US3568450A - Method for counterstressing in situ rock for support of underground openings - Google Patents

Abstract

IN ROCK SURROUNDING AN UNDERGROUND EXCAVATION WHICH MUST HAVE A MAN-MADE SUPPORT SYSTEM TO ENSURE A REQUIRED DEGREE OF STRUCTURAL STABILITY AND WHICH IS SUBJECTED TO CERTAIN STRESS CONDITIONS, AN INFLATABLE DEVICE OR ROCK JACK IS INSERTED INTO A DRILL HOLE OR SLOT IN THE ROCK. THE ROCK JACK IS INFLATED SLIGHTLY BY FLUID PRESSURE TO TAKE UP ANY SPACE BETWEEN THE JACK AND THE IN SITU ROCK AND IS SUBSEQUENTLY PRESURIZED TO THE LIMIT OF ITS CAPABILITY. HIGH COMPRESSIVE FORCE IS TRANSMITTED TO THE ROCK, THUS CREATING COMPRESSIVE STRESS IN MONOLITHIC ROCK AND INCREASING CONFINEMENT OF AND FRICTION BETWEEN PROXIMAL BLOCKS OF ROCK WHICH ARE SEPARATED BY NARROW FRACTURES TENSION CRACKS, JOINT PLANES OR SIMILAR STRUCTURAL DEFECTS. SUCH ACTION MAINTAINS OR INCREASES THE NATURAL CAPABILITY OF THE ROCK TO SUPPORT ITSELF AND TO REMAIN STABLE BY UTILIZING THE INHERENT COMPRESSIVE STRENGTH OF THE ROCK.

Description

March 9, 1971 J. M. WHITING 3,568,450

METHOD FOR COUNTERSTRESSING IN SITU ROCK FOR SUPPORT OF UNDERGROUND OPENINGS Filed May 28, 1969 6 Sheets-Sheet l FIG. I

INVENTOR JERRY M. WHITING ATTORNEY Mirth 9; T971 J M. WHITING 3,568,450

METHOD FOR COUNTERSTRESSING IN SITU ROCK FOR SUPPORT OF UNDERGROUND OPENINGS Filed May 28, 1969 6 Sheets-Sheet 2 POTENTIAL SHEAR PLANE/S R= PLASTIC RADIUS FIG. 2 A PRIOR 28 ART PRIOR ART "LAMINATED" BEAM FIG. 3

INVENTOR "LINEAR" ARCH JERRY M. WHITING RM QBMWI 1h ATTORNEY March 9, 1971 J M, wmnNG 3,568,450"

METHOD FOR COUNTERSTRESSING IN SITU ROCK FOR SUPPORT OF UNDERGROUND OPENINGS Filed May 28. 1969 6 Sheets-Sheet 3 INVENTOR JERRY M. WHITING ATTORNEY March 9, 1971 J. M. WHITING 3,568,450

METHOD FOR COUNTERSTRESSING IN SITU ROCK FOR SUPPORT OF UNDERGROUND OPENINGS Filed May 28 1969 6 Sheets-Sheet 4 BRAZED SEAL FIG. 9A

FIG. 95

BRAZED SEAL ROCK JACK IN DRILL HOLE FIG. 8

FIG. IO

INVENTOR JERRY M. WHlTlNG ATTORNEY J. M. WHITING METHOD FOR COUNTERSTRESSING IN SITU ROCK March 9, 1971 FOR SUPPORT OF UNDERGROUND OPENINGS Filed May 28, 1969 6 Sheets-Sheet 5 FIG;

INVENTOR JERRY M. WHITING ATTORNEY March 1971 J. M. WHITING 3,568,450

METHOD FOR COUNTERSTRESSING IN SITU ROCK FOR SUPPORT 0F UNDERGROUND OPENINGS Filed May 28. 1969 e Sheets-Sheet e a, C: if.

