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diameter borehole to a depth of 3000e5000 ft. The drill rig is manufactured in the
United States by J. H. Fletcher Company in Huntington, WV. Nearly 100 drill rigs
are in use in all major coal mining countries, including the United States, China, India,
Australia, and South Africa. Besides coal mine degasification, horizontal boreholes
can be used for water drainage and advance exploration for faults, washouts, and other
geological anomalies [3,4].
The equipment used to drill long horizontal boreholes can be divided into four ma-
jor groups: the drill unit, the auxiliary unit, the bit guidance system, and the downhole
drill monitor (DDM).
The drill rig provides the thrust and torque necessary to drill 3- to 4-in. diameter
boreholes to a depth of 3000e5000 ft. The auxiliary unit provides the high-pressure
water to drive a drill motor and flush the cuttings out. It also holds a gas and drill cut-
ting separation system. The bit guidance system guides the drill bit up, down, left, and
right as desired to keep the borehole in the coal seam. The DDM measures the pitch,
roll, and azimuth of the borehole assembly. In addition, it indicates the approximate
thickness of coal between the borehole and the roof and floor of the coal seam by using
a gamma ray sensor that measures radiation from the roof or floor. The half-depth of
gamma rays in coal is typically 8 inches. In recent years, many other uses of in-mine
horizontal boreholes have come into practice, such as in situ gasification of coal,
improved auger mining, and oil and gas production from shallow deposits [4].

15.2.1.1 The Drill Rig

Fig. 15.1 shows the drill unit. It is mounted on a four-wheel drive chassis driven by
Staffa hydraulic motors with chains or torque hubs. The tires are 15 by 18 in. in
size and provide a ground clearance of 12 in. The prime mover is a 50 hp
explosion-proof electric motor which is used only for tramming. Once the unit is
trammed to the drill site, electric power is disconnected and hydraulic power from
the auxiliary unit is turned on. Four floor jacks are used to level the machine and raise

Figure 15.1 The drill unit for in-mine horizontal drilling.