It's logical to assume that this backfill was semi-liquid when it flooded the premises. Plus, it's not limited to this tunnel: all of Liverpool stands on it. And Prague. And Moscow. And Omsk. And Kazan. And after 1824, when Liverpool Royal Infirmary was opened (the dishes are found here), there was one similar event: the silting up of part of San Francisco Bay around 1851.

(A long read, large file) The penultimate stage of the flooding is dated 10,000 years ago, but they usually avoid dating of the last, uppermost sea terrace. In which case, the total flooding of Beringia 8,200 years ago is an acceptable assumption. Electrical discharges moving with space velocity liquidate the most part of threat pointwise: the most dangerous meteorites blow up from the bottom side, and most of them do not reach the surface of the planet.

The first stage of the catastrophe took place around 1764, creating the Khvalyn Sea in place of the Caspian Sea and killing a mass of people to the south and north of the Manych River, through which the snow-soil mixture was carried.