Sometimes I ask myself if my passion for engineering is just the result of an Electra complex, because my father is a geotechnical engineer and I always dreamed of following his footsteps,” says Giulia Viggiani, newly-appointed professor of infrastructure geotechnics at Cambridge University. “Of course my enthusiasm for scientific studies helped me to pursue my career in this field.”
Viggiani has worked as professor of geotechnics at Tor Vergata University in Rome for 10 years, where, together with colleagues in structural engineering, she has helped establish the ‘Tunnelling Engineering Research Centre’ to address and coordinate the many departmental research activities on tunnelling connected to geotechnics, structures, transport and environmental engineering.
She has recently moved to Cambridge to join the geotechnical group currently headed by Robert Mair who retires next year. “Professor Mair spent his life in the tunnelling industry. I’m honoured to take over from him at Cambridge University, which is a very interesting place to be,” Viggiani says.
“Cambridge University offers several centres and networks to work with. For example, the Centre for Smart Infrastructure and Construction (CSIC), an international centre for excellence providing organisation operating in the infrastructure and construction sectors with tools and information necessary to take advantage of the latest technical developments in asset management and sensor technology, or the UK Collaboratorium for Research on Infrastructure and Cities (UKCRIC).”
“Moving to Cambridge is important not just because the British tunnelling industry is so active, but also because the themes and goals of my research are very well-matched with the current activities of the geotechnical research group here. I hope that I may contribute positively to maintain and further develop the world leading role of Cambridge’s group in producing high quality, innovative, and industry-relevant research in geotechnical engineering.
“At the same time, I also believe that my research will benefit from the huge potential offered by the innovative sensing and monitoring systems available at CSIC and by the availability of excellent laboratory facilities, including the Turner Beam Geotechnical Centrifuge of the Schofield Centre and the planned National Research Facility for Infrastructure Sensing on the West Cambridge Site.”
BACKGROUND
Viggiani graduated in Civil Engineering at Università di Napoli Federico II, Italy in 1989. While completing her PhD in geotechnical engineering at the City University in London, under the supervision of her professor, John Atkinson, she had an opportunity to help another professor, John Burland of Imperial College, on a project on the stabilisation of the Leaning Tower of Pisa. “He had been appointed member of the International Committee to Safeguard the Leaning Tower of Pisa, and had received a huge amount of technical paperwork in Italian, so I helped him with translation and interpretation, which was an amazing great experience for a young geotechnical engineer.”
“After a few years professor Burland invited me back to London to participate to the CIRIA-LINK team on Monitoring Building Response to Tunnelling, in connection with the construction of the Jubilee Line Extension. I joined the monitoring group at Imperial College at the very beginning of 1996 and worked there for nearly two years. It was a great experience, which enriched my scientific and technical skills. Before that, my research had mainly been focused on laboratory work on fundamental aspects of the mechanical behaviour of soils.
Joining the IC group gave me the opportunity to get involved in more engineering oriented research and also put me in touch with many people in the in underground construction and tunnelling sector. I guess it is not by chance that, towards the end of 1996, I became one of the two Italian representatives on the then Technical Committee TC28 of the ISSMGE on Tunnelling in Soft Ground.”
CURRENT AND FUTURE RESEARCH
Viggiani’s research interests and areas of expertise span from soil mechanics to geotechnical engineering. “I regard the technical challenges stemming from geotechnical engineering as opportunities to advance fundamental understanding in soil mechanics, and in turn, have always tried to transfer any fundamental developments back to the engineering practice, in the belief that they can provide powerful tools to optimise design and control construction processes,” Viggiani says.
“The main thrust of my current research is on the applications of soil mechanics to geotechnical engineering, and deals mainly with underground construction, foundation engineering and earthquake geotechnical engineering. I have been involved in some of the most important current infrastructural projects in Italy, including the design and construction of Lines 1 and 6 of Napoli Underground and of Line C of Roma Underground, and the design of the foundations, anchor blocks and terminal structures of the Strait of Messina Bridge.”
The participation in these projects gave her the opportunity to carry out research on tunnelling and construction processes, tunnelling induced damage assessment and connected mitigation and remedial measures, and performance-based design of geotechnical structures under seismic actions, using a combination of field monitoring and laboratory observations, theoretical analyses, and physical and numerical modelling.
Viggiani is currently working on a number of interconnected themes including the development of advanced numerical modelling of mechanised tunnelling, the study of the impact of tunnelling on structures and of construction processes involving coupled phenomena in soils, such as artificial fround freezing, and the evaluation of the effectiveness of mitigation measures.
