Table of Contents
Problems of mine backfilling and fill material deposited underground, the characteristics and relationship between the deposited fill material and overlying ground, and the practical applications and limitations of hydraulic backfilling as ground support are discussed.
Ground Pressure and Backfill
There are many theories explaining the formation and distribution of stresses around underground excavations. It has to be taken into consideration that these theories are based on observations under different ground conditions. A detailed discussion of these theories is beyond the scope of this paper; only the “dome theory” explaining the stresses in inclined stopes, typical of hydraulic back-filling application, will be outlined here. All these theories have one thing in common, however they demonstrate that ground movement and subsidence can be considerable reduced through good backfilling.
When an excavation is started underground, the existing equilibrium is disturbed, because a part of the natural support has been removed. The existing stress field is disrupted, and stresses which formerly passed through the portion of ground now excavated must now pass around the edges of the excavation. This causes a concentration of stresses in the rock ahead of the advancing face.
When the closure starts, there is little resistance to expansion because of the voids in the backfill material. Progressive wall closure increases the resistance of the fill through elimination of voids, and the stress zone becomes flatter. Backfill thus reduces the penetration of expansion domes and stress zones into the walls.
From this short discussion of stresses in underground working, we can deduce some criteria necessary for efficient ground control and support. Backfilling should be such that closure or subsidence is reduced to minimum. This is accomplished by:
- keeping the open area to a minimum,
- backfilling at the same rate as excavating,
- completely backfilling the mined-out space, and
- using compact backfill material with low compressibility, so that the backfill will take the full load as soon as possible.
The advantages of hydraulic backfilling as ground support are as follows:
- Properly placed material has excellent supporting qualities, because it can be densely packed in both vertical and inclined mine voids.
- Faster-mining eliminates or reduces timbering, because the hangingwall does not have time to settle and develop heavy pressure on timber.
- Higher production results. The non-productive part of the mining cycle can be shortened, as the capacity of the working place is increased. Fewer stopes have to be maintained to meet the production demand.
- It affords more complete extraction of ore. Floor and sill pillars can be taken out easily, and protective pillars can be mined out and empty space backfilled.
- Fill can be placed in mined-out stopes through boreholes, without requiring the men to enter the place.
- Waste rock or old, large-sized fill can be compacted with hydraulic fill and the void space reduced.
- Hydraulically placed material can penetrate the cracks in ground and reinforce it. This is advantageous in pillar mining.
Disadvantages of hydraulic backfill are the following:
- The whole backfill mass should be regarded potentially as quicksand held in place, by bulkheads, if the fill does not compact or cement, and if insufficient drainage is provided where seepage of water is present. Percolation channels may also develop along the stope walls. These channels gradually increase in size and provide room for expansion of fill and absorption of water.
- Hydraulic backfilling does not work very efficiently in horizontal seams, since it is difficult to fill tightly to the back and to place fill close to an advancing working face.
The shortcomings of hydraulic backfilling lead to the development of pneumatic stowing. In pneumatic stowing, the backfill material is conveyed in a pipeline by high-velocity air flow and discharged in the void to be filled. A serious disadvantage of pneumatic stowing, however, is its high consumption of compressed air and resulting high energy costs.
Characteristics of Hydraulic Backfill Material as Support
The material used as hydraulic fill ranges in size and characteristics from coal washery refuse to mill tailings. Backfill is a bulk material that always occupies more space than the original material in situ and may exhibit considerably different properties, however.
If the material used for backfill has higher compressive strength than the pressure imposed on it underground, the size distribution of the material determines the space occupied by the material and the supporting qualities of the material.
Techniques of compressibility testing were developed primarily in soil mechanics to test the qualities of soils, and data comparing compressibilities of various materials used as mine backfill are available in the technical literature.
The pressure applied on fine-sized fill material is distributed to a larger number of contact points than in coarse fill. Fine particles have better lateral support among themselves, and this results in increased resistance to pressure, i.e., lower compressibility.
Consolidation refers to the compaction that takes place within the material itself. The retained moisture, slime content, and placement technique are the principal factors that influence the degree of consolidation.
Limited slime and moisture contents are important in fill consolidation for two reasons: (1) it is necessary to retain moisture, and (2) closer packing is obtained since the fines occupy voids between the larger particles.
Segregation in Fill Material
The tests were conducted for two purposes:
- to find the quantity of solids escaping from the mine void being filled and its relationship to the discharge rate and concentration of solids in fill slurry, by weight; and
- to study the segregation of solids, based on screen analyses, taking place during the filling operation, determining if any segregation occurs, and if it can be detected by model studies.
It could be observed that during the discharge of slurry into the model, large particles were rolled or dragged down to the bottom of the slurry stream more rapidly than the small ones. This effect is due to significant changes in the stream velocity gradient near the bed surface; it is a sorting phenomenon very similar to the “film sizing” known in ore dressing.