Water Boreholes

Water wells and boreholes

The methodology for drilling water wells and boreholes is dependant on a number of factors including:

• site location
• water requirements
• water table depth
• ground conditions

All of these factors are taken into consideration when tailoring a system to meet your needs.

Establishing ground conditions

The first stage of the process is to assess the geology of the proposed site. On initial contact we would take a postcode/grid reference for your property and using our in house geological maps and knowledge ascertain the geological strata for the proposed site. We also look at what other water wells have been drilled in the area and at what depth water was found to give us 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 we would ask that you obtain a survey from the British Geological Survey who will send you a report of the area. However this is not necessary on all occasions.

Drilling the borehole

The 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’ and is explained further in the next section.

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. Our specialist equipment can enable drilling from diameters ranging from 4” to 15”.

Techniques

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 and casing installation

Once the borehole has been drilled to the required depth, the tools are removed and the 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 an 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.

Test Pumping

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 borehole pump

A borehole pump, put simply, is a submersible pump, of narrow enough diameter to be lowered inside the well liner. The pump is lowered into the well, 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.

The water well ‘finish’

The finish of the well can be above or below ground, though we prefer to finish above ground on farms, in line with Environment Agency advice, minimising the risk of contamination to the well from run-off containing slurry, chemicals, fuel etc.

The borehole pump can be set up to deliver water to a tank or reservoir, controlled by a float switch, or to supply directly into your system, controlled on a pressure switch.

Water for human consumption

Where water is required for the purpose of human consumption, it must be tested and any subsequent treatment applied.