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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 B1 is a branch of the existing Line B, and the section currently under construction is characterized by the excavation of 2 tunnels (with a total length of approximately 6 km) and 3 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 2008. 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 6,050m and is characterized by a maximum slope of 4%, numerous horizontal curves (with minimum radius of 130m) and a low planimetric and altimetric wheelbase, between 8.70m and 11.30 m, at the station entry/exit areas.

In particular, the planimetric and altimetric profile involves reaching the maximum depth (40 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 15 and 40 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:

- “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 2” 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 1.

Table 1. Soil formations

Formations Frequency % Even Track

Frequency % Odd Track

Frequency % Total

Alluvioni Recenti - AR 16.3% 30.6% 23.6%

Pyroclastic – P - 7.1% 3.5%

Upper layer “Paleo-Tevere 2 formations” – P2S -

18.9% 24.0% 21.5%

Lower layer “Paleo-Tevere2 formations” – P2L -

11.8% 23.4% 17.7%

Base layer “Paleo-Tevere 2 formations” – P2G -

45.9% 22.0% 33.7%

From the hydrogeological point of vista, the layer types described above are characterized by the constant

presence of groundwater, with permeability recorded between 1.0 E-05 cm/sec and 3.0 E-02 cm/sec, as shown in detail in Table 2.

Table 2. Permeability index Formations Permeability

cm/sec Alluvioni Recenti - AR 1.0 E-05 Pyroclast – P - 1.0 E-04

1.0 E-03 Upper layer “Paleo-Tevere 2formations” – P2S -

7.3 E-03

Lower layer “Paleo-Tevere 2formations” – P2L -

5.6 E-04

Base layer “Paleo-Tevere 2formations” – P2G-

3.0 E-02

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

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 4,120 m), where, however, excavation was conducted on a continuous basis.

TBM excavation parameters In general, as shown in Table 3, as well as in Figures 1 and 2, the machine parameters recorded during

excavation were always under the alert levels assigned to this type of TBM. Table 3. TBM Excavation Parameters Even Track Tunnel Odd Track Tunnel Parameters Averag

e value Max. value

Min. value

Parameters Average value

Max. value

Min. value

Excavation time (min) 40 100 19 Excavation time (min)

38 86 19

Penetration speed (mm/min)

36 72 11 Penetration speed (mm/min)

39 72 16

Total Thrust (KN) 18777 36456 9407 Total Thrust (KN) 15302 34329 5650 Active Articulation Thrust (KN)

1222 2100 648 Active Articulation Thrust (KN)

1018 1921 608

Cuttinghead RPM (rpm) 1.90 2.30 1.20 Cuttinghead RPM (rpm)

1.90 2.40 1.00

Torque (KN*m) 2710 4200 1130 Torque (KN*m) 2280 3910 960

Figure 1. Even track TBM Boring Parameters

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TBM Boring Parameters

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Anello (n°)

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/ Cu

ttin

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ead

RP

M

(rp

m)

Articulation Thrust (KN) Total Thrust (KN)

Cuttinghead RPM (rpm) Torque (MN*m)

Figure 2. Odd track TBM Boring Parameters

Figure 3. Even Track – Correlation Torque vs EPB Pressure top and TBM Penetration speed

METROPOLITANA DI ROMA - LINEA B1 -Galleria Binario DispariTBM Boring Parameters

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Correlation Torque vs EPB Pressure top and TBM Penetration speed

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op

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Alluvioni Formazione Del Paleotevere 2 Piroclastiti Recenti (AR)

Figure 4. Odd Track – Correlation Torque vs EPB Pressure top and TBM Penetration speed

Figure 5. Torque of the cutterhead for both tracks

METROPOLITANA DI ROMA - LINEA B1 -Galleria Binario Dispari

Correlation Torque vs EPB Pressure top and TBM Penetration speed

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Penetration speed (mm/min)Torque (MN*m)EPB Pressure top (bar)

Alluvioni Recenti Formazione del Paleotevere 2

Furthermore, the analysis of Figures 3 and 4 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 2 Formation, thus leading to a decrease of the TBM speed of penetration.

Nevertheless, as shown in Figure 5, 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 bi-

component 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 8-10 seconds. Table 4. Backfilling grout volume injected Even Track Tunnel Odd Track Tunnel Parameters Volume (mc) Ring N# Parameters Volume (mc) Ring N# Maximum value 7.5 1268 Maximum value 7.9 1163 Minimum value 2.8 1609 Minimum value 2.9 1971 Average value 4.1 Average value 4.1 Theoretical value 4,23 Theoretical value 4,23 Table 5. Backfilling grout Injection pressure Even Track Tunnel Odd Track Tunnel Parameters Pressure (bar) Ring N# Parameters Pressure (bar) Ring N# Maximum value 5.23 1370 Maximum value 4.67 945 Minimum value 1.18 2015 Minimum value 1.30 2273 Average value 3.12 Average value 2.98

From the data shown in tables 4 and 5 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, (4.10 m3), virtually corresponding to the theoretical value (4.23 m3), considering that the difference between the two is 3.10%.

