A mechanism for directional breaking of rocks by blasting

Prévia do material em texto

A MECHANISM FOR DIRECTIONAL BREAKING OF ROCKS BY BLASTING 
A. L. Isakov and G. V. Basheev UDC 622.235 
As is well known, the occurrence of local failure in uniform brittle materials, in par- 
ticular in hard rocks, is caused by a factor such as stress concentration in areas of ensuing 
failure. A classification was given in [1] of directional breaking of rocks by blasting which 
revealed two principal variants for creating stress concentration leading to directional crack 
formation. The first is connected with mechanical weakening of the peripheral zone of the 
blast-hole or borehole in the directions of planned crack formation, accomplished either prev- 
iously or at the instant of blasting. The second does not require the presence of stress con- 
centrators in the peripheral zone of the blast-hole or borehole, and it is accomplished using 
special devices and attachments placed within the charge cavity. Superposition of these var- 
iants describe all of those directional crack formation mechanisms currently known with blast- 
ing of single blast-hole or borehole charges in a uniformly brittle material. Also described 
in [i] was one of the most effective and easily effected methods in practice for directional 
breaking using strong shells in blast-holes and boreholes having longitudinal cut slits orien- 
tated in the plane of planned failure. In this way the start of failure (radial crack initia- 
tion) was dependent on achieving a pulse of tangential tensile stresses in the blast-hole per- 
iphery of a certain critical value J, [i] which was calculated on the basis of considering the 
piston action of detonation products in the blast-hole cavity. However, strict argumentation 
of the conclusion about the contribution of the piston action of detonation products to the 
mechanism of directional crack initiation and unimportant wave and gasdynamic factors due to 
the limited size of the article was not given. 
In this article, a description and the results of two types of experiment are provided. 
Their aim is to elucidate the situation in evaluating the qualitative and quantitative Con- 
tribution of all of the factors noted above to the process of directional failure using shells 
with longitudional cut slits. 
The experiments of the first type are directed towards establishing the effect of purely 
dynamic factors on the test process. With the aim of removing the action of tensile hoop stres- 
ses caused by the pressure of explosive detonation products in the charge cavity, the follow- 
ing experimental scheme was adopted (Fig. i). A thln-walled metal tube simulating a shell 
with a penetrating diametral cut was placed between two plates of organic glass. The internal 
cavity of the metal tube was filled with PETN, and the whole structure was drawn together with 
metal bolts preventing the plates coming apart during explosion. With a tube length of 30 nun 
as a result of explosion of a PETN charge (Pes = 0.9 g/cm 3) of diameter from 3 to 5 mm placed 
within it, clear bands of soot form in the organic glass plates opposite the slits, and in 
some cases there are small irregularities in the organic glass surface. No other mechanical 
damage was observed during the tests. This points to the fact that the outflow of detonation 
products moving along the cut sllt from the charge cavity towards the organic glass plates and 
applying to them a hlgh-veloclty impact, does not in itself create clearly defined damage with 
a capacity to play the role of initiated cracks. This also applies to the air shock wave, in 
some cases ahead of the gas flow. It is concluded that without special concentration of the 
gas flow and shock waves these factors are not decisive in the case studied for preparing di- 
rectional failure of rocks by blasting. 
Thus, by excluding gasdynamic factors from explaining the mechanism for occurrence of 
directional cracks using weakened shells, we move on to study the role of the piston effect 
of detonation products in the process being considered. 
The second type of experiment was devoted to studying the static picture of strains e 8 
for the blast-hole opening periphery occurring on application of a uniformly distributed pres- 
sure to the internal surface of weakened shells. The main interest was in comparing maximum 
g~ ax and minimum e~ in values of tangential tensile strains with a change in such factors as 
Institute of Mining, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk. 
Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, No. 6, pp. 
30-33, November-December, 1985. Original article submitted June i0, 1985. 
496 0038-5581/85/2106-0496509.50 @ 1986 Plenum Publishing Corporation 
Fig. i. Diagram of the experiment for 
evaluating the role of explosive gases 
in creating directional cracks. 
shell material, its thickness, and cut slit parameters playing a role in weakened shells. It 
is not difficult to assume that maximum values e~ ax will correspond to points of the blast- 
hole periphery located opposite cut slits in the shell, and e~ In is at the maximum distance 
from these points. Then the ratio s~in/e~ax will be an indicator of the efficiency of creating 
strain (stress) concentration in the blast-hole contour caused by the presence of a weakened 
shell within the blast-hole. 
