ANCHOR SUPPORT

V. V. GUBIN

(Presented by the scientific seminar of the departments of development of stratified deposits, development of ore deposits, mine construction, safety engineering, and mine ventilation)

A systematic increase in coal production and growth in labor productivity are impossible without further technical progress in the coal industry. In this respect, along with mechanization of the processes of coal mining, the introduction of new types of metal support in working and preparatory faces is of great importance.

The volume of work on supporting and controlling rock pressure in coal mines is very large. The total area of the supported roof of extraction workings exceeds 1.5 million m². The number of workers engaged in support and rock-pressure control, and in the related operations of cutting and delivering support materials into the mine, exceeds the number of workers directly engaged in coal extraction.

This shows how important it is to replace timber support with metal support. This replacement provides a number of substantial advantages, which are well known.

At present, the creation and introduction of new types of metal support is proceeding along the following lines:

a) mechanization of the work of installing, removing, and resetting supports through the use of special machines and mechanisms that perform individual operations;

b) the use, instead of prop and crib support, of movable mechanized supports adapted for the most complete mechanization of supporting and roof control, as well as for ensuring mechanization of conveyor movement;

c) the use of support without props, or so-called anchor support.

It should be noted that the first two approaches are characteristic of working faces in coal mines developing flat and inclined seams, the first approach being less promising than the second. As for anchor support, in recent years it has found increasingly wide application in the USSR and especially abroad. Taking this into account, we shall attempt to systematize and summarize the available published and practical data on this new type of support.

Operating Principle of Anchor Support

Anchoring of the roof (or floor) of a mine working is the fastening together of the individual rock layers composing it. This fastening is carried out by means of a metal anchor, one end of which is fixed in the rock by a lock of special design. A square washer with a nut is placed on the other end of the anchor at the level of the rocks being fastened. In this way the rocks are tightened by the bolts and become more rigid.

In its operating principle, anchor support differs sharply from ordinary prop support.

The main purpose of prop support is to support the roof rocks so as to prevent their collapse. At the same time, it is not assumed that the supporting support can eliminate deformation of the roof rocks. On the contrary, it is understood that during the joint deformation of the support–roof system the permissible strength limit of the support material will not be exceeded.

The use of anchor support, however, provides for the elimination of deformation of the side rocks. By anchoring the rocks, the negative character of the manifestation of rock pressure is prevented. This is the principal advantage and the difference between anchor support and prop support.

When the roof is supported by anchor support in workings of rectangular or trapezoidal shape driven through stratified rocks, the role of a beam is performed by a rock slab, whose necessary strength is ensured by fastening the individual rock layers together. The increase in strength of a rock slab through the use of anchors may be judged from the fact that the resisting moment of an anchored beam is greater than that of an unanchored one consisting of the same number of individual layers. Just as the prestressing of reinforcement is carried out before a reinforced-concrete structure takes up the load, so the side rocks must be tightened by anchor support before rock pressure develops, i.e., as soon as possible after the next exposure of the rocks.

An anchor bolt works mainly in tension, but bending and shear deformations are also not excluded, depending on the structure of the rocks and the shape of the cross section of the mine working (in particular, development workings). Thus, for example, with an arched cross-sectional shape of a mine working and horizontal rock stratification, shear stress is considerably reduced. It is therefore very important that each bolt not only ensure proper compaction of the rocks, but also be positioned in a direction corresponding to the geological conditions.

On the basis of practical data, it may be said that anchor support is installed most effectively in strong (but not excessively hard) rocks. However, even in relatively weak rocks this type of support works reliably, as is confirmed by the practice of conducting mining operations in the Ruhr basin.

Tests in sandstones and shales show that under the action of an external load the support lock slowly slides downward. The distance through which the lower end of the anchor bolt moves, i.e., the settlement of the support, is a function of the applied load. The magnitude of support settlement includes not only the displacement of the lock but also the elongation of the anchor itself. In addition, in tests of expansion-type support, sliding of the bolt along the expansion element takes place.

