Underground water inflow to a mine

picture : accident because water inflow to a mine

Past experience indicates that there are four distinct modes of underground water inflow to a mine which has an important influence in designing pumping capacity. These inflow modes are:

a) Constant Rate of Inflow over a Long Period

This mode of flow is under free flow conditions which is characterized by dripping or seepage depending upon the inflow quantities. Mine development workings under an open aquifer or over a confined aquifer may experience this mode of flow.

b) Occasional Large Make of Water

Characterized by a high initial flow reducing to normal flow over a short period of time (Fig. 1). At an initial stage AB, the rate of water inflow increases rapidly as the head of water overcomes the resistance offered by the flow channels in the intervening strata , until a peak flow rate at Cis observed. This flow rate is usually of a short duration and rapidly decreases until a normal flow rate consistent with strata permeability is obtained. This type of flow is normally observed when natural or induced mining fractures intercept afinite accumulation of water such as:

-bed separation cavities

-solution cavity

-old mine workings.

This type of flow is also observed when a protective layer or a pillar between an old working and the mine is eroded due to hydraulic pressure and induced mining stresses ; for example, a barrier pillar between old workings and the present extraction panel in a steep seam conditions.

c) Large Solution Cavities

The water inflow from large caverns in Karstified rock is usually a concentrated flow for a limited period followed by a decrease in inflow rates . The residual inflow r a t e from a dewatered Karst aquifer i s only a fraction of the initial flow. The drainage control under such circumstances can be achieved by grouting from advanced boreholes, sealing the openings by underground dams whenever water is encountered and provision for large pumping capacities

d) Water Inflow Throuph an Erosive Protective Layer

Mining in the vicinity of soft argilleceous strata or protective layer in a steep seam condition allows water to erode the existing or induced mining fractures and discontinuities, thus permitting water to flow through the barrier without offering any marked resistance to the water flow. The inflow in such circumstiances starts with the roof convergence initiating the seepage at a low rate then followed by a high rate of inflow. The increase rate of inflow is accompanied by an increase in solid contents of water due to erosion of fractures and discontinuities until an equilibrium state, characterized by low rates of inflow and reduction in solid content has been reached. The above discussion shows that the mode of water inflow has a great influence on the design capacity of main standage, peak pumping capacity and a sediment settler for a mine pumping system.