1、Strategic versus Tactical Approaches in Mining 2006, Page 1Cadia Extended Pit Instability Monitoring and ManagementF. Pothitos Newcrest Mining Ltd, Cadia Valley Operations, AustraliaS. Webster Newcrest Mining Ltd, Cadia Valley Operations ,AustraliaL. Meagher Newcrest Mining Ltd, Cadia Valley Operati

2、ons ,AustraliaT. Li Newcrest Mining Ltd, Cadia Valley Operations ,Australia1INTRODUCTIONCadia Valley Operations are located approximately 250km west of Sydney and 25 km south of Orange. The Cadia Extended (CX) pit is located immediately north of the main Cadia Hill Pit. Mining in the CX Pit commence

3、d in January 2003 and was completed in July 2004 during which a total of 23 million tonnes of material was mined. The dimensions of the pit are 400m in length, 350m in width and 105m in depth. The strategic purpose of the CX pit was to fill in a production gap. This required accelerating the project

4、 from concept to operation over a short period. The design of the pit attempted to take into consideration the geotechnical uncertainties and short life required. There are instabilities and localised failures the second half of the pit life. ? Monitoring and management aspects.2GEOLOGY AND STRUCTUR

5、EThe CX pit comprises of Ordovician monzonite and lesser volcanics unconformably overlain by Silurian sediments near the pit surface.The weathering profile is relatively shallow and controlled by the major structures that intersect the area. A creek line through the central area of the pit required

6、dieversion along the western wall in the early stages of mining.Up to four deformation events are recognised in the Cadia district (Hewson, 2004). Of these the most significant was Ordovician-aged faulting coeval with mineralisation, and Devonian-aged regional thrusting that caused the Cadia Extende

7、d deposit to lie on the haingingwall of a major west-dipping fault (Cadiangullong Fault). For all geotechnical sectors the mapped shear and fault orientations are shown in Figure 1. Spatial analysis identified four structural domains (Table 1).Strategic versus Tactical Approaches in Mining 2006, Pag

8、e 2Figure 1 Stereonet showing faults and shears from mapping dataTable 1 Structural domains and main defect setsDomainDefect SetDipDipDirection1175+10170+15NE250+10195+20385+15245+202155+15160+20SE240+10340+153140+15160+15SW255+15340+20360+20060+204145+15165+20NW255+15190+25365+15215+203DESIGN AND C

9、ONSTRUCTIONThe overall design strategy was to have a two staged pit. The first stage was designed to test the slope parameters and geology confidence level in relation to grade and orebody continuity so that the data collected and the slope performance could be used for the second stage of the pit.

10、A circular haul road was Strategic versus Tactical Approaches in Mining 2006, Page 3designed to traverse all the geotechnical sectors to share the uncertainty by ensuring that no section of the pit had a high inter-ramp slope. Within the monzonite joints were discounted as a control for batter scale

11、 stability (15m bench heights). The design focused on shear and fault structures generally greater than 15m in length and in conjunction with the structural domains a kinematic based pit design was a component of the pit design approach. As the pit was developed numerous batter scale stability issue

12、s arose which were proactively managed with onsite geotechnical support. This approach was pertinent for complementing the level of knowledge in relation to the operating environment. When the pit design and production were reviewed prior to commencing the second stage, the consideration of the prev

13、ailing geotechnical and other factors resulted in committing to a single stage pit (Figure 2).Figure 2 Plan of pit design and structural domains (100m grid)Once slope stability issues were identified, localised modifications to the design were made attempting to buttress the toe of the potentially u

14、nstable slope.Blasting practices were considered aggressive, utilising large diameter (222mm) holes. Modified production blasts were used against pit walls. Blasting practices could have impacted on the stability of the slope.Domain 1Domain 4Domain 3Domain 2Strategic versus Tactical Approaches in Mi

15、ning 2006, Page 44PIT SLOPE MANAGEMENTFrom the start of mining an active pit slope management program was implemented as part of the slope stability major hazard management plan. This included;Regular pit inspections covering batters, berms and pit crests on daily, weekly and monthly schedules.Speci

16、fic pit inspections such as on ramps and known instabilities after blasting and/or rainfall events.Pit face mapping of bench-scale and larger geological structures.Implementation of a pit slope monitoring program, with progressive upgrading as instability developed.4.1 Slope MonitoringSlope monitori

17、ng comprised of:Wireline extensometersPrism monitoring Piezometers Installation of monitoring systems varied according to pit inspections and operational requirements. The intensity of monitoring was reduced for stable sectors and increased for the potentially unstable sectors as the failure mechani

18、sms were understood.4.1.1ExtensometersThe wireline extensometers were linked to a telemetry system which provided real time data. However due to the slope failure mode, and the location of the extensometers, the vector direction of movement of the slope (down and out) resulted in little displacement

19、 recorded. Thus highlighting the need for multiple monitoring systems, and understanding the failure mode.4.1.2PrismsInitially manual prism monitoring was conducted twice weekly and covered a broad area of the pit. When slope stability issues arose requiring more frequent monitoring, a geodimeter ro

20、botic prism monitoring system was utilised. It monitored approximately 14 prisms from the opposite side of the pit at a slope distance of 300m (Figure 3) and was collected approximately on an hourly basis.The system was linked to Quickslope software which allowed for live interpretation of data in t

