Note: Descriptions are shown in the official language in which they were submitted.
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Improved rock bolt with ploughing anchors
Introduction
The present invention relates to an improved deformable rock bolt with
anchors arranged for ploughing in the hardened grout in the borehole in which
the
bolt is arranged for reinforcing the rock.
General background
In mined underground cavities, the lithostatic pressure which balanced the
rocks before the mining process excavated the mining cavity, a significant
imbalance
arises from the moment the mining cavity is formed. This particularly relates
to
tunnels and mines with an overburden of often more than 1000 m of rocks. The
lithostatic pressure is thus of considerable magnitude, and the rock may
deform by
rock bursts or slow, but significant rock deformation depending on the
lithology and
mechanical properties of the rock.
In general, the maximum accumulated rock deformation occurs at the tunnel
walls where the pressure gradient is high, and which is also the only
available
underground space to receive the deforming rock. The accumulated deformation
becomes smaller with the distance from the wall, please see an illustration in
Fig. 1.
When the displacement increments are taken into account, the displacement
increments may look like what is illustrated in Fig. 2. The displacement
increment is
large in the incremental range nearest to the cavity wall, and the
displacement
increment decreases with increasing distance from the wall.
Background art
The so-called deformable bolt or "D-bolt" described in the applicant's
international patent application PCT/N02007/000461 filed 22.12.2007, describes
a
rock bolt for being grouted in a borehole in a rock. Essential of the D-bolt
are the
following features:
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- An elongate cylindrical massive stem with a preferably threaded bolt head
comprising a nut and a washer arranged for pre-tensioning the rock bolt in the
borehole. A face plate for the bolt head is usually required.
- The stem comprising three or more extensive lengths of stem portions. Each
stem
portion is followed by an integrated anchor. Each anchor is of short extent
compared
to the extent of the stem portions, and the anchors are distributed with
separations
along the length of the stem.
- The anchors are arranged for being locally anchored relative to their
corresponding
local borehole wall portions. This is for taking up load arising due to rock
deformation.
- The stem portions are more or less slick so as for enabling slipping
relative to the
grout or the borehole. Thus each of the elongate, slick stem portions may
constrain
local rock deformation through elongation, the stem portions being free to
extend
between pairs of a locally anchored, fixed, preceding anchor and a locally
anchored
consecutive anchor which is also fixed.
Two significant features of the above-mentioned deformable bolt are: Firstly,
it has the ability to take up both shock deformation such as rock bursts. A
rock burst
may incur damage to bolts used in the prior art. A commonly used rib bar of
the prior
art may break when cracks suddenly open in the rock, due to the short
available
portion of the rib bar to take up deformation. Secondly, the above-mentioned
deformable bolt has the ability to take up long-term deformation which occurs
when
the rock steadily creeps, and distribute the local rock deformation on a
relatively long
bar segment between two consecutive anchors fixed in the grout but relatively
moving.
One significant advantage of the D-bolt is due to the fact that if one fixed
anchor
should loose its grip during deformation, a consecutive anchor will probably
hold. A
second significant advantage of the D-bolt is, that contrary to rib bars which
are held
firmly by the grout throughout their length and thus vulnerable to break due
to large
local extensional deformation, long sections of the deformable bolt may slide
longitudinally in the borehole grout, and distribute any longitudinal
deformation of the
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rock, whether the deformation is due to a shock burst or due to long-term
creep. The
deformable bolt of the above prior art is to be deformed by extension through
the
elastically recoverable range, and stop there if mounted in a sufficient
number in the
rock, but, if too few bolts are arranged there is a risk that some bolts may
locally pass
the yield limit and eventually pass the failure strength. The anchors of the
prior art bolt
are designed to be solidly fixed in the borehole grout, for never to move to
any
significant degree.
German patent DE3504543C1 describes a slick single stem anchor with an
inner anchor and an outer anchor by a fixed bolt head. The anchors are
arranged as
bends of increasing amplitude away from the stem axis and are arranged to lose
their
grip consecutively, and to lose their grip at the anchor end facing the slick
stem first.
