Venice is built on unstable ground. Facades are distorted, floors uneven and cracks are numerous.
Precise surveying and monitoring enables the origins of this deformations to be understood and indicates if facades where already built tilted inwards to avoid a more dangerous tilting outwards. Iron ties, original or added, hold floors and walls together.
Much of the deformation took place during construction (brick and stone courses are uneven at lower levels and become more and more horizontal at each successive floor: evidence that the foundations were settling during construction. The later addition of floors or new constructions nearby could cause significant additional settlements.
The deformations of several decimetres that we measured in our surveys took centuries or very traumatic events to develop and are usually not a cause for alarm (the much-debated sinking of Venice is another topic: subsidence in the highly industrialized post-war period was mostly due to the extraction of cheap ground water for industrial use and the phenomenon seems to have arrested since restrictive laws were introduced).
If the foundations do not show any more differential settlement, localized actions, like tying tilted walls to the floors and roof etc. will often be sufficient to improve resistance to earthquakes (the most devastating one for Venice being recorded at the beginning of the 16th Century).
Things become more alarming if new cracks appear or old ones start to reopen.
A very efficient way of measuring ongoing deformations is periodic high precision levelling (1/100 of a mm). Tilting and cracks can also be monitored, but in most cases they show the consequences of horizontal settlements of the foundations.
Why do some foundations still settle in Venice? Equilibrium in the unstable ground is very delicate: although the wooden piles and planks used by builders until the 19th century to consolidate the ground under load-bearing walls are mostly found in a good state of conservation, they rarely reach the deeper and more resistant layers as do modern foundations (on concrete pillars, for example). Foundations along canal-sides are constantly being eroded by the activity of the tides and the waves generated by motorboats. The construction of septic tanks (the only way waste water is treated in Venice and mandatory for the frequent transformation of housing into hotels and restaurants) close to foundations or the carelessly executed consolidation of waterfronts are other causes.
The latter was the cause of the settlements and consequent crack formation in the following constructions we happened to monitor: