offsets. Figure 18 shows the measured BPC records for
the 034500 (4) and 122020 (B) events along with the best
fit offsets (least squares) using dislocation slip models.
For the redundantly oriented BPC’s, only cells 6 and 8
were used in the fits since their total pressures were
closest to the average total pressure for the other cells of
the array. The 034500 event was modeled as a vertical-
rectangular dislocation 297 m on a side, centered at the
located event. The model for this event allowed the
relative amounts of strike slip and dip slip to be obtained
from the fit. The best fit at 95% confidence has 1.09
(£1.18) mm left-lateral strike slip and 1.6 (+0.99) mm
northeast-downward dip slip. The large uncertainty in fit
can be attributed to measurement error and to inadequacy
of the homogeneous elastic media of the dislocation model
for describing the actual response of the fractured rock
mass.

The 122020 event (fig. 188) was modeled as a vertical-
rectangular dislocation, 28 m on a side, centered at the
location for that event. The dislocation is oriented N. 40°
W., consistent with the vertical nodal plane of the
mechanism. (The other nodal plane would require
right-lateral strike slip motion only. Such motion results
in BPC offsets in the opposite sense from those observed
for all but cells 5 and 6.) The orientation and relative
amounts of northeast-downward dip slip and left-lateral
strike slip were determined by the composite focal
mechanism as shown in figure 13. The total amount of
slip that best fits the offsets at 95% confidence was 0.45
(£0.55) mm.

There are several possible reasons for the large
uncertainties in the fit for this event:

1. Measurement error.

2. The homogencous elastic model may not ade-
quately represent the complex stope geometry in the vicin-
ity of this event.

3. There may have been a localized inelastic response
of the rock mass at the BPC array because of its close
proximity to the 122020 event.

4. The cells are very close to the fault, and their
response may be strongly influenced by the details of the
fault geometry. This affects the offsets computed by dis-
location modeling as well, because in this case the cell
array turns out to be 15 m from the top edge of the dislo-
cation and 20 m from the center, in an area of rapidly
changing stress field. Since the position of the disloca-
tion center with respect to the hypocenter is not well
constrained, a better fit might be obtained by adjustment
of this parameter. Similarly, there is an uncertainty in the
size and shape of the dislocation patch, where a rectangu-
lar patch is used as a matter of convenience. The role of

uncertainty in the patch geometry is probably dominated
by edge effects, i.c., exactly where the patch boundaries
fall relative to the BPC array.

5. Deviation from the assumed planar slip surface may
play a role. For example, the orientation of the fault may
have shifted away from the fault plane solution after slip
began.

6. Cells 5 through 8 are less smooth in their be-
havior than the others for this event: During the previous
10-min interval, they show changes in pressure of about
the same magnitude as the observed changes during the
10-min interval that encompasses the event.

Fit

PRESSURE

Scale, kPa

U [ 200

TIME, h

Figure 18,—Fits of measured pressure changes on BPC array
computed using dislocation sources. (Curves are labeled with
cell numbers; curves are offset for clarity and do not indicate
relative or absolute pressure differences among cells. Fitted
offsets are shown to left of each curve used in fit) A, Fit for
034500 event. Amount of slip was 1.1 mm of left-lateral strike slip
and 1.6 mm of northeast-downward dip slip; B, fit for 122020
event. Relative amounts of strike slip and dip slip were
constrained by fault plane solution (see figure 13).



PRESSURE

1 L

Scale, MPa
[ 10

Aug. Sept. Oct.

Nov.

Dec. Jan.

Figure 9.—Time history of pressure recorded manually for BPC array from installation at end
of July 1989 through January 1990. An offset on several cells Is evident, coinciding with time
of October 5, 1989, rock burst, which took place near 46-99 stope. (Curves are labeled with cell
numbers; curves are offset for clarity and do not Indicate relative or absolute pressure

differences among cells.)

Table 1 and figure 5 indicate the significant events that
followed the destress of February 2, 1990. The Gutenberg-
Richter magnitude-energy scale (10) shows that by far the
largest energy-release event is the one at 034500 on
February 7 (table 1). This event released 350 times the
energy of the next largest event. The location of this event
coincides with the plane that marks the locus of damaging
seismic events occurring in the mine during a 20-month
study period discussed above (5). Because this event and
the event at 122020 later the same day caused damage,
these two events are the subject of the analysis that
follows.

Noteworthy events that are believed to be related
to the two damaging events on February 7 are shown in
figures 10 (for the 034500 event) and 11 (for the 122020
event), The five events shown in figure 10 are distin-
guished from the hundreds that triggered the routine-
monitoring system during the 0300 hour because either
their energy was sufficient so that they were detected on
the surface seismograph, or the energy as monitored by
the routine-monitoring system was unusually high. These
five events can be enclosed by a sphere 185 m in diameter;
this volume is comparable to that expected to be affected
by the strain release in the event. The events related to
the 122020 event, shown in figure 11, have very similar

SEISMIC EVENTS

waveform first-motion polarity-pattern distributions (11) to
the 122020 event. Since they are in the same place, within
the location uncertainty, this is taken as evidence that the
mechanism is the same for all these events.

EVENT MECHANISMS

Event focal mechanisms were estimated for the 034500
and 122020 events. The principle and methods for deter-
mining focal mechanisms are described in various litera-
ture; one report is by Bath (72).

Although it is well known that the velocity structure
in the vicinity of the mine is strongly heterogeneous (and
possibly weakly anisotropic) (13), an accurate velocity
model is not yet available. In lieu of such a model, a
constant velocity of 5.02 km/s was used for both the loca-
tion of events and for focal mechanism determinations;
this value was determined from plots of travel time versus
distance for calibration blast tests performed earlier in
a nearby and overlapping microscismic array. The inaccu-
racy of the velocity model means that computed takeoff
angles and azimuths are inaccurate by an unknown amount
corresponding to the difference between the constant
velocity model used and the true velocity structure.


is not favored. (This neglects the effects of pervasive
fracturing on the stiffness of either the ore vein or the
country rock.) There is a tendency to deflect away from
the stiff region back into the low-stiffness material. Thus,
any cracks that initially extend in the direction of the maxi-
mum principal stress may deflect away from high-stiffness
country rock, Once deflected, such a crack is no longer
optimized for releasing stored elastic strain energy because
of the maximum principal stress.

To make effective use of principal stress data and the
basic principles of fracture mechanics, the objective of
destressing must be taken into account. Ifit is to release,
in a controlled fashion, the stored clastic strain energy that
is capable of being released in the largest expected rock
burst (i.e., without triggering a large rock burst), then the
destress blast pattern should be large enough and of the
appropriate layout. This may be impractical, in which case
alternative methods should be considered. What is the
volume of rock that releases strain energy during the
largest (M,, ~ 3) rock bursts at the Galena Mine? Swan-
son (6), using several different approaches to estimating
this volume, suggests that a magnitude 2.9 rock burst may
be driven by clastic strain energy release from a volume as
large as 2.4 x 10’ m? or a sphere of ~300 m diameter.
This characteristic source dimension is not unique, how-
ever, and will depend upon, for a given event magnitude
(or seismic moment), the stress or strain drop associated
with the release mechanism (i.e., the lower the stress drop,
the larger the volume), The volume of stress release
associated with a magnitude +3 event with a small to

modest stress drop will not likely be released controllably
by any amount of destress blast drillholes in a single pillar.
Destress blasting may, however, influence the initiation of
such an event; to do this in a planned way would require
a sophisticated understanding of the current stress and
stability state of the structure and how to modify these.
The February 7, 1990, 034500 M,, ~ 2.9 event produced
significantly elevated microseismic activity in stopes en-
compassing a volume of 2 x 10’ m®. The stored energy
associated with a smaller magnitude but potentially very
destructive event with high stress drop centered on a single
stope may conceivably be released controllably by destress
blasting in that particular stope.