Experiments were carried out on plates of organic glass 1 • 1 m and 30 mm thick with a 
hole in the center simulating the blast-hole. Hole diameter was varied from 50 to 65 mm. With 
the aim of embracing as wide a range of material strength properties as possible, shells were 
made of three different materials, fluoroplastic, molded concrete, and lead. Pressure within 
the shells was created by means of a hydraulic pick-up with diameter 40 mm having an external 
shell made of vacuum resin with a pressure transfer coefficient of 0.97-0.98. As a result of 
this creep, the resin densely filled all of the cuts in the prepared shells guaranteeing 
transfer of pressure at their surfaces (the construction of the hydraulic pick-up is described 
in more detail in [2]). The magnitude of the pressure in the tests described was varied from 
5 to i0 MPa. Hoop strains e 8 in the hole periphery were measured at two typical points, i.e., 
opposite the cut slits in the shell and in the areas most distant from them (Fig. 2). Strain 
gauges KF 4M-I-I-100-V-12 with a 1 mm base and a nominal resistance of i00 ~ were used for 
measurements. Gauges were glued from two sides on the plate surface in the direct vicinity of 
the hole boundary. 
min max 
Given in Fig. 3 are processed results for measurements of ~8 and e e with a hole diam- 
eter of 65 mm (ratio of internal to external diameter of the shell is 0.62) for different 
shell materials and sharpness angles of the cut slits. Comparison of the graphs obtained in- 
mln max dicates that shell material has a marked effect on the strain concentration index e 8 /e~ 
on the hole periphery. Material with high strength properties makes it possible to achieve 
the highest strain concentration indices, whereas material with weak strength properties 
(e.g., fluoroplastic) does not generally make it possible to obtain even a small concentration 
of tensile strains in the area of cut slits located in the shell. Sharpness angles for the 
cut slits only have an effect with values not exceeding 30-35 ~ . An increase in these angles, 
starting from 45 ~ , does not change values of the strain concentration index. Tests with 
shells of the same materials, but thinner, indicated that stress concentration is directly 
dependent upon shell thickness, i.e., the thinner the shell, the lower is the effectiveness 
of creating strain concentration on the hole periphery. 
In conclusion, some comments are made. In describing experimental studies for the mech- 
anism of directional crack initiation we are talking about strainconcentration and the index 
characterizing the effectiveness of its creation e~ln/g~ ax. This is connected with practical 
considerations which do not make it possible to estimate directly stress concentration in the 
case being considered. However, the presence of stress concentration in a qualitative sense 
makes it possible to speak about the presence of stress concentration in areas of measurement 
(given below is a numerical estimate making it possible with tolerable accuracy to compare 
mln/s~max osmln /osmax values of strain concentration e 8" and indices). This provides a basis for 
broadening all of the strains giQen in terms into a6 understanding of stresses. We use Hooke's 
law described on principal axes [3]. Then assuming the hypothesis of a plane stress state 
(o= = 0) for the plat~ (see Fi . 2) in which experiments are carried out, for determination 
of~tenslle" stralns" esmln and ~6~mw in the hole periphery, simulating the blast-hole, we obtain 
497 
IZ 
i....-1 
r. 
0 
I 
i i �9 
I g 
of 
qD2 
o3 
0 JO ~0 309'* 
Fig. 2 Fig. 3 
Fig. 2. Diagram of the experiment for measuring tensile strains in the hole con- 
tour. i) Block of organic glass; 2) weakened shell; 3) strain gauges. 
mln. max 
Fig. 3. Graphical dependence of the value of e 8 /e0 sharpness angle of the cut 
slits in a shell of different materials, i) Fluoroplastic; 2) molded concrete; 3) 
lead. 
ee q - ve r 
~ ---- t - - v ~ E ; 
er + v~ e 
O'r=ffi E . 
t - - v z 
( l ) 
min 
Assuming that e 0 
the value 
min 
=--g , and using the first equality of (1), we write an expression for r 
.m in i max 
G~ln/Grmax ffi ~ l~e 
max max i+~r /Be (1 ~). (2) 
max. max 
In order to replace e /e 8 in terms of known values, we use the second equality of (i). 
