Titanium rods were drilled through the foundation stone blocs and anchored at the foundation’s corners by titanium plates, being initially prestressed to 250 kN (28 tons) each. Depending on the foundation’s structural responses, stresses in the titani- um rods could be adjusted over a few years. 2 Corner chambers have 10 –15 m (107 – 161 sq ft) area, with different shapes according to surface constraints. Lateral walls of the chambers are formed with two or three rows of secant DCM columns having a diameter of 0.40 or 0.50 m (16 or 20 in), 8 – 9 m (26 – 30 ft) deep. Average compressive strength of the treated soil is in the order of 0.5 MPa (72 psi). From a geotechnical point of view, the main project issues were: • Watertightness of the chambers during excavation, to avoid any losses of fines due to water inflow • Stability of lateral walls of the chambers during excavation • Resistance to uplift of the chambers in temporary conditions taking into account the periodic “high water” • Control and limitation of total and differential settlements of existing foundations (bell tower and Procuratie Nuove) to very small values, as com- pared to recorded seasonal movement due to tide, temperature, wind, etc. • Control and limitation of vibrations induced by construction The dead weight of the reinforced concrete chamber walls and the chamber cover guarantee resistance to uplift at the corner chambers after the project is com- pleted. Stability in temporary construction conditions, after chamber excavation and during the reinforced concrete walls construction, was guaranteed by the dead weight of the treated soil (taken as a massive block), and, possibly, by side friction against the alignment of the DCM columns. The strength of the treated soil, in terms of unconfined compressive strength, was considered less critical than the density issue, since expected stresses in treated soil columns during chambers excavation are relatively low (<2.0 MPa or 290 psi). 72 • DEEP FOUNDATIONS • JULY/AUG 2012 Figure 1. Typical profiles of CPTU around the San Marco Bell Tower Worksite in the Heart of Venice The jobsite is in the very heart of Venice, in the Piazza San Marco in front of the Basilica, packed every day by thousands of tourists. The authorities required the job footprint to be minimized, to reduce disturbance to the normal life of the city and of the tourists. Trevi selected a small/medium size drilling rig (unusual for DCM columns) that could produce the small diameter columns needed to minimize disturbance at the historic site. Transporting the rig (SM 21 Soilmec–Trevi Group) from the San Marco Basin to the bell tower required a special platform to distribute the weight of the rig (nearly 220 kN or 25 tons) and fulfill the municipal regulation that limits the maximum allowable pressure on the square paving “masegni” (trachyte grey stones) to 3 kPa (0.45 psi). The workers built the platform using a 150 mm (6 in) layer of sand and a layer of neoprene, covered with wooden beams, over which steel I-beams were placed, blocked laterally with retainers anchored to the wooden beams. The 160 m (175 yd) route was covered in four stages of 40 m (43.7 yd) each. After each stage during the night, workers disassembled the platform, then reassembled it in front of the machine. The entire stretch took less than a week to arrive at its destination. The other major concern was the effect on the existing monuments, in particular the bell tower and the nearby Procuratie Nuove with the Marciana Library, in terms of induced settlements and rotations. The two monuments have different foundation systems (Figure 2): • The bell tower is founded on driven wooden piles, penetrating the sand layers below 6.0 – 7.0 m (20 – 23 ft) deep; the original foundation was reinforced with 300 additional piles during the 1912 reconstruction. • The Procuratie Nuove sits on shallow foundations, with an expected depth slightly smaller than the design excavation level of the corner chamber on the south side of the bell tower. Along this side the overall strength of the intervention was improved by incorporating 50 cm (20 in) diameter DCM columns and increased steel pipe reinforcement. Preliminary laboratory tests deter- mined the best cement content to obtain the strength required by the designer, and to assess the achievable density of the mixed soil. The preliminary noise and vibration tests suggested that the expected vibration level during DCM column construction was negligible and would not appreciably disturb residents and tourists. Full-scale tests measured the vibrations and the noise generated by the rig during the column installation, and confirmed those results. Test results confirmed the possibility of achieving the required minimum compressive strength of 0.35 MPa (50 psi) at 28 days curing and the minimum unit 3 weight of 18 kN/m (1.52 tons/cu yd). The main work on the Piazza San Marco started in early August 2009. Due to the possible presence of underground obstacles, the columns were drilled to the full depth using the minimum quantity of water. The cement slurry was added during retrieval at fixed speed. As the lower half of the col- umns is in fine sand and the upper half in cohesive, soft, organic material, the tool was rapidly moved twice up and down to enhance the homogenization of the cemen- ted fresh soil column. Special mixing tools have been developed to cope with the obstacles.