If the objective is to trigger a large seismic event, then
the destress strategy should be designed to optimize trig-
gering; this is not necessarily consistent with softening the
pillar. Other methods such as water infusion may be more
suited to accomplishing this objective. At the present time
it is not possible to be specific, without being unduly spec-
ulative, about which method should be used because this
requires a firm understanding of the mechanisms of rock
bursting. While the state of understanding is growing
rapidly in this area and the tools highlighted in this report
are being used to help characterize rock burst mecha-
nisms, current understanding is not sufficiently developed
at this point to make detailed recommendations for
changes in destress strategies. It is quite apparent that
an understanding of rock burst mechanisms is the first step
required for developing triggering or avoidance methods
in the most effective and safe manner.

CONCLUSIONS

Examples of the use of new analysis techniques, such as
focal mechanisms and dislocation modeling, for quantita-
tively describing mining-induced ground deformation have
been presented. These techniques, which are still under
development and adaptation for use in the mining environ-
ment, are being evaluated by the Bureau to determine if
they can provide the information needed to evaluate
existing stress control methods and to design new methods.

Seismic observations and measurements of changes in
ground stress were made in conjunction with a destress of
the 46-99 stope of the Galena Mine. Similar observations
were made of seismic events that occurred during the 2%
weeks following the destress. Two of the seismic events
during this period caused damage to the mine. These two
events were studied in depth to identify their relationship
to one another and the destress. Focal mechanisms for
the damaging seismic events were used as constraints for
dislocation models computed to fit the observed pressure
offsets on an array of BPC’s. These computations showed
that the larger damaging seismic event was probably an

important factor in promoting the occurrence of the
smaller damaging event later the same day.

Given the observed small stress change due to the
destress on February 2, 1990, and the evidence from the
state of stress and fracture mechanics considerations, some
recommendations about destressing can be made. First,
adequate knowledge of the existing local stress field is
necessary for designing appropriate hole patterns for blast
destressing. While in situ stress can be accurately meas-
ured at individual points using, for example, overcoring
techniques, this is probably impractical for the majority of
situations where destressing will be attempted. Further, it
does not provide adequate three-dimensional local stress
field information. Knowledge of the far-field stresses from
available tectonic and regional seismic data, combined
with numerical modeling of the effect of mining in the
area to be destressed, may be a more practical approach.
Any additional knowledge from borehole breakouts to
analyses of in-mine or stope-level seismic and microseismic
mechanisms is desirable. Second, blast hole patterns for


SEISMICITY AND STRESS CHANGES SUBSEQUENT TO DESTRESS
BLASTING AT THE GALENA MINE AND IMPLICATIONS
FOR STRESS CONTROL STRATEGIES

By F. M. Boler! and P. L. Swanson’

ABSTRACT

The U.S. Bureau of Mines conducts research at the Galena Mine, Wallace, ID, with the aim of
mitigating the effects of rock bursting. Destress blasting is commonly used as a stress control technique
at the mine. A digital seismic array and an array of borehole pressure cells (BPC’s) had been installed
near the site of a stope undergoing mining and periodic destressing. The instrumentation was being
monitored at the time of a destress blast of the 46-99 stope. No significant seismic events occurred
coincident with the destress. However, the destress was followed by a 2¥%-week period of increased
seismic activity, including two damaging events on February 7, 1990, at 034500 (hour, minute, and
second) and 122020 Pacific standard time. BPC measurements indicated coseismic ground pressure
changes on the order of 200 to 300 kPa associated with the damaging events. Fault plane solutions and
dislocation models established that stress changes induced by the event at 034500 may have been
significant in promoting the occurrence of the event at 122020. Theoretical investigations suggest that
applying knowledge of the existing stress field, an understanding of rock burst mechanics, and fracture
mechanics principles can improve destress effectiveness.

1Geophysicist, Denver Research Center, U.S. Bureau of Mines, Denver, CO.


EVENT INTERACTION

The dislocation parameters that fit the BPC data
for the 034500 event were used to calculate the induced
changes in stress field components in the vicinity of the
122020 event. The results are used to determine if the
stress changes produced by the 034500 event inhibit or
enhance the prospects for slip of the type observed for
the 122020 event. Figure 19 shows the shear and normal
stresses on the 122020 fault induced by the dislocation on
the 034500 fault (see "Fits to Borehole Pressure Cell Data"
section). This dislocation results in a decrease in normal
stress of somewhat less than 0.2 MPa across the 122020
fault. This would enhance the possibility for slip in any
direction on this plane. This dislocation also induced an

B

Dis

Computation
plane

o

a,

100

Scale, kPa

increase in shear stress favoring southwest-downward dip
slip of 0.4 MPa (maximum) and a decrease in shear stress
favoring left-lateral strike slip of 0.3 MPa (maximum).
The composite focal mechanism for the 122020 event
shows a combination of left-lateral strike slip and
southwest-downward dip slip. Ignoring the preexisting
tectonic and mining-induced stresses acting on the 122020
fault, the potential for the observed southwest-downward
dip slip was enhanced by the 034500 dislocation, while the
potential for the observed left-lateral strike slip motion
was inhibited by the 034500 dislocation. Two components
of the induced stress field enhanced the possibility for the
observed slip, suggesting that the 034500 event was
important in promoting the occurrence of the 122020 event
later that day.

f]

islocation.
plane

Figure 19.—Stress field on 28- by 28-m fault plane for 122020 event, induced by dislocation slip on 297- by 297-m fault plane of
034500 event. A, Plan view of 4300 level showing plane of dislocation and plane of computation; B, oblique perspective view looking
parallel to plane of computation; C, induced normal stress. Distance in direction perpendicular to computation plane (vertical line) of
point of gray-tone surface Is proportional to induced stress. Since surface ls completely to left of computation plane, induced
stress is all tensile; D, induced strike slip shear stress. induced stresses are in the sense to favor right-lateral strike slip; E, induced
dip slip shear stress, Induced stresses are in the sense to favor southwest-downward dip slip.



IMPROVING DESTRESS EFFECTIVENESS

The destressing method of drilling holes into pillars and
blasting has been used for over 20 years in the Coeur
Alene district. Effort is rarely expended in determining
whether existing information can help determine whether
the destressing will be effective. With knowledge of the
state of stress and the principles of fracture mechanics, it
is possible to make educated judgments about whether a
particular destressing operation is capable of accomplish-
ing either (1) inducing strain energy release through a
coincident seismic event, or (2) softening the remaining
pillar rock so that strain energy cannot be stored.

