Prof. M. I. Evdokimov-Rokotovsky

Rock Popping

Tomsk
1928

Prof. M. I. Evdokimov-Rokotovsky.

Rock Popping

Introduction.

In underground workings an interesting phenomenon is sometimes observed: unexpectedly and quite spontaneously, with rumbling and cracking, layers of rock rebound in the form of slabs, sometimes of larger, sometimes of smaller size; in the shape of their lenticular cross-section they are thin and crumbling at the edges, while in the middle they consist of a thick layer of undestroyed rock.

This phenomenon has long been known and is called: in Russian, “rock popping”; in German, “Bergschläge”; in French, “Bendons”; in English, “Bumps.”

I became interested in this phenomenon. Having acquainted myself with both the Russian and the foreign literature on this question, I became convinced that rock popping is a very complex phenomenon, unfortunately still little studied. A scientific explanation and a disclosure of the essence of popping have not yet been given. On the contrary, the discrepancy among the views of various authors on the nature of this phenomenon indicates that further collection of data from the practice of mine workings and tunnel construction is still required, as well as scientific study of the question of popping.

Rock popping is of both theoretical and practical significance. As a specialist in tunnels, I have more than once had to deal, both in practice and in scientific work, with the question of rock popping. No less interest than I have should also be felt by my colleagues in underground works—mine and mining engineers. The aim of the present article is, above all, to interest our engineers in “rock popping,” to acquaint them as fully as possible with this phenomenon, and to give them the opportunity, in their work, to notice and study popping. By this the aim of my present work will be achieved. However, at the end of the article I shall also permit myself personally to express my own considerations (hypothesis) concerning the causes and circumstances of the occurrence of rock popping.

Data from Practice.

First of all, it is necessary, of course, to begin with data from the practice of underground works. Such are mine workings and tunnels. The practice of mine workings provides rather rich material on the question of rock popping.

The phenomenon of popping was observed with complete distinctness in one of the prospecting drifts of the Novorossiisk Mine near the station “Vechernii Kut” of the Ekaterininskaia, now Dnepropetrovsk, railway, in the Krivoi Rog iron-ore district. This drift, 228 meters long, was begun in 1902. The last works in it date to 1904. The driving was carried out across the strike of the beds of typical ferruginous quartzite (jaspilite) along the contact

its diabase dike, cutting the quartzites in a direction approximately perpendicular to the strike of the beds. The dip angle of the ferruginous quartzite is 60°–65° to the W. In places the beds are bent into intense small folds of extremely complex form, the axes of which (the hinges of the bends) dip northward, more precisely toward the NW. The dike, in turn, dips steeply to the N. at an angle of 80°–87°. The rock of the diabase dike, in places where the quartzite is mineralized and in places of ore deposits, is usually kaolinized to a greater or lesser degree. Where the ferruginous quartzites have been altered neither by processes of mineralization nor by weathering, the diabases are likewise more or less fresh; the ferruginous quartzites are broken by a series of cracks, more or less close to vertical and more or less parallel to the diabase dike. The distance between adjacent cracks near the dike ranges from a few cm to 10 and even to 30 cm and more. The procedure in driving the crosscut, owing to the considerably lower density of the diabase in comparison with the quartzites, was as follows. The excavation was carried out through the diabase, and only the contact plane of the quartzite was exposed. Then shot holes were drilled in the quartzites, and by blasting a more or less fresh jaspilite mass was exposed. Sometimes the face in the diabase advanced ahead of the unblasted jaspilite wall by 10–15 meters, i.e., for a period of up to one week. In such cases, in places where the jaspilites showed no signs of alteration and, equally, where the diabases were distinguished by freshness and showed no traces of kaolinization, rock popping was observed. From the exposed, but as yet unblasted, contact plane of the jaspilite, slabs of jaspilite suddenly and spontaneously broke away. With rumbling and cracking they were hurled with force toward the opposite wall of the crosscut over a distance of not less than a meter, and in most cases across the full width of the crosscut, and, striking the wall, broke up and fell down. The phenomenon was always accompanied by cracking and rumbling, at first greatly frightening the workers, who, before becoming accustomed to it, hurriedly left the work site.

The detached slabs had a thickness of 3 to 6 cm. The length of the slabs varied, on average up to 70 cm and even up to 1 meter, and their width up to 40 and up to 70 cm. There were cases in which the slabs reached still larger dimensions. The height of the crosscut was 1.7 meters, and the width 1.25 meters. There were no fewer than 10 cases of popping which the technical supervisor and workers had occasion to observe. Inspection of the crosscuts confirms that numerous cases of rock popping occurred at other times as well. It was noted that in almost all cases the jaspilite possessed fine platy structure. In any event, the phenomenon of popping can be connected with an extremely dense, strong, fresh, but platy jaspilite.

