Looks suspiciously like a drill hole ! {i'm comparing it to the capped off holes that were used when the "Old Quay" at Whitehaven was grouted, to stabilise the structure.
[shoe is about 100mm across at the widest point].
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A series of 18 images showing erosion and penetrations of the concrete cap, which has allowed scouring of the underlying fill leading to some significant voids and un-supported areas.