent anticline where gradients are sufficiently favorable (<16⁰) to permit underground mining using continuous miners and shuttle cars.

The characteristic high relief found in the Coalfield is typically mountainous. This is evident in Figure 4(c), a photo taken along the highway near Grande Cache.

Mining has taken place in the Smoky River Coalfield since 1969. Much of this mining has employed the open cast method. The No. 5 Mine, operated by Smoky River Coal Limited is the only underground coal mine currently operating in the Coalfield.

The mines described in this project extract coal from seams that exhibit a wide variety of depositional and tectonic environments. The distinctiveness of the environments is reflected in seam thickness, coal quality and structure, and the composition of the strata lying adjacent to the seams. Vertical profiles of the seams are shown in Figure 7, and some general properties of the coals are listed in Table 3. A brief description of each seam and its immediate roof and floor strata is presented below.

1.  Hub Seam

The Hub Seam is believed to have been deposited in a fluvial environment on a slowly subsiding coastal plain. This origin is based on the absence of marine fossils, the presence of sandstone-filled paleo river channels in its roof strata, and the coarsening-up sedimentation sequences, which suggest deposition in a lacustrine bay-fill setting.

The thickness of the Hub Seam averages about 2.2 meters. The seam is generally clean but stone bands are present in the eastern portion of the reserve block. The coal is bright banded with thin layers of durain and lenses of fusain. Pyrite is common as scales along cleat planes, as discontinuous bands, nodules, and lenses. The coal is blocky and hard and does not possess a well-developed cleat system. In some areas, the coal is fractured and shows evidence of internal movement. The Hub Seam is considered to be a thermal quality coal. Sulfur and ash content average 3.8% and 7.5% respectively, but these values rise rapidly in areas of stone bands, and where sandstone occupies the immediate roof. The sulfur is not considered to be of marine origin, but derived from underlying gypsum and anhydrite deposits.

The immediate roof strata are generally composed of mudstone and siltstone, but locally these rocks have been replaced by sandstone. A major infilled, paleo river channel crosses the reserve block, and locally erodes the entire coal seam.

The immediate floor of the Hub Seam varies from a weak mudstone to moderately strong siltstone seatearth.

2.  Phalen Seam

The Phalen Seam at Phalen Colliery was also deposited in a fluvial environment, and possesses seam structure and roof strata similar to Prince Colliery.

The Phalen Seam averages about 2.4 meters in thickness. Thickness may increase to 3 meters where a prominent stone split thickens near the roof of the seam. The coal is bright banded and contains several distinct durain bands that serve as excellent marker beds. Fusain lenses containing pyrite are common. The seam is considered to be a high-grade thermal coal and possesses good metallurgical characteristics. Sulfur and ash content average 2.0% and 7.5% respectively. The coal is moderately soft and has a very well developed cleat system.

Roof strata are composed primarily of mudstone and siltstone, but may locally be replaced by sandstone. The Phalen sandstones are thicker and stronger than those overlying the Hub Seam, and tend to be considerably less permeable and porous. These characteristics have made the Phalen sandstone prone to outbursting and a source of strong roof weightings on its longwall faces.

The Phalen Seam floor is a strong siltstone seatearth.

B.  Comox Coalfield

1.  No.1 Seam

No.1 Seam is believed to have been deposited in a fluvial environment but much closer to the seacoast than coals of the Sydney Coalfield. Proximity to the seashore is evident by the presence of marine pelecypod and gastropod shells in the strata immediately overlying the seam, but its low sulfur (<1%) content indicates a depo-center just beyond the influence of sulfate rich seawater. These observations suggest that coal formation took place in a fresh water environment but was terminated by a temporary rise in sealevel, which resulted in the drowning of the peat swamp.

The thickness of No.1 seam averages about 3.5 meters. The coal is generally bright-banded with dull inter-beds of durain. Calcite and pyrite are present along bedding planes, vertical fractures, and cleat faces. Slickensided and polished surfaces are present on some fractures. Resin has been observed locally in sections of the seam. Within the current mine area (7, 8, and 9 Panel), three mudstone partings occur within the seam. The lowest of the partings, the main parting, is characteristically soft and plastic, and slowly exudes from the seam shortly after the coal is mined. The stone partings thicken to the east and eventually terminate mining in that direction. No.1 seam is considered a high quality thermal coal. Sulfur and ash content are <1% and 10-12% respectively. The coal is blocky and hard and possesses a moderately to well developed cleat system.

Roof strata are composed primarily of mudstone and siltstone. In some areas, a thin (0.4-0.6 m) Rider seam occurs in the roof. The roof rock tends to be weak and has to be removed during mining when this rider is within 0.3 m of seam. As the rider rises higher above the seam, roof rock becomes more competent.

The seam floor is composed of weak mudstone and carbonaceous mudstone. This rock tends to become soft when wet.

2.  No. 3 Seam

No. 3 seam was probably deposited behind a barrier beach that was subject to occasional marine incursions (storm surge). Close proximity of the sea is inferred from

• the high sulfur content of the coal (probably derived from seawater sulphate)
• the thick blanket of sandstone overlying the seam (either deltaic or the result of a barrier beach over-riding the peat swamp).

The thickness of No. 3 seam averages about 4.0 meters but sections up to 6.5 meters have been observed in boreholes. The seam contains several stone partings composed primarily of sandstone. No. 3 seam is hard and blocky and considered to be a thermal coal. It is generally bright-banded with dull inter-beds (durain ?). Calcite and pyrite are present along bedding planes, vertical joints and cleat faces. Pyrite lenses are common. Some fracturing with polished, slickensided surfaces are also present. Sulfur and ash content of the seam is highly variable. Ash is controlled by the thickness of intercalated stone bands, and may range from 10-20%. Sulfur content is higher than No.1 seam, and generally ranges from 1.2% - 3.5%.

