Maurizio Monina S.E.L.I Spa, Rome, Italy. Remo Grandori S.E.L.I Spa, Rome, Italy. Aristodemo Busillo S.E.L.I Spa, Rome, Italy

Transcription

1 HIGH PRESSURE EPB TUNNELING UNDER SENSITIVE BUILDINGS Maurizio Monina S.E.L.I Spa, Rome, Italy Remo Grandori S.E.L.I Spa, Rome, Italy Aristodemo Busillo S.E.L.I Spa, Rome, Italy Andrea Sciotti Roma Metropolitane, Rome, Italy Roberto Ginanneschi S.E.L.I Spa, Rome, Italy Metro Line B is a branch of the existing Line B, and the section currently under construction is characterized by the excavation of tunnels (with a total length of approximately 6 km) and stations (Conca d Oro, Gondar and Annibaliano). The works of the new metro line are located in a heavily built-up area on an axis from Piazza Conca d Oro to Piazza Bologna, and affecting Via delle Valli, Viale Libia, Piazza Annibaliano and Viale XXI Aprile. Excavation of the tunnels started was in the early months of 8. The choice of the system for excavation, mucking system and logistical management of the activities required to make the tunnels of this line was defined on the basis of various factors, including the altimetric profile of the route, the geological, geotechnical and hydrogeological characteristics of the terrain and above all the evaluation o the interferences of the excavation with previously existing structure. The evaluation of the aspects mentioned above led to the decision to use mechanized excavation with the EPB method, with the use of a mucking system with continuous conveyor belts and a vertical conveyor belt at the initial station (Conca d Oro Station). ROUTE The route of the two tunnels has a total length of 6,m and is characterized by a maximum slope of %, numerous horizontal curves (with minimum radius of m) and a low planimetric and altimetric wheelbase, between 8.7m and. m, at the station entry/exit areas. In particular, the planimetric and altimetric profile involves reaching the maximum depth ( m) in the stretch going under the Aniene River, while the point of minimum coverage is at the Bologna Station, with a depth of approximately 8 m with respect to the current ground level. GEOLOGY The line tunnels planned in this project have a depth ranging between and meters below ground level. The geological formations involved in the excavation, from the top downwards, and under the surface layers consisting of fill, are as follows:

2 - Alluvioni recenti (Late Pleistocene Holocene) - these are recent sediments of the Aniene River and its tributaries, with soil consisting of plastic clayey silt, interlayered with sandy silt. - Tuff coverage (Middle Pleistocene) - related to the volcanic activity of the Sabatini Hills and the Alban Hills; brown pyroclastic formations interlayered with volcanic ash. - Paleo-Tevere sediments (Middle Pleistocene) - these formations include a series of heterogeneous deposits deriving from different phases, subdivided as follows: a) Upper layer: consisting of silty clays and/or clayey silt interlayered with sand; b) Lower layer: consisting fluvial and lake sediments, mainly clay and clayey silt; c) Base layer: characterized by limestone gravel embedded in very fine sand and/or sandy silt. The underlying layer of these formations, which will not in any case be involved by the works, consists of Pleistocene clay and gray-blue clayey silt formations. The distribution of the soil types involved in the excavation of the line tunnels is shown in Table. Table. Soil formations Formations Frequency % Even Track Frequency % Odd Track Frequency % Total Alluvioni Recenti - AR 6.%.6%.6% Pyroclastic P - 7.%.% Upper layer Paleo-Tevere formations PS - Lower layer Paleo-Tevere formations PL - Base layer Paleo-Tevere formations PG - 8.9%.%.%.8%.% 7.7%.9%.%.7% From the hydrogeological point of vista, the layer types described above are characterized by the constant presence of groundwater, with permeability recorded between. E- cm/sec and. E- cm/sec, as shown in detail in Table. Table. Permeability index Formations Permeability cm/sec Alluvioni Recenti - AR. E- Pyroclast P -. E-. E- Upper layer Paleo-Tevere formations 7. E- PS - Lower layer Paleo-Tevere formations.6 E- PL - Base layer Paleo-Tevere formations. E- PG- ANALYSIS OF THE MAIN EXCAVATION PARAMETERS Because of the requirements of planning and design related not only to the tunnel excavation operations, but also the ones for the execution of the preliminary works, especially the construction of the various parts of the Conca d Oro Station, the excavation of the two tunnels (Even Track tunnel & Odd Track tunnel) along the stretch between Conca d Oro-Gondar, was conducted on an alternating and not continuous basis with the two TBMs. This

