Mercia Mudstone Group

Minor fault in the Cropwell Bishop and Blue Anchor formations of the Mercia Mudstone at Cropwell Bishop, Nottinghamshire.

Download the Engineering geology of British rocks and soils: Mercia Mudstone Group 6.4 MB pdf

Plot of standard penetration test SPT 'N' value against depth for mudstones of the Mercia Mudstone Group.

The Mercia Mudstone Group in the UK is characterised by a sequence of brown and red-brown, calcareous clays and mudstones, with occasional beds of impersistent green siltstone and fine-grained sandstone. It was deposited between 200 and 250 million years ago in the Triassic period.

It underlies much of central and southern England and on which many urban areas and their infrastructure are built.

Although Mercia Mudstone appears to cause few serious geotechnical problems compared with other, higher plasticity, clays it is significant to the construction industry because it is frequently encountered in civil engineering activities involving foundations, excavations and earthworks.

Outcrop of the Mercia Mudstone Group in Britain.

1. Wessex Basin
2. Somerset/Avon/South Wales
3. Worcester/Knowle Basins
4. Stafford/Needwood Basins
5. Cheshire Basin
6. West Lancashire Basin
7. Carlisle Basin
8. East Midlands/NE England

Geological hazards and geotechnical problems

• Mercia Mudstone may vary between classification as soil or rock (based on BS5930:1999) depending on its detailed lithology and state of weathering. As a result of this, in some cases, weaker material may be found below stronger material. Consequently sampling and testing is difficult because the material may not be suited to either soil- or rock-specific techniques.
• Some parts of the Mercia Mudstone may be subject to shrinking and swelling; with changes in moisture content to a sufficient degree that structural damage to buildings or disruption in some types of construction works is caused.
• The Mercia Mudstone Group contains evaporite minerals, mainly halite (sodium, chloride) and gypsum (hydrous calcium sulphate). These lithologies can cause significant problems for construction due to dissolution.
• Mercia Mudstone may contain aqueous solutions of sulphate and sulphuric acid in the ground in sufficient quantity for potential chemical attack on concrete.