State of Stress Estimates

A stress province map of the continental United States
was recently published (18). Stress provinces are sub-
regions of the continent that possess similar principal
stress directions and relative magnitudes. The stress
provinces are formed in response to tectonic processes.
Stress data used to establish these provinces include
earthquake focal mechanisms, borehole breakouts, in situ
stress-relief measurements, hydraulic fracturing, and other
stress indicators. The map in reference 18 shows a ques-
tion mark in the region surrounding northwestern Mon-
tana and the northern Idaho Panhandle. The question
mark is present not for a lack of stress measurements, but
because of inconsistencies in the available data (19). In-
consistencies may occur in at least two ways. First, in a
stress field that has equal horizontal components, a pre-
ferred orientation of directional indicators of stress is
not expected. Second, the stress measurements in this
region, most of which are stress-relief measurements in
Coeur d’Alene mines, may be influenced by stress field
heterogeneity induced by mining. Some studies specifically
acknowledge this influence. In addition, overcoring stress
measurements generally receive the lowest quality rating
for use in determining regional tectonic stress (18) because
many measurements are not presented with evidence of
their consistency over suitable distances.

It is easy to understand how individual point meas-
urements of stress made in extensively fractured and
faulted ground (fig. 2) may not be representative of
regional stress, particularly when measurements are made
near high-stress gradients. The Bureau is examining
methods to measure principal stresses in fractured and
faulted ground by inversion of focal mechanism param-
eters obtained for groups of distributed microseismic
events. Such analyses provide data useful for planning and
evaluating destressing techniques, circumventing the com-
plications of making conventional point measurements of
stress in inelastic and nonhomogeneous ground.

State of Stress and Destress Strategies

Knowledge of the approximate principal stress direc-
tions is essential to design effective destressing strategies.
This knowledge is particularly important when relying up-
on the release of stress by blast-induced cracks; principal
stress directions strongly influence crack propagation
directions and thus determine the manner in which
stresses are rearranged. With a measure of the local (on
the scale of a stope) principal stress directions, one may
devise a destress strategy based on the actual local con-
ditions that takes advantage of these conditions. The
stress field of interest is not necessarily the regional stress
field because not only do the workings perturb this stress
field, but stoping areas are often in a stress shadow or in
an area of stress amplification because of other nearby
heterogencities, particularly mined-out veins, which may or
may not be backfilled. In the case of the Galena Mine at
the present stage of development, most of the veins are
close enough together and of sufficient size for stress
interaction to occur. While knowledge of the stress field
applied on the scale of the entire mine is certainly ben-
cficial, it is not sufficient for engineering stress control
measures in individual, closely spaced stopes.

In the present study, the principal stress directions
acting in the pillar were estimated from borehole
breakouts that formed when the vertical destress blast
holes were drilled. Borehole breakouts, or stress-induced
¢longation of boreholes, form in a direction perpendicular
to the maximum principal compressive stress (20-21). In
all but one of the destress holes, breakouts were oriented
parallel to the strike of the vein, indicating a maximum
principal stress component, resolved in the horizontal
plane, that was perpendicular to the strike of the pillar.
The exception to this trend was a borehole separated from
the others by an intervening fault. Its breakout was
perpendicular to the others, suggesting that at the time the
holes were drilled, the fault surface provided a disconti-
nuity in stress. This observation is an example of the need
for multiple consistent stress measurements over a repre-
sentative distance for high-quality stress determinations.
With the principal stress directions in hand, one can
estimate the influence of the stress field on blast-induced
crack formation and propagation in the pillar. It is well
known from fracture mechanics that tensile fractures tend
to propagate parallel to the direction of maximum princi-
pal compressive stress. This is also a very stable crack
geometry in that, once the dynamic crack propagation
phase ceases, the driving force for crack extension in-
creases only with further increases in applied load (22). It
is also known that propagation of cracks from a region of
low stiffness (for example, the ore vein) into a region with
higher stiffness (for example, the quartzite country rock)


Scale, kPa

[ 400

PRESSURE
Foy US

ETE

niiy ¢ f

1 1 n L
3 4 5 6

ee FEBRUARY

Figure 15.—Digitally logged BPC data for February 1-9, 1990. Period of k isy data on
February 3 and 4 Is the weekend, when minimal crews were working. (Curves are labeled with cell
numbers; curves are offset for clarity and do not indicate relative or absolute pressure differences

among cells. Times corresponding to 034500 and 122020 events on February 7 are shown as
vertical lines.)

N

TTT qt Tt TTT TTT

6 [ zo _
Le pO
Bf} +> ——7Prm |
4 ee ee mene ee
payer

PAA

PRESSURE

1 —\_____
Dd
1 3 5 7 9 11°13 45 17 19 21 23

TIME, h oe

Figure 16.—BPC data for February 7, 1990. (Curves are labeled with cell numbers; curves are
offset for clarity and do not Indicate relative or absolute pressure differences among cells. Times
corresponding to 034500 and 122020 events are shown as vertical lines.)



4300 level 46-99 sill drift |
Pillar
Destress holes
: |
2
Cs
c
oO 10
[a rr
Scale, m

Figure 4.—Schematic vertical section view of 46-99 stope du

ring February 2, 1990, destress blast. Eight 10-m

boreholes and three 4-m boreholes were filled with total of 125 kg of explosives for destress.

ACKNOWLEDGMENTS

The authors acknowledge ASARCO Incorporated for
their cooperation with the Bureau in providing informa-
tion, access, and assistance for the data-gathering phase
of this research. Clinton Sines, engineering technician,
Denver Research Center, provided invaluable on-site

expertise and assistance. Louis Estey, geophysicist, Denver
Research Center, wrote the three-dimensional rendering
and data-fitting software used in data analyses. Robert
Simpson, geophysicist, U.S. Geological Survey, Menlo
Park, CA, provided the three-dimensional dislocation code.

INSTRUMENTATION

SEISMIC INSTRUMENTATION
Surface Seismometer

The seismic sensor deployed in an adit near the surface
is a Teledyne Geotech S-510° short-period, vertical-velocity
gauge. Its location is 1.3 km vertically above and 0.2 km
laterally northwest from the 46-99 stope. The signal is
amplified, filtered, and displayed via drum recorder. The

3Reference to specific products does not imply endorsement by the
US. Bureau of Mines.

gain is adjusted so that microseismic events associated with
ordinary production blasting are just visible above the
background noise level. Local magnitudes for most lo-
cated events fall in the range -0.5 to 2.9 (6). The largest
magnitude event in the mine during a 20-month study
period, M, = 2.9, occurred 5 days after the February 2,
1990, destress blast of the 46-99 stope pillar. Its hypo-
center was 110 m from the stope [February 7, 1990, 034500
(hour, minute, second), in figure 5]. (All times in this
report are Pacific standard time.)


80

60
40)

20

[|

1989

SEISMOGRAPH AMPLITUDE, mm

Destress

1990 1991

Figure 1.—Selemic activity In vicinity of 46-99 stope during
January 1989 through October 1990. Number of large amplitude
events In 20 days after February 2, 1990, destress represents
unusual concentration of such events.

