ON THE MECHANISM OF ROCK DESTRUCTION DURING PERCUSSIVE-ROTARY DRILLING WITH DRILL HAMMERS

O. D. ALIMOV

For the successful solution of the grand tasks set for the coming years before the mining, coal, oil, and construction industries of our Motherland, increasing productivity on the basis of mechanizing heavy and labor-intensive processes is acquiring enormous importance. In this connection, drilling-and-blasting operations are becoming widely used in mining and construction. But if the blasting process makes it possible to carry out enormous work in the shortest intervals of time, then the preparatory work—drilling blast-holes and boreholes—represents the most labor-intensive and prolonged process, especially when destroying hard and very hard rocks. It is enough to point out that there are rocks in which, to break off \(1 \, m^3\), it is necessary to drill up to \(8 \, m\) of blast-hole, while the drilling rate in these rocks is only \(10\text{–}12 \, mm\) per minute. The significance of these figures becomes especially clear if one takes into account that in many modern mines and even individual shafts thousands of cubic meters of useful mineral are extracted per day [3], and the total volume of rock extracted by means of drilling-and-blasting operations already in the first postwar five-year plan amounted to several hundred million cubic meters.

The figures cited show the urgency of rationalizing drilling-and-blasting operations, especially in the development of rocks of medium and high strength. In this, the following directions are indicated:

1. Reducing the volume of drilling operations by selecting the most rational dimensions and arrangement of blast-holes and thereby increasing their productivity. As practice shows, rational blast-hole parameters also make it possible to significantly reduce the work connected with breaking and transporting rock after blasting.

2. Increasing the drilling rate by selecting more rational drilling regimes, improving and increasing the durability of the drilling tool, and improving and creating new designs of drilling machines and drilling methods.

The selection of rational drilling regimes, and the successful improvement and creation of new tools and percussion machines, are possible only on the basis of a comprehensive study of drilling practice, the physical properties of rocks, and the mechanism of rock destruction under one or another method of drilling.

For drilling blast-holes and boreholes in rocks of medium and high strength, at the present time the percussive-rotary method is most often used. In the present work we consider the mechanism of destruction of rocks of medium and high strength during percussive-rotary drilling.

Existing theories on the mechanism of rock destruction during percussive-rotary drilling

By the mechanism of rock destruction during drilling is understood the process of interaction between the tool of the drilling machine and the rock, as a result of which the latter is destroyed.

To explain the mechanism of rock destruction during percussive-rotary drilling, the theory given by N. S. Uspenskii [12]; [13] is most often used. According to this theory, the drill bit, in the form of a symmetrical wedge, Fig. 1, under the action of the force \(P\), penetrates into the rock, pushing apart and crushing its particles. The penetration of the tool continues until “the entire live force of the bit has been absorbed by this work of destroying the rock and overcoming accompanying harmful resistances” [13]. N. S. Uspenskii divides the destruction of rock under impact penetration of the bit into

Fig. 1

two periods: “during the first blow the bit penetrates into the rock, overcoming resistance to crushing. After rotation through a certain angle, the bit penetrates again; in doing so it already acts as a wedge, shearing part of the rock toward the depression from the preceding blow” [13], Fig. 1.

Assuming that, for penetration of the bit into the bottom hole, the condition

\[ \frac{P}{F}>\sigma_{\text{cr}}, \]

is necessary, where
\(P\) — acting force, kg;
\(F\) — contact area, cm\(^2\);
\(\sigma_{\text{cr}}\) — resistance of the rock to crushing, kg/cm\(^2\). N. S. Uspenskii, on the basis of a mathematical analysis of the forces acting during penetration, Fig. 1, determines the depth of penetration of the drill per blow as

\[ h=\frac{P}{2d\sigma_{\text{cr}}}\cdot \frac{\cos \frac{\alpha}{2}\cos\varphi} {\sin\left(\frac{\alpha}{2}+\varphi\right)}, \tag{1} \]

where \(h\) — depth of penetration, cm,
\(d\) — diameter of the blast hole (borehole), cm,
\(\alpha\) — included angle of the drill blade, degrees,
\(\varphi\) — angle of friction of the drill on the rock, degrees.

It follows from formula (1) that the depth of penetration of the drill bit in one blow is proportional to the force of impact \(P\), inversely proportional to the diameter of the borehole, to the resistance of the rock, and increases as the angle of sharpening \(\alpha\) decreases.

Assuming that the force \(P\) at the moment of impact varies from \(P\) to zero, the work expended on penetration is

\[ A_{\text{in}}=\frac{P\cdot h}{2}= \frac{P^2 \cos \frac{\alpha}{2}\cos\varphi} {4d\cdot \sin\left(\frac{\alpha}{2}+\varphi\right)\cdot \sigma_{\text{comp}}}. \tag{2} \]

Under the action of the bursting force \(H\), the sectors of rock \(aoc\) and \(dob\), enclosed between the indentations from the blows, are chipped off, Fig. 1b. At the same time, N. S. Uspenskii believed that the chipping of the rock would occur only behind the rear face of the drill-bit blade, while on the opposite side its second force \(H\) would be eliminated by the resistance of the rock in its solid mass.