Roof strata are composed almost entirely of sandstone, while the floor is typically mudstone and locally sandstone.

C.  Smoky River Coalfield

1.  No. 4 Seam

No. 4 Seam was deposited in a shallow, low energy, brackish, delta plain environment. This environment is substantiated by the presence of fossil assemblages in the shale roof. (Langenburg, Kalkreuth, Wrightson, 1987).

The No. 4 seam thickness averages about 4.9 meters at No. 5B-4 Mine. A profile of the seam, taken in the mine, shows the coal to be bright banded with intercalated layers of durain, and lenses of fusain. Two thin (<5 cms) stone partings were observed about a meter below the roof and above the floor. As many as five layers of highly sheared, soft, platey coals were noted within the seam section. These probably represent planes along which movement associated with folding and thrust faulting took place. No. 4 Seam is a high-grade metallurgical coal. Sulfur and ash content average <1% and 10% respectively. The coal is generally soft and possesses a well developed cleat system.

Roof strata are composed primarily of mudstone and siltstone, which have been extensively sheared by tectonic movement. The floor of the seam is composed of gray shale and siltstone with thin beds of coal.

V.  Methods of Mining

The mines described in this study employ two basic mining methods. These methods are well suited to efficiently maximize the recovery of coal reserves within the geological setting in which they are found. The methods used are summarized in Table 4, and a brief description of each is given below.

MINE NAME	COAL SEAM MINED	MINING METHOD
2 North	No. 1 Seam	Retreat Room and Pillar
4 South	No. 3 Seam	Retreat Room and Pillar
5B-4	No. 4 Seam	Retreat Room and Pillar
Prince Colliery	Hub Seam	Retreat Longwall
Phalen Colliery	Phalen Seam	Retreat Longwall

Table 4: Mining Methods Used to Extract Coal

The retreat longwall mining method is employed in the Sydney Coalfield at both Phalen and Prince Collieries. It involves the driving of long single-entry roadways to outline the rectangular block of coal to be mined. (In Australian and U.S mines two or more roadways may be driven). These roadways are generally driven sub-parallel to seam strike and joined at one end by a wall face roadway. The wallface is equipped with hydraulic roof supports and a shearer. Man, material, and coal transportation facilities are installed in the gateroads. The coal is mined by a shearer which cuts 1-meter wide slices along the entire length of the wallface. After each cut, the roof supports are advanced and the roof is allowed to cave behind the roof supports (See Figure 8).

Longwall face width is a major delimiting factor in the submarine Sydney Coalfield because tensile strains (at the seabed) induced by mining must be kept within specified limits to prevent seawater from entering the mines. Longwalls typically produce about 1million tonnes of coal per year in the Sydney Coalfield, but much higher production rates have been achieved using this method in other countries. Typically, up to 70% of the coal is removed using this mining method.

At Prince Colliery, longwall panels are driven for lengths of up to 3000 meters. The longwall currently prepared for mining has a nominal width of 200 meters. The panel gateroads are driven in-seam using Dosco dintheading machines. Roadway width is a nominal 4.5 meters. The roof is supported by rectangular steel sets installed at about 1-meter spacing. The coal ribs are not re-enforced.

At Phalen Colliery, longwall panels have reached a length of 3500 meters, and a face length of 350 meters has been achieved. Longwall gateroads are driven in-seam, at a width of about 4.9 meters, using continuous miner machines. 2.4-meter long, full column resin steel bolts support the roof. The roadway coal ribs are also bolted using steel and resin bolts.

This method of mining is employed in both the Comox and Smoky River Coalfields. It involves the development of mining panels in a typical room-and-pillar fashion. The pillars within the panel are then extracted by systematically removing the coal (typically the "Christmas Tree" cut pattern) and allowing the roof to collapse. In this way, about 60% to 95% of the coal in the panel can be removed (See Figures 9). The percent of coal recovery varies depending on roof geology.

At the 2 North and 4 South mines at Quinsam, panels are developed using continuous miner machines and shuttle cars. Roadway widths are typically driven 5.5 - 6 meters wide. The roof is supported by 8-foot long resin, point anchor rebar, and steel mesh (mesh in 2 North only). Because the coal seams are so thick (>3 meters), the upper portion of the seam (about 2 meters) is removed during development with the remainder of the floor coal mined as the panel is retreated. Coal extraction in 4 South mine is restricted to about 40%, because of the thick sandstone overlying the coal. Pillars of unmined coal, 20-30 meters wide, separate individual panels.

At Smoky River No. 5 B Mine the mining method is the same as that used at Quinsam. The only difference is the number of entries driven into the mining panels.

VI.  Mine Visits

In May of 1999, the author toured the Quinsam and Smoky River Coalfields, and spent several days at each of the mine sites. These visits included discussions with engineering and geological staff, and examination of underground mining operations. In the following section of the report, each of the mining operations is described with particular emphasis placed on ground control issues at the mines.

A.  Phalen Colliery

Phalen Colliery is situated near the center of the Sydney Coalfield adjacent to the town of New Waterford (See Figure 10). It has operated since 1984, and during this time has extracted about 18,000,000 tonnes of coal from 14 longwalls. The principal working areas of the mine are located entirely beneath the sea, at a maximum depth of about 700 meters below sea level, and about 4.5 kilometers from the coastline (See Figure 11).

