The primary objective of mine dewatering is to allow safe and efficient mining to be done Dewatering may impact the groundwater table which is shared with the local communities and environment. This article outlines the important parameters to consider when designing an efficient underground mine dewatering system. An optimal system involves high operational efficiency, low maintenance and low overall costs. Excess water entry into an underground mine can cause costly issues for the operating company. A mineral deposit may not be feasible to extract if the inflowing water exceeds economic pumping capacity. It is important for the mine planning department to have an accurate estimate of groundwater flow that will be encountered in the underground mine and to have a method of dewatering the mine if deemed necessary. When designing an optimal underground sump and pumping system it is of utmost importance to know the water inflow quantities and their sources, the total head and the characteristics of the mine water.
Sources of Water
For water to cause problems in underground mine sites there must be a water source nearby and there must be a mode of entry for the water to reach the mine. Inflow rates of water depend on the type of water sources present, and the route traversed to reach the mine.
Sources of water near mine sites depend on the regional climate and hydrogeology. The presence of any of the following near the excavation can be a source of water:
- Heavy rain fall
- Aquifers, springs, artesian wells
- Surface accumulations (lakes, rivers, seas, oceans)
- Drainage from hydraulic backfill
In order for the water to reach the underground mine, there must be a mode of entry for the water to travel through. Many mine sites have water sources present nearby that pose no problem as the country rock that the ore is hosted in is impermeable. Water may reach excavations through the following pathways:
- Permeable country rock
- Major faults/fracture zones
- Direct contact between excavation and water source
Impact of Water on Operations
Wet working conditions for underground mines are undesirable for various reasons and the problems that occur depend on the resistance to erosion of the rock type encountered, the pH of the water, and the inflow rate of the water. Problems that occur from excess water in an underground mine include:
- Dangerous working conditions
- Erosion of roadways
- Difficulty in ore handling
- Increased price of explosives
- Reduced operating life and efficiency of machinery
- Possible floor heave
- Water seepage can carry backfill into the sump which can lead to a solid blockade
- Flooding of mine
- Instability in mine
Predicting Water Inflows
Hydrological factors as well as the effects of planned mine work must be considered when predicting water inflows. Predictions often lack the data to accurately gauge the magnitude of inflows. As the project progresses new information will be available to improve the reliability of predictions. Potential sources of data may include:
- Historic rainfall and intensity data
- Information on rivers and lakes in the area
- Temperature, humidity and evaporation rates
- Hydrogeological information of domains including permeability and storage
- Soil characteristics
- Topographical Information
- Groundwater characteristics such as: general flow directions and magnitude, levels, temperature, and salinity
- Historical earthquake magnitude and frequency
Inflow from a precipitation event can be estimated based off site specific characteristics.
V = C * i * t * K
V = Volume of Inflow
C = Catchment Area
i = Duration of Event
t = Intensity of Event
K = Runoff Coefficient
Precipitation intensity and duration is based on historical records therefore mine life must be considered in predicting potential inflow volume. A long duration mine has a higher probability of facing a 1-in-100-year precipitation event. A risk assessment must be completed to assess the consequences and likely hood of each event. The risk assessment will dictate how robust the water management should be.
When planning a water management system, there is a tradeoff between water storage capacity and pumping rate.
V = C * i * t * K − PumpedVolume − OtherInflows
The water table is a horizontal plane where the ground reaches full water saturation. A pumping well will depress the water table. In a homogeneous, isotropic and horizontally infinite aquifer, the depression will take the shape of an inverted cone. The slope takes the shape of a logarithmic equation with the slope increasing closer to the well. Figure 1 displays the observed and computed water table draw-downs around a pumped well.
When the well first begins pumping the water table is horizontal and slowly begins to depress. This continues until the well is dewatered or until the pumping rate matches the water inflow. The amount of water stored within the ground material is dictated by the material’s porosity and inter-granular spaces. Effective porosity is the amount of water that a material can store and then release. Shale or clay, with very fine particle, has a high porosity but very small inter-granular spaces. Small inter-granular spaces prevents the movement of water therefore reducing the effective porosity.
A pumping test is done by pumping a well until the water table reaches equilibrium and the pumping rate equals the inflow rate. The water level is measured at surrounding wells to monitor the rate and magnitude of change. These observations can be used to understand rock uniformity and effective porosity. Pumping tests are limited due to the time and costs required.
Pressure tests inject water into a diamond drill hole over a certain interval. Fluid pressure and flow rate is monitored as the water permeates into the ground. In high permeability rock, a low pressure is used to have a controlled measurable flow. These observations can be used to understand rock uniformity and effective porosity.
Tracers can be put into the groundwater to identify the direction and rate of ground water flow. A tracer must :
- Be recognizable after dilution
- Move with water
- Be composed of environmentally friendly agents
- Be unhindered by physical, chemical or bacteriological reactions triggered by contact with the rock or water
- Be easy to use, low cost, minimal equipment, and soluble in water
Commonly used tracers include fluorescein, chloride ion and dextrose.
Control and Collection of Water
Water management plans must consider all potential sources under typical conditions and while assessing the risk of extreme events.
Surface water inflow can be mitigated through the following strategies:
- Trees can be planted in low elevation areas to increase evapotranspiration
- Build an evaluated portal when a ramp begins in an open pit
- Pre-grout shafts
- Do not impede natural water ways unless necessary
- Divert rivers, drain swaps and lakes in close proximity
Exploration drillholes have the potential to allow inflows. Drillholes in high risk areas should be fully or partially grouted. It is important to record hydrological data during exploration. High pressure water has the potential to erode strong rock and metal which must be considered in mine planning.
Grouting can be used to limit underground inflow but should include release holes to prevent a buildup of pressure. The released water can be directed toward a sump. Grouting may be used to control water in shaft sinking, limit flow past bulkheads and plugs, prevent leaking from reservoirs, and plug unplanned release holes.
