BGS GroundInvestigation 3DGroundModel Rockhead

BGS depth-to-bedrock model strengthens early ground risk screening

*BGS Superficial Deposit Thickness Model output showing regional variation in superficial deposit thickness across Great Britain. Source: BGS*

The British Geological Survey has released Version 8 of its Superficial Deposit Thickness Model, giving engineers and geoscientists an updated national view of how much unconsolidated ground lies above bedrock across Great Britain.

The major addition is the Rockhead Elevation Model, a new raster layer showing the modelled elevation of geological rockhead relative to Ordnance Datum. In practical terms, it helps users understand the likely depth to the top of solid bedrock, a boundary that often controls foundation choices, excavation behaviour, groundwater movement and ground investigation planning.

Superficial deposits can be highly variable. They may appear as thin veneers, thick glacial sequences, river sediments, marine deposits, made ground or valley infill. This variability matters because the transition from superficial material to bedrock is usually where engineering properties change sharply. Strength, permeability, porosity, lithology and conductivity can all shift across this boundary.

The updated dataset is most useful at desk study, regional planning and corridor screening stages. It can help identify areas where bedrock may be shallow, where thick drift deposits may affect foundations, or where buried valleys and complex superficial sequences may require more targeted investigation.

Comparison of BGS SDTM Version 8 layers, showing observation-driven and terrain-adjusted superficial deposit thickness models, data distance as an uncertainty indicator, and the new rockhead elevation model used to assess depth to bedrock across Great Britain. Source: BGS

The SDTM Version 8 package includes four main layers. The observation driven model interpolates superficial deposit thickness from borehole records and mapped deposit extents. The terrain adjusted model also considers ground surface elevation using OS Terrain 50 data. The new rockhead elevation model provides the modelled bedrock surface. The DataDist layer indicates the distance to the nearest source of thickness information, such as a borehole or mapped deposit boundary, giving users a practical indication of uncertainty.

A key improvement is the updated borehole selection process. BGS has incorporated newer interpretations of archived borehole logs and excluded low-confidence, conflicting or inappropriate records that may have affected previous versions. This makes the model more robust than earlier releases.

However, its limitations are important. The dataset is intended for national, regional and city-scale use at around 1:100,000 scale, with average vertical accuracy of about +/-5 m. It should not replace project-specific ground investigation where precise rockhead levels are required.

The value of the updated model is therefore not that it removes uncertainty. It makes uncertainty easier to see earlier. Used properly, it can improve scoping, guide intrusive investigation, support 3D ground modelling and help engineers ask better questions before committing to detailed design.

Sources: bgs.ac.uk, geomechanics.io