Early efforts using longwall mining had been very successful at Phalen Colliery. After 1992 however, the situation progressively deteriorated when ground control issues raised serious concerns about worker safety and productivity. These concerns were generated by the effects of a thick (up to 27 meters) sandstone bed located in the immediate roof of the seam, and impacted both development and longwall sections. The inability of the mine to effectively deal with these issues led to its closure in December of this year. A brief description of the impact of this sandstone, and a description of ground control issues is presented below.

1. The sandstone provided competent roof conditions in development sections, but unfortunately was also a reservoir for methane gas. When main entry development roadways passed the 700-meter depth line, the sandstone exhibited a tendency to outburst. Hundreds of tonnes of rock were violently ejected from the working face, and large quantities of gas were liberated to the roadways during these events (See Figure 12). The ejected rock and gas presented a major hazard to mine workers, and severely reduced the rate of roadway advance. Because of these concerns, mine expansion to the north was severely inhibited, and was eventually stopped.
2. The thick and strong (85 MPa) sandstone delayed caving of the immediate roof during longwall mining. This led to the development of intense cantilever forces (up to 2000 tonnes) which exceeded the capacity of the hydraulic roof supports. Additional stress was also added by remnant pillars in abandoned mine workings overlying the seam. The high stresses created by the sandstone cantilever and overlying pillars, and the subsequent convergence of the roof supports, caused the roof to fracture and unravel for heights of up to 8 meters above the seam (See Figure 13).

Falling roof rock extended over half the length of the face and led to the formation of large cavities. In order to resume mining, the falling roof had to be removed and the cavities stabilized. Rehabilitation work was both hazardous and time-consuming. It placed a severe strain on the economics of the operation, and the psychology of the workforce. After experiencing 8 roof falls, and implementing a variety of preventative measures without positive results, longwall mining was terminated at Phalen Colliery in September 1999.

3. Albeit not as severe as the weightings and outbursts, the presence of elevated horizontal stress constituted another ground control issue at Phalen Colliery. This compressional stress is a remnant of tectonic activity, and has been found to effect ground conditions in roadways during development and upon longwall retreat. The impact of horizontal stress is highly related to the angle that roadways make with the direction of principal horizontal stress. Roadways driven parallel to the stress direction tend to be most stable, while those driven normal to the stress are least stable.

At Phalen, the principal horizontal stress direction is northeast southwest and about twice the vertical stress in magnitude. Typically, longwall gateroads are driven east west. This drivage direction forms an acute angle with the stress direction and results in moderate to severe coal rib spalling, and shearing of the roof along one side of the roadway during development. Bolting the coal rib and the addition of roof support on the stress-notch side of the road controls these effects. During longwall retreat, horizontal stress concentrates in one of the gateroads, and can lead to the development of severe "cutter roof" along one of the ribs (See Figure 14). Supplementary support is added to control this movement. In general, with the use of ground monitoring techniques, and appropriate remedial support, the impact of horizontal stress has been kept under control.

4. Phalen Colliery mines coal about 130 meters below abandoned, flooded workings of the Harbour Seam. Remnant coal pillars left in these workings has concentrated overburden stress and transmitted that stress downward to the roadways and longwalls at Phalen Colliery. Stress effects occur in the form of increased loading of roof supports on longwalls, and ground control problems in development roadways. In general, the impact of these vertical stresses can be effectively handled by predictive monitoring and by supplementing ground support.

   There is a special case however, where their impact is more severe. When longwalls at Phalen Colliery mine under these pillars in areas of thick sandstone, pillar stress is added to the cantilever stress induced by the sandstone. In particular cases, this stress surcharge may be sufficient to cause a roof fall where one may not normally occur. In addition, longwalls working under these pillars have experienced inrushes of water from the overlying flooded workings. This is believed to be the result of increased hydraulic conductivity caused by stress-induced fracturing of the interburden beneath the pillars. The surprising thing about these stresses is that they are capable of being felt over the relatively wide interval separating the mines. It is the opinion of the author that the high percentage of sandstone in the interburden plays a significant role in this stress transfer.

B.  Prince Colliery

Prince Colliery is located on the western end of the Coalfield, about 30 kilometers west of the city of Sydney (See Figure 10). It has been operating for almost 25 years, and has extracted about 19,000,000 tonnes of coal from 24 longwalls. The main workings of the mine are located entirely beneath the sea, and mining depth is about 350 meters below sea level. Current workings are located about 7 kilometers from the seacoast (See Figure 15).

This colliery was initially developed as a small room and pillar operation. In the early 1970’s efforts to use this method proved unsuccessful due to poor roof conditions and soft floor. Rock bolts supported the roof in these early workings, but after experiencing several major roof failures was abandoned and replaced by steel sets. Subsequently, all roadways at Prince have been driven under steel sets (rectangular and arch-shaped). Longwall mining has been very successful at Prince because of the relatively flat gradient (<5 degrees), the absence of faulting, and roof strata that is prone to easy caving. Ground stability issues at the mine have centered on two principle factors.