The purpose of this report is to show how quantitative
evaluation of the effectiveness of a destress is possible.
This is accomplished through monitoring the stress state
and changing stress conditions using full waveform micro-
seismic data as well as quasi-static rock-mechanics data.
Through seismic and stress observations and subsequent
modeling, the degree of stress interaction between the
seismic events following the destress is examined. Digitally
recorded accelerometer waveforms are used to constrain
the focal mechanisms for the damaging events and several
associated microseismic events. These mechanisms, along
with the event locations, are used to define fault planes
on which dislocation slip, constrained by stress-change
measurements, is determined. These analyses show how
Processes surrounding destressing can be quantitatively
described. With further observations, including knowledge
of the near and far-field stresses, quantitative evaluation
of destress effectiveness is possible. Further, knowledge of
the state of stress in the area to be destressed, along with
the mine and host rock structure, including fractures and
faults, can lead to the design of better destress methods.

BACKGROUND

GEOLOGIC SETTING

Hobbs (4) discusses the geology of the district. Fig-
ure 2 shows a location map with faults in the vicinity of
the Galena Mine. The country rock consists of Precam-
brian Belt Series quartzites and minor argillites. Much of
the faulting displacement postdates mineralization. Fig-
ure 3 shows the mapped vein geology in the vicinity of the
46-99 sill drift. The vein is near vertical and strikes
approximately N. 45° E. Vein offsets are indicative of
postmineralization faulting; bedding plane faults striking
approximately N. 45° W., cutting the vein at nearly 90°, are
most common. The apparent sense of offset is typically
right lateral; the maximum apparent offset (plan view) for
faults in the 46-99 stope area is approximately 4 m.

LOCUS OF MAJOR SEISMIC ACTIVITY
WITHIN MINE

The relationship between the locations of stopes ex-
periencing rock bursting and the major fault trends in the
immediate vicinity of the Galena Mine was recently inves-
tigated (5). Seismic data recorded on the surface seismo-
graph over a 20-month observation period were correlated
with rock bursts and other large seismic events that were
located using dense arrays of accelerometers. Seventy
percent of the stoping areas associated with the 29 largest
seismic events [local magnitude (M,) ~ 1.8 to 2.9] were
found to occur on a single, well-defined, near-vertical

plane during this time interval. The plane had dimensions
of 1.6 km in length and 0.4 km in height and was centered
1.5 km below surface. The strike of the plane, N. 48° W.,
generally paralleled several 10-km-long faults mapped on
the surface (fig. 2) and identified underground. The few
events that did occur away from this plane during this time
period took place on another structurally significant fea-
ture, the Silver Vein (dimensions of 300 by 800 m.)

While examination of the mine’s geologic maps does
not reveal a single major well-defined fault surface that
extended continuously throughout the entire plane of
major seismic activity, there are many faults of the
appropriate strike and location, Wherever a significant
fault is not in the appropriate position in a stope, the maps
indicate the potential to accommodate slip over the same
general trend and area via a series of blocky structures
with bounding surfaces of similar orientation. Such is
the nature of the structure in many mines of the Coeur
dAlene district. Using relationships between inferred
rupture plane dimensions and seismic magnitudes pub-
lished in the mining seismology literature, Swanson (6)
shows that slip is conceivably mobilized over this entire
plane.

During one 3-month period of high seismic activity, the
Galena shaft, which is intersected by the plane of large
seismic activity, experienced ground control problems that
shut it down for repair for several months. Fault gouge
from the major near-vertical fault strand that it intersects
was expelled into the shaft directly above the plane of


INTRODUCTION

The U.S. Bureau of Mines conducts rock burst research
in the Coeur d’Alene silver district of northern Idaho.
Some of the Bureau’s research effort has focused on meth-
ods of destressing burst-prone pillars (1). As part of
its program to improve safety in mines, the Bureau con-
ducted research to show how quantitative evaluation of the
effectiveness of a destress is possible.

The purpose of destressing is to eliminate or, at a mini-
mum, reduce the rock burst danger to miners. The ulti-
mate test of the success of a destress is whether damaging
events do or do not occur once work resumes in the stope
after a destress. If no damaging events occur for an ex-
tended (but unspecified) time period once work resumes,
then the miners have been protected.

It is not currently possible to guarantee that a destress
will achieve the desired objective, nor is it possible to
tell whether the destress has achieved the desired objective
until a sufficient time has passed without a damaging
event. It would be desirable to (1) have a quantitative
means to evaluate the effectiveness of a destress blast
before work resumes in the stope, and (2) be able to tailor
the design of a particular destress to the site characteristics
to maximize its effectiveness.

The results of the research of the early 1970’s by Blake
(2) showed that pillar destressing by means of blasting
could be helpful in controlling rock bursts. There are two
ways that destress blasting may achieve the desired objec-
tive. First, the stiffness of the pillar might be reduced by
fracturing caused by the blast. As in laboratory rock sam-
ple testing with a stiff loading machine, when the stiffness
of the pillar (rock sample) is lower than the surrounding
rock (loading machine), then unstable failure of the pillar
(rock sample) is impossible. Leighton (2) pointed out that
stresses previously borne by the pillar may be increased to
dangerous levels in the surrounding rock mass by the pillar
destressing. Thus, while the pillar itself might not fail
unstably, some part of the surrounding rock mass might.

Second, the dynamic nature of the destress, in combina-
tion with the fracturing generated, might trigger a large
seismic event that effectively releases strain energy that
might otherwise be released in a rock burst. Cook (3)
suggested that the destress could be viewed as a success
only if destressing released seismic energy in excess of the
energy contained in the blast itself. This is the criterion
used in practice by mine management to judge destressing
effectiveness at the time of the blast.

Since these early studies, destressing has been under-
taken frequently at the Galena silver-lead-zinc mine, in

Italic numbers in parentheses refer to items in the list of references
at the end of this report.

Wallace, ID, the site of the present study, as well as other
mines worldwide. The mining method at the Galena Mine
is overhand cut and fill of the near-vertical silver-bearing
veins, The general progression on a vein is from the sur-
face downward, but individual stopes are mined upward
between horizontal access drifts separated by a vertical
distance of 90 m.

Destress blasting is typically used when two conditions
occur simultaneously: (1) the overhand pillar size reaches
a critical height of 10 to 20 m (J); and (2) an increase in
numbers and rates of microseismic events detected with an
accelerometer array surrounding the stope occurs. The
second condition alone may be sufficient for a destress to
be made,

On February 2, 1990, the 21-m-high pillar in the 46-99
stope of the Galena Mine was destress blasted. This was
the second attempt at destressing this stope in 2 months;
a previous attempt took place in December 1989. A sub-
sequent destress was attempted in June 1990. No "signifi-
cant" seismic event was observed coincident with any of
the three destressing attempts. From the point of view of
success of destressing, the size of a "significant seismic
event" has been determined through experience by mine
personnel. [In this report, “significant seismic event" refers
to an event that generates displacements sufficient to peg
the surface seismometer recording system (an event with
an approximate local Richter magnitude greater than 0.7).]
While no significant events coincided with the February 2
destress, several significant events occurred over the
following week. Also, when mining resumed with a pro-
duction blast on February 19, a significant event that
caused damage in the raise followed blasting by less than
30 min, Figure 1 shows the significant events in the 46-99
stope from the period January 1989 through October 1990.
The intensity of activity following the destress of
February 2, 1990, is noteworthy.