:1 Q T 7 7 IL A. A m 4 m, II III l INVENTOR JERRY M. WHITING WWM A T TORNE Y United States Patent O 3,568,450 METHOD FOR COUNTERSTRESSING IN SITU ROCK FOR SUPPORT OF UNDERGROUND OPENINGS Jerry M. Whiting, Dallas, TeX-, assignor to Atlantic Richfield Company, a fractional part interest Filed May 28, 1969, Ser. No. 828,513 Int. Cl. E2ld 21/00 US. Cl. 61-45 4 Claims ABSTRACT OF THE DISCLOSURE In rock surrounding an underground excavation which must have a man-made support system to ensure a required degree of structural stability and which is subjected to certain stress conditions, an inflatable device or rock jack is inserted into a drill hole or slot in the rock. The rock jack is inflated slightly by fluid pressure to take up any space between the jack and the in situ rock and is subsequently pressurized to the limit of its capability. High compressive force is transmitted to the rock, thus creating compressive stress in monolithic rock and increasing confinement of and friction between proximal blocks of rock which are separated by narrow fractures, tension cracks, joint planes or similar structural defects. Such action maintains or increases the natural capability of the rock to support itself and to remain stable by utilizing the inherent compressive strength of the rock.

BACKGROUND OF THE INVENTION The invention pertains to mining and particularly relates to supporting underground openings.

Rock below the surface of the earth is under stress. The magnitude of the stress often becomes appreciable at depths where mining operations are carried out. The origin of thi inherent stress has been the subject of research for several years. Many causes have been suggested. The most important are outlined below:

(a) The uniform inward pull on the earths crust by the force of gravity creates both vertical and lateral stresses on rock at depth. Thus, a two-dimensional section of the earths crust can be likened to a circular steel ring subjected to external radial forces. In place of the external load, however, it is the weight of the rock itself which creates the radial (vertical to an observer) component. The weight of the overlying material tends to compress the rock, and when squeezed in a vertical direction, the rock tries to expand laterally. This expansion is resisted by the surrounding rock so that some magnitude of compressive lateral stresses are present on any confined volume of rock at depth.

(b) Deformation of the earths crust accounts for inherent stress at some localities. Folding and faulting are evidence of intense stress buildup. Stresses which have remained stored in the elastic deformation of the rock or which are the result of presently active faulting may be present in some portions of the earths crust.

(c) Expansion during crystallization or recrystallization during cooling of magmas r metamorphic changes have been suggested as a cause for inherent stress.

It should be clearly pointed out that inherent stress at a particular underground location may be caused by any combination of these factors. For this reason, attempts to compute inherent stress at certain depths by using only the weight of overlying material as a basis have often proved to be grossly inaccurate. Some mines in the Alps, for example, have accumulated data which show that the greatest portion of inherent stress encountered there is caused by active tectonic movement. Nevertheless, the weight of overlying material plays the prime role 3,568,458 Patented Mar. 9, 1971 in producing inherent stress at most subsurface locations.

When an opening is made in a material which is under stress, there results an internal rearrangement of the stress distribution which creates a stress concentration in the proximity of the opening. The stresses immediately adjacent to the opening may change from compression to tension, or may increase or decrease in magnitude several times. The resulting distribution depends on the shape and orientation of the opening, the original (inherent) stress distribution, and Poissons ratio of the rock.

Stress concentrations caused by discontinuities were not known for many years after the start of structural and machine design. Even after the phenomenon was recognized and applied to machine design, it was many years before anyone realized the structural similarity of a rivet hole in a piece of metal and a circular opening in the earths crust.

The difference between the inherent stress and the resulting stress after an opening has been formed in rock may be referred to as induced stress. It is of prime importance in ground support and rock pressure problems. Woodrutf succinctly states:

The realignment and concentration of pre-existing stresses during excavation of underground openings appears to be the most important single factor tending to destroy the structural stability of rock around an opening.

(Woodruf, Seth D., The Causes of Structural Deterioration of Rock Around Mine Openings, Engineer of Mines Thesis, University of Idaho, Moscow, Idaho, 1956, pp. 1-88.)

The above fact should be considered in light of the fact that the tensile and shear strengths of most rocks are only about 4 percent and 10 to 15 percent, respectively, of the rocks compressive strength. Because of this anistropic behavior of rock, the presence of minor tensile stress and moderate shear stress is far more important than compressive stress in determining the stability of an opening.

