Note: Descriptions are shown in the official language in which they were submitted.
31~
BACKGROUND OF THE INVENTION
The present invention relates to stabilization of rock
structures for mining operations, and the like, and more
particularly to rock stabilizers of the type wherein an
elongated, hollow tube frictionally engages a bore hole in the
rock formation.
In underground mining and other tunneling operations it
is necessary to support the roof, and sometimes the walls, to
prevent rock falls or cave-ins. This is normally accomplished
in current practice by drilling holes into the rock structure
surrounding the tunnel at certain interva~s and anchoring an
elongated member such as a bolt, rod or tube in the hole to
hold a plate in supporting engagement with the rock face
surrounding the drill hole. The more common anchoring means
include mechanical expansion anchors, hardenable resin grouting
mixes, and frictional engagement of tubular structures with the
drill hole surface.
Among the latter types of anchors or stabilizers are
those disclosed in ~.S. Patents ~los. 3,349,567 of Munn,
3,922,867 and 4,012,913 of Scott, and 4,126,004 of Lindeboom.
The stabilizer disclosed by Munn comprises a hollow steel tube
of annular cross section having a bore hole into which it is
inserted; the tube is expanded into contact with the bore ho]e
wall at a number of discrete poin~s by magnetic fields induced
by high voltage electrical discharges from a wand inserted
into the tube. The stabilizers of the Scott and Lindeboom
patents are all hollow tubes of annular cross section larg~r
than the bore hole; the tubes are forcibly driven into the hole,
thereby undergoing circumferential compression and plastic
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~-~ d/eb -1- . ,
deformatior~, to frictionally engage the bore hole wall along
the entire inserted length.
Hollow tube stabilizers such as that of the l~unn
patent are in frictional contact with the bore hole wall at only
a few discrete points, rather than continuously along their
entire length and are thus of limited holding force. The Scott
& Lindeboom stabilizers, while providing continuous contact and
holding force along their entire length, must be forcibly driven
into the hole. Therefore, caution must be exercised to avoid
bending, buckling or other structural damage to the tube as it
is contacted with a suitable driving tool, especially in the
early stages of insertion. Furthermore, although pneumatic
impact tools of sufficient driving force to effect insertion of
the tubes are generally available in hard rock, metal mines, such
is not often the case in coal mines which normally employ
hydraulic equipment for installation of expansion anchor or
resin grouted systems. As insertion progresses, the force
required ~o drive the tube further into the hole will increase
due to the larger area of frictional contact between the tube
and hole walls.
It is a principal object of the present invention to
provide a tubular rock stabilizer which is placed in frictional
engagement with the interior of a bore hole without encountering
high frictional forces upon tube insert~on.
Ano~her object is to provide a tubular stabilizer which
is initially of smaller cross section than a bore hole in the
rock structure, thereby allowing uninterrupted insertion r and
which is thereafter expanded to engage the bore hole wall along
the entire inserted length of the tube.
A further object is to provide a hollow tube stabilizer
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which may be placed in fri.ctional contact with the wall of a
bore hole in a rock structure, after being subjected to elastic
and plastic deformation, with less driving force than similar
tubular anchors presently in use.
Additional objects are: to provide a hollow metal tube
stabilizer for frictional engagement with a bore hole in a
rock formation which exerts a greater holding force per pound of
metal (and therefore per unit cost) than prior art designs;
to provide a hollow tube rock stabilizer which will function
better than prior art designs in bore holes which vary in
dlameter from end to endt particularly those which taper to a
larger diameter at the upper end; to provide a hollow tube
stabilizer which may be increased in its frictional engagement
with a bore hole in a rock structure at any time subsequent
to initial installation; to provide a hollow tube, frictional
contact, rock formation stabilizer which is more adaptable to
use in longer sections than prior art designs; to provide a
hollow tube stabilizer which is initially under tension when
installed; and, to provide a hollow metal. tube rock stabilizer
wherein the entire axial strength of the tube is utilized.
In a more general sense, the object is to provide
novel and improved rock stabilizers of the hollow tube type
which frictionall~ engage the wall of a bore hole in a rock
structure.
