Underground Space – the 4th Dimension of Metropolises – Barták, Hrdina, Romancov & Zlámal (eds)© 2007 Taylor & Francis Group, London, ISBN 978-0-415-40807-3

Geological and geotechnical conditions in underground structures design ofRome subway: the B-Line extension

R. Enrione, A. Eusebio & L. SoldoGeodata S.p.A., Turin, Italy

R. FunicielloUniversitá degli Studi Roma Tre, Rome, Italy

L. SolimeneComune di Roma – Dipartimento VII, Rome, Italy

ABSTRACT: Underground works, in complex urban areas, can present prominent levels of risk connected withvarious ambits: low overburden, buildings facing the line, interference with acquifers, soil of poor geotechnicalcharacteristics, presence of man-made cavities. The knowledge of the geological – technical context is the firstand inevitable stage of study, for the development of an underground construction project, by which it is possibleto achieve a high level of sensibility about the underground space and to propose mitigation measures for therisks associated, both during the construction phase and during exploitation. The case of Rome subway B-lineextension is presented, where a rigorous procedure, involving geotechnical investigations and consecutive phasesof study, has been applied both for the preliminary design and for the construction phase (PAT – Protocol forAdvancement of Tunnel excavation). The illustrated approach has been consolidated from the experience of manysimilar completed projects, and has become a standard practice.

1 THE WORK AND THE TRANSPORTATIONSYSTEM OF ROME

1.1 The “Mobility System” of Rome

The transportation planning of a city like Romepresents a high level of complexity and requires thechoice of strict planning tools.

The “Mobility System”, drawn up and approved bythe local administration, is the reference programmaticdocument, in which the different existing transporta-tion networks, the actions for their development andupdating find a rational placement and harmonization.

According to the “Mobility System”, the subwaynetwork proves to be a necessary means of transport,which is able to connect, in a rather short time, thehighly populated areas of the suburbs to the city center.This aim is being further developed, as shown in theambitious enlargement plan (Fig. 1).

1.2 The subway B-line extension

The B-line extension project coherently fits in theabove described mobility scenario, enabling the oldline to settle close to the existing ring-road highway

(Grande Raccordo Anulare, GRA) and thus re-presenting an important nodal exchange (Fig. 2).

The B-line extension includes 3 km of new line, 3intermediate stations and a new depot. In this way, theSan Basilio, Torraccia and the new Casal Monasterodistricts will be served for the first time, with the viewof a contextual requalification of their urban and socialcontext and also installing new services.

From a technical point of view, the new metropol-itan section will be realized entirely underground, withexcavation between retaining structures and below thecover slab (top-down method). This will allow the sur-face conditions to be restored quickly, limiting thedisturbance to the public to a relatively short time andensuring the separation of underground works fromordinary daily activities.

1.3 The assignment

The engineering services, necessary to the prelim-inary and definitive design of B-line extension, hasbeen assigned by the local administration (Comunedi Roma) to the group of companies composed ofC. Lotti & Associati, Geodata, Siteco and A2G.

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Figure 1. The subway network enlargement plan and thelocation of the B-line extension.

Figure 2. The B-line extension design in the Casal Monas-tero area.

2 TERRITORIAL CONTEXT, POSSIBLEDIFFICULTIES AND BASIC PHILOSOPHY

The project of the new subway section had to facemany difficulties, connected to the insertion of theworks in the already complex territorial context ofreference, due to specific technical uncertainties,which were pointed out since the very first planningstages, as being associated with an initial level ofhigh risk.

More specifically, the major difficulties wereproved to be directly or indirectly connected tothe geological-technical context that consequentlyassume high priority on the design agenda, evenbefore specific studies and geotechnical investigationtook place.

Figure 3. Collapse of a cavity in Rome (Via Bricci).

The most important uncertainties, directly con-nected to geological-technical context, are thefollowing:

– type of soil intercepted by the excavation, withhighly variable level of cementation and poorgeotechnical characteristics;

– presence of an unconfined aquifer at the tunnellevel and possibly other deeper aquifers, even inpressure;

– verified presence of man-made cavities in the site,single or complex ones, also of big size (Fig. 3).

