This has come slower than any of us might have hoped:
What stabilises a slender pier in a viaduct. Obviously, as displayed within Archie, the adjacent span provides propping, but this is a very conservative view. It is impossbile to push a slice of a pier over. any live load effects will be distributed over the full width of the pier, or nearly so. If the distribution doesn't reach the full width at the pier head it will continue down the pier so the disturbing effect per metre width will progressively reduce.
Perhaps more importantly, the spandrels and parapets, even if detached from the arch, provide an effective prop from pier to pier. If the spandrels are completely free from the arch, the limiting force will be controlled by friction at the pier head, but it will still be large.
When assessing the multi span aspect of a viaduct, the "lane width" should be altered to the tota width of the bridge divided by the number of vehicles that can fit side by side. The result will still be conservative becasue there are alternative load paths as described above.
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