: Other objects will in part be obvious and will in part
appear hereina~ter
SUMMARY OF THE INVENTION
Broadly speaking, therefore, the present invention may
be considered as providing a method of stabilizing and suppor-ting
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a rock formation comprising the steps of: (a) boring a hole
into a surface of the rock formation to a predetermined
nominal diameter; (b) fabricating a hollow, elongated, tubular
stabilizer having an insertion portion extending for at least
a substantial ~ortion of its length, the insertion portion
having a maximum external transverse dimension not greater
than the nominal diameter and comprising a wall extending for
substantially the full length thereof and adapted for radially
outward elastic and plastic deformation; (c~ placing the
insertlon portion in the hole; and (d). moving a mandrel through
the insertion portion to effect the deformation of the wall
radially outwardly throughout the axial extent of the wall
through which the mandrel is moved, thereby placing the wall
in frictional, non-piercing engagement with the interior of
the wall to provide an anchoring force on the rock formation.
The above method may be carried out by utilizing
stabilizer means for installation in a bore hole of predeter-
mined nominal diameter in unconsolida-ted underground rock strata
to prevent separation and falling of portions thereof, the
means comprising: (a) an elongated, hollow, body member having
an insertion portion extending for at least a substantia~
portion of its length, the insertion portion having a maximum
transverse dimension not signi.ficantly greater than the pre-
determined diameter, whereby the insertion portion may be
inserted into the bore hole with little or no axial force;
(b) the insertion portion comprising a non-perforated wall
extending for substantially the full length thereoE and adapted.
for radially outward elastic and plastic deformation; and
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3~'~3
(c) mandrel means movable through the insertion portion of
the body member to effect the deformation of the wall thereof
radially outwardly about substantially its entire perimeter
throughout the axial extent of the wall through which the
mandrel means is moved, whereby the strata is stabilized
solely by frictional, non-piercing engagement by the exterior
surface of the wall with the interior of the bore hole.
BRIEF DESCRIPTION OF THE DRAWINGS
.
Figure 1 is an elevational view of a first embodiment
of the tubular stabilizer of the invention;
Figures 2 and 3 are bottom and top plan views,
respectively, of the stabilizer of Figure l;
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Figure 4 is an elevational view o~ the stabilizer of
Figure 1 in axial section on the line 4-4 thereof, shown
inserted in a bore hole in a rock structure, also shown in
section prior to expansion of the tube;
Figure 5 is a sectional view on the line 5-5 of Figure 4;
Figure 6 is an elevational view in section, as in
Figure 4, after expansion of the tube;
Figure 7 is a sectional view on the line 7-7 of Figure 6;
Figure 8 is an elevational view of a second embodiment
of the stabilizer, also shown in section within a bore hoie
with the mandrel partially inserted;
Figure 9 is a sectional view of the stabilizer of
Figure 8, taken on the line 9-9 thereof;
Figure 10 is an elevational view showing an alternate
type of expansion device; and
Figures 11-13 are a series of cross sectional plan
views of a modification of the stabilizer of Figures 1-7 shown
prior to, during and after expansion within a bore hole.
DETAI~,~D DESCRIPTION
.......... ........ .
Referring now the drawings, the reference numeral 10
denotes the hollow, tubular body portion of the stabilizer
embodiment of Figures 1-5. Body 10 is constructed from hollow,
metal tube stock which, in its conventional form, is fabricated
in standard lengths by bending flat stock to circular cross
section and welding the edges together. A suitable material -
for use in fabricating the stabilizer of the present invention
is 1020 CRS electric welded tubing having an outside diameter of
1.50", wall thickness of .068" and ultimate tensile strength of
. . ~ .
sd/ ~
60,000 to 70,000 psi by pull test.
