The following piece is repeated from an AMC Consultants newsletter. I post it here in its entirety as, in my opinion, it deals with a significant technical approach of interest to all involved in underground mine backfilling. The original is by Mehmet Yumlu, Principal Mining Engineer at AMC.

Safe and efficient placement of paste fill requires a detailed understanding of paste fill characteristics from production to final fill exposures. Mobilization of uncured paste fill as a result of a bulkhead failure is a safety hazard and can lead to significant consequences, including danger to personnel, property damage and production losses and delays.

Since inception of paste filling in 1999, there have been three major bulkhead failures incidents at Cayeli Mine. Fortunately, there was no injury to personnel when the failures occurred. The failures however were very violent and resulted in severe damage to equipment and caused significant production delays (Figures 1 and 2).

Figure 1 - Property damage caused by a bulkhead failure (transformer)

Figure 2 - Property damage caused by a bulkhead failure (Fargo truck)

Post failure investigations revealed that, in all Cayeli cases failure occurred in small and blind stopes during last stage of filling while tight filling. Stopes were overfilled by about 10% and there were problems with air relief holes. These bulkhead failures were attributed one or a combination of factors; fast filling rate; continuous filling or inadequate plug fill cure time; overfilling during the tight filling stage due to the absence, blockage or inadequate use of air breather holes; lack of adequate fill management and fill monitoring controls; inadequate bulkhead design.

Based on AMC’s recommendations and guidelines Cayeli set up comprehensive field instrumentation and monitoring program in order to investigate the development and magnitude of paste fill pressures inside and behind the bulkheads during and after fill placement (Figure 3).

Figure 3 - Paste fill pressure instrumentation setup

AMC has helped Cayeli to analyse and interpret the field pressure instrumentation data (Figure 4). The results indicated that the loading conditions in the paste fill and the resultant lateral loads on the bulkheads are complex and depend such factors as tailings properties (type, particle sizing and specific gravity), paste fill recipe (slump, solids content, and cement type and content), filling rate, filling placement sequence and stope size and geometry.

Figure 4 - Paste fill pressure results for Test 2

Based on Cayeli pressure monitoring test results, the following general conclusions can be drawn (Table 1):

  • Despite the same fill recipes and fill rise rates used, bulkhead pressures were significantly different. The difference is attributed to different stope size and filling sequences.
  • Continuous filling with no cure time leads to higher bulkhead pressures. This is due to the higher pore water pressure and slower pore water pressure dissipation with ongoing filling.
  • At the same fill rise rate, staged filling results in lower bulkhead pressure. This is attributed to the higher fill strength in the plug fill. The rest time enables pore water pressure dissipation and longer fill cure times. The weight of ongoing filling is partially distributed to the stope sides as a result of arching.
  • Temperature changes during cement hydration can impact on pressure readings. Tests show that an increase in temperature increases pressure and a decrease in temperature decreases pressures.

Table 1 - Summary of pressure monitoring test results

Prior to the bulkhead failures, Cayeli shotcrete bulkheads were designed for a maximum working pressure of 32 kPa. Fill placement was continuous and managed by restricting fill rise to a maximum of 0.43m per hour. Pressure-monitoring results indicated that bulkheads could be subjected to up to 100 kPa, even at the restricted fill rise rate of only 0.35m per hour.

Based on the evaluation of pressure monitoring test results and findings from the bulkhead failures, Cayeli has made the following design and operational changes in their paste fill system: (1) revised shotcrete bulkhead design; (2) new fill recipe; (3) revised fill placement procedures; and (4) new fill risk management and monitoring procedures. Site-specific new operating practices now in place allow safe and efficient fill placement.

To date, Cayeli has filled more than 50 blind stopes without any failure incidents. The revised bulkhead design and fill placement sequence has therefore shown to be successful.

Going forward, Cayeli is considering instrumenting every single bulkhead with a pressure cell, transferring the pressure data to the paste plant and filling until a threshold bulkhead pressure is reached. Ongoing pressure tests are planned for this purpose. The mine is also looking to automatically monitor the airflow in blind stope breather holes as an indicator of potential over pressuring.

Only a combination of properly engineered bulkhead design, bulkhead construction and better management of fill placement can provide safe and efficient filling practices. Bulkhead pressures are very site specific and any bulkhead design should be based on actual pressure monitoring results.

AMC has the expertise and knowledge to help clients optimise their paste backfill system from the conception of designs through to the implementation in order to achieve a safe and efficient backfill system.