Another case of rock popping was observed in 1913–1916 at the “Dubovaya Balka” iron mine. The popping occurred in quartzites at a depth of about 220 meters. The rock exfoliated with rumbling and cracking in pieces of lenticular cross-section. Passing through the working, one could hear a faint crackling: the rock was crumbling off in small layers. From time to time there was a crack equal in strength to a rifle shot, and then slabs up to 40 and even up to 70 cm long would spring off. Rock popping was not always observed immediately after excavation, but after a time, sometimes up to two weeks later. In all, up to 10 cases of popping were observed.

During the sinking of the “Elpedifor” shaft of Paramonov, now the “Artem” shaft of the Vlasovka Ore Administration of Donugol, rock popping was likewise observed for some time. Unfortunately, I was unable to obtain detailed data concerning this place of popping. It is known only that up to 6 cases of popping of hard rock were observed. The phenomenon was each time accompanied by a characteristic crack like a gunshot. The popping extended along the side walls of the crosscut and immediately after the excavation. After 5–8 days, popping was no longer observed at all.

L. A. Yachevsky*) reports the following data from his practice in the Dombrovo hard-coal basin. The “Kazimir” mine reaches, by its workings, a depth of 320 m. The “Reden” seam has a thickness of 14–16 m, with a dip of up to 23°. When a still untouched part of the seam is penetrated by new workings, it is observed that the coal rather readily breaks off in the face in the form of more or less substantial slabs. At the same time a crackling is heard, and often the sound effects reach the force of rifle shots. Work in the face is rather dangerous. If, in the extraction pillar of a thick coal seam, only a single horizon of it is taken out, then, when the subsequent horizons of that same pillar are extracted, the phenomenon of shooting is no longer observed. The coal in the faces of the new pillar already possesses other physical properties than the coal in the parts of the seam remaining after extraction of the first horizon. Thus, during the very winning work, a change occurs in the physical properties of the coal. This change of properties is expressed in the fact that the coal in the still untouched pillar does not yet clearly reveal stratification; but after one horizon has been extracted, the stratification appears distinctly in the remaining part of the coal. At the same time, it is remarkable that whereas the coal in the fresh part of the seam readily lends itself to extraction, in the parts worked subsequently the coal is won with much greater difficulty and requires the expenditure of twice as much effort. The foregoing is conveniently illustrated by a drawing (Fig. 1). ABCD is part of a thick hard-coal seam. The seam is worked in layers parallel to its bedding. When mining is carried out in the seam along the drift EFGC, the work is accompanied by sharp shooting of the coal; but when the part of the seam corresponding to the parallel drift has been worked out, then, when driving the drift HIKE, the phenomenon of shooting is no longer present.

Fig. 1. Scheme of hard-coal mining in the Dombrovo basin. During extraction of EFGC, strong “shooting” of the country rock is observed. During extraction of the seam HIKE, there is no longer any “shooting.”

An examination of faces in which shooting is observed and in which this phenomenon is not noticed showed the following: on approaching the face in the fresh part of the coal seam, dry, sharp crackling sounds began to reach my hearing, says L. A. Yachevsky. At the face itself the crackling was a continuous phenomenon. From the face, more or less substantial fragments of coal broke away. At the moment when I was standing at the face, there came a strong shot, which involuntarily made me jump back. The workers employed at the face explained to me that this shot, equal in sound intensity to a rifle shot, was still very weak and that shots can be considerably louder. Examination of the face showed that in the large mass of coal the stratification is noticeable without difficulty, and that it is due to an alternation of thin, irregular, rapidly wedging interlayers of a shiny variety of coal with layers of dull coal. The slabs and even small fragments of coal that had fallen away from the face possessed a peculiar shape. They were bounded by irregular curved surfaces and could be called irregular lentils—lenses. The coal of the face is broken up by more or less vertical cracks, but these cracks influence only to a slight degree the lens-shaped form of the slabs of coal. Likewise, the stratification of the coal does not influence the form of the slabs of coal that fly off. Schematically in the face—

) Yachevsky, L. A., “On the Phenomenon of Shooting in the Hard-Coal Mines of the Dombrovo Basin,” Mining Journal*, 1905, XII, pp. 362–368.

Fig. 2. Diagram of the order in which slabs peel off during “popping” of rock.

The general order in which the coal slabs peel off is shown in Fig. 2, which represents a section along the axis of the drift. The curved lines correspond to the shape of the slabs peeling away from the face. An inspection of the face in the part of the seam under conditions of NIKE showed that work at the face is not accompanied by sound phenomena and that the coal does not spontaneously peel away from the face. On the contrary, in such a face the coal is worked with difficulty, not as in a face of the first type, where a fairly light blow of a hammer was enough for the coal to begin peeling off in considerable quantity.

