Earth Pressure Balance Machines Research Articles

A new approach for estimating the transverse surface settlement curve for twin tunnels in shallow and soft soils

Increasing demand on infrastructures has led to increased attention to shallow soft ground tunneling methods in urbanized areas. Especially in metro tunnel excavations, it is important to control the surface settlements which are observed before and after excavation, which may cause damage to surface structures. Unlike motorway, sewage and other infrastructure tunnels, metro tunnels generally have to be excavated as twin tunnels and must have a larger diameter. Metro tunnels also have shallow depth. Due to their shallow depth, metro tunnels generally have been constructed in weak rocks or weak soils in cities. The construction of twin tunnels will generate ground movements which have the potential to cause damage to existing surface and subsurface structures. To solve this settlement problem, experts have used the Earth pressure balance machine (EPBM) and the slurry balance machine. In such excavations, especially in twin tunnels, the main challenges for constructers are estimating the maximum surface settlement, controlling the interaction of transverse surface settlement and shaping the settlement curve. Incorrect estimation of these parameters can lead to significant problems above the tunnels and in nearby structures. This paper focuses on surface settlement measurements, on the interaction of twin tunnel transverse surface settlement and on the relationship between shield parameters and transverse surface settlement for parallel tunnels using EPBM shields in clay and sand soils in shallow depth. Also, a new equation is proposed for estimating the transverse settlement curve of twin tunnels. The results from this proposed equation are compared with the results of field observations. The transverse settlement curve values obtained from the proposed equation have good agreement with the actual results for the Otogar–Kirazli metro case studies.

EPB tunneling challenges in bouldery ground: a new experience on the Tabriz metro line 1, Iran

Underground construction of the Tabriz metro line 1 involved boring twin tunnels with two earth pressure balance machines in alluvial deposits. Although this type of machine is recommended for excavation in fine soils, its performance was evaluated in this project where part of the ground consists of alluvial deposits with particle size ranging from silt to boulder and even rock blocks. It was found that the tunnel boring machine (TBM) performance was highly reduced due to the variation in ground conditions. Many types of problem were observed during the excavation in coarse grained alluvial deposits including high cutter-head torque, excessive wear of tools, breaking of TBM parts and ground instabilities in the form of large settlement and ground collapse. A trial effort was implanted to stabilize the overburden ground by the injection of cement grout, but this proved to be unsuccessful. In both cases, the excavation operation was delayed. With the experience gained from the excavation of the first tunnel and use of foam with a high foam expansion ratio as a soil conditioner in closed mode, the performance of the TBM in the second tunnel improved.

Case Study of Damage on Masonry Buildings Produced by Tunneling Induced Settlements

This study analyzes the structural response of a group of masonry buildings subjected to real ground movements experienced during the construction of the L9 Metro tunnel in Barcelona, bored by a earth pressure balance machine (EPB) tunnel boring machine (TBM). The studied one-story small dwellings represent a common building typology frequently used in those days in Barcelona's outskirts (more than 1000 were erected). Real settlement profiles are compared with the ones provided by empirical methods, which estimate the shape and the area of the trough according to the ground properties and the volume loss. The first aim of the study is to evaluate the effectiveness of two techniques used to predict damages in buildings resulting from tunneling subsidence: 1) the equivalent beam and its subsequent refinements, and 2) the use of a non-linear finite element (FE) macro-model. The real structural damage presented in the buildings is compared with the predictions given by these two methods. Main model parameters have been determined by means of characterization experiments developed on the site and in the laboratory, thus giving a much higher significance to the analysis. The obtained predictions present a high correspondence with the actual damage registered, particularly in crack patterns and widths.

Greenfield ground response to EPBM tunnelling in London Clay

Earth-pressure-balance machines (EPBMs) are frequently used in preference to more conventional non-pressurised tunnel-boring machines (TBMs) to minimise ground movements in the urban environment. The greenfield ground response to EPBM tunnelling in London Clay from a field monitoring research study is presented and discussed. Vertical and horizontal displacements and pore pressure changes were measured with an extensive array of surface and subsurface instruments. Surface settlement troughs observed above the tunnels were small (wmax< 10 mm) and can be modelled using the commonly adopted inverse Gaussian curve, but a complex kinematic mechanism took place within the subsurface ground. In the near vicinity of the pressurised closed-face tunnel-boring machine an ‘expanding' displacement field was observed, in contrast to the ‘contracting' field often observed and associated with open-face tunnelling. This expanding response is dependent on TBM variables, such as face pressure, and should be contemplated when predicting ground movements and assessing subsurface structures such as piles and existing tunnels.