From the drawing in Fig. 1 it follows that

\[ H=\frac{P}{2}\operatorname{ctg}\left(\frac{\alpha}{2}+\varphi\right). \tag{3} \]

Thus the chipping force \(H\) is proportional to the impact force \(P\) and increases as the angle \(\alpha\) decreases.

Proceeding from the chipping force \(H\) and the resistance of the rock to chipping, \(\sigma_{\text{ch}}\), N. S. Uspenskii substantiates the number of blows per one revolution of the drill bit:

\[ n=\pi\cdot d\left(\frac{d}{2}+2h\right)\frac{\sigma_{\text{ch}}}{H}. \tag{4} \]

Despite these mathematical derivations, N. S. Uspenskii indicates that the magnitude of the angle of rotation is more often determined experimentally, and the angle of sharpening of the drill-bit blade \(\alpha\) is not taken as less than \(70^\circ\). With increasing hardness of the rock, the angle \(\alpha\) is increased to \(110\text{–}120^\circ\). The latter is explained by the creation of the necessary strength of the bit blade.

It should be noted that at the time when these propositions were formulated by N. S. Uspenskii (1907–1908), blast holes and boreholes were drilled by the percussive method not only in rocks of medium and high hardness, but also in rocks of low strength possessing considerable plastic properties. The rotational speeds of the drill bit when drilling blast holes and boreholes were so insignificant that the forces from their rotation could be neglected. Therefore, the picture of the mechanism of destruction of rocks during percussive drilling, given by N. S. Uspenskii, was of a general character; it was not specified for one or another method of percussive drilling, nor for one or another category of rock strength.

As a result of subsequent improvements and investigations, the machines, as well as the methods of drilling, changed. Rotary drilling, as the more productive method, completely displaced percussive drilling in rocks of low and partly medium strength. At present, percussive drilling of blast holes is used for rocks of medium and high strength. The speed of percussive machines has increased considerably. The number of blows in modern rock drills reaches 2000–2200 blows per minute, and the number of revolutions of the drill bit is 200–300 per minute. Thus, the practice of drilling and the design of mining machines have posed to mining science the question of a deeper investigation of the mechanism of destruction of rocks during percussive-rotary drilling.

Therefore the great interest is understandable that has been aroused in recent years by a number of works [7; 8; 9; 10; 14, etc.] devoted to the mechanism of rock destruction and to the theory of percussive drilling.

A. I. Medvedko [7] believes that penetration of the bit into the rock is due mainly to the compaction of voids present in the rock in the form of pores and natural cracks. At the same time, partial chipping out of rock particles may occur along the edges of the blade, Fig. 2. The case of penetration with local chipping is an undesirable case, since in this case, owing to the absence of a wedging action,

Fig. 2

the effect of rotating the bit, calculated to produce complete chipping of the sector of rock formed between two blows, will not be fully utilized. Therefore, from the standpoint of drilling efficiency, in the opinion of A. I. Medvedko [7], it is expedient to choose the sharpening angle of the bit from the condition of ensuring self-jamming of the particles chipped out during penetration, and the angle of rotation from the condition of complete chipping of the entire prism of rock located between two indentations in the horizontal plane (Fig. 3). Using the basic propositions of the theory of elasticity, A. I. Medvedko finds

Fig. 3

for the described mechanism of destruction the relations between axial pressure, the temporary compressive strength of the rock, the zone of destruction, and the sharpening angle of the bit. On the basis of these relations the angle of rotation of the bit is determined.

In the opinion of I. S. Pokrovskii [9], the theory of rock destruction in percussive drilling given by N. S. Uspenskii does not explain the character of destruction of hard rocks. I. S. Pokrovskii’s basic premises in this case are the following: “A bit, as a solid body, can ‘enter’ a liquid body, ‘displacing’ part of the liquid; it can ‘penetrate’ a plastic body, such as clay, wax, etc., and displace part of it without rupture of continuity,

but the bit cannot “enter” or “penetrate” into hard rock and displace part of it without rupturing its continuity. Upon striking hard rock, the bit must destroy its continuity, and only in this way can it make an indentation in the rock. The second stage—shearing of the rock at the next blow of the bit—can take place only on the condition that the bit “penetrates” into the hard rock as into a plastic mass, and the form of its destruction will be an exact imprint of the bit blade, since only under this condition will the lateral planes of the bit blade press on the side walls of the rock. In hard rocks such a process of destruction is impossible, since the bit blade will not leave an exact imprint in the rock without destroying its continuity, and its lateral planes will not exert lateral pressure on the rock.”

From the point of view of I. S. Pokrovskii, the bit blade always has a certain blunted contact area. When the bit strikes the rock, Fig. 4, the rock area $\overline{AB}$, equal to the area of the bit blade, receives the pressure that will compress the rock. As a result, a prism $ABC$ of maximum compressive stresses is formed in the rock along the entire length of the bit blade; this prism is broken out and will act on the remaining rock as a wedge. The lateral planes of

Fig. 4

this wedge $AC$ and $BC$, after loss of continuity along them, will in turn press on the surrounding rock; moreover, the prisms $ACK$ and $BCL$ will be broken out and, finally, the rock will spall along the plane $DK$ and $LE$. Thus, it is assumed that, when rocks are destroyed by indentation, displacements of its parts are obtained at some angle $\alpha$ to one another. The value of this angle for a given rock is constant. Starting from the sum of