3 was to avoid creating interference between the two excavation faces. The non-continuity of advance has undoubtedly influenced the excavation parameters, and therefore in order to ensure a more correct interpretation, the data for the sections of between Gondar Station and Bologna Station (total length, m), where, however, excavation was conducted on a continuous basis. TBM excavation parameters In general, as shown in Table, as well as in Figures and, the machine parameters recorded during excavation were always under the alert levels assigned to this type of TBM. Table. TBM Excavation Parameters Even Track Tunnel Parameters Averag Max. e value value Min. value Odd Track Tunnel Parameters Average value Max. value Min. value Excavation time (min) 9 Excavation time (min) Penetration speed 6 7 Penetration speed (mm/min) (mm/min) Total Thrust (KN) Total Thrust (KN) 9 6 Active Articulation 68 Active Articulation Thrust (KN) Thrust (KN) Cuttinghead RPM (rpm).9.. Cuttinghead RPM.9.. (rpm) Torque (KN*m) 7 Torque (KN*m) METROPOLITANA DI ROMA - LINEA B - Galleria Binario Pari TBM Boring Parameters Articulation Thrust (KN) Cuttinghead RPM (rpm) Total Thrust (KN) Torque (MN*m), Total thrust (KN) / Articulation Thrust (KN),,, Torque (MN*m) / Cuttinghead RPM (rpm), Anello (n ) Figure. Even track TBM Boring Parameters

4 METROPOLITANA DI ROMA - LINEA B - Galleria Binario Dispari TBM Boring Parameters Articulation Thrust (KN) Cuttinghead rpm (rpm) Total Thrust (KN) Torque (MN*m), Parametri di scavo TBM,,,, Torque (MN*m) / Cuttinghead RPM (rpm) Figure. Odd track TBM Boring Parameters METROPOLITANA DI ROMA - LINEA B - Galleria Binario Pari Correlation Torque vs EPB Pressure top and TBM Penetration speed Penetration speed (mm/min) Torque (MN*m) EPB Pressure top (bar) 8, 7 Penetration speed (mm/min) 6,,, Torque (MN*m) / EPB Pressure Top (bar), Alluvioni Formazione Del Paleotevere Piroclastiti Recenti (AR) Figure. Even Track Correlation Torque vs EPB Pressure top and TBM Penetration speed

5 METROPOLITANA DI ROMA - LINEA B - Galleria Binario Dispari Correlation Torque vs EPB Pressure top and TBM Penetration speed Penetration speed (mm/min) Torque (MN*m) EPB Pressure top (bar) Penetration speed (mm/min) Alluvioni Recenti Formazione del Paleotevere,,,,, Torque (MN*m) / EPB Pressure top (bar) Figure. Odd Track Correlation Torque vs EPB Pressure top and TBM Penetration speed Figure. Torque of the cutterhead for both tracks

6 Furthermore, the analysis of Figures and shows that the cuttinghead torque of the TBM has been more significantly affected by the geological and geotechnical characteristics of the soil excavated than by the variations of EPB pressure rates recorded during the excavation. In particular, in both tunnels this parameter showed an increase in the most relevant levels, i.e. the clays normal consolidated or overconsolidated in the Formation of Alluvioni Recenti deposits, and cemented gravels in the Paleo-Tevere Formation, thus leading to a decrease of the TBM speed of penetration. Nevertheless, as shown in Figure, during the excavation operations by both the TBMs, the torque values recorded in relation to the rotation speed of the cutterhead were well within the range set by the maximum allowable values for this type of TBM. Injection pressures and volumes of the backfilling grout The ring space between the excavation section and the extrados of the lining segments was backfilled with bicomponent mortar, injected during the excavation phase by special nozzles positioned on the shield tail. The bi-component system consists in the injection of a grout (component A) and of a setting accelerator (component B) through separate lines to the rear part shield tail where, using nozzles, the grout is sprayed with component B, triggering the reaction behind the shield. Lab tests, confirmed by tests conducted on site, have shown that gelling time is approximately 8- seconds. Table. Backfilling grout volume injected Even Track Tunnel Odd Track Tunnel Parameters Volume (mc) Ring N# Parameters Volume (mc) Ring N# Maximum value Maximum value Minimum value.8 69 Minimum value.9 97 Average value. Average value. Theoretical value, Theoretical value, Table. Backfilling grout Injection pressure Even Track Tunnel Odd Track Tunnel Parameters Pressure (bar) Ring N# Parameters Pressure (bar) Ring N# Maximum value. 7 Maximum value.67 9 Minimum value.8 Minimum value. 7 Average value. Average value.98 From the data shown in tables and and figures 6 and 7 herebelow, we can conclude the following: for both tunnels, the same average value of the volume of backfilling grout injected was observed, (. m ), virtually corresponding to the theoretical value (. m ), considering that the difference between the two is.%. This can be considered in line with and correlated to the values of weigh extracted during the excavation phases, which remain constantly under the theoretical ones indicated in the design guidelines. During the excavation of the Even Track Tunnel and the Odd Track, the injection pressures were constantly above of the ones forecast by the design in the various sectors of tunnel, with positive results regarding the backfill of the ring gap behind the lining, and therefore regarding the surface settlements triggered by the excavation.