Tensile stress may not be present on the peri hery of all openings; but it will often be present at some period in the history of an opening in suflicient magnitude to cause tensile cracks if one of the principal stresses, which can be at any orientation, exceeds the other (assuming analysis of a two-dimensional cross-section) by a ratio of about 3 to l or more. Woodruif (ibid.) comments on such a situation as follows:

the maximum stress at the periphery of an opening is a compressive stress tangent to the periphery. This stress is located at the ends of the horizontal axis [where the vertical exceeds the horizontal stress by a sufiicient ratio] in the case of circles and ellipses, and at the corners in the case of rectangular openings.

Thus, physical failure of an opening takes place because of excessive induced stresses; these may be shear, compressive, or tensile. However, shear and tensile stress are the most critical if they are present in sufficient magnitude; this possibility is very likely in many situations and depends on the ratio of principal stresses which are acting on the rock adjacent to the opening. In a stress field where the horizontal stress equals the vertical stress, a compressive stress is present around the entire periphery. However, where only a uniaxial stress is present, a tensile stress concentration is created in a zone centered about the axis of the stress. (Obert, L., Duvall, W. I., and

3 Merrill, R. H., Design of Underground Openings in Competent Rock, US. Bureau of Mines Bulletin 587, 1960, pp. 1-17.)

Fundamental as it may appear, it should be empha sized that gravity acts on any rock in the overhead portion of an opening which has been loosened or displaced by tensile fracturing and/ or shearing. The material overhead, therefore, constitutes the greatest potential safety hazard. Walls and ribs may crush, slab oif, slough and even allow the back (roof) to converge on the opening. Floors may crack and some times heave. But, troublesome as these failures may be, they cannot compare with sudden uncontrolled roof failures. In summary, therefore, the most troublesome features attending ground failure are low tensile strength of rock and roof or back failures.

Any opening in a medium under stress creates induced stresses by its very existence. Nothing can be done to prevent induced stresses (compressive, tensile, and shear stress concentrations) from appearing at the periphery. Rather, there are three alternative approaches:

(a) Artificial supports can be used to hold broken rock in place after it has cracked due to excessive induced stresses. This has been, and still is, the most widely used solution in mining. It requires little knowledge of stressstrain phenomena because each particular solution is mainly a question of economics.

(b) A second approach is to create induced stresses which are beneficial (or least destructive) to the stability of the opening by planning the location, shape, size and orientation of the opening. Most of the emphasis in the engineering field of rock mechanics to date has been toward this type of solution.

A third approach has scarcely been touched upon in research or practical work thus far; that is, to prevent or change undesirable induced stresses by use of some mechanical device. This method is most analogous to prestressing concrete structures, a technique that is widely applied in the construction industry. (Lin, T. Y., Prestressed Concrete Structures, John Wiley and Sons, Inc., New York, N.Y., 1955, pp. 1-79.) It would supplement the second approach and have the marked advantage of not having to make the mining sequence or plan tailored to the openings. Often there is no choice as to where an opening should be located, or how it should be oriented; and the desirable shape structurally may be undesirable from a production viewpoint. It is the third approach that is the subject of this invention.

The concept of modifying induced stresses mechanically so that their effect on the stability of an opening is minimized was reported by J. I. Reed during the 1956 Symposium on Rock Mechanics. (Mine Openings Stabilization by Stress Redistribution, Quarterly of the C010- rado School of Mines, July 1956, pp. 65-97.) What Reed did was to reduce high compressive tangential stresses and resultant high shear stresses at the periphery of an opening in an isostatic stress field by mechanically inducing compressive stresses some distance from the openmg.

The greatest number of rock failures, however, are encountered in openings located in a nearly uniaxial stress field 'where tensile stresses or strains tend to cause fracturing and subsequent displacement of pieces initiates failure of the overhead supporting rock structure. Thus, in most instances, the need is to counteract tensile stresses or strains by increasing, as opposed to reducing, compressive stress (rock confinement) sufficiently to enable natural structures such as linear arches or Voussoir arches to develop the shear stresses such that they are capable of resisting. (Woodruff, Seth D., Methods of Working Coal and Metal Mines, vol. 1, Pergamon Press, London, 1966, pp. 282-287.)

The present invention is an extension of the above concept. It was reduced to practice and successfully proved by the inventor while he attended graduate school.