Fig. 5

the shear planes of the prisms $ABC$, $ACK$, $KDA$, $BCL$, and $BLE$ and the resistance of the rock to shear—$\sigma_{\mathrm{sr}}$, I. S. Pokrovskii determines the total resistance of the rock to penetration and the necessary “impact force”—$P$

\[ P = P_1 + P_2 + P_3, \]

where $P_1$, $P_2$, $P_3$ are the forces necessary for shearing the prisms along the slip faces. “If the impact force proves greater than the resistance of the rock” [9], then the blade, having destroyed the volume of rock $DKLE$, will move onto the area $KL$, Fig. 5, and may produce further destruction. For a more rational

For the utilization of impact energy, I. S. Pokrovskii considers it necessary to turn the drill between blows in such a way as to ensure double chipping, Fig. 6.

N. E. Cherkasov, K. B. Shliapin [10], and A. I. Ostroushko [8], developing and refining the basic propositions expressed by I. S. Pokrovskii, regard some of the conclusions as debatable and erroneous.

In our opinion, most authors [7, 9, 10], when considering the mechanism of destruction, in turn make a number of errors and inaccuracies:

a) The penetration of the drill into the rock and the mechanism of destruction of the latter are considered in isolation from the conditions created by the preceding blows, which have already disturbed the integrity of the surface of the blasthole bottom by numerous

Fig. 6

cracks, which considerably reduce the resistance of the rock to penetration.

b) When considering the interaction of the bit with the rock, the force acting on the bottom is taken as a constant value, independent of the process of interaction of the two bodies. In doing so, neither the considerable rate of deformation nor the characteristic mechanical properties of the colliding bodies that appear under these conditions are taken into account. The calculation formulas for determining the resistance of rocks to penetration express it in terms of force in kg and the resistance of the rock to uniaxial compression and shear in kg/cm². Since, moreover, no methods are given for determining the impact energy required to create the given force, and the value of the resistance of rocks under complex deformations differs considerably from the resistance under simple uniaxial deformations, the formulas obtained on the basis of the above theories are very difficult to use in practice for determining the required energy of a single blow and the angle of rotation of the drill between blows.

c) When considering the mechanism of destruction, the energy of the rotating drill is completely disregarded; yet, during the very brief stoppage of the drill caused by penetration of the bit, this energy may create additional forces.

At present there is no single opinion on the choice of the angle of rotation of the drill between blows. In contrast to I. S. Pokrovskii, N. E. Cherkasov and K. B. Shliapin consider it more expedient to choose the angle of rotation from the condition of one-sided chipping along the rear face of the blade, since in that case, allegedly, less force is expended on chipping and the main part of the energy goes into further penetration of the drill.

The absence of a unified point of view on the mechanism of rock destruction in percussion drilling is explained to a considerable extent by the great diversity of rocks, tool shapes, and methods of creating the force required for penetration. However, such a situation makes it extremely difficult to design and improve drilling machines and to determine the most expedient drilling regimes. On the other hand, we believe that questions of the theory of percussion drilling can at present be substantially refined on the basis of the works of Russian and Soviet scientists on the physics of the solid state, the strength of materials, and investigations of the mechanical properties of rocks. For this, it is first of all necessary:

1. To specify the conditions for the use of percussive-rotary drilling

1. To determine the nature of the principal deformations in percussive-rotary drilling and the principal mechanical properties of rocks manifested in this process.

2. To determine the relationships between the energy expended in percussive-rotary drilling and the magnitude of the deformation produced.

Application of percussive-rotary drilling

Percussive-rotary drilling is understood as the making, in rocks, of cylindrical holes—blastholes or boreholes—where the destruction of the rock is carried out by a tool in the form of a wedge, driven into the surface of the bottom hole by the energy of an impact. In the interval between two impacts the tool is turned through a certain angle. Rotation of the drill may be continuous or intermittent.

At present the following are classed as percussive-rotary drilling:

1. Drilling of blastholes and boreholes of 30–80 mm by drilling hammers (perforators) in rocks of medium and high strength, classes I–VIII according to the unified classification of rocks by drillability (quartzites, basalts, diabases, granites, etc.) [11]. In this case the energy from the drilling hammer is transmitted to the tool through a steel rod—a drill rod, 25–32 mm in diameter and, depending on the depth of the blasthole, from 0.4 to 2.5–3 m long in ordinary drilling and up to 35 m in drilling deep boreholes [1].

Drilling hammers provide a number of blows from 1000 to 2500, with the energy of a single blow 3–10 kg/m, and a number of revolutions of the drill from 100 to 300 per minute.

1. Percussive-cable drilling of deep boreholes. The essence of percussive-cable drilling is that the drilling tool—the bit together with the drilling assembly, weighing 500–1000 kg or more—falls from a certain height to the bottom of the borehole, strikes the rock, and crushes it. The drilling assembly is raised by means of a cable. The number of blows is 58–60 per minute. Between two blows the assembly is turned through a certain angle.

By the percussive-cable method, boreholes 150–250 mm in diameter are drilled in rocks of high and medium strength, from class I to XI according to the unified classification by drillability [3].

1. In exploration work, for drilling boreholes of small depth in rocks of low hardness, manual percussive-rotary drilling is sometimes used [4]. The latter takes its name from the set of tools used for this purpose—the “drilling set,” which provides both rotary and percussive drilling of the rock.