In forming body 10 the tubing outer wall is indented,
as by a pressure roll, along axial lines at four equally spaced
intervals about its periphery for the entire fabricated length
oE the tubing. The inwardly indented or folded areas are
denoted generally be reference numeral 12. Although steel
tubing may be fabricated in virtually any desired length, and
body 10 of the stabili~er may be provided in various lengths,
in the usual case it is more convenient to fabricate the
tubing in lengths considerably longer than the desired length of
body 10. For example, the tubing may be fabricated in 30 foot
lengths and cut, preferably after the axial folds are made, to
lengths of 5 or 6 feet.
After the tubing is folded and cut to the desired final
length of body 10, one end thereof is expanded outwardly to the
original circular cross section of the tubing and the other end
is crimped or tapered to a smaller dimension, for reasons
explained later~ The open and crimped ends are respectively
denoted by reference numerals 14 and 16. Ends 14 and 16 are
hereafter termed the lower and upper ends, respectively, of
body 10 sinces these are the positions they assume when
inserted upwardly into a drill hole in the intended manner.
Lower end 14 may be expanded by forced insertion of a mandrel
of appropriate dimensions.
Steel ring 18, having an inside diameter, substantially
equal to the outside diameter of end 14 in its original or
expanded state, is placed around end 14 and securely fastened
thereto, as by spot welding at a number of points. Ring 18 may
be a continuous circle or formed from flat stock bent to
sd~ 6-
circular configuration without welding or otherwise joining
the abutting ends. It may be placed around end 14 after the
latter has been expanded to its original circular cross section
but is preferably positioned around end 14 prior to expansion
so that the outer surface of end 1~ is expanded into tight
engagement with the inner surface of the ring prior to welding
the two together.
After the expanding mandrel is withdrawn a substantially
smaller mandrel 20 is inserted into end 14 and forced a short
distance into the folded portion of body 10~ As seen in
Flgures 4-6, mandrel ~0 is in the form of a plug of circulcar
cross section which tapers from a first diameter at the end
facing lower end 1~ of the stabilizer to a second, smaller
diameter at the other end. Mandrel 20 is retained within body
10 near lower end 14, as shown in Figure 4, until the stabilizer
is ready for use by means of a slight radial crimp or
indentation of the tube body below the mandrel, or other
appr~priate means for maintaining the mandre~ i.n association
with body.10~ ~eturning lower end 14 of body 10 to its
original circular cross section provides a greater area for
welding ring 18 to the stabilizer body, making an efficient
coupling of the ring to tube forces, as well as providing a
gradually tapering area for insertion of the mandrel into the
^folded portion of the body.
Of course, the mandrel and body portions of the
stabilizer may be kept separate until ready for use with the
mandrel being placed manually in the lower end of the stabilizer
body just prior -to or after insertion thereof into the bore
hole. This has the advantage of allowing the selection of a
.
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sd/~ ~ -7-
1 3~
mandrel having a size matched to the hole size from a
plurality of different sized mandrels, since hole size may be
expected to vary somewhat even in holes made by the same drill.
However, the possibility of operator error in choosing a
mandrel of the wrong size is also introduced, as well as the
undesirable requirement of additional manual operations at the
point of use. It has been found that mandrels of a single size
will operate in a satisfactory manner in holes which vary as
much as 7% in diameter, which is the largest variation
encountered under normal circumstances in holes drilled to
the same nominal diameter. Thus, it is preferred that the
mandrel be frictionally engaged, or otherwise captured or
permanently associated, with body 10 of the stabilizer at the
time of fabrication rather than being assembled therewith at
the point of end use. Small indentations such as those shown in
Figure 4 at 21 are made in two opposite folded areas 12 for
this purpose.
In operation, a bore hole such as that denoted in
Figures 4 & 6 by reference numeral 22 is drilled into a rock
formation such as a mine roof to a norninal diameter slightly
larger than the largest; cross sectional diameter of the folded
portion of body 10, i.e., the portion other than expanded lower
end 14. A steel plate 24 of any conventional design used as a
load bearing support in applications such as that of the present
;nvention is placed in association with the stabilizer by
passin~ body 10 through an opening in the plate. The diameter
of -the opening is approximately e~ual to the outside diameter
of lo~,~er end 14, whereby plate 24 is supported upon ring 18.