Fr. Bernhardi provides exceedingly interesting material on the basis of his personal observations in the mines of Upper Silesia. In working thick coal seams at depths not exceeding 100 m, drifts with double track stood firmly without any supports. At depths of 150 m, a system of working that was quite suitable at lesser depths proved dangerous and led to disasters. At greater depths, especially at a depth of 240 m, a peculiar stress began to appear in the coal. In newly driven drifts the coal begins, as it were, to increase in volume, and the coal mass is forced into the drift. No support can withstand the pressure of the coal, but if the coal mass is removed from a drift that has been blocked as if by squeezed-out fallen coal, then such a drift becomes stable and may be left without support. This observation shows that the process of driving the drift is the impulse that causes the release of internal stresses in the coal mass, and as soon as this potential energy has been expended, the coal seam comes into a state of equilibrium and the degree of stability of the working depends exclusively on the general properties of the hard coal as a rock.

No less interesting data were obtained from observations in the Mysore gold mines in India, which work gold-bearing quartz. Here the popping of the rock was at one time observed with extraordinary intensity and with indescribable force. It is remarkable that if, on both sides of the place where the popping was taking place, headings were driven into the mass, the popping ceased. This could be interpreted in the sense that there exist certain particularly hard-to-detect directions of the forces of rock pressure.

In the Comstock mine in California, in hornblende andesites, the phenomenon of popping was observed with particular intensity at a depth of about 220–240 meters.

In some mines near Lake Superior in North America, developed in melaphyre and diabase, the phenomenon of popping is observed at a depth of 1450 m; moreover, all data indicate that where the popping of the rock is observed, the vertical rock pressure is very slight.

In the Belgian quarries of Quenast, and in the Dinant marble quarries, situated on the open surface at sites of ancient workings, popping of the rock was repeatedly observed with all the features characteristic of this phenomenon: crackling, shots, and lens-shaped slabs of rock that had sprung off.

One could cite several dozen more examples from the practice of mining operations; however, most of them, unfortunately, have been examined superficially and one-sidedly. Many of the circumstances under which the popping occurred were not noted and remain unknown.

No less interesting material on the question of rock popping is provided by tunnels. Lying deep within a mountain mass and possessing large dimensions in comparison with mine workings, tunnels, for a whole series of other preceding circumstances, sometimes give even more vivid examples of rock popping than mines.

In the St. Gotthard tunnel in Switzerland, 14,892 m long, in 1872–82, during its construction, popping was observed in talcose granite—protogine. Later, popping, and quite strong popping at that, was also observed in gneisses and in granite. The phenomenon, once it appeared, usually lasted a long time, for several weeks, until complete cracking and spalling of the rock to a certain depth occurred. Popping was observed in the floor of the adit, but particularly intense popping occurred from the side walls, where in most cases it also began. The lentil-shaped pieces of rock that sprang off with a rumble and crackling were thin at the edges, like a knife blade, crumbling, and with sound, solid rock in the middle. It is curious that if one took such a detached and rebounding slab and placed it against the spot from which it had just sprung off, it turned out that it did not fit into its socket.

Cases of rock popping were also observed in many other tunnels of the St. Gotthard railway.

In the Leggistein tunnel, 1095 m long, and in the Pfaffensprung tunnel, 1487 m long, situated near the station of Wassen on the famous Wassen loops on the line of the St. Gotthard railway, the phenomenon of rock popping was observed in very dense, compact, crack-free, tough gneiss-granites, in the pilot adit immediately after the rock excavation, at a depth of about 400 m. The rock pressure here was insignificant, and a light type of tunnel lining was used. The phenomenon of popping was observed here independently of the season and of temperature.

In the Wattingen tunnel, 1090 m long, driven in gneiss-granite and situated on the same loops near Wassen as the two preceding tunnels, popping was observed not immediately after the beginning of rock excavation in the pilot adit, but two to three months after the tunnel had been fully excavated to its complete profile. Rock pressure was not observed here at all; the rock was fresh, unweathered, and bore no traces of metamorphism, as a result of which the first type of tunnel construction was applied; in other words, in view of the strength of the rock, the tunnel was left without lining. However, because of rock popping that began two to three months later, the entire tunnel had to be lined in order to protect the lives of the workers.

Rock popping is not always expressed in slabs of lentil-shaped section. Tecklenburg¹ reports rock popping, but without a lentil-shaped section of the pieces that sprang off, in the geological description of the Kräberg tunnel on the Odenwald railway line. This tunnel, 3100 m long, is situated in variegated sandstones.

Rock popping was also observed during the construction of the Karawanken tunnel in limestone, the Wochein tunnel in dolomite, and the Tauern tunnel in gneiss-granite. Popping in these tunnels usually began in the side parts already in the pilot gallery. In the Tauern tunnel, however, popping sometimes also occurred from the floor and even from the roof of the pilot adit. In Fig. 3 the place of popping in the pilot gallery of the Tauern tunnel is visible. In this tunnel popping was especially strong at a distance of between 3–4 km from the northern portal, at a depth of about 1200 m below the surface. As the tunnels were driven, rock popping took in larger surfaces, but became less intense. With respect to the intensity of pop—

...one must suppose that the reason for the decrease in intensity was the comparatively long interval of time that had elapsed from the beginning of the excavation of the pilot gallery to the full excavation of the tunnel. Photograph 4 shows the appearance of the spalling slabs at the moment of their separation from the rock. The rock pressure in the three tunnels mentioned above, in those places where rock popping was observed, was insignificant. This is evident from the fact that the lining of these tunnels, over the lengths corresponding to the places of popping, was made of the light type.

