Gault Formation

Arlesey Pit, Bedfordshire. Chalk resting unconformably on Gault clay.

Download the Engineering geology of British rocks and soils: Gault clay 50 MB pdf.

Casagrande 'plasticity' plot of Gault; plasticity is generally high to extremely high.

The Gault is a sequence of clays, mudstone and thin siltstones with bands of phosphatic nodules deposited between 100 and 112 million years ago in the Cretaceous period.

Its outcrop stretches south-westward from East Anglia through Wessex to west Dorset and surrounds the Weald in an arc from north-east Kent westwards through Surrey to Hampshire where it turns south and returns eastwards through west and east Sussex.

The study of the geotechnical properties in the database has shown that regional variations in geotechnical properties exist. These variations appear to be largely a function of depositional environment, lithology and stress history.

Outcrop locations of the Gault and Upper Greensand formations.

Geological Hazards and Geotechnical Problems

• Gault has ancient and recent landslides that are shallow translational in type with rotational elements at the back scarp and flows at the toe region. Multiple retrogressive rotational failures occur on some slopes. At major, inland escarpment or coastal cliffs, large, deep seated, landslide complexes have developed as at Folkestone Warren near Dover and at Ventnor on the Isle of Wight.
• Gault is prone to shrink–swell processes due to significant amounts of smectite clay in parts of its sequence. In the south-west, where the Gault becomes siltier and more sandy, it is only of medium to high expansive potential; the remainder of the outcrop falls almost entirely in the very high expansive potential category.
• Gault may contain aqueous solutions of sulphate and sulphuric acid in the ground in sufficient quantity for potential chemical attach on concrete.