2
METHODOLOGY
Cave Shape Monitoring and Interpretation
Knowledge of actual cave shape is critical to ensure safety
during development and guarantee the cave connection
between existing cave of PB1 and PB4 cave. The drift devel-
opment is maintained as close as possible to the mobilize
zone/ cave back boundary. However, limitation is placed
in the yielded zone boundary so that the drift can be exca-
vated safely and preserved for long-term purpose. In figure
2 explain the detailed cave shape boundaries, which the
cave zones comprise the boundary of cave back, zone of
fractured/ yielded rock, and elastic zone. This figure was
modified after Cumming-Potvin et al. (2018) where caving
induced damage intensity increase at area closer to cave.
The existing DMLZ cave shape has been estimated
using monitoring instrumentations such as Open hole sur-
vey, Network Smart Markers (NSM), and Total Domain
Reflectometer (TDR). In the absence of these instruments,
seismic tomography and HOD data was utilized to define
inelastic boundary. In early DMLZ-PB1 cave establish-
ment, open hole survey has been abundantly distributed in
the cave perimeters to measure the cave propagation. The
cave back position was not difficult to determine, therefore,
the cave shape model represented the measured cave back.
However, as the cave mature and become taller it is difficult
to find the cave back/ mobilized zone. Shearing and shifted
between two fractures plane inside the open holes blocked
the camera from penetrating further to the cave back posi-
tion. This phenomenon has been anticipated where only
fractured zone can be investigated hence the cave back is
no longer represents the cave back but the outer boundary
of fractured zone (orange line in Figure 2) at which bore-
hole camera cannot pass through the fractured zone.
Mining against the active caves pose high risk of sub-
sidence with difficulties to exactly design the end if exca-
vation drift due to cave shape irregularity. Lesson learned
from similar case of connecting the E26 lift 2 caves and
new block of E26 Lift 1 in the Northparkes mine (Webster,
Snyman, Francois, &Saadatfar, 2022) has been advanta-
geous. However, development of PB4 drifts is carried out
towards active mine of PB1 and PB2. Consequently, uncer-
tainty level increases because of a continuous lateral cave
induced damage propagation which endangered personnel
in a hazardous zone during development work. Therefore,
intense cave monitoring and probe holes prior every round
blasting whilst controlling the draw rates of PB1 south
drawbell are critical to reduce the risk.
The DMLZ-PB1 cave propagation is parallel against
the PB4 east footprint (2815/L–2835/L). Propagation rates
is being fully monitored using three different types of cave
monitoring that drilled from MLA and DOZ levels. Prior
2018, the cave monitoring instruments are Open hole and
TDR which mostly to cover cave growth monitoring in
PB1. The Network Smart Marker (NSM) commenced in
early 2019 which intended for PB2 cave monitoring. The
NSM holes is coupled with cave tracker of which com-
binations can provide cave growth progression and cave
flow behaviour. The detailed cave monitoring locations are
delineated in Figure 3 and amount of active monitoring
holes are outlined below:
• Open holes survey: 36 holes available from DOZ
and MLA level. Additional 5 holes to cover lower
wedge volume (LWV) below PB4 extraction level
have been drilled in 2023.
• TDR: 57 sensors available since cave initiation in
2015. There has been no additional TDR drilling
Figure 2. Conceptual cave shape definition showing the line of cave back, intense
fractured limit, and the seismogenic zone (modified after Cumming-Potvin, 2018)
METHODOLOGY
Cave Shape Monitoring and Interpretation
Knowledge of actual cave shape is critical to ensure safety
during development and guarantee the cave connection
between existing cave of PB1 and PB4 cave. The drift devel-
opment is maintained as close as possible to the mobilize
zone/ cave back boundary. However, limitation is placed
in the yielded zone boundary so that the drift can be exca-
vated safely and preserved for long-term purpose. In figure
2 explain the detailed cave shape boundaries, which the
cave zones comprise the boundary of cave back, zone of
fractured/ yielded rock, and elastic zone. This figure was
modified after Cumming-Potvin et al. (2018) where caving
induced damage intensity increase at area closer to cave.
The existing DMLZ cave shape has been estimated
using monitoring instrumentations such as Open hole sur-
vey, Network Smart Markers (NSM), and Total Domain
Reflectometer (TDR). In the absence of these instruments,
seismic tomography and HOD data was utilized to define
inelastic boundary. In early DMLZ-PB1 cave establish-
ment, open hole survey has been abundantly distributed in
the cave perimeters to measure the cave propagation. The
cave back position was not difficult to determine, therefore,
the cave shape model represented the measured cave back.
However, as the cave mature and become taller it is difficult
to find the cave back/ mobilized zone. Shearing and shifted
between two fractures plane inside the open holes blocked
the camera from penetrating further to the cave back posi-
tion. This phenomenon has been anticipated where only
fractured zone can be investigated hence the cave back is
no longer represents the cave back but the outer boundary
of fractured zone (orange line in Figure 2) at which bore-
hole camera cannot pass through the fractured zone.
Mining against the active caves pose high risk of sub-
sidence with difficulties to exactly design the end if exca-
vation drift due to cave shape irregularity. Lesson learned
from similar case of connecting the E26 lift 2 caves and
new block of E26 Lift 1 in the Northparkes mine (Webster,
Snyman, Francois, &Saadatfar, 2022) has been advanta-
geous. However, development of PB4 drifts is carried out
towards active mine of PB1 and PB2. Consequently, uncer-
tainty level increases because of a continuous lateral cave
induced damage propagation which endangered personnel
in a hazardous zone during development work. Therefore,
intense cave monitoring and probe holes prior every round
blasting whilst controlling the draw rates of PB1 south
drawbell are critical to reduce the risk.
The DMLZ-PB1 cave propagation is parallel against
the PB4 east footprint (2815/L–2835/L). Propagation rates
is being fully monitored using three different types of cave
monitoring that drilled from MLA and DOZ levels. Prior
2018, the cave monitoring instruments are Open hole and
TDR which mostly to cover cave growth monitoring in
PB1. The Network Smart Marker (NSM) commenced in
early 2019 which intended for PB2 cave monitoring. The
NSM holes is coupled with cave tracker of which com-
binations can provide cave growth progression and cave
flow behaviour. The detailed cave monitoring locations are
delineated in Figure 3 and amount of active monitoring
holes are outlined below:
• Open holes survey: 36 holes available from DOZ
and MLA level. Additional 5 holes to cover lower
wedge volume (LWV) below PB4 extraction level
have been drilled in 2023.
• TDR: 57 sensors available since cave initiation in
2015. There has been no additional TDR drilling
Figure 2. Conceptual cave shape definition showing the line of cave back, intense
fractured limit, and the seismogenic zone (modified after Cumming-Potvin, 2018)