U.S. patent number 3,837,258 [Application Number 05/268,839] was granted by the patent office on 1974-09-24 for rock bolts.
Invention is credited to Chester I. Williams.
United States Patent |
3,837,258 |
Williams |
September 24, 1974 |
ROCK BOLTS
Abstract
A method of reinforcing a rock formation with rock bolts
establishing a locked-in pre-stress condition inhibiting the
initiation of movement of the formation, and structural features of
a rock bolt assembly capable of use in the practice of the
method.
Inventors: |
Williams; Chester I. (Grand
Rapids, MI) |
Family
ID: |
26678098 |
Appl.
No.: |
05/268,839 |
Filed: |
July 3, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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8345 |
Feb 3, 1970 |
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Current U.S.
Class: |
411/44;
405/259.3; 405/259.5 |
Current CPC
Class: |
E21D
20/00 (20130101) |
Current International
Class: |
E21D
20/00 (20060101); F16b 013/04 () |
Field of
Search: |
;61/45B,63
;85/75,77,76,66,84,85 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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239,150 |
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Jun 1962 |
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AU |
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737,207 |
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Jun 1966 |
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CA |
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1,005,474 |
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Apr 1957 |
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DT |
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Primary Examiner: Taylor; Dennis L.
Attorney, Agent or Firm: Morse; Glenn B.
Parent Case Text
CROSS-REFERENCE
This application is a continuation in part of U.S. Pat. application
Ser. No. 8,345, filed Feb. 3, 1970.
Claims
I claim:
1. In combination with a rock bolt having a threaded end portion,
an improved anchor device including a cone member in threaded
engagement with said end portion, an expansible shell surrounding
said cone member and having an inner conical surface parallel to
the exterior surface of said cone member under all conditions of
expansion of said shell, and abutment means normally axially fixed
with respect to said end portion and engageable with the end of
said shell to confine said shell between said cone member and said
abutment means; the improvement being characterized in that the
exterior surface of said shell has a plurality of frustro-conical
surface portions all of which slope uniformly inwardly at an angle
not greater than 10.degree. relative to the longitudinal axis of
said rock bolt toward the end of said shell closest to the outer
end of said rock bolt, the radially outermost ends of said
frustro-conical surfaces being of equal radial distance from said
longitudinal axis, the arrangement being such that, upon axial
movement of said cone member toward said abutment means, said shell
is forced radially outwardly uniformly along the axial length
thereof bringing the exterior surface of said shell into engagement
with the walls of a bolt hole, said frustro-conical portions
applying compressive force to the adjacent portions of said bolt
hole wall along lines normal to said frustro-conical surfaces.
Description
BACKGROUND OF THE INVENTION
It is generally recognized that a stratified or fractured rock
formation can be held in place by the use of bolts extending from
the surface inward to a sufficient depth to suit the requirements
of the particular conditions. Bolts of 40 feet in length are not
uncommon. It has also been established that rock laminae can be
locked together with bolts to function as a self-supporting beam to
form the roof of tunnels or other excavations. It is common
practice to provide anchor devices at the inner ends of the bolts
so that some degree of tension can be developed between that point
and a bearing plate placed over the surface opening of a hole in
which the bolt is installed. These bolts are frequently given a
protective covering and a bonding to the rock by injecting grout in
the hole around the bolt rod.
A peculiarity of materials under stress has presented a problem in
the design and installation of these rock bolts. It is fundamental
that a bolt rod must be stretched in order to develop tension. It
follows that a bolt installed without some degree of pre-stress
will exhibit no restraining power on the rock formation until the
formation actually begins to move. This is precisely the condition
that the bolt is installed to prevent. It is not only desirable to
preserve the solidity of the rock formation insofar as is possible,
but it is also desirable to obtain the benefits of the well-known
"stick-slip" friction characteristics which result in making it
much easier to hold a movable object in place prior to actual
commencement of movement than it is afterward.
