PEOPLE, PROJECTS AND EQUIPMENT Deep Soil Mixing Helps Restore Venice’s San Marco Bell Tower After a thorough investigation, the designers (Studio Macchi and Studio Geotecnico) stated that no foundation improvements were needed; only an intervention to stop the progression of the cracks. The Ministry of Infrastructure and Transport — Venice Water Authority, through the concessionary Consorzio Venezia Nuova, drew up the plans for this restoration. Trevi S.p.A (from Trevi Group, Italy) has been involved in the technological design and execution of all the specialized work. Soil Profile and Geotechnical Framework Soil Mixing is underway in Venice’s historic center The bell tower in Venice, Italy’s, Piazza San Marco dating back to the 12th century, is currently being strengthened to improve its foundations. The construction of the original tower started at the beginning of the 10th century and lasted to the beginning of the 12th century. On July 14, 1902, at 9:47 am the tower collapsed and 14,000 tons of bricks and debris dropped ruinously over the square, causing the the original 224 m (2,411 sq ft) to 410 m (4,413 sq ft). The reconstruction began in 1903 and was completed in 1908. The new masonry was locked together to the origi- nal, inserting new stone blocks to the old ones. However, the designers who oversaw collapse of the northeast foundations from 2 2 the repair extended the foundation area and not the height; this later caused cracks on all the four sides of the new foundation. These cracks have been monitored from the early 1950s, and show constant increase with time. In the 1990s, authorities decided to undertake a new intervention. The intervention work is underway in extremely sensitive geotechnical con- ditions, characterized by the presence of soft and fine lagoon sediments, with extensive historic fill, especially in central Venice. The typical soil profile in the area of the Piazza San Marco was identified using boreholes and CPTU tests carried out in four stages, from 1993 to 2006. The soil profile is as follows: • From Piazza San Marco pavement level at +0.90 m (2.95 ft) mean sea level (MSL) to approximately 5.0 m (16.4 ft), fill is made up of sandy-clayey silts (locally medium to fine sand with silt). Presence of masonry debris, trachyte blocs, wooden piles and unreinforced concrete. • Below fill to 6.0 – 7.0 m (20 – 23 ft) depth from ground level (GL), a layer of soft sandy clayey silt and/or silty clay with organic debris and peat. • From 6.0 – 7.0 m (20 – 23 ft) depth from GL to 10.0 m (33 ft), medium to fine sand. • Below 10.0 m (33 ft) deep, an alternating silty clay, clayey silts and silty sands were found. A typical CPTU profile around the bell tower is shown in Figure 1. The old footings of the bell tower during the 1903 reconstruction Titanium Rebar The intervention consists of installing two levels of prestressed titanium rebars into the foundation block, to increase the overall flexural stiffness of the foundation. Titanium was selected for its high resistance to corrosion, even in extremely aggressive environments such as saltwater. The first level was to be placed at about 40 cm (16 in) below the square floor and the second level about 2.30 m (7.5 ft) deeper, below the water table and surrounded by an existing block of massive concrete. It was necessary to drill through the concrete to insert the bars along the four sides of the foundation in the correct position. The designers selected deep cement mixing (DCM) columns, reinforced with steel pipes to provide watertightness, lateral support, as well as bottom resistance to uplift for the seven pits (“chambers”). The pits, in turn, were used to install the titanium bars. The chambers are mostly below the water table, and will remain accessible after the project is completed, to allow for monitoring of the titanium bars and to allow future inter- ventions, if necessary. AUTHORS: Daniele Vanni and Maurizio Siepi TREVI S.p.A.,Cesena, Italy Stefano Macchi Studio Tecnico Macchi, Milan, Italy Valeriano Pastore, Studio Geotecnico Italiano S.r.l., Milan, Italy DEEP FOUNDATIONS • JULY/AUG 2012 • 71