4 (Whiting, Jerry M., counterstressing Rock for Ground Support, Ph.D. Thesis, Stanford University, Department of Mineral Engineering, June 1968.)

SUMMARY OF THE INVENTION As described below, rock bolts create a moderate compressive force (i.e. 10,000 lb.) normal to the rock surface which helps hold rock units in position. A new method has been found, however, in which a high compressive force (i.e. 150,000 lb. or more) is transmitted tangential to the rock surface in those portions of the opening, particularly in the roof or back, which are subjected to induced tensile stress or strain. Such a method is designated as counterstressing in situ rock, and accomplishes one or more of the following useful results:

(a) Absorbs induced tensile stress, thus delaying or eliminating tensile fracturing and consequent reduction of the particle-to-particle fraction which maintains shear strength.

(b) Increases the effective shear strength of the rock by increasing the lateral confinement of the rock.

(0) Increase the lateral compressive stress components in a linear arch or Voussoir arch by pressing together the proximal blocks of rock comprising such a structure and maintaining sufiicient lateral thrust on abutment regions to hold such a structure in place.

(d) Preserves the structural integrity of the rock by forcing every unit tightly together into a single structural unit.

(e) Eliminates or reduces the amount of rigid supports such as sets, props, and the like, that are used, or increases spans and recovery.

One practical device for afiecting counterstressing of in situ rock is a rock jack. A rock jack is simply an inflatable, fluid pressure maintaining, high strength sheet metal container which is inserted into a drill hole or prepared slot in the rock. Any air space between the jacks faces and the rock is filled with relatively unyielding solid material such as hardened grout, cement, concrete, or epoxy resin. The jack is filled with similar material in a fluid, unhardened condition and is subsequently pressurized to the limit of its capability, somewhere between 3500 or 5000 p.s.i. or higher, depending upon its design.

During pressurization, the jack expands slightly while taking up normal yields of materials and cracks between proximal blocks of rock. The total compressive force transmitted to the rock is the product obtained by multiplying the area of the face of the jack by the contained pressure per unit area. For example, 2 /a-inch wide by 28-inch long jacks have been tested and found to exert a total force more than 150,000 lb. at 3500 lb. per sq. inch. A jacks effectiveness in accomplishing the results listed above is directly related to the total force it is capable of exerting and maintaining up to the limit of the surrounding rocks capacity to withstand such force. Thus, care is taken to have the jack filled with material which will undergo minimum shrinkage (and, hence, cause minimum pressure reduction upon hardening.

No method of ground support is a cure-all, including this invention. Experienced men must identify rock and stress conditions which indicate counterstressing is an appropriate technique. In general, those conditions will be similar to those where the now familiar method of rock bolting can be successfully applied, excluding thinly bedded strata which may buckle upon application of high tangential compressive force. Blocky, brittle rock, particularly where the blocks are sufiiciently large to encase entire rock bolts and, consequently, where there is some danger of apparently properly bolted rock masses falling out of place, seems to offer an ideal area of application. In most situations, it is envisioned that rock bolts and rock jacks would be used together to provide the best possible support for high value openings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings.

FIG. 1 illustrates the prior art rigid set method of supporting an underground opening.

FIGS. 2A and 2B illustrate the prior art yieldable set method of supporting an underground opening.

FIG-S. 3 and 4 illustrate the prior art method of supporting an underground opening by using rock bolts.

FIGS. 5, 6, and 7 illustrate the counterstressing or rock jack method of supporting an underground opening according to the present invention.

FIG. 8 shows side elevations of a rock jack of the type used by the inventor.

FIGS. 9A and 9B are drawings of the inlet and outlet ends of the rock jack shown in FIG. 8.

FIG. 10 is a schematic drawing illustrating the positioning and expansion of a rock jack in a drill hole.

FIGS. 11A and 11B are drawings of a frame to aid in forming rock jacks.

FIG. 12 is a drawing of a valve which allows rock jacks to be plugged while under pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Besides eliminating the opening altogether as is done by hydraulic or rock fill, there are only three general types of artificial supports when one stops to consider how all of the many adaptations function: (a) rigid, (b) yieldable, and (c) rock bolts.