1. Like Phalen, mine workings at Prince are subject to elevated in-situ horizontal stresses. Measurements taken at the mine indicate that the direction of principal horizontal stress is northeast southwest, and about 2 times the vertical stress. The impact of these stresses has gradually increased, as the mine approached the axis of a tectonic syncline. Longwall development gateroads driven east west, at an acute angle to the direction of principal stress, have experienced significant closure in the form of floor heave and roof sag (See Figure 16). During longwall mining, additional loading is induced by the front abutment and has caused excessive roadway convergence. The magnitude of convergence has made access to the longwall face difficult, and restricted the ability of the gate-end equipment to move efficiently. This situation has made it necessary to brush the floor of the gateroads, and install supplementary ground support. These actions constitute an additional operating cost, but more importantly reduce longwall mining rate. In 1994, the direction of the longwall panels was re-oriented north-south to place the mine at a more favorable direction to horizontal stress, and this seems to have significantly reduced the severity of closure in the gateroads (See Figure 17).
2. A major sandstone-filled, paleo-river channel crosses the Prince Colliery reserves in a northeast southwest direction. Locally, erosive scouring has totally removed the coal seam. An example of coal seam erosion is shown in Figure 18. This sandstone and its erosive channels have severely restricted the placement of longwall panels at the mine. Indeed, Prince has become a one-sided (west) mine because of the extensive sandstone influence on the east side of its main entries. Mining has historically been arranged to avoid areas where the sandstone is present.

C.  No. 2 North Mine

No. 2 North Mine is located about 27 kilometers southwest of the city of Campbell River. It was opened in 1990. Current workings are located about 3 kilometers from the surface, and at a depth of about 120 meters (See Figure 19). The main access portals were driven into the highwall of an open pit, which extracted the near surface coal of the No.1 Seam. The author had an opportunity to inspect the face of the open pit where the geology of the mine roof was well exposed. A thick layer of strong mudstone roof containing numerous beds of ironstone (siderite) overlay the immediate roof of the coal seam. Marine fossil shells were also observed in the mudstone. A thick bed of sandstone occurred immediately on top of the mudstone. A normal fault was observed in the face; its displacement was estimated to be about 1 to 2 meters.

The travel underground began at the portals, and continued down the mine to development and active mining areas in 7, 8, and 9 Panels. The gradient of the seam ranged from 5 to 10 degrees. Dan Payne (geotechnical engineer) and Jim MacMillan (mine geologist) accompanied the writer. Observations made during this travel are as follows.

1. The roof of the mine was well supported with roof bolts and monitored with telltales. The author was immediately impressed by the obvious stability of the roof in the main entries; despite the fact that they had been standing for several years and had suffered the slaking effects of a humid mine atmosphere. This observation supported the first impression that the mudstone roof was indeed a competent member with good support characteristics. Locally, portions of the roof had fallen out during development. These "fallouts" were slickensided and polished, and were associated with pot-and-kettle roof, and shear zones related to faulting. The mine has adopted a comprehensive remedial program to maintain roof stability. This involves scaling down of loose roof, re-bolting and screening, installation of cable bolts, and placement of cribs and posts.
2. Several small displacement faults were examined (See Figure 20). Roof conditions were inferior in these areas, especially where the strike of fault planes was sub-parallel to the direction of drivage. Locally, faulting had caused roadway gradient changes, a deterioration of coal pillar stability, and produced barren zones within the coal mining horizon. Faulting is of particular concern during de-pillaring because it can cause unpredictable movement of the roof. Mapping the location of these faults, and conveying their location to shiftbosses, face workers, and mining staff is a priority activity for the mine geologist.
3. The No. 1 coal seam is about 3.5 meters thick and contains three prominent stone partings. Thickness of these partings varies from place to place but exhibit an overall thickening to the east. Mining stops when the thickness of the bands causes ROM ash to exceed the cut-off limit. The lowest of the three bands contains a core of soft plastic mud. The mud from this split actually creeps slowly into the roadway shortly after being exposed by mining (See Figure 21). The author has not observed a band of this type before. Its occurrence has been found to increase the problem with coal rib spalling. (It would be interesting to carry out X-ray defraction analyses of this mud in an effort to explain its origin).
4. The coal ribs of the main entry roadways were in exceptionally good condition. (See Figure 22). This statement is made by way of comparison to the often poor ribs of the Hub and Phalen Seams in the Sydney Coalfield. The strong ribs are probably due to shallow depth of mining, and the low HGI and rank of the coal. My overall impression is that the No. 1 Seam is a relatively strong, blocky coal.
5. In areas adjacent to the de-pillaring operations, large vertical cracks were observed in the coal pillars. These often extended from the roof to floor of the seam and formed large blocks that fall from the rib without warning. Spalling was evident not only at pillar corners, but also along pillar roadsides. The mine was obviously concerned about the hazard this presented to the safety of the work force. They had a comprehensive rib-monitoring program in place that called for the sounding and marking of hazardous ribs, and for the placement of posts or cribs to protect walkways. Safety alert memos were also issued to keep the work force mindful of the hazard. Cleat dilation and tensile failure of pillar edges probably caused these rib conditions. The forces driving dilation are most likely related to load transfer from the main roof as it bends into the gob area. The presence of the soft mud parting also amplifies this effect by providing of slippery surface along which horizontal movement can take place.
6. The mining area was examined to obtain some idea of the de-pillaring sequence. It was immediately apparent that considerable skill on the part of both supervisors and workers is required to conduct this process safely and efficiently. Since roof caving is essentially a process that occurs when underlying support is removed, it is obvious that roof lithology and structure play a major role in this process. Realizing this fact, mine engineering, geotechnical, and geological staff prepare detailed drawings of rock structure (especially the location of faults, slip planes, and other planes of weakness), and convey this information to workers on a regular basis. A photo taken in a typical de-pillaring section is shown as Figure 9.
7. No obvious evidence for the existence of elevated horizontal stresses was evident in this mine. It is suspected that the tectonic history of the area accounts for its absence. Since the majority of faulting present is of the normal type, tectonic movement was primarily extensional in nature as opposed to compression, which is the source of horizontal stresses. Some thrust faulting was apparent, but it suspected that it probably predates normal faulting.

