After a bit of research - why are some quay walls made of arches?
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Description: Egerton Dock, BirkenheadDescription: Egerton Dock, Birkenhead
When the ground is soft, piles have to be driven in and then these are bridged using arches, sometimes they are subsequently filled in, sometimes they are not.
The Egerton Dock arches do have back walls and in the roof have a hole. The dock wall has a number of manhole covers and two larger plates (which are above the two arched section), indicating it is quite likely hollow. Pure speculation but the roof holes may just be to reduce the slap/splashback effect of waves in the confined environment.
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