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Geotechnical Slope Remediation Via RC-drilling And "Controlled Slope Failure Blasting"
Abstract and Figures
In December of 2009 an estimated one million ton slope failure activated a 10 million ton mass impacting use of the main ramp system in a Northern Nevada open pit. The failure left behind a 400 foot (122 meters), vertical to sub vertical scarp feature and shut down production in the lower benches of the pit. The Geotechnical group recommended that no equipment should go within an 80 foot (24 meter) safety zone of the scarp crest due to stability concerns. This meant that the traditional approach of load/haul could not be used to remediate the slope failure. Following careful analysis, the drill and blast group was then tasked with finalizing the scarp feature remediation planning. The key considerations determining the viability of blasting for stabilizing the crest were: Safety – Ability for equipment and personnel to perform remediation activities while remaining outside of the 80 foot (24 meters) safety zone Material type – The rockmass is an alluvial material that lends itself well towards this style of blasting Crest and face topography – Face profiles provided evidence that lent itself towards a blasting approach Drill Selection – Exploration RC drill rigs were selected due to their ability to drill angled holes down to the depth required and at the recommended hole diameter
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... Since safety restrictions precluded access to an area within 25 m along the crest, it was determined that a drill and blast approach was a potential solution. The concept was to use angle-drilled blast holes that were sequentially timed to initiate small, controlled slope failures around the headscarp crest to develop a shallower slope that could be remediated using typical mining methods (Acorn, 2011). A key blast design criterion was to minimize damage to the wall behind each blast. ...
In 2009 the Gold Quarry open pit mine, located 11 km northwest of Carlin, Nevada, USA, experienced multiple large-scale slope instabilities of the upper east highwall that reduced gold ore extraction for nearly 18 months. The slope instabilities occurred within a weak, consolidated sedimentary sequence that exhibits strength characteristics that are transitional between soil and rock. Instability initiated as mining exposed the lower clay-rich sub-units of the Carlin Formation (Fm.). This deformation created preferential flow paths that allowed groundwater from the upper sandy sub-units to infiltrate low permeability clay-rich sub-units, thereby enhancing deformation of the slope toe which in turn destabilized the upper portion of the highwall. The outcome was a 160 m high slope instability that had a lateral run-out of 850 m. The effort to return the pit to ore production required both the geotechnical and hydrogeological investigations and the preliminary remediation mining activity to be concurrent. This approach required the development of detailed safety procedures and a requirement to modify the remediation design as new results were obtained. An initial challenge was to mitigate a near vertical, 90 m headscarp with blast induced, localized slope failures. Back-analyses applying numerical modeling indicated that the failure surface did not coincide with initial interpretations based on field observations; drilling results eventually confirmed this alternative failure geometry. The final remediation design incorporated shallower slope geometries and an approximately 3 Mt buttress along the base of the Carlin Formation and bedrock contact to reinforce the lower clay-rich sub-units. The outcome is a stable highwall within the Carlin Fm. following nearly ten years of repeated slope instability, and an example of the necessity to conduct appropriate geotechnical and hydrogeological studies during the early stages of a new layback evaluation or new open pit development.
... Since safety restrictions precluded access to an area within 25 m of the failure escarpment crest, it was determined that a drill-and-blast approach was a potential solution. The concept was to use angle-drilled blast holes that were sequentially timed to initiate small, controlled slope failures around the headscarp crest to develop a shallower slope that could be remediated using typical mining methods (Acorn, 2011). A key blast design criterion was to minimize damage to the wall behind each blast. ...
In 2009 the Gold Quarry open pit mine, located 11 km northwest of Carlin, Nevada, USA, experienced multiple large-scale slope instabilities of the upper east highwall that reduced gold ore extraction for nearly 18 months. The slope instabilities occurred within a weak, consolidated sedimentary sequence that exhibits strength characteristics that are transitional between soil and rock. Instability initiated as mining exposed the lower clay-rich sub-units of the Carlin Formation (Fm.). This deformation created preferential flow paths that allowed groundwater from the upper sandy sub-units to infiltrate low permeability clay-rich sub-units, thereby enhancing deformation of the slope toe which in turn destabilized the upper portion of the highwall. The outcome was a 160 m high slope instability that had a lateral runout of 850 m. The effort to return the pit to ore production required both the geotechnical and hydrogeological investigations and the preliminary remediation mining activity to be concurrent. This approach required the development of detailed safety procedures and a requirement to modify the remediation design as new results were obtained. An initial challenge was to mitigate a near vertical, 90 m headscarp with blast induced, localized slope failures. Back-analyses applying numerical modeling indicated that the failure surface did not coincide with initial interpretations based on field observations; drilling results eventually confirmed this alternative failure geometry. The final remediation design incorporated shallower slope geometries and an approximately 3 Mt buttress along the base of the Carlin Formation and bedrock contact to reinforce the lower clay-rich sub-units. The outcome is a stable highwall within the Carlin Fm. following nearly ten years of repeated slope instability, and an example of the necessity to conduct appropriate geotechnical and hydrogeological studies during the early stages of a new layback evaluation or new open pit development.