D.  No. 4 South Mine

No. 4 South mine is a small mining operation located about 2 kilometers south of No. 2 North Mine. It was not being worked on the day of the visit. Current workings were located about 1 kilometer from the portal, and at a depth of about 100 meters (See Figure 23). No. 4 South uses the same method of mining practiced in 2 North except partial depillaring is carried out. The underground visit began at the portals, and continued down the mine to the face of the Mains development. Dan Payne (geotechnical engineer) and Jim MacMillan (mine geologist) accompanied the writer. Observations made during this travel are as follows.

1. At the mouth of the mine, the coal seam was severely split with partings of sandstone. These partings gradually disappeared further inside the mine. The seam roof was composed almost entirely of light gray, medium grained sandstone, and was obviously exceptionally competent. The roof of the mine was supported with roof bolts but screening was not required. As expected, the sandstone did not exhibit any significant deterioration due to weathering.

2. A major fault was encountered in the Mains about half way into the mine. It had caused a significant change in seam gradient. The roof around the fault zone was slabby and prompted the placement of additional support. No other sizable faults were observed during the visit.

3. The No. 3 coal seam is about 4 meters thick in this mine and contains several prominent stone partings composed of light gray sandstone. At the time of visit, efforts were being made to mine the 7 to 9 foot section of clean coal located above the lower sandstone parting. This was being done to avoid the soft, wet, and inferior quality coal lying below the parting. A picture of the coal seam showing the partings can be seen in Figure 24.

4. The physical character of No. 3 coal seam is similar to No. 1 seam. The strength of the coal was evident (Figure 24) where we examined a small area of de-pillared coal to left of the Mains. The coal shows little sign of rib spalling despite its small size. This is, of course, partly due to the presence of a strong roof.

5. In the most advanced development drivages, good exposure of the sandstone roof were examined (See Figure 25). The extensive presence of this sandstone was providing excellent roof conditions in the roadway drivages. Locally, sandstone had eroded away a portion of the seam, but it is surprising that more extensive erosion had not been encountered. The strong sandstone roof is not the preferred situation for retreat mining because sandstone does not cave easily after the coal is removed. Extensive mining under such roof, could lead to a major, mine structure failure. For this reason, coal recovery was only partial in the de-pillaring areas.

6. As was the case in 2 North, no obvious evidence for the existence of elevated horizontal stresses was evident in this mine.

E.  No. 5B Mine

No. 5B mine is located about 20 kilometers north of the town of Grande Cache. It was opened in 1998 to extract coal from the No. 4 coal seam. At the time of mine visit workings were located about 0.5 kilometers from the surface, and at a depth of about 300 meters. The general layout of the workings is shown in Figure 26.

The travel underground began at the portals, and proceeded down Main Entry No. 3 to the active mining areas of No. 4 South Panel. Dan Payne (geotechnical engineer), Chris Mark (NIOSH), Peter Cain (geotechnical engineer) and Andrew Beaton (mine geologist) accompanied the writer. Observations made during this travel are as follows.

1. The roof of the mine was well supported with roof bolts. The author was impressed by the irregular and blocky nature of the roof. The abundance of multi-directional, shiny, polished, and slickensided parting planes suggested it had been difficult to hold during development (See Figure 27). These planes were caused by pot-and-kettle roof, and shear zones related to thrust faulting and folding. Although individual pieces of the rock appeared to be strong, the presence of the shear planes made the rock mass weak. Roof integrity in the entry was maintained by close monitoring with telltales and convergence monitors, and installing cable bolts cribs and posts as required.

2. One small thrust fault was encountered during the travel, however the great height of the roadway prevented its close examination. A typical thrust fault found at the mine is shown in Figure 28. Roof conditions are generally inferior around faults, especially where fault planes strike sub-parallel to the direction of drivage. Faulting and associated roof shears are the primary structural features effecting ground support at the mine. In accordance with this concern, comprehensive geotechnical mapping of these features is carried out, and their location conveyed to shiftbosses, face workers, and mining staff. In areas where the roof was free of shear planes and faulting, it was found to be in remarkably good shape.

3. A profile of the seam was measured at a location immediately outbye the face in one of the Main entry drivages. This profile has been described above in this report.

4. The coal ribs of the main entry roadways were not as strong as those at Quinsam, but were similar to the stress-notch side of roadways driven at Phalen Colliery. Considering the high (90-100) HGI of No. 4 coal, it was surprising to find them as strong as they were.

5. The active mining area was examined to get some idea of the de-pillaring sequence used. Several older gobs (2 and 3 South) were also inspected. These clearly showed that after coal extraction, the immediate roof caved well. There did not appear to be any large massive blocks, which might be indicative of load transfer to adjacent pillars.

6. There was some evidence, albeit minor, for the existence of in-situ horizontal stress in the roof along the right side of Main Entry #3. It was hotly debated, among members of the team, whether it was present at all. This was surprising to me, because intuitively one would expect horizontal stress to be high in a coalfield exhibiting such dramatic compressional structures. It is possible that most of the stress has been relieved by movement and deformation along the major thrust sheets.

7. A trip was arranged to examine some of the large open pit mines in the area. This was important because No. 5B mine was of limited areal extent at the time of the visit, and roof exposures were generally inaccessible because of the great height of the roadways. This excursion provided an opportunity to assess the composition and structure of the roof and floor strata surrounding No. 4 seam. It also proved to be an effective forum for Chris Mark to illustrate the finer points of the Coal Mining Roof Rating system. The structure in these pits must simply be described as spectacular. This is evident in the photo shown in Figure 29.