Prior anchoring devices have generally proven inadequate to sustain
the full load capability of the bolt rod, so that reliance had to
be made on a bond between the bolt and the rock formation through
the surrounding grout in order to develop the full load capability
of any particular rock bolt installation. As long as reliance upon
the grout was necessary, it is clearly out of the question to
develop more than a relatively minor degree of pre-stress. Rock
bolt installations were developed around these limitations. Once
the grout had "set," further pre-stress could not be considered due
to the fact that the bolt became locked to the rock formation along
its full length. It was therefore incapable of any further
stretching, which would be necessary for any increase in stress. A
further problem associated with conventional anchor devices is the
general impossibility of grounting the anchor along with the bolt
rod. This is due to the fact that these devices did not adequately
provide for flow of the grout along the full length of the bolt
assembly, including the anchor, nor for a passage of the grout
along beside the anchor. Either a hollow bolt rod or a separate
conduit must be provided extending over the full length of the
assembly, or it is obvious that the grout cannot be deposited
beyond the end of the bolt assembly with any degree certainty. It
is also obvious that these conditions render adequate inspection of
a completed bolt assembly, particularly with regard to the
completion of the grouting, almost impossible. In summary, prior
rock bolt assemblies have not been designed such that the adequacy
of the installation is immediately evident to an inspector whose
observations are necessarily confined to the surface area of the
installation.
SUMMARY OF THE INVENTION
The present invention provides a method which can preserve the
initial placement of a rock formation by applying pre-stress to an
intensity corresponding to a working load of the rock bolt rod,
which is related to the yield strength of the rod material, thus
utilizing the primary strength characteristic of the bolt that
would not involve displacement of the rock formation. The
components of a rock bolt assembly are adapted to permit this
degree of pre-stress, and also permit the injection of grout to the
point of full recirculation within the bolt hole throughout the
length of the bolt assembly, including the anchor device. The bolt
is also adapted to be disengaged from the hole in the formation
prior to grouting, if it is discovered during the setting of the
anchor device that the rock formation is too soft or fractured to
sustain the necessary intensity of set. This latter feature is
obtained by utilizing a rotatively set anchor in combination with
rod-section couplings having differential disengagement torque.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional elevation showing the installation of a rock
bolt assembly in a hole in a rock formation inclined upwardly from
the entrance.
FIG. 2 is a sectional elevation on an enlarged scale over that of
FIG. 1, showing the un-expanded condition of an anchor device.
FIG. 3 is a view of the mechanism shown in FIG. 2, in the expanded
condition.
FIG. 4 is a transverse section on the plane 4--4 of FIG. 2, on an
enlarged scale.
FIG. 5 is a schematic illustration showing a typical installation
of a pattern of rock bolts to secure a rock formation over a
tunnel.
FIG. 6 is a perspective view of an anchor provided with a resilient
shim sleeve.
FIG. 7 is a perspective view of the shim sleeve shown in FIG. 6, on
an enlarged scale.
FIG. 8 is a fragmentary axial section through an an expansible
shell with one form of a peripheral concavity.
FIG. 9 is an axial section of an expansible shell with a modified
form of peripheral concavity.
FIG. 10 is a diagram illustrating the stress condition in a rock
formation resulting from the installation of a pattern of rock
bolts.
FIG. 11 is a view of the surface components of a rock bolt
installation, and illustrating the application of grout.
FIG. 12 is a plan view of a surface bearing plate.
FIG. 13 is an exploded view showing a stopper for insertion in the
entrance of the bolt hole, in conjunction with a vent tube
insertable in the side passage in the stopper.
FIG. 14 is a plan view of a bevel washer used with the surface
components of the rock bolt assembly.
FIG. 15 is a side elevation of the washer shown in FIG. 14.
FIG. 16 is a perspective view of a form of bearing plate capable of
accommodation to large angles of deviation from a perpendicular
relationship between the plane of the surface of the rock formation
and the axis of the bolt rod.
FIG. 17 is a sectional elevation of a form of coupling uniting
sections of the bolt rod.
FIG. 18 is a sectional elevation showing a modified form of locking
arrangement for controlling back rotation of a coupling connection
uniting two rod sections.
FIG. 19 is a sectional elevation showing a further modification of
coupling arrangement uniting adjacent rod sections.
FIG. 20 illustrates a modified form of anchor assembly providing a
relatively greater shell area, and adapted to prevent pull-through
of the cone member in soft rock conditions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a typical installation of a rock bolt assembly
in a rock formation 30, which has been prepared by drilling a hole
31 from the surface 32 to a sufficient depth to involve the desired
amount of the rock formation in the securing effect of the rock
bolt installation. The rock bolt assembly includes the anchor
device 33, a bolt rod 34 (which may be in one piece, or a series of
axially interconnected sections), a surface plate 35, a bevel
washer 36, and a standard nut 37. FIG. 1 illustrates the condition
of the installation immediately after "grouting," in which a charge
of liquid cementitious material is injected into the hole 31
surrounding the bolt rod and the anchor device to provide a
protective sheathe around these components, and to bond them to the
rock formation throughout the length of the assembly. To facilitate
the grouting operation, a sealant packing 38 is jammed into the
entrance of the hole 31 around the bolt rod, and the bearing plate
35 is placed down over it. The flexible tube 39 traverses this
packing, and becomes a means for carrying the grout into the
hole.