As such it resembles the cone bolt mentioned below, but it can not be pre-
tensioned.
Another bolt in common use, particularly in South African mines, is the so-
called "cone bolt", which comprises a bolt with a surface head and associated
face
plate, a slick stem of required length, and an inverted cone which presents a
significant widening from the stem diameter to a larger diameter at the inner
end of
the bolt. The cone bolt's inner end is designed to cooperate with the shear
resistance
presented by the borehole grout and shall have an anchor bearing capacity not
exceeding the failure strength of the slick stem. However, if the cone bolt's
head
breaks, which it may be susceptible of doing first as the surface deformation
is larger
than the rock deformation deeper from the wall surface, the entire purpose of
the
cone bolt is lost: neither shock bursts nor creep may be counteracted.
Further, any
previously restrained deformation held back by the cone bolt will accelerate
after the
cone bolt tension is lost due to the head being lost.
However, even though the above-mentioned D-bolt has its significant
advantages, for some mining conditions there is a need for even further
extending the
operating range of the rock bolt. For any deformable rock bolt of the prior
art the
longitudinal rock extension deformation may either eventually break the bolt,
the bolt
head may eventually be pulled into the borehole, or the inner end of the bolt
may
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eventually loose its hold in the grout. The latter problem is a particular
problem to the
cone bolt. Further, for some national regulations, utilizing bolts to their
failure
strength may not be allowed.
A significant mechanical problem is that even though each long bolt section
of the prior art D-bolt is able to slide in the grout while the anchors are
fixed, for
unevenly distributed rock deformation one section may become loaded past its
failure
strength, thus the integrity of the original bolt and thus the local rock mass
is only
partly preserved through the remaining sections being fixedly anchored in the
borehole.
Short figure captions
The invention and some features of the invention in relation to features of
the prior art are illustrated in the accompanying drawing figures:
Fig. 1 illustrates the accumulated distribution of rock displacement in a
tunnel wall as a function of the distance from the tunnel wall. Please notice
that the
derivative of the function is largest near the wall.
Fig. 2 illustrates in increments (corresponding to a rock bolt's sliding
sections) the displacement of three sections in a tunnel wall.
Fig. 3 shows distribution of the load in the bolt sections in case of fixed
anchors. The right hatched, left hatched and vertical hatched columns in the
diagram
refer to the loads induced by the elongations in the corresponding bolt
sections,
respectively. All anchors are fixed. Fig. 3 illustrates the resulting
incremental
distribution of load in bolt sections of a rock bolt of the prior art as
illustrated, such as
the D-bolt using fixed anchors. Here, the outer long section is loaded to very
near the
yield load, and may pass over the yield load limit upon further deformation.
Fig. 4 shows load transfer between sections and the distribution of loads in
the sections in case of movable anchors. The right hatched, left hatched, and
vertical
hatched columns in the diagram refer to the loads induced by the elongations
in the
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corresponding bolt sections, respectively. Some or all anchors are ploughing.
Fig. 4
illustrates resulting incremental distribution of load in bolt sections of a
rock bolt
according to the present invention, in which the anchors are designed for
having a
bearing capacity in the grout being less than the failure strength of an
adjacent stem
portion. Thus when the outer, left section has been heavily loaded and has
approached the yield load, the first anchor's (2a) bearing capacity limit has
been
exceeded and the anchor has moved in the direction of the outer section as
illustrated. Thus load has successively been transferred from the outer stem
section
to the intermediate stem section without exceeding the yield strength of the
outer
stem section. Likewise, as load has successively been transferred from the
outer
section to the intermediate section, the load at the intermediate section has
exceeded
the bearing capacity of the second anchor (2b) which has slid outwards in the
direction of the surface, and thus part of the load has been transferred to
the third
stem section, which, in this illustration, has not reached the yield load yet.