All other conditions being equal, the lower the strength of the rocks, the greater the amount of settlement.

Design of various types of anchor support

The principal elements of anchor support are: the anchor (a bolt rod), the lock, and a nut with washer.

Anchor bolts are made of soft round steel with a diameter from 18 to 25 mm. Their length depends on the nature of the rocks being fastened and varies within the range from 0.7 to 3.0 m, the most common length being 1.2–1.8 m. If the height of the working is less than the required length of the anchor bolts, then they are used in sections. The magnitude of the load that a bolt can withstand depends on its diameter and on the quality of the metal.

Anchor bolts are fastened in boreholes by means of a lock of special design. Depending on the design of the lock, the existing varieties of anchor support used in the USSR and abroad may be reduced to two types: 1) the wedge-slot type, in which close contact with the side rocks of the borehole is achieved by driving a wedge into the slot at the upper end of the bolt (Fig. 1); 2) the expansion type, in which fastening of the bolt in the borehole is carried out—

Fig. 1.

Fig. 2.

is effected by the expansion of the additional part by means of internal wedging (Figs. 2, 3, 4).

When using wedge-type anchor support, an important role is played by the choice of the correct ratio between the diameters of the borehole and the anchor bolt in relation to the dimensions of the wedge. For a given driving force of the anchor bolt in rocks of a given strength, the fastening of the lock becomes stronger the larger the contact surface is between the bolt “jaws” spread apart by the wedge and the side rocks of the borehole.

Fig. 3.

Fig. 4.

In world practice, wedge-type anchor support has become more widespread than other types of expansion support, which is explained by the simplicity of the design and the ease of its manufacture.

Figures 2, 3, and 4 show designs of expansion-type anchor supports. In the lock design with an expansion sleeve (Fig. 2), cast two- or four-bladed sleeves with a roughened surface are placed on the upper end of the anchor bolt. The expansion of the blades in the borehole is accomplished by means of a conical nut, which enters the sleeve when the bolt is rotated; the bolt is provided at its free end with a hexagonal or square head. Slipping of the nut is prevented by two lugs located on it.

The design of a lock with a double cylindrical wedge is shown in Fig. 3. The expansion part consists of two obliquely cut halves of a cylinder. The half with the smaller diameter is screwed onto the bolt. When the bolt is rotated it moves downward, while the outer half of the cylinder, provided with a projection, is held by the end of the bolt.

Figure 4 shows the design of an expansion-type lock, “Boltex,” manufactured by a French firm [8]. Bolts with such locks are equipped with an upper sliding sleeve, which is rapidly clamped in the walls of the borehole before failure of the short rod between the two wedges occurs. Then the second sliding sleeve is actuated, and the required expansion of the bolt is achieved. A bolt of this type provides a better distribution of stresses over a larger surface. A significant disadvantage of the design is its complexity.

In addition to the indicated varieties of expansion-type anchor support, the literature sources [4, 5] describe the design and principle of operation of the “Podvesok” AP-1, AIA-2, and locks proposed by Engineer Gorfinkel. The latter are very imperfect in design, unreliable in operation, and are unlikely to find industrial application.

A significant disadvantage of all varieties of expansion-type anchor support is the complexity of their design, which considerably limits their spread.