7 8 METROPOLITANA DI ROMA - LINEA B - Galleria BinarioPari Backfilling Grout and Pressure Analisys Volume (mc) Injection Pressure (bar) 7 6 Volume (mc) / Injection Pressure (bar) Figure 6. Even Track Backfilling Grout and Pressure Analysis 8 METROPOLITANA DI ROMA - LINEA B - Galleria Binario Dispari Backfilling Grout Volume & Pressure Analysis Volume (mc) Injection Pressure (bar) 7 Volume (mc) / Injection Pressure (bar) Figure 7. Odd Track Backfilling Grout and Pressure Analysis This seems to be confirmed in Figures 8 and 9, showing, by way of example, on the Gondar-Annibaliano stretch, the surface settlements recorded on the tunnel axis, both with the support pressure of the face, and with the of injection pressure of the backfilling grout. The graphs, in fact, not only show a correlation between the parameters considered, but also a direct effect of the support pressure of the face and, above all, of the injection pressure of the backfilling grout on surface settlements, thus confirming, among other things, a marked lack of concern and an evident response in the terrain affected by the excavation operations.

8 METROPOLITANA DI ROMA - LINEA B - Galleria BinarioPari EPB pressure and Injection pressure vs Surface settlements Surface settlements (mm) Injection Pressure (bar) EPB Pressure top (bar) Surface settlements 8mm) ,,,,, EPB Pressure top (bar) / backfilling grout Injection pressure (bar) Figure 8. Even Track EPB pressure and injection pressure vs surface settlements METROPOLITANA DI ROMA - LINEA B - Galleria Binario Dispari EPB pressure and Injection pressure vs Surface settlements Surface settlements (mm) Injection Pressure (bar) EPB Pressure top (bar) Volume (mc) / Injection Pressure (bar) EPB Pressure top (bar) / backfilling grout pressure (bar) Figure 9. Odd Track EPB pressure and injection pressure vs surface settlements Muck weight Considering that the system of muck removal, using continuous conveyor belts, does not provide for measurement of the volume removed, the monitoring of its weight is a fundamental aspect in excavation management. Therefore, two scales were installed on the main conveyor belt on the TBM back up, allowing the continuous monitoring of the weight of the material removed during excavation. The theoretical value of the weight of material to be removed for each thrust, indicated in the guidelines, is purely indicative, since it was obtained with a rate for undisturbed (and unconditioned) terrain of.8 t/m;

9 therefore, without considering the volume of liquid injected and necessary for conditioning. Analysis of the data shows that in both tunnels the values recorded during excavation are lower than those indicated by the designer. Table 6. Muck Weight and Liquid Injected Even Track Tunnel Odd Track Tunnel Parameters Muck weight (ton) Liquid injected (ton) Parameters Muck weight (ton) Liquid injected (ton) Average value 76.. Average value Theoretical 9. Theoretical 9. value value Likewise, Figures and show that in the stretches of tunnel excavated in the Paleo-Tevere Formation, the values of the weights excavated are lower compared to those recorded in the clays of Alluvioni Recenti sediments. This aspect confirms the greater compacting effect of the Paleo-Tevere soils, due to the thrust and the pressure produced during excavation on the face by the TBM, and can be correlated with the partial reduction of the ring gap and thus the volume of backfilling grout injected. In particular, if we consider the weights of the material extracted, minus the liquids injected during the excavation and necessary for the conditioning, we obtain average values corresponding to 79% of the theoretical value for the Even Track Tunnel and 76.% for the Odd Track Tunnel. METROPOLITANA DI ROMA - LINEA B - Galleria Binario Pari Muck Weight and Liquid injected Analisys Muck w eight (ton) Real Muck w eight (ton) Total w eight w it liquid (ton) EPB Pressure top (bar), Muck Weight (ton) 9 8,, EPB pressure top (bar) 7 6, Ring n Figure. Even Track Muck weight and liquid injected Analysis

