SLOPE FAILURE TYPES:

Complex landslide

complex

SLOPE FAILURE NAME:

Complex landslide

DEFINITION:

Consists of a failure which can be a landslide or rockfall that changes its initial characteristics while moving downslope.

MAIN CHARACTERISTICS:

Such movements are the largest, heaviest and most destructive failures that happen in the Giant’s Causeway. They usually start as a deep rotational slide or massive topple, which always happens at the top of the cliff. Due to the great mass of rocks and the steep slope, the resultant material quickly breaks into a mass of debris that moves downslope causing secondary failures, mainly rockfall on lower cliffs and landslides on debris mantled screes and slopes. Finally the failed material deposits forming a new scree which buries the previous surface. Such scree presents a grain-classification from top to toe, with coarser material arriving at the toe of the scree, while fine material usually stops near the apex of the scree.

CAUSES:

Usually prolonged and / or intense rainfall is the most common cause. Frequently this is complemented with concentrated subsurface moisture flow along so - called percolines (SMITH et al., 1994). Percolines may themselves concentrate in faulted or fractured zones and may appear as marshy areas on gentle slopes.

AREAS PRONE TO FAILURE:

Complex landslides happen when stress is not released by smaller, less pernicious landslides. That is why very weathered substratum areas, like palaeosoils, or till covered areas are not prone to such a type of failure: here stress can be released as shallow translational slides or mudflows. Thus the most prone areas are well developed columnar basalt cliffs, with no debris mantle over them. Thin interlayered soils help promote complex landslides.

The Amphitheatre, and the eastern part of Port Noffer are the most prone areas in the Giant’s Causeway. Also the eastern part of Portnaboe could be a future prone area, due to the wall that stops the occurrence of smaller failures, increasing the stress on the slope.

OCCURRENCE:

During and after prolonged rainfall, enhanced by episodes of high rainfall intensity.

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