The rigid method of support is illustrated in FIG. 1. The total elastic deformation of rock around subsurface opening 21 is so slight that a few small voids in the blocking absorb all of the initial movement before any appreciable load is transmitted to support means 22. During this slight movement of the rock, stress concentrations develop which exceed the structural capacity of the rock; the rock breaks; and it comes to bear against the beams and blocking. Therefore, rigid sets, stulls, posts, props, and similar supports only hold broken material in place. They do not utilize the compressive strength of the rock except by holding the broken pieces in position so that a few pieces may bear against one another and form a sort of loose linear arch or Voussoir arch as indicated by the force lines.

Rigid supports are only economical or effective where the burden of broken material does not exceed the strength of the support. Thus, it can be stated that they generally support the static load of a very limited amount of overlying material.

Yieldable supports 23, such as depicted in FIGS. 2A and 2B, perform a very specialized function. When opening 24 is created in rock 25 which is subjected to high magnitude, nearly isostatic stress conditions, high tangential compressive stresses develop at its periphery. 'If the high shear stresses which result exceed the rocks shear strength, the rock fractures at the edge of the opening and progressive breakdown is initiated. Yieldable supports 23 comprised of wood lagging 26 and sectional steel arches 27 resist displacement of the fractured rock and provide a radial confining stress as the fracturing expands into opening 24. This radial confining pressure helps maintain a natural rock arch around opening 24 and reduces the distortion of rock 25 beyond the plastic radius R. Therefore, a shear stress is maintained within the unfractured rock some distance from the opening which is below the rocks strength.

A pertinent point is that rigid sets (FIG. 1) would have the same effect. However, the enormous force which can be caused by the convergence of the broken rock causes early buckling and consequent loss of rigid supports. Yieldable supports (FIGS. 2A and 2B) are designed to relieve their load when a maximum load is reached (when properly installed) and thus they last longer. Often convergence of the rock stops before the sets total yielding capacity is used up.

Referring to FIGS. 3 and 4, rock bolts 28 perform still a third distinct function. (Lang, T. A., Theory and Practice of Rock Bolting, AIME Transaction, vol. 220, 1961, pp. 333-348.) Bolts 28 are inserted into drill holes and expanded to tie many small units together into a single stronger unit, FIG. 3, or hold structural units of rock in place, FIG. 4. Thus, in FIG. 3, laminated beams of overlying strata 29 are formed over opening 30 and, in FIG. 4, linear arches of fractured or jointed blocks of rock 31 are formed over opening 32. Therefore, material which would become further disarranged and fall out of place, or have to be supported by sets, props, etc., is maintained in position by forces acting perpendicular to openings 30 and 32.

The success of yieldable supports (FIGS. 2A and 2B) and rock bolts (FIGS. 3 and 4) in holding ground previously requiring much more extensive and expensive methods results from the unique way these supports function. Both enable the rock to help support itself. Thus, they carry only a portion of the load that would ultimately bear upon a conventional rigid support system (FIG. 1). Of these prior art techniques, rock bolting (FIGS. 3 and 4) best utilizes the strength of rock. A rock bolt, though, provides a rather limited force, which is directed normal to the rock surface, i.e. parallel to the axes of each rock bolt, to hold rock units together, and even the slightest slippage of its anchorage often sharply reduces or eliminates its support capability.

The method of the present invention is illustrated in FIGS. 5, 6, and 7. Inflatable devices 33, referred to hereafter as rock jacks or fiat jacks, are inserted into holes or slots 34 drilled in rock 35 about the periphery of underground openings 36, 37, and 38. Rock jacks 33 are inserted radially into holes 3 4 in the roofs of mine openings 35, 36, and 37, primarily in the overhead quadrants thereof. They are spaced at sufiiciently close intervals to create a region of compression which is suflicient to counteract the tensile stress in rock 35. When jacks 33 are pressurized, high compressive forces are transmitted to the rock in a direction generally lateral or tangential to the surface (face) of openings 36, 37, and 38, as indicated by the lines of force. The result is that proximal blocks of rock are pressed tightly together, increasing and/ or maintaining the frictional contacts between rock particles so as to increase the rocks capability for resisting structural breakdown. This counterstresses the rock and greatly improves stability.