10 METROPOLITANA DI ROMA - LINEA B - Galleria Binario Dispari Muck Weight and Liquid Injected Analisys Muck w eight (ton) Real Muck weight (ton) Total Weight with Liquid (ton) EPB Pressure top (bar), Muck Weight (ton) 8 6,, EPB pressure top (bar), Ring n Figure. Odd Track Muck weight and liquid injected Analysis Conditioning of the soil In the tunnels analyzed, the conditioning proved to be more difficult in the stretches characterized by the presence of silty clays of the recent alluvial sediments, compared to the situation in the gravels and sandy silts of the Paleo Tevere Formation. This aspect is above all due to the different permeability of the single formations excavated. The low permeability of Alluvioni Recenti sediments has led to a number of problems in achieving the optimal physical characteristics of the material conditioned (e.g.: high cohesion, low permeability and a consistency bordering between liquid and plastic) necessary to guarantee the even distribution of the ground pressure on the excavation face, and at the same time avoiding flooding of the excavation chamber. Table 7. Soil Conditioning Parameters Even Track Tunnel Alluvioni Recenti Formation Paleo-Tevere Formation Pyroclastic deposits Parameters UM Value Parameters UM Value Parameters UM Value TA Concentration %. TA %. TA Concentration %.8 Concentration FER FER. FER FIR % FIR % FIR % Surfactant Volume lt/ring 9 Surfactant Volume lt/ring 6 Surfactant Volume lt/ring Table 8. Soil Conditioning Parameters Odd Track Tunnel Alluvioni Recenti Formation Paleo-Tevere Formation Parameters UM Value Parameters UM Value TA Concentration %. TA %. Concentration FER. FER. FIR % 7 FIR % Surfactant Volume lt/ring 9 Surfactant Volume lt/ring

11 Therefore, from an operational point of view, this situation has required a type of conditioning characterized by: - injection of foam exclusively through the dedicated lines on the cutterhead, in order to ensure entry of already conditioned material in the excavation chamber; - rather low FER values ( ), aimed obtaining a rather thick and stable foam, and above all to prevent the formation of air in the excavation chamber, also improving the maintenance of support pressure on the excavation face (see Figures and ); - a degree of conditioning that is not too high (FIR: -), with no excessive injection of foam, in order to prevent its separation from the material (see Figures and ); - conducting excavation without injecting water on the face and/or in the chamber together with the foam, in order to prevent phenomena of separation between the liquid phase and the material excavated, which would have led to the worsening of its homogeneity inside the chamber. This aspect is enhanced in the clay soils of the Alluvioni Recenti sediments; - conditioning obtained without the use of polymers added to the foam; - use of higher volumes of foam through the nozzles located in the central part of the cutterhead, in order to keep the central load windows free, limiting possible blockage caused by the excavation material. In some cases, the occurrence of this phenomenon led to an overall worsening of the TBM excavation parameters, with considerable increases both in the thrust rates and in torque, and consequently the reduction of the penetration speed, making maintenance measures necessary, executed in hyperbaric conditions, for the removal and cleaning of the windows. 9 METROPOLITANA DI ROMA - LINEA B - Galleria Binario Pari Soil Conditioning Parameters FIR ( % ) FER Liquid Injected (mc) FIR ( % ) 8 6 FER / Liquid Injected Figure. Even Track Soil conditioning Parameters

12 9 METROPOLITANA DI ROMA - LINEA B - Galleria Binario Dispari Soil Conditioning Parameters FIR ( % ) FER Liquid injected (mc) FIR ( % ) FER / Liquid injected (mc) Figure. Odd Track Soil conditioning Parameters