The most significant aspect indirectly connected tothe geological-geotechnical context is the presence ofmany buildings along or facing the line of the work.

It is therefore clear that the full knowledge of thegeological-technical context is an indispensable pre-requisite for the design. This is not only for normativereasons or design completeness (as often requiredtoday), but also for the management and mitigationof the risks during construction.

This is especially effective if performed since thevery first steps of a project, that is since the Pre-liminary Design, when specific studies and geotech-nical exploration can be successfully used to reduceuncertainties while work’s structural designing can bedirected at best.

This process will ensure the most effective defin-ition of:

– risks connected to the construction, with referenceto the design;

– functional level provided by the line;– resulting economical and financial schedule.

The value of this approach can further increase onceextended to the construction phase by a constant moni-toring of the excavation parameters and by the analysisof the work’s actual conditions (PAT Method: Proto-col for Advancement of Tunnel excavation, Grassoet al. 2002a,b). These are strategic activities for therisk management and its mitigation, as they ensure a

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real-time comparison between the design scenario andthat encountered during construction.

The B-line extension of Rome subway, designedwith a deep awareness of the above described phil-osophy, well illustrates the Geodata’s methodologicalapproach.

3 THE METHODOLOGICAL APPROACH

The methodological approach followed for the designof the B-line extension is based on a well defined suc-cession of phases; each phase was established on theresults of the previous one. In this way a sequential“cascade” process was introduced, by which the activ-ities of every single phase proved to be efficientlyrationalised (in terms of quantity, typology and costs)as systematically addressed by the issues of the priorphase (Fig. 4).

The process, by favouring a constant comparisonwith the Owner, benefited from the contributions ofthe whole project management team, thus permittinga better definition and management of risks.

In the following pages the method outline will bedescribed and subsequently account will be given ofthe level of knowledge that has been reached by itsapplication.

3.1 Consolidation of basic information andpreliminary geological-geotechnical model

The first phase of the process is the collection of allthe information and data concerning the geological-geotechnical context, that were available since thePreliminary Design started off.They are: general bibli-ography, available stratigraphic evidences, technicalstudies and previous design documentation (in particu-lar, Feasibility Design). This first phase proved to be astrategic one, because it led to an immediate awarenessof the context in a very short time.

Technical and reconnaissance surveys were alsoperformed, so that the gathered information could beset in its proper context and the knowledge of the “geo”scenario could be further investigated in a rationalway, minimizing the impact on the urban activities andreducing time-cost for the geotechnical exploration.

A platform of GIS (Geographic Information Sys-tem) was prepared for ground data filing, consultationand real-time updating.

It should be noted that in complex areas, such asRome, with buried channels and erosion forms, com-plex lateral eterophies between alluvial and volcanicsediments, etc. reliance must be placed on engineeringgeological interpretation of available information, pre-diction on the basis of known geological relationshipsand careful interpolation and extrapolation of data.

Figure 4. Methodological process flow-chart.

Figure 5. Extract of the Geological Map: actual alluvialdeposit (hatched bright zone) and volcanic sequence (hatchedgrey-scale zone).

For this reason inside the investigations for thedesign of Line B the value of the geological surveyingphase and the presence in the geological and geotech-nical design team of specialist in Structural Geologyand Sedimentology were emphasized: also in urbanareas, with limited rock and soil outcrops the “geo-logical survey” (improved in this environment withtrenches, shafts and other indirect data) made it pos-sible a correct interpretation of the results obtainedfrom the deep investigations. At the end of the firstphase a preliminary geological-technical model wasdefined, substantially represented by a geological map(Fig. 5) and by the corresponding profile along the axisof the work.

3.2 The project and the plan of site investigation

After the “geo” context was defined, even if prelim-inary, the design phase started off; its developmenttook into account the potential risks that had emergedin the previous stage. The hypothesis of the projectwas since the beginning aimed at reducing risks of theconstruction phase.

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In parallel, once the uncertainty margins of the“geo” context were determined, a specific investiga-tion plan was developed and validated by the Owner;it included different steps: geophysical (Stage I) andgeotechnical ones (stages II and III).

3.3 Update of the geological-geotechnical model

Once the investigation steps I and II were performed,a first update of the geological-technical model fol-lowed, based on which significant design indicationsto manage risks were given.

