Investigating Slopes on Lias Group Clay Affected by Solifluction

Geo-Integrity have recently investigated a site in Northamptonshire that is underlain by Lias Group geology.  These soils consist mainly of clays, mudstones and limestones of Late Triassic and Early Jurassic age, deposited between 180 and 205 million years ago. The outcrop of the Lias extends in a continuous band from the south coast of Dorset to Yorkshire; including many Counties local to us; Buckinghamshire, Northamptonshire, Warwickshire, Rutland, Gloucestershire, Oxfordshire and Leicestershire.


The Lias Group rocks are recorded as having the highest incidence of land-sliding in the UK. The Whitby Mudstone Formation may have as many as 42 landslides per 100 km² of outcrop.  This is mostly due to previous Ice Age processes still affecting the slopes today.  One such process is solifluction.  Solifluction is a geological phenomenon that occurs when soil on slopes becomes saturated, loses cohesion, and slowly begins to flow downhill. During the Ice Age when slopes were exposed to constant freeze/thaw and high volumes of groundwater, solifluction was widespread on clayey soils.


Although these slopes are now mostly semi-stable, this ancient process can have significant consequences for the stability of slopes, posing threats to infrastructure, ecosystems, and communities, as any earth-working or construction process can reactivate these old slip planes and lead to land-sliding.


To mitigate the adverse effects of solifluction, the construction of soil retaining structures is often necessary.  Structures such as sheet pile walls or gabion walls are required to hold back these ancient semi-stable soils.  Geo-Integrity have recently been undertaking a site investigation on such a site, where the slope angle was around 6-8˚ and the geology was Whitby Mudstone Formation.  The attached photographs show the old relic slip planes that were encountered.  We can now model the slope with our in-house Trimble surveying kit and Geo5 Slope Stability computer software and establish where on site is at most risk and what type of retaining structures will be required.


If you have a similar site give us a call on 01280 816409 or contact us by email



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