A number of underground visits were conducted to backfill stopes in order to investigate the effectiveness of the placed backfill. In these stopes the zone of backfill influence was estimated by pushing, as deeply as possible, an inclino-rule between the backfill and the hangingwall of the stope. This gives an approximation of the point at which the backfill and the hangingwall comes into contact. The readings are shown in Figure 6.2.1 (the point of contact between the backfill and the hangingwall is measured from the face of the backfill). The panels in which the readings were taken are approximately 2200 m below surface.
0 2 4 6 8 10 12
0-20 20-40 40-60 60-80 80-100 >100
Distance of no contact between fill and hangingwall (cm)
Number of readings
5 m 10 m 30 m
Figure 6.2.1 In situ data of the point of backfill – hangingwall contact with respect to the side of the backfill, measured along dip for different stopes.
The zone of influence of backfill is assumed to be zero if it does not take any load, i.e.
immediately after installation of the backfill. It is understood that if the backfill is very compressed, it will exert more pressure on the hangingwall and thus its zone of influence will increase.
The parameter c is defined as the average distance between the edge of the backfill and the point of contact between the hangingwall and backfill.
The averages of the readings, c, taken for the panels in which the backfill is 30 m, 10 m, and 5 m from the face, are 25 cm, 50 cm and 70 cm respectively (Figure 6.2.1). Thus, it can be said that the point of contact between the backfill and the hangingwall is behind the face of the backfill for all cases.
A graphical representation of the zone of backfill influence is given in Figure 6.2.2.
Backfill
No contact between backfill and hangingwall
Stope face Zone of backfill
influence
Vertical stress
Figure 6.2.2 Graphical representation of the zone of backfill influence at the stope face (section on strike).
Assuming the backfill is in contact with the hangingwall at the edge of the backfill, the zone of backfill influence can be expressed as:
z = b tan ϕ, (6.2.1)
where b is the bedding thickness, and ϕ is the friction angle of the bedding plane contact Thus, using the notation as in Chapter 4, the effective zone of influence of the backfill, zb, is the difference between z and c (as defined earlier).
zb = z – c, (6.2.2)
zb = b tan ϕ – c. (6.2.3)
For example, consider a 1 m thick bedding plane with an interface friction angle of 45°. The value of c for the panel in which the backfill is 10 m from the face is 50 cm (Figure 6.2.1). The effective zone of influence of the backfill is:
zb = 1 m – 0,5 m = 0,5 m
Thus, the zone of influence of the backfill would extend approximately 50 cm ahead of the backfill face.
Western Area Gold Mine (WAGM) installs cemented backfill in the stopes. This is a stiff backfill and is usually installed such that the hangingwall and the backfill are in contact throughout the panel. Although this is the case for most of the panel, at the up-dip and down-dip side of the panel the backfill and hangingwall are not in contact. This is shown in Figure 6.2.3.
The numerical analyses (Section 6.3) indicate that the zone of backfill influence does extend ahead of the position of the backfill face. It is shown that the backfill exerts pressure on the rock immediately above it and slightly ahead of the point of backfill-hangingwall contact. If the backfill is extremely stiff or taking very large stresses, i.e. large deformation of the backfill has occurred, the zone of influence of backfill extends further ahead of the contact.
Evaluation of the data obtained from WAGM (see Figure 6.2.4) indicated that the point of backfill-hangingwall contact on the up-dip side of the panel was up to approximately 2,3 m behind the actual face of the backfill. If the zone of backfill influence extends only 1 m ahead of the point of contact, there is at least another metre that is not supported.
Backfill
Pack Area of no contact between
backfill and hangingwall
Gully
Figure 6.2.3 Section view of stope showing the lack of contact at up-dip and down-dip sides of panel between the backfill and the hangingwall.
The measurements taken on the down-dip side of the panel (Figure 6.2.4) show that the point backfill-hangingwall contact starts at a point approximately 2,2 m behind the face of the backfill.
Thus, although at first glance the up-dip side of the panel appears to be more problematic, the down-dip side also results in a substantial distance of hangingwall that is under very little or no influence of the backfill. This could, however, be due to poor installation, using incorrect density, i.e. too much water, which results in high shrinkage of the backfill, or the reef dipping at a low angle to the horizontal. If the dip of the reef is near horizontal, the correct installation of the backfill is problematic since the backfill flow is dependent on gravity.
0 20 40 60 80 100 120
0-20 20-40 40-60 60-80 80-100 >100
Distance of no contact between backfill and hangingwall (cm)
Number of readings
Downdip Updip