How Are Exploratory Holes From Geotechnical Testing Dealt With, Once Finished?

by | Sep 29, 2020

Geotechnical testing requires below the ground investigative work. In order to decide various spoil properties and geological make-up. This is in the form of sonic drilling, rotary drilling, window sampling, cable percussion or trial pitting. This is in order to recover suitable samples and undertake the required testing.

The borehole must be treated properly and cannot simply just be left open. They have to be dealt with correctly following the end of a geotechnical investigation. The team here at Borehole Solutions, wanted to talk through the various processes in which exploratory holes can be either utilised or decommissioned. Some boreholes can be converted to be used as monitoring points. Even though this was not the original purpose of the borehole.

Often the borehole is installed with a well point for the ongoing monitoring of water levels. As well as gas and movement through the use of inclinometers. All of which Borehole solutions can undertake for the client.

Groundwater monitoring systems (also known as Groundwater Quality Monitoring Points – GQMPs) can be split into two major monitoring objectives.

 

  1. Strategic Monitoring Objectives

This relates to general monitoring, keeping an eye on trends over time and on the overall, long-term quality of the groundwater.

 

  1. Defensive Monitoring Objectives

Defensive monitoring pertains to so-called ‘problem’ areas (or potentially problematic areas). These may be in high-polluting areas, areas near to chemical plants, for instance, where keeping an eye on any water contamination is imperative.

 

Decommissioning Boreholes

All boreholes must be decommissioned safely and appropriately in order to be made safe and prevent cross contamination of aquifers in particular. This is most commonly done through the process of sealing and capping.

 

Removing Casing From Geotechnical Testing

Removing the casing from a borehole after geotechnical testing is complete, will seem like a simple task. In deeper boreholes, and in softer geological conditions, it is more complex. The removal process is carried out carefully making sure the borehole walls don’t collapse. How the casings are removed depends on the conditions, in difficult conditions, we can use our high-quality hydraulic jacks to maintain the borehole’s structural integrity.

 

Backfilling/Sealing

The boreholes which have been installed have to be decommissioned in accordance with the EA guidance document ‘Decommissioning Redundant Boreholes and Wells’. This guidance 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

In order to ensure the borehole is free from obstructions, All head works and pipes must be removed. Boreholes should be returned to mimic the original natural strata to maintain ground flows .

Restoration requires a variety of materials to be used. This is so that permeable aggregates are positioned alongside to aquifer horizons and low permeability materials are positioned alongside to low permeability horizons.

Alternatively, the entire borehole or well can be backfilled with low permeability materials. This prevents significant vertical or horizontal movement of groundwater through or along the borehole. The materials used to backfill must be clean and non-polluting. Suitable materials include pea gravel, sand, shingle, concrete, bentonite, cement grout and uncontaminated rock.

 

Plugging/Capping

Finally, once complete the borehole top must be sealed to prevent potentially contaminated surface run off entering the borehole system, it is standard practice to install and a plug and cap. Which the depth will be provided by the Borehole Solutions Engineer dependant on the location and end use of the area i.e agricultural uses require a 2m depth of treatment.

A job isn’t done until it’s done. That’s one of our mantras here at Borehole Solutions. We know that drilling the holes is only half of the job, and that dealing with them when we’re finished is just as important. You can tell a good geotechnical contractor from a poor one by looking at how they deal with their completed boreholes! So, if you’d like to find out more about our geotechnical testing services, then get in touch! Contact Borehole Solutions today on 01733 200501. Or take a look at our instagram for more insights into our previous work.