卡迪亚露天坑不稳定性监测及管理

1、Cadia 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 Operations ,AustraliaT. Li Newcrest Mining Ltd, Cadia Valley Opera

2、tions ,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 commenced in January 2003 and was completed in July 2004 during whi

3、ch 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 from concept to operation over a short period. The design

4、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 STRUCTUREThe CX pit comprises of Ordovician monzonite and lesser vo

5、lcanics 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 dieversion along the western wall in the early stages of mi

6、ning.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 Extended deposit to lie on the haingingwall of a major west-dippin

7、g 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).Figure 1 Stereonet showing faults and shears from mapping dataTable 1 Structural domains and main defect setsDomain

8、Defect SetDipDipDirection1175+10170+15NE250+10195+20385+15245+202155+15160+20SE240+10340+153140+15160+15SW255+15340+20360+20060+204145+15165+20NW255+15190+25365+15215+203DESIGN AND CONSTRUCTIONThe overall design strategy was to have a two staged pit. The first stage was designed to test the slope pa

9、rameters 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. A circular haul road was designed to traverse all the geotechnical sectors to share the uncertainty by ensuring that n

10、o section of the pit had a high inter-ramp slope. Within the monzonite joints were discounted as a control for batter scale 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

11、based pit design was a component of the pit design approach. As the pit was developed numerous batter scale stability issues arose which were proactively managed with onsite geotechnical support. This approach was pertinent for complementing the level of knowledge in relation to the operating enviro

12、nment. When the pit design and production were reviewed prior to commencing the second stage, the consideration of the prevailing 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

13、issues were identified, localised modifications to the design were made attempting to buttress the toe of the potentially unstable slope.Blasting practices were considered aggressive, utilising large diameter (222mm) holes. Modified production blasts were used against pit walls. Blasting practices c

14、ould have impacted on the stability of the slope.Domain 1Domain 4Domain 3Domain 24PIT 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, b

15、erms and pit crests on daily, weekly and monthly schedules.Specific 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 upgra

16、ding as instability developed.4.1 Slope MonitoringSlope monitoring 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 incre

17、ased for the potentially unstable sectors as the failure mechanisms 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 move

18、ment of the slope (down and out) resulted in little displacement 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

19、issues arose requiring more frequent monitoring, a geodimeter robotic 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 Quicks

20、lope software which allowed for live interpretation of data in the following format:Slope distance (adjusted distance),XYZ 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.F

21、igure 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 - piezometersThere were four piezometers in the vicinity of the pit

22、 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 alarming thresholdsThe slope deformed in a ductile manner w

23、ith 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 prism monitoring system had a 7mm velocity and a 10mm 3d t

24、hreshold 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 slope movements (Figure 3 and 4). Using the time to fai

25、lure 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 time for the failure to be filmed (Figures 6 to 8).01

26、2345678024681012TimeDisplacementD3D2T3T2T1TFD1Figure 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 (4.1.5.2)(2T2 -T1-T3)The total movement of the slope at the onset of failure was approximately 200-400mm, and at time

27、 of failure approximately movement was 800-1000mm.Figure 6 Photo prior to failureFigure 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

28、dislocation within the slope. This would have aided in understanding 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 eleme

29、nts of the failure involved an active sliding block with a passive block 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 identif

30、ied the presence of flat dipping structures (Figure 12).Figure 9 Section through failure zoneFigure 10 Stereonet showing prism vectors prior to failureFigure 11 Stereonet showing prism vector movements post failureFigure 12 Stereonet showing mapping from adjacent Cadia Hill pit post failure6OPERATIO

31、NAL COMMUNICATIONCrew briefing sessions were held routinely for:Pole of flat dipping shears.Explaining pit slope movements in the open pit environment,Failure modes in relation to slope movement rates,Monitoring and alarming systems,Procedures for managing the instability.An evacuation plan was also

32、 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 capture on film, it has been used for information sessions to crews. This provides the operators with a visual unde

33、rstanding 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 site systems require the risks to be identified and risk reduction plans put in place. The elements of the procedu

34、re 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 was to be routine review of monitoring results.Rainfall triggers to vacate the pit were:oIntense rainfall at 10

35、mm/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 value of ore on the last bench and the expected outcome of extracting it. This highlighted that the cost outcome fo

36、r 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 the primary structural controls aided in focusing the back analysis work. Planar failure analysis incorporating

37、 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 rock mass bridge was critical to stability,More detailed back-analysis work is required. 9CONCLUSIONSThe 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 pit slope management program to identify change and react to both favourable an