In addition to the above-mentioned types of metal anchor support, in mining practice there are known cases of the use of wooden and reinforced-concrete anchors. Wooden anchors with wedges were used at the Day mine in the USA to support the roof and reinforce the sides of the working [3]. The round-section anchors were made of fir and had a diameter of 50 mm. At each end of the anchor bolt, mutually perpendicular slots were cut to prevent the bolt from splitting in the longitudinal direction. The depth of the slot at the end inserted into the roof was 410 mm; at the free end, for the second wedge, which presses the bearing plate against the roof and serves to tension the bolt, it was 300 mm. The width of the wedges was 500 mm, and the thickness at the base was 25 mm. When anchor bolts of this type are used in preparatory workings, it is recommended that all support elements be impregnated with antiseptics. Reinforced-concrete anchors were used in some foreign mines to support the roof and floor, including at one of the metal-ore mines of Norway in 1945 in a section with a weak roof represented by shale. Boreholes were drilled to a depth of 2–3 m with a spacing between them of 2–3 m. The borehole diameter was 32 mm. Into a borehole sealed with a quick-hardening plastic plug, a loop of wire rope 13 mm in diameter was inserted, and through a thin tube, under a pressure of 20 atm, quick-setting cement was supplied.

In the case of reinforced-concrete anchors used to secure soil rocks in the mines of Bavaria, the Ruhr, and Lorraine, fastening is carried out by means of corrugated rods of 14 and 19 mm

steel, inserted into the borehole and grouted with quick-setting cement.

The results of using reinforced-concrete anchor support have shown that it is the best and most economical means of combating soil heaving.

Conditions for the use and installation of various types of anchor support

Wedge-slot anchor support is recommended for use in rocks of medium strength and in strong rocks (close in strength to sandy shales and sandstones). In strong granites and dense quartz the effect of loosening the anchor bolts is reduced, since with a comparatively small load on the support ($1—3\ t$) slipping of the lock is observed. Moreover, very often in strong and very strong rocks there is in general no need to use any support.

Installation of support with a lock of this design is carried out as follows. A bolt with a steel wedge fitted into the slot is driven, by a pneumatic or electric percussion tool, into holes drilled in the side rocks, the diameter of the holes being $6—7\ \text{mm}$ greater than the diameter of the bolt. The wedge, entering the slot, presses the split end of the bolt against the walls of the hole, thereby ensuring quite sufficient anchoring strength. After driving the bolt into the hole it must be tightened by screwing the nut onto the thread of the lower end of the bolt to the limit, with the aim of ensuring a tension of about $4\ t$. Experience shows that, to ensure such a tension, the magnitude of the applied torque should be $30—35\ \text{kg m}$ for bolts $25\ \text{mm}$ in diameter. When nuts are tightened by hand, the required torque can be provided by using a wrench with a bent head and a lever $70—80\ \text{cm}$ long. It is most advisable to tighten the nuts with a special mechanical tool. In mines equipped with pneumatic facilities, mechanical tightening of nuts can be carried out with pneumatic bolters, widely used in our metro construction.

For more reliable anchoring of anchor bolts in less stable rocks, various designs of expansion-type locks are used. Installation of expansion-type anchor support is, as a rule, simpler than that of the wedge-slot type, because instead of two operations (driving the bolt and tightening the nut), in most designs only tightening the nut or the head of the bolt is required.

For all types of anchor support it is characteristic that, several days after installation, the initial tension of the bolts sometimes decreases, and it becomes possible to make an additional one or two turns of the nut. Therefore it is recommended to require the support workers, before installing the next row of support, to check and tighten the nuts on the bolts of the three or four preceding rows.

The time required for installing anchor support, as practice has shown, varies within wide limits and depends on a number of factors (strength of the rocks, type of drilling machines, organization of work, etc.). For example, the duration of installing an anchor bolt (including drilling the holes) was, in experimental work by MAKNII, $5—8$ minutes; when the support was introduced at Mine No. 2 of Estonslanets, $0.33$ man-hours; and, according to published data, at foreign mines, $0.16—0.53$ man-hours. With pneumatic drilling of holes for installing bolts in comparatively strong rocks, labor productivity decreases somewhat.

At the Mirgalimsai mine, the time required to install one bolt averaged 0.64 man-hours; at the Lotaring iron-ore mines, 0.53–0.8 man-hours.