Grout should always be injected at the low end of a hole, with
provision being made for the exhaust of the entrapped air as the
charge of grout advances. Since the hole is inclined upwardly from
the entrance in the FIG. 1 installation, grout is injected through
the tube 39, which represents the low end of the hole in this form
of installation. As the grout progressively fills the hole, air is
withdrawn through the conduit formed by the hollow interior of the
bolt rod 34. The injection of the grout continues through the tube
39 until the hole is completely filled, which is indicated by
movement of grout down through the bolt rod to the point where it
emerges at the surface, in the manner illustrated in FIG. 11. The
injection of the grout is accomplished by any conventional form of
grout pump, which has a delivery tube 40 provided with a convenient
adapter 41 for receiving the tube 39. A pump of the type described
in my U.S. Pat. No. 3,227,426 is recommended. The stream of grout
42 emerging from the end of the bolt rod 34 provides a positive
indication that the hole 31 has been completely filled. After the
grouting operation has been completed, the hole in the bolt rod 34
from which the grout is shown emerging in FIG. 11, together with
the tube 39, are plugged as shown in FIG. 1 to sustain at least
some degree of grout pressure within the hole 31 until a complete
set has taken place. After this has occurred, the tube 39 has no
further use. The diameters of the holes in the bolt rod 34 and in
the tube 39 are of the order of a quarter of an inch, or larger
(depending on rod diameter) and it has been found that plugs in the
general shape of golf tees have been very effective and easy to
handle in performing this sealing operation. These can be removed,
if desired, after the grout has set. If the hole 31 were inclined
downwardly from the entrance, the flow of grout would be reversed.
It would be injected through the bolt rod, since that would form a
conduit leading to the lower extremity of the hole in the rock
formation. Emergence of the grout through the tube 39 would then
give the indication of a completed grouting operation. In either
case, it will be noted that this completion is assured by at least
a momentary complete recirculation of grout within the hole 31
throughout the length of the rock bolt assembly. Some forms of rock
bolt assemblies utilize a solid rod, rather than the hollow form
illustrated in the drawings. When the solid rod is used, a side
tube of the general nature of the tube 39 is lashed in some
convenient form to the bolt rod, and preferably extends over the
full length of the entire rock bolt assembly. One advantage to the
use of the hollow bolt rod is the freedom of the assembly from
displacement of the full-length tube which is likely to occur as
the anchor device 31 is "set" by rotation of the rod from the
surface.
The securing of the anchor device 33 is illustrated in FIGS. 2 and
3. The components of the anchor device are shown installed on the
threaded end 43 of the rod section 34. A cone member 44 has a
threaded central opening extending throughout its length, and is
normally in threaded engagement with the rod section 43. A thrust
ring 45 is also in threaded engagement with the rod section 43, and
is disposed at the extremity of the threaded portion of this
section. The expansible shell 46 surrounds the cone member 44, and
is axially interposed between the cone and the thrust ring 45.
Relative rotation between the rod section 34 and the cone 44 will
result in movement of the cone 44 toward the axially-fixed thrust
ring 45, resulting in the expansion of the shell 46. This shell is
"C" -shaped in cross-section, as best shown in FIG. 4. To assure
that the cone does not rotate within the shell, the cone is
provided with a key ridge 47 located within the discontinuity 48 of
the "C"-shaped cross-section of the shell 46. This discontinuity is
provided to facilitate the expansion of the shell, and works in
conjunction with the point of weakness established by the axial
groove 49 on the opposite side of the shell. This portion of the
shell functions somewhat in the manner of a hinge as the cone 44
advances to the left, as shown in FIGS. 2 and 3. Fracturing takes
place initially in the shell adjacent the notched area 49, as the
cone proceeds from the FIG. 2 to the FIG. 3 position. The setting
of the anchor must proceed to the point where the shell is jammed
solidly against the wall of the hole 31 with a sufficient intensity
of force to permit the anchor assembly to resist the full working
load of the bolt rod 34, which corresponds preferably to 75 to 80
percent the yield strength of a selected steel rod commonly
anywhere from an inch to 2 inches or more in diameter.