Fig. 5a illustrates embodiments of the prior art D-bolt which have been
modified to the present invention so as for having movable anchors relative to
the
grout, according to the present invention.
Fig. 5b illustrates two embodiments of the present invention represented by
a rock bolt with a bolt head, and optional near-surface anchor and paddle-
shaped
anchors arranged consecutively between slick stem portions. The upper rock
bolt has
single paddle anchors and the lower rock bolt illustrated has double paddle
anchors.
Fig. 6 illustrates an embodiment of an integrated single bending paddle
anchor according to a preferred embodiment of the invention. The single
bending
paddle anchor is shown in a first side view and a second side view orthogonal
to said
first side view, and in section view.
Fig. 7 illustrates corresponding views of an integrated double bending
paddle anchor along the lines of Fig. 6.
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Fig. 8a illustrates an installed rock bolt according to the invention which
shall illustrate the bolt newly arranged in grout in a borehole.
Fig. 8b illustrates a taut, deformed, slid rock bolt. Fig. 8b illustrates the
same bolt after significant longitudinal expansion of the rock itself, and a
significant
longitudinal extension of the rock bolt while still fixed at least at the
inner anchor. At
least two anchors have slid.
Fig. 9 is a graph of pull load (in kN) as a function of joint opening (in mm)
in
a pull test using twin-paddle anchor bolts with 0 22 mm, 0.3 m segment. Fig. 9
illustrates a force versus length diagram for a pull test using a double
paddle anchor
and a 0.3 m segment for two test samples of 0 22 mm. For Sample B, the load
increased steeply to over 250 kN (a stress of approximately 660 MPa) at
slightly less
than 40 mm joint opening, and failed at about 52 mm. For Sample A, for which
the
resin was accidently mixed to an unsatisfactory degree, the anchor had a lower
bearing capacity of about 200 kN in that particular grout (a stress in the
stem of
approximately 525 MPa), the tension built up slower and the anchor started
ploughing
at about 35 mm but maintained its resistance at about 200 kN, and did not
loose its
grip until a total movement of 200 mm when the test was terminated.
Brief summary of the invention and advantages of the invention
A solution to several of the above-mentioned problems is presented by the
present invention which is a rock bolt for being grouted to set in grout in a
borehole in
a rock, comprising the following features:
- an elongate massive stem with a surface anchor the stem having two or more
extensive lengths of stem portions;
- each stem portion followed by a borehole anchor for setting in the grout,
the
borehole anchors distributed with separations along the length of the stem;
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- the stem portions arranged for slipping relative to said grout of said
borehole, so as
for each of the stem portions to constrain local rock deformation through
elongation of
the stem portions between pairs of a locally anchored preceding anchor and a
consecutive anchor;
- one or more of said borehole anchors is arranged for having a bearing
capacity in
said grout being less than a failure strength of an adjacent stem portion, to
enable
one or more of said borehole anchors to move during rock deformation so as for
redistributing strain between stem portions.
Advantages of the invention
Redistribution of stress:
A first advantage of the invention is that by enabling one or more of the
borehole
anchors to move during rock deformation will allow for redistribution of
stress and
strain from a more stressed stem portion to a less stressed stem portion,
given that
the stress difference between the unevenly stressed stem portions is higher
than the
bearing capacity of the anchor. This will enable the rock bolt of the
invention to stay
in operation for an extended deformation length and thus for an extended time
compared to rock bolts of the prior art, without exceeding its failure
strength. In other
words, the rock bolt of the invention allows elastic strain redistribution
without
exceeding the yield limit of the rock bolt.
High pull force during ploughing:
A second advantage of the invention is that even though some the anchors may
slowly plough through the hardened grout in the borehole when the bearing
capacity
of the anchors exceeded, this ploughing action occurs at a pull load near to
but below
the failure strength of the rock bolt stem (please see Fig. 9), thus the work
required to
make the anchor plough through the hardened grout is very high, thus the work
required to plough the anchor through a given distance is very high and
restricts rock
displacement to a significant degree without exceeding the failure strength of
the rock
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bolt stem anywhere along the stem. This provides a higher overall deformation
capacity and deformation work resistance than the prior art.