8. In addition to the open pit visit, a road side exposure of No. 4 seam provided an excellent opportunity to observe a thrust fault (See Figure 30). The thrust fault plane clearly passed through the coal, and indicates that at least some of the shear zones observed in the seam profile in No. 5B mine were produced by thrust fault slippage within the coal seam. One can also observe the fractured roof of the seam where the fault plane exits the coal. At least in this example, it seems readily apparent that a roadway driven against the dip of the fault plane will experience more unstable roof conditions than one driven with the dip.

VII  Properties of the Coal Seams and Surrounding Strata

Data describing the mechanical and physical properties of rock types commonly found in the three coalfields were collected for comparison purposes. These data include uniaxial compressive strength, tensile strength, Youngs Modulus, Poissons Ratio and density. The information has been summarized in a separate table for each of the major rock types (sandstones, shales, mudstones, siltstones, and coal) present. These tables can be found in Appendix 1. Although they are not the only tests used in ground control design, these are among the most commonly used. Following are some comments on these data.

1. The number of tests conducted varied for each coalfield and rock type. All tests were reported to conform to ASTM testing procedures and standards. Point load tests were not included in the tables. In general, there are more tests available for the Sydney Coalfield. This is probably because mining has taken place for a longer time at Sydney, and its mines have been subject to many serious ground support problems associated with sandstone roof and mudstone floor.

2. The quantity of coal strength testing in the coalfields is very limited. Undoubtedly this is due to the difficulty in obtaining suitable test specimens. In general when coal strength testing is done, it is the stronger bands that are tested because these bands are more often recovered as cores. The greater number of test results for the Quinsam coals probably reflects the characteristically low HGI and rank of No.1 and No. 3 seams. The rank and HGI of both the Sydney and Smoky River coals are both considerably higher.

3. The test results shown in the tables indicate fairly standard values for coal measures strata and coal. Testing however, has been carried out on a variety of sample types (cores from boreholes, sub-cored blocks of rock, rectangular squares) and sizes (35- 75 mm in diameter). It was not always possible to determine if test results had been adjusted to meet the standard 50-mm size. Some percentage of the test result range is probably due the effect of sample size. Because of the well-known relationship of increasing strength with decreasing size, all test results should be adjusted before the values are used for design purposes.

4. The data show a relatively wide range in test values for each rock type. To a large degree this is due to the fact that test results are summarized under four general headings. In reality, these are generic headings and the data ranges reveal the wide variability of the rocks they claim to describe. Pure varieties of sandstone, siltstone, shale, mudstone, and coal are relatively rare in nature. These headings are more properly interpreted as end members between which there are an infinite variety of intermediate types. The strata enclosing most economic coal seams were deposited in chaotic environments, and the composition of the rocks that eventually form has been influenced by numerous factors including sediment source, local ecology and meteorology, diagenetic processes, burial history, post-depositional tectonics, etc.

   To illustrate this variability, Table 5 has been prepared. It contains descriptions of sandstone used by the author during core logging and mapping exposures. For each of these descriptions, a different rock strength would result. A good average strength for sandstone would eventually emerge if a sufficient number of these sand-types were tested. Even this average however, would not be appropriate for designing mine support systems, or assessing the best method for mining a given coal block. For example, if the strength and other physical properties of the Prince Colliery sandstone were used to assess the Phalen mining block, gas outbursts and longwall weighting would not be expected. Such an assumption would have obviously been a gross error in judgment.

   These observations are made to emphasize that generic classifications, such as those presented in the tables, may lead to an over, or an under-estimation of rock strength in the design of ground support systems. These tables must be used with a full appreciation of this limitation. They give a rough estimate of rock properties for the average case. For individual cases, ground support design should be based on site specific testing of the rock.

VIII.  Standardization of Geological Assessment

Standardization may be defined as doing things the same way every time. As regards to geological assessments in underground coal mines, we might infer from this definition, that geological assessments would all be carried out in the same way, using the same procedures in the three coalfields. I believe that standardization should not be based on formal adherence to rigid procedure; but rather be grounded on a few basic goal-oriented principles and allow considerable leeway for initiatives that recognize the distinctiveness and individuality of each of the coalfields.

The objective of geological assessment is to describe and characterize the rock mass by using appropriate exploration techniques and assessment methods. It provides mining and geotechnical engineers with the information they need to carry out their own important roles. It recognizes that geological assessments are part of an on-going process, and constitutes a continuous cycle of input and feedback where deficiencies are recognized and adjustments are made to improve the process.

To be most effective, geological assessment should begin with the design of exploratory core drilling programs, and whenever possible, input from mining and geotechnical staff should be sought at an early stage in this design. At a nominal cost and a little extra effort, these programs can prove to be an invaluable asset to the engineers charged with the task of designing the mine and its support systems. Some basic guidelines for designing exploration programs are discussed below.

In general, exploration boreholes should secure cores no smaller than 50 mm, and be followed by a suite of geophysical logs that includes natural gamma, wide-spaced and focused density, geodip, and directional logs. Electromagnetic and neutron logs should be considered if water and gas problems are anticipated, and sonic logs if seismic surveys are planned. Other logs may be appropriate for local conditions.

During core logging, lithological descriptions should be consistent between boreholes. Some lithological unit classification system like the numbering system used at Smoky River would be of great benefit in insuring this consistency. Special note should be made of the parameters listed below because of their importance in performing rock mass ratings. All of these parameters should be assessed before detailed lithology logging is carried out.

• Percent core recovery
• RQD
• Bedding dip
• Fracture frequency
• Fracture dip
• Condition of fracture walls (smooth, rough, listric, slickensided, etc)
• Location of major discontinuities
• Natural rock staining
• Point load testing of all intervals within 8 meters of the seam roof. This procedure is recommended in the CMRR system. Test results can also be used with the RMR system.
• Selection of representative samples for laboratory testing.

