Mechanized tunnelling in soft soil. General aspects & history of the machine’s development
The first concept for a shield was developed in 1806 by Sir Marc Brunel wherein soil was excavated with a screw. In the coming years, several shield concepts were developed and the introduction of compressed air to support the face. With these developments also came obstacles such as ground loss (settlement), blow outs, safety and human factors.
In the 1980-90s different designs were developed and used and Tunnel Boring Machines (TBM) became an accepted means of soft ground excavation. A variety of types of TBMs could tunnel through a number of ground conditions ranging from clay, silt, sand and gravel, in dry conditions and under the water table. Ground conditioning systems were developed to meet the requirements of a projects and closing the gap between EPB and Slurry type TBMs.
Further innovation on TBMs is required for their application on new project challenges, as well as a commercial environment which encourages it.
Face pressure design in mechanized tunnelling
The lecture will introduce a simple computational method as well as design diagrams for the estimation of the necessary face support pressure in closed shield tunnelling. Furthermore, it shall improve understanding of the mechanics of failures occurring sometimes under extremely unfavourable geological conditions. For slurry shields attention will be paid to the time-dependent effects associated with the infiltration of slurry into the ground ahead of the face. For EPB-shields the effect of seepage flow, which may occur in the case of a coarse-grained ground, will be discussed.
Slurry Shield tunnelling design
The slurry shield has its field of application in unstable soft ground combined with most demanding urban conditions. In this lecture, the technological details will be explained and the development of slurry shields will be highlighted up to recent record-breaking projects. To enable full understanding of the concept of slurry shield tunnelling, the mechanism of slurry face support, slurry transport of the cuttings and last but not least the principles functioning and operation of slurry treatment plants will be shown. The third part of the lecture will deal with important design aspects related to typical risks such as sticky clay, wear, very coarse ground and ground with high fines content.
EPB Tunnelling design. Investigations, soil conditioning and backfilling technology
The lecture will be developed with the following items:
- Description of the machine: main elements and particular aspects regarding the EPB
- Soil conditioning as a key point for a correct machine use (choesionless soil, clay, rock masses)
- Investigation to be carried out for a correct design of and EPB tunnel.
- Problems that can occur during and EPB excavation.
- Monitoring the machine parameters.
- Backfilling technology and application. Example of Rome metro.
- Description of some relevant job sites case histories.
- Presentation of the largest EPB in the world and discussion on the critical aspects that have to be faced when a large diameter machine has be used.
Segment lining design,
Tunnel lining with segmental rings behind TBMs are reinforced concrete elements and must be calculated and designed according to the standards of reinforced con-crete constructions. In tunnelling, however, specific circumstances must be taken in account, which make design much more complicated:
The determination of loads during ring erection, advance of the TBM, earth pres-sure and bedding of the articulated ring is difficult. The ring model and the design input values must be studied carefully according to the parameters of the sur-rounding soil.
The segmental ring design is a complex planning. After the determination of the necessary clearance and geometry, the designer must fit the ring to the TBM ge-ometry and the transport options. The interaction between TBM and segmental ring must not be disregarded.
The connection possibilities between the segments need to be studied together with sealing gaskets and intermediate plates.
The longitudinal joints between the segments of a ring react as articulations. Be-cause of the high normal forces, bending moments can be transverse over the joints, but the stability of the ring depends on bedding reactions. The bedding pa-rameters are calculated from the Young’s modulus of the surrounding soil. Even if powerful calculation methods exist, e.g. FEM-calculations, the determination of correct bedding parameters may be difficult.
Geological & geotechnical issues related to the design of ‘Martina’, the world’s largest EPB TBM (15.62 m in diameter)
The lesson describes the geological and geotechnical assessments performed to define
the TBM to employ for the excavation of the “Sparvo Tunnel”, which has an exceptional
outer diameter of 15.62 m.; the tunnel lies on the A1 Milan-Naples motorway in the section between La Quercia (Bologna) and Barberino di Mugello (Florence).
The technical characteristics and operating parameters are identified on which the design
of the machine were based, with particular attention paid to the most critical geotechnical context consisting of the “Argille a Palombini” formation. This possesses poor strength and deformation parameters and, with overburdens of 100-120 m., “squeezing” phenomena are highly probable, which could cause serious problems to the advance of the machine. It was therefore necessary to perform keen assessments in order to determine the characteristics of the machine (geometry, maximum thrust, pressure at the face, etc.).
Metro Roma case history. Settlement study and analysis of stability of buildings
A brief introduction of the two following lectures about Rome’s Metro case history, with some general information concerning the features of the on-going Development Program of the Network, the constraints and the challenges to be faced, the design approach, the management of the construction process and some specific technologies carried out.
Evaluating the effects of tunnelling on historical buildings: the example of a new subway in Rome
In this lecture the approaches adopted for evaluating the effects of tunnelling on the historical buildings located in the centre of Rome are discussed. Specifically, reference is made to about 35 masonry buildings of relevant historical value and built in between the 15th and the 19th centuries that are encountered in lot T2 of the new metro line C of Rome underground.