From tunnel practice it has been observed that rock popping occurs only occasionally in the presence of high vertical rock pressure. Usually these two phenomena, i.e., popping and vertical rock pressure, as it were mutually exclude one another.

During the construction of the Simplon Tunnel, 19.8 km long, cases of rock popping were especially frequent over a great length of the tunnel. The popping occurred either soon after blasting operations during the driving of the pilot gallery, or after fairly long intervals of time—6 and even 8 months after blasting—when all work of any kind on excavating the tunnel to full section had already ceased.

An examination of the slabs of Simplon rock that had flown off leads to the conviction that the mountain mass, in the places where the rock popped, was experiencing very high internal stresses. The stresses were sometimes so great that deformation was observed even on small pieces. At the same time, however, it was observed that along the lengths in question the mountain mass produced no vertical or lateral rock pressure whatever on the tunnel lining. The tunnel could here have been left entirely without lining. Such lining was installed in these places solely to protect workers and trains from pieces of rock spalling from the surface of the excavation.

By careful inspection of the places (photographs 5 and 6) from which the rock popping originated, it was possible to establish that the popping is usually confined to a thin outer layer of rock exposed in the tunnel.

The phenomenon of popping in the Simplon Tunnel¹ was observed both in Tunnel I (photograph 7) and in Tunnel II (photograph 8). Engineer F. Rothpletz² describes as follows a case of popping that he personally observed in July 1914. Work on excavating Simplon Tunnel II to full section on July 1, 1914, was being carried out at a distance of 3.05 to 3.50 km from the south portal.

“On July 1, 1914, at 5 hours 20 minutes in the evening, in the part of Tunnel II between 3.29–3.32 km from the south portal, there came a crash equal in strength of sound to a cannon shot, accompanied by so strong a shaking of the mountain mass that the phenomenon extended for more than 1 km, and at such a distance was felt quite clearly.”

From the shaking of the air, all the acetylene lamps throughout Tunnel II were extinguished by the air blast.

The consequences of this “shot” of the rock, this most remarkable case of popping, were as follows: at 3.30 km the bottom of the tunnel rose by 24 cm, the east wall shifted westward by 25 cm, and the west wall eastward by 17 cm. As a result, the passage of Tunnel II at 3.30 km narrowed from 60 cm to 30 cm. Some supports in the crown were displaced. Two struts in the lower

¹ The Simplon Tunnel consists of two single-track tunnels, located 17 m apart from each other. Tunnel I was constructed in 1893–1905, and Tunnel II was built in 1912–1921. When Tunnel I was being built, a gallery for the second tunnel was driven simultaneously with it. Subsequently this gallery was excavated to full section.

² F. Rothpletz. “Bergschläge in Simplontunnel.” Schweiz. Bauzeit. 1914. Bd. LXVI, No. 5, pp. 68–70.

the heading gallery broke. There were cases of stones falling from the sides and from the roof, and two workers were injured. As a result of the heaving of the bottom of the tunnel, the 800 mm gauge rails were shifted, even though loaded mine cars were standing on the rails; moreover, the cars came off the rails.

In Tunnel I, as a result of the shock, a slab was displaced that covered a large drainage channel measuring 70 cm in width and 80 cm in depth. The slab fell over a length of 20 m; as a result, the water flowing through the channel became dammed up and flooded Tunnel I. It should be added that the water discharge at this point varied with the seasons from 800 to 1500 liters per second. At that time the discharge was just near its maximum.

During the construction of Tunnel I, no particularly intensified rock popping was observed at the places corresponding to those indicated above. This has been positively established by questioning the participants in the construction of Tunnel I, and is also confirmed by the fact that in Tunnel I, along the stretch corresponding to the place of strong popping in the second tunnel, lining of type No. 2 was used, i.e., the lightest type of lining (type No. 1—without lining).

Rock popping in the Simplon Tunnel was observed quite often. This phenomenon could easily be checked here, and at times even entirely eliminated by timely lining.

In photograph 9 is shown the rock popping observed in the Second Simplon Tunnel at a distance of 9.672 km from the northern portal, in the upper part of the working, at the moment when the crown of the tunnel was being laid out.

A very curious case of rock popping was observed during the construction of the Wochein Tunnel.—This tunnel, 6339 m long, lies in Tithonian limestones, marls, dolomites, dense limestones, and clayey shales. During the driving of the tunnel in November 1903, oolitic limestones were encountered. At the boundary of the Triassic and Jurassic formations, approximately in the middle of the tunnel, strong rock popping was observed over a length of about 100 m. The popping always began several weeks after the exposure of the rock. With a rumble and cracking, more or less large pieces sprang off. But the most remarkable case occurred on February 1, 1904. A block of about 8 cubic meters sprang off. The rumble, equal in force to a cannon shot, was heard several kilometers from the tunnel.—This case ended quite safely.

