Note: Descriptions are shown in the official language in which they were submitted.
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ROCK BOLTING METHOD AND ROCK BOLT
FILED OF THE INVENTION
This invention concerns a method in rock bolting accord;ing to
the preamble of claim 1. The invention also concerns a rock
bolt according to the preamble of claim 6.
DESCRIPTION OF BACK GROUND ART
From US-A-4,459,067 (Atlas Copco AB) is previously known a
rock bolt having a tubular bolt portion, which has a closed
cross section and which is intended to be expanded for bearing
against the inside of a bore hole in a rock structure.
In the expanded state of the tubular bolt portion, the
previously known rock bolt is capable of supporting loads in
order to i.a. stabilize a rock face belonging to a rock
structure in connection._with tunnelling, mining, drifting and
the like.
AIM AND MOST IMPORTANT FEATURES OF THE INVENTION
It is an aim of the present invention to further develop a
method in rock bolting, wherein rock bolts of the general kind
which is described in the above mentioned US-A-4,459,067 are
used.
It is also an aim of the invention to provide a rock bolt as
initially stated which allows secure and versatile use at the
same time as it provides good economy.
These aims are achieved in a method according to the invention
by determining the length of the tubular bolt portion such
that the maximum fictional force between parts of the tubular
bolt portion that bear against the inner wall of a bore hole,
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upon pulling inside this bore hole, is limited so that it is
below a predetermined maximum value.
Hereby it is achieved that a rock bolt having a tubular bolt
portion can be manufactured and dimensioned such that great
pulling forces, which a rock bolt typically can be subjected
to at powerful but in space limited movements in the rock
structure it is intended to stabilize, does not cause breakage
of the rock bolt but instead results in controlled sliding of
the tubular bolt portion, a limited distance inside the bore
hole, whereupon the rock bolt is again capable of resisting
essentially the same load as prior to the sliding, so that
such movements in the rock structure will be tolerable.
The advantages of the invention are besides that rock bolt
failure is avoided also that the number of rock bolts per
surface unit can be reduced, and that the dimensions of rock
bolts that are used can be reduced, since the intended and
resulting yielding thus allows rock movements in a controlled
manner. Thereby the rock structure does not have to be
stabilized in an exaggerated and a more costly manner.
It is preferred that the length of the tubular bolt portion is
determined as the result of at least one measurement,
preformed during pulling of at least one rock bolt having a
tubular bolt portion of a predetermined in at least one bore
hole in the same or in a corresponding rock structure.
Starting out from values resulting from this drawing
measurement or these drawing measurements, the rock bolts to
be used at the stabilization can thereupon be produced and
adequately dimensioned with respect to the length of the
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tubular bolt portion, so that the intended sliding at intended
loads is achieved.
It is preferred that said predetermined value is the yield
point load for the tubular bolt portion, which means that the
material is optimized to the existing conditions in an
advantageous way. The yield point load for the tubular bolt
portion of a rock bolt intended here can be determined by a
person skilled in the art through in per se previously known
load tests.
It is preferred that a number of drawing tests, as for example
5 - 10 drawing tests, are performed, in order to obtain more
reliable values for dimensioning. Starting out from such a
plurality of drawing test results, the distribution in the
results is considered in order to determine the marginal to
the yield point when the length of a tubular bolt portion
which is to be used for each rock bolt, is to determine.
At greater distribution, a greater margin to the yield point
is typically necessary. In that case it could be suitable to
dimension a rock bolt such that, starting from the mean value
of the drawing test, maximum calculated frictional force is as
an example 60 - 70% of the yield point load. At smaller
distribution, which also is to considered as normal
distribution, starting from the mean value of the drawing
tests, maximal calculated fictional force can be 80 - 90% of
the yield point load. It shall be noted that this is exemplary
and that it is within the scope of the invention that other
values can be considered.
It is also preferred that the length of the tubular bolt
portion is determined and dimensioned such that said
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frictional force exceeds a predetermined minimum value in
order to avoid unnecessary sliding movements at less powerful
rock movements.
The corresponding advantages are achieved through the
invention through a rock bolt having corresponding features.
Through an inventive rock bolt, great and sudden rock
movements and be controlled.' After a sliding movement, the
capability of the rock bolt to take up load is essentially the
same as it was before the sliding movement.
A device for performing said method includes means for
engagement with a rock bolt having a tubular bolt portion
inserted into and expanded inside a bore hole, means for
applying a pulling force to said rock bolt, means for
detecting sliding of said rock bolt as a result of application
of said pulling force and means for detecting and measuring
the pulling force that corresponds to the detected sliding.
This device allows effective determining of the effective
length of a rock bolt which has the ability of yielding
through sliding at greater movements in the rock structure.
The invention will now be described further with the aid of
embodiments and with reference of the annexed drawings.
BRIEF DESCRIPTION OF DRAWING
On the annexed drawing:
Fig. 1 shows a section through a rock structure with a rock
bolt according to the invention,
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Fig. 2 shows a device for performing a method according to the
invention in connection with a rock bolt, and
Fig. 3 shows a sequence in order to illustrate the method in
5 rock bolting according to the invention.
