Shaft pillar stability - Geohart Consultants

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Shaft pillar stability - Geohart Consultants

Shaft “Stability” Pillars - A Geotechnical RiskManagement ProcessWouter HartmanAssociate Geotechnical EngineerTownsville – May 2007


Coffey MiningShaft Pillars Background• We know that shafts are crucial important excavations and a fairlyexpensive investment.• Many considerations govern their design and operation.• We make use of rock mechanics principles in the shaft location and inin the design of shaft pillars or other protection systems.• The principle involved in the use of shaft pillars as a means of shaftsystem protection is to keep the sum of mining induced and virginstresses below damaging levels.


Coffey MiningShaft Pillars Background• Protection of a shaft against normal induced strata deformations istraditionally accomplished by leaving a shaft pillar of unmined groundadjacent to the shaft barrel.• The size of the pillar is chosen so as to limit mining induceddeformations in the shaft and at the same time to maintain tolerablelevels of stress near the stoping block• As mining proceeds stress levels in the pillar steadily increases,which in the case of under designed pillars can cause deteriorations inthe condition of the shaft barrel and other important excavations inclose proximity e.g. dams, u/g winder excavation, shaft access etc.


Coffey MiningShaft Pillars Background• Damage to shaft systems can becaused by increased or decreasingstress, resultant deformation,displacement within the rockmass (e.g.major faults intersecting the shaft).P.S. Stress is a derived quantity and is strongly dependant onmining geometry.Stress = Load / Area


Coffey MiningShaft Pillars – Unstable ShaftsMechanisms of Failure• Mining in proximity of Faults which slices up the rock mass andintersecting shafts – e.g. Mt. Isa X41 (Bruneau et al 2003), Vaal ReefsNo. 5-Shaft• The Effect of stress redistribution around a very large mined andbackfilled orebody such as the 1100 orebody in the vicinity of theX41 shaft, No. 10-Shaft Impala incline shaft


Coffey MiningShaft Pillars – Unstable ShaftsMechanisms of Instability•TiltTilt is defined as induced displacement over length.Tilt = du / lwhere, l is the length of shaft under consideration and du induceddisplacementIt is not the absolute value of tilt that is important but the rateof change of tilt that must be kept within acceptable limits


Coffey MiningShaft Pillars – Unstable ShaftsMechanisms of InstabilityShaft OvalingShaft ovaling occurs due to differencesin horizontal compression.Assuming the amplitude of the ovalingradial displacement is “a”, the change ofthe concrete lining centre line is shown inthe figure


Coffey MiningShaft Pillars – Shaft Damage Criteria


Coffey MiningSome Personal Shaft & Shaft Pillar Experience


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• Vaal Reefs – Anglo Gold (Ashanti) – No. 2-Shaft & 5-Shaft (1993-1996) –Essentially Tabular orebody – Reef Dips between 7-35 degreesThe No. 5 Shaft Pillar wasrectangular in shape andapproximately 850m long by 550mwide. The main shaft is situatedwithin the 5 Shaft Fault loss.The No. 2 Shaft Pillar was semicircular in shape and had adiameter of approximately 900m.What this cross section don’t showis that the shaft pillar wasessentially cut in plane by adolerite dyke with 4-7m verticaldisplacement.


Coffey MiningSome Personal Shaft & Shaft Pillar ExperienceImpalaPlatinum1997-2000


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• Impala Platinum – No. 9-Shaft (Merensky & UG2 Reef) – EssentiallyTabular orebody – Reef Dips between 9-11 degrees towards theeast


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• Impala Platinum – No. 10-Shaft (Merensky Reef) –Tripple decline system


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• Impala Platinum –- No. 1A Vent Shaft, 6m diameter Raisebore Shaft 1060m deep –Geotechnical Investigations, experiencing failures within the shaft to around80m below – following completion of the raisebore – groundwater intercept– Shaft walls “exfoliate”….2-Shiftbosses injured during night shift – 50mpillar- No. 11B – Vent Shaft – 1200m Deep – 5.8m diameter – ConventionalSink – Geotechnical Investigations, 75mm thick steel fibre shotcrete…50m pillar- No. 14B – Vent Shaft – 1200m deep, Conventional Sink, GeotechnicalInvestigation, 50m pillar


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• March 2001 - Dendrobium – BHP Billiton – Ventilation Shaft – Workingconducted for Walters Construction – Mining Division ContractorsThe 180 metres deep shaft, costing $4.07 million, 4.25 metres in diameter,Initial proposal was for raiseboring – Recommendation not to Raisebore- Very High Risk – BHP opted in the end to blind bore…• No Shaft Pillar – Shaft positioned well outside the predictedincremental subsidence line


Coffey MiningSome Personal Shaft & Shaft Pillar Experience


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• July 2003 – Newmont Pajingo – Jandam Fresh Air Rise – VentilationShaft – Review of all Ventilation shafts constructed at Pajingo prior tothe JANDAM Return Air Rise….First 70m very poor rock…notconducive to raiseboring and was seen as an unacceptable risk….• No Shaft Pillar – Shaft positioned well within the footwall and outsidelong-hole ope stoping influence zone and in close proximity to themain decline


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• August 2004 – Olympic Dam Operation – WMC – Clark Shaft – 7.5mdiam.• Shaft Tilt• Shaft Ovaling• Geological Structures• Blasting vibration• Seismicity• Other importantexcavations aroundthe shaft (i.e. ore bins,crusher, loadingtransfer excavation


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• October 2004 – Latrobe Valley – Victoria – 1300m deep, 4m diametershaft – Geotechnical Investigation• Lateral displacementwithin various stratahorizons• Shaft Tilt• Geological Structures• Ground water


Coffey MiningSome Personal Shaft & Shaft Pillar Experience• 2006 – 2007- Kestrel Coal Mine Vent Shaft – Review of Geotechnical Investigation- Ballarat Vent Shaft –Review of Geotechnical Investigation


Coffey MiningGeotechnical Risk Management Process


Coffey MiningGeotechnical Risk Management Process


Coffey MiningGeotechnical Risk Management Process (cont.)


Coffey MiningGeotechnical Risk Management Process (cont.)


Coffey MiningGeotechnical Risk Management Process (cont)Example geotechnical process map of shaft pillar design


Coffey MiningGeotechnical Risk Management Process (cont)Example geotechnical process map of shaft pillar design

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