RonR, over here in Holland, in our river delta country, there are lots of old river arms with bog underneath.
The first serious bog is miles away from here, we live on the driest desert sand possible. Anyways a guy building a house 3 km away from here, hit an old river arm and had to put 9 meter concrete poles into a solid sand layer under the bog, to get enough foundation for an extra room against his house...
It was only a matter of a few meters away, before the subsoil turned soild again.
What also happens is that they put houses in bog areas on poles... then they alter the level of surface water (draining the ditches, cities require a lower water table than grasslands) and the peat sponge starts to shrink... it has happened in the 50's that the soil sank 1,5 meter after a house was built in a newly drained wetland..