Because significant events that potentially endangered
miners occurred after work had resumed in the stope, the
destress cannot be viewed as successful. By applying tech-
niques to quantitatively evaluate the state of stress before
and after a destress, it may be possible to identify such
hazardous situations.

The significant events that followed the destress were
recorded by a surface seismometer and, in most cases, by
an underground digitally recorded accelerometer array.
Stress-change-monitoring equipment was deployed in the
vicinity of the stope and was monitored during destressing
and during the following weeks.


Scale, kPa

[ 100

/,

15

| ue

TIME,

16
h

Figure 17.—BPC data for February 2, 1990. (Curves are labeled with cell numbers; curves are
offset for clarity and do not indicate relative or absolute pressure differences among cells. Time
corresponding to destress Is shown as vertical line.)

(19 MPa) is about twice that of cell 5 (10 MPa); the total
pressure for cell 7 (32 MPa) is about 15% higher than that
of cell 8 (27 MPa).] The observed offsets due to the

February 7 and other events are not significantly different
for these duplicate oriented cells in spite of the differences
in total pressure.

DISLOCATION MODELS OF FAULT OFFSET

By using dislocation models of seismic sources, static
changes in elastic field values (after minus before) caused
by fault slip can be computed; for an example, see refer-
ence 14, These models are based on analytic expressions
for the elastic fields developed by a rectangular disloca-
tion (5). Computations in this report were done with a
FORTRAN program described by Erickson (16). Such
models were used to find the amount of fault slip nec-
essary to fit the observed pressure offsets at the BPC
array. The available seismic measurements (event location
and fault plane solution) were used to constrain the loca-
tion, size, orientation, and sense of slip for the dislocation
models, Dislocation models were also used to evaluate the
effect of the 034500 event on the fault plane site of the
122020 event.

FITS TO BOREHOLE PRESSURE CELL DATA

The focal mechanisms determined above were used to
define fault planes for each of the 034500 and 122020
events. Magnitudes for these two events were obtained
using the coda duration (of seismic signal after first
arrival) fit by Swanson (6). Slip areas were estimated
using magnitude-area relations (17). Fault plane solution
information was used to constrain the possible sense of
slip for the 034500 event, but not the relative amounts of
strike slip and dip slip, since these were not constrained by
the available polarity data. For the 122020 event, the fault
plane solution was used to constrain the relative amounts
and sense of strike and dip slip. The magnitude of slip
was then determined by the best fit to the measured BPC


Table 1.—Destress blast and postdestress seismic events with M, 20.6

Date! Time? M, EJ Damage Comments
900202 ...... 144910 04 25x10 None reported ........ Destress blast.
900204...... 035210 12 4.0.x 10° Roof fall in stope . None.
900207...... 034500 29 1.4.x 10? 5-6 cars muck down... . Largest event in mine during
. 20-month period.
122020 0.9 1.4x10° — Track uplifted above stope None,
135421 0.6 5.0x10° None reported ..... wee Do.
144608 12 0 4.0x108) ..do.... teeee Event occurred 25 min after

first production blast fol-
lowing destress.

E Energy.
M, Local magnitude.

1Format of this field is yymmdd, where yy = year, mm
Format of this field is hhmmss, where hh = hour, mm

4300 level

month, and dd
minute, and ss

day.
second.

Figure 3.—Plan view map of vein exposures in drifts and crosscuts of 4300 level in vicinity of 46-99 sill drift.


the most effective crack growth pattern must be designed
with consideration of the existing stress field and the
available access. Third, any means of monitoring the

response of the rock mass to the destress, such as ground
pressure or microseismic monitoring, is highly desirable so
that destress effectiveness can be evaluated.

REFERENCES

1. Blake, W. Destressing Test at the Galena Mine, Wallace, Idaho.
Soe. Min. Eng., AIME Trans. v. 252, 1972, pp. 294-299.

2. Leighton, F. W. A Case History of a Major Rock Burst. BuMines
RI 8701, 1982, 14 pp.

3. Cook, N. G. W., E. Hoek, J. P. G. Pretorius, W. O. Ortlepp, and
M. D. G. Salamon, Rock Mechanics Applied to the Study of Rock-
bursts. S. Afr. Inst. Min, and Metall. J., v. 66, 1966, pp. 436-528.

4. Hobbs, S. W., A. B. Griggs, R. E. Wallace, and A. B. Campbell.
Geology of the Coeur d’Alene District, Shoshone County, Idaho. U.S.
Geol. Surv. Prof. Pap. 478, 1965, 139 pp.

5. Swanson, P. L., and C.D. Sines. Repetitive Seismicity (M ~ 2-3)
and Rock Bursting Along a Plane Parallel to Known Faulting in the
Coeur é’Alene District, ID. EOS Trans. Am. Geophys. Union, v. 71,
1990, p. 1453.

6. ___. Characteristics of Mining-Induced Seismicity and Rock
Bursting in a Deep Hard-Rock Mine. BuMines RI 9393, 1991, 12 pp.

7, Steblay, B. J., B. T. Brady, and T, Swendseid. Innovative Micro-
seismic Monitoring System. Paper in Proceedings of the 2nd Interna-
tional Symposium on Rockbursts and Seismicity in Mines (Minneapolis,
MN, 1988). Balkema, 1990, pp. 259-262.

8 Boler, F. M, and P. L. Swanson. Computer-Automated
Measurement- and Control-Based Workstation for Microscismic and
Acoustic Emission Research. BuMines IC 9262, 1990, 9 pp.

9. Haramy, K. Y., and R. O. Kneisley. Hydraulic Borehole Pres-
sure Cells: Equipment, Technique, and Theories. BuMines IC 9294,
1991, 26 pp.

10, Gutenberg, B., and C. Richter. Seismicity of the Barth. Princeton
Univ. Press, 1949, 273 pp.

11. Billington, S., F. M. Boler, P. L. Swanson, and L. H. Estey. P-
Wave Polarity Patterns From Mining Induced Seismicity in a Hard-Rock
Mine, Paper in Proceedings of the 31st Symposium on Rock Mechanics.
CO Sch. Mines, Golden, CO, 1990, pp. 931-938.

12, Bath, M. Introduction to Seismology. Birkhaeuser (Basel), 2d ed.,
1979, 428 pp.

13. Estey, L. H., P. L. Swanson, F. M. Boler, and S. Billington, Micro-
seismic Source Locations: A Test of Faith. Paper in Proceedings of the
3st Symposium on Rock Mechanics. CO Sch. Mines, Golden, CO,
1990, pp. 939-946.

14, Chinnery, M. A. The Stress Changes That Accompany Strike Slip
Faulting. Bull. Seismol. Soc. Am. v. 53, 1963, pp. 921-932.

15. Mansinha, L., and D. E. Smylie. The Displacement Field of In-
clined Faults. Bull. Seismol. Soc. Am., v. 61, 1971, pp. 1433-1440.

16. Erickson, L. L. A Three-Dimensional Dislocation Program With
Application to Faulting in the Earth. M.S. Thesis, Stanford Univ., Stan-
ford, CA, 1986, 167 pp.

17. Utsu, T,, and A. Seki, A Relation Between the Area of After-
Shock Region and the Energy of Main Shock. J. Seismol. Soc. Japan
(Zisin), v. 7, 1955, pp. 233-240 (in Japanese).

