Challenge Beneath San Giusto

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Many European cities are taking hard looks at the issue of planning sustainable mobility. Many of the Old Continent’s most important cities are adjusting their management policies to reflect modern guidelines. The new requirements include reducing atmospheric pollution and the use of cars; safeguarding the citizens’ right to travel efficiently within their cities; and setting a limit to traffic congestion and urban blight.

Trieste, an Italian city with approximately 200,000 inhabitants, is located on the Adriatic Sea in the northeastern tip of Italy and is nearly surrounded by Slovenia. The city has historically represented a bridge between western and south-central Europe, and at times seems more central European than Italian. Thanks to its location and history, Trieste, more than other Italian cities, can recognize the need for sustainable mobility.

In 2010, the municipality of Trieste started construction of Park San Giusto, a futuristic car-parking solution. When completed, this structure will be the largest underground parking lot in Italy and the second largest in Europe. It will feature 718 parking spaces arranged on five levels beneath San Giusto hill, right in the middle of Trieste’s historical center. Given the challenges and benefits of building in the center of urban areas, other cities are watching the project closely. If the project is successful, similar structures may be undertaken in other Italian cities.

A New Approach to Urban Parking
The structure of Park San Giusto will have an overall volume of 100,000 m (3,500,000 ft). It will consist of two parallel tunnels, each measuring 120 m (390 ft) long, 20 m (65 ft) wide and 15 m (50 ft) high. Each tunnel will contain five levels of underground parking accessed by two one-way ramps serving as the entrance and exit. Pedestrians will access the garage by an elevator at the top of San Giusto hill. The site is currently under construction 24 hours a day. The tunnels’ excavation, which began after a meticulous geological survey, is being carried out without the use of explosives. Workers use excavators equipped with hydraulic hammers or cutter heads to loosen and remove the material in the tunnels. The excavation is scheduled to be finished by spring 2014, with the completion of the structure and opening of the parking garage scheduled for 2015.

Valentino De Odorico, the owner of TSD Servizi, a proactive topographic surveying company based in Udine, was contacted by the Park San Giusto project management company to carry out the surveying needed to support the safety procedures of the two tunnels. When De Odorico accepted the project, he knew he was accepting more than just a job–he was accepting a real challenge.

Goals and Complexities
Digging tunnels is, by definition, problematic. Besides the actual excavation, topographical surveys must be performed on the tunnel during the work to avoid subsidence or convergence (slow, inward deformation of the rock surrounding the tunnel) and the subsequent collapse of the structure.

Usually, the development of a tunnel proceeds longitudinally. The top heading, which responds to the higher arch of the tunnel, is dug first during the progression of the tunnel. The excavated profile is then covered with reinforced shotcrete, making it possible to excavate the invert (the lower portion of the tunnel). Once it is reinforced and joined to the top heading, the invert forms the completed tube and ensures proper weight distribution on the structure. Making convergence measurements is standard procedure and an integral part of tunneling operations. Once a section of the tunnel is complete and the excavation profile is reinforced with shotcrete, it is then possible to proceed with the next sections of the tunnel.

While the process for Park San Giusto generally follows this approach, it features a variable that greatly increases the complexity of the work. Instead of proceeding section by section, the longitudinal excavations are carried out continuously. After the reinforced shotcrete is in place in the top heading, the excavation proceeds to increase the tunnel’s depth to create the space needed for the multi-level parking. This construction method at Park San Giusto does not provide the top heading with an adequate basis to carry the load of the rock above. As a result, the excavation profile is reinforced while the digging proceeds both longitudinally and in depth. The convergence measurements, which must be taken continuously, are fundamental in controlling the non-negligible amount of subsidence. During this critical phase of construction, the normal stress caused by rock movement is not the only source of concern. The reinforced shotcrete layers are also subjected to stress caused by the movement of machinery and vehicles operating within the San Giusto hill construction site.

Adding to the complexity, a number of minor service structures–connections between the two tunnels, the main entrance, a pedestrian passageway and a 60-m- (200-ft-) deep air well–are also under construction. This combination of elements increases the amount of convergence within the two tunnels.