“Together with researchers of National Technical University of Athens, I am currently contributing to the development of 3D FE procedures to model TBM tunnelling,” she says. “These include the realistic simulation of the main physical phenomena occurring during EPB shield tunnelling, such as the application of a variable support pressure at the face, the presence of a physical gap between the excavation boundary and the permanent lining – due to over-cut at the cutter-head, shield tapering, and installation of the segmental lining within the shield – and the execution of tail void grouting.
“We are simulating the contact between the shield and the ground introducing appropriate normal and tangential contact laws, while tail void grouting is modelled using a timedependent setting law for the grout and an initial stress state in the grout elements to represent the injection pressure. Since a significant proportion of ground deformation due to tunnelling in fine grained soils is associated with consolidation settlements, fully coupled consolidation analyses are carried out with realistic rates of advancement of the shield.
“Although numerical results are very promising at this stage, the comparison with real field data is crucial for the ultimate validation of the proposed methods. We plan to apply these numerical techniques to full scale class A predictions of the ground and monument response to construction of the tunnels of Line C of Roma Underground.”
Viggiani has been leading one of the work packages of a EUR 10M (USD 11.7M) Large European Collaborative Project involving 22 partners across nine countries, including universities, R&D laboratories, large companies and SME. This project addressed key scientific and technical challenges on the theme of mechanised tunnelling such as the development of an advanced multi-sensor ground prediction system for TBMs, of a suite of systems for modelling and thus controlling the impact of tunnelling on surrounding structures, and the creation of a decision support system for tunnel maintenance management. A large experimental campaign has been undertaken at ENTPE Lyon to investigate the impact of tunnelling on reduced scale models of piled structures, using a small scale EPB shield advancing inside a large tank filled with sand, in which instrumented piles or small pile groups are pre-installed at different locations around the tunnel. The response of the piles in terms of settlement, axial load, longitudinal, and transverse bending moment, was monitored during advancement of the TBM. The results obtained so far show non-negligible effects in terms of displacements and bending moments associated to the changes of stress in the direction of TBM advance.
PROJECTS
“I have recently contributed to activities connected to the design and construction of Napoli Underground, and I am a member of the Geotechnical Working Group of the International Technical and Scientific Committee set up to assist the designers of Line C of Roma Underground, in close co-operation with the general contractor, Metro C SpA,” Viggiani says.
At present, Napoli Underground includes six underground transit railway lines, a commuter rail network, and four funicular lines, with planned upgrading and expansion work underway.
“The idea of a fully integrated urban rail network was proposed in the 1950s as part of the post-war regeneration effort; plans were first formulated in the 1960s, but funding, planning, and development problems all caused long delays,” Viggiani says. “Construction began in 1976 and the first 4km-long rapid transit line opened in 1993, running between Colli Aminei and Vanvitelli Stations; two years later, the line was extended to reach Piscinola, for an overall track length of 13km.”
The City Transport Plan, approved by the Municipality in 1997, included three main phases of re-development. Phase 1 consisted of an expansion to five lines, to take the network up to 53km of track, with 68 stations (23 newly built), and 12 interchange nodes, and was completed by 2002. Phase 2 was designed to increase the network to seven lines, with 84 stations, and 16 interchange nodes, and is currently under way. Phase 3 will see the network expanded to 10 rail lines with 93km of track, and a further 30km of new light rail linking 114 stations, with 21 interchanges.
Once completed, Line 1 of Napoli Underground will form a closed ring connecting the northern outskirts of the city, the area of the hills, the historical centre, the administrative district, and the airport, for a total length of about 40km and 25 stations. The first 22km of the line, between Piscinola and Dante Stations, were completed relatively quickly and were fully operating by 2002. The next 6km, between Dante and Garibaldi Stations, proved to be much more problematic. This is because all five stations included in this work had to be excavated through loose granular deposits, well below the water table, and in an extremely densely built urban environment.
At present, four out of five stations are open to the public, bringing the number of operating stations on Line 1 to a total of 18, while it is expected that the last station on this stretch of the line, at Duomo, will be completed in 2020.
The preliminary design of the line between Duomo and Garibaldi consisted of shallow tunnels constructed by cut-andcover. At a later stage, it was decided to bore the tunnels within the Yellow Tuff formation, because this would both minimise direct interferences with the archaeological layer and reduce limitations to the surface traffic during construction. The good mechanical properties of the tuff also reduced the risk of settlements and hence potential damage to nearby structures. The main drawback associated with this design is that the line is quite deep and for a long stretch it is well below the groundwater table, with hydraulic heads between 25 and 30m. The very high pore pressures, together with the random occurrence of fractures in the tuff, made it necessary to bore the running tunnels with closedshield EPBMs.
The station tunnels and passageways were enlarged by conventional mining and had to be constructed with the aid of a variety of ground improvement methods, including chemical injections, cement grouting, and the extensive use of artificial ground freezing (AGF).
“The works on Napoli underground brought about international co-operative research bringing together constitutive modelling, laboratory tests and field data involving Universitat Politécnica de Catalunya Barcelona, Università di Roma Tor Vergata, and Seconda Università di Napoli,” Viggiani says. “The participation of technical personnel and engineers involved in the design and construction of Napoli underground permitted to collect all monitoring data on the applications of AGF.
“The predictive capabilities of an existing fully coupled thermo-hydro- mechanical model of the behaviour of frozen ground were tested against the experimental results of a number of triaxial tests carried out at different temperatures and confining stress. The predictions of the model compared very favourably with the experimental observation and the softening behaviour of the material on thawing was remarkably well reproduced by the numerical simulations.
The constitutive relations are implemented in a visco-plastic form, mainly to regularise integration of the material law on softening; this feature of the formulation may be enhanced to model the time-dependent behaviour of frozen soils, and it is envisaged that the role played by temperature may be accounted for by introducing the dependency of fluidity on suction.
“The triaxial tests for Napoli Underground were carried out in a double-walled triaxial cell working under temperature-controlled conditions developed by Tecno-in SpA, which, in its present configuration, has several limitations. For instance, due to water freezing in the drainage lines, it is not possible to measure volume changes using external volume gauges and, contrary to conditions on site, freezing proceeds from the outer boundary of the sample towards its centre.
“Building on this experience and to overcome these limitations, funding has been secured to design, build, and set-up a prototype stress-path-controlled triaxial system for frozen soils at Tor Vergata University.”
Line C of Roma Underground, whose preliminary design was approved by the municipality in October 2002, is currently under construction. To date, the existing network consists of only two lines, Line A and Line B, intersecting at Termini Central Railway Station. Line B was built in the 1930s by cut-and-cover whereas the bored tunnels of Line A date back only a few decades.
The new Line C runs north-west to south-east of the city, for a total length of more than 25km and 33 stations. The project is divided into seven contracts; its easternmost stretch, between Monte Compatri-Pantano and Giardinetti Stations (contract T7), is on surface, while the remaining part of the line is excavated using two EPB shields of a diameter of 6.7 m. The stretch between Giardinetti and Parco di Centocelle was completed in November 2014, while the following six stations, from Mirti to Lodi, were opened to the public in June 2015, thus completing contracts T6 to T4. The part of the line between San Giovanni and Fori Imperiali (contract T3) is currently under construction, at different stages of advancement: San Giovanni station was opened to the public at the end of 2016, while it is expected that the stretch between San Giovanni and Fori Imperiali will take another five years to be finished.
“Construction of contracts T3 and T2 is very problematic because the running tunnels and the stations will have to be built in the historical centre of the city, with significant problems connected to buried archaeological remnants, the geotechnical characteristics of the soil, consisting of alluvia from the Tiber River, construction below the water table, and the necessity of minimising the effects at the surface on the historical and monumental heritage,” says Viggiani. “Contract T3 interacts mainly with monuments of Roman Age, including some of the most famous archaeological landmarks in Roma, such as the Coliseum, the Basilica di Massenzio, and the Foro di Cesare, while contract T2 underpasses the historical centre of the City, potentially affecting a large number of ancient masonry buildings of historic and artistic value built between the XV and the XIX century.”
Because of the exceptional archaeological and historical value of the structures potentially affected by the construction of the line, the design had to include a detailed study of the interaction between the construction activities and the monuments. As prescribed by the grantor of the project (Roma Metropolitane), the general contractor (Metro C) set up an international, multidisciplinary steering technical committee with the assignment of implementing all necessary procedures to safeguard the historical buildings.
The main tasks of the steering committee were to evaluate the influence of the construction of Line C on the existing monuments and historical buildings, suggest, where necessary, appropriate mitigation measures of geotechnical and structural nature, develop a comprehensive and redundant monitoring scheme to follow in real time the response of the buildings to construction, and assist the general contractor in the evaluation of the monitoring data to optimise construction sequences and procedures. To accomplish these tasks, five working groups were set up, whose activities were coordinated by the steering committee, including experts in preservation and restoration of monuments, tunnelling, geology, structural and geotechnical engineering, geomatics and monitoring.
“The geotechnical engineering working group, of which I was a member, was charged with the tasks of the mechanical characterisation of the deposits and the definition of the relevant soil parameters, the prediction of the displacement field affecting the different buildings during and after construction, the evaluation of the feasibility of different mitigation measures to be implemented when the soil-structure analyses yielded unacceptable displacement fields, and the development of a geotechnical monitoring scheme,” Viggiani says.
