Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to improvements in
the field of borehole extensometry. More particularly, the
invention is concerned with an improved borehole extensome-
ter for monitoring relative mass displacements.
Borehole extensometers are instruments generally
used to measure rock movements -that may take place as a
result of surface and undergrouncl excavation, foundation
loading, movement of natural slopes, etc. The relatlve
displacements of the rock are measured at various depths in
a borehole and these displacement measurements allow to
determine the relative stability of the rock surrounding the
opening, to define those zones which are being compressed or
put in tension and to possibly delineate the destressed
volume of rock created by the excavation itself.
A borehole extensometer typically comprises three
main components: a displacement sensor, one or more anchors
at various depths in the borehole and rods or wires each
interconnecting a respective anchor with the displacement
sensor. The displacement sensor is usually mounted inside a
reference head located at the borehole collar and can be
designed for either electrical or mechanical readout. The
relative displacements of the rock, as transmitted to the
rods or wires, are measured by the displacement sensor as a
change in the distance between an anchor within the borehole
and the reference head at the borehole collar.
A borehole extensometer consisting of a single
anchor and a rod or wire extending between the anchor and
the reference head is termed "single-position borehole
extensometer". Extensometers with a plurality of anchors
within the rock are termed "multiple-position borehole
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extensometers" and are preferred for measuring rock move-
ments since the variation of displacement with depth can be
determined.
As the displacement sensor is located in a head
assembly protruding from the collar of the borehole, it can
be easily damaged, for instance by moving trucks, especially
in mining areas. Even if fixed by rock bolts or grout at
the borehole collar, the extensometer head is generally not
sufficiently stable to blast vibrations and can thus be
displaced by violent shock waves created by blasting,
resulting in erroneous measurements.
Moreover, since a rod or wire extending between
each anchor within the borehole and the reference head at
the borehole collar is used to transmit to the sensor the
relative displacement of the anchor caused by rock movement,
long rod- or wire-type installations usually suffer from a
lack of instrument precision due to unavoidable friction in
such installations. In long vertical holes, buckling of the
rods and excessive weight may also cause problems.
It is therefore an object of the present inven-
tion to overcome the above drawbacks and to provide an
improved borehole extensometer which does not require a
surface reference head and is capable of providing in-bore-
hole displacement measurements.
It is a further object of the invention to
provide such a borehole extensometer which also features
adequate mechanical stability to blast vibrations, easy
installation and complete retrievability as well as high
accuracy.
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1~7~8~7
In accordance with the invention, there is
provided a device positionable in a borehole for monitor-
ing relative mass displacements axially along the borehole
between at least two spaced-apart anchor points, which
device comprises at least two retrievable anchor members ar-
ranged in spaced-apart opposite relationship to define the
at least two anchor points, the anchor members each having
radially displaceable borehole wall contacting means and
including mechanical means for radially outwardly displacing
the borehole wall contacting means to engage the borehole
wall and thereby fix the anchor member in the borehole
against the wall thereof. A linear displacement sensing
means is arranged between the anchor members, the displace-
ment sensing means being connected to one of the anchor
members and including an elongated axially movable sensor
element extending substantially centrally of the borehole,
the sensor element being normally biased in a direction
toward the other anchor member for contact engagement
therewith. Relative mass displacements are transmitted to
the sensor element and are measured by the displacement
sensing means as a variation in distance between the anchor
members within the borehole.
Thus, according to the invention, the displace-
ment sensing means measuring relative displacement be-tween
anchor points are located in the borehole and not in a head
assembly protruding from the collar of the hole. This
feature provides protection of the measuring system and
consequently increases its reliability especially in thoses
cases where the device is subjected to shock waves created
by blasting.
127~9187
In a preferred embodiment of the monitoring
device according to the invention, the anchor members each
have a cylindrical body and a plurality of elongated axially
extending borehole wall contacting elements each having
spaced-apart borehole wall contact surfaces are arranged
circumferentially of the body in spaced relation to one
another, at least one of the oorehole wall contac-ting
elements being radially displaceable. The mechanical means
preferably comprise axially movable cam means arranged in
the body and cam follower means connected to the radially
displaceable borehole wall contacting elemen-t and cooperable
with the cam means upon movement thereof for radially
outwardly displacing the element to engage the borehole
wall. The anchoring capacity of such mechanical anchor
members is very high and the borehole wall contacting
elements are designed to adjust to small deformations of the
borehole while still exerting the necessary anchoring force.
The use of such anchor members also allows for a complete
retrievability of the monitoring device according to the
invention.