21、he following format:Slope distance (adjusted distance),Strategic versus Tactical Approaches in Mining 2006, Page 5XYZ movement (north, east, RL),3D movement (Figure 3), andVelocity (Figure 4).Figure 3 Graph showing 3d vector movementInflection point in monitoring data showing onset of failure.Strate

22、gic versus Tactical Approaches in Mining 2006, Page 6Figure 4 Graph showing velocity movementThe system allowed for live data analysis, pre-set triggering for and the alarm capabilities. The monitoring of the prisms and the alarm worked up until the time the slope failed.4.1.3Hydrology - piezometers

23、There were four piezometers in the vicinity of the pit with one located in the vicinity of the failure. The results showed no reaction to rainfall and no change in levels prior to or following the failure. However slope movements increased in reaction to rainfall events.4.1.4Movement rates and alarm

24、ing thresholdsThe slope deformed in a ductile manner with base movement rates in the order of 1 to 2mm/day. Following a triggering event (blasting, rainfall or excavation) the movement rates increased to over 5mm/day and subsequently decreased to the base rate up until the onset of failure. The pris

25、m monitoring system had a 7mm velocity and a 10mm 3d threshold alarm. If this was activated it triggered an audible alarm of the mine radio system.4.1.5Time to failure analysisThe use of the time to failure analysis is shown in Figure 5. The onset of failure was identified through an inflection in s

26、lope movements (Figure 3 and 4). Using the time to failure equation (4.1.5.2), the mode of failure and the quality of monitoring data allowed for predicting the failure within 6 hours, 3 days prior to the failure. On the day the accuracy of the prediction was within 30 minutes which allowed adequate

27、 time for the failure to be filmed (Figures 6 to 8).Strategic versus Tactical Approaches in Mining 2006, Page 7012345678024681012TimeDisplacementD3D2T3T2T1TFD1Figure 5 Diagram showing how the parameters for failure prediction are identifiedD2 = (D3-D1)/2 (4.1.5.1)Time to failure = T1 + (T2-T1)2 (4.1

28、.5.2)(2T2 -T1-T3)The total movement of the slope at the onset of failure was approximately 200-400mm, and at time of failure approximately movement was 800-1000mm.Strategic versus Tactical Approaches in Mining 2006, Page 8Figure 6 Photo prior to failureStrategic versus Tactical Approaches in Mining

29、2006, Page 9Figure 7 Photo of failure in progressFigure 8 Photo following the failureAnother monitoring system that could have provided valuable information was the slope inclinometer. This system could have identified the area of dislocation within the slope. This would have aided in understanding

30、the failure mechanism and scale of the instability. The slope radar would also have been suitable for use.5FAILURE MODEThe failure mode was primarily a deep seated structural control in combination with rock mass failure. The elements of the failure involved an active sliding block with a passive bl

31、ock at the toe of the slope (Figure 9).The deep seated structure is supported by:Scale of the failure,Vector movements of the prisms (figures 10 and 11), andSubsequent to the failure pit exposures in the main Cadia Hill Pit identified the presence of flat dipping structures (Figure 12).Strategic ver

32、sus Tactical Approaches in Mining 2006, Page 10Figure 9 Section through failure zoneStrategic versus Tactical Approaches in Mining 2006, Page 11Figure 10 Stereonet showing prism vectors prior to failureFigure 11 Stereonet showing prism vector movements post failureFigure 12 Stereonet showing mapping

33、 from adjacent Cadia Hill pit post failure6OPERATIONAL COMMUNICATIONCrew briefing sessions were held routinely for:Pole of flat dipping shears.Strategic versus Tactical Approaches in Mining 2006, Page 12Explaining pit slope movements in the open pit environment,Failure modes in relation to slope mov

34、ement rates,Monitoring and alarming systems,Procedures for managing the instability.An evacuation plan was also developed for when pit slope alarms were activated. The plan highlighted muster points within the pit and sections of the ramp were machinery should not stop.Since the failure and its capt

35、ure on film, it has been used for information sessions to crews. This provides the operators with a visual understanding of how a slope can fail and how monitoring systems are used.7PROCEDURESSite specific procedures are linked to the Slope Stability Major Hazard Management Plan. Essentially the sit

36、e systems require the risks to be identified and risk reduction plans put in place. The elements of the procedure were:Subsequent to blasting at the base of the pit there was to be an 18 hour stand-off, review of monitoring and inspection of the haul road.Whilst working at the base of the pit there

37、was to be routine review of monitoring results.Rainfall triggers to vacate the pit were:oIntense rainfall at 10mm/hr oroSustained rainfall 2mm/hr over an 8 hour period.As the pit progressively developed the impact on slope movements increased. A risk assessment was undertaken in relation to the valu

38、e of ore on the last bench and the expected outcome of extracting it. This highlighted that the cost outcome for extracting the last bench did not significantly out way the risk of the unwanted event occurring. Thus the last bench of the pit was left behind and not mined.8BACK ANALYSISUnderstanding

39、the primary structural controls aided in focusing the back analysis work. Planar failure analysis incorporating two planes was used to represent the fault plane and the rock bridge, assigning cohesion and friction angles. The results highlighted:For c =100kpa ranged between 20 to 30,Width of the roc

40、k mass bridge was critical to stability,More detailed back-analysis work is required. Strategic versus Tactical Approaches in Mining 2006, Page 139CONCLUSIONSThe CX pit highlighted the pertinent aspects of a comprehensive geotechnical investigation to reduce technical risk. However when mining a pit with or without a comprehensive investigation will require a pi