This principle is based on the rather obvious assumption that rock mass discontinuity type and character has been determined by geological history. It proposes therefore, that the types of discontinuities that will likely be encountered during mining can, to a large extent, be predicted beforehand if geological history of the deposit is known. For example, in a high energy fluvial depositional environment, sharp contacts between lithological units are commonplace and lateral change in roof rock composition can be extreme; whereas in bay-fill lacustrine deposits, coarsening upward sequences generally produce more gradual vertical changes and offer less variation in the horizontal direction. This type of information is especially useful in the rating of discontinuities for rock mass classifications.

In all three coalfields, tectonic history has played a major role in the distribution of the coal deposits and the character of their roof. At Quinsam for example, structural geology is characterized primarily by normal faulting, and the extent of unstable roof around those faults is less extensive than at Smoky River where low-angle thrusts influence a larger area of roof. In the Sydney Coalfield, where structural style is characterized primarily by folding, elevated levels of horizontal stress is still preserved in the coal measures and exerts a significant effect on both roof and floor stability.

Knowledge of sedimentary and structural history can be a major benefit to the engineers who are faced with designing coal recovery and ground support systems. To optimize the use of this geological data, a number of drawings displaying the information should be available and kept up to date. Although the number and type of maps and drawings may vary with local conditions, the following basic geological drawings should be present at all mining operations.

• Cross sections should be constructed showing the interval from topographic surface to at least 50 meters below the coal seam, and parallel to dip and strike of the deposit. All major sedimentary units and faults should be displayed. Cross sections provide an excellent vehicle for visually displaying otherwise complex geological environments.

• Structural geology plans should be prepared showing the following information.

  • the position and dip of all known faults planes

  • rosette or Schmitt net diagrams indicating the strike and dip of major fault planes, joint systems, and coal cleat

  • structure contours at either the base or top of the coal seam

  • the direction and magnitude of in-situ horizontal stress, if it is present.

• Isopach maps showing the interval between the coal seam roof and the base or top of any strong or weak layers in the roof and floor of the coal seam. These maps can be used to determine the potential impact of major sedimentary discontinuities (sandstone erosional bases, rider seam packages, etc) and zones of weakness on the ground support plan.

• Isopach maps of the thickness of all strong and weak sedimentary units in the roof and floor. Of special importance are all sandstone beds greater than 5 meters in thickness. Such units can have a major impact on caving conditions in all mining systems where full roof caving is practiced.

The qualitative geological descriptions of rock, which are so important in the establishment of depositional environment and structural setting, do not easily lend themselves to the tools used by the geotechnical engineer for designing ground support plans. This has been universally recognized and has resulted in the development of number rock mass classification systems that quantify geological descriptions so they can be used for ground support purposes. These systems are described in most standard ground support textbooks. One particularly good reference is "Underground Excavations in Rock" by E. Hoek and E.T. Brown.

As regards to coal mining, the author is most familiar with the CSIR Geomechanics or RMR system proposed by Bieniawski. This system was initially designed for hard rock applications, but has also been widely used in coal mine support system design. The other widely used system is the Coal Mine Roof Rating (CMRR) developed by Molinda and Mark. This system includes all the features of RMR but has been specifically designed for the special set of discontinuities present in coal mining strata. It is receiving more and more prominence as its case history database expands, and will undoubtedly be the system of choice for coal mine operators in the future.

To illustrate the use of a rock mass classification system, the author has used the RMR system to rate one or two cores taken from roof strata at each of the mines included in this report (the core logs are attached in Appendix 2). The ratings (shown in Table 6) were derived from the 2.4 meters of strata immediately overlying the coal seams. This interval was used because it is the interval most commonly bolted with primary roadway support. Roof assessment extended above the 2.4-meter interval however, because some adjustment may be appropriate for discontinuities present immediately above the bolted beam. It is interesting to note that the cores from No. 5B Mine at Smoky River were independently rated by Peter Cain (the mine geotechnical engineer) and were found to compare closely with those determined by the author. The ratings were as follows.

Borehole #	Forgeron	Cain
5-97-51C	44	40
5-97-41C	32	34

This comparison is included to demonstrate that roof rating systems can be duplicated fairly well by independent investigators.

Although one or two samples is insufficient to assess conditions mine-wide, they do represent a random sampling of roof, and can be used to make a general comparison of roof types at the three mines. These ratings show that a great variety of roof types exist in the three mining districts, and that one support system cannot cater to them all. Good roof support practice recognizes this fact, and a well engineered mine is continuously monitored to determine the health of its support systems and adjustments are made as a matter of course.

Initial roof support designs are rarely totally in tune with the ground they intent to support. They may represent an over-design or under-design for the conditions actually present. For this reason, the performance of installed support is assessed using a variety of ground response monitoring systems (telltales, ground movement monitors, convergence poles, etc,). Monitoring is usually carried out under the supervision of the geotechnical engineer, but results should be fed back to the geologist or person who initially determined the roof rating. With information of this kind, the roof support plan can be modified or adjusted, and secondary and remedial support installed to maintain a safe and effective ground support system.

In addition, ground conditions based on relatively wide-spaced geological boreholes seldom capture the true complexity of the overall roof geology. Frequent geological mapping of development roadways and mining areas is therefore crucial to stay in tune with the subtle character of the deposit. This information must be placed on appropriately scaled maps, and communicated to engineering staff and workers at the mining face.