Accordingly, the present invention also provides
in another aspect thereof an anchor member positionable in a
borehole for defining an anchor point, comprising a cylin-
drical body; a plurality of elongated axially extending
borehole wall contacting elements arranged circumferentially
of the body in spaced relation to one another and each
having spaced-apart borehole wall contact surfaces, at least
one of the borehole wall contacting elements being radially
displaceable; axially movable cam means arranged in the
body; and cam follower means connected to the at least one
borehole wall contacting element and cooperable with the cam
~7~3~87
means upon movement thereof for radially outwardly displac-
ing the one element to engage the borehole wall and thereby
fix the anchor member in the borehole against the wall
thereof.
The anchor member according to the invention
preferably comprises three borehole wall contacting ele-
ments, one of the elements being radially displaceable and
the other two being fixed. Each borehole wall contacting
element is advantageously provided with two spaced-apart
borehole wall contact surfaces arranged adjacent opposite
ends of each element. The radially displaceable element is
connected intermediate the ends thereof to the cam follower
means whereas the two other elements are fixed intermediate
the ends thereof to the body and are disposed relative to
the displaceable element such that movement of the cam means
to radially outwardly displace the latter element causes all
three elements to be resiliently urged against the borehole
wall, thereby providing positive engagement of the contact
surfaces with the borehole wall. The borehole wall contact-
ing elements therefore lock the anchor member to the bore-
hole wall to prevent anchor slippage due to blast vibra-
tions.
According to a preferred embodiment, the cam
means comprises a spindle having a tapered end defining a
cam surface and the cam follower means comprises an arm
slidably mounted in the body for radial movement, the arm
being connected at one end to the displaceable borehole wall
contacting element and being formed at the other end with a
sloped surface engaging the cam surface. Thus, axial inward
movement of the spindle causes the arm to move radially
outwardly and thereby displace the element to engage the
~2~9187
borehole wall. Preferably, the spindle is threadably engaged
in the body for rotation about its longitudinal axis such
that rotation of the spindle in one direction causes the
spindle to move axially inwardly of the body and the arm to
move radially outwardly, thereby engaging the displaceable
element with the borehole wall. Reversing the direction of
rotation of the spindle causes the spindle to move axially
outwardly of the body and the arm to move radially inwardly,
thereby disengaging the element from the borehole wall.
In the monitoring device according to the inven-
tion, the sensor element is preferably provided at a free
end thereof with an enlarged head for contact engagement
with a portion of the spindle which is opposite the tapered
end and protrudes from the body of the~other anchor member.
The provision of an enlarged head at the free end of the
sensor element enables the sensor element to remain in
contact engagement with the spindle of the other anchor
member in those cases where mass displacements created by
shear forces are transverse to the longitudinal axis of the
sensGr element. Thus, the monitoring device according to the
invention still remains operative for measuring the axial
component of such mass displacements.
The displacement sensing means can be electrical,
mechanical, optical or acoustic,but is preferably of the
electrical type and mounted in a watertight housing con-
nected to one of the anchor members, the sensor element
extending through the housing for contact engagement with
the other anchor member. Such electrical displacement
sensing means preferably comprises a linear voltage displa-
30 cement transducer having a slidable core rod coupled withthe sensor element.
37
In a particularly preferred embodiment, the
monitoring device includes a plurality of anchor members
arranged in spaced relation to one ancther to define multi-
ple anchoring points, and linear displacement sensing means
arranged between two successive anchor members for providing
multiple point displacement monitoring. Displacement mea-
surements are thus made in the borehole, in sections distri-
buted along the borehole length.
According to yet another preferred embodiment,
the displacement sensing means are detachably connected to
the anchor members to define a plurality of modular displa-
cement sensing units each associated with a respective one
of the anchor members. Due to such modular assembly, it is
possible to choose the displacement sensing unit in each
measuring section to match the scale of the displacement
expected along the borehole. The result is an optimization
of sensitivity at each measuring point along the borehole.
The device according to the invention can be used
not only for rock slope stability monitoring and rock
displacement measurements around tunnels and shafts, but
also for the monitoring of underground vaults for nuclear
waste disposal and for stability assessment of internal
cracks in concrete dams.
Further features and advantages of the invention
will become more readily apparent from the following des-
cription of preferred embodiments as illustrated by way of
example in the accompanying drawings, in which:
Figure 1 is a side view of a monitoring device or
extensometer according to the invention, seen installed in a
30 borehole;
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Figure 2 is a fragmentary perspective view of the
extensometer illustrated in Fig. l;
Figure 3 is a fragmentary longitudinal section
view of the extensometer;
Figure 4 is a sectional view taken along line 4-4
of Fig. l; and
Figure 5 is a sectional view taken along line 5-5
of Fig. 1.
