Projects

IT98 Venice

01.01.98

Diving into Venice Lagoon

The need for a sewer system in Venice and its most popular tourist resorts around the lagoon has been critical for many years. After prolonged delays caused by Italy's need to satisfy Maastricht criteria, which resulted in severe budget constraints on infrastructure projects, the most urgent schemes were recommenced and/or re-tendered in the autumn of 1997.

On the basis of the existing "Master Plan for the Reclamation of the Venetian Lagoon" and a sub-master plan for the Cavallino Peninsula, several construction lot tenders were called. HOBAS was awarded contracts to supply piping material for two major lots. The most important of these is "The Cavallino Sewer System".

A new separate sewer system

The old existing sewer was designed as a combined system, carrying both storm water and sanitary sewage. Now completely worn out, most of the old system's joints were either leaking or broken and uneven settlement of the pipes precluded the possibility of any type of relining. In view of effluent treatment considerations, the client decided to build a split sewer system with one part for sanitary sewerage (handling domestic and industrial wastewater)
which is processed by a new treatment plant and a second for storm water, which is discharged either into the lagoon or sea without treatment. The long pipe lengths of the sanitary sewer system required a pipe material with inherent corrosion resistance. HOBAS was the answer.

Corrosion and abrasion resistance

Experience has shown that along the coastal area of the Veneto region, conductive metal piping materials are highly susceptible to electrolytic pit corrosion. This type of corrosion, apart from that caused by high and/or medium-tension lines, also occurs because the fluctuating levels of the brackish lagoon water aquifers conduct stray electrical currents, aggravating the pit corrosion problem.

As so often before, the non-conductive HOBAS CC-GRP Pipe System was the answer to this problem. At the site geological analysis showed a high content of very abrasive sand. The abrasion resistant liner resin of HOBAS CC-GRP Pipe System again provided a benefit for the client.

Hydraulics

The hydraulic gradient designed for the trunk main sewers was only 0.2%, i.e. 2 m/km. The client's main concerns were to provide optimum pipe flow velocity conditions which would accommodate the differing requirements of the various (tourist) seasons, without creating the bad odors and increased corrosion which can arise if deposition of solid matter occurs, particularly at times of low flow. The low coefficient of roughness k = 0.01 mm (according to Colebrook-White) combined with the larger inside diameter of HOBAS CC-GRP Pipe System, when compared to other piping products, was also highly beneficial to the client. Otherwise a hazard, inner corrosion can be neglected for the inherently corrosion-resistant HOBAS CC-GRP Pipe System. The problem of offensive odors was solved by using a closed system design with sealed inspection shafts and manholes for the locations where the pipeline was situated in the tourist and recreation zones on the Cavallino Peninsula.

Short construction times

In order to avoid the tourist seasons, construction could only be carried out from late October to the end of March. HOBAS CC-GRP Pipe Systems were laid to schedule, despite heavy rainfalls and very soft soils. The limiting factor for the speed of pipe installation was the time required for earth works. Leak tightness tests were conducted without encountering any problems.

Operation and cleansing

One of the most important considerations for the client, as the end user, was the ability to operate modern, high-pressure cleansing devices such as the 'canal jet'. The proven high abrasion resistance and greater thickness of HOBAS Liner Resin was one of the decisive reasons for the client in selecting HOBAS CC-GRP Pipe Systems. In Italy and the Veneto region in particular, HOBAS CC-GRP Pipe Systems of all types were and are well known. Nevertheless, having realized the obvious benefits of the centrifugally cast HOBAS CC-GRP Pipe Systems, far-sighted clients have now started installing HOBAS piping material again, thus continuing the long established use of the HOBAS Product in Italy.