4 PRELIMINARY GEOLOGICAL-GEOTECHNICAL MODEL

In the following paragraphs the results of consolida-tion of the basic information will be showed, withreference to the geological-technical investigations.The analysis of every ambit of study ended up withthe characterization of the difficulties connected to theconstruction, on which the subsequent investigationsteps was planned.

4.1 The geological scenario

The area of interest is located inside the Thyrrhenianmargin of Apennines. Its complex geomorphic, strati-graphic and structural elements can be traced back tothe geodynamic evolutionary characters of the Apen-nines chain of the late Pliocenic-Quaternary period(Ventriglia, 2000).

Above the oldest carbonate levels, not relevantfor the design, terrigenous sandy-clayey sediments ofPliocene can systematically be found (Argille di MonteVaticano, Marne Vaticane, Argille azzurre). Theseunits act as substrate of reference for the subsequentelements. Over them, pre-volcanic alluvial depositsoccur (Unità di Santa Cecilia), consisting of gravel,clay and sand, organized in complex stratigraphicrelationship.

The pyroclastic sequence that follows is of a laterage and is linked to the past explosive volcanic activ-ities, which are characterized by old altered tuffs (fromUnità di Tor de Cenci to Successione di Sacrofano),pozzolane (Pozzolane Rosse) and lithoid tuff (TufoLionato). Finally one can find the actual alluvial sedi-ment and the more recent man-made filling, oftenremarkably thick.

Considering the complexity of the stratigraphic fea-tures, a geophysical and geotechnical investigationwas necessary to reorganize the geological elementsconcerning the project.

4.2 Geomorphology and underground cavities

The area is characterized by the presence of riverAniene’s right tributaries, which flow in relatively deep

Figure 6. Cavities found along the line with buildings above(Pietralata area).

Figure 7. Map of cavities surveyed (hatched grey zone) andrisk of being intercepted.

flat valleys, whose sides have often been changed byextraction activities. Old or current instability phe-nomena are not visible, neither along vertical cavesfaces, because of the good technical characteristics ofthe volcanic soils.

In the concerned area underground cavities canoccur; they can be caused by the extraction of lithoidtuff (Tufo Lionato) and Pozzolane Rosse, especiallyin the areas of S. Basilio and Casal Monastero, wherethe great thickness of the volcanic units favoured theextractions (Fig. 6).

Single and isolated cavities can also be found every-where, especially inside the pyroclastic sequence; theywere generally excavated to function as a well or waterstorage tank.

Since the beginning, the identification and map-ping of man-made cavities proved to be a necessarystep, both for technical (difficult realization of sup-port works and excavations) and for safety reasons(buildings near the excavation areas, undergroundutilities networks, etc.). This activity was performedthrough surveys, within the planned site investiga-tions, systematically integrated with the GIS platform(Fig. 7).

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4.3 Geotechnical and hydrogeological issues

The involved lithologies show a high degree of het-erogeneity in terms of mechanical characteristics andcementation, in particular:

– pre-volcanic alluvial sediments show prevalentsilty granulometry, low density and high watercontent; occasionally cemented levels. Generallythey present poor mechanical characteristics (bothresistance and deformability);

– pyroclastic sequence shows a stratified aspect, withalternation of altered (incoherent, mechanicallypoor) and competent (lithoid tuff) levels;

– actual alluvial deposits show clayey-silty granu-lometry, good plasticity and high deformability;

– man-made fills present a highly variable granulom-etry, with incoherent behaviour.

Some potential, major, construction consequencesderive from this complexity: for example, the pres-ence of lithoid levels makes difficult the excavationwith traditional tools, as well as the possible escape ofbentonite-slurry used for the excavation of retainingwalls.

A detailed geotechnical investigation was there-fore necessary, in order to examine every encounteredlythology, both in situ and in laboratory.

The potential hydrogeological problems can berelated to underground water at the level of thedesign (interference with excavations and supportingworks, dam effect, hydraulic overpressure); possibledeterioration of the setting by underground cavities.

To manage risks at best, all the drillings were pro-vided with stand-pipe and Casagrande type piezome-ters, for a continuous monitoring of piezometric levels.