In concluding the data reported from practice, one cannot fail to mention several facts that are interesting and important from the theoretical point of view, when the circumstances under which rock popping should have occurred coincided with the presence of jointing and schistosity in the rock. It turns out that in such cases the character of the phenomenon changed sharply. Popping, in the sense adopted by us, was no longer observed; instead, displacement of the rock occurred. The examples are rare, but characteristic. It has already been said in its proper place that rock popping in general is a little-studied phenomenon, despite the fact that this phenomenon is accompanied by a strong sound effect, which, it would seem, inevitably attracts universal attention. There is even less reason for displacements of jointed or schistose rock to have been noticed and recorded.

Not having a large amount of data to illustrate the phenomenon of popping in schistose rocks, I nevertheless am able to cite an example from the practice of constructing the Simplon Tunnel. In heading II, at a distance of 7.7 km from the southern portal, displacement of the rock was observed in a limestone shale (phot. 10). In the vicinity, where the rock was dense, strong, and not schistose, spalling of slabs was observed, i.e., the phenomenon of popping in the full sense of the word.

General Results of Observations

As a general result of all the numerous cases of observation of the phenomena of rock popping, the following may be said:

The phenomenon of popping cannot be connected with any particular rock. It may be considered proven that this phenomenon is observed in every kind of stone rock, but predominantly in the strongest, most brittle, homogeneous, solid, and massive rocks.

It is rather difficult to compile a complete list of all the rocks in which popping has occurred. Such are quartzites, quartz sandstones, gneisses, dense limestones, diorites, porphyries, granites, dolomites. Further, though more rarely, the phenomenon of popping has been observed in slaty gray sandstones and even in clay shales. It has quite often been necessary to observe popping in the hornblende andesites of the Comstock mine in California, in tunnel No. 27, 588 m long, in Cincinnati, situated in ordinary sandstone, in the lead mines of Przibram, occurring in graywacke, in the lead mines in Derbyshire, occurring in shales and diabases. Finally, popping has quite often been observed in many hard-coal workings.

It is very interesting to connect the phenomena of popping of rock with rock pressure. All persons who have observed popping insistently say that the phenomenon of popping and strong vertical rock pressure seem to exclude one another. Some authors go so far as to assert outright that the phenomenon of popping is observed under conditions “which completely exclude the possibility of speaking of any rock pressure whatever.”

In appearance, popping proceeds in exactly the same way both in the Simplon Tunnel, where the height of the overlying rock mass reaches 2000 m, and in the Belgian quarries of Quenast, where the workings are carried out at the surface.

The time when popping begins varies. In the overwhelming majority of cases, popping begins immediately after the rock has been excavated. However, there are far from isolated cases in which popping began only a long time after the driving of an underground opening. Weeks and even months passed without the slightest signs of popping, and suddenly, quite unexpectedly, the popping of the rock began with furious force.

The intensity of popping varies. In most cases it is expressed by rumbling and crackling. Sometimes there is a continuous slight crackling of the rock, accompanied by fine crumbling and its flaking off. Sometimes, on the contrary, a rumble like a rifle shot is heard in the adit. Cases in which the sound effect reached the force of a cannon shot, as happened in the Simplon and Vosges tunnels, are exceptional.

It is quite curious to note that, although popping of rocks is encountered in mines and tunnels situated at the most varied depths, for some reason the phenomenon of popping is especially widespread at a depth of 230–240 m. This is precisely what is observed in the mines of Upper Silesia, in the Westphalian basin, in the Ruhr basin, in the Dąbrowa basin, in the Donets basin, and in many other places. This phenomenon is very remarkable. If, in the indicated districts, workings are followed at the above-mentioned depth or below it, then the phenomenon of popping is either not observed at all, or is observed unusually rarely.

Along with this, popping is observed in some mines near Lake Superior in North America, at a depth of 1450 m, in the workings of Mansfield at a depth of 1350 m, in the St. Ingbert mine in the Rhenish Palatinate at a depth of 1250 m, and in the Comstock mine in America, at a depth of 900 m. Finally, as has already been said, popping is observed in the Belgian quarries of Quenast, situated on the open surface, in places of daytime extraction of rock.

Thus, noting the remarkable prevalence of popping at a depth of 240 meters, it is nevertheless not possible to regard the phenomenon of popping as a function of depth.

The place where popping begins and occurs, in most cases, is the side parts of underground workings. Here, along the walls, popping usually begins and in most cases reaches its maximum and particular intensity. Very rarely, and as an exception, popping was observed at the floor of underground workings. As for the ceiling of an adit, it seems that only two cases are known in all, when very weak and barely perceptible popping was detected. It would be more correct to say that popping does not occur on the ceiling of an adit.

