Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen, Henna Punkkinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.
Since most mines operate below the groundwater table, the water needs to be pumped to dewater the mine to gain access to the ore. Additionally the purpose of protection pumping of the groundwater is to separate the polluted groundwater hydraulically from unpolluted groundwater and change the groundwater flow in the desired directions (Figure 1). The pumped groundwater may be required to be processed. (Finnish Environment Institute 2001).
Pumping creates a depression of the groundwater table, which affects the connected surface waters systems. The size of the depression and the area affected depends on the volume of the pumped water. The dewatering may decrease flows in streams, wetlands, and lakes that have hydraulic connection with groundwater. In the worst case it can lower the water table in the vicinity of a water supply or irrigation wells, leading to complete drying-up of the wells. In some cases, dewatering of the mine has created subsidence, either due to compaction of fine-grained sediments or a collapse of voids as buoyant support is withdrawn. (Younger et al. 2002)
Figure 1. Protection pumping of the groundwater to hydraulically separate the waters of the landfill from the environment. (Modified from Finnish Environment Institute 2001)
Contaminated groundwater is pumped into a treatment plant to be cleaned after which the water is discharged back to the soil, conducted to the surface waters or into continued treatment. The treatment method of water depends on the detrimental substances contained by it. (Penttinen 2001)
The pump and treat methods are the most general handling methods for contaminated groundwater and only during the last few years have new groundwater handling methods been developed to replace these. The pump and treat -methods are used to prevent spreading of detrimental substances into a wider area (protection pumping) with groundwater and to remove the detrimental substances from the groundwater. (Penttinen 2001) The pump and treat methods are not suitable for detrimental substances which sorb themselves tightly to soil particles and are not transported with the groundwater. (Penttinen 2001)
Contaminated groundwater can be treated with several separate methods. These methods are evaluated in more detail in the Water treatment section of the Closedure web pages.
Choice of dewatering technique
Groundwater can be lowered with gravitational drying, by pumping out of the sump, open excavation, filter well or from drilled wells, using a vacuum method or with electro-osmosis.
Lowering of the groundwater surface has to be done according to the plan made and the plans have to be based on soil surveys and observations of the groundwater levels. The suggestive equations to the amounts of water to be pumped, to the spheres of actions (distances) and to the decrease of the water level are presented in geotechnical manuals (e.g. Martio 2011).
The choice of dewatering method to be used depends on the soil and rock conditions as well as the grading of the soil and the water permeability, groundwater conditions and objectives of the project and the desired extent of drying. Also levels of the groundwater between which the lowering is made affect the choice of the dewatering method. Because of biological activity or chemical precipitation (for example iron and manganese), the clogging of groundwater wells is a general problem that can disturb pumping. Groundwater wells have to be installed so that they can be cleaned, if necessary. (Penttinen 2001)
Figure 2 shows the useable areas of different dewatering methods on the basis of the soil grading. In Figure 3 the dewatering methods are shown with regard to the water permeability of the soil and the required ground water lowering.
Figure 2. Useable areas of dewatering methods. 1) Open pumping; 2-4) Ejector wells: 2) Filter well pumping, 3) Transitional area, 4) Vacuum pumping; 5) Electro-osmosis. (Modified from Hartikainen & Ruoppa 1974).
Figure 3. Useable areas of different pumping methods. 1) Dewatering not feasible and may not be necessary, 2) Ejectors (vacuum necessary), 3) Single stage well points (vacuum beneficial), 4) Two stage well points (vacuum beneficial) / Deepwells, 5) Deepwells, 6) Sump pump, 7) Excessive seepage flows: cut off walls or wet excavation may be necessary. (Modified from Preene 2014).
When the grain size of the soil and water permeability increase, the area and the amount of water to be pumped increases and thus several efficient pumping wells may be needed side by side. The reduction of the grain size and water permeability of the soil also leads to the same result because the sphere of influence of the pumping decreases.
Pumping from an open excavation
Pumping directly from the excavation is the simplest way to lower the groundwater level. Lowering of the groundwater takes place by pumping water out of the sumps in this method. In the case of an excavation, the excavation will be surrounded at ditches or underdrains with which the water is conducted into the pump wells. The pump wells must be surrounded with fine-grained soils as filter layers or with filter fabrics that prevent entrance of fine soil into the pumps. In coarse-grained soils, seepage water can be pumped directly away from the excavation with a sufficient pumping capacity if the difference in altitude between the bottom of the excavation and the groundwater surface is small.