From the foregoing it is evident that percussive-rotary drilling is understood to include several different methods of drilling which, despite their common character—the penetration of a tool into rock under the action of impact—also possess a number of specific features. These features undoubtedly also affect the mechanism of destruction. In what follows we consider the mechanism of destruction only in percussive-rotary drilling of rocks of medium and high strength by drilling hammers, although some conclusions may also be extended to other methods of percussive drilling.

Mechanical properties of rocks

Rocks of high strength, in mineralogical composition, structure and texture, and mechanical characteristics, are very diverse and are, as a rule, anisotropic hard bodies. Common mechanical properties of most rocks of high strength are high density (low porosity) and considerable resistance

penetration. Most rocks and rock-forming minerals, when tested under simple types of deformation (compression, tension, bending) with “static” application of the load, deform chiefly elastically, up to brittle failure. However, the relationship between stress and deformation in this case is more complex than Hooke’s law. Under dynamic application of external forces, as some experimental data show, the linear Hooke law is obeyed for many rocks [15].

Rocks offer the greatest resistance to penetration and compression. In tension, shear, and bending, the strength of rocks is 10–20 times less than in compression. The considerable resistance of rocks to penetration, which exceeds the resistance to simple compression by 10–18 times, is explained by the volumetrically stressed state of the rock at the point of contact with the tool.

As a number of studies [15] show, under all-round compression the strength of rocks increases. Under high all-round compression—up to 4000–10000 atm—some rocks (marble, limestones) exhibit plastic properties. However, the strength of rocks as a result of plastic deformation not only does not increase, but, on the contrary, decreases. The mechanical characteristics of rocks under dynamic loads have so far been studied only very poorly.

The brittle failure of most rocks under static load, and the absence of hardening under plastic deformations that may arise in a complex, volumetrically stressed state, give grounds for assuming that the strength of most rocks of medium and high hardness at significant deformation rates (impact) differs only slightly from their strength under deformation by static loads.

Some authors believe [7] that, since the rate of propagation of deformations is a finite quantity, then with a reduction in the duration of the load action (impact), deformation, including failure, in impact drilling spreads over a comparatively small volume, which decreases as the velocity of the striking body increases. As a characteristic example they cite the observed phenomenon in which a bullet fired from a rifle makes only a small hole in glass, whereas the same bullet, thrown by a person’s hand at a much lower velocity, can shatter the entire pane. This is used to explain the low efficiency of percussive-rotary drilling and to cast doubt on the advisability of increasing the penetration rate of percussion drilling machines.

However, these conclusions are hardly correct, since in impact drilling a relatively small volume of rock, located in direct contact with the cutting edge of the drill bit, is subjected to destruction; therefore an increase in the deformation rate under impact, in comparison with static penetration of the tool, is unlikely to cause a decrease in the volume destroyed and, on the contrary, can only reduce losses to elastic deformation of the remaining part of the rock.

Thus:

1) The principal deformation in impact drilling is the penetration of the tool into the rock from the surface of the bottom of the blasthole.

2) The resistance of the rock to penetration is the greatest in comparison with its resistance to other types of deformation.

3) The brittle failure of most rocks even at insignificant deformation rates, the considerable rate of deformation under impact, and the relatively small volume of rock destroyed per blow in impact drilling indicate that the velocity of collision between the tool and the rock in impact drilling should not exert any significant influence on the strength of the rock and the drilling rate.

4) The elastic deformation of most rocks of medium and high strength, up to failure, and the small deviation from Hooke’s law under dynamic loads make it possible to use certain conclusions of the theory of elasticity [2; 5] to reveal the general regularities of rock deformation in percussive-rotary drilling. Of course, in this way it is possible to reveal only qualitative regularities characteristic of rock as a solid body. Quantitative interrelations, in view of the great variety of rocks and their inhomogeneity and anisotropy, can be established only on the basis of special experiments.

Application of the Principles of the Theory of Elasticity to Consideration of the Mechanism of Failure During Penetration

The distribution of stresses and the limiting state during the impact indentation of one elastic body into the surface of another is described by the theory developed by Hertz, A. N. Dinnik, and N. M. Belyaev [5; 2]. It is assumed here that “the elastic state of the bodies near the impact surface throughout the impact is very close to the state of equilibrium that would arise in both bodies under their slow compression” [5].

According to this theory, the most probable places for the onset of failure during indentation are: on the pressure surface, its contour; and, in the volume of the body, a point situated on the axis of symmetry at some distance from the pressure surface, \(Z = (0.5—1.0)a\), where \(a\) is one-half the width of the strip of contact between the surfaces. The maximum shear stresses along the axis of symmetry, depending on the shape of the contacting surfaces, exceed the maximum shear stresses on the surface by 1.5–2 times. These conclusions have been confirmed by numerous experiments and by the practice of mechanical engineering and construction.

On the basis of these propositions L. A. Schreiner describes the mechanism of failure of an elastic-brittle body during the indentation into its surface of a cylinder with a flat base [15], Fig. 7.