We have no experience in extracting installed anchor support, since mainly wedge-slot type support is used. In foreign practice, extraction of installed support is resorted to quite often. Thus, according to data of the U.S. Bureau of Mines, before the abandonment of stoping chambers under the room-and-pillar mining system, about 300,000 expansion-type bolts were extracted during 1953; in some mines of the Ruhr, before workings are abandoned, it is possible to recover for reuse up to 85% of the installed support.

Mutual arrangement of anchor support

In order to ensure reliable operation of anchor support, it is necessary that the length of the bolts and their number per unit area correspond to definite conditions. One should strive to adopt such a length of anchor bolts that fastening of the lock is ensured in the strongest rocks. The spacing between rows and between individual bolts in a row depends mainly on the character of the rocks being supported. In practice, the area per bolt ranges from 0.5 \(m^2\) to 3.0 \(m^2\), with the most common arrangement being on a grid with square area from \(1 \times 1\) to \(1.5 \times 1.5\) m.

At present, despite the comparatively extensive practical experience in the use of anchor support, a theoretical substantiation of its individual parameters is almost absent both in foreign and in our literature, apart from individual works [6, 7].

On the basis of the conclusions and reasoning of G. N. Kuznetsov [7] in determining the full bearing capacity of the roof of underground workings, V. N. Semevskii assumes that the maximum tensile stresses due to bending in the lower layer of the roof, of width equal to unity, are, when the locks of anchor bolts are fastened in the rocks of the main roof that do not deform after the driving of the working,

\[ \sigma_{max} = \frac{q l^2}{2h}, \tag{1} \]

where \(q\) is the load per unit area, equal to the product of the unit weight by the thickness of the rocks of the immediate roof supported by the bolts;

\(l\) is the distance between anchors;

\(h\) is the thickness of the lower layer, the danger of whose collapse as a result of bending must be eliminated.

From expression (1) one can determine the distance between anchors, which will be equal to:

\[ l = h \sqrt{\frac{2\sigma_n}{k_3 \gamma h_1}}, \tag{2} \]

where \(\sigma_n\) is the temporary tensile resistance of the rock of the lower layer;

\(k_3\) is the safety factor.

For a definite grid of anchor arrangement, the length of the latter can be found by the formula:

\[ l_\sigma=\frac{2\sigma_n h^2}{k_3\gamma l^2}+l_1+l_2, \tag{3} \]

where \(l_1\) is the length of the segment projecting into the working (determined from structural considerations);

\(l_2\) is the amount by which the borehole is sunk into the rocks of the main roof, taken in practice as equal to \(200\text{–}400\) mm.

The latter quantity may approximately be determined from the expression

\[ l_2=\sqrt{\frac{q}{\sigma_n}}. \tag{4} \]

In determining the length of anchor bolts, a calculation performed under the assumption that the rock slab fastened by the anchors is under the action of a load whose magnitude depends on the span of the working, the conditions of occurrence, and the properties of the enclosing rocks may be of more general significance. If the “anchored” slab is considered as a roof beam and the maximum stress is found from the formulas of strength of materials, then the rock slab may be regarded as a beam fixed at the supports. However, in view of the uncertainty of the conditions of fixity, the calculation for maximum stress must be made more cautiously, intermediate between those obtained with and without allowance for fixity,

\[ \sigma_{max}=\frac{\gamma H L^2}{2h^2}, \tag{5} \]

where \(\gamma\) is the unit weight of the rocks;

\(H\) is the depth of the working;

\(L\) is the span of the working;

\(h_1\) is the thickness of the anchored rock slab.

It would not be difficult to determine from expression (5) the value of \(h_1\) for the given conditions if the degree of increase in the bearing capacity of rocks when they are fastened by bolts were known. However, the degree of such strengthening is unknown, and it has to be assigned arbitrarily. It is considered sufficiently cautious to take the temporary resistance of the anchored slab as equal to 25% of the arithmetic mean of the values of the temporary resistance of the given rock and of steel,

\[ \sigma=0.25\,\frac{\sigma_n+\sigma_{cm}}{2}. \tag{6} \]

Under this assumption the required thickness of the anchored slab will be

\[ h_1=2L\sqrt{\frac{k_3\gamma H}{\sigma_n+\sigma_{cm}}}. \tag{7} \]

In such a calculation it is recommended to take the factor of safety as no greater than 2, since a considerable factor of safety has already been allowed in the very construction of the formula.