DESCRIPTION OF EMBODIMENT
In Fig. 1, reference number 1 relates to a rock structure,
wherein a rock bolt 2 is inserted inside a bore hole, the
inner wall of which is referenced 3. The rock bolt 2 includes
a tubular expandable bolt portion 4 and a tension rod 5 which
adjoins to the tubular bolt portion 4 over an adapter sleeve
6. On Fig. 1 is shown a tension rod in the form of a number of
tubular sections 5', in pairs joined with the aid of a joining
sleeve 5". This arrangement allows the use of rock bolts
having a great total length also in narrow drifts, which
otherwise would limit the length of a tension rod in one
piece. Inside, most inwardly, in the bore hole 3 the tubular
bolt portion 4 is completed with a terminal sleeve 7.
At the outside of the bore hole 3 a rock plate 8 is as usually
applied, which through preloading of the tension rod 5 exerts
an inwardly directed stabilizing pressing force against an
outer part of the rock structure 1.
In Fig. 1, 9 indicates a crevice in the rock structure in
order to somewhat illustrate the inherent instability of the
rock structure 1. According to the invention, movements in the
rock structure, which tend to displace the rock plate 8
axially outwards, that is to the right in Fig. 1, and that
subject the rock bolt to a sufficiently high force, result in
a sliding of the tubular bolt portion 4 against the inner wall
of the bore hole 3. In particular this occurs if the force
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exerted on the rock bolt by the moveable portion of the rock
structure, exceeds a minimum value.
This way a controlled yielding of the rock bolt occurs through
the sliding movement, such that the rock bolt thereby is
capable of resisting great forces and movements of the
structure without failing.
In Fig. 2 is shown a device 14 for measuring frictional forces
that are present in a certain type of expandable rock bolts in
connection with a certain contracture. This device is intended
for dimensioning of rock bolts according to the invention and
in particular the length of the tubular bolt portion.
According to the invention, a process is undertaken in order
to determine the effective length of a tubular bolt portion of
a rock bolt before the positioning of a number of rock bolts
in a rock structure in order to stabilize the rock face, as an
example in tunnelling or drifting in mining.
The measurement is made such that a test specimen in the form
of a rock bolt 10 having a tubular bolt portion of a
predetermined length is inserted inside a bore hole 12 in the
rock structuring 13 in question. The length of the tubular
bolt portion is suitably chosen such that sliding of the rock
bolt is safely achieved at a force that is below the yield
point of the tubular bolt portion. If the yield point would be
reached before sliding is obtained, a new rock bolt with a
shorter tubular bolt portion 11 will be selected.
The device for measuring 14 includes a tension rod 15 for
adjoining to the expandable tubular bolt portion, a supporting
portion 16 for applying against the side of the rock
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structure, an engagement portion 17 for the engagement with
the tension rod 15 and a pulling mechanism 18 which, for
example with hydraulic means, applies a pulling force to the
rock bolt 9. A control unit 19 senses the applied pulling
force and detects when sliding occurs, and at which pulling
force this occurs.
Starting from the results of the measurement, which can be
transformed to fictional force per length unit tubular bolt
portion, rock bolts can be dimensioned with adequate length of
tubular bolt portion so that a pulling force corresponding to
sliding for the chosen rock bolt with a tubular bolt portion
of selected length as an example corresponds to about 60 -
about 90% of said yield point load. Selection of an adequate
length can as an example be made practidally through lists,
diagrams or the like.
In Fig. 3 is illustrated a sequence for carrying out the
method according to the invention.
Position 20 indicates the start of the sequence.
Position 21 concerns inserting and expanding a tubular rock
bolt having a certain predetermined length in a predrilled
bore hole in a rock structure.
Position 22 concerns axially pulling of the rock bolt in a
direction outwardly.
Position 23 concerns detecting of sliding of the rock bolt and
the pulling force, at which sliding occurs.
Through the interrupted line 26 is indicated that a plurality
of drawing tests can be performed.
Position 24 concerns calculating of the length of the tubular
bolt portion for a rock bolt, either through manual control in
tables, diagrams etc. or automatically through a circuit which
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is connected to the control unit. This is performed starting
from the drawing test or the drawing tests and associated
detections performed in positions 22 - 24.
Position 25 concerns termination of the sequence.
The invention can be modified within the scope of the claims.
Thus, the device for performing the method can be.designed
otherwise as long as it includes means for exerting a pulling
force, detecting sliding and measuring pulling force at
sliding.
It is preferred that the yield point is used as reference when
dimensioning of rock bolts according to the invention. It is,
however, not excluded that also other values that are
significant for a rock bolt can be used for dimensioning. As
an example the break load of the rock bolt could be used,
wherein the tubular portion of the rock bolt could be
dimensioned to have a length corresponding to a frictional
force which is a certain determined part of the break load. It
is also not excluded that dimensioning starts from the fact
that the fictional force is to comprise a certain determined
part of a significant value of the strength of the rock.
The rock bolt according to the invention could be of different
embodiments. It is for example not excluded that a rock bolt
according to the invention includes more than one tubular bolt
portion arranged one after the other.