Fig. 7

At the beginning of indentation, the pressure along the contour is higher than over the rest of the contact surface; therefore the shear stresses arising in the depth of the body along the pressure contour reach limiting values earlier than on the axis of symmetry. In brittle bodies this leads to shearing along the pressure contour, as a result of which a wedge-shaped depression is formed, while in plastic bodies plastic deformation begins. It is considered that the moment of shearing, or the beginning of plastic flow along the contour, does not affect the general course of deformation in the volume of the body under the base of the cylinder. Beginning from this moment, the pressure on the contact plane for brittle bodies abruptly, and for plastic bodies continuously, becomes uniformly distributed. Uniform pressure can also be attained from the very beginning of indentation,

if the planes at the base of the cylinder are given a certain curvature. “The volume of the body, bounded at the surface by the base of the cylinder and in the depth of the body by a hemisphere whose great circle is the base of the cylinder, is under conditions of stronger all-around compression. Therefore, when at the pole of the hemisphere, at point ‘c,’ Fig. 7, the tangential stresses reach the limiting value in planes located at an angle of 45° to the axis of symmetry, they cannot exceed the body’s resistance to shear along these planes up to the surface of the body, since these planes pass through a region of very strong all-around compression, where the body’s resistance to shear is considerably higher than at point ‘c.’ With a further increase of the load, the tangential stresses reach limiting values also in other planes, located to the axis of symmetry at angles greater than 45°, until, along a certain conical surface, they emerge to the surface outside the plane of contact. At this moment the stressed state of all-around compression changes into uniaxial compression; the entire volume of the body bounded by the conical surface is destroyed, its central part being transformed into a fine powder” [15].

In rocks, owing to anisotropy, the formation of shear planes cannot proceed equally in all directions, and failure must occur along the planes with the minimum resistance to shear. In the destruction of rocks by tools of various shapes, with different angles of sharpening, the process of destruction is more complex and peculiar; nevertheless, there is every reason to assume that the general character of the stressed state and of failure will be similar to that in the indentation of a cylinder.

On the basis of what has been set forth above, the following conclusions may be drawn:

1. Since N. S. Uspenskii’s theory assumes penetration of the drill bit blade only through crushing without brittle failure of the rock, and then chipping of part of the rock only through the wedging action of the drill-bit wedge, it is applicable for explaining the process of penetration and the mechanism of failure only in the drilling of rocks possessing considerable plasticity and porosity (soft and viscous rocks).

2. A. I. Medvedko, considering the process of tool penetration only through porosity and fissuring, bases his conclusions chiefly on the picture of failure given by N. S. Uspenskii. Therefore A. I. Medvedko’s works are a further development of Uspenskii’s theory for rocks of low strength with great fissuring and porosity. Undoubtedly, porosity and fissuring influence the process of penetration of a drill into rocks of medium and high strength, but they are not the principal factor.

3. The works of I. S. Pokrovskii [9], I. E. Cherkasov, K. B. Shlyapin [14], and I. A. Ostroushko [8] give a more correct picture of the process of destruction in percussion drilling of rocks of medium and high strength. The latter can be refined on the basis of the considerations set forth above and of special experimental investigations.

Experimental Investigations

To verify the basic propositions of the existing theories on the mechanism of rock destruction in percussive-rotary drilling, and the considerations expressed by us, a series of experiments was carried out. Experiments on the impact penetration of a drill bit into rock and on determining the influence of the rate of deformation on the process of destruction were conducted on a drop tester built for this purpose. The considerable height of the drop tester (18 m) ensured the obtaining of different impact energies both in magnitude and in the ratio of mass to velocity. “Static” penetration of the drill bit into rock was carried out on a Gagarin press; in this process, ...

deformation–force diagram. The influence of the drill rotation speed, as well as the influence of the angle of rotation of the drill between blows, on the mechanism of rock destruction and the speed of percussive-rotary drilling were investigated on a test stand for electropneumatic drill hammers in the laboratory of the Tomsk Polytechnic Institute. Drilling was carried out in marble, granite, and diabase. As the tool, drills with replaceable single-chisel bits were used, armed with VK-15 hard-alloy plates, 40–45 mm in diameter.

As can be seen from the diagram in Fig. 8, during the “static” indentation of the bit into marble, penetration at first occurs under an almost constant load (section 1–2); during this time, compaction and extrusion of the rock, half-destroyed from the surface, occur from under the bit blade. In section 2–3, further compaction of the rock surface and elastic deformation occur, as a result of which at point 3 there is abrupt

Fig. 8

destruction of the rock under the drill blade. At the same time, chipping of the rock is observed on the specimen, symmetrical to the blade of the drill bit, and on the load indicator—a sharp decrease in pressure (section 3–4). In section 4–5, extrusion and compaction of the rock destroyed under the blade again occur, and then elastic deformation again (section 5–6), up to a new chip or splitting of the specimen. The sections of extrusion and compaction of the destroyed rock were the smaller, the smaller was the sharpening angle of the blade of the drill bit.

When the bit is pressed into diabase, Fig. 9, the elastic deformation of the rock under the bit blade and brittle chipping occurred more clearly than during the destruction of marble. In this case, chipping was accompanied by a sharp sound and by rebound of the destroyed rock particles from under the bit. Because the destroyed rock was rapidly removed from under the blade, on the diagram, Fig. 9, there is no longer any section associated with compaction of the destroyed rock, which was characteristic during the destruction of marble; instead, immediately after each destruction (sections 2–3, 4–5, 6–7, 8–9), elastic deformation again follows (sections 3–4, 5–6, 7–8, 9–10), which leads to new destruction. The magnitude of the abrupt penetration, as a rule, was the greater, the larger was the plane of the preceding contact of the bit blade with the rock. The unloading curve 6–7 shows-

shows that the deformation of the rock preceding chipping was mainly elastic, with some residual deformation. A similar dependence was obtained when the bit blade was pressed into granite.

