A2.9GT1/A2.4VE1 GEOLOGICAL TECHNIQUES IN SITE INVESTIGATION

1. A2.9GT1/A2.4VE1GEOLOGICAL TECHNIQUES IN SITE INVESTIGATION LECTURE 10 ENGINEERING GEOLOGY OF LANDSLIDES1: PRESENT DAY

2. Summary • Review of landslide risk factors • Morphology of landslides • Geotechnical classification of landslides • Case studies • Slides in homogeneous clays • Slides in layered strata • Slides in glacial till

3. Introduction • Landslides, whether active or stabilised, are one of the most important features that need to be recognised during an SI • Many natural slopes have evolved by some process of landsliding • Thus history of any possible landsliding is of fundamental importance to the geological assessment of a site.

4. Introduction • Landslides are the result of both environmental and geological factors • Presence of elevated pore water pressures or seepage • Progressive failure • Presence of weak or incompetent strata • Oversteepening by erosion • Renewal of loading

5. Introduction • Evidence for any of these factors should be actively sought during a desk study, since the actual landslide morphology may have become degraded over time

6. Morphology of Landslides

7. Morphology of Landslides • Morphological classifications usually divide landslides into: • Circular slips • Non-circular slips • Planar (slab) slides • Flow slides • The bulk movement varies from purely rotational to purely translational, or something in between.

8. Morphology of Landslides • Circular slips • occur in cohesive sediments • low brittleness (little loss of strength on failure) • homogeneous internal structure • Circular slips can be single or multiple • Seen on natural slopes in homogeneous clays or as shallow, late stage events in the weathered zone • Distinguished by evidence of rotation such as back-tilting

11. Morphology of Landslides • Non-circular slips • Occur in layered cohesive sediments • Often associated with a competent base layer that prevents the formation of a circular slip • Alternatively with a weaker layer that guides the slip surface • Common on many natural slopes • Distinguished by evidence of both rotation and translation - back tilt and counter-scarp

13. Morphology of Landslides • Multiple non-circular slides • involve a sequence of retrogressive slides • usually founded beneath a caprock layer • the slides may move singly or as a group • may form complexes that cover large areas • Large multiple slide complexes are common along the English south coast due to the local patterns of geology and erosion.

16. Morphology of Landslides Planar (slab) slides • these follow some shallow, planar structure in the ground • such structures may relate to the active layer under earlier cold-climate conditions • solifluction in particular creates shear surfaces in the upper few metres of the ground • planar sliding should always be suspected on former solifluction slopes • it is unfortunately not easy to see on many slopes due to subsequent slope development

17. Morphology of Landslides Mudflows • cohesive soil of high plasticity • fully remoulded at high water content • often form as secondary features during the process of block disruption • may continue in a steady state if removal by erosion is in balance with supply by block breakdown

18. Morphology of Landslides Flowslides • semi-cohesive soil of low plasticity • fully remoulded at high water content • high brittleness (loses strength on failure) • often form as major collapse features from a large landslide • have a characteristic shape with a low surface profile • frequently arise from cliff failures in sandy glacial tills

19. Geotechnical Classification of Landslides

20. Geotechnical Classification of Landslides • Divided into first-time vs reactivated slides • First-time failure causes new fabric disruption • Reactivated slides exploit existing shear surfaces near their residual strength.

21. Geotechnical Classification of Landslides • Failure can be short term or long term • In short term failure the pore pressure has a transient value • In long term failure the pore pressure has achieved its long term equilibrium value.

22. Coastal slopes in homogeneous clays The Hutchinson Model

23. The Hutchinson Model • This model was developed for coastal and recently abandoned cliffs in the London Clay • It is applicable to other coastal cliffs in cohesive clays of moderate to high plasticity. • It is not applicable to cliffs in layered strata or to low plasticity clays such as glacial tills

24. The Hutchinson Model • It emphasises the influence of toe erosion as a basic mechanism that can remove the protective covering of secondary flows • This will expose the intact clay to further wave attack to a greater or lesser extent and so stimulate further landsliding.

25. The Hutchinson Model • After failure, the slipped mass undergoes a slow period of progressive breakdown until the intact clay is again exposed and the cycle is repeated. • Thus the model is cyclical in form, with a cycle time of about 50 years, depending on the rate of removal of the slipped debris.

26. The Hutchinson Model • The model is built around three levels of toe erosion: • Strong • Moderate (equilibrium) • Weak or zero (abandoned cliff)

27. The Hutchinson Model Strong toe erosion: • sufficient for complete removal of the debris from earlier landslides and steepening the in situ clay • the next landslide occurs when the slope has become sufficiently steep • this generates a cycle of rotational landslides • the cycle time is determined by on the time required for toe erosion to occur and for the time for the pore pressure to recover following the unloading caused by the earlier landslide.

29. The Hutchinson Model Equilibrium toe erosion • This removes the covering of slope debris produced by block disruption • This debris is delivered by secondary mudslides • The rate of removal is equal to its delivery, thus maintaining a steady state. • The key point is that the fresh clay is not removed from the base of the slope and so the slope does not become any steeper.

30. The Hutchinson Model Zero erosion • There is a failure to remove the debris of earlier slides, so allowing the accumulation of a protective covering of colluvium. • Thus the slope angle is reduced towards its long term equilibrium value. This is assisted by further shallow rotational and translational slides at the top of the slope. • The whole process is termed free degradation. It is characteristic of so-called abandoned cliffs provided they have formed after the last glaciation and thus have not been subject to solifluction.

32. The London Clay cliffs of the Thames estuary

33. The Thames Estuary • These cliffs provide examples of landsliding and slope development in an homogeneous clay undergoing different rates of erosion. • These examples formed the basis for the Hutchinson model • Three sites are considered: • Warden Point (Isle of Sheppey) - strong erosion • Beltinge (North Kent) - equilibrium erosion • Hadleigh (Essex) - zero erosion / free degradation

35. WARDEN POINTstrong erosion

36. Warden point lies at the NE tip of the Isle of Sheppey and exhibits cliffs in London Clay up to 30m or more in height. • It is exposed to very strong wave attack from the NE • This has stimulated a cycle of deep circular landslips

38. Warden Point deep rotational slip

45. HERNE BAYequilibrium erosion

46. Herne Bay is located on the south side of the Thames estuary and is open to wave attack from the NE, although more sheltered than Warden Point. • Continuous moderate erosion caused the development of a mudslide system that protected the cliff and was in equilibrium with the rate of erosion • This has now been removed by new coastal defences.