The coalfields of northern West Virginia, Pennsylvania, Ohio and Kentucky contain millions of tonnes of high quality bituminous coal. Most of the coal is located under at least 700 to 900 ft (200-300 m) of coal measure rock formations. The coal con¬tains natural methane gas in large quantities. Modern longwall mining liberates this gas at high rates. Maintaining sufficient ventilation as the coal is rapidly mined is critical for these longwall operations.

A bleeder system using 2 m or 3 m diameter steel-lined shafts equipped with high-pressure centrifugal fans is an increas¬ingly popular method of supplementary ventilation for many of the longwall mining systems. When a bleeder shaft is used, the only entries developed are those actually needed for the longwall, greatly reducing development time.

When longwall production starts, the bleeder shaft exhausts the air necessary to ventilate the panel. As complete panels are extracted, the bleeder entries deteriorate, calling for increased output from the fan. The blades of the centrifugal fan can be adjusted to higher pressure and volume while running which further enhances safety by reducing fan downtime.

A bleeder system can be described as a combination of entries, ventilation controls, and fans which are used to clear methane gas from areas that have been second-mined. Areas left after second-mining are known as gobs. Fresh air is swept through the gobs to dilute or carry away methane gas accumulations. Until the bleeder shafts were introduced, the contaminated air had to be coursed through main entries and sometimes moved for miles to a main mine fan.

Generally, the bleeder entries are constructed prior to second-mining. They are mined around the perimeter of a gob area. Fresh air enters the gob as it passes the longwall face, and is pulled through the shields. On a pillar section, fresh air enters the gob through the curtains strung across the pillar line.

In both cases, the air traverses the gob and flows to regulators which are strategically placed in stoppings built between the bleeder entries and the gob. The regulators provide control to set up a balanced air flow through the gob. The contaminated air is coursed through the bleeder entries and then fed into the main entries and exhausted at the portal.

If a bleeder system fails, hazardous concentrations of methane gas may build in the gob area. These normally would not pose a problem, except for occasions when the barometric pressure changes or there is a fault in the ventilation system, a fan stops, a roof fall closes off a vital entry, or a crushed stopping causes a short circulation of the air. At these times the methane would flow out into the face areas, placing into miners at risk.

Bleeder shafts, together with an appropriate fan, provide the required pressure differentials across gob areas to adeptly ventilate second-mined areas. They also reduce the risk of bleeder system failure by eliminating the need to move return air through submains and mains. The methane-laden air is exhausted through the bleeder shaft. The air is pulled through the shaft by a fan on the surface. In this application, the centrifugal fan has found renewed acceptance. They generate high head pressures and relatively low volumes and are quieter than axial flow fans.

Blind shaft drilling methods have proved to be a reasonable alternative to more time-consuming conventional shaft construction. The shaft is drilled and lined well in advance of mining. It is then intersected during development of the longwall panel section. One bleeder shaft is sometimes sufficient to ventilate as many as four full panels. Raise drilling is seldom considered for bleeder shafts because of the underground support required to remove the cuttings from the raise operation. Blind shaft drilling can provide the immediate ventilation relief required to establish new longwall panels effectively.

Most of the drilled shafts have been steel lined completely from surface to the mine level. The hydrostatic liner, when cemented in place, provides a virtually maintenance-free shaft for the life of the mine. The shaft has to be maintenance-free because it becomes inaccessible from the bottom soon after longwall mining begins, due to the deterioration of the bleeder entries. Water leaks or liner problems can therefore not be tolerated for the life of the shaft.

Several miles of bleeder shafts have been drilled in the eastern coalfields by Zeni Drilling Co. of Morgantown, WV U.S.A. The majority have been 6 ft (1-8 m) finished, inside diameter. The shafts have all been blind-drilled using reverse circulation as a cutting disposal method. The tool strings used by Zeni consist of up to 100 t of lead-filled drill collars. The cutterheads are equipped with disc cutters and the assembly is stabilized at the top by a non-rotating stabilizer and at the bottom with a stinger rotating in a pre-drilled pilot hole.

Zeni usually uses a pilot hole in the coal measure shafts. The pilot hole serves several purposes. For instance, it may be drilled to a tight deviation tolerance and surveyed to determine the straightness of the finished shaft, and it is used as a positive bottom stabilization for the drilling assembly. The use of the pilot as a guide also allows the drill collar weight to be utilized completely for cutter loading rather than being held back for pendulum guidance of the drill string.

The pilot hole also enhances performance by creating a position for the cutting pickup inlet in the stinger at the drill floor level. Because the cuttings are picked up at the lowest point in the bottom profile, the regrinding of cuttings, which is a common problem in blind drilling, is minimized.

The pilot hole requires at least two weeks of added programme time to drill and may be considered by some to be a waste of time in blind drilling. In many areas of the world, and in some formations, this would be true. However, in the eastern coal fields, the pilot hole does enhance performance. In addition, the confidence that the pilot hole provides for shaft straightness makes the extra time very worthwhile.