This method is best suited for coarse sandy soils and gravel soils, in which the digging depth is maximum 2.5-3 m below the groundwater table. The method is also applicable to tight moraines that conduct water poorly. This method usually cannot be used in fine-grained friction soils because of the risk of slope failure or hydraulic breaking of the bottom (Hartikainen & Ruoppa 1974). However, the method is suitable to be used in coarse friction soils with a high water conductivity, because the flow rate which is necessary to cause the hydraulic bottom fracture is not usually reached. In such case the biggest problem will be to reach sufficient pumping capacity. The necessary capacity can be reduced with compaction injection of the soil or with watertight support walls that surround the excavation and are positioned deep enough (Hartikainen & Ruoppa 1974). In soils that have a dense stratified structure, the hydraulic breaking of the bottom does not take place, making this method also suitable for tight tilly soils. The surface part of the moraines with fine grains can, however, lose strength. This is best prevented by making a filter layer at the bottom of the excavation. The filter layer has to be built as quickly as possible after finishing the excavation work. The filter layer prevents fine soil grain from becoming loose at the bottom of the excavation. (Hartikainen & Ruoppa 1974)
The calculating methods of the flow rate needed are inaccurate and only indicative, so it is also more advantageous to use more wells in which the pumping power required can be controlled.
Pumping from filter wells
The groundwater table can be lowered with filter wells if the soil is so fine-graded that the pumping is not possible directly from the excavation because of hydraulic breaking. The flow of water from the soil into the well takes place by the action of gravity due to the pressure difference. This method can be effectively used in gravel and sand layers, in which the water permeability is 10-3-10-6 m/s (Hartikainen & Ruoppa 1974). The decrease in the groundwater surface has to be large enough to prevent formation of a directed critical hydraulic pressure against the slopes of the excavation or against the bottom (Hartikainen & Ruoppa 1974).
The surface of the groundwater can be lowered to one target depth or step by step using more installed well lines. In the case of one well line, no more than 30-35 m deep pipe wells are used (Hartikainen & Ruoppa 1974). If an impermeable layer on the building site is very near the required lowering level, the filter pipes cannot be set deep enough.
The filter wells are usually installed in a line in the vicinity of the upper edge of the ramp of the excavation. They are made from strainer pipes with a diameter of 15-20 cm and installed within 5-15 m from each other using work pipes with a diameter of 30-100 cm, or from strainer pipes with a 4-5 cm diameter installed in the soil within 1-2 m from each other using water flushing. In both cases the strainer pipe is surrounded with a suitable filter gravel or filter sand. A filter pipe is embedded inside the work pipe, which is then installed in the soil and a filter layer is installed in-between. A suction pipe is installed innermost. The filter pipes are connected to each other with a pipe that serves as a suction pipe or pressure pipe depending on the pumping direction (Hartikainen & Ruoppa 1974).
Large diameter filter pipes are best suited for soil layers that have water permeability (k) > 10-4 m/p. In these soils, ejector pumps and submersible pumps are used and pumping depths up to 60-100 m can be obtained (Hartikainen & Ruoppa 1974).
The filter well method can be simplified by combining a suction pipe and a filter pipe and by sinking them into the ground without the work pipe. In such case, the diameter of the well and the yield of water will decrease so it can be used in finer granular soils.
Suction pumps are usually used in connection with small diameter filter pipes. In this way the groundwater surface decrease obtained is no more than 3.5 m in the middle of the excavation. If one wants to lower the groundwater surface more than this, more pipeline systems can be installed at different levels around the excavation. In such case the maximum depth of the filter wells will be about 9 m and the greatest lowering of the groundwater that is achieved in practice is about 3.0-3.5 m level per pumping level (Hartikainen & Ruoppa 1974).
Pumping from deepwells
If the required level for the lowering of the groundwater is over 6-7 m, it can be carried out alternatively with deepwells, not merely with multi-phased filter wells. In the deepwells, a submersible pump is placed on the bottom of the deepwell which is made from shield pipe with at least 400 mm diameter. Each well serves separately in contrast to the filter pipes method, in which several wells are connected to the same pump. If the water permeability of the soil is smaller than about 10-4 m/s, the water flowing by gravity will be too slow so that the pumping from the filter well cannot succeed.
