SLOPE FAILURE TYPES:

Single / Multiple block release.

block_release

SLOPE FAILURE NAME:

Single / Multiple block release.

DEFINITION:

Generically called “Rock falls” (FLAGEOLLET and WEBBER, 1996), they consist of a free movement of rock material from steep slopes such as cliffs.

MAIN CHARACTERISTICS:

There is a wide nomenclature of rock falls depending on their size, shape and parental material, but in the case of Giant’s Causeway reported block releases are invariably small sized, below 10 m³, and happen mainly on basaltic flows, secondly on interlayered soils and rarely on till outcrops. Scar shapes depend on the cracking pattern of rock material or, in case of till and old soils, on the infilling material.

Sizes depend more on the causes of failure: freeze / thaw cycles use to develop single block releases, while overloading and soil liquefaction due to high rainfall events result on larger, multiple block releases.

CAUSES:

Severe storms, with high wind and rainfall episodes seem to be the most important factors, followed by freeze – thaw cycles.

AREAS PRONE TO FAILURE:

This is an almost ubiquitous type of failure, which happens on any rock present at the Giant’s Causeway. However, Lower Basalt outcrops, especially when they are weathered to form soils, and the “Entablature” formations in Middle Basalts, are very prone to these block releases. For instance, the road that connects the Visitor Centre to the Causeway below Weir’s Snout, or the lower path below The Organ, are very prone to these failures.

OCCURRENCE:

During or after high storm events, long rainfall terms, and early in the morning.

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