This is not a construction site for the faint-of-heart! Working underground means that temperatures can reach unbearable limits. Moving vehicles, construction activities and shotcrete fill the space with dust and deafening noise. The lighting is dim and, on top of everything else, the work goes on day and night without interruption. This is the Dante’s circle in which TSD Servizi must carry out their challenging work.

Measuring Convergence
TSD Servizi’s primary job of measuring convergence requires preliminary work every evening before entering the construction site. Given the constant movement within the workspace, no benchmarks or control points can be positioned inside the tunnel. The team is thus forced to enter the tunnel by running an open traverse that originates from an external, known control point located near the main entrance of the site.

The team must start from the beginning every day. This requirement makes each survey seem like the very first one, continuously facing the same uncertainties and meeting the narrow tolerances imposed by the nature of the work. Open traverses are common in tunneling work, and the TSD Servizi team uses rigorous procedures to ensure the accuracy of their work. Multiple measurements and frequent checkpoints helped to reinforce the traverse.

The survey team uses Trimble S3 and Trimble S6 total stations for its work of measuring rock convergence and tracking the various points required for the tunneling operations. Initially, while creating the open traverse (carried out with a traverse kit, 2 tripods and the total stations), the surveyors were faced with unexpected displacements in the measurements. Investigation revealed that the displacements were related to the difference in internal and external temperatures. To avoid this problem, the team decided to reinforce the traverse with additional points.

On a typical survey at Park San Giusto, the first convergence measurement is taken inside the tunnel entry. The operation continues with the open traverse, measuring the pedestrian service tunnel, the main tunnels and the other internal areas. In order to measure the amount of convergence, prisms are positioned along the top heading and on the sidewalls of the tunnel. This operation is carried out using the Measure Rounds option of Trimble Survey ControllerTM software, which automatically conducts face 1 and face 2 observations on the selected prisms. The d
ata is then post-processed to understand the extent of convergence within the tunnels.

The Tracking Phase
In addition to the fundamental convergence measurements, TSD Servizi is also tracking the various points required for the tunneling operations. This includes points marking the tunnel axes to guide the digging, establishing topographic elevations and setting out points needed for the installation of future tunnel components and fixtures.

The tracking methodology is almost the same as that used to measure convergence. If the target is observable from one of the previously surveyed points within the open traverse, tracking can be carried out easily. Otherwise, the open traverse must be extended to reach the target.

The tracking procedure highlights the Trimble S3 and S6 total stations’ (both with an angular precision of 2") performance. The instruments can be set up quickly, an important benefit in the busy work environment. The Trimble field software helps as well, working with three-dimensional lines based on the original design of the parking garage. The instrument and software can immediately specify to the on-site workers points corresponding with the top heading excavation profile and compare the existing profile with the design. Office post-processing is not necessary, which saves a considerable amount of time. The same methodology is used to track the shuttering machine rail used to pump concrete onto the tunnel walls, the beams, columns and riveting.

Along with technical difficulties, the TSD Servizi team faces many logistical issues. The operational environment increases the complexity of the surveying work. The sensitive surveying instruments can be affected by vibrations produced by the vehicles, which due to the limited space have trouble just moving around. TSD Servizi’s surveyors, together with the managers of the construction site, are constantly seeking the best ways to carry out the measurements and digging operations simultaneously.

The Results
Based on the results obtained by TSD Servizi, tunnel designers correct the structures and implement geotechnical and structural reinforcement work. This may call for inserting more riveting or tie-beams where needed in the concrete facing. So far, results have revealed enough convergence to make it necessary to constantly modify the original design of the parking garage. Such a complex job puts the men and instruments to the test. In spite of the difficult conditions, the accuracy and speed of the measurements must be guaranteed. With its advanced survey systems, TSD Servizi is meeting the challenge.

Fulvio Bernardini is a freelance writer, translator and editor based in Italy.

A 2.812Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE