Rio Terà S.Polo
In the last years the delicate Venetian ecosystem has underlined the maintenance and cleaning issues of “rii tera’”. During XVIII and XIX century, above all under the Austrian domination, the “rii tera’” have been transformed in pedestrian crossing thanks to interments and coverings.
During XVII century Riello delle Erbe was completely covered because of the unstable water flow and frequent silting up. In this way water was forced through underground channel with a semicircular vault built entirely of brick. Over the last two centuries, up until 1963, the channel, known as Rio Tera’ San Polo, was occasionally cleaned in order to remove mud and debris which inevitably clogged it.
The removal of the sediment, caused by water stagnation in pipes and covered canals, is a problem involving operational difficulties and important expenditure. The environmental conditions do not permit to use or limit the use of ordinary machines, both mechanical and manual. Moreover digging in channels with traditional methods oblige to work in underground and unhealthy environment. Other more advanced systems which use scraping buckets need big spaces and cause troubles to pedestrians and residents because of the noise and smell.
After almost forty years from the last digging Insula, together with Gerotto Federico company, planned and acted a cleaning operation which has freed the obstructed rio tera’ San Polo. For the first time Insula used in the Venetian historic centre an innovative technology of mud removal.
It consists in a modular system created to have an appropriate distribution of weight in order to be moved in the narrow spaces of Venice, first by boats and then in pavements of limited capacity.
Through an opening in the underground vault and through the paving on top this equipment activates a fan which sucks the silt removed by an hydraulic mini-digger in a watertight tank.
From here the mud is pumped in a pipe which carry it directly to a barge moored at Fondamenta delle Erbe far from the yard (about 80m). In the last phase the silt is carried through barges to an authorised dump.
The operation involved the following stages:
- Creation of cofferdams at two points, in rio delle Erbe and riello Sant’Antonio, to enable the drying of the entire underground channel.
- Cleaning of the main channel without intermediate cofferdams. For the safety of workers and to have a continuos change of air there were no blocks with sluice gates along the longitudinal axis.
In the pavement on top of the vault four openings of limited dimensions have been acted to introduce the suction and mud removal equipment. At the end of the work these openings have been closed by manhole covers to have an easy entry to the channel in the future.
Unloading of the material with the methods explained above and the use of the suction excavation.
An other example of channel reactivation is that of rio terà S. Leonardo (230m long and 6-13m wide). It is one of the busiest thoroughfare of Venice. Its width permits to frontagers thriving trade and the use of the square shared with the permanent local market. During the works of sewer maintenance and paving restoration it was verified the precarious state of channels which could, because of the new works, risk their functioning with consequent dangerous irregular reflow and problems to public health and buildings. The main underground channel, a brick vault 1,6 m wide, was completely full of sediments (mud height 1,60m) avoiding regular water flow.
Therefore it was necessary to reactivate as soon as possible, at least partially, the hydraulic functioning of the channel (ending part 1,10m).
It was used the suction excavation method. The yard with the suction equipment was placed at the end of rio terà S. Leonardo occupying a central part of rio terà Farsetti (6m wide 30m long), that is only 1\10 of the space occupied by a traditional yard.
The channel was drainned closing with a cofferdam Canal de Cannaregio and maintained dry by an automatic draught pump.
The speed and the reduction in the number of operators have been the main features of this method. The work was well-done cleaning completely the vault without further manual finishing.
The channel was emptided using the same method of Rio Terà S.Polo thanks to a video-controlled suction mini digger.