Note: Descriptions are shown in the official language in which they were submitted.
20625~2
METHOD AND APPARATUS FOR MBASURING THREB
DIMENSIONAL STRESS IN ROCK SURROUNDING A BOREHO~E
The invention relates to a method and an
` apparatus to measure three dimensional stress surrounding
a borehole.
BACKGROUND AND PRIOR ART
The state of stress in rock plays an important
role in the behaviour of the ground in response to the
creation of underground openings such as found in mi ning
operations or civil engineering projects. For the design
of underground openings it is therefore essential that the
pre-excavation in situ stress be either measured or
estimated. The inability to measure or estimate
satisfactorily the state of stress in many circumstances
is one of the major impediments to the utility of many of
the theoretical and mathematical models which have come
into use for the design of underground excavations.
Stress is an intangible quantity which cannot be
measured directly. It is only the manifestation of stress
which is measured and used to estimate the stress. The
most common methods employ stress relief or compensation
techniques which result in strains which can be measured.
By knowing the mechanical properties such as the modulus
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of elasticity and the Poisson's ratio of the rock in which
the measurements were carried out one can deduce the in
situ stress.
In most mining and civil engineering
applications the in situ stress is measured with
overcoring methods. These methods require the
installation of a strain measuring device in a borehole
and the subsequent drilling of an oversized hole over the
existing hole to obtain an annulus which is stress
relieved. The resultant strain is measured with the
device inside the annulus. The stress regime can then be
calculated from the strains measured as a result of the
relaxation of the rock.
There are several different overcoring
procedures and devices all of which have in common that
they require the presence of a diamond drill throughout
the testing procedure and, with the exception of the USBM
gauge, the measuring devices cannot be recovered. As a
consequence, these determinations are very costly and time
consuming and can rarely be carried out on a routine
basis.
Against this background a novel instrument was
recently developed at James Cook University, Australia,
which does not rely on overcoring methods and is fully
recoverable. A full description of the method and
associated apparatus is given by H. Bock et al in the
2 0 6 2 5 4 2
-
Proceedings of the 4th Australian & New Zealand Conference
on Geomechanics in Perth, Australia, 1984. The novel
device is now manufactured and marketed by Interfels,
Germany as the James Cook/Interfels Type 096 Borehole
Slotter (hereinafter referred to as the Slotter). This is
the instrument upon which the above invention is based and
it is therefore described in more detail.
The Slotter works on the (St. Venant) principle
that, when a crack or slot is created in the wall of a
stressed borehole, virtually total stress relief will
occur immediately adjacent and normal to the crack or
slot. This results in a deformation of the rock which is
controlled by the state of stress the rock was prior to
the creation of the crack or slot as well as the
mechanical properties of the rock material.
The Slotter is designed to cut, by means of a
diamond saw blade, an axial slot into the sidewall of a
borehole and to measure the tangential deformation of the
rock caused by the resultant relaxation. The strain
measurement is done with an integrated, specifically
designed recoverable strain sensor. Repeated slots are
cut around the circumference of the borehole wall and the
deformation results are combined. To arrive at the two
~;~ensional stress field normal to the axis of the
borehole, the measured deformations, expressed in
microstrains, which are a function of the stress acting
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~,
perpendicular to the respective slots, and the physical
properties such as the modulus of elasticity and the
Poisson's Ratio of the rock mass, are used for a close
form mathematical solution. To determine the three
~;mensional stress field in the rock mass, similar
measurements are carried out in two additional, non-
coplanar, non-coangular boreholes, and the results of
three boreholes are combined by regression analysis.
Although the slotter is in many respects more
efficient and cost effective than the overcoring methods,
it has the disadvantage of needing three separate
boreholes to determine the three dimensional state of
stress in a rock mass. In view of the cost of drilling
the required "H~' size (96-106mm) diamond drill holes,
which can be as high as $5,000 per hole, considerable
savings could be realized if the number of boreholes
required could be reduced. In addition, the time needed
to carry out measurements could also be reduced
substantially if the procedures would not have to be
repeated in three different boreholes. Lastly, it is
difficult to combine the results of three independent,
diverging boreholes into a single three dimensional stress
tensor, especially when the rock is anisotropic and
structured, which may cause significant differences in
local stress orientations and magnitudes.