Fig. 9

Thus, the results of the experiments show that the destruction of rocks of medium and high strength during static penetration of the tool.

occurs mainly as a result of elastic deformation, in a stepwise manner. The character of the destruction itself and of the subsequent removal of the destroyed rock from beneath the drill bit blade depends on the properties of the rock and on the angle of sharpening of the drill bit blade.

Fig. 10

Destruction during the impact penetration of a symmetrical bit, just as during static penetration, has the form of symmetrical chipping from the surface of the hole relative to the bit blade and crushing of the part of the rock located beneath the blade, Figs. 10 and 11 (the images in Figs. 10, 12, and 13 are given as developments along the circumference of radius \(R\), Fig. 11). The chipped-off part of the rock

Fig. 11

Fig. 12

is finely crushed, and part of the rock located near the blade of the bit is turned into a fine powder, which, in the destruction of harder and drier rocks—granite and diabase—is rapidly forced out from under the blade, whereas in the destruction of softer and wetter—“viscous”—rocks, for example marble, it does not have time to be forced out from under the blade of the bit and, as a result, is strongly compacted, creating a considerable area

Fig. 13

of contact between the bit and the rock, Fig. 12. In the presence of cracks and depressions close to the blade of the bit from previous blows, the rock is destroyed asymmetrically, with increased chipping toward the side of lesser resistance. In this case, slipping of the bit is observed along the stronger side of the bottom hole toward the side of lesser resistance, which leads to a certain twisting of the rod.

Fig. 14

The minimum volumetric work of destruction (the work expended on destroying a unit volume), at one and the same impact energy but at a different angle of rotation of the drill between blows, corresponds to bilateral chipping, i.e., to the case when two blows form depressions with symmetrical chipping, and the third is delivered to the protrusion that has formed, Fig. 13. With an increase in impact energy, the volumetric work of destruction changes periodically, Fig. 14.

The magnitude of the amplitude and period of oscillation of the curves of the volumetric work of destruction depends on the properties of the rock, the size, shape, and degree of dulling of the tool. As can be seen from Fig. 14, the change in the values of the volumetric work in destruction with a dulled bit—1—is considerably greater than in destruction with a sharply ground one—2. For certain values of impact energy, the volumetric work of rock destruction when drilling with a dulled bit is less than when destruction is carried out with a sharp one. The mean value of the volumetric work of destruction—3 when drilling with a sharply ground bit for the given rock was, in most cases, almost constant as the impact energy increased from 2.5 to 20–25 kgm.

The values of the maximum and minimum volumetric works of destruction and the corresponding impact energies in the destruction of rocks by a single-cutter bit, 43 mm in diameter, are given in the table.

Table

The periodic change in the volumetric work of destruction as a function of impact energy can be explained by the fact that the process of destruction during dynamic penetration of a tool into brittle rocks proceeds in the same discontinuous manner as under static penetration. However, in contrast to static penetration, the region over which deformations propagate, in the form of residual cracks, is smaller under dynamic action than under static action. The latter may be explained by the significant influence of the inertial forces of the destroyed rock particles, which leads to considerably smaller deformation regions and to localization of destruction at the surface in the immediate vicinity of the blade of the drill bit.

The size of the rock-spalling zone from a single blow was estimated by the spalling sector \(\beta\), Fig. 11. It was found that, with an increase in impact energy, the spalling zone increases. The increase in the zone occurs discontinuously and corresponds to the regions of minimum work of destruction.

At one and the same impact energy, but with a change in the striker velocity before impact from 4.4 to 18.8 m/sec, i.e., by more than a factor of 4, no increase in the volumetric work of destruction was found.

After impact, the striker, as a rule, rebounded from the drill, and the drill from the bottom hole. The magnitude of the rebound of the striker from the drill and of the drill from the bottom hole was not a constant quantity for the given rock, material, and masses of the colliding parts, and depended mainly on the position of the drill before impact, the magnitude of the initial contact of the bit with the bottom hole, and the character of rock destruction. Thus, for example, for granite, when a striker weighing \(Q_1 = 970\) g was dropped from a height \(H = 4\) meters onto a drill of length \(l = 370\) mm and weight \(Q_2 = 1370\) g, the magnitude of the striker rebound varied from \(h = 2\) to 220 mm (25, 50, 62, 2, 8, 15,

105, 220, 36, 10, 218, 12, 30, 20, 30, 10, 20, 28, 12, 20), which corresponded to a change in the coefficient of restitution \(K = \sqrt{\dfrac{h}{H}}\) from 0.0022 to 0.234, i.e., by more than a factor of ten.