Referring first to Fig. 1, there is illustrated a
multiple point borehole extensometer generally designed by
reference numeral 10 and seen installed in a borehole 12.
The extensometer 10 comprises a plurality of mechanical
anchor members 14 arranged in spaced relation to one another
to define multiple anchoring points, the innermost anchor
member 14' defining a bottom anchor. The extensometer also
comprises a plurality of linear displacement sensing modùles
16 each arranged between two successive anchor members. The
displacement sensing modules 16 are connected either direc-
tly to the anchor members 14 or indirectly thereto by means
of extension tubes 18,18' of varying lengths. As shown, each
displacement sensing module 16 extends substantially cen-
trally of the borehole 12 and has an axially movable sensor
rod 20 normally biased in a direction toward an opposed
anchor member for contact engagement therewith. Relative
mass displacements are thus transmitted to the sensor rods
20 and are measured by the displacement sensing modules 16
as a variation in distance between the anchor members 14,14'
within the borehole 12. Displacement measurements are
therefore made in the borehole, in sections distributed
along the borehole length. Where a relatively long extension
tube 18' is interconnected between an anchor member 14 and a
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displacement sensing module 16, a centralizer 22 is mounted
over the tube to prevent its sagging, the centralizer 22
receiving the extension tube 18' in sliding engagement
therethrough.
As shown in Figs 2, 3 and 4, the anchor members
14,14' each have a cylindrical body 24 formed with a collar
26 and comprise three axially extending shoes 28,28' adapted
to contact the borehole wall 30 ancl arranged circumferen-
tially of the body 24 in equidistantly spaced relation to
one another, the shoe 28' being radially displaceable and
the two other shoes 28 being fixed to the body 24. The shoes
28,28' are each provided with two spaced-apart borehole wall
contact surfaces 32 arranged at the ends of each shoe.
The anchor members 14,14' further include mecha-
nical means for radially outwardly displacing the shoe 28'
to engage the borehole wall 30 and thereby fix the anchor
member in the borehole 12. Such mechanical means comprise a
spindle 34 having a tapered end defining a cam surface 36
and formed at the other end with a pin 38 of hexagonal
cross-section, the spindle 34 being threadably engaged in
the collar 26 for rotation and axial movement, and an arm 40
slidably mounted in the body 24 for radial movement, the arm
40 fixedly holding the shoe 28' by means of screws 42 and
being formed with a sloped surface 44 engaging the cam
surface 36. Thus, clockwise rotation of the spindle 34
causes the spindle to move axially inwardly of the body 24
and the arm 40 to move radially outwardly, thereby engaging
the shoe 28' with the borehole wall 30. Radial outward
movement of the arm 40 is limited by a screw 46 acting as a
stop. Since the shoes 28,28' are provided with spaced-apart
contact surfaces 32 and are connected intermediate their
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1279~
ends to the body 24 and arm 40, all three shoes can be
resiliently urged against the borehole wall 30 by forcably
rotating the spindle 34, thereby providing positive engage-
ment of the contact surfaces 32 with the borehole wall 30.
Reversing the direction of rotation of the spindle 34 causes
the spindle to move axially outwardly of the body 24 and the
arm 40 to move radially inwardly, thereby disengaging the
shoe 28' from the borehole wall 30. The arm 40 therefore
acts as a cam follower cooperating with the spindle 34 upon
movement thereof to radially displace the shoe 28'.
The anchor members 14,14' are also provided with
dowell pins 48 which are press-fitted into the collar 26 and
serve for coupling the anchor members to an installation
tool (not shown). A tubular coupling element 50 extends from
the body 24 centrally thereof for connection to either a
displacement sensing module 16 or to an extension tube 18 or
18'. As shown in Fig. 3, the extension tube 18' is provided
at its connecting end with a threaded insert 52 which is
welded to the tube and threadably engages the coupling
element 50; the extension tube 18 shown in Fig. 1 is simi-
larly provided with such a threaded insert. In the case of
the bottom anchor member 14', the tubular coupling element
50 can of course be omitted.
