The **Red Lake Mine** in eastern Ontario currently does nearly sixty percent of its mining using overhand cut and fill, twenty-five percent using underhand cut and fill, and about fifteen percent using pillar recovery.

To contain the paste backfill they place fill fences that incorporate rebar anchored to the walls and floor and a minimum of 10 cm of shotcrete. The paste includes between five and ten percent cement with an unconfined compressive strength of 2 MPa. The achievable pouring rate is 40 cubic meters per hour.

As part of his master thesis at the University of British Columbia, Paul Hughes working under Professor Rimas Pakalnis instrumented nine typical fill fences and sought to establish the adequacy (or improvements to) current fill fences.

**At this link** is a PowerPoint presentation on his work. They set out to ask these questions:

- What are the loading mechanisms of paste against barricades?
- What is the capacity of the paste fill fences at Red Lake?
- Based on the loading mechanism of the paste, do the fill fences pose a risk of failure?

The most interesting finding (to me at least) is that the Rankine earth pressure coefficient, Ka, is not unity, but about 0.5 even when the paste is being slurried in Let me explain the significance of this.

The Rankine earth pressure coefficient is, at its simplest the ratio of the horizontal stress to the vertical stress. Obviously in a fluid Ka is unity. i.e., the horizontal stress is the same as the vertical stress. In a soil, Ka is theoretically the ratio of one minus the sin of the angle of friction of the soil divided by one plus the sin of the angle of friction of the soil.

Using data from instruments they installed on a trial fill fence Paul and his coworkers measured the fill height and the lateral earth pressure on the fence. Hence they are able to calculate Ka. And they come up with an average value of 0.548. This implies that the angle of friction of the fill when places is about 17 degrees. That is pretty small, but enough to help reduce the lateral pressures on the fence.

Thus they are able to make these recommendations to the mine as a result of their work:

- Fill pressures during backfill do not depend on the rate of fill placement
- The current fill fence construction method is suitable for the loads applied by current fill
- Continuous pouring is advised for underhand cut and fill stopes as it will eliminate hazards associated with ground fall due to cold joints.