How are Boreholes Decommissioned after Geotechnical Testing?

by | Jul 17, 2023

Once a geotechnical testing investigation has been complete, any exploratory holes you’ve drilled can’t remain open. They must be dealt with correctly and safely due to the risk they pose to humans and animals alike. This doesn’t always mean just filling them in and walking away, however. There are a number of ways in which a borehole can be utilised or decommissioned. The team at Borehole Solutions have prepared this blog to detail exactly what options you have.

Converting into a Groundwater Quality Monitoring Point (GQMP)

Keeping track of water levels via GQMPs is one option for repurposing an out-of-use borehole. The main objective of these points is to ensure that representative groundwater samples can be collected. Groundwater monitoring objectives can be divided into three main categories. These are strategic monitoring, defensive monitoring, and investigative drilling. More information on these techniques is available here.

Decommissioning a Borehole

All boreholes must be decommissioned safely to prevent any risk of cross contamination of aquifers. The most common way of doing this is through case removal, backfilling and capping.

Removing the Borehole’s Casing after Geotechnical Testing

Once geotechnical testing has been completed, your first need to remove the casing from the exploratory hole. This may seem simple, and in many cases it is. In softer geological conditions or deeper boreholes, however, it can be a bit more challenging. At Borehole Solutions, our go-to approach involves using specialist hydraulic jacks to remove the casing whilst maintaining the borehole’s integrity.

Adhering to EA Guidance

Regardless of the geotechnical testing undertaken on a site, out-of-use boreholes must be decommissioned in a specific way. This is outlined in the EA guidance document titled ‘Decommissioning Redundant Boreholes and Wells.’ Essentially, how you decommission a borehole depends on its specific characteristics. There are, however, five objectives that must always be satisfied. These include:

  • Any open holes need to be removed.
  • The borehole cannot act as a conduit for groundwater contamination.
  • Contaminated and uncontaminated groundwater from different aquifers must be prevented from mixing.
  • The flow of groundwater from one geological horizon to the next must be slowed.
  • You must prohibit groundwater waste from overflowing from any artesian wells.

Backfilling Exploratory Boreholes

When it comes to sealing the borehole, one option involves backfilling it with non-polluting materials with low permeability. This is done to prevent any significant movement of groundwater upwards or horizontally throughout the borehole. Sand, shingle, cement and uncontaminated rock are all suitable options for backfilling.

Capping the Borehole

Once the Borehole has been successfully decommissioned in line with the EA guidance, it can then be sealed. The standard procedure for doing this is to install a plug and cap, with the depth dependent on the location. This is to prevent any contaminated surface runoff from entering the borehole.

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If you’d like more information about our geotechnical testing services, get in touch! Call us today on 01733 200 501 or click here to send us an enquiry directly.