TAILINGS overloading . To solve this , the dedicated underfilter conveyor speed and direction was optimised based on the relative direction of the cake discharge . For one direction , the conveyor runs faster at 9 m / min during cake discharge to spread the cake load and then decreases to 2 m / min to slowly discharge the filter cake onto the take-away conveyor . In the other direction , to spread the cake load the conveyor runs in the opposite direction of the cake discharge . Then the conveyor reverses towards the discharge point to discharge the cake onto the take-away conveyor . This retroactive optimisation ensured that the coal fly ash filter plant now operates as designed and produces cake that meets the environmental regulatory guidelines .
Dams and the mysterious 1.5 factor of safety
Bryan Ulrich , Owner of Bryan Ulrich LLC - Tailings Solutions and a well known figure in the industry having led tailings teams at Stantec and before that for many years with Knight Piesold , has discussed the issue that for dams and tailings dams , it is still almost universally accepted that for static , long-term , steady-state conditions a minimum safety factor of 1.5 is required .
Groups that are in agreement with this value include : The US Army Corps of Engineers ( USACOE ); The Australian National Committee on Large Dams ( ANCOLD ); The Canadian Dam Association ( CDA ); The Brazilian Standard ABNT ’ s NBR 13028:2017 ; The US Bureau of Reclamation ( USBR ); and The Federal Energy Regulatory Commission ( FERC ).
Ulrich points out that the Global Industry Standard on Tailings Management ( GISTM ) does not actually provide recommendations for safety factors under any condition . Requirement 4.5 of the GISTM indicates that one should : “ Apply design criteria , such as factors of safety for slope stability and seepage management , that consider estimated operational properties of materials and expected performance of design elements , and quality of the implementation of risk management systems . These issues should also be appropriately accounted for in designs based on deformation analyses .”
This is quite similar to the advice provided by the USACOE ( 1970 ). That document uses the phrase ‘ factors of safety ’ or ‘ factor of safety ’ nearly 100 times , thus the importance of this factor is of considerable importance to the document . The document includes the following passage : “ Appropriate values of computed safety factors depend on the : design condition being analysed ; estimated reliability of shear strength design values ; embankment height ; presence of structures within the embankment ; thoroughness of investigations ; stress-strain characteristics
and compatibility of embankment and foundation materials ; probable quality of construction control ; and judgment based on past experience with earth and rock-fill dams .”
It adds : “ In the final analysis , the consequences of a failure with respect to human life , property damage , and impairment of functions are important considerations in establishing acceptable factors of safety for specific projects .” Ulrich says it should be noted that the same document recommends a minimum safety factor of 1.5 for static , long-term , steadystate conditions , but it is clear that the minimum value hinges on numerous factors that require special consideration .
Herza et al ( 2017 ) state : “ The minimum acceptable FoS for dam design were anecdotally determined in the USA in mid-20th century by back-calculating the FoS of existing dams . It was found that the FoS of 1.5 provided sufficient contingency and was generally considered acceptable .” Unfortunately , the authors do not provide a reference for this back calculation assessment .
Feld ( 1965 ) wrote ( shortly after the mid-20th century ): “ A computed factor of safety of 1.3 for cuts and fills and 1.5 for earth dams is normal , although these figures are based on a number of uncertainties . These values are accepted however , because a factor of safety of 2.5 would make the cost of embankments and slopes so high that they would not be built .” Shortly after that , Sowers and Sowers ( 1970 ) concurred that a 1.5 factor of safety was sufficient .
Regarding the field of aerospace engineering , which Ulrich says seems somewhat analogous to
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APRIL 2024 | International Mining 61