Popping that has begun can possibly be suspended and even completely stopped in two ways: either by the successful placing of lateral boreholes, or by means of lining.

Unfortunately, one can only state that lateral stopes to the right and left of the places of popping had a decisive effect on the further course of the popping of the rock, but there is no possibility of giving any more precise indications with respect to the tectonics of the mass at the given place, owing to the absence of rigorously verified material.

In tunnels it was repeatedly observed that timely lining delayed and even stopped the popping of the rock. Unfortunately, here too the persons who made the observations provide no data whatever on the internal structure of the rock mass. With respect to tunnel lining as a means of delaying and even stopping the popping of rock, I shall note that injection of the rock with cement mortar was not carried out in these places. The arches were laid with wedging, and thus small air spaces remained behind the side walls and behind the vaults.

Finally, the question of temperature. Here we have only fragmentary data, and not systematic and planned observations. Still, it may be said that, apparently, the temperature of the rock and the phenomenon of popping have no connection whatever. In the Simplon Tunnel the popping of the rock began at its temperature of \(+23^\circ\mathrm{C}\) and at an air temperature of about \(+20^\circ\mathrm{C}\). The maximum temperature in this tunnel was, for the rock, \(+55.4^\circ\mathrm{C}\), and the air temperature \(+50^\circ\mathrm{C}\). The popping of the rock occurred at different temperatures in one and the same place. It has by no means been established that changes in the temperature or humidity of the air were in any way reflected in the intensity of popping. In the quarries of Kenast, popping is observed at different times of the year, at the most varied temperatures of the hot rock and of the surrounding air, at the most varied degrees of humidity and heights of atmospheric pressure. The same may also be said with respect to the popping of rock in the hornblende andesites of the Comstock mine in California.

Hypotheses on the causes of popping in rocks

Analyzing the phenomenon of rock popping, we must state that the materials obtained from observations in tunnel and mine practice are still insufficient for a complete and comprehensive clarification of the causes and circumstances of rock popping. This explains the variety, contradictoriness, and uncertainty of the theoretical views existing in the literature on this interesting phenomenon.

The famous Zurich professor of geology Albert Heim points to the phenomenon of popping as one of the proofs of the manifestation of mountain pressure—

at depth¹. This reasoning comes down to the fact that at a certain depth rock experiences a stress corresponding to the stress of the overlying masses due to their weight. Heim assumes that a body is solid only so long as it is subjected to unilateral pressure, but it becomes plastic and even, to a certain degree, fluid if it is subjected to pressure from all sides, and in this case the pressure exceeds the strength of the rock. Then the rock acquires the properties of perfect plasticity.

In order to examine in greater detail the processes to which rocks are subjected in their changes at depth, it is necessary to recall the categories into which Prof. A. Heim divides rocks according to those of their properties that determine the extent and character of their changes. These properties are as follows: first, either the strength or the weakness of the rock—a property determined by the force of cohesion of its constituent parts, but not dependent on the mechanical conditions in which the rock is found. Second, either the brittleness or the plasticity of the rock—a property not determined by the force of cohesion, but, in contrast to the first property, dependent on the surrounding mechanical conditions. The first and second properties are not in any dependence on one another, and therefore exist in various combinations in one and the same rock; for example, granite and quartzite possess strength in combination with brittleness, marl and sandstones possess strength and plasticity, mica—weakness and brittleness, and clay weakness and plasticity.

Hard, strong, and brittle rocks, under pressure exceeding the strength of the rock, acquire the properties of plasticity if the rock is subjected to pressure from all sides; but if we open the rock with a face, it produces popping, which, in Prof. A. Heim’s opinion, is one of the proofs of the manifestation of rock pressure at depth.

Opponents of Heim’s theory of the pressure of rocks as a function of depth resolutely remove the phenomenon of popping from the category of proofs of the real existence of rock pressure at depth. They point out that the phenomenon of popping occurs in analogous, if not identical, forms both at a depth of 2000 meters in the Simplon Tunnel and on the surface in the quarries of Kenast. Further, they point out that popping is usually confined to a comparatively insignificant layer of the rocks exposed in an adit.

There is a hypothesis that the cause of rock popping is, to some extent, the change in temperature following the driving of a tunnel adit. Supporters of this hypothesis point out that the temperature of rock is usually elevated as a result of the earth’s internal heat. Knowing the geothermal gradient, the conditions of occurrence of the rocks, and a whole series of other factors, we can calculate with a sufficient degree of accuracy what the temperature of the rock is at a given depth. With the conduct of tunnel workings the temperature changes; cooling occurs, and this plays a certain role in the phenomenon of rock popping.

Objections are raised to this, with reference to the Simplon, the quarries of Kenast, and a whole series of tunnels and mines where the most varied combinations of temperatures were observed that did not affect the phenomenon of popping.

