HDD Solves Pipeline Relocation In Venice Canal

When it became necessary to dredge and deepen the Bacino di Evoluzione No. 3 ship canal in Venice to accommodate larger ships, an existing IES (Italiana Energia e Servizi S.p.A.) oil pipeline crossing the canal had to be relocated.

Before proceeding with the project, IES officials carried out a careful analysis and decided horizontal directional drilling offered the best solution to install the pipeline at depths specified by port authorities. LMR Drilling GmbH was charged to carry out the final design and to realize the HDD. The works were performed during August and September 2009.

Site Location
The site is located in the central part of the Venice lagoon, three miles south of the town of Mestre and three miles west of Venice center itself. The pipeline crosses the Evoluzione 3 Canal and connects Oil Island with the Romagna Pier. The Venice lagoon originated about 6,000 years ago. Originally it was much less extended because the sea water level was substantially lower than today and large areas, now under water, were instead dry lands.

As sea water levels rose, the depositional environment changed from continental conditions, with fluvial and fluvio-glacial transport and sedimentation, to a low energy marine environment.

Due to the environmental changes, two sedimentary complexes are present, even though they are similar as regards the lythological and grain size composition. At the bottom, late-Pleistocene marine deposits are found above the Holocenic lagoon deposits. Both of them consist of silty-sandy-clay soils with low consistency and high vertical and lateral variability. The passage from the former to the latter is marked by a well-defined level called “caranto,” of overconsolidated clay due to drying which represents the paleo-ground formed between the two depositional cycles.

Within the lower depositional complex, many peat layers with pieces of wood are present.

As regards the hydrogeological conditions, no continuous aquiferous is present, due to the high soil variability; only local perching ponds, not connected, can be found. The “caranto” level of high consistency and impermeable material sometimes confines artesian waters.

Geotechnical Characteristics
The geotechnical characterization of the site was based upon specific investigations (boreholes and tests) as the result of geotechnical studies performed in close areas for the Protective Intervention of Venice on behalf of the Venice Water Authority and available information from scientific literature.

As regards the stratigraphy, a high-grain size variability can be noted up to and over the maximum HDD depths, with prevalence of clayey and sandy silts between 130-160 feet. Such variability has repercussions on the geotechnical parameters, as can be noted in the characterization in Figure 1A by Cola and Simonini 1995.

The assessment of the geotechnical parameters, used for the HDD design, is based on results of the tests performed in the boreholes close to the HDD line (S.P.T., Torvane, permeability) and of the tests, both on site and laboratory, carried out for the Protective Intervention of Venice.

Due to the high variability of the encountered soils, no continuous layers with specific geotechnical parameters were found; only a distinction between cohesive and granular soil was made, whose values of the main parameters are listed in Table 1.

Table 1 - Values of the main geotechnical parameters