The geology and a possible mechanism of the Hatfield Stainforth colliery landslide

Unfortunately, overnight no new images have appeared of the Hatfield Stainforth landslide, so it is difficult to know whether the landslide has now stopped moving.  The morphology of the landslide continues to suggest to me that this is a bearing capacity failure.  I have taken a quick look at the excellent BGS Geology of Britain Viewer, which produces the following map (I have marked the approximate location of the colliery at which the landslide at occurred):

copyright BGS: http://mapapps.bgs.ac.uk/geologyofbritain/home.html

The map shows that underlying the location of the spoil tip is a layer of alluvium - basically soft sediments such as sands and gravels deposited by ancient rivers.  These are comparatively weak materials, especially when wet.  So, my hypothesis is that the weight of the spoil tip has caused the development of a failure through these materials, which has then generated a landslide with a rotational geometry.  The sketch below is my suggested interpretation, with the spoil sitting over a layer of alluvium.  The very wet weather over the last few months has led to a high groundwater level, which in turn has reduced the strength of the alluvium.  This has then failed with the geometry shown in the upper sketch - the rotational landslide has then progressively moved to generate the geometry in the bottom sketch:

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