While the initial setting operation is performed by rotation of the
rod 34, an examination of the configuration of the components of
the anchor assembly shown in FIG. 2 will make it obvious that a
subsequent movement of the rod to the left, once the shell 33 has
been solidly placed against the rock formation, will result in
further movement of the cone into the shell (accompanied by a
movement of the thrust ring 45 away from the adjacent end of the
shell as the axial rod movement takes place). It is nevertheless
preferable that the initial set of the anchor assembly by rotation
should be sufficient to resist all of the applied loading, where
this is practically obtainable. This degree of anchor set will
permit a quicker pre-stressing operation, which is accomplished by
progressively tightening the nut 37 to a predetermined torque to
produce a loading of preferably 75 to 80 percent of the yield
strength of the particular steel selected for the bolt rod 34. In
the larger rock bolts (above one inch in diameter), it becomes very
difficult to rotatively set the anchor to the full working load. A
setting torque of around 750 foot-pounds will seat the anchor to
the point that subsequent pre-stressing will increase the set of
the anchor by directly pulling the cone into the shell until the
necessary resistance is generated. The testing of an installed bolt
for pre-stress is then done with a hydraulic jack, rather than with
a torque guage, noting the loading that is applied without further
stretch of the bolt rod. These torque requirements are sufficiently
high to present a real problem in the axial securing of the thrust
ring 45. Because of the intensity of the forces involved, there is
a strong tendency to frictionally induce continued rotation of the
thrust ring with respect to the bolt rod during the setting
operation, and to thus strip out the threaded engagement between
the thrust ring 45 and the bolt rod. It is preferable to provide a
shoulder against which the thrust ring can advance, and also
provide a diameter-length relationship of the thrust ring such that
the threaded length is approximately equal to the diameter. This
relationship makes it possible to keep the diameter of the thrust
ring sufficiently low to be less than the unexpanded diameter of
the shell 46, and thus facilitate the insertion of the anchor
assembly into the hole 31, and eliminate any substantial
interference with the axial flow of grout to or from the end of the
anchor through the unclosed "C" area of the shell and over the
hardened slip washers and thrust rings. To further facilitate
relative rotation between the thrust ring 45 and the shell 46, it
is preferable to incorporate a pair of hardened slip rings 50-51,
which may be coated with oil or grease to minimize the torque
transfer between the shell and the thrust ring.
The proportions of the thrust ring specified above operate best in
conjunction with a particular slip relationship on the periphery of
the cone 44. The elements of the cone 44 are disposed at an angle
with respect to the axis of the rod 34 of less than 15.degree.,
with 10.degree. giving the best performance. The conical periphery
52 engages a similarly shaped surface on the inside of the shell
46, the walls of which are wedge-shaped in axial cross-section. The
fracturing of the shell as the expansion proceeds therefore
generates a group of wedges spaced around the periphery of the
conical surface 44 as the anchor takes hold of the walls of the
hole 31. During the entire expansion of the anchor, these wedge
sections move out in position parallel to the original un-expanded
condition, with at least the major portion of the periphery 53 of
the shell remaining parallel to the axis of the bolt rod 34.
The surface components of the bolt rod assembly are shown in FIGS.
12 to 15. The surface plate 35 has a "keyhole" shaped opening
including the central portion 54 sized to receive the bolt rod 34.
The lateral extension 55 from the central portion 54 is too small
to receive the bolt rod, and is provided for accommodating the
flexible tube 39. This arrangement prevents the rock bolt from
migrating to a position in which the flexible tube 39 is pinched
off. The lateral extent of the portion 55 of the opening is such
that it reaches beyond the diameter of the bevel washer 36 shown in
FIGS. 14 and 15. The tube is thus permitted to emerge from the
plate 35 at a position where it cannot be pinched off by the rod,
the bevel washer, or by the nut 37. This arrangement is shown and
claimed in my U.S. Pat. No. 3,234,732. Using a single washer 36,
the alignment of the components to accommodate a particular angular
relationship between the bolt axis and the surface of the rock
formation can only be approximate. By the use of a pair of these
washers, however, it becomes possible to adjust the assembly to an
exact angular relationship so that the bearing forces are
transmitted uniformly around the opening 54, rather than
exclusively at one point. The two washers constituting the pair can
be rotatively adjusted with their beveled faces interengaged so
that the slant between the two outer faces are then exactly in
conformity with the alignment of the bolt axis and that of the
plate pressed against the face of the rock formation. With the pair
maintained in this relative angular position, they can then be
rotated together to the correct position of the pair with respect
to the plate. The arrangement of a pair of these washers for
adjustment in this manner normally will require a slightly greater
clearance between the diameter of the bolt rod and the inside
diameter of the washers, as the washers assume a canted
relationship on the rod.