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3~L~
Body 10 is then inserted into hole 22 until plate 24
is engaged with the surface of the rock formation surrounding
the entrance to the hole. The inward taper of upper end 16
assists in guiding body 10 into the hole. No axial forces of
any consequence are exerted upon the stabilizer during
insertion into a straight hole since, as previously stated, the
hole diameter is slightly larger than, and in no event smaller
than the largest cross sectional dimension of the portion of
body 10 inserted therein. Some axial force may be required if
the bore hole axis is curved. The elements at this point are
as shown in Figures 4 & ~. Mandrel 20 is then contacted by a
suitable driving implement (not shown) such as a pneumatic
impact tool, or a hydraulic drill boom of the type com~only
used in connection with the drilling and mechanical bolting
operations in coal mines. An appropriate force is applied by
the driving implement to move mandrel 20 through body 10,
expanding folded portions 12 thereof to an extent sufficient
to bring a substantial portion of the outer surface of body ]~
into engagement with the wall of bore hoIe 220 Mandrel 20 is
moved to a position near upper end 16, as seen in Figure 6.
The mandrel may be pushed all the way through and out of the
upper end of body 10 without adverse effects.
With respect to Figure 4, it will be noted that ring
18 is in direct, supporting contact with plate 24, which in
turn contacts and supports the surface of the rock formation
surrounding the drill hole. Since the surface of ring 18
which contacts plate 24 lies in a plane normal to the axis of
body 10, the forces transmitted between the ring and plate will
" ' sd/,',¦~j g
1.~'7~ ~3~
not be uniform about the periphery of the ring if -the axis of
the bore ho]e is not perpendicular to the plane of the surround-
ing rock surface. In actual practice, in fact, this is often
the case. The transmitted forces may be distributed more
evenly by providing a spherical rather than a flat contact area
between the plate and its supporting element. One example of
such arrangement is shown in Figure 6 wherein washer 2S is
inserted between ring 18 and plate 24. Washer 25 has an
internal, stepped shoulder for engagement with ring 1~ and a
spherical outer surface for contact with plate 24. In such
applications, a spherical indentation is provided about the
opening in plate 24, as indicated in the drawing. The use of a
separate element such as washer 25 may be avoided, if desired,
by forming a similar spherical surface directly on the portion
of ring 18 which contacts plate 2~.
It is preferred by that the extent of expansion of body
10, even in the largest holes, is such that portions of at least
some of folds 12 remain after full expansion. That is, the outer
surface of body 10 is not returned to a fully circular configur-
ation. Body 10 need not be expanded symmetrically and, in
practice, does not expand so in most cases. Mandrel 20 tends to
migrate toward the side of body 10 where the weld is formed
when the tubing is fabricated. Figure 7 indicates a typical
expansion pattern. However, this does not deter in any manner
from the performance of the stabilizer, or the holding force
exerted thereby on the surrounding rock formation.
A lubricant is preferably provided on the inner wall
of body 10 in order to minimize the force required to drive the
mandrel through the body. Ordinary greases and oils have been
r
sd/ ~ 10-
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found to provide little, if any, efect in reducing the driving
force required. However, significant results are obtained by
the use o~ lubricants such as those employed in drawing steel
tubing over mandrels during fabrication thereof. An example
of such lubriaant suitable for use in the present invention is
that sold under the trademark Reactobond 909 by O.M.I. Parker
Division of Oxy Metal Industries Corp., of Madison Heights,
Michigan. ~ convenient means of applying the lubricant is to
dip the en-tire body 10 therein, thus coating both the inside and
outside s~rfaces. Although coating the outside has no effect on
the force ~equired to drive the mandrel, of course, it does
provide a measure of protection, as from corrosion, and does not
reduce the holding power of the installed stabilizer.
