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SubLevel Cave



Ore is divided into sublevels with comparatively close vertical spacings, normally 8 - 15 m. Each sublevel is developed with a regular network of drifts, covering the complete ore section. In wide orebodies the drifts are laid out as crosscuts traversing the ore from a footwall drift; in narrow deposits the drifts will run parallel to the strike. Boreholes ranging in diameter from 64 mm - 100 mm are used. In the smaller hole diameters ANFO can be pneumatically conveyed, whereas in larger hole diameters, bulk emulsion explosives are used. One of the problems in loading rings with blasting pattern geometries that converge near the drift roof is the energy concentrated at the collar.
SLC pattern design - alternating top sills and bottom sills. Sublevel ring showing Concentration of Explosive in collar region.

Spacing and Burden

Defined as the distance between adjacent blastholes measured perpendicular to the burden. Spacing to burden ratio or S/B can be anywhere from 1.0 to 1.2 but most often about 1.15. For strong or massive ore, spacing and burden ratios should be equal to 1. If highly jointed, spacing and burden distances should include the distances between major joints. S/B ratio does not take into account;

Methods for Determining Spacing and Burden Laying Out Patterns

The diagram below shows the different schemes that are used to deal with toe spacing dimensions. There are three schemes; 
Schematic showing methods of spacing toes of holes.
Blasting patterns should be determined according to or taking into account the following;
Collar height should be at least 1.6 times the burden distance. In most cases collars should be in the range of 2 times the burden distance. Corner holes are usually delayed such that they are the last to detonate. Since these holes are usually short, collar height should be lower in the outer holes rather than higher. The tendency is to clean up the corners so that there is no burden remnant from the previous blast.

Paper on Draw Control

A good description of sublevel cave is given by a proposal to study an optimization process of sublevel caving draw. This paper can be obtained via the link - DrawControlOptimization  .
The paper "Draw Control Optimization along the Production Drift in Sublevel Caving Mines' by  Shekhar, G., Gustafson, A. , Boeg-Jensen, P.  and Schunnesson, H. - details some outstanding work regarding draw control in a well known Swedish mining operation and is worthwhile reading. 

Quality of Upholes in Sublevel Cave

Loading upholes using bulk emulsion produces some challenges. Emulsion need to stay up over time either using their own viscosity or using an aid to plug the holes after they are load. Loading from the top down is the preferred method since pumping up from the collar allows explosives to wander into cracks. In one case the author has personally seen 100 mm holes being from the collar only the find them oozing out  rock bolt holes ore even other unloaded blastholes. A typical uphole is presented in the following video with the camera actually entering the apex of a sublevel cave hole.
Condition of holes at the apex.

Bad Ground

The video here is self explanatory. This hole is one that was adjacent to a blasted ring. It becomes almost impossible to have competent ore to load when blasting designs are heavily concentrated with explosives in the collar zones.
Sublevel cave very bad ground. This is the reason why it is better to load from the end of the hole to the collar along with staggering collar loads.

Additional References

1. Queen's University description from MiningWIKI  Sublevel Cave Mining - Queen's University, Kingston, Ontario - CANADA
2. Atlas Copco You Tube Video
Atlas Copco animation for Sublevel Cave Mining
3.  Sublevel Caving Technique-Simplicity and Low Cost are the Essence -
Author: Partha Das Sharma,,Blog/
4. Kiruna Iron Ore Mine - Sweden   Kiruna Mine - Largest Sublevel Cave Mine