Geotechnical Study Of Mine Dumps In Dabwam Village Of Riyom Lga Of Plateau State For The Back Filling Of Mine Pond In The Area

Statement of the problem

The Plateau Mineral Mining project at the Dabwam village will require operations to extend further to greater depths (1500m below surface) and hence the need for an extensive use of systematic backfill of the stopes. Dabwam village has been currently facing poor ground conditions that ultimately make the mining process challenging and dangerous. This ground conditions will therefore affect stable stope spans, sequence of mining and ground support practises. However, ore recovery will also be reduced from around 80% by prompting the mine to leave larger rib pillars (> 5m) of ore for stabilisation of the stopes as depth of mining operations increases. This research was therefore initiated by Plateau Mineral Development Company to establish the necessity of backfilling in the steep dipping areas and also depending on the mining method used to clarify what would be the behaviour of the hanging wall, footwall and pillars assuming that the mine does not carry out any backfilling.

Objective of the study

The main objective of this study is the geotechnical study of mine dumps in Dabwam village of Riyom LGA of Plateau State for the back filling of mine pond in the area. The specific objectives of this study are to:

1. Model the stress distributions and displacement around the mine openings using numerical methods (Phase2).

2. Evaluate the zone of stability of the individual stopes at different depths using the empirical approach (Mathew Stability graph).

3. Compare the outcomes of the two methods and make recommendations.

4. Optimise ore extraction by selecting possible backfill material types that can be used at the mine in case modelling and empirical analysis proves the need to backfill the stopes.

Research questions

In order to accomplish this research work, the following research questions were born in mind:

1. Are the stopes in the steep dipping areas stable without any filling?

2. Is there any long term effect if the stopes are not backfilled?

3. If backfilling is needed, what are the estimated quantities, strengths, and at what point in terms of mining depth should the filling be recommended?

4. Is sequencing of stopes mining an option?

Methodology

In order to achieve the above set objectives, the methodology used followed a systematic way of reviewing literature as well as collecting data from the mine that was used in the numerical modelling and Mathew’s stability graph method. Stopes from different levels (950mL, 1050mL, 1150mL, 1250mL and 1350mL) were modelled using Phase2 computer software to analyse the stresses and displacements within the stopes as depth of mining increased. The behaviour of these stopes was used to make conclusions concerning the approximate depths of instability as mining depth increased.

On the other hand, the Mathew’s stability graph method was used to determine the different zones of stability that these stopes at different mining depths can plot on. In this method, the stability number was determined following the procedures described by (Potvin, 1988). This stability number was plotted against the hydraulic radius of each stope on the Stability graph. Each zone of stability for the individual stopes was noted and discussed for the purpose of drawing conclusions.

Laboratory testing for uniaxial compressive strength of intact rock samples as well as core logging and structural mapping was used to evaluate the parameters that were needed in the evaluation techniques.

The outcomes of the two techniques were compared to each other in order that the approximate depth of instability can be suggested. Backfilling of the stopes located in the unstable zones was recommended on the bases of the outcomes of these evaluation methods and the backfill material suggested was that already analysed by Plateau Mineral Development Company. The full methodology used in this research if discussed in detail under Chapter 3.

Significance of the study

A better understanding of the stope behaviour (hanging wall, footwall and pillars) will definitely go a long way in the ultimate allocation of the backfill resources and also it helps the mine to minimise the possible unnecessary cost of backfilling. However, the outcome can also contribute to the proper stabilization of the underground openings. More importantly, the research will facilitate the execution of the proposed primary/secondary ore extraction system which will obviously improve the copper ore production as the number of stabilising pillars will be reduced or the size of the pillars reduced hence helping the mine to achieve its target of ramping up production.

Since production is to be performed at greater depths where high induced stresses will be experienced, modelling of the stopes to determine depth at which backfilling should commence will contribute to efficiency of mining operations and it will also permit the accessing of high grade of copper ore which from exploration is deeper than the current levels of operation. The need to ascertain backfilling can also contribute towards proper waste management hence better environmental management efforts as little waste will find its way to the ponds.

Scope of study

The research work is focusing on the backfill needs for the current and future mining methods in the steep dipping areas of Plateau Mineral Development Company. Steep dipping areas for this study are the part of the ore body exceeding 45 degrees (dip angle). Stopes modelled are those from the deeps which is from 0mS to 1000mS. The deeps are also selected because it is this portion of the Dabwam village deposit with the steepest outline of the pond. However, the effect of stresses was much anticipated from this section because it has the greatest depth so far on the Dabwam village mine (1040mL).

The study of the backfill needs will include stope dimensions, backfill strength and timing requirements, exposure and geotechnical characteristics of the rock.