The effectiveness of grout is limited by the expertise of the operator. The operator must select the grout nature, pumping rate, and pressure to match the situations conditions and objectives. It is difficult for the operator to visually inspect the grout for quality assurance.
A mine should have a contingency plan in case of inflow magnitudes beyond the water control systems capabilities. Constant monitoring must be done to ensure the contingency plan is triggered. Detailed evacuation procedures should be included in employee training.
Mine dewatering is a production critical function; inflow of waters needs to be removed to maintain a safe production environment. In order to ensure redundancy, the pumping system must include spare pumps and parts. In a three-unit system, any two pumps need to be able to provide the maximum inflow. As the number of the pumps increases in a system, the percentage of spare can be lowered. However, many small pumps can be inefficient and drives up the operating cost.
When designing a pumping system, the following factor should be considered:
- Depending of the criticality of the system, enough redundancy must be provide. There needs to be enough pumps so that when one pump is in repair, the others can handle the load.
- Pumps and electrical equipment need to be flood proof.
- There needs to be proper control, which includes automatic start-and-stop and alarms.
- NPSH should be calculated and positive suction head should be provided.
- The pumping room should have a sloped floor and high-tension items should be protected in case of jets and splashes.
- Sufficient ventilation should be provided and hot air need to be properly discharged.
When designing a pumping system, the first step is to decide whether the solid in the water should be remove to enable the use of clear water pumps. This decision depends on several factors such as the volume of the water, the life of the mine, the depth of the mine and the properties of the water. The second step would be deciding whether to use a single lift system or pump in stages. When considering design, the following are a few suggestions:
- Deep mine with a high inflow of water favors clarification and clear water pumps, which requires high capital cost but low operating cost.
- Shallow mine with a short mine life favors dirty water pump.
- Mines with top-down sequence and progressive mine development favors pumping in stages.
- Shallow mines favors single-lift pumping.
Many types of pump available for underground mining, the following are short non-exhaustive summary of the pumps available.
Portable Pumps: These are usually vertical centrifugal pumps with open impellers powered by compressed air or electricity.
Reciprocating Positive Displacement Pumps: Positive displacement pumps are used for dirty water. They have high abrasion resistance and moderate capacity.
Single-Stage Horizontally Split Centrifugal Pumps: These are clear water pumps are directly attached to an electric motor for compactness. They are highly efficiency with a capacity of 200L/s and 150m head.
Horizontal Multistage Centrifugal Pumps: These are very similar to the last pump but with multistage. They are able to provide a higher head than the single stage.
It is important to understand that the cost of pumps is a small portion of a pumping system. For a pump, motor and starter unit, the cost of the pump is usually only a 30% to 50% of the total. For an underground pumping plant that includes sumps, clarification, power supply, excavation and sufficient ventilation, the pump cost may only be 5%-15%. The efficiency of the pump will significant affect the operating cost (Power cost). Simpler pumping system with less pumps and high head are usually favorable. Large pumps with matching motor and pumps can achieve over 80% efficiency.
Bulkheads and Plugs
From tests in deep gold mines in South Africa several conclusions about plug behavior were deducted . These plugs were constructed in strong quantize rock. Water leakage through rocks fractures can be related to the pressure gradient. It is generally easier to make the bulkhead strong to resist thrust than to stop it from leaking.
After the inrush at the West Drieffontein mine in November 1968, four plugs were developed where many of them were neither hitched, tapered nor reinforced. These plugs withstood a pressure of more than 6.9 MPa. Mud, honeycomb, and laitance, which is a weak layer due to excess water, can lead to a leakage along the floor or the roof.
Pressure on rock surface causes rock movement which results in water breaking through rock fracture. Several stages of cement grouting can seal this leakage. The effect of grouting seems to be in filling the fractures. Failure of gaskets, threaded plugs and other fittings can be linked to the loss of bulkheads subjected to more than 6.9 MPa.
Injecting cement-sand grout into clean, strong, angular rock results in stronger concrete with less time than directly placing concrete. Ordinary Portland cement is usually used. The concrete settle time should be 17.2 MPa in 28 days.
Recommendations from the West Driefontein include:
- Plugs should be long to keep the pressure gradient moderate. A gradient between 900 and 1400 KPa/m were effective.
- Mud and loose rock should be removed.
- Pipes, gaskets, and valves should be tested at a higher pressure than the expected.
- Larger excavations should not be subjected to pressures higher than the maximum hydrostatic head
- The use of expanding cement with aluminum powder seems promising.
== See Also ==
- ↑ 1.0 1.1 1.2 1.3 1.4 . P. L. McCarthy and M. G. Dorricott (2011). Dewatering Underground Operations. In P. Darling, SME Mining Engineering Handbook Third Edition (pp. 765-780).
- ↑ . (1993). Mine Water and the Environment.
- ↑ Robertson Geoconsultants. Quecreek Mine Flooding Disaster. Retrieved January 2013, from: http://www.robertsongeoconsultants.com/index.php?page=page&id=69
- ↑ . Lucas, J.R. and Adler, L. (1973). SME Mining Engineering Handbook, Vol. 2. New York: SME-AIME.
- ↑ 5.0 5.1 . Theis, C.V. (1935). The relationship between the lowering of the piezometric surface and the rate and duration of discharge of a well using ground-water sotrage. Trans. Am. Geopys. Union 16:519-624.
- ↑ 6.0 6.1 6.2 . Garrett, W.S., and Campbell Pitt, L.T. (1961) Design and construction of underground bulkheads and water barriers. In Transactions, 7th Commonwealth Mining and Metallurgical Congress. South African Institute of Mining and Metallurgy. Pp. 1283-1299.