“In close co-operation with the structural engineering group, the geotechnical engineering group was also required to develop a consistent and scientifically sound methodological approach to be followed to evaluate how the construction of the line would affect the existing historical buildings, thus assessing the expected damage category.”
In the subject of mitigation measures, Viggiani is carrying out field trials, including both compensation grouting and barriers, along a dedicated stretch of Contract T3 of Line C.
“Properly conducted field trials prior to tunnelling are therefore vital to prove the feasibility of compensation grouting and its efficiency in the long term, but also to evaluate the effects of the grout sleeve tubes and the proposed grout mixes, and to validate assumptions regarding grout spread,” Viggiani says. “The proposed field trial of compensation grouting will include a dummy foundation and a monitoring system of surface and subsurface displacements. The circular shaft from which the grout sleeve tubes will be inserted has already been constructed in Largo dell’Amba-Aradam, where very similar soil conditions exist as for the nearby Mura Aureliane at Porta Metronia, which we intend to protect from settlements using this technique.
“The other mitigation measure under examination is the installation of barriers between the tunnel and the building, by introducing alignments of piles, jet-grouted columns, discrete micro-piles, or plastic (bentonite) diaphragm walls. The results of centrifuge and numerical studies show that barriers can be very effective in reducing tunnelling induced ground movements. However, these are often implemented in practice on the basis of empirical knowledge more than as a result of a rational design process.”
The effectiveness of a barrier consisting of 48 concrete piles with a diameter of 600mm, a length of 34.5m and a spacing of 900mm, will be tested in an instrumented dedicated section, located at the beginning of Contract T3. Parallel to the site work, small scale physical modelling and numerical modelling of barriers are also being carried out at ENTPE Lyon and at Tor Vergata. The effects of the distance of the barrier from the tunnel axis, its length, stiffness and nature of the soil-barrier interface (rough or smooth) were examined numerically to orient design of the field trial and extend the parametric study carried out by physical modelling.
ACHIEVEMENTS
Viggiani’s role as a recognised expert in the field of geotechnical engineering for infrastructure has been acknowledged both at national and international level. She has prepared general reports for several international conferences on infrastructure geotechnics, and has been invited to deliver lectures in many countries. The research she carried out in cooperation with Gopal Madabhushi of Cambridge University and Riccardo Conti of Tor Vergata, on the behaviour of flexible retaining structures under seismic actions, received the 2012 ICE TK Hsieh Award.
Viggiani delivered a keynote lecture at the XVI European Conference on Soil Mechanics and Geotechnical Engineering, which took place in Edinburgh in September 2015. “I talked about applications of artificial ground freezing in tunnelling related to Naples underground,” she says. “I was honoured to be one of the three keynote lecturers at this prominent international event. I consider it one of the highest points of my career, although, of course, there are other things I take great pride about.
“Because of my commitment to the activities of the technical committee, I was gratified to have been awarded the prize for the best presented paper at the 5th TC28 IS on ‘Geotechnical Aspects of Underground Constructions in Soft Ground’ in 2005 in Amsterdam, and am still very proud of having organised its 7th edition in Roma in 2011.”
TUNNELLING INDUSTRIES
“Italy still has serious infrastructural deficiencies; in particular, in the field of transport infrastructure, there are significant faults in the congestion of major metropolitan urban areas and low quality of the regional public transport,” Viggiani says.
In 2015, the Italian Strategic Infrastructure Programme identified 25 priority projects, for a total cost of EUR 70.9bn (USD 80.55bn) and financial hedges amounting to EUR 48bn (USD 54.53bn). In this context, the amount of resources allocated to the extension of metropolitan underground lines was very significant; for the underground lines of Napoli, Bologna, Milan, Turin, Palermo, and Florence these amounted to approximately EUR 9.5bn (USD 11.12bn), while the total cost of the sole Line C of Rome Underground was estimated at EUR 2.7bn (USD3.16bn).
“I believe that Italian companies have established expertise in tunnelling and excavation works; for example, in ground treatment and artificial ground freezing, we have world leading companies that are currently involved in tunnelling projects all around the world. I believe that the expertise of Italian industry in this market should be maintained and, if possible, expanded as it can play a role in the national economy.
“To carry out big construction projects in Italy, the main problems are often the political management and bureaucracy that can slow down or even put on hold public works. Instead it seems to me that in the UK there is a custom of virtuous interaction between governmental action, research and the construction industry, tunnelling included.”