The study of the effects induced by tunnelling on these buildings was carried out following procedures of increasing complexity. At a first stage simplified analyses, based on empirical procedures, were performed assuming green field conditions that are neglecting the presence of the buildings. At a second stage, the interaction between the tunnels and the historical buildings was studied through numerical FE analyses in which the building characteristics were included using an equivalent solid entirely embedded into the soil. These analyses were carried out under both 2D and 3D conditions. In the numerical analyses, account was taken of long term effects that may develop when tunnelling is carried out in fine-grained soils. In these conditions
the tunnel may in fact act as a drain thus inducing a progressive reduction of pore water pressure with associated consolidation settlements.
The main steps of the adopted procedure are illustrated in the lecture focusing on the results obtained for Palazzo Grazioli that is located close to Piazza Venezia.
Predicted and observed settlements induced on two old masonry buildings by the Metro C tunnels construction, in Rome
The lecture reports the main results obtained in a numerical study aimed to predict the effects induced on two old masonry building by the tunneling operations for the construction of Metro C line, in Rome. After the description of the buildings, the tunnels and the geotechnical context, the relevant aspects of the 3D finite elements numerical analyses carried out are illustrated. The most important features simulated are the advancement of the tunnel front, the pressure to support the front, the TBM geometry (conicity of shield), the tail void grouting and the building structure. The lecture outlines the relevance of preliminary analyses carried out using mesh of different density and tolerated error, in order to optimize the complete analyses, that is achieving an acceptable compromise between calculation time and accuracy of the results. The reinforcement of the foundations by micropiles, adopted as a passive protection system, for one of the two buildings is illustrated. Finally, the numerical predictions are compared to the field data obtained by the monitoring system during the tunnels construction.
Bologna underground railway by-pass case history. Description of the work and use of compensation grouting
The Bologna underground railway project was part of a new high speed rail link under construction between Turin, Milan, Rome, Naples and Salerno; the so named "national subway" has recently started operations while works are almost completed for the new Bologna underground station and pass and have just started for the new Florence underground station and pass.
This will be described.
A section in Bologna passes beneath a number of railway bridges and follows the alignment of the existing railway. The most important of these bridges is a 112 m long and 11 m wide masonry viaduct, with 8 – 10 m high embankments at each end. The new high speed rail twin tunnels of diameter 9.1 m were constructed parallel to the viaduct and directly beneath its alignment. The ground generally comprises Made Ground up to 8 m thick over a substantial depth of alluvial deposits, which are predominantly dense gravelly sand or very sandy gravels.
Construction of the twin tunnels, using EPB tunnelling machines, was expected to generate large settlements, typically around 20 mm but potentially up to 50 mm for volume losses of 1%. Such settlements would have induced excessive distortions of the viaduct, which was a cause of concern, particularly as suspension of train services was not permitted. There were also major concerns about potential cracking of the masonry arches, some of which were already cracked. Compensation grouting was therefore implemented during tunnelling, with field trials to ascertain its effectiveness before the EPB machines approached the viaduct. This will be described.
In summary, the compensation grouting achieved a high degree of control of settlement effects induced by the tunnelling. The sensitive masonry viaduct only experienced small and acceptable levels of distortion, and the existing rail services continued without interruption. The compensation grouting demonstrated (a) the innovative use of directional drilling to install curved grout tubes and (b) the successful application of compensation grouting to granular soils, for which there has been generally less experience in comparison to clay soils. Directional drilling is of practical importance for projects in congested urban areas where it might not be possible to construct shafts while curved TAMs drilled from ground level may be feasible.
Risk management and real time monitoring of settlements. Application to relevant case histories
To assure optimum safety and reliable (time & cost) tunnelling in an urban area, a comprehensive real-time monitoring program fully integrated with a Risk Management Program (RMP) is required. RMP provides a flexible design method to predict and identify the relative residual risks and counter-measure while facing the actual construction behaviour.
The monitoring program for tunneling should assess the risk damage for buildings, that consists mainly of two groups:
- Building Condition Survey(BCS) to check the real conditions of buildings prior, during, and after tunnel construction, and
- Building Risk Assessment (BRA) to estimate the potentially expected damages on the basis of settlement predictions and the intrinsic vulnerability of the structures through Vulnerability Index (VI)
Since the main concern in mechanized tunneling, in urban and shallow condition is associated with the face stability, so face control and excavation volume are of by far the most important issue. The key TBM parameters such as earth pressure at the face, volume of grout behind segmental lining and weight of muck excavated are to be continuously monitored.
PAT (Plan for Advance of Tunnel) which is a integrated tool updating design and construction parameters makes it possible to fulfil a real-time monitoring system (GDMS), a software based on web technology and incorporating a GIS (“geographical information system” in which all data are referenced to their position as well as to time). Monitoring data stored by GDMS were used in critical circumstances and to propose a proper counter-measure.
Finally, the important role of Risk management Program will be, in turn, described trough some recent cases-histories