There is a hypothesis that popping is a consequence of the redistribution of stresses in rocks, depending on the driving of the adit. According to this hypothesis, the phenomenon of popping is an example of the accumulation in the deep bowels of the earth of potential energy in the form of mechanical forces. In such an explanation, popping acquires the significance of a major geological fact, directly connected with dislocations of the earth’s crust, usually considered under various names.

Unfortunately, this hypothesis has been insufficiently developed and has its objections. It must be pointed out that here one speaks of the “deep bowels of the earth,” whereas in fact rock popping is observed predominantly at a depth of 240 meters and even on the surface of the earth, for example in the quarries of Kenast.

The author’s supposition as to the causes of rock popping (hypothesis).

Considering the phenomenon of popping as a whole, regardless of depth, one must acknowledge that, in the actual working of the earth’s interior at various depths, we encounter rocks in a particularly stressed state. The potential energy concealed in them, when the rock is exposed, is expressed in the form of mechanical force. Such accumulators of energy are concentrated in rocks that are hard, strong, fresh, and homogeneous. We may encounter these foci of potential energy at any depth, but especially at a depth of 240 meters.

I am inclined to ascribe the principal cause of the formation of such foci of potential energy to modern tectonic processes. We observe rock popping not only in massive rocks, but also in sedimentary ones: in limestones, in sandstones, in clay shales, in dolomites, in coal, in gneiss, in mica and chlorite schists, in hornblende schist, and especially in quartzites.

It is therefore necessary to reject, in the most decisive manner, any possibility of admitting the formation, connected with the solidification of molten magma, of special “contractions” of the crust, and not to seek this cause of rock popping in the deep interior of the earth.

The presence of popping in sedimentary rocks may serve for us as a certain guiding thread.

Popping is absent in plastic rocks and is observed exclusively in strong and brittle rocks. This, apparently, may be interpreted in the sense that foci of potential energy arising from causes which we are trying to discover, so to speak, “dissipate” and disappear, owing to the plasticity of the rocks, through displacement of its particles.

Popping is observed from the side walls. This too should serve as a guiding thread for us. It will be especially important for us that, if on the right and on the left of the place of popping lateral cuts are made in the rock, then the popping ceases.

All this taken together leads me to the thought whether the cause of popping may not be the stress of the rock mass during the formation of folds (tangential forces).

Rock pressure in the mass of a mountain is distributed according to very complex laws, with varying intensity and in various directions. In my work “The Pressure of Rocks...” I noted, and showed by examples, that pressure may be vertical, lateral, from below upward, or directed along our underground working. In the formation of folds, if, for example, we recall Daubrée’s well-known experiments, we may accept that rock pressure is also directed horizontally.

Thus, my supposition concerning the forces of rock pressure directed horizontally during the formation of folds in a rock mass is entirely admissible.

If the hypothesis I have put forward is accepted, then all the circumstances under which rock popping occurs find their full explanation.

In order to verify my supposition expressed here, I carried out the following experiment: I took a hard rock and began to compress it (Fig. 11); then, at the place A—A, rock popping will be observed.

rocks. Lens-shaped plates with rumbling and crackling begin to spall off if the pressure has not been brought to the point of complete destruction of the rock.

For the experiment I took a fine-grained granite. The length of the piece was 39 cm. The end faces measured 12 × 16 cm. At the narrow place where the neck was made, the section is 8 × 7 cm. Having clamped it in the press, at the place A—A we obtain the characteristic rock popping.

Let us repeat the experiment with marble. The picture obtained is the same: until complete destruction of the piece has occurred, the exfoliations go inward in plates 1½ or 1 mm thick. Crumbling at the ends, thin and transparent, they are hard and strong in the middle of the lens. If such a spalled-off plate is applied to the place from which it has just spalled off, it turns out that it has become somewhat larger and does not fit into its own socket (Fig. 12).

Let us now consider how far the hypothesis I have put forward agrees with observations in practice.

If we look at the drawing (Fig. 13), which schematically depicts the formation of a fold, we see that the place of popping will be in the zone where there are no forces of a clearly expressed vertical rock pressure. This agrees as well as possible with observations from practice. In this respect Prof. of Geology A. Heim was mistaken in trying to regard rock popping as proof of the manifestation of forces of vertical rock pressure at depth.

The rock that spalls off has a lens-shaped form. This form is usual in strong, fresh, and homogeneous rocks, which is easy to verify experimentally. If the rock is destroyed, weathered, not sufficiently fresh, it will crumble. What has been said agrees splendidly with reality. When the rock is fresh, we actually have lens-shaped plates. In the variegated sandstones of the Kraberg tunnel and in some mines, where the rock is not sufficiently strong and fresh, we do not see plates, but obtain a fine crumbling talus.