18. Zoback, M. L., and M. D. Zoback. Tectonic Stress Field of the
Continental United States. Paper in Geophysical Framework of the
Continental United States. Geol. Soc. Am. Mem. 172, 1989, pp. 523-539.

19. Zoback, M. L. (U.S. Geological Survey). Private communication,
1991; available from P. L. Swanson, BuMines, Denver Research Center,
Denver, CO.

20. Gough, D. L, and J. S. Bell. Stress Orientation From Bore-
hole Wall Fractures With Examples for Colorado, East Texas, and
Northern Canada. Can. J. Barth Sci, v. 19, 1982, pp. 1358-1370.

21, Haimson, B. C., and C. G. Herrick. Borehole Breakouts—A New
Tool for Estimating In-Situ Stress? Paper in Proceedings of the Inter-
national Symposium on Rock Stress and Rock Stress Measurements
(Stockholm, Sept. 1-3, 1986). Centek, 1986, pp. 271-280.

22. Rooke, D. P., and D. J. Cartwright. Compendium of Stress Inten-
sity Factors. Hillingdon Press, London, 1976, 330 pp.



REPORT OF INVESTIGATIONS/1993

Seismicity and Stress Changes Subsequent
to Destress Blasting at the Galena Mine
and Implications for Stress Control
Strategies

By F. M. Boler and P. L. Swanson



KEY
@ Rib-mounted accelerometer
@ Borehole-mounted accelerometer

Figure 6.—Oblique perspective view of mine openings, rib- and borehole-mounted accelerometers in 46-99 digitally recorded

acceleromet

STRESS-MONITORING INSTRUMENTATION

The Bureau’s borehole pressure cell (BPC), described
in detail by Haramy (9), was used to monitor stress
changes caused by destressing and seismic events. The
BPC consists of a stainless steel, oblong, pancake-shaped
bladder encased in a cylinder of grout, molded to be just
under the size of the borchole (60 mm). Cell emplace-
ment includes inserting the package in the borehole and
aligning, and pumping the bladder to a preselected setting
pressure, typically selected to approximate the overburden
pressure (9).

Figure 7 shows the position and orientation of the
eight BPC’s installed at the end of July 1989, Cells were

ray, and BPC hole array. [For scale, mine openings are 3 m (10 ft) high.]

emplaced in three mutually perpendicular 6-m-long
boreholes. The geometry of the BPC array was selected
based on the observation that the majority of faults trend
N, 45° W. and are near vertical. Laboratory observations
show that faults tend to form at 30° to 45° to the maximum
principal stress direction. If the fault orientation is rep-
resentative of the present stress field, then BPC’s oriented
at 45° to the fault will be sensitive to pressure changes in
the principal stress orientations. This concept is illustrated
in figure 8,

The overburden pressure in the vicinity of the 46-99
stope is estimated at 34 to 37 MPa (from the overburden
thickness of 1.5 km and the rock density of 2,400 to 2,600


4000 level

4300 level

900207

Figure 5.—Oblique perspective view of mine openings and events with M, 20.6
detected by surface selsmograph in 46-99 stope area during tme period between
February 2, 1990, destress and first day of production (February 19) in stope following
destross. Event identifiers show date as top six-digit number (year, month, and day)

\d time (hour, minute, and second)

otto six-digit number. Lengths of axes are

uncertainty for event source location in axis direction. Event with largest location
uncertainty (900207, 034500) Is outside of most of array. [For scale, mine openings are

3m (10 ft) high.}

Stope-Level Routine-Monitoring
Accelerometer Array

The 46-99 stope is one of eight stopes that are each
monitored with a stope-level accelerometer array with a
characteristic dimension of approximately 150 m laterally
and 180 m vertically. Steblay (7) describes the character-
istics of this monitoring system. The 46-99 array consists
of 16 rib-mounted Wilcoxon 793 M40 accelerometers with
40 V/g sensitivity over 3 to 6,000 Hz. Arrival times of
signals are determined by hardware electronic circuitry
when a floating threshold is exceeded. These relative
times are transmitted to a computer, which calculates and
displays locations in near real time for analysis by mine
personnel. The arrival times from this system were used
in a few cases in which the digital system did not record a
particular event of interest.

Digitally Recorded Stope-Level
Accelerometer Array

The routine-monitoring stope-level array was aug-
mented by two additional rib-mounted accelerometers and
four borehole-mounted, three-component accelerometer
packages. The analog signals from these accelerometers
were digitally recorded using a 12-bit computer-automated
measurement- and control (CAMAC)-based data acquisi-
tion system (8). Figure 6 shows the locations of the
accelerometers in the augmented array. The digitizing
rate was 50 kHz or 20 ys between samples. The system
provides for on-scale recording of events in the approx-
imate Richter magnitude range -5 to -2. With this setup,
significant events within the stope are always clipped
(exceed the dynamic range of the digitizers).


Kellogg

\ ceed

Scale, km

Figure 2.—Faulting in Coeur d’Alene mining district, Wallace, ID. (All lines indicate faults. Circle Indicates the

Galena Mine.)

seismic activity and damaged 28 timber sets. These obser-
vations suggest that the major structural discontinuities
in this mine play an important role in accommodating
mining-induced deformation.

DESTRESS OF FEBRUARY 2, 1990

The 46-99 stope of the Galena Mine was destressed on
Friday, February 2, 1990, after the last production shift of
the day. Figure 4 shows the hole pattern for the destress
blast. The total amount of explosives in all the holes was
125 kg. This stope is 1.3 km below the shaft collar and
1.5 km below the ground surface, which slopes steeply up
away from the shaft in the direction of the stope. The
vein had been mined 77% of the 90 m from the 4600 to
the 4300 level, leaving a pillar 21 m high at the time of
destressing, close to the critical height (1). Additionally,
the stope was showing fairly high rates and numbers of
microseismic events.

The usual indicator of the success of destressing used
by mine management is the presence of a significant
seismic event on the surface seismograph coincident with

destress blasting or before work resumes in the stope (2-4
days). No such seismic event was observed coincident with
the destress blast on February 2.

However, within 3 weeks of the February 2 destress,
several significant events were detected with the surface
seismograph and identified as originating in the 46-99
stope area. Locations for five events subsequent to the
destress for which magnitudes could be determined are
shown in figure 5, Because of hardware limitations, not all
of these events were recorded by the stope digital seismic
accelerometer array (described in the next section). In the
cases where digital data were not available, locations are
from a stope-level automatic system, which identifies ar-
rival times electronically (7). Table 1 lists the events
shown in figure 5, their magnitudes, seismic energy,
amount of damage, and their locations. Especially for the
largest events, the locations may not be representative of
the site of the largest moment release. (Moment is the
product of the shear modulus, the slipped area, and the
magnitude of slip.) Within the accuracy of the location
method, each location is representative of the event
nucleation site,


DISCUSSION

The broader purpose of this work is to show how quan-
titative observation and analysis of destressing attempts
can lead to a better understanding of the process and to
the ability to design better destress methods. For the
February 2, 1990, destress, there is BPC and seismological
information available for analysis. The following sections
present a review of the seismological data for evidence of
a connection between the destress and subsequent events.
Possible ways to improve destress effectiveness are also
discussed.

