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Mining NZ
» Spring 2017
Gold » OceanaGold
North Wall project well on track
Karen Phelps
OceanaGold has constructed a noise bund using reconstituted topsoil material, which is currently located above the slip to the north east where it will remain until it is required for rehabilitation purposes.
Undertaking a project on unstable ground in a mine
located in an urban area with strict environmental
requirements while keeping the public happy may
sound pretty difficult. But that’s exactly the type of
project that has been taking place at OceanaGold’s
Waihi open cast mine.
Martha Mine had a well-documented large slip
on April 26, 2016, which saw two million tonnes
of rock fall. This large rock fall follows a smaller
rock fall on 2 April 2015 after which mining in the
open pit ceased. The current project, the North Wall
Interim Remediation Cutback, is to remove unstable
material in the north wall and stabilise it.
Kevin Storer, site services manager at
OceanaGold Waihi, says that due to the dynamic
nature of the north wall failure and its response
to mining, the stability designs and subsequent
remedial works have had to be modified ‘on-the-
run’ and as a result have had to change as work
progressed.
Any significant changes to design have been
peer reviewed and provided to stakeholders by
OceanaGold as necessary.
Kevin says that the west was the highest risk
area in terms of operation with long vertical drops
and unstable working conditions.
“The plan has been to continue to unload this
area of the wall by way of excavation above and
behind on stable ground out towards the unstable
areas. This left a safe area for operators to retreat
if sudden movement were to occur while drilling or
excavating.
“The current design to lower the west is de-
signed to achieve 60 degree batter angles with
five metre wide benches, depending on ground
conditions and levels of safety assessments as we
progress.”
Kevin says that the west area will join in with the
central area via a ramp and will open safe access to
the existing south perimeter road. The central area
of the failure has continued to fret away slowly and
the remedial works undertaken to-date have shown
a significant reduction in movement.
“Existing and new prisms have been installed
on the crest and have showed very little movement
before excavations got under way which indicated
this area to be relatively stable.
“As excavations have progressed, very hard
competent ground has been reached which is very
difficult to scale. Currently the bench is at final
design with a single bench cut which ranges from
15-20 metres wide as recommended by the lead
design consultant.”
He says to the east area of the failure a large
block has been showing creep into the failure area.
To help stabilise this block and prevent further
regression it needs to be unloaded, also from the
top.
“Currently the working bench will allow access
to the bottom level, however this has needed to be
redesigned as the ground conditions in the far east
are poor (saturated clay materials).
“Batters angles are 60 degrees immediately
around the eastern block, but are angled shallower
to 30-45 degrees in the east due to the weaker rock
mass. The width of each bench is five metres,”
explains Kevin.
Glenn Kiernan, general manager of Inline Drilling,
the company that is undertaking the drill and blast
component of the project, says that the project has
been particularly challenging as blasts need to take
place as close as 80 metres to the boundary wall.
Major risk assessments were undertaken
prior to the project getting underway and several
innovative methods of tackling the project devised.
He says that the fact that Inline Drilling had
experience working on the site for over 17 years
stood the company in good stead to come up with
the best solutions in conjunction with OceanaGold.
Two standard top hammer drilling rigs were
fitted with rotary drill heads to make the drilling
operations significantly quieter so only engine noise
was emitted. Local explosive companies were used
to gain the best cost effective and supply delivery
options to suit the varied challenges for each part
of the project.
Global Seismic Solutions provided the
Daveytronic electronic detonators which provide
extremely accurate inter-hole delay timing for better-
controlled blast induced vibration levels to ensure
the project adhered to mine consent conditions for
the project. Because smaller charge weights were
necessary when operating close to boundaries
Glenn says blast hole loading needed extreme
accuracy. The Orica Mining Services underground
mini loader emulsion pumping system provided
charge weight accuracies in 250 gram strokes.
While normally these units are used
underground, Glenn says that Orica Mining Services
got these certified for use on an open cast mine.
The Orica bulk emulsion product is also
non-fuming making the blasting on site more
environmentally friendly. On the lower benches
where higher charge weights were not a problem,
local supplier Prime Explosives supplied product
pumped from a 4WD bulk delivery truck.
Inline Drilling was the overall drill and blast
manager therefore streamlining the supplier
client management process allowing the client
to negotiate their own explosive pricing direct
from suppliers of their choice. Inline Drilling then
provided overall expertise and advice specific to
implementation of these tasks which has required
significant degrees of discipline and accuracy due
to the compliance sensitive nature of the tasks.
“We have to be extremely accurate in what we
do working so close to the boundary and unstable
Currently the bench is at final design with a single
bench cut which ranges from 15-20 metres wide as
recommended by the lead design consultant.”
areas on a project such as this. There is no room
for mistakes,” says Glenn.
Health and safety was a major focus and
included specialised procedures for anchoring the
drill rigs to the bulldozer such as escape plans,
how attachments were to be completed, daily
geotechnical assessments, spotters etc.
OceanaGold has been monitoring the north wall
using a radar system and other devices. The radar
provides important accurate information at every
stage of the project. In addition prisms monitored
the site along with a human ‘spotter’ observing and
searching for areas of movement.
A number of prisms were damaged as a
result of the North Wall slip and construction of
the Western Ramp and as a result OceanaGold
has progressively re-established a more robust
prism monitoring network. Precise levelling and
monitoring of ground conditions is another control
measure in and around the slip area, this as well as
the prism and radar monitoring of the slip area, will
continue to be carried out.
Although OceanaGold spokesperson Kit
Wilson says that mining within the pit will not be
undertaken in the coming years, the obligations
and procedures associated with an active mine site
remain in place and OceanaGold continues to be
responsible for the site.
The company has constructed a noise bund
using reconstituted topsoil material (approximately
12,000m3), which is currently located above the
slip to the north east where it will remain until it is
required for rehabilitation purposes.
The topsoil stockpile and base surface has been
surveyed to ensure minimum loss of material and
grassed for stability and visual effects.
The North Wall Interim Remediation Cutback
project is scheduled to be completed by the end of
August. The volume of the stockpile material during
the project has been estimated at approximately
130,000 cubic metres.
Kit says that with the complex nature of the
north wall failure, the company will continue to
monitor and further stabilise the wall if required
to mitigate any potential further regression to the
north.