The displacement sensing module 16 comprises a
linear voltage displacement transducer (LVDT) 54 having a
slidable core rod 56 coupled with the sensor rod 20; a
linear potentiometer can of course be used instead of a
LVDT. The transducer 54 is secured to a support 58 by means
of a screw 60 and is mounted in a watertight housing 62
closed with a nozzle 64, seal rings 66 being provided
between the housing 62 and the support 58 and nozzle 64. The
-- 10 --
127~L8~
sensor rod 20 sealingly extends through the nozzle 64,
sealing being achieved by means of a seal member 68 arranged
in the nozzle 64. As best shown in Fig. 2, the sensor rod 20
is provided at its free end with an enlarged head 70 having
a flat surface 72 for contact engagement with the spindle 34
of an opposed anchor member 14. A helicoidal spring 74 is
arranged in the housing 62 for normally biasing the sensor
rod 20 outwardly, the spring 74 abut-ting the shoulder 76 and
acting on a disk 78 secured to the sensor rod; the disk 78
is formed with an annular recess 80 for seatingly receiving
the spring 74. A multi-conductor cable 82 electrically
connects the transducer 54 to a remote readout unit (not
shown), the electrical cable 82 being connec-ted to the
displacement sensing module by means of a watertight plug
84. As shown in Fig. 3, the electrical cable 82 passes
through the extension tube 18' to extend through an aperture
86 formed in the coupling element 50 of the anchor member
14, and then through one of six axial holes 88 formed in the
anchor member.
The centralizer 22 which is used to maintain the
central positioning of the extension tube 18' within the
borehole 12 comprises a cylindrical guide sleeve 90 receiv-
ing the tube 18' in sliding engagement, and three axially
extending resilient blade elements 92 arranged circumferer.-
tially of the guide sleeve 90 in equidistantly spaced
relation to one another. The blade elements 92 are resilien-
tly urged into contact engagement with the borehole wall 30
to position the guide sleeve 90 substantially centrally of
the borehole 12. In order to minimize friction between the
guide sleeve 90 and the extension tube 18', a cylindrical
bearing 94 of TEFLON (trade mark) is arranged therebetween,
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as best shown in Fig. 5. The screws 96 which secure the
blade elements 92 to the guide sleeve 90 are also used to
fix the inner TEFLON bearing 94. A pair of stop rings 98,98'
are secured to the extension tube 18' on either side of the
centralizer 22 in spaced relation thereof; the stop ring 98
is used to properly position the centralizer 22 during
installation of the extensometer 10 in the borehole 12
whereas the stop ring 98' is used for retrieving the centra-
lizer 22 together with the extensometer.
The installation procedure of the extensometer 10
is simple and straightforward due to the modular design of
the instrument. Having previously established the anchor
points, the bottom anchor member 14' is first installed. it
is mounted on and locked, by means of the dowell pins 48, to
an installation tool (not shown) consisting of a hollow tube
and then pushed in the borehole 12 to the desired location.
To fix the anchor member 14' against the borehole wall, an
inner setting rod having at the end thereof a hexagonal
socket is inserted in the installation tube such that the
hexagonal socket fits onto the hexagonal pin 38 of the
anchor member. Holding the external tube, the inner rod is
rotated clockwise to rotate the spindle 34 and thus radially
outwardly displace the shoe 28' until it contacts the
borehole wall 30. The inner setting rod is further tightened
to provide a strong anchoring. The external tube is unlocked
from the anchor member by turning counterclockwise and it is
withdrawn from the borehole 12 together with inner setting
rod.
The next step consists in mounting a displacement
sensing module 16 on the length of extension tube 18 re-
quired to provide the necessary spacing between the lower
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and upper anchor points of the monitoring section. In doing
so, the electrical cable 82 of the module 16 is either
passed through each additional length of extension tube or
is left along its side. If required, centralizers 22 are
also mounted on the extension tube at regular intervals.
Following the same procedure as described above for instal-
ling the bottom anchor member 14', the next or upper anchor
member 14 is mounted and locked to the installation tool and
the whole assembly consisting of the anchor member 14,
10 displacement sensing module 16 and extension tube 18 there-
between is pushed in the borehole 12 until the sensor rod 20
of the module 16 makes contact with the lower anchor member
14'.
The electrical cable 82 of the module 16 is then
connected to a portable readout unit (not shown). While
checking the display of the readout unit, the external
setting tube is pushed slightly until the transducer 54 is
positioned correctly within its operating range. When this
position is reached, the inner setting rod is rotated
20 clockwise to tighten the upper anchor member 14 solidly in
the borehole 12. The external setting tube is unlocked and
withdrawn together with the inner setting rod, leaving the
deepest monitoring section fully installed in the hole. The
procedure is repeated for all the subsequent monitoring
sections up to the collar of the borehole, taking care of
passing all the electrical cables 82 through the holes 88
provided in -the anchor members 14 as the installation
progresses. Complete retrieval of the extensometer 10 can be
done following the reverse procedure.
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Depending on the operating range of the transdu-
cer 54 used, the extensometer 10 can measure relative mass
displacements as small as 0.001 mm (0.00004 inch) with a
stability of the same order.
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