Page 175 - Failure Analysis Case Studies II

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Fig. 1. Stress-strain curves for an ideal elastic-plastic material (I) and an elastic-brittle rock (11).

of contact between the tool and the rock surface. Hence, the volume of rock removed by the passage
of the tool is greater than the volume of rock penetrated. Penetration rates can be estimated from
factors such as the specific boring energy, the power delivered to the working face, and the uniaxial
compressive strength of the rock. The latter, however, is not an accurate guide to boreahility, which
can vary by a factor of 3 or more for rocks of similar compressive strength.

2.2. The reaming operation
The principle of underground raise production involves two basic operations: firstly, the drilling
of a pilot hole, and, secondly, back reaming. A diagrammatic representation of a raise borer being
used for the slope drilling of a pilot hole is shown in Fig. 2. During the pilot hole drilling cycle, drill
rods connect the raise boring machine with a bottom-hole assembly consisting of ribbed stabilizers,
roller reamer and pilot bit. The rock debris is flushed to the surface and collected in a settling drain.
Any common flushing medium can be used, Le. air, water or foam.
After the pilot hole has been completed, a raise boring head is used to back ream the required
raise between the underground levels. The raise boring head has a number of rock cutters to facilitate
the reaming operation (Fig. 3).

3. BACKGROUND
The catastrophic failure of the raise boring machine occurred during the reaming of a 3.66m
diameter by 266 m long hole at a dip angle of 88" to the horizontal (Le. almost vertical). All 32 bolts
on the raise borer drive head failed after 119 m of reaming had been completed. Prior to the failure,
no abnormalities had been reported and the operation had been running smoothly.
The raise borer had been subjected to a major overhaul approximately 2 years before the failure.
An exploded view of the drive head assembly, with respect to the derrick and base plate, is shown
in Fig. 4. To facilitate the overhaul, the equipment was moved from its underground location to
the surface. All the drive head bolts were replaced. A cutaway diagram of the drive head installation,
showing the relative positions of the cover, drive head bolts and body, is shown in Fig. 5.
Since overhaul, the raise borer had been used to ream a series of smaller diameter (2.44m) holes
92, 97, 89 and 91 m in length. The medium used for flushing was mains water. After these holes
were reamed, the equipment stood underground for a period of 3 weeks. The next hole that was
reamed was 97m long, and was produced using mine service water for flushing. Catastrophic failure
of the raise borer occurred during the reaming of the subsequent 3.66m diameter by 266m long