MEASURED PRESSURE CHANGES
DURING DESTRESS

Figure 17 shows that pressure changes due to the
destress, as monitored at the BPC array, were no more
than the noise level, which was at about 100 kPa at that
time. Blake (1) computed the stress field in a pillar before
and after destressing using a two-dimensional, finite-
element model. A hydrostatic stress of 52 MPa was ap-
plied to the model. Maximum stresses in the pillar as a
function of pillar height were computed, along with the
contours of the stress field. For the pillar height at the
time of the February 2 destress, the maximum stress in the
pillar expected from Blake’s model is 165 MPa. In the
model, after destressing, the maximum stress in the pillar
was reduced to about 60 MPa, assuming that the destress
reduced the Young’s modulus of the pillar by more than
80% because of fracturing. No such dramatic reduction in
stress was observed on the BPC array at the time of the
February 2 destress, This array samples ground within
5 m of the pillar and 25 m of the center of the destress
round, Unless the BPC array was shielded somehow from
the effects of the destress, the measurements suggest that
the destress reduced Young’s modulus (induced fracturing)
by an amount far smaller than the 80% assumed by Blake.
Such shielding is possible, given the fractured nature of
the ground and the ability of fractures to act as stress
discontinuities.

RELATIONSHIP OF DESTRESS
TO SUBSEQUENT SEISMIC EVENTS

There is evidence both for and against the notion that
the destress triggered the subsequent seismic events. This
evidence is summarized as follows.

Evidence Supporting Connection Between
Destress and Subsequent Events

The evidence supporting a connection is that the
destress may have marked the onset of a flurry of seismic

activity unmatched in the previous year in this stope.
The activity following the destress is intense and unusual
(fig. 1). Eight significant events occurred in the vicinity
of the 46-99 stope from January 1989 through February 1,
1990, Seven events occurred beginning with the destress
on February 2 through February 19, 1990. Seven addi-
tional events occurred from March 1990 through the end
of the survey period in October 1990. Similar activity
did not follow the other two destresses of the stope in
December 1989 and June 1990, It cannot be ruled out
that the flurry of activity may be coincidentally associated
with the destress. Generally, mine management expects a
large seismic event coincident with an effective destress
blast. However, destresses are always scheduled on a
Friday's last shift so that the possibility of delayed events
or large “aftershocks” happening during a working shift is
reduced. The first significant event following the destress
was a small one that occurred 37 h later; the largest event
of the sequence took place 4% days after the destress.

Evidence Against Connection Between
Destress and Subsequent Events

There are four main lines of evidence against a con-
nection between the destress and subsequent events:

1. The time delay that occurred between the destress
and subsequent events is significant since no large event
occurred at the time of the destress, assuming that the
relationship between the flurry of activity in February 1990
discussed above was coincidental.

2. The magnitude of the pressure change observed with
BPC’s at the time of the destress was negligible, and per-
haps not enough of a change to induce a flurry of seismic
activity.

3. If a stress discontinuity is invoked to explain the
lack of observed pressure changes at the time of the de-
stress blast, an explanation must be made as to why the
February 7 events resulted in observable pressure changes
when the same (assumed) stress discontinuity should shield
the array from effects of these events.

4, By using the measured stress changes on the BPC
array from the destress to estimate stress change at the
site of the 034500 event hypocenter, assuming a homoge-
neous elastic medium, magnitudes are yielded on the order
of a few kilopascals. This magnitude of stress change
would be unlikely to trigger the 034500 event. However,
it is more likely that the destress triggered the February 4
event (table 1), which in turn may have triggered the
034500 event on February 7.


16.
17.
18,
19.

Digitally logged BPC data for Febroary 2 1-9, 1990 .
BPC data for February 7, 1990 .
BPC data for February 2, 1990 .
Fits of measured pressure changes on BPC array computed using dislocation sources .
Stress field on 28- by 28-m fault plane for 122020 event, induced by dislocation slip on
fault plane of 034500 event

Destress blast and postdestress seismic events with M, 20.6

ILLUSTRATIONS—Continued

TABLE

Page

4
14

16

17

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

ft

kg/m?

kHz

km/s

foot

hour

hertz

joule

kilogram

kilogram per cubic meter
kilohertz

kilometer

kilometer per second

kPa
m

m?

mm
MPa
HS
psi
Vie

Kilopascal
meter

cubic meter

minute

millimeter

megapascal

microsecond

pound (force) per square inch

volt per acceleration of gravity



It is only possible to estimate event mechanisms for
those events that were recorded by the digital system, since
otherwise no first arrival polarity information is retained.
Unfortunately, the 034500 event was not recorded by the
digital system. To determine a possible fault plane solu-
tion for the 034500 event, the foreshock at 030326 (fig. 10),
for which digital data were available, was used. This fore-
shock was located in the same place as the 034500 event,
within the location uncertainty. A fault plane solution was
also estimated for the damaging event at 122020.

The mechanism for the foreshock to the 034500 event,
shown in figure 12, is poorly constrained. The figure
shows two possible end members based on one nodal
plane (N. 55° W.) that is reasonably consistent with the
plane locus of large seismic events occurring throughout
the mine identified by Swanson (5). Clearly there are
many other possible solutions consistent with the limited
data. Readings for stations with near-grazing incidence at
the accelerometer (+ 10°) are not shown.

The 122020 event mechanism is inferred from a com-
posite of six events, including the 122020 event. This
composite, shown in figure 13, indicates a fairly well-
constrained mechanism. The northwest-striking nodal
plane is consistent with the N. 45° W. predominant strike
of mapped faults in the mine. Given the uncertainty in the
velocity structure, and the limitations of the constant
velocity model used, it is encouraging to sce that this
composite can define a solution fairly well.

Figure 14 shows the 034500 and 122020 event mecha-
nisms superimposed on a geologic map of the 4300 level.
The trace of the plane defined by the largest seismic
events in the mine (5) is also shown.

|_| { i

4000

level

46-99 sill drift

fs

Figure 10.—Oblique perspective view of mine openin:
events temporally related to 034500 event. [For scale, mine
openings are 3 m (10 ft) high.) (Six-digit numbers refer to hour,
minute, and second.)

4300 level

46-99
sill drift

Figure 11.—Oblique perspective view of mine openings and events with similar polarity pattern
distributions to 122020 event, indicating same focal mechanism (when events are located in same place).
‘These events were used to generate a composite focal mechanism for 122020 event. [For scale, mine

openings are 3 m (10 ft) high.)


46-99 sill drift

122020

4300

level

ae

fe) 10
be

Scale, m

14,—Fault plane solutions for 034500 and 122020 events with plan view map of vein exposures and

drifts and crosscuts of 4300 level In vicinity of 46-99 sill drift.

Figure 15 shows the time history of BPC measurements
from the beginning of February through February 8, 1990.
There is a certain amount of "cultural" noise that occurs
when shift crews are working (probably electric motor
noise). This is evident in comparison with times such as
the weekend from February 3 to 4, when the signals were
significantly quieter. This noise limits the resolution
of offsets caused by specific seismic events.