Examples from the Practice of Using Roof Bolting

It is known from the literature that roof bolting was first used in 1912 for supporting mine workings at the Friedensgrube mine in Upper Silesia [5]. In the USSR, roof bolting was first proposed in 1940 by Yakovlev, a technician at the Kirov apatite mine, who proposed supporting the roof of the main adit with “reinforced-concrete piles.” In 1947–1948, the Mining Institute of the Academy of Sciences of the Ukrainian SSR used roof bolting experimentally in several mines of the Makeevugol trust. At the Tarny-Auz mine, the possibility was established of strengthening an exposed roof by driving metal bolts into it.

Industrial application of roof bolting began in 1953 at Mine No. 2 of the Estonslanets combine. At the end of 1954 and the beginning of 1955, VUGI carried out tests of this support under the conditions of steeply dipping seams in the Kuzbass. The experiments were conducted at the Kalinin mine of the Prokopyevskugol trust, in the III Internal seam, which, in terms of the stability of its side rocks, is one of the unfavorable ones. In February–March 1956, also in the Kuzbass, at the Vakhrushev mine, in the 1st Internal seam, on the proposal of mining engineer K. I. Rubin, a system of strip extraction along the rise with the use of roof bolting was tested. Before the introduction of the above-mentioned system, this seam was mined by a system of long pillars along the strike, with the level divided into 3 sublevels and with sublevel pillars left. The transition to the new mining system with the use of roof bolting provides the following economic effect:

1) a reduction in coal losses by 14%;
2) an increase in labor productivity by 37%;
3) a reduction in the cost of 1 t of coal by 7 rubles 02 kopecks, or 41.5%.

Despite its advantages, roof bolting has not become widespread in our country, which, as some authors [5] assert, is explained only by insufficient awareness of this very promising type of support. As for foreign practice, the situation there is different. Roof bolting has now become widely used industrially in the USA. Since 1952, more than 2 million bolts have been installed each month in U.S. mines. Coal output from faces supported by this system in 1954 reached 26% of the total output of bituminous coals. Roof bolting has been introduced in 433 mines, and in 109 of them this support alone is used. It is used both in the driving of development workings (drifts) up to 7 m wide and in stoping faces—rooms 6–8 m wide and from 60 to 160 m long. By the end of 1951 alone, in 170 mines of West Virginia, 5 million bolts had been installed to support 1325 km of drifts and rooms, and 5–6% of the state’s total coal output (about 100 million t) came from mines where roof bolting was used.

In addition to coal mines, roof bolting is also used in lead-zinc mines in Missouri and Colorado, and in iron-ore mines in the Upper Lake region and in the state of Alabama.

The introduction of roof bolting in the driving of workings through rock provides a 75% reduction in expenditures for support and maintenance of workings, while at the same time increasing driving rates by 30%. The increase in the rate of advance of rock faces is achieved by reducing the time spent on support, since drilling holes for roof bolting and installing bolts are carried out while the face of the working is being drilled.

The use of anchor support in tunnel driving in the USA began in 1950. For example, during the driving of the Delaware tunnel for New York’s water supply, 124,000 anchor bolts were installed over 20 km of tunnel to secure gently dipping shales and sandstones. At first, metal arches on wooden posts were used here, and later, under the same geological conditions, a transition was made to anchor support with shotcreting. This transition made it possible to reduce the tunnel cross-section from 15.5 to 14 m². At the same time, the consumption of steel decreased from 520 to 75 kg, and that of concrete from 4.1 to 2.6 m per running meter of tunnel. The labor input for driving 1 running meter decreased from 137 to 108 man-hours. The cost of driving decreased by 30%.