... Since safety restrictions precluded access to an area within 25 m of the failure escarpment crest, it was determined that a drill-and-blast approach was a potential solution. The concept was to use angle-drilled blast holes that were sequentially timed to initiate small, controlled slope failures around the headscarp crest to develop a shallower slope that could be remediated using typical mining methods (Acorn, 2011). A key blast design criterion was to minimize damage to the wall behind each blast. ...
In 2009, the Gold Quarry openpit mine experienced multiple large-scale slope failures of the upper east highwall that reduced gold ore extraction for nearly 18 months. The slope failures occurred within a weak, consolidated sedimentary sequence that exhibits strength characteristics that are transitional between soil and rock. Instability initiated as mining exposed the lower, high plasticity subunits of the Carlin Formation. This deformation created preferential flow paths that allowed ground water from the upper sandy subunits to infiltrate low-permeability, clay-rich subunits, thereby enhancing deformation of the slope toe, which, in turn, destabilized the upper portion of the highwall. The outcome was a 160 m high slope failure that had a lateral run-out of 850 m. The effort to return the pit to ore production required geotechnical and hydrogeological investigations and the preliminary remediation mining activity to be concurrent. This required the development of detailed safety procedures and a requirement to modify the remediation design as new results were obtained. An initial challenge was to mitigate a near vertical, 90 m headscarp with localized, blast-induced slope failures. Back-analyses with numerical modeling software indicated that the failure surface could be shallower, which contradicted the initial failure interpretations. Eventually, drilling results confirmed this alternative failure geometry. The final remediation design incorporated shallower slope geometries and an approximately 3 Mt buttress along the base of the Carlin Formation and bedrock contact to reinforce the subunits with residual strength properties. The results are a stable highwall within the Carlin Formation following nearly 10 years of repeated slope failures, and an example of the necessity to conduct appropriate geotechnical and hydrogeological studies during the early stages of a new layback evaluation or new openpit development.
During the past twenty-five years, the Gold Quarry open pit has experienced numerous slope instabilities that developed within the Carlin Formation. The Carlin Formation is characterized as a lacustrine, volcanoclastic sequence composed of gravels, silty-clay units, and overlain by a variably indurated, clay-altered volcanic tuff. The upper formation is composed of interbedded silt, sand, and gravel layers. Structural deformation occurred concurrent with deposition during Basin and Range tectonics; it has been subjected to additional internal soft-sediment deformation and substantial differential loading. Overall a low permeability formation, compartmentalized groundwater is present in some sand and gravel sub-units and structurally controlled flow paths. Geotechnical investigation and visual observation of slope instabilities in recent years have identified the lowest sub-units to have moderate to high-plasticity and exhibit strain-softening of material to near or at its inherent residual strength. The result is progressive slope instability that develops over a period of weeks, months, or years. More recent slope failures provided an opportunity for dramatically advancing the geological understanding of the Carlin Formation. The new knowledge combined with previously understood mitigation techniques have resulted in vertical exposures of Carlin Formation between 90 m to 150 m to be stabilized with shallow slopes reinforced with rockfill toe buttresses and structurally targeted dewatering, while implementing a rigorous monitoring program.
Geotechnical Outline of the December 25 th 9-points failure via email
- Russell Sheets
Russell Sheets, 2010, Geotechnical Outline of the December 25 th 9-points failure via email
Nine Points Slide Stabilization
- John Floyd
John Floyd, 2010, "Nine Points Slide Stabilization", Report to Newmont Mining Gold Quarry Mine,
pg 1
Loading Plan for Blast 1-Nine Points Slide Stabilization-9
- John Floyd
John Floyd, 2010, Loading Plan for Blast 1-Nine Points Slide Stabilization-9.875 inch blastholes,
ANFO, Excel file
Loading Plan for Blast 1 -Nine Points Slide Stabilization -9
- John Floyd
John Floyd, 2010, Loading Plan for Blast 1 -Nine Points Slide Stabilization -9.875 inch blastholes,
ANFO, Excel file