Figure 16 shows the BPC measurements for Febru-
ary 7. Changes coinciding in time with the observed
events, within the 10-min resolution of the measurements,
are observed for both of the damaging event sequences
(034500 and 122020). The earlier offset is correlated
with the 034500 event rather than the 034654 event
(fig. 10) because of the large seismic energy of the 034500
event (table 1). All of the pressure changes observed are

BOREHOLE PRESSURE CELL OBSERVATIONS

positive for the 034500 event. For the 122020 event, a
decrease in pressure was observed for cells 5 and 6, which
have identical orientations. The magnitude of pressure
changes for both events on all the cells is a few hundred
kilopascals maximum. Although the 034500 event had a
much larger magnitude, the 122020 event was much closer
to the BPC array; hence, the resultant observed pressure
offsets are ‘of the same magnitude for both events. Fig-
ure 17 shows a similar time period around the destress
blast on February 2. From a comparison of figures 16 and
17, the seismic events (fig. 16) caused a far larger coin-
cident change in stress than did the destress blast itself.
Comparison of offsets on duplicate cells 5 and 6 and
cells 7 and 8 (fig. 16) indicate that the magnitudes of the
measured pressure offsets are not significantly affected by
the total absolute pressure. [The total pressure for cell 6


CONTENTS

Abstract ..
Introduction .
Background .
Geologic setting .
Locus of major seismic activity within mine
Destress of February 2, 1990 .
Acknowledgments . . .
Instrumentation ....
Seismic instrumentation .
Surface seismometer
Stope-level routine-monitoring accelerometer array
Digitally recorded stope-level accelerometer array .
Stress-monitoring instrumentation
Seismic events .
Event mechanisms os
Borehole pressure cell observations
Dislocation models of fault offset .
Fits to borehole pressure cell data.
Event interaction .
Discussion oe
Measured pressure changes during destress .. . . . wee
Relationship of destress to subsequent seismic events beeen
Evidence supporting connection between destress and subsequent events
Evidence against connection between destress and subsequent events .
Improving destress effectiveness . :
State of stress estimates ....
State of stress and destress strategies
Conclusions .
References ..

ILLUSTRATIONS

Seismic activity in vicinity of 46-99 stope . . .
Faulting i in Coeur d’Alene mining district .... .
Plan view map of vein exposures in drifts and crosscuts of 4300 level in vicinity of 46- 99 sill drift.
Schematic vertical section view of 46-99 stope .........
Oblique perspective view of mine openings and events with M, 20.6 detected by surface seismograph in
46-99 stope area... eee cece cee eceeeueeuees .
Oblique perspective view of mine openings, rib- and borchole-mounted accelerometers in 46.99 digitally |
recorded accelerometer array, and BPC hole array ......... .
Oblique perspective view of BPC array geometry ...... bees
Selection of cell orientations .................. eee
Time history of pressure recorded manually for BPC array from installation at end of July 1989 though
January 1990 Lee cece cece eens .
10. Oblique perspective view of mine openings and events temporally related to 034500 event ..
11. Oblique perspective view of mine openings and events with similar polarity pattern distributions to

Yeypr

s

een

122020 event .

12. Two possible fault. plane solutions for 030326 foreshock to 034: 00 event .

13. Composite fault plane solution for 122020 event consisting of readings for that event and the events
shown in figure 11

14, Fault plane solutions for 034500 and 122020 events

Auaw

~)


Report of Investigations 9448

Seismicity and Stress Changes Subsequent
to Destress Blasting at the Galena Mine
and Implications for Stress Control
Strategies

By F. M. Boler and P. L. Swanson

UNITED STATES DEPARTMENT OF THE INTERIOR
Bruce Babbitt, Secretary

BUREAU OF MINES


kg/m*), This area is believed to be under high horizontal
tectonic stress. A nominal setting pressure of 35 MPa
(5,000 psi) was used with six of the eight cells (1, 2, 3, 4,
6, and 7). In addition, two cells (5 and 8 in figure 7) in
redundant orientations to two other cells (6 and 7) were
emplaced with a nominal setting pressure of 28 MPa
(4,000 psi) as a check on the effect of the absolute setting
pressure on cell output.

Prior to February 1990, dial pressure gauge readings
were manually recorded. Figure 9 shows the time history
of manually recorded cell pressure readings. Digital log-
ging of data began in February 1990, with readings logged
for each cell every 10 min. Cell pressures were converted
to voltages using Bourns 35 MPa (5,000 psi) pressure
transducers (gauges with a larger maximum pressure were
not available). Gauge outputs were recorded by the
CAMAC system using a 32-channel digital voltmeter
module. The pressure transducer installed at cell 2 did
not return reliable readings because of electrical problems;
therefore, it will not be discussed further.

By the end of January, the absolute pressures for the
various cells had diminished, in some cases significantly
(cells 5 and 6), below the installation setting pressure.
This was partially in response to a rock burst in the
vicinity on October 5, 1989. It was expected that the
magnitude of pressure changes (due to seismic events, for
example) would be affected by the absolute magnitude of
Pressure in the cell. Data from the cells in redundant
orientations did not bear out this expectation, at least for
the range of total pressures and pressure changes observed
(sce "Borehole Pressure Cell Observations" section).

A % 8
|
|
|
i) a 2» o>

2
S
Q
%
ry
%
2
%
ry
?

Figure 7.—Oblique perspective view of BPC array geometry,
with eight cells (numbered 1-8) mounted in three mutually per-
Pendicular 6-m-long boreholes. Cells 1 and 2, in vertical bore-
hole, have horizontal sensitive axes oriented east-west (cell 1)
and north-south (cell 2). Cells 3 and 4 are located In N. 45° W.
trending borehole, Their sensitive axes are perpendicular to one
another, and both are at 45° to vertical. Cells 5-8 are located In
the N. 45° E. borehole. Cells 5 and 6 have duplicate orientations,
as do cells 7 and 8. Sensitive axes of cells 5 and 6 are perpen-
dicular to those of cells 7 and 8. Sensitive axes for all four cells
in this hole are at 45° to vertical. [For scale, boreholes are 4.6
‘m (15 ft) long.] (See figure 6 for location of BPC hole array.)

c

Figure 8.—Selection of cell orientations (numbered 1-4). A, Faults typically form at an angle « = 30° to 45° to maximum principal
stress (c,) direction; B, for vertical N. 45° W. faults, cells 1 and 2 are orlented to sense principal stresses that might be responsible

for strike-slip motions;
(view looking N. 45°

» similarly, cells 3 and

oriented to sense principal stresses that might be responsible for dip-slip motion
|). Cells 5-8 are oriented to sense stresses in plane of fault.


The mechanisms shown in figure 14 were used to con- February 7, fit to observed coseismic BPC pressure
strain dislocation models of the damaging events from changes.

A B

+ Compression
__¥ Dilatation

Figure 12.—Two possible fault plane solutions for 030326 foreshock to 034500 event. Readings with angle of incidence >10° not
shown. Both solutions have N. 55° W. striking nodal plane, selected to coincide with plane that is apparently the locus of locations
of largest events occurring within mine. A, Dip-slip mechanism; B, strike-slip mechanism.

KEY
+ Compression
s v Dilatation Ss

Figure 13.—Composite fault plane solution for 122020 event consisting of readings for that event and the events shown In figure
11. A, All readings; B, readings with angle of incidence at accelerometer >10°.