In Canada, anchor support has been used fairly widely in coal mines since 1949. In the period from 1950 to 1955, more than 1 million tons of coal were mined from rooms supported by this system. The total length of workings supported by anchor support under the room-and-pillar system of mining is about 100 km, where about 376 thousand anchors were installed. In addition to coal mines, in Canada it is also used in a number of gold and nickel mines. For example, at the Kerr-Addison mine, where this support began to be used in 1952. In 1953, 5,000 bolts were installed there, and in 1954 their monthly consumption amounted to 1,200 units. Predominantly wedge-slot-type bolts are used.

In France, although still at the stage of industrial trials, anchor support is used in coal mines, in iron-ore mines in Lorraine, and since 1949 also in the potash mines of Alsace, both in development workings and in stopes (the “Simon” and “Roselle” mines).

One cannot fail to note the experience of using reinforced-concrete anchor support, which has been tested more extensively with good results in the coal mines of Bavaria, as well as in the Ruhr basin, to prevent floor heave in haulage workings. For example, in a working 2.4 m wide at the floor, 800 reinforced-concrete anchor bolts were installed over a section 180 m long. Bolts 1.5 m long were placed four in a row, with a spacing between rows of 0.9 m. The greatest amount of heave in the supported area did not exceed 10 cm, whereas in the unsupported part the heave reached 1.4 m [3].

A comparison of the cost of expenditures for securing the floor with anchors with the cost of work to blast it, on the basis of experiments carried out at the Neumühl mine in the Ruhr (the “Gretchen” seam), showed that the cost of anchoring the floor is approximately 2.5 times less than the expenditure for blasting it [3].

Conditions of Use

As mining practice has shown, anchor support can be used under various conditions and for various purposes. It can be used in the mining of both coal and ore deposits, both in stopes and in development workings.

In foreign practice, anchor support is currently used relatively widely in the driving and construction of railway and water tunnels, in the construction of large structures—for example, in the construction of a hydroelectric system in the Snowy Mountains (Australia).

From the standpoint of intended purpose, anchor support may be used as auxiliary, temporary, and permanent

support. The limits of application of anchor support from the standpoint of the stability of the surrounding rocks are fairly broad. It can be used and is used in rocks of varying stability: in weak rocks, in rocks of medium stability, and in strong rocks; both in bedded and in monolithic rocks. On the basis of data from foreign practice, it may be asserted that anchor support can be used for supporting permanent workings driven through fissured and disturbed rocks. It can also be used when driving preparatory workings with swelling ground, in order to prevent heaving of the floor of the working. Nevertheless, from the standpoint of support strength and the effective use of anchor support, it is desirable to apply it in rocks of no less than medium stability, but not in excessively strong rocks, since in the latter there is often no need at all for the use of any support. In individual cases, anchor support may also be used to counteract lateral pressure.

In conclusion, it should be said that, taking into account the considerable advantages of anchor support, efforts should be made toward its broader industrial application.

REFERENCES

1. Verigo K. N. Mining iron ore by a system with an open stope, Gornyi zhurnal, No. 12, 1951.

2. Volkenau A. V., Shirokov A. P. Testing anchor support in the Kuznetsk basin, Ugol’, No. 9, 1955.

3. VUGI. Application of anchor support abroad, Ugletekhizdat, 1956.

4. Rosinskii N. L. Support-free suspended support of mine workings, Ugletekhizdat, 1949.

5. Semevskii V. N. Rod support, Gornyi zhurnal, No. 6, 1953.

6. Semevskii V. N. Rod support in permanent workings, Gornyi zhurnal, No. 1, 1956.

7. Semevskii V. N. Investigation of the parameters of rod support (author’s abstract of a doctoral dissertation), Leningrad, 1955.

8. Information of the Central Institute of Technical Information.