Avoca methods of mining

Avoca methods of mining

Gideon Gipmai Yowa
(August, 2017)
Senior Instructor, Mining Eng. Dept., PNG University of Technology. PNG

1.     Overview

Avoca Method of mining is a variation of sublevel stoping method with the introduction of waste rockfill as backfill. This method is popular in high grade epithermal vein deposits where the strength and conditions of the walls of the open stopes are weak to moderate and require backfilling of waste rock to stabilise the walls, minimise the ore dilution and maximise the ore recovery. The stopes are drilled with longholes as either upholes or downholes. A slot is established to create the initial void for the subsequent rings firing. The rings are fired in slices and are bogged clean. Filling commences right after the recommended stable strike length is reached.

Avoca is suitable for narrow ore bodies. Wider ore bodies (>10m) may find it challenging to fill the voids further to the walls in time for the next ring firings. Production schedules and pressures may prevent complete filling of the walls and subsequently compromise the wall stability. Moreover, ore loss can be encountered on the walls of a wide stope due to the bogger (loader) inability to turn and bog the materials on the edges.

Avoca method is applied on stopes of high grade. The grade has to be high to meet the cost of introducing fill into the voids as well as dilution. Dilution is expected from bogging the waste rock fill due to uncontrolled monitoring of ore and waste fill contact.

2.    Pure Avoca Method

An Avoca Method can be either a Pure (True) Avoca or Modified Avoca depending on the direction of mining and filling.  

In a Pure Avoca Method the mining retreats towards the access while the backfilling advances from the opposite end and chases after the stoping brow.   There is always a pre-determined void established between the fill and the stope brow prior to firing the next set of rings. The filling and mining cycle repeats until the stope strike is completely mined out.   

There has to be two accesses available and are developed from the footwall drive (FWD) for a Pure Avoca Method. One access is used for drilling, blasting and bogging, while the other access serves for backfilling. The filling volume and tonnage has to be monitored closely to prevent overfilling that potentially chocks the brow or under filling that potentially exposes an unstable stope span. The filling is stopped when the brow is reached. The brow must not be choked in order to have void to fire the next rings. The cycle repeats until the stope is completely mined out and filled.

The figure 1 below shows a typical plan of level development for a Pure Avoca Method. There are two accesses into the stoping blocks (ore blocks in green colour) and they are developed from FWD. Some additional infrastructure development can be added like ore pass drive, and paste reticulation drive depending on the mine site operation.

Fig1:   Pure Avoca development layout-plan vie

3.    Modified Avoca Method

In a Modified Avoca Method, there is one access to the stope for drilling, blasting, bogging and backfilling. Unlike a Pure Avoca, there is no separate access for filling. The filling is done from the stope brow. The rings are fired to the recommended strike length and the ore is completely bogged out. The void is then filled completed with waste rockfill. The brow in this case is completely choked off. 
In order to fire the next set of rings, the waste rock fill is re-slotted (bogged at the toe) to create the void. The next set of rings are fired and bogged until the require strike length is reached. The process repeats until the entire stope is mined out. 


Fig2:   Modified Avoca development layout-plan view

4.    Modified Avoca-Chock Blast Method

In some cases, chock firing is applied where the next set of rings are fired against the rockfill without re-slotting the rockfill. Since rockfill have some void within (15-30%), the rings are fired against the rockfill aiming to compact or chock the blast. The benefit of chock blast is the reduction of cost in re-slotting the waste. However, this method is usually not preferred as it introduces more dilution from the rockfill waste, freezes the blast due to inadequate void, as well as in many cases back break and damages are induced on the holes on the adjacent ring.  

            5.    Backfilling 

The common fill type is the waste rock fill. The rockfill comes from the waste generated from lateral or vertical developments. Often waste from surface waste stockpiles can be back hauled into underground to fill stopes when there is shortage of waste material underground. Generally back hauling is not preferred. A proper mine planning and scheduling can prevent backhauling and managing waste underground. 

Prior to backfilling, a cavity monitoring system (CMS) or void pick up has to be made for stope reconciliation purpose and also determining the volume of void to be filled.  The void wireframe is used frequently during the filling and bogging stages to make sure waste fill is not bogged as ore and also blasted ore are fully recovered.

            6.    Span Stability - Hydraulic Radius (HR)

The geotechnical assessment of the stable span determines how far a hangingwall shall be exposed along the strike and height of the stope. Filling is introduced soon after the recommended stable span has been mined out.

Hydraulic radius (HR) is widely used in the rock mass characterisation and is a modification to the NGI (Q-System) where a stability number (N) is determined. The “N” value is based on the modified “Q” classification system and hence takes into consideration the stress conditions and open void geometries. HR is simply the area of the opening divided by the perimeter of the opening. HR is used to determine the height and strike of a stoping block.

Avoca methods of mining Avoca methods of mining Reviewed by Gideon Gipmai Yowa on August 04, 2017 Rating: 5

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