If the rock, under the action of horizontally directed forces of rock pressure, has been brought to the magnitude of the temporary resistance of the rock to shearing \((K_s)\), then upon exposure of this rock its popping occurs, accompanied by rumbling and crackling. If the stress of the rock up to the limits of temporary resistance had been brought about by the moment of exposure of the rock, then the popping of the rock begins immediately upon its exposure. Popping may begin not at once, but after some time, in the case where the stress of the rock had not been brought to the limits of temporary resistance to shearing. Popping will begin if the temporary resistance is reached, and this will occur in two cases: first, either with further driving of the underground working to full profile, or, second, if somewhere a displacement of the rock occurs, or in general such a process (Fig. 14) in which the force in the horizontal direction will increase. Such, apparently, was the case in the Simplon Tunnel.

The hypothesis I have put forward concerning the action of tangentially directed forces, in connection with the formation of folds, as the cause of rock popping, finds full confirmation, among other things, also in the fact that, by making lateral cuttings near the place of rock popping, we stop this popping of the rock. Indeed, by creating an air space and isolating the place of popping from the forces of horizontal pressure, we eliminate the cause which results in the disappearance of popping as a consequence of this pressure.

It is appropriate here to mention that even light lining of a tunnel at the place of rock popping can delay and even stop this popping. If the hypothesis I have put forward is accepted, such an influence of the lining becomes clear to us. Indeed, let us make the following calculation. If we take the temporary resistance of the rock to shearing as 100 kg per...

sq. cm.¹), then the force required for the spalling of the rock will be equal to 100 kg per cm²; and since this force is directed at an angle of about \(5^\circ 40'\), then, resolving it by the parallelogram rule (Fig. 15), we find that the force directed normally to the surface where the rock bursting occurs will be only 10 kg per sq. cm. Such is the force that must be applied in order to extinguish completely the spalling stress. It is clear that a tunnel lining, even with slight resistance, can already easily bring the rock, in terms of its stress, beyond the limits close to the temporary resistance of the rock to spalling, and thus even a light tunnel lining can stop the bursting of rocks.

Everything just said gives us an explanation of one more interesting circumstance in the phenomenon of rock bursting. I mean that, during bursting, the rock is in a visibly and clearly distinguishable stressed state only to a small depth. With the adoption of my hypothesis this also becomes understandable and receives the following explanation: horizontally directed forces of rock pressure are inclined at a small angle to the surface where bursting is observed. Hard and brittle rock will crumble and rebound in thin plates only to a small depth, while farther on, deeper into the rock, although the rock will experience rather high stresses, spalling will not occur, since temporary arches will form there and the rock in these temporary arches will work in compression (Fig. 15). The temporary resistance of rock to compression, as is known, is much higher than those forces which, with a certain combination and direction, can destroy the rock by spalling its outer layers.

There is one more question closely connected with the phenomenon of bursting, to which my hypothesis also gives an answer. I mean that bursting is very often observed at a depth of about 240 m. This is a mysterious value, and we cannot justify or verify it by mathematical calculation. In view of my hypothesis on the causes of rock bursting, it is possible that 240 m is the zone where a wave of dislocations in the earth’s crust is taking place; however, this question is complex and cannot be resolved at once.

Thus, from a number of circumstances accompanying the bursting of rocks, we are convinced that the hypothesis I have set forth concerning horizontally directed forces of rock pressure in the formation of folds can indeed be considered as the cause of rock bursting, and it explains well all the circumstances accompanying rock bursting.

¹ The temporary resistance to spalling of rock, according to the data of K. Bach (Machine Parts, Their Calculation and Design), may be taken as follows:

Drawing 3. Location of rock popping in the pilot gallery of the Tauern Tunnel.

Photograph 4. View of rebounding slabs at the moment of rock popping in the Tauern Tunnel.

Photographs 5 and 6. Places where rock popping occurred in the Simplon Tunnel.

Photo 7. Rock popping in Simplon Tunnel I.

Photo 8. Rock popping in the calcareous schists of Simplon Tunnel II.

Fig. 9. Rock popping in the upper part of the Simplon Tunnel during the laying of the arch.

Fig. 10. Appearance of schistose rock under conditions corresponding to rock popping.

To the article by Prof. M. I. Evdokimov-Rokotovsky.

In the left drawing:

• “Location of spalling / fracture plane.”
• “Back fracture line.”
• Dimensions shown: 16 cm, 12 cm, 39 cm, 8 cm.

Drawing 11.

In the right drawing:

• Dimensions shown: 9.7 m, 0.35 m, 0.50–0.40 m.

Drawing 12.

To the article by Prof. M. I. [[unclear: surname]]-Rykotovsky.

Place of blasting of the rock

Fig. 13.

Place of blasting of the rock

Fig. 14.

\(100\)

\(T\)

\(5^\circ 40'\)

\(P = 100 \sin 5^\circ 40'\)

\(p = 10 \text{ kg/cm}^2\)

\(T\) — the horizontal stress force of rock pressure
(tangential force).

Fig. 15.

1. For details on this, see: Prof. M. I. Evdokimov-Rokotovsky, Pressure of Rocks. Published 1927, Tomsk. ↩↩