Turning now to Figures 8 and 9, a second form of
stabilizer is shown. In this embodiment, body 26 is of round
cross section, comprising a length of conventional, hollow, steel
- tubing. Mandrel 28, in this case being spherical in shape, is
inserted i~ expanded lower end 28 of body 26 which again has
affixed th(ereto a ring 30 and support plate 32. Of course,
~20 mandrels of any suitable shape which produces the desired
expansion ~ay be used with either of the disclosed stabilizer
body embodiments. Body 26 has a diameter initially smaller,
or not greater, than tha~ of bore hole 34 in the rock
formation to be stabilized. Thus, body 26 may be inserted in
hole 34 wlthout exerting any substantial axial forces until plate
- 32 is engaged against the surface of the rock formation surround-
ing hole 3~. Mandrel 28 is then forced through body 26 to a
position near the upper end thereof, stretching ~he tube wall
outwardly into tight frictional engagement with the wall of
hole 34.
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3~L~
Since the body of the stabilizer is to be inserted
without substantial interference, its outer diameter should be
equal to approximately the smallest hole encountered under
normal drilling conditions. As previously stated, hole size may
be expected to vary up to perhaps 7%. Steels of suitable
strengths for use in such applications have sufficient ductility
to accommodate such variations in hole size before ultimate
failure of the metal and still exert the necessary axial holding
forces on the surrounding rock structure. However, a much
greater variation in the driving force required to move the
mandrel through the body is encountered when using a circular
cross section stabilizer body than when using the folded body of
the pre~ious embodiment. For this reason, i.e., more uniform
mandrel driving force over the anticipated range of hole size
varLation, the folded body embodiment is preferred over the
circular cross section body. In addition, there is a substan-
tially lower possibility of inducing harmful cracks in the rock
strata surrounding undersize holes with the folded embodiment.
The mandrel used to expand the stabilizer body may take
any of a variety of forms, in addition to those of the tapered
plug and spherical mandrels $hown in the accompanying drawings,
and any suitable form may be used with either of the two
disclosed embodiments of the stabilizer body. It is preferred
~that a relatively inexpensive mandrel be utilized and remain in
the tube body after expansion. That is, it is preferred that
the mandrels be expendable and, as previously mentioned, that a
single, standard size (diameter) of mandrel be used with each
tube size, for use with a particular nominal diameter of bore
hole. ~owever, the scope of the invention is also intended to
.,
sd/~ 12-
,
encompass designs utilizing recoverable mandrels which are
either driven through the body from the lower to the upper end
and then withdrawn or pulled from an initial position at the
upper end through the lower end. In the latter case, it is
obvious]y necessary to provide some means for restraining the
stabilizer body against axial movement out of the hole as the
mandrel is drawn therethrough.
In Figure 10 is shown an alternate embodiment of
reuseable mandrel in the form of tapered screw 36 having a
diameter at its upper end smaller than the inside diameter of at
least the lower end of the tube body, and a maximum diameter
equal to the desired extent of expansion of the body. Screw 36
is rotated as it is inserted into and through the tube, thereby
effecting the desired expansion, and may be counter-rotated to
assist in its withdrawal at which time the minimum inside
dimension of the stabilizer body is substantially equal to the
maximum diameter of the screw. The expander screw could be
attached to and rotated by conventional mine bolting machines
- currently in use. If necessary, means would be provided in
association with the bolting machine for gripping the ring on
the lower end of the body to restrain the Iatter agalnst
rotation, at least in the initial stage of advancement of the
screw until a sufficient portion of the body has engaged to
bore hole ~all to prevent rotation. A captured, expendabie
mandrel could also be provided in the form of a screw or other
rotating structure.
In some mine roof support applications, the bore hole
is made in two steps, drilling to a first depth at a first
diameter to provide a so-called starter hole and then drilling
sd/ ~ -13~
1 3~0
through the end of the starter hole to a deeper depth at a
smaller diameter in order to reduce the possibility of cracking
the rock strata near the supported surface in small or under-
sized holes. In such cases, the stabilizer engages only the
smaller diameter drill hole wall, which may begin several feet
from the entrance to the starter hole. When the present
invention is utilized in such applications, the mandrel may be
inserted in the tube~at the time of fabrication to a depth
equal to that of the starter hole wherein the stabilizer is to be
be employed. This has the advantage of requiring less time and
energy for mandrel insertion at the point of use.
