Questions and answers: excavation principles 25 1express the energy in watt seconds. A joule is also the energy expended
by 1 W for 1 s, or 1 J = 1 W s. Hence, 39 J is equivalent to 39 W s, and so
the 100-W light bulb has to be illuminated for 39/100 s = 0.4 s.
Notice that if the compression test is conducted at a strain rate of
5 x = 400 s, or just less than
7 minutes, to complete, representing a power requirement of 39/400 =
0.098 W, which is about the same as a small battery-powered radio.
(c) The specific energy is the energy required to pulverize the rock/
unit volume = 39 J/(the volume of rock specimen). The volume is
n x (0.05/2)2 x 0.1 = 2 x m3. Therefore, the specific energy' is
39/(2 xs-l, the test will take 0.002/5 x= 19.9 x 104 J/m3 = 0.20 MJ/m3.
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.A15.2 The amount of energy used is 48.5 x 3.92 = 190 MJ. The
amount of rock removed is 125 m3. Thereforc, the specific energy
is 190/125 = 1.52 MJ/m3, which is about 8 times greater than the
0.20 MJ/m3 value obtained in A15.1.
The difference in the specific energies calculated in A15.1 and in this
answer, results from the losses that are incurred in blasting. In the com-
pression test, we anticipate that most of the energy used for the test will
be absorbed in specimen degradation but, in any case, the energy actually
absorbed by the specimen was used for the calculation, i.e. the energy
represented by the area beneath the complete stress-strain curve. For the
blasting calculation, the total energy was used, and much of this energy
will be lost in the form of gas pressure and stress wave dissipation.
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.
(i) For the case where the TBM has four 500 V electrical motors
providing the torque to drive the cutting head. Each motor
uses 10 A when rotating freely, and 11 0 A when the head is
cutting. Neglect the thrust for this case and take the cutting
rate as 3 m/h.
(ii) For the same cutting information in part (i) but with an ad-
ditional continuous thrust of 2.7 MN and an increased cutfing
rate of 3.6 m/h.
I Note that the specific energy computed here applies to this particular testing configur-
ation and rock type. A change to the testing regime with the same rock type would result
in a changed specific energy. Thus, specific energy is not an intrinsic material property.