5 THE SITE INVESTIGATIONS

5.1 Step I – The geophysical investigations

The first step of investigation was performed by com-bining geophysical surveys of georadar and geoelectri-cal methods, developed along work’s longitudinal axis;the results were reciprocally merged for a better resol-ution. Site calibration by the selected methods wasconducted, accomplished by two field-tests focusedon searching for man-made cavity.

Geoelectrical tomography (multi-electrod withspacing <3 m) highlighted resistivity values of 50–200 �m for the top layers (pyroclastic sequence) andlower values for the deeper layers, 5–50 �m (pre-volcanic alluvial deposit, completely saturated). Nosignificant cavity was detected.

Two resistivity anomalies were identified in the pre-volcanic alluvial layer (400–600 �m): subsequentlytested with boreholes, they turned out to be portions ofa very loose soil, partially washed out by groundwaterflow (Fig. 8).

Figure 8. Geophysical section with resistivity anomalyidentified, subsequently investigated with test boring.

A multi-array georadar survey was then carried out,for a multiple frequency scanning of the undergroundcontext. At first, frequencies at 100–200–600 MHzwere adopted, with a lower penetration but higher reso-lution. Later, an ultra-low frequency antenna wasused (25 MHz) for not build-up area only, where nodisturbance-factors should be present for the radiosignal. Its maximum penetration reached 10 m.

On the whole, the georadar survey has highlighteda complex network of underground utilities as well asthe stratification of the upper portion of the pyroclasticsequence.

5.2 Step II – Geotechnical investigaion

According to the findings of the previous step, a firstgeotechnical investigation was conducted, with coreborings and laboratory tests: geophysical anomalieswas directly explored as well as the most interestingareas.

In total, 27 test boreholes were drilled, all ofwhich provided with piezometers (open stand-pipeor Casagrande stand-pipe, occasionally coupled inthe same borehole, for simultaneous logs of differentaquifers).

SPTs, Lefranc, Lugeon and pressiometer tests weresystematically performed in the boreholes, for a bet-ter definition of the geological and hydrogeologicalcontext.

5 piezocone tests (CPTUs) were carried out on thealluvial deposits.

Ultimately, for archaeological reasons, 9 additionalborings were drilled around the presumed-ancientbuilding of Casal Monastero.

6 UPDATING OF THE GEOLOGICAL-TECHNICAL MODEL OF REFERENCE

At the end of the process the preliminary geological-technical context (Fig. 9) was updated (Fig. 10).

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Figure 9. Profile of the preliminary geological-technical model.

Figure 10. Profile of the updated geological-technical model.

The higher level of details permitted a consider-able reduction of the uncertainty margin as well asrisks associated with the construction of the subwayextension.

In parallel, the GIS archive was kept up to date,with a final reconstruction of the geological-technicalmodel, through 3D rendering.

7 CONCLUSIONS

The proposed method has shown how a rigorousprotocol of study, rationally developed, could permita timely identification of the critical aspects for thedesign, the further studies and management of thesecritical aspects in the design process, till the selec-tion of the most appropriate typological solutions forminimizing the connected risks. This has been imple-mented step by step, with an easy-to-follow-againmethodology, both by the Owner and by the Designer.

The whole process was followed by the continu-ous development of the geological-geotechnical model

in GIS environment. This activity will proceed in thefurther design stages and will be completely accom-plished in the construction phase, with the imple-mentation of the PAT protocol that will en-sure themanagement of risks and any unforeseen situationsthrough the most efficient counter-measures, prede-fined in the design stage.

REFERENCES

Funiciello, R. & Giordano, G. 2005. Carta geologica delComune di Roma.

Grasso, P., Chiriotti, & Xu, S. 2002. Riduzione e con-divisione dei rischi residui associati allo scavo di untunnel meccanizzato in ambito urbano attraverso l’usodi un protocollo di avanzamento. Convegno Nazionale diGeotecnica, L’Aquila.

Grasso, P., Mahtab, M.H., Kalamaras, G.S. & Einstein, H.H.2002. On the Development of a Risk Management Plan forTunnelling. 28th ITA General Assembly and World TunnelCongress, Sydney.

Ventriglia, U. 2000. Geologia del territorio del Comune diRoma.

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