Another groundwater dewatering technique is the vacuum method, which is also called the wellpoint method. This method is used for work-time drying of excavations and to prevent hydraulic breaking of the bottom of excavations. It is used in fine-grained soils (for example in silty soil or in fine sand) in which groundwater is not obtained to decrease with the filter wells. Because of the small water permeability of these soils and because of capillary forces, the pressure difference based solely on gravity does not produce the necessary ground water flow. The suitable permeability of the soils is 10-5-10-7 m/s. The water flow is intensified by causing a vacuum to the suction points installed in the soil, which in turn absorb the water from soil. (Hartikainen & Ruoppa 1974)
In the vacuum method, the suction points are embedded around the area to be dug or to the edge of it in lines with spaces of 2-5 m. The suction points are small filter pipes of about 50 mm in diameter. The filter part is installed below the future digging level in the water conducting layer (sand) or the strainer part is surrounded otherwise with filter sand.
The suction points are usually about 5-7 m long and their perforated strainer part is about 1-2 m in the bottom. The space between the suction points is about 2 m, which at the same time is the diameter of the sphere of influence of the vacuum. The installation hole is sealed from its top so that air will not penetrate to the suction point.
The suction points can be installed with flushing water or with the help of the bore. Water flushing is the more common way. The standpipes of the suction points are connected with joint hoses to the body pipe. In connection with every joint hose there is a valve that can be used to connect the suction point in question or to adjust its suction effect when out of use. The body pipe is connected to a suction pump. The suction pump used in the method consists of the suction pump and the water pump to remove water. When the pump’s theoretical vacuum is about 0.8-0.9 bar due to the pressure defeat the water surface of the groundwater can be lowered only 6 metres below the level of the suction pump. If the desired lowering depth of the groundwater surface is greater than 6 m, the excavation work must be installed around the excavation when progressing to different levels. Also in deep excavations, the suction points are often installed phase by phase. In the supported excavations, the suction points are usually installed to both sides of the support walls, outside and inside.
Discount of groundwater on electricity osmosis
Electro-osmosis can be used for lowering the groundwater in fine-graded soils in which water permeability is 10-7-10-9 m/s. It is based on the phenomenon in which direct current makes the water in the soil flow from a positive electrode (from the anode) to a negative electrode (to the cathode). Perforated filter pipes are installed in the soil as cathodes and steel pipes, such as steel bars between them as anodes. Water flows to the perforated filter pipes. The force of the current has to be at least 150 amperes and the field intensity’s 0.1-1.0 V/cm. Power consumption in the range of 3-30 kWh/m3 has been measured (Hartikainen & Ruoppa 1974).
The electro-osmosis method has been seldom used because the lowering of the groundwater is not usually needed in practice in ground conditions that correspond to the suitability area for this method. It can be used to assist other methods.
Finnish Environment Institute 2001. Kaatopaikkojen lopettamisopas. Ympäristöopas 89. Suomen Ympäristökeskus. ISBN 952-11-1021-X (nid.) ISBN 952-11-1022-8 (PDF). Edita Oyj, Helsinki.
Hartikainen, J. & Ruoppa, A. 1974. Pohjarakennus. Suomen Rakennusinsinöörien Liitto RIL ry. Jyväskylä. Gummerus. 459 s. ISBN 951-758-003-7.
Martio, J. 2011. Pohjavesitilanteen tarkastelu alikulkusiltapaikoilla. Liikenneviraston tutkimuksia ja selvityksiä 13/2011. Liikennevirasto, Helsinki. ISBN 978-952-255-635-6.
Penttinen, R. 2001. Maaperän ja pohjaveden kunnostus. Yleisimpien menetelmien esittely. 227. Suomen Ympäristökeskus. ISBN 952-11-0943-2. Oy Edita Ab, Helsinki.
Preene, M. 2014. Controlling groundwater on construction sites. June 2014. At http://www.slideshare.net/MartinPreene/controlling-water-on-construction-sites.
Younger, P.L., Banwart, S.A. & Hedin, R.S. 2002. Mine Water: Hydrology, Pollution, Remediation. Kluwer Academic Publishers, Dordrecht, The Netherlands.