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

1.1 Background to the study

The tremendous need for mineral resources as a result of the rapid population growth and economical development in Nigeria, exploration and exploitation of various minerals for several decades has greatly accelerated. The exploitation of minerals on the Jos Plateau dated as far back as 1902 Calvert (1977). Some on large/medium scale while some small scale or artisanal mining in the form of “Lotto” mining, as a result, a broad array of environmental problems related to human mining activities, such as air and water pollution, land resources and inhabited environmental devastation generated by non reclamation of these explored and exploited areas, and other hazards posed by these opened and unreclamed mining ponds etc, are daily becoming matters of concern, and furthermore, as the need for these mineral resources gets on the rise, much pressure is places on the environment with urbanization getting higher.

The goal of mining is to extract valuable minerals from the earth’s crust and this process widely results in the formation of voids both underground and on the surface. Filling of these voids is usually done and the main function of the fill material in mines is to assist in controlling the stability of mine related voids (Potvin, et al., 2005). The use of different types of mine fill and their functions and ultimately the engineering requirements are closely related to the mining methods and mining sequence. Generally mine fill is applicable to artificially supported mining methods and the main functions of fill in artificially supported methods includes ensuring long term regional stability, limiting excavation exposure and also waste disposal.

Many factors, such as rock mass condition, stope geometry, in-situ stress condition, blasting, stope exposure time and geological structures influence stope stability and dilution (Jucheng, 2004). Since backfilling of stopes can help in arresting instability, it is vital to assess the stability of mine stopes and openings underground to identify if backfilling is necessary. In underground mining operations, the mined-out spaces need to be backfilled to maintain the stability of surrounding rock mass and increase the ore recovery (Liung & Mamadou, 2015). This research will mainly consider the effect of rock mass condition, stope geometry and in- situ stress on stability of the stopes to aid on backfilling of the mining voids.

The mining site in Dabwam village takes the form of an anticline and it has a strike length of nearly 11km. On the other hand, the dip of the ponds varies between 15? and 65? and the average dip is 25? at No.3 shaft and 55? at No.1 shaft production areas. The thickness of the ponds varies between 3m-12m and the average is approximately 7m-8m. The mining operations are currently at 950m depth at No.1 shaft and 590m at No.3 shaft and another shaft, No.4, reaching to 1500m has been sunk to get access to mineralisation at greater depth.

Structurally, the ponds are divided into five units. At the bottom is the finely banded dolomitic, calcareous sandstone, in places highly weathered to a brown micaceous clay, ‘A’ Unit varying in width from 0.3m to 1.0m and rock mass rating of 21-40. In some places the rock mass rating is less than 20. Above the ‘A’ Unit is the 1.0-1.5m thickly bedded, ‘B’ Unit. The ‘C’ and ‘D’ Units comprising inter-bedded strong siltstone and dolomite bands, lie below the strong siliceous ‘E’ Unit within which the Assay hanging wall usually lies. The hanging wall formation consists of quartzite and dolomitic sandstone bands, which in some places completely koalinized, resulting into poor hanging wall condition.

There are also three main aquifers at Dabwam village and also the geo-hydrological setting at the mine results in high inflows of water into the mine operation pumping nearly 300 000m3 of water daily. These three aquifers are namely:

? Footwall Quartzite Aquifer

? Footwall Aquifer

? Hanging wall Aquifer

The mine in Dabwam village is in the process of developing other additional areas of production and undertaking key projects to extend the life of the mine and to allow an increase in tin production that is aimed at producing 7.5 Mtpa over the coming five years. Among these projects is the Plateau Mineral Mining Project (PMMP) which is targeted at expanding copper ore production by accessing the ore body that lies further down underneath what the current operations are currently mining. This then involved the sinking of the No. 4 shaft which reaches a depth of 1500m and the commissioning of the 6.0 Mtpa concentrator to enhance mining output and recovery.

Plateau Mineral Development Company has therefore engaged several consultants during the past decade in order to undertake several work to meet its targets and plans of ramping up production. Due to the shape and outline of the ponds, the mine uses a number of mining methods. These methods include:

? Sublevel open stoping (SLOS)

? Long-hole open-stoping (LHOS)

? Post pillar cut and fill (PPCF)

? Modified over-cut benches (MOCB)

? Continuous retreat up dip etc.

However, the conclusions and opinions from the consultant work was for the notion that the mine should adopt mining with backfill in order to break up the resource into several mining blocks so as to maximise resource extraction by removing the need for stability pillars. This is because, currently, the mining operations at Dabwam village have reached critical depths shown by the ever increasing induced mining stresses posing challenging ground conditions that require stabilisation. Reports from different departments have shown that recovery has been low due to dilution and therefore the consulting companies have strongly advised Dabwam to carry out a mine-wide systematic backfill operation into stoping activities.

This research was therefore initiated by Plateau Mineral Development Company to establish the need of backfilling in the steep dipping areas and also depending on the mining method used to clarify what would be the behaviour of the stopes assuming that the mine does not carry out any backfilling in these areas of the underground operations. It is the aim of this proposed research also to show at what point, in terms of depth, the stopes will pose a risk to the surrounding mining activities if not backfilled. Therefore, it is the scope of this research work to ascertain the need for backfilling in the steep dipping areas of the mine.