BDA Webinar – Interpreting Groundwater Levels from Geotechnical Data

February 20, 2025

The latest British Drilling Association (BDA) webinar took place on 19^th February 2025, titled “Interpreting Groundwater Levels from Geotechnical Data”.

With a record number of attendees, the webinar was presented by Dr Martin Preene, a Technical Director at Coffey Geotechnics Ltd, a Chartered Civil Engineer, Chartered Geologist, and UK Registered Ground Engineering Adviser, with 39 years of experience in groundwater control. Dr Preene is the lead author of CIRIA Report C750 – Groundwater Control Design and Practice and co-author of Cashman & Preene – Groundwater Lowering in Construction.

Understanding Groundwater Levels

The presentation began with an overview of why groundwater level data is crucial in geotechnical and geoenvironmental design. Dr Preene highlighted its relevance to:

• Temporary works – groundwater control and waterproofing of permanent works.
• Geoenvironmental considerations – such as contaminant migration.
• General geotechnical behaviour – groundwater influences effective stresses, which dictate how soils and rocks respond to loading.

Challenges in Obtaining Representative Groundwater Levels

Groundwater levels are dynamic and subject to multiple influences, making them challenging to measure accurately. The presentation covered key factors that affect readings, including:

• Temporal variability – levels can fluctuate seasonally or due to human activities.
• Drilling and testing effects – which can artificially alter groundwater levels.
• Complex geological conditions – such as layered aquifers and aquitards influencing flow patterns.

The Importance of the Conceptual Model

A key takeaway was the necessity of a conceptual model to properly interpret groundwater levels. Dr Preene emphasised that conceptual models:

• Help visualise subsurface conditions in words, pictures, or diagrams.
• Differentiate between aquifers and aquitards – understanding groundwater movement in different formations.
• Help prevent misinterpretation – for example, a borehole in fine sands reported as ‘dry’ during drilling may not indicate the true groundwater conditions.

Interpreting Groundwater Level Data

Dr Preene explored various sources of groundwater level data, including:

• Drilling and testing data – including water strikes and trial pit observations.
• Monitoring wells – highlighting different well types (e.g., piezometers vs. standpipes) and their response times.
• Geophysical methods – such as CPTu probing and packer tests, which offer alternative insights into subsurface conditions.

The importance of accurate datum recording, timing of measurements, and potential sources of error was also discussed, particularly regarding automated readings from vibrating wire pressure sensors and dataloggers.

Final Discussion and Q&A

The webinar concluded with an interesting discussion and Q&A session, where attendees raised practical challenges they face in groundwater monitoring. Dr Preene addressed issues related to data reliability, instrument calibration, and regulatory standards.

A recording of this webinar will be available soon, visit – https://www.britishdrillingassociation.co.uk/news/category/webinars/