Once an exploratory hole has been drilled and the geotechnical testing is complete, it can’t simply be left open. The Borehole Solutions team have put together this blog to detail how to deal with a borehole properly.
There are various processes in which an exploratory hole can be utilised or decommissioned. Perhaps the borehole can be converted into a monitoring point, in order to keep track of water levels, for example. GQMPs (Groundwater Quality Monitoring Points) can be split into two major monitoring objectives: strategic monitoring and defensive monitoring.
The former relates to general monitoring, the overall, long-term quality of the groundwater and simply keeping an eye on trends over time. The latter pertains to potentially problematic areas, where staying up to date on water contamination is essential. High-polluting areas, such as the land near chemical plants, fall into this category.
Safely and appropriately decommissioning borehole is essential in order to prevent cross-contamination of aquifers. The most common process of carrying this out is through sealing and capping.
Removing Casing From Geotechnical Testing
Following the completed geotechnical testing, removing the casing from the exploratory hole will seem like a simple task. It can be much more complex in softer geological conditions, however, or in deeper boreholes.
The removal process must be carried out very carefully to ensure the walls of the borehole don’t collapse. The removal process differs depending on the difficulty of the conditions. For particularly difficult removals, we often use our high-quality hydraulic jacks in order to maintain the structual integrity of the borehole.
Backfilling/Sealing Exploratory Holes
All boreholes must be decommissioned in accordance with the EA guidance document ‘Decommissioning Redundant Boreholes and Wells’. This document states that:
“Each borehole or well has its own particular characteristics which must be considered when planning how to decommission it, but the method used should be capable of achieving a minimum of each of the following objectives:
- Remove the hazard of an open hole (safety issues).
- Prevent the borehole acting as a conduit for contamination of groundwater.
- Stop the mixing of contaminated and uncontaminated groundwater from different aquifers.
- Slow the flow of groundwater from one geological horizon to another.
- Prohibit the wastage of groundwater from overflow from artesian boreholes.”
All headworks and pipes must be removed to ensure the exploratory hole is free from obstructions. Once decommissioned, a borehole should mimic the original natural strata, in order to maintain ground flows.
A variety of materials will be used in the restoration. Permeable aggregates will be positioned alongside aquifer horizons, whilst low permeability materials will be positioned alongside low permeability horizons.
Alternatively, however, the entire borehole may be backfilled with low permeability materials. This will prevent any significant horizontal or vertical movement of groundwater through the borehole. Any material used for backfilling must be non-polluting, with suitable options including sand, shingle, pea gravel, bentonite, cement, cement grout and uncontaminated rock.
Plugging/Capping Exploratory Holes
Once the exploratory hole has been successfully decommissioned, the top of the borehole must be sealed. This will prevent any possibility of contaminated surface runoff entering the borehole system. Standard practice dictates the installation of a plug and cap, the depth of which will be dependent on the location and end-use of the area.
At Borehole Solutions, we understand that drilling the hole is only half of the job! Dealing with them once we’re finished is just as important as the investigations themselves. If you would like to find out more about the vast array of geotechnical services we provide, please get in touch! You can reach us today on 01733 200 501.