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.

Figure 6. Even Track – Backfilling Grout and Pressure Analysis

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.

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Backfilling Grout and Pressure Analisys

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Backfilling Grout Volume & Pressure Analysis

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Figure 8. Even Track – EPB pressure and injection pressure vs surface settlements

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 1.8 t/m3;

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EPB pressure and Injection pressure vs Surface settlements

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nts

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EPB pressure and Injection pressure vs Surface settlements

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Surface settlements (mm)Injection Pressure (bar)EPB Pressure top (bar)

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 76.0 3.4 Average value 73.8 3.6

Theoretical value

92.1 Theoretical value

92.1

Likewise, Figures 10 and 11 show that in the stretches of tunnel excavated in the Paleo-Tevere 2 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 2 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 76.2% for the Odd Track Tunnel.

Figure 10. Even Track – Muck weight and liquid injected Analysis


Muck Weight and Liquid injected Analisys

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ck

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ht

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Figure 11. 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 2 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-Tevere2 Formation Pyroclastic deposits Parameters UM Value Parameters UM Value Parameters UM Value TA Concentration % 1.3 TA

Concentration % 1.5 TA Concentration % 1.8

FER 3 FER 3.4 FER 2 FIR % 20 FIR % 21 FIR % 32 Surfactant Volume lt/ring 49 Surfactant Volume lt/ring 56 Surfactant Volume lt/ring 100

Table 8. Soil Conditioning Parameters Odd Track Tunnel Alluvioni Recenti Formation Paleo-Tevere2 Formation Parameters UM Value Parameters UM Value TA Concentration % 1.5 TA

Concentration % 1.5

FER 3.1 FER 3.0 FIR % 27 FIR % 20 Surfactant Volume lt/ring 59 Surfactant Volume lt/ring 45


Muck Weight and Liquid Injected Analisys

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ck

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t (t

on

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Therefore, from an operational point of view, this situation has required a type of conditioning characterized by:

- injection of foam exclusively through the 5 dedicated lines on the cutterhead, in order to ensure entry of already conditioned material in the excavation chamber;

- rather low FER values (3 – 4), 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 12 and 13);

- a degree of conditioning that is not too high (FIR: 20-35), with no excessive injection of foam, in order to prevent its separation from the material (see Figures 12 and 13);

- 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.

Figure 12. Even Track – Soil conditioning Parameters


Soil Conditioning Parameters

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FIR

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Figure 13. Odd Track – Soil conditioning Parameters


Soil Conditioning Parameters

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Figure 14. Alluvioni Recenti sediments (clayey silts) Figure 15. PaleoTevere 2 (gravel) inside the on the surface TBM belt exit of the screw 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 75% (from 18 MN to 31 MN); - the reduction of the speed of advance by 50% (from 35 mm/min to 19 mm/min); - the increase of torque by 80% ( from 2.4 MN*m to 4.3 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 15 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 3 to 5 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.

Figure 16. Even Track - Hydraulic monitoring cell system

Figure 17. Odd Track - Hydraulic monitoring cell system

As we can see in Figures 16 and 17, 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 pre-treatment 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

FAB 9 ‐ Tazze livellometriche

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Quota [mm]

TL901

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FAB 5 ‐ Tazze livellometriche

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Quota [mm]

TL501

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As we can see in Figures 18 and 19, 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 0.15% and 0.40% (max loss of design volume: 0.60%).

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.

Figure 18. Even Track – Surface settlements in tunnel axis Gondar - Annibaliano

Figure 19. Odd Track – Surface settlements in tunnel axis Gondar - Annibaliano

METROPOLITANA DI ROMA ‐ LINEA B1 ‐

Galleria Binario Pari

Surface settlements in tunnel axis Gondar‐Annibaliano

‐20

‐15

‐10

‐5

0

5

875

885

895

905

915

925

935

945

955

965

975

985

995

1005

1015

1025

1035

1045

1055

1065

1075

1085

1095

1105

1115

1125

1135

1145

1155

1165

1175

1185

1195

1205

1215

1225

1235

1245

1255

1265

1275

1285

1295

1305

1315

1325

1335

1345

Ring n°

Surface settlemens (m

m)

ANNIBALIANO

METROPOLITANA DI ROMA ‐ LINEA B1 ‐

Galleria Binario Dispari

Surface settlements in axis tunnel Gondar‐Annibaliano

‐20

‐15

‐10

‐5

0

5886

896

906

916

926

936

946

956

966

976

986

996

1006

1016

1026

1036

1046

1056

1066

1076

1086

1096

1106

1116

1126

1136

1146

1156

1166

1176

1186

1196

1206

1216

1226

1236

1246

1256

1266

1276

1286

1296

1306

1316

1326

1336

1346

1356

1366

1376

1386

Surface settlements (mm)

ANNIBALIANO

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.

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