1. Sevenoaks Bypass Landslide
Robert Mullin
&
Matthew Woodhouse

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

3. 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.’

4. Geology and Topography

5. 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

6. 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

7. 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

8. 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

9. 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

11. 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

12. 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”