The stopper 56 shown in FIG. 13 can be used in place of the mass of
sealant packing 38 illustrated in FIG. 1. The stopper 56 functions
in the manner of a cork, and is provided with the small opening 57
extending axially within one side wall for receiving the tube 39.
The plug 56 is preferably of rubber or some material of similar
characteristics. Another possible variant in surface assembly is
illustrated in FIG. 16, in which a bearing plate 58 is used in
conjunction with a bolt rod 59, where the angular relationship is
greater than that which can readily be accommodated by the bevel
washer shown in FIGS. 14 and 15. The nut 60 (either directly, or
through a suitable heavy washer) bears against the inclined arcuate
flanges 61 for the transmission of bolt forces through the bearing
plate 59 over to the rock formation. Bearing plates of this type
are usually fabricated of relatively heavy malleable cast iron.
The inevitable variations in conditions in a rock formation which
may be expected in the installation of large numbers of rock bolts
are such that it frequently becomes necessary to disengage an
anchor that has only been partially set. This situation occurs when
the rock formation appears to be too soft to take the full
intensity of set of the anchor, or the anchor appears to have been
lodged in a fractured area that did not have sufficient density to
accommodate the necessary pressures. In such cases, it is desirable
to back-rotate the anchor assembly from the FIG. 3 to the FIG. 2
positions. Since it is common practice to make up the lengths of
relatively long bolt rods in a number of sections interconnected by
couplings, it is obvious that a problem arises the moment one
attempts to back-rotate the anchor assembly. To assure that all of
such back-rotation takes place at the anchor rather than at any one
of the couplings, the arrangements shown in FIGS. 17, 18, and 19
can be used. In FIG. 17, the threaded ends of the rod sections 61
and 62 terminate at such a point that these rod sections are
threaded into the coupling 63 to the maximum extent prior to the
interengagement of the ends of the rod sections 61 and 62. The
wedging action developed at the ends of the threaded portions of
the rod sections produce a jamming action when the coupling is
solidly tightened of a sufficient intensity to create a friction
resistance to back-rotation which is in excess of the back-rotation
torque required to unset the anchor assembly. A similar effect is
produced in quite a different way by the arrangement shown in FIG.
18, in which the rod sections 64 and 65 are interconnected by the
coupling 66. No attempt has been made in this arrangement to
control the length of threaded interengagement, but the lock washer
67 is interposed between the ends of the rod sections. As the
coupling is tightened down in the illustrated position, the
presence of the lock washer tends to inhibit back rotation such as
would loosen the coupling. In FIG. 19, an effect is produced which
is quite similar in principle to that illustrated in FIG. 17. In
this case, however, the rod sections 68 and 69 are threaded to any
convenient length in excess of half the length of the coupling 70,
and the coupling itself is provided with a discontinuous threading
leaving the central portion 71 with incomplete threads. Each of the
rod sections 68 and 69 is thus threaded in as far as it will go,
and the coupling is then given a severe tightening torque,
resulting in a binding action at the incomplete threads in the
center portion of the coupling. The central hole at the rod ends
should be bevelled (countersunk) to minimize flow resistance to
grout in all cases.
FIGS. 8 and 9 illustrate modified forms of the anchor shell which
facilitate the development of the frictional and pressure-centered
retaining forces characteristic of the anchor assembly described in
connection with FIGS. 2 and 3. It is common practice in the design
of anchors to utilize a saw-toothed exterior, with the generally
radial faces arranged to confront the pull-out forces operating
against the anchor. In other words, the anchor shell is expected to
act something along the line of a broach or file. Applicant has
discovered that this principle is less effective than the use of a
completely opposite orientation of the peripheral irregularities.