A predetermined tangential compressive force such as may be desired can be achieved by increasing the pressure within the jacks, making the jacks longer and/or wider, or by increasing the number of jacks. A rule of thumb is to arrange multiple jacks so that their spacing is equal to less than about three times and, preferably, less than about twice the length of one of the jacks. (Twodimensional photoelastic model studies have shown that the loss in compressive stress one jack lengths distance from the center of the jack is about 36 percent.) Desirably, the rock jacks should be at least two or three feet long and should be set 90 to the edge of the opening. Typically, they are pressurized to 35005000 p.s.i. (Maximum pressures should be used in order to achieve economy in the number of jacks.)

Rock jacks of the type shown in FIG. 8 are easily made by flattening seamless metal tubing 39 and brazing end plugs 40 and 41 into position. A 2 /s-inch by 28-inch rock jack may be manufactured in the following manner:

Sixteen gage, 1 /2-inch diameter, seamless steel tubing 39 is cut to length. Inlet end 40, FIG. 9A, is machined from steel to slip snugly into tube 39. It is provided with an external Ai-inch pipe thread and its center is tapped and threaded for A-inch pipe plug 42. By slipping inlet end unit 40 past the end of tube 39 for /2 to inch, a well is formed for brazing a seal. Outlet end 41, FIG. 9B, is a simple cylindrical plug which is machined from steel to slip snugly into the opposite end of tube 39. A

fillet is cut on the external outside diameter to provide means for forming a strong brazed seal to the tube. The center is tapped and threaded for As-inch pipe plug 43. Tube 39 thus fabricated, with inlet and outlet ends brazed into place, is heated to about 1600 F. When tube 39 is uniformly hot, it is flattened on two opposing sides to a thickness of inch, resulting in the configuration shown in FIG. 8.

A simple frame, FIGS. 11A and 11B, is used to ensure uniformity and ease in the flattening process. The frame consists of a 3% -inch by 26-inch piece of /2-inch thick steel plate 44 which has been ground off to a large radius curve on the upper side of each end. A l-inch by 3 /z-inch angle 45 is welded upright at each corner of plate 44. The clearance L between the angles slightly exceeds the diameter of tube 39 so that it can be dropped quickly into place. Thereafter, a matching /2-inch steel plate 46 is dropped into position over tube 39. The assembly is placed between the jaws of a suitable fast acting press (not shown) and the upper plate is compressed to where it contacts %-inch spacer bar 47.

The resulting rock jack, FIG. 8, has parallel faces about 2% inches wide and 24 inches long, an area of 57 sq. In.

One convenient method of using the rock jack, FIG. 10, is to first encase the entire jack, except the %-inch pipe threads of inlet end 40, FIG. 9A, with high strength concrete (sand and cement). A 2%-inch diameter cylindrical mold can be used for this purpose. This step would not be necessary, however, if the jack and drill hole already have approximately the same cross sections. In the latter case, the jack is simply inserted directly into the drill hole without the use of filler material. (Hence, the advantage of drilling holes in the form of slots.) Referring to FIG. 10, 2 /s-inch diameter holes 48 are bored at selected locations in the overhead portion of an underground opening to a depth of 28 inches or more. The rock jacks, sandwiched in a concrete cylinder 49, are inserted into holes 48. A valve is then attached which allows the jack to be filled, pressurized, and plugged at peak pressure.

One such valve, developed by St. Joseph Lead Com pany, is shown in FIG. 12. The valve receives inlet end 40 of the rock jack and provides a means of forcing pipe plug 42 into the inlet while maintaining the full pressure of the pump. Rod 50 holds pipe plug 42 in jaw means 51. Grout, epoxy resin, or other fluid is introduced via port 52 and forced into the rock jack while port 53 provides an outlet for flushing. When the jack is pressurized to the desired level, lever 54 is actuated to force plug 42 into contact with inlet end 40. Rod 50 is then turned with a wrench to seat plug 42. The valve may then be removed from the pressurized rock jack and attached to the next jack to be pressurized.

When the jack is thus filled and the outlet ends tightly plugged, the assembly can be slid into drill hole 48, FIG. 10, and its faces oriented at right angles to the desired direction of compressive force. Very slight pressurization causes the jack to expand slightly and holds the assembly tightly in position. Once in the proper position, the jack is pressurized to the limit of its capability, i.e. 3500 to 5000 p.s.i., producing compressive forces normal to the face of the jack as shown in FIG. 10.