Part functionality after break:
A third advantage of the invention is that given the not very likely event
that one of
the stem sections should break (e.g. due to combined longitudinal extension
and
shear faulting across the borehole) the anchors at either side of the fault
would still be
intact and the entire function of the rock bolt would not be lost, as would be
the case
with the cone bolt.
Extended effective range:
A fourth advantage of the invention is that if the rock bolt of the invention
is strained
so far that the inner anchor starts ploughing, the rock bolt of the invention
does not
break but it still works while the inner anchor continues to plough under very
high pull
load near to, but below the failure strength of the rock bolt stem. Thus the
restricting
work length of the rock bolt according to the invention is extended as
compared to the
prior art D-bolt, and the work required to deform and displace the inner
anchor and
the remaining anchors of the rock bolt of the invention with several ploughing
anchors
far exceeds the work required to deform and move the single anchor cone bolt.
In
other words, the rock bolt of the present invention extends the work length of
the
entire bolt to exceed the total longitudinal extension of rock surrounding the
borehole.
Unidirectional anchor stamping:
A fifth advantage of the rock bolt according to an embodiment of the invention
shown
in Figs. 6 & 7 is that the anchors may be manufactured by stamping the stem
from
one side only, forming the single or twin-paddle anchors in one operation
without
turning the stem. This may reduce the number of operations for each bolt
produced
and is a significant advantage during manufacture of the several tens of
thousands of
rock bolts required for some mines.
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Description of preferred embodiments of the invention
The invention is a rock bolt for being grouted to set in grout (g) in a
borehole in a
rock. The grout (g) may be a two-component epoxy resin for being mixed while
in the
borehole, or a cement, a polyester, or masses for filling part or all of the
annulus
between the borehole wall and the rock bolt. The rock bolt according to the
invention
comprises the following features:
- an elongate massive stem (1) with a surface anchor (3), the stem (1)
comprising two
or more extensive lengths of stem portions (1 a, 1 b, ..., 1 i), in which each
stem portion
is followed by a borehole anchor (2a, 2b, ..., 2i) for setting in the grout
(g), said
borehole anchors distributed with separations (La, Lb, ..., L;) along the
length of the
stem (1). The stem portions (1 a, 1 b, ..., 1 i) are arranged for slipping
relative to the
grout of said borehole, so as for each of the stem portions (1 a, 1 b, ..., 1
i) to constrain
local rock deformation through elongation of said stem portions between pairs
of a
locally anchored preceding anchor (3, 2a, 2b, ...) and a consecutive anchor
(2a, 2b,
2c, ..., 2i). So far, the rock bolt of the invention is rather similar to the
D-bolt of the
prior art. One of the essential novel features of the present invention over
the D-bolt
is that one or more of the borehole anchors (2a, 2b, 2c, ..., 2i) are arranged
for having
a bearing capacity in said grout (g) being less than a failure strength of an
adjacent
stem portion ((1 a or 1 b), (1 b or 1 c), ... (1 i). This will enable one or
more of the
borehole anchors (2a, 2b, ..., 2i) to move during rock deformation so as for
redistributing strain between stem portions (1 a, 1b, ..., 1i). Please see
embodiment in
Fig. 5a and 5b. Preferably, two or more consecutive anchors separating
consecutive
sections of the rock bolt stem are movable in the grout, as illustrated in
Fig. 8b. In
Fig. 5a, even the last anchor (2c) slips. The strength of the last anchor is
designed
less than the failure strength of the last long segment so that the segment
never fails
rather than the last anchor slips when it subjects to a high load.
The bearing capacity of the borehole anchors (2) in the grout (g) may be
adapted to
be less than the failure strength of the adjacent stem portions. (1) in
several ways:
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- Adjusting the size or shape of the anchors (2) to the mechanical properties
of the
hardened grout so as for the anchors to lose their bearing capacity at a given
load,
i.e. modifying the anchors in order to adapt their bearing capacity in a
given, perfectly
mixed and hardened grout to be sufficiently less than the failure strength of
the bolt
stem so as for allowing the anchors to plough above a given pull load.