It is therefore the object of the present
2062S~2
invention to provide a method and an apparatus, based on
the slotting principle, which would allow the
determination of the three ~;m~nsional stress regime in
rock i) in a single borehole, ii) without the requirement
of an on-site diamond drill and iii) with a device that
can be recovered and reused.
SU~ARY OF THE INVENTION
The method in accordance with the present
invention for measuring three ~;m~n~ional stress in rock
surrounding a borehole comprises cutting slots into the
wall of a borehole at different angles relative to the
borehole axis so as to effect stress relaxation and strain
deformations which are representative of the stresses
normal to the angle of the respective slots and measuring
the respective deformations adjacent to the slots that are
being cut into the borehole wall at a sufficient number of
angles to satisfy the mathematical requirement of six
independent equations to solve for the three ~;men,sional
stress tensors so as to determine the three ~imensional
stress field surrounding the borehole from measurements in
a single borehole.
Cutting of the slot is done by inserting into
the borehole an instrument having a first device for
cutting an axial slot in the wall of the borehole and a
second device for cutting a second slot at an angle of
about 45 to the axis of the borehole. Then after axial
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rotation of the instrument another slot is cut at about
325 relative to the axial slot and perpendicular to the
former angular slot, thus creating a series of three slots
of independent angles. In order to arrive at the m;n;mlim
required number of angles, the three-angle cutting
procedure is repeated at different axial borehole
rotations, preferably but not exclusively, at the two
rotational angles of 60 i 10 and 120 i 10 from the
rotational angle of the first slot series.
The apparatus for carrying out the above method
comprises a tubular main housing adapted to be inserted
and maintained in position in a borehole during
measurement, and two stress relief mechanisms mounted
coaxially within the housing. The first one comprises a
slot cutting device for cutting an axial slot into the
borehole wall parallel to the borehole axis, a strain
sensor assembly adapted to be located adjacent to the
centre of the slot to measure the deformation of the rock
normal to the slot while the slot cutting is taking place,
and means for moving the slot cutting device against the
borehole wall. The second one comprises a slot cutting
device for cutting a slot into the borehole wall at an
angle of about 45 to the borehole axis, a strain sensor
assembly adapted to be located adjacent to the centre of
the 45 slot to measure the deformation of the rock
material normal to the 45 slot while the slot cutting is
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taking place, and means for moving the slot cutting device
against the borehole wall.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be disclosed, by way of
example, with reference to a preferred embodiment
illustrated in the accompanying drawings in which:
Figure 1 is graphic representation of the stress
relief orientations when slots are cut at different angles
into the sidewall of a borehole;
Figure 2 illustrates a preferred sequence of
slot cutting; and
Figures 3a and 3b are schematic diagrams
illustrating top and side views, respectively, of an
embodiment of an apparatus for measuring three ~;m~nsiona
stress in rock surrounding a borehole.
DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to Figure 1, diagram (a) illustrates
the orientation A of stress relief when a single slot 10
is cut axially into the wall of a borehole 12. The
corresponding strain deformation may be measured by
placing a suitable strain sensor 14 adjacent the center of
slot 10. Diagram (b) illustrates the orientations A and
B of stress relief when an additional slot 16 is cut at an
angle of 45 to the axis of the borehole. The
corresponding strain deformation may be similarly measured
by placing a strain sensor 18 adjacent the centre of the
~_ 2062542
slot 16. Diagram (c) illustrates the orientations A, B
and C of stress relief when an additional 45 slot 20 is
cut after rotation of the instrument by 180 from the
first two slots. In order to arrive at the m; n;mllm
required number of angles, the three-angle cutting
procedure is repeated at different axial borehole
rotations, preferably but not exclusively, at the three
rotational angles of 60 i 10 and 120 + 10 from the
rotational angles of first slot series. A graphic
representation is given in Figure 2. Sequence a
illustrates the axial slots at the top and the bottom of
the borehole after cutting the first series of axial and
45 slots. Sequence b illustrates axial slots at 60 and
240 from the top of the borehole after cutting the second
series of axial and 45 slots. Sequence c illustrates the
axial slots at 120 and 300 from the top of the borehole
after cutting the third series of axial and 45 slots.