Similar to (2) we obtain 
re |n : max 
'mln# rain g0 /~@ 
Or I~r oma*/olnaffi' (1 --%'), (3) 
%' "7- ~r 1c'0 
min max 
where o and o r r 
above. 
are values of radial stresses in the hole contour at the points indicated 
Since cuts in shells reach the hole boundary and at the tip they have almost zero area 
max 
of contact with the walls of the hole, it is possible to assume that o r =-Po , where po is 
pressure in the cavity. Then according to [i] 
mint max (ro/Ro)~ml(l+m)o Or lo t = (4) 
By substituting (4) in (3), where Ro and ro.are shell internal and external radii, m = sin 
(~ is internal friction angle for the shell material), we obtain 
~ / 8 m~ = - - ( i ., ,D ,_ , lm/( l+m) _min i .max (5 ) 
- - v l tXiOffO/ ~0 /~# - - V, 
which with substitution in (2) gives 
omin/omax 
ra in / max ~0 /~0 
i -I- v [1 - - (Rolro)'mi(l+m) e~" i " l~"" i ] " (6) 
498 
min. max 
Assuming 0.5 as an average value for e 8 /e 8 and taking the actual range for change in 
value Ro/ro (1.5-2), the denominator of the fraction in the right-hand part of expression (6) 
may be estimated "from above" by a value of i-0.3~ (an estimate "from below" gives i). Whence 
follows the existence of a conditional equality 
rnln! max mini max (7) 
~8 /~O ~ 88 188 , 
valid with an accuracy no worse than 10%. 
i . 
2. 
3. 
LITERATURE CITED 
A. L. Isakov, "Directional failure of rocks by blasting,'! Fiz.-Tekh. Probl. Razrab. 
Polezn. Iskop.. No. 6 (1983). 
A. L. Isakov, E. P. Taran, and R. Yun, "The question of modeling mechanical action of 
blasting by static loads," Fiz.-Tekh. Probl. Razrab. Polezn. Iskop., No. 5 (1983). 
N. I. Muskhelishvili, Some Basic Problems of Mathematical Elasticity Theory [in Rus- 
sian], Nauka, Moscow (1966). 
COMPARATIVE EFFECTIVENESS OF THE METHODS OF EXPLOSION 
CONTROL IN HOLE SHOOTING 
A. V. Mikha!yuk and N. A. Lysyuk UDC 622.235.5:622.32(3+4) 
Explosions in near-hole zones have long been used to change the permeability of reser- 
voir rocks and enhance flow in productive wells for the extraction of oil, gas, and other 
fossil resources [1-4], but the effectiveness of this method still remains unclear. This is 
due to a number of factors. 
First, the explosion inside a hole presents a serious danger to the casing string (and 
to the protective string wherever it is used); although there are some means of protection, 
using them makes the restoration of holes after the shooting difficult. Blasting to improve 
productivity has to be conducted in uncased holes (or intervals) that are free of equipment 
and mostly require no special protective measures. 
Second, the capability of blasting used to improve permeability depends on the proper- 
ties of rocks and the relative effects of the compacting and decompacting action of a blast. 
The shooting of wells, therefore, should be planned in dense rocks exhibiting brittle destruc- 
t ion . 
Third, the efficacy of using the energy of a blast to improve well productivity depends 
on the technology. For geotechnical wells, operating with forced liquid circulation in the 
near-hole zone (underground sulfur melting, extraction of heat from the earth's interior, un- 
derground solution of low-grade ore, etc.), well shooting improves permeability even if the 
rock is virtually impermeable. Blasting to improve rock permeability around the wells for ex- 
traction of oil, gas, and water with a radius of influence that may be tens and hundreds of 
times as large as the destroyed zone is justified if the bed has a natural permeability; oth- 
erwise, regardless of the rock permeability in the near-hole zone the influx of fluid to the 
hole is impossible. 
When the study of the surrounding rocks and the well structure has determined that both 
conditions are present, one normally tries to set up the blasting operation so as to provide 
the highest efficacy of well shooting. An objective estimate of the effect is the increased 
flow (or injection) of the well; there have been several suggestions as to ways this can be 
predicted. 
Since in the crushing zone, where the rocks are fragmented, the permeability is higher 
than the natural permeability (by several orders of magnitude), it was suggested in [5] that 
this zone should be taken for the new radius of the well. The uncertainty of the physical 
base leads to arbitrariness in the choice of this radius. The uncertainty is eliminated by 
S. I. Subbotin Institute of Geophysics, Academy of Sciences of the Ukrainian SSR, Kiev. 
Translated from Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh, No. 6, pp. 
34-40, November-December, 1985. Original article submitted March i, 1984. 
0038-5581/85/2106-0499509.50 �9 1986 Plenum Publishing Corporation 499