Sevenoaks Bypass Landslide Robert Mullin & Matthew Woodhouse

Geology and Topography Hythe Beds consisting of: Ragstone (Sandy Limestone) Hassock (Calcareous Sandstone) Sandy Silty Clay Atherfield Clay Weald Clay

Geology and Topography Hythe beds: cambered Landslipped On underlying Atherfield clay Cambered blocks slipping into fissures Large vertical fissures Ubiquitous Solifluction sheet overlain by Solifluction lobes Solifluction: ‘The slow, downhill movement of soil or other material in areas typically underlain by frozen ground.’

Geology and Topography

Geology and Topography Solifluction Lobes Consist of Angular Chert fragments + Ragstone blocks in a sandy silty clay matrix Streams thrown out of the Hythe Beds forming ridges + isolated hills Capped with head deposits of chert + other stones in a clay matrix Remnants of extensive sheets of periglacially soliflucted debris

Geology and Topography Erosion of surface and retreat of the escarpment Further, and later solifluction debris Atherfield and Weald Clays Overlays brecciated Weald Clay – contains principal slip surfaces These shear surfaces separate the lobe from the sheet – likely to be reactivated by earthworks

Construction The aim was to: - But: - Build a Dual Carriageway At an acute angle to the contours Built mainly of fill But: - The landslipped nature of the slope was not properly appreciated Instabilities occurred Road had to be realigned

Initial failure Solution Early fill placement triggered movement of some of the soliflucted soils The failures were similar to bearing capacity failures Failures were small relative to the previously landslipped ground Solution Remove all fills (both unfailed and failed) Excavate a trench along line of the foundation to remove the weak foundation soils

Induced failure The trench was As wide as a dual carriageway 10m deep This was well below the main slide surface of the soliflucted surface material This allowed them to slip out

Solution Stabilisation impractical Road was realigned New split carriage way Already constructed overhead bridges had to be abandoned and demolished Significant cost and time delay

Lessons leant Problems will always arise when building upon a landslipped area Excavation resulted in a greater failure then the embankments because a space was provided for the slide to move into “Geotechnical engineers are now much more cautious of even quite gentle slopes”