Referring to Figures 3a and 3b, the apparatus is
contained in a tubular main housing 30, which is adapted
to be inserted, and to be maintained in position by means
of hydraulic jacks 32, in a, preferably but not
exclusively, "H" diamond drill borehole (not shown) at a
depth of up to 25 metres.
The stress relief mechanisms within the
apparatus are divided into two sections, one to create and
measure the strain perpendicular to an axial slot,
~A
.
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referred to as the axial mechanism 34, and one to create
and measure the strain perpendicular to a slot angled at
45 to the borehole axis, referred to as the angular
mechanism 36.
The axial mechanism 34 is comprised of a
tangential strain sensor assembly 38, similar but not
exclusively identical, to the strain sensor of the
Slotter, which is pressed by means of a hydraulic jack 40
or other device against the wall of the borehole in such
a way that it is located within approximately 3 mm from,
in the centre of, and perpendicular to the slot to be cut.
The axial mechanism is further comprised of a slot cutting
device which consists of a 75 to 80 mm diameter, 0.8 to
1.0 mm thick diamond impregnated cutting wheel 42 attached
to a motor 44 in such a way that it allows the cutting of
a radial slot approximately 25 mm deep and up to 75 mm in
length into the borehole wall parallel to the borehole
axis. The motor is mounted on a wedge assembly 46 or
other device which is operated by a hydraulic or electric
motor 48 to move the cutting wheel against the borehole
wall. The deformation of the rock material adjacent and
normal to the slot is measured with the strain sensor
while the slot cutting is taking place. The procedure is
deemed terminated when there is no longer any deformation
registered while the slot is being cut. The deformation
is measured directly in micro strains.
~ 2062542
The angular mechanism 36 is similar to the axial
mechanism, however with the difference that it comprises
a tangential strain sensor assembly 50 which is mounted in
such a way that it allows the measurement of the
deformation of the rock in the centre of, and
perpendicular to a radial slot to be cut at an angle of
45 to the borehole axis. The slot cutting device is
similar to the axial mechanism with the difference that it
comprises a cutting wheel 52 which is attached to the
motor in such a way that it allows the cutting of a radial
slot approximately 25 mm deep and up to 75 mm in length
into the borehole wall at an angle of about 45 to the
borehole axis. The cutting and measurement procedure is
similar to that of the axial.
To obtain the minimum number of deformation
measurements at independent angles - with sufficient
redundancy - to solve for the three stress tensors, ~1, ~2
and ~3 the preferred overall test procedure is the
following:
- The apparatus is inserted into the horizontal or
sub horizontal "H" size borehole to
the appropriate depth.
- The first axial and angular slots are
cut at the top of the borehole.
- The apparatus is then rotated in the
borehole by 180 and another axial
,_ 2062542
11
and angular slot is cut at the bottom
of the borehole.
- The apparatus is advanced 10 cm
deeper into the borehole and the
above procedures are repeated at 60
and 240 respectively from the top of
the borehole (sequence 2, Fig. 2).
- The apparatus is advanced 10 cm
deeper into the borehole and the
above procedures are repeated at 120
and 300 respectively from the top of
the borehole (sequence 3, Fig. 2).
Although the invention has been disclosed with
reference to a preferred embodiment, it is to be
understood that it is not limited to such embodiment and
that other alternatives, within the scope of the following
claims, are also envisaged.
