Questions 15.1 -1 5.1 0:
excavation p ri nci p I es
415.1 The complete stress-strain curve has been obtained for a cyl-
indrical specimen of intact granite tested in uniaxial compression. The
specimen is 100 mm long and 50 mm in diameter. Assume that, for the
purposes of calculation, the curve can be approximated to the bilinear
form below. The uniaxial compressive strength is reached at 0.1% strain
and 200 MPa stress. When the curve reaches 0.2% strain, the rock mi-
crostructure has been destroyed and all that remains are small flakes of
crushed mineral grains.
(a) Calculate the energy under
displacement curve. This is the
energy required to completely des-
troy the rock specimen. Express MPa
your answer in joules.
(b) For how long would a do-
mestic 100-W light bulb have to
be illuminated to use up the same
amount of energy?
(c) What is the specific energy
of the rock, i.e. the energy required/unit volume to pulverize the rock?
stress
the equivalent complete force- A
Strain, %415.2 During bench blasting in a quarry, it was found that 48.5 kg of
explosive is required to break 125 m3 of marble. Given that the explosive
used was ammonium nitrate-fuel oil (ANFO) with an explosive energy
of 3.92 MJ/kg, calculate the specific energy for these circumstances, state
whether this is greater or less than the answer to Q15.1, and explain why
there is a difference.
415.3 A 5-m-diameter tunnel is being excavated in limestone by a
full-face tunnel boring machine (TBM), which operates by exerting a
torque and a thrust.
(a) Calculate the specific energy required to break the rock in the
circumstances given by the data in (i) and (ii) below.