Effects of dispersing foams and polymers on the mechanical behaviour of clay pastes

The use of soil conditioners during excavation with earth pressure balance machines offers a number of advantages, including increased advancing rates, higher safety and settlement minimisation in a variety of ground conditions. Dispersing polymers and foams are added to optimise the strength and improve the deformation properties of the supporting soil paste. In this paper, possible effects of some commercially used products on clays are discussed. Whereas for most clays conditioning is based on lubrication (leading to reduced shear strength), for calcium bentonite a high polymer dosage can lead to the counterproductive effect of increasing the material's strength. In the case of kaolin, the change of mechanical behaviour is much more dramatic than simply reducing the strength; instead, the soil transforms into a viscous material with both liquid and solid properties. This unexpected phenomenon is characterised in detail and the working mechanisms of the used dispersants are identified. Shear-strength measurements at different polymer concentrations, with different kaolins and commercial clay dispersants are presented. The results suggest an enhanced particle-lubrication mechanism, arising from ion exchange and altered electrostatic forces between the clay surfaces and the surrounding water layer; this picture is confirmed by additional adsorption and microscopic measurements.

TBM tunnelling-induced settlements in coarse-grained soils: The case of the new Milan underground line 5

In this paper measurements of tunnelling-induced settlements collected during the construction of the new Milan underground line 5 are presented. The twin tunnels are being excavated in coarse-grained soils partially under the water table with an earth pressure balance (EPB) machine. In the following the attention is given only to the monitoring data recorded under free-field conditions (i.e. no structure on the soil surface) during and after the construction of the first tunnel. Settlement measurements are back-analysed using the classical Gaussian empirical predictions, both in the transverse and longitudinal directions, providing a very detailed description of the EPB tunnelling performance under such geotechnical conditions. In particular, a translated Gaussian cumulative curve, to be related to the influence of pressure applied at the face, is proposed to better match the evolution of settlements during tunnel advancement. Finally, the influence of different excavation parameters (such as face pressure, grouting pressure, machine thrust) on the estimated volume loss and shape of subsidence trough is investigated.

Discrete element model for performance analysis of cutterhead excavation system of EPB machine

With the aim of probing for design theory of cutterhead excavation system of earth pressure balance (EPB) machine, the parameters representing the system performance are concluded firstly. Then a 3D model for cutterhead excavation system of EPB machine is presented with the discrete element method (DEM) software PFC3D, which is capable of simulating the tunneling ground in site, the machine structure and the excavation operation. The performance parameters indicating stability of excavation face, soil discharging rate, cutterhead system torque and cutter wear are measured by running the DEM code. The results obtained with the DEM model are accord with situ data. It indicates that the DEM model is a promising method replacing the field experiment to analyze the influences of the structural parameters on system performances, which are essential for structure optimization design of the cutterhead system of EPB machine.

Effects of important factors on surface settlement prediction for metro tunnel excavated by EPB

Due to population growth, people demand more and more transportation services, there can be no doubt that the requirement for tunnels will also grow. Mechanized excavations using EPB have been successfully applied, especially in urban environments where there is less surface space available, over the past twenty years. Because of excavation speed and less hazardous working environments, use of the Earth Pressure Balance Machine (EPB) is a more popular method in metro, railway and road tunnels in urban areas. Control of surface settlement is very important when tunnels are excavated in urban areas or beneath important structures. This research mainly discusses effects of important properties like tunnel depth, overburden pressure, tunnel dimension and face pressure using empirical, theoretical and numerical methods used to control surface settlement. Results of these methods are compared with observation data.

Reduction of the clogging potential of clays: new chemical applications and novel quantification approaches

Earth pressure balance machines are increasingly being used for tunnelling in difficult soils and