BORE HOLES - SUSTAINABLE WATER SUPPLIES

 

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Terramec drilling rig and pump in operation

 

 

Water is essential to life, for drinking, washing and heating. We take it for granted in developed countries, but then you should try surviving from a water hole using a bucket and rope. I have. It was a dreadful chore, until the installation of a pump.

 

As water rates rise and our climate changes a water well borehole offers a cost effective solution for irrigation of crops and for use as drinking water. Up to 20m3/day of water can be abstracted without consent or license which could save you £1000ís of your bills. According to the the Water Act of 2003 anyone is allowed to extract up to 20,000 litres or 20m3 per day without license or charge. If you wish to extract more than this, you will need to obtain an Abstraction License from the Environment Agency.

 

WATER TABLE

 

Rain flows from the land into rivers and soaks into the soil providing essential water for plants to grow. Water that is not taken up by the plants will soak into the ground further through a process called infiltration. This water seeps through the soil and downwards through rocks. The process can be likened to water being held in a sponge. As the water travels through the rock at some point lower down saturation occurs and that is height of the water table.

Water will travel through the ground from the point at which it was infiltrated until it reaches a point of discharge which in most cases is a spring or river.

The water table depth is dependant on the type of rocks that form the ground conditions of any given site. Some rocks are impermeable, meaning that water can hardly flow through them, where as many rocks are permeable with small holes allowing water to flow through them. These permeable rocks that contain groundwater are called aquifers. The holes in the rocks can be between small grains of rock such as sandstone or through fissures (cracks) in rock such as chalk.

Consequently, the depth to which you drill to install a borehole is dependant on the rock type and water table depth. A borehole is normally drilled 30m below the water table to allow for draw down as the water gets pumped to the surface.

 

 

 

UK geological regions

 

 

GROUND CONDITIONS

The first stage of the boring process is to assess the geology of the proposed site.  It is a good idea to note what other water wells have been drilled in the area and at what depth water was found to give some history/background to the areas hydrological characteristics.

The geological conditions of the site determine the depth and technique needed for drilling to obtain a water supply. You can find out more in the 'what's involved section'.

If there is any doubt over the ground conditions a survey from the British Geological Survey may be necessary. They will send you a report of the area. However this is not necessary on all occasions.

DRILLING

 

A borehole is drilled using a large drill bit which cuts through the rock creating a hole held open by casing which is inserted as the borehole develops. In many cases where the rock is hard the casing will only be inserted for the first few metres and the hole will be naturally supported lower down. In other, softer, ground conditions casing will have to be inserted further to ensure the borehole does not collapse.

 

The drill continues to cut through the rock and the waste material from the drilling process e.g. crushed rock/stone needs to be removed from the borehole. The technique used for this process is called 'flushing.'

 

Water boreholes are generally drilled to a diameter of 200mm (8'') but this can vary depending on the amount of water required and the geology of the area. Specialist equipment can enable drilling from diameters ranging from 4'' to 15''.

METHOD

 

The borehole will be drilled using either mud or air flush techniques. 'Flush techniques' relate to the process used to remove the waste material that has been cut away in the drilling process.

 

Different flushing methods are used to remove the cuttings (waste) from the borehole depending on the geology of the site. Air flush technique uses compressed air to operate a down-hole air hammer on the end of the drill string that helps to break up the rock formation. The compressed air that is used to operate the down-hole air hammer also blows the crushed rock fragments out of the hole to the surface along with any water that flows into the hole during drilling.

 

Mud flush uses water and polymer to aid the drilling process and clear cuttings from the borehole as it is drilled. Mud flush technique uses a drill bit made from toughened materials such as tungsten to break through the substrata. Once the drill bit has broken through the substrata, the drill fluids are circulated through the drilling pipework into the borehole and back to the surface, at the same time washing the drilling residue or cuttings upwards and out of the hole. This fluid also serves as a formation stabiliser preventing possible cave-in of unstable sands or crumbly rock before the well casing or well screen is installed. In addition this fluid acts as a lubricant for the drill bit.

 

Depending on the geology of the site where surface formations are unstable, such as sands, a temporary steel casing may have to be installed to stop the borehole collapsing until a solid, stable material is encountered.

WELL SCREEN OR CASINGS

 

Once the borehole has been drilled to the required depth, the tools are removed and a well screen and casing can be installed. The well screen is installed in the lower section of the borehole. The well screen has a 5'' diameter (in a 8'' borehole) with 1mm precision cut holes to allow water to percolate into the borehole. Casing is then installed in the upper sections of the borehole. This is a solid pipe that prevents any surface water and contaminants entering the borehole.

 

The gap between the 5'' casing and well screen and the 8'' borehole wall is then back filled with washed pea shingle. The upper solid casing annulus is then grouted to ground level, preventing any surface contamination from entering the well.

 

 

 

A narrow diameter bore hole pump

 

 

TEST PUMP

 

All wells, once construction is complete, need to be test pumped in order for the pumping characteristics to be assessed. The standing water level, pumping water level and assessed sustainable yield all need to be known in order for a suitable pump to be selected for the desired application.

 

On a domestic well, 20m3 per day or less, a pumping test of 4 hours is usually adequate to record the pumping performance of the well. On larger abstractions where the Environment Agency require further information, pumping may be necessary for a longer period.

 

THE PUMP

 

A borehole pump is a submersible pump, of narrow diameter which is lowered inside the well liner,  below the water level and suspended a minimum of 5 metres from the bottom of the well. Attached to the pump is a pipe to bring water to the surface and a cable to supply power to the pump. The weight of this suspended system is taken by a wellhead set in a small concrete slab on the surface.

 

COST

 

A guide cost for the South East is usually between £70 and £100 per metre for a water borehole.

 

 

SERVICE SUPPLIERS

 

OT Drilling Ltd

Meadowsweet
The Street, Wormshill
Sittingbourne
Kent, ME9 0TT
enquiries@otdrilling.co.uk

Tel: 01622 884944 or 07970 782581

 

 

 

A traditional well head in Sussex

 

 

 

 

 

  

 

 

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