Venicea city founded according to tradition on 25 March 421 AD, is famous throughout the world for being a city that seems to float above the lagoon, mainly thanks to wooden poles driven into the lagoon sediment. This has allowed the city to acquire over time the nickname of “the forest in reverse“. The technique used is called constipationwhich replaces the natural seabed with an artificial one made up of a dense network of poles (about 9 per m2). But from an engineering point of view, how was all this possible? And is it true that wooden poles still stand today despite being immersed in water?
How the foundations of Venice were built
The foundations of the ancient Venetian structures are called “deep”We are talking about foundations made with wooden piles, with diameters from 10 to 25 cm and variable lengths from approximately 1m up to 3.5m in extreme cases. The types of wood used varied both according to the availability of the material and the importance of the work to be carried out on the surface. In fact, more important works led to the use of fir, larch and pine poleswhile works of lesser importance were founded on elm, ash or acacia poles. The common feature was to build poles practically side by side, so much so that under a load-bearing wall one could find up to 25 poles per square meter!
In general, a pole can be beaten or drilled inside the land. In the Venetian case, the technique of percussion drillingtypical of beaten poles: practically these wooden poles they were literally hammered into the groundFrom a mechanical point of view, percussion driving increases the resistance that the pile is able to offer through lateral friction with the ground.
Above the structure of fixed poles, the support surface of the elevated structure was then completed using various layers of plank or stones, ensuring the right solidity to the whole and maximizing the performance of the palisade. The thickness of this completion layer could even exceed 50 cm.
The problem of deterioration of the poles
The ingenious aspect of these foundations is that the piles they are not exposed to air or water but they are totally immersed in this muddy layer in which bacteria cannot develop Aerobicthat is, bacteria that require oxygen, which could damage the wood. Think that they have been preserved so well that analyses on the elm and beech poles present under the bell tower of San Marco have allowed us to date them to the year 970 ADthat is, they are more than a thousand years old and still do their job! For this very reason it is said that the poles of Venice are eternal… Even if this is not exactly true.
There are various research bodies, such as Corila, The CNR and some universities, which have studied the phenomenon of wood degradation and have actually realised that in the mud it is true that there are no aerobic bacteria… but there are bacteria anaerobes that instead they don’t need oxygen! Now, these actually degrade the wood, although given the particular context this happens very slowly, especially for coniferous wood.
This is not an insurmountable problem, on the contrary: for years now, steps have been taken Consolidation interventions wooden foundations and alternative and more modern foundations are increasingly widespread – especially for new buildings – which use other materials that are more resistant in this environment, such as concrete.
Alteration of bricks
Obviously even if the wooden poles do not come into direct contact with water there is still a problem for the buildings, since the brick levels which come into contact with water can instead to get upset.
In fact, brackish water he manages to climb up from the brickssoaking them like a sponge. At that point the water evaporates and settles sodium chloride – that is, salt – in the brick which, over time, swells until it cracks. Precisely for this reason, a layer is often placed which acts as a barrier and prevents humidity from rising: there are various technical solutions but one of the most traditional is to use the Istrian stone which is a very compact and not very porous limestone rock that prevents water from rising further. If you look carefully you can still see it today on Venetian houses!