The greatest rebound corresponded to a larger area of the initial contact of the bit crown with the rock, which resulted from a blow on the drill whose crown rested in the depression from the preceding blow, Figs. 10, 12. A smaller rebound corresponded to greater destruction of the rock and occurred when the crown was positioned on a protrusion between two depressions from previous blows, Fig. 13. A greater volumetric work of destruction was also accompanied by a greater relative rebound of striker 4, Fig. 14. Thus, the magnitude of the coefficient of restitution may serve, to a certain extent, as an estimate of the appropriateness of a given impact energy for a given condition of rock destruction. The magnitude of the striker rebound decreased markedly with increasing drill length.

The magnitude of the drill rebound from the rock, as a function of impact energy, varied periodically, with maxima and minima of the values corresponding also to the maximum and minimum work of destruction.

In conducting experiments on the impact penetration of the drill into rock, when the drill before impact had different rotational speeds (equal to the rotational speed of the drill in modern drilling hammers), it was found that the destruction in this case also has the character of bilateral chipping, with some increase in the destroyed volume in front of the leading face of the blade of the drill bit crown.

In view of the fact that the minimum work of destruction at one and the same impact energy, as a rule, corresponded to the case when two impacts produced bilateral chipping, and the third destroyed the remaining protrusion on the surface of the blast hole, Fig. 13, it was assumed that the most expedient angle of rotation of the drill between impacts should be selected from the condition of bilateral shearing, taking into account the dependence of the size of the sector of chipped rock on the impact energy. Thus, for example, at an impact energy of 5–6 kgm, in drilling granite with a hardness on Prof. Protodyakonov’s scale of \(f = 14\text{–}18\), the sector of chipped rock per impact, with a blast-hole diameter of 40–45 mm, was 30–40°. On this basis, it may be assumed that the most expedient angle of drill rotation in drilling with a single-chisel bit crown should be 32.7 or 40°.

Indeed, when the drill is turned between impacts by 40°, impacts 1–5, Fig. 15, with bilateral chipping of the rock, deepen the crown into the face surface. Thereafter each subsequent impact—from the 6th to the 9th—is delivered in the zones between two depressions from previous impacts, which ensures effective chipping of the remaining part of the rock and reduces the volumetric work of destruction during drilling. Similar conclusions may also be drawn for other relationships between impact energy, sector of chipped rock, and angle of drill rotation. From this point of view, when drilling with a drill having a single-chisel bit crown, the most expedient angles of drill rotation between impacts may be: 31.5°; 40°; 32.7°; 27.8°; 24°; 21.2°; 19°; 17.2°; 15.65°, etc.

To verify these considerations, experiments were carried out on a specially designed drilling rig, the design of which made it possible, at one and the same impact energy and feed force, to vary the angle of drill rotation between impacts. As can be seen from Fig. 16, the magnitude of the angle of drill rotation between impacts has a substantial influence on the drilling rate. Thus, for example, at one and the same impact energy \(A = 6\) kgm and number of blows per minute \(n = 1200\), the drilling rate of granite with \(f = 14 \div 18\) varied from 175 to 90 mm, i.e., almost by a factor of 2, Fig. 16. The greatest fluctuations in the rate

drilling and, at the same time, its greatest absolute values were observed when the drill rotation angle between blows varied from 17 to 42°—when drilling rocks of high and medium hardness (granite and diabase)—and up to 54–56° when drilling rock of low hardness (marble). The maximum drilling rates, as a rule, corresponded to drill rotation angles that ensured effective destruction of the rock under two-sided chipping, i.e., to angles \(\alpha = 24^\circ;\ 27.8^\circ;\ 32.7^\circ;\ 40^\circ\) or close to them. The value of the rotation angle increased with an increase in impact energy and a decrease in rock hardness. Increasing the drill rotation angle beyond 43–56° in all cases led to a reduction in the drilling rate.

Fig. 15

Fig. 16

In carrying out these experiments it was also established that:

a) The resistance to rotation of the drill during percussive-rotary drilling with continuous rotation of the drill is 1.5–2 times less than when rotating the drill with the same feed force, but without impacts.

b) When the rotational speed of the drill is changed at one and the same feed force, number and energy of impacts, the moment of resistance to rotation of the drill remains almost constant, Fig. 16, increasing or decreasing slightly at certain ratios of the number of impacts and drill revolutions.

c) The feed force of the machine does not have a direct influence on the process of rock destruction in percussive-rotary drilling, but only ensures operation of the drilling hammer in one regime or another. The feed force that is optimal from the standpoint of drilling speed corresponds to the case in which, during drilling, a certain separation of the body of the drilling hammer from the limiting collar of the drill is ensured. As a consequence of this, and also as a result of the always present rebound of the drill from the rock, the blow of the striker on the drill occurs when there is a certain gap—1–5 mm—between the drill blade and the rock. In this case, after collision the striker and the drill in effect exchange kinetic energy. The striker stops, while the drill moves toward the rock and destroys it. When drilling horizontal blastholes, as a rule, rebound of the striker after impact on the drill is not observed.

Similar relationships were obtained by us also in the study of two pneumatic drilling hammers—PR-30k and PM-508.

Conclusions

On the basis of the physics of solids [6; 15], materials on the mechanical properties and destruction of rocks [12; 13; 7; 9; 8; 14], the fundamental propositions of the theory of elasticity [2; 5], and the experiments we have conducted, we arrived at the following conclusions concerning the mechanism of rock destruction in percussive-rotary drilling.

