Percolation Rate For hydraulic backfill, the most important design criteria is percolation rate. A minimum percolation rate of 10 cm /hour is a common rule of thumb in many mines. At this rate, the actual drainage of the fill is good and there is low potential for liquefaction. If the percolation rate is too slow, it slows down mine productivity. It can create high pressures on bulkheads (potentially dangerous condition). Percolation rate measurements are normally performed in a lab using a perspex tube. A common standard size for the tube is 6 cm in diameter by 30 cm in length. The test technique involves [placing backfill in the tube and allowing it to settle in place. The bottom of the tube is covered with a woven geotextile or burlap fabric to retain the backfill but allow free passage of water. A constant head of water is maintained above the backfill column. Water percolates through the backfill column and is collected at the bottom at various time intervals. Percolation Rate, k = (Q)(L)/(Dt)(H)(A) cm/hr |
| where | Q | = | volume of water collected per time interval (cm) | | Dt | = | time interval of collection, (hr) | | L | = | height of emplaced backfill column in tube, (cm) | | H | = | fixed height of water column, (cm) | | A | = | cross-sectional area of percolation tube, (cm2) |
The initial percolation rate will be high and will decrease gradually as the water moving through the column densifies the backfill. The test should be continued until a steady state percolation rate is obtained. Percolation Test Setup
Backfill Strength Backfill strength is required in longhole stopes to maintain a free standing height of fill in the primary stope while ore in the secondary stope is extracted. The factor of safety against planar failure through the backfill can be calculated according to the following equation. 
where, C=cohesive strength of fill B= width of fill in stope H=total exposed height of stope f = friction angle of fill L=strike length of fill in stope g = fill bulk density y = plane failure angle He = effective sliding block height 
The stability of a free standing backfill pour can also be determined using equations developed from physical model tests. Based on centrifugal modeling tests conducted by Mitchell (1983), the stability can be related to the unconfined compressive strength of the backfill. The critical unconfined compressive strength (UCS) is given by: where, UCS = unconfined compressive strength, Pa H = height of fill exposure (m) L = length of fill exposure or stope width (m) g = unit weight of fill (N/m3)
As an example, based on a typical total tailings unit weight of 21,090 N/m3 and an average fill height of 60m, the critical UCS is calculated in the table below. Design strengths for the fill are provided assuming a safety factor of 1.2. Example Strengths for Backfill versus Stope Width Fill Exposure Length (m) | Critical UCS (kPa) | Design UCS (kPa) | 10 | 217 | 260 | 20 | 380 | 456 | 30 | 506 | 607 | 40 | 608 | 730 |
These design strengths in are appropriate for fill at the bottom the stope where stresses are highest. In practice, the cement content will vary within the stope with the lower portion having the highest cement content and the upper portion having the lowest cement content. A high strength cap may be placed on the top 0.3m of fill to improve the bearing capacity and to facilitate vehicle travel. Introduction to Mine Backfill Types of Mine Backfill Backfill Properties | 