- By controlling the mechanical properties of said grout (g) to get desired
mechanical
properties when eventually hardened, during the preparation of said grout (g),
say,
using a two component epoxy resin, controlling the ratio of resin to hardener.
- By designing the anchors to deform mechanically at a given load in order to
reduce
the bearing capacity at a given load.
In a preferred embodiment of the invention, one or more of said borehole
anchors
(2a, 2b, 2c, ..., 2i) being arranged for having a bearing capacity in said
grout (g) being
not only less than a failure strength, but also less than a yield strength of
an adjacent
stem portion ((1 a or 1 b), (1 b or 1 c), ... (1 i)).
In a preferred embodiment of the invention, the borehole anchors (2a, 2b, ...,
2i) are
materially integrated with the stem portions, i.e. the anchors are formed from
a same
smooth bar material blank (0) as the stem portions (1 a, 1 b, ..., 1 i) are
formed from.
This is an advantage from a manufacturing point of view. Further manufacturing
advantages are described below.
In a preferred embodiment of the anchors of the bolt according to the
invention,
please see the embodiment illustrated in Fig. 6, the bolt comprises one or
more
paddle-shaped anchors (2, 22) formed from said smooth bar material blank (0)
by
bending a first short portion (20) of said smooth bar (0) to a first side of
said smooth
bar's (0) axis, the length of said first short portion (20) corresponding to a
length of
said borehole anchor, and clamping said first short bent portion (20) to a
desired
thickness less than, and to a desired width greater than, a diameter of said
smooth
bar material blank (0).
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In another preferred embodiment of the anchors of the bolt according to the
invention,
please see the embodiment illustrated in Fig. 7, the bolt comprises double
paddle-
shaped anchors (24) formed from said smooth bar material blank (0) by bending
a
first short portion (20) of the smooth bar (0) to a first side, and a second
small portion
(23) adjacent to said first short portion (20), to a second side of said
smooth bar's (0)
axis, the combined length of said first and second short portions (22, 23)
corresponding to a length of said borehole anchor, and clamping said first and
second short bent portions (20, 23) to desired thicknesses less than, and to
desired
widths greater than, a diameter of said smooth bar material blank (0).
The outer bar section of long extension may have the surface anchor (3) as its
outer
anchoring point, but in an embodiment shown in Fig. 5b, the rock bolt of the
invention
comprises a short outer stem portion (1 o) adjacent to said surface anchor (3)
and an
auxiliary borehole anchor (20) similar to said borehole anchors (2a, 2b, ...,
2i)
adjacent to said first stem portion (1a). The auxiliary outer borehole anchor
(2o) may
be arranged not to slip in the grout. In other words, the auxiliary borehole
anchor
(20), which is arranged adjacent to the surface anchor (3) and arranged
between the
short outer stem portion (10) and the first stem portion (1 a), may be
arranged for
having a bearing capacity in the grout (g) being higher than a failure
strength of the
main stem portion (1a). With such an embodiment of the invention, one or more
of
the anchors (2a, 2b, ... ) further inside the borehole may be displaced in the
grout and
transfer load in order not to exceed the failure load of the stem portions
(1a, 1b, ...).
In an embodiment of the invention the innermost anchor (2i) may be arranged
with a
bearing capacity in the grout (g) being higher than a failure strength of the
innermost
stem portion (1 i). This situation may be envisaged such as in Fig. 8b where
the
innermost anchor (2i) is fixed in place relative to the borehole wall and the
grout.
