Venice is built on unstable ground. Facades are distorted, floors uneven  and cracks are numerous.

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Casa a San Giobbe before restoration (photo Joaquim Moreno 1997)

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.

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Deformations Casa a San Giobbe (Leo Schubert 2006)

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.

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Rio di Santa Caterina (foto Leo Schubert 2015)

 

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.

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High precision levelling over a period of 5 years (palazzo Merati, Leo Schubert)

 

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:

Damage assessment Corte Botera ( Leo Schubert 2008)

Damage assessment Corte Botera ( Leo Schubert 2008)

 

Damage assessment Palazzo Donà dale Rose ( Leo Schubert 2008)

Damage assessment Palazzo Donà dale Rose ( Leo Schubert 2008)