1. In percussive-rotary drilling of blastholes in rocks of medium and high hardness, before each impact the bottom-hole area of the blasthole is an uneven surface whose integrity has been disrupted by numerous cracks formed as a result of preceding impacts.

Before impact, owing to the preceding rebound of the rod, the drill bit is not in contact with the bottom hole, but is at some distance from it—1–5 mm, depending on the feed force. After the blow by the striker on the rod, a certain energy is imparted to the latter, and it begins to move toward the bottom hole with a certain velocity. Part of the energy is thereby lost in overcoming resistance to the motion of the drill in the blasthole. When the bit meets the bottom hole of the blasthole, the blade of the bit deforms the surface of the bottom hole, displacing and compacting the rock particles. This is aided by the preexisting fracturing and porosity of the rock.

The magnitude of the force between the bit and the rock increases gradually as the rock is compacted and also as the area of contact between the bit and the rock increases. The motion of the rod and bit is arrested when all the energy of motion received from the striker is transformed into the energy of destruction and the energy of elastic compression of the rock and of the drill rod. In this case the following cases are possible:

a) The energy received by the drill is greater than is necessary to overcome harmful resistances and to compact the rock at the place of contact. Then the surplus energy goes into elastic deformations of the rock and the drill. The drill bit, coming into contact with the rock over the surface \(AOB\), Fig. 17, creates in the volume of the rock a nonuniform volumetric stress state. The nonuniformity of the volumetric stress state is intensified at the expense of resista-

...resistance to rotation of the rod that arose as a result of penetration. The maximum shear stresses in the deforming volume of rock first arise at a certain distance from the cutting edge of the bit—$h$. This distance depends on the shape of the drill blade and on the area of contact between the drill and the rock after compaction. As the load increases, the shear stresses reach the limiting value for the given material at other points of the volume as well, until, along certain inclined planes, they emerge abruptly

Fig. 17

onto the surface outside the contact plane. As a result of this, the part of the volume $AKD$ and $BUE$ shears off, while the other part, $ABUCK$, is turned into a fine powder, which in harder and more brittle rocks—granite, diabase, etc.—is easily squeezed out from under the blade of the bit.

Fig. 18

If not all of the kinetic energy has been spent on the first fracture and the translational motion of the drill continues, then the blade of the bit begins to deform the newly formed surface, Fig. 18. However, for secondary shearing along the plane $C_1D_1$ and $C_1E_1$, and likewise for each subsequent shearing, an already greater force is required. If the energy of the drill remaining after the first chip is insufficient to create such forces, then the part of the energy expended on the elastic deformation of the drill and the rock is restored and the drill rebounds from the bottom.

In the destruction of softer and more viscous rocks, for example, marble, the particles of the destroyed volume $ABUCK$, owing to considerable friction between the particles, the bit, and the bottom, do not have time to be squeezed out from under the blade of the bit. During secondary penetration the bit compacts the already destruc-

…valuable part of the rock, creating a considerable contact plane, Fig. 19, which, as a rule, precludes the full use of the energy of the bit for secondary spalling. This can to some extent explain why, in percussive drilling of softer rocks—for example, marble—the drilling rate differs little from the drilling rate in stronger, but brittle, rocks.

b) With a large quantity of rock particles not removed from the hole and with a small impact energy, a considerable part of the energy is spent in overcoming the resistance to the motion of the bit and in compacting the destroyed rock. At the end of the motion and of the elastic deformation of the bit and the rock, the crown

Fig. 19

comes into contact with the bottom along a certain surface; however, the force arising in this case does not create a limiting stress in the sliding planes. Destruction in this case has the character of surface crushing; the drilling rate is insignificant.

1. From the standpoint of the mechanism of rock destruction, the drilling rate of holes in rocks of medium and high strength can be significantly increased by increasing the energy of a single blow. Thus, for example, for holes up to 45 mm in diameter, it is possible to increase the impact energy to 25–30 kgm, i.e., almost fivefold in comparison with the impact energy of most modern drilling hammers. In this case, an increase in impact energy is possible both by increasing the mass and by increasing the velocity of the striking body, at least up to 18–20 m/sec as against 6–8 m/sec in modern drilling hammers.

For a final solution of the question of choosing the impact energy for one machine or another, it is necessary to develop the problem of the transfer of impact energy from the machine striker to the tool and of the strength of the colliding parts.

1. It is not advisable to use an impact energy of less than 2.5 kgm in drilling hammers, since in this case the most significant part of the energy of each blow is expended on friction and on secondary comminution of the already destroyed rock. To reduce energy losses to friction, it is advisable to use a large part of the energy of the bit in the first jumps of destruction during penetration. For this purpose, when increasing the impact energy it is necessary to increase the area of the initial contact between the crown and the rock. As a result, the increase in the impact energy of the drilling hammer should reduce the influence on the drilling rate of the drill cuttings located in the hole and should significantly increase the operating time of the drill crown between regrindings.

1. The angle of rotation of the drill between two blows is most expediently chosen from the condition of two-sided chipping of the rock relative to the blades of the drill bit. As the optimal angles of rotation for drilling various rocks with a single-chisel drill bit of diameter 40–45 mm, the angles of rotation \(\alpha = 24^\circ;\ 27.8^\circ;\ 32.7^\circ;\ 40^\circ\) may be recommended.

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