It is also possible to combine the two above embodiments in that the auxiliary
anchor
(2o) and the innermost anchor (2i) being arranged to have a bearing capacity
in the
grout (g) exceeding the failure strength of the stem (1, 10, 1 a, 1i). In such
a bolt,
when the rock expands, the outer, auxiliary anchor (20) near the surface
anchor (3),
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and the inner anchor (2i) will reside generally fixed in the grout (g) while
the anchors
(2a, 2b, ...) lying between the outermost and innermost anchors (2o, 2i) are
allowed
to slide in the grout to redistribute strain along the stem when the rock
expands.
The desired ploughing properties may be achieved by controlling/designing the
shape
of the anchors, or by designing some or all the anchors to plough through the
given
resin given a certain axial load, being less than the failure strength of the
stem
portion.
Alternatively the outer borehole anchor (20) may be arranged to slip in the
grout so
as for the auxiliary outer stem portion to be slightly extended (or
contracted) under
varying load during rock deformation.
The pattern of lengths of the stem portions may be varied according to the
requirements presented by the local geological conditions and/or mining
regulations.
According to an embodiment of the rock bolt of the invention, one or more of
said
stem portions (1 a, 1 b, ...) may have a longer initial length than a
consecutive stem
portion (1 b, 1 c, ..., 1 i) , please see the embodiment illustrated in Fig.
5b. In another
embodiment of the invention, one or more of the stem portions (1 a, 1 b, ...)
may
generally the same length as a consecutive stem portion (1 b, 1 c..., 1 i).
In an embodiment of the invention the stem portions (1) have a higher
deformation
capacity per unit length as compared to said anchors (2), i.e. the stem
portions are
elastically extendible, and the anchors are hard and resilient, but slip
before the stem
portions exceed their yield limit.
In a preferred embodiment of the invention two or more of the stem portions (1
a, 1 b,
..., 1 i) have essentially the same failure strength, particularly this
relates to
consecutive stem portions.
In preferred embodiments of the invention, the borehole anchors (2a, 2b, ...,
2i) are of
short extent, say between 5 and 20 per cent length compared to the length of
said
stem portion (1 a, 1b, ..., 1i).
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In one preferred embodiment of the invention, the surface anchor (3) of the
rock bolt
comprises a threaded portion (3a) at the borehole surface portion of the stem
(1), the
threaded portion (3a) provided with one or more nuts (3b) and one or more
washers
or sheaves (3c). This allows pre-tensioning of the bolt (1) when mounted in
the
borehole. Alternatively, the rock bolt may have a surface anchor (3)
comprising a
fixed head (3f) at the borehole surface portion of the stem (1). Anyway, in
order to
transfer load to the wall and maintain the integrity of the rock surface with
possible
shotcrete and metallic nets, the surface anchor (3) may be provided with a
face plate
(3d). The rock bolt of the present invention may be used with several kinds of
face
plates of the background art.
A simple example of use of the rock bolt according to the invention is
illustrated in
Fig. 8. The rock deformation in the present example is according to Fig. 1.
The
surface anchor (3) moves outward to a larger degree than the first main anchor
(2a).
The generally outer, first main stem portion (1a) will thus be strained most
of all stem
portions, please see Fig. 4. The novel features of the rock bolt according to
the
invention resides in the ratio of the anchoring properties of the anchors (2a,
2b, ..., 2i)
with respect to the grout, preferably an epoxy resin, compared to the failure
strength
of the stem portions (1 a, 1b, ..., 1i) of the rock bolt. The result of anchor
(2a) having
a bearing capacity less than the failure strength of stem portion (1a) is that
anchor
(2a) will move outwards when stem portion (1 a) is strained over the bearing
capacity
of anchor (2a), please see Fig. 8 and Fig. 4. The result of anchor (2a) having
a
bearing capacity which is also less than the failure strength of the
consecutive stem
portion (1b) will result in anchor (2a) to move outward while straining
consecutive
stem portion (1 b). This will unload some of the strain from the more heavily
stressed
stem portion (1 a) and strain stem portion (1 b). When approaching the yield
load of
section (1 b), the consecutive anchor (2b) will start ploughing outward and
transfer
load to the consecutive section (1 c).
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