In other words, the pull-out strength of the anchor based only upon
the shear strength of the immediately surrounding rock formation is
likely to develop less retaining force than a high degree of
pressure exerted normally against the sloped surfaces 72-74 of the
shell 75 shown in FIG. 9. In FIG. 8, the shell 76 has an arcuate
depression 77. Preferably, both this depression and the
notched-shaped irregularities 72-74 of FIG. 9 are annular. In the
case of the sloped surfaces 72-74, it is preferable that these be
kept at around 10.degree. with respect to the axis of the bolt rod.
Correspondingly, tangents to the curved surface 77, particularly at
the inner (right-hand) extremity of the shell, should be at
approximately 10.degree. with respect to the axis of the bolt
rod.
FIGS. 6 and 7 illustrate arrangements that may be used when it
appears that a bolt hole has been drilled somewhat oversized with
respect to the anchor unit to be installed. The "C"-shaped shim
sleeve 78 has a discontinuity at 79 which permits circumferential
expansion along with the expansible shell 80 around which the
sleeve has been installed, as shown in FIG. 6. The anchor assembly
is the same as that illustrated in FIGS. 2 and 3, and includes the
cone 81 and the thrust ring 82 assembled to the bolt rod 83. The
shim sleeve may be provided with a random number of holes as shown
at 84 to facilitate the gripping action against the rock formation,
but this is entirely optional. Inwardly turned tabs as shown at 85
and 86 are provided at both ends for axial interengagement with the
ends of the shell 80 to locate the shim sleeve with respect to the
anchor assembly.
FIG. 20 illustrates a form of anchor assembly that can be used
particularly well in relatively soft rock formations where a
greater surface area on the shell is desirable. Comparison between
FIG. 20 and FIG. 1 will bring out the relatively greater axial
length of the shell in the FIG. 20 modification. The slope of the
peripheral surface of the cone 86 is received within the similarly
shaped interior surface of the shell 87, which is restrained
axially by the presence of the thrust ring 88. FIG. 20 illustrates
the use of a solid bolt rod 89, in which the axial positioning of
the thrust ring 88 is supplemented by the presence of the forged
flat 90. The cone 86 is provided with a shoulder 91 formed by a
cylindrical enlargement of the diameter of the cone at the point,
and this shoulder is discontinuous at the point 92. The reason for
this discontinuity is to allow for the flow of grout through an
area which might otherwise be obstructed by the presence of the
shoulder 91. In order to assure the flow of grout completely around
all surfaces of the cone which are exposed, it is preferable that
the key ridge 93 be disposed opposite the discontinuity 92, and
that the discontinuity be somewhat wider than the key ridge at that
point. To further facilitate the flow of grout, it is preferable to
provide the inclined surface 94 to define the end of the key ridge
93, rather than permitting this member to come to an abrupt
shoulder at that point. The height of the key ridge 93 then may be
considered to decrease to zero approaching the discontinuity
92.
FIG. 5 illustrates a typical installation of a series of rock bolts
to maintain the integrity of the rock formation over the upper
portion of a tunnel. The bolts assemblies 95-105 are all installed
in the rock formation in a manner similar to that shown in FIG. 1,
but in varying attitudes with respect to the horizontal. Grouting
techniques will vary accordingly, as described previously. The
pre-stressed installation of these bolts is capable of binding the
laminae of the rock formation 106 together to produce the effect of
an arch beam extending over the top of the tunnel in a manner shown
schematically in FIG. 10. In FIG. 19, a group of bolts are shown
installed along the dotted line axes which are generally radial to
the curvature of the tunnel, but may vary considerably from this
arrangement as suggested in FIG. 5. The inside surface of the
tunnel is indicated at 107, and each bolt establishes a compression
area from its point of application at the tunnel surface extending
in a conical pattern at approximately 45.degree. from the surface
plate. These compression areas are shown defined in dotted lines in
FIG. 10. At the point where the compression areas of adjacent bolts
overlap, the cross hatched area 108 is produced, in which all of
the area is under the compression established by the bolt pattern.
A similar condition exists at the opposite ends of the bolts. The
presence of the cross hatched area causes the rock formation to
function in the manner of a structural beam. The bolts not only
establish the necessary compression to lock the laminae together
for the transmission of shear forces, but the bolts also tend to
prevent sections of the rock formation from falling away from the
surface 107. This is further prevented, in most tunnel
installations, by securing wire mesh across the exposed rock bolt
ends, and securing it at the threading of the bolt rods outward of
the retaining nuts. This wire mesh can be supplemented by a layer
of plastering, if desired.
* * * * *