The casings used for final lining of the shafts are fabricated in fuly hydrostatic steel. The hydrostatic design is achieved by increasing wall thickness with depth and adding external solid bar stiffening rings at strategic intervals along the csing column. Zeni Drilling Co. develops complete designs for all the hydrostatic steel casing used, and maintains a heavy metal fabrication facility complete with plate rolls, machining and automatic welding facilities. All of the design and fabrication work on the steel shaft linings is done at Zeni's heavy metal workshop.

At present work is proceeding on five bleeder shafts in the Northern West Virginia area. Four of these shafts are 6 ft (l-8 m) inside diameter and one is 8 ft (2-4 m) inside diameter. .All of the shafts will be lined with steel hydrostatic casing. The depths of the shafts range from 750 ft (229 m) to 940 ft (286 m). The completed casing for one shaft can typically weigh well over 600,000 Ib (270,000 kg) suspended from the drilling rig for installation. The casing is floated into a water-filled shaft. The bottom of the casing is capped with a removable bottom bulkhead. As the casing is added and lowered, the water level inside the casing is kept lower than the water level outside the casing. The differential pressure pushes up on the bulkhead thereby floating the casing. The hook load on the drilling rig can then be adjusted as desired by changing the water level inside the casing. In order to float, however, the casing must be able to withstand the differential pressure imparted by the water on the outside of the casing. This pressure can be higher than the actual pressure anticipated when the casing is in service over the life of the ventilation shaft. Installing the casing by flotation serves as a pressure test of the liner system.

When the casing has been placed to the top of the coal seam it is cemented in position with neat cement. The cement is pumped in place using high pressure pumps similar to oil field cementing practice. The annulus between the casing and the rock is completely filled from bottom to tap with the neat cement mix. When the casing has been dewatered, the miners may cut through under the casing bulkhead and remove the bolts holding it in place. With the bulkhead removed the shaft is ready for use.

An 8 ft (2-4 m) shaft was recently completed in Pennsylvania for a large longwall mine near Morgantown. The shaft was drilled to a diameter of 9-6 ft (2-9 m) to allow for the installation of the 8 ft (2-4 m) hydrostatic casing. The depth of the shaft from the surface pad to the top of the coal seam was 752 ft (229 m). The shaft was steel lined for I00% of its depth. The construction of the shaft was accomplished using the blind drilling, lining and cementing methods described above. A pilot hole was used here, and required approximately 2-5 weeks to complete. Some water loss was experienced in the pilot hole and steps were taken to grout the loss zone before drilling into it with the full-size cutterhead. This points up another advantage of the pilot hole in that the water loss zone was detected and could be dealt with while it was relatively easy to correct.

The actual shaft drilling phase of the project required slightly over five weeks to complete. The average performance of the large diameter drill was approximately 30 ft (9 1 m) per 24 h/d or 1-25 ft (0 38 m) per hour. This is an overall average which includes all downtime and tripping for cutters. Actual drilling performance based on a random sampling from the drilling log reflected approximate 2-4 ft (0 73 m) per hour. The casing phase of the project consumed just less than two weeks. The 8 ft (2 4 m) diameter casing required significantly more time to weld than a standard 6 ft (1 8 m) diameter casing. The casing was installed by floating with the drilling rig holding a maximum of 200,000 lb (90,700 kg) of the total weight of 740,000 lb (335,600 kg). The floating process therefore supported 540,000 lb (245,000 kg) of the casing weight. The cementing phase consumed about one week of schedule. Due to limitations in water storage and cement transport the backfill process was done in five separate stages. In total over 17,000 ft3 (481 m3) of cement grout was placed in the annulus between the casing and the rock.

The drilling water was treated and disposed of during the lining and cementing process. As the csing was run into the hole, the displaced water was transferred to the cuttings pond where it was treated and discharged. The water used to add to the inside of the casing was fresh water which did not need treatment for disposal. The remaining water in the annulus was displaced by the cement and treated during the cementing process. This process allows the water to be treated concurrently with the lining and cementing operations so that large storage areas for water were not required and the water treatment was virtually complete when the shaft was finished. Final dewatering of the csing was accomplished with a bailer. Undermining the shaft with a continuous miner and the installation of a ventilation fan completed the bleeder shaft construction.

The entire project from mobilization to demobilization was completed in 14 weeks. The working schedule for the drilling and lining operations was 24 h/d five or six days per week. The remaining operations, mobilization, surface casing, pad construction and rig set-up were accomplished on a one shift/ day schedule.

The use of drilled shafts is not restricted to bleeder systems. They are also being used to provide a boost of intake air and emergency escape facilities. In one application, a 7ft (2 13m) diameter shaft was substituted for an entry when a mining company was tunnelling through a hard rock zone to get to coal reserves. This shaft provided a fresh air intake and an emergency escape-way, while reducing the capital and time required to begin mining the reserve.