Although in both illustrated embodiments, the body
sections of the stabilizer are of closed form in cross section,
the invention also contemplates the use of open forms. That is,
the abutting edges of the hollow tube formed by bending the
initially flat stock to the circular configuration need not be
welded to forma closed cross-section tube. In such cases, the
sta~ilizer body would include an open axial slot along its
entire length, which would provide manufacturing economies over
~elded tubing. Although such designs would not be practical in
- .stabilizer bodies or circular cross section since the slot
would simply be widened as the body is expanded and would tend
to spring back and reduce the holding force, such is not the
case with folded embodiments such as first described herein.
Such a design is shown in Figures 11-13, wherein the
sta~ilizer body is denoted by reference numeral 38. Body 38 is
formed as a rolled section from flat strip stock, providing two
adjacent but unjoined edges 40 and 42. As in the first
embodiment, four inward folds or indented areas 44 are formed
axially along the length of body 38.
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lV
In Figure 11, body 38 is shown in its free state, as
inserted into the bore hole prior to expansion. Figure 12
illustrates the appearance of body 38 just ahead of the mandrel
as it is driven therethrough. Edges 40 and 42 have opened
somewhat and are spaced further than in the initial, undeformed
state;as body 38 is expanded to bring contact surfaces 46 into
engagement with the wall of the bore hole. Body 38 is shown in
Figure 13 as it wouId typically appear are complete expansion
by the mandrel. As the metal is pushed outwardly into folded
areas 44, edges 40 and 42 are brought together and continued
expansion brings the body cross section to a final configuration
approximating that of Figure 5. In such constructions, as the
mandrel is advanced it will normally shift toward the side at
which the edges are unjoined, as indicated in Figure 13. Also,
edges 40 and 42 may tend to t~rn inwardly after being brought
into contact. In order to prevent edges 40 and 42 from riding
over one another, i.e., overlapping as the body is expanded they
may be provided with bent-over flanged portions or other
suitable means (not shown).
From the foregoing description it is apparent that the
present invention provides an economical and convenient means
for stabilizing rock formations, including the support of mine
roofs, and the llke. The hollow tube-type anchor is inserted
in the drill hole without undergoing significant axial stress,
and is radially expanded into permanently gripping contact with
all or a portion of the length of the bore hole wall over all or
most of its external periphery. Using a single, standard size
stabilizer body and mandrel, the system will operate satis-
actory in hole sizes which vary up to at least 7~ in diameter.
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In practice, it has been found that suitable designs of this
type for use in nominal 1.425" diameter holes may be constructed
from the previously mentioned 1020 CRS electric welded tubing
having an initial outside diameter of 1.50" with four axial folds
producing a maximum cross sectional dimension of 1.360i'
(+ .005"), and minimum outer dimension of .~9". In such a
stabilizer body, a force of about 2,000 lbs. is required to
advance a mandrel 1.020" in diameter when the body is not
radially restrained, i.e., not inserted in a hole, about 3,000
lbs. when the stabilizer body is within the largest hole
(1.475") and about 4,000 lbs. when in the smallest hole (1.375").
Designs such as the embodiments of Figures 1-7 made
of steel having 65,000 psi breaking strength have been found to
exhiblt a holding or stabilizing force on the rock formation
equlvalent to or greater than that of current oversize,
forcibl~ inserted tubular anchors such as those of previously
mentioned Patents Nos. 3,922,867 and 4,012,913 while requiring
approximately one pound less steel per 5 foot length. ~ven
greater savings in material, and therefore in cost, may be
realized by employing higher strength materials. While the
driving force required for insertion of the oversize tubular
anchors increases steadily as the anchor is inserted, the force
re~uired to advance the mandrel through the initially undersize
stabilizer of the present invention in substantially constant
over the entire range of travel. Other advantages include
the capability of increasing or restoring the holding force of
the stabilizer by driving larger mandrels through the tube body
subsequent to initial installation, and the use of longer
stabilizers ~hich are increasingly subject to buckling or other
failure when high axial forces are exerted thereon.
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