A workforce, well trained in the fundamentals of geology and ground control, is more apt to work safely than one that has not been trained. Training raises the level of awareness in the workforce making it more alert to subtle changes in mining conditions, at identifying the significance of those changes (especially as regards to safety), and knowing what actions to take to mitigate their impact. A trained workforce is also a major asset to geologists and mining engineers. Since workers are present in their sections on a daily basis, they can provide information that is not readily seen by technical staff during their less frequent visits. To cultivate this asset, geologists and engineers must make a special effort to communicate regularly with the face workers, and be proactive in the provision of information such as geological mappings, and the results of ground support monitoring.

IX.  Conclusions

1. The Comox, Smoky River, and Sydney Coalfields represent a contrast in structural style and depositional environment.

2. The Sydney Coalfield was formed in a fluvial sedimentary setting on a slowly subsiding coastal plain. Tectonically, the coalfield was not appreciably disturbed. Its structural style is characterized by gentle folding caused by northeast southwest compression. Remembrance of this tectonic history remains in the form of unrelieved excess horizontal stress. Ground control issues involve sedimentary discontinuities, gas stored in sandstone, thick sandstone beds in the roof strata, adjacent colliery interaction, and the effects of horizontal stress.

3. The Comox Coalfield was formed in a fluvial environment with tentative contact with marine conditions. Its tectonic history is believed to be one of gentle compression followed by extension. These activities have produced a coalfield characterized primarily by normal faulting, and an absence of horizontal stress effects. Ground control issues revolve around mine pillar instability adjacent to de-pillaring sections, and the difficult to predict roof conditions associated with faulting.

4. The Smoky River Coalfield was also deposited in a fluvial environment. Tectonically, the coalfield is characterized by spectacular examples of sediment compression and shortening which has left a legacy of intensely folded and thrusted rocks. Ground control issues revolve around ground fracturing and weakening effects associated with tectonic history. Surprisingly, excess horizontal stress has not been identified as a particular problem in this coalfield. It is possible that stress has been consumed in deformation or shed by intense movement along the large thrusting sheets.

5. The types of mining employed in these coalfields, appear to be in tune with the geological environment in which they are found. In the tectonically disturbed western coalfields, the retreat room and pillar mining method is used. This system provides the flexibility that is necessary to successfully mine in an environment where seam and roof structure can change quickly. The relatively inflexible longwall mining system has been successful in the Sydney Coalfield because large panels of coal can be developed without the concern of faulting or other major seam disruptions.

6. The geotechnical people encountered in the process of preparing this report are all conscientious, and competent individuals who are well schooled in the science and art of ground control. They take great pride in their work, and aggressively promote the professional interchange of information, technological developments and ideas. They are also proactive in the use of expert opinion to help improve the effectiveness and safety of their ground support designs.

7. A concern for worker safety was obvious at all mines visited. This concern could be seen at all levels of management, and by workers at the mining face. It was evident in the extensive ground monitoring and geological mapping programs in place, and the initiatives taken to train and convey information to the workforce.

8. It is proposed that the standardization of geological assessments be principle and goal oriented. In this way, geological assessment procedures will remained focused on their primary objective, while at the same time retain the flexibility needed to incorporate the distinctiveness of individual coalfields.

1. Cathyl-Bickford, C. G.; Coal Geology and Coalbed Methane Potential of Comox and Nanaimo Coal Fields, Vancouver Island, British Columbia; Coalbed Methane; 1991.

2. Cullen, M.; Deisgn Methods to Optimize Underground Layout and Support at the Quinsam Coal Mine; CANMET Report, 1998.

3. Parkes, D.R.; Review of Mining Operations and Planning at the Quinsam Coal Corporation, Campbell River, B.C.; 1995.

4. Smith, G. G.; Coal Resources of Canada; Geological Survey of Canada; Paper 89-4; 1989.

Physical and Mechanical Properties of Rocks Found in the Comox, Sydney and Smoky River Coalfields

Mechanical and Physical Properties of Sandstone - Table 7

Mechanical and Physical Properties of Siltstone - Table 8

Mechanical and Physical Properties of Mudstone/Shale - Table 9

Mechanical and Physical Properties of Coal - Table 10

Lithology Logs of Boreholes used to Determine RMR of Selected Roof Strata in the Comox, Sydney, and Smoky River Coalfields

Borehole No. PH-145 Phalen Colliery, 8 East Panel (1 of 2)

Borehole No. PH-145 Phalen Colliery, 8 East Panel (2 of 2)

Borehole No. PH-206 Phalen Colliery, 8 East Panel (1 of 2)

Borehole No. PH-206 Phalen Colliery, 8 East Panel (2 of 2)

Borehole No. PH-142 Prince Colliery, 1 North Panel (1 of 2)

Borehole No. PH-142 Prince Colliery, 1 North Panel (2 of 2)

Borehole No. PH-136 Prince Colliery, 1 North Panel (1 of 2)

Borehole No. PH-136 Prince Colliery, 1 North Panel (2 of 2)

Borehole No. QM1 Quinsam 4 South Mine - Mains, 23 Xcut C Road, 20' inby (1 of 1)

Borehole No. QM2 Quinsam 2 North Mine - 4 Mains, 31 Xcut, Supply Rd, 25' East (1 of 2)

Borehole No. QM2 Quinsam 2 North Mine - 4 Mains, 31 Xcut, Supply Rd, 25' East (2 of 2)

Borehole No. 5-97-41C Smoky River Coal Mine (1 of 2)

Borehole No. 5-97-41C Smoky River Coal Mine (2 of 2)

Borehole No. 5-97-51C Smoky River Coal Mine (1 of 2)

Borehole No. 5-97-51C Smoky River Coal Mine (2 of 2)