The total force exerted by the rock jack is determined by the area of its faces and the pressure maintained upon solidification of its grout or epoxy resin contents. For example, a jack with a face area of 57 sq. in., pressurized to 3500 p.s.i., with a 20 percent loss of pressure upon hardening and shrinking of its contents, would exert 57 3500 0.80=159,600 lbs.

The forces obtainable can be varied by controlling the the jack length, width (diameter), pressure capability, and percentage loss of pressure due to shrinkage of filler material. Relative to the latter factor, water can be used for medium term installations with no shrinkage problems. For permanent installations, care should be taken in 8 choosing a filler material such as neat cement or epoxy resin, and particularly in filling the jack to avoid air bubbles, so that shrinkage and pressure loss is minimized.

As the rock jack is pressurized, compressive stresses are created in adjacent rock which tend to counteract deleterious tensile stresses or increase lateral confinement and thus the contact between proximal rock blocks or particles. Rock shear strength is determined by the frictional stresses developed between rock particles as shear stresses tend to move these particles in opposite parallel directions. Frictional stresses are proportional to the magnitude of stresses tending to press the rock particles directly together. Thus, the confining compressive normal stresses created by rock jacks tend to maintain or increase the shear resistance of the natural rock structure in overhead portions of underground openings which is supporting its own weight plus possibly some amount of overlying material.

This explains why the roofs of many underground openings will continue to remain in place even after fractures have developed. This phenomenon is attributed to the development of a linear arch (or, as some prefer, a Voussoir arch).

Note that a linear arch of fractured rock is held in place by the lateral components which are created as the arch deflects or sags across the open span. If these lateral components are too low, one or more blocks of rock will shear out of place from their own weight and the arch will collapse. Rigid supports such as sets, linings and stulls limit the displacement of pieces in the arch by directly opposing gravitational forces. Prior art devices, such as rock bolts, tie pieces of the arch together and tend to prevent dislocations which drastically reduce the archs natural strength. Rock jacks, on the other hand, perform an additional vital function: that of substantially increasing the lateral (tangential) compressive stress component which forces the blocks of the arch together into a stabile structural unit and presses this unit against the abutments. FIGS. 5, 6, and 7, as already noted, illustrate typical underground openings which can be counterstressed by rock jacks to increase their stability and are referred to by way of summary.

What is claimed is:

1. A method for improving the stability of underground openings by counterstressing in situ rock comprising drilling a hole normal to the rock face of an opening in the upper quadrant thereof, inserting a rock jack into said hole, filling said jack with a suitable fluid, and pressurizing said jack to a predetermined level, thereby creating a compressive tangential stress in the rock about said opening.

2. A method for improving the stability of underground openings by counterstressing in situ rock comprising drilling a plurality of holes in a plane normal to the rock face of an opening in the roof thereof, inserting a rock jack into one of said holes, filling said jack with a suitable fluid, and pressurizing said jack to a predetermined level, and repeating said procedure for each of the remaining holes, thereby creating a compressive tangential stress throughout the rock structure enclosing said opening.

3. A method for improving the stability of underground openings by counterstressing in situ rock comprising drilling holes of desire-d depth and orientation into the rock face of an opening, especially the overhead portion, inserting a rock jack into each said hole, adding solid filler material to take up any space between the jack faces and the rock, and filling the jacks with a suitable fluid and pressurizing them to a predetermined limit, thus transmitting high compressive force to proximal blocks of rocks.

4. A method for improving the stability of underground openings by counterstressing in situ rock comprising drilling at least one hole in the overhead face of the opening which is to be stabilized, inserting a flattened metal tubular member into the hole, adding material to opening.

fill the air space between the tubular member and the bore of the hole, and pressurizing the tubular member With fluid so that it expands in volume over substantially all its longitudinal length, thereby transmitting significant compressive forces tangential around the faces of the References Cited UNITED Howlett 6145 McLean 6145X Turzillo 6l35 Munn 6145 Casagrande 61-35 Lamberton 6145X Lamberton 61-35 US. Cl. X.R.

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