13 Figure. Alluvioni Recenti sediments (clayey silts) the surface TBM belt exit of the screw Figure. PaleoTevere (gravel) inside the on muck pit CRITICAL ASPECTS IN THE ROUTE The route of the tunnels examined here has some zones that should be considered as especially critical; these are the stretch crossing under the Rome-Florence railway line and under the foundations of the Ponte delle Valli bridge and of some important historical buildings. The stretch under the Ponte delle Valli was especially difficult due to the direct interference between the Odd Track Tunnel (at a higher level) and the piles foundation of the bridge. The design phase provided for consolidation measures for the distribution of the loads on the piles interfering with the excavation face both depth-wise and far from the tunnel, as well as improving the geotechnical characteristics of the soil. Furthermore, an operational procedure was defined and implemented for stopping the advance of the TBM at the points where the foundation piles were encountered at the face, with intervention in hyperbaric conditions to remove the metal reinforcement of the piles that could have damaged the cutterhead. The inspections described above showed the presence of concrete foundation piles at the excavation face level but without metallic reinforcement, so that boring could be conducted without special measures. Nevertheless, the effectiveness of the consolidation was confirmed during excavation by the variation of some parameters, such as: - the increase of the thrust by 7% (from 8 MN to MN); - the reduction of the speed of advance by % (from mm/min to 9 mm/min); - the increase of torque by 8% ( from. MN*m to. MN*m). Furthermore, the cement and chemical mixtures present in the soil after consolidation underwent reactivation due to the conditioning used during excavation, thus producing a gradual obstruction of the TBM windows (as confirmed by subsequent inspections under pressurized conditions) and thus contributing to the worsening of excavation performance rates. Therefore, while on the one hand the improvement of the geotechnical conditions of the soil after treatment helped to guarantee practically zero surface settlements, on the other it definitely had a negative impact on the excavation operations. In the final part of the two tunnels, the TBMs crossed under a series of buildings having considerable historical and architectural importance, with serious interference occurring due to the very shallow layer between the top of the excavation and the base level of the foundations (from 8 to m). Because of this problem, compensation grouting was conducted in these areas, involving injections of pre-treatment and pre-compensation cement mixtures, executed before boring by the TBMs, and aimed to improve the geotechnical characteristics of the soil above the tunnel chambers, and to provide pre-lifting to be exploited during the excavation phase (from to mm). In order to jointly monitor the effects of the excavation and the Compensation Grouting operations on the soil and the structures, a complex monitoring system consisting of livelometer gauges was installed on the buildings involved, associated with a series of tiltmeters, together with software for data recording and processing, and an automatic system (with optical and acoustic devices) to monitor the threshold values.

14 FAB 9 Tazze livellometriche Quota [mm] TL9 TL9 TL9 TL9 TL9 TL96 TL97 TL98 TL99 TL9 TL9 TL9 6/7/ /7/ /8/ /8/ /8/ /8/ /8/ /8/ /9/ 9/9/ /9/ 9/9/ Figure 6. Even Track - Hydraulic monitoring cell system FAB Tazze livellometriche TL /7/ /7/ /8/ /8/ /8/ 9/9/ 9/9/ Quota [mm] TL TL TL TL TL6 TL7 TL8 TL9 TL TL TL Figure 7. Odd Track - Hydraulic monitoring cell system As we can see in Figures 6 and 7, the operation of the TBMs caused very slight subsidence in the buildings, much lower than the alert threshold set in the design phase. In particular, the absolute value of the settlements caused after excavation was less than or equal to the lifting induced in the building during the pretreatment and pre-compensation phases conducted before the excavation in these areas. The results obtained have therefore ensured the safety of the buildings involved, and the conditions required for undertaking compensation measures were not reached. All this is closely connected with the regular execution of the excavation, which was conducted maintaining the supporting pressure on the face constantly near to or equal to the maximum value set by the designer in the guidelines, and characterized by the constantly lower levels of material removed as well as the complete backfilling of the ring gap with backfilling grout. SURFACE SETTLEMENTS

15 As we can see in Figures 8 and 9, the above conclusions are confirmed by topographical monitoring on the surface, indicating slight settlement rates along the tunnels, and in any case lower than the design thresholds. These can also be correlated with the slight losses of volume, at between.% and.% (max loss of design volume:.6%). In particular, the settlements and the consequent losses of volume may be considered as insignificant also for the excavations under the areas previously defined as critical, providing positive responses on the methodology and conducting of the excavation operations. METROPOLITANA DI ROMA LINEA B Galleria Binario Pari Surface settlements in tunnel axis Gondar Annibaliano Surface settlemens (mm) ANNIBALIANO Ring n Figure 8. Even Track Surface settlements in tunnel axis Gondar - Annibaliano METROPOLITANA DI ROMA LINEA B Galleria Binario Dispari Surface settlements in axis tunnel Gondar Annibaliano Surface settlements (mm) ANNIBALIANO Figure 9. Odd Track Surface settlements in tunnel axis Gondar - Annibaliano

16 As already stated previously, the excavation was, in fact, conducted maintaining the support pressures on the excavation face and backfilling inject pressures behind the lining near or corresponding to the maximum values indicated in the design for the various stretches of tunnel. Considering the highly sensitive nature of the soils excavated, and the rather unfavorable geotechnical characteristics, the result has been the limitation of the volume lost on the front, and in the section deriving from the convergence of the excavation and the TBM', and subsequently, with the final lining ring, reducing surface settlements to very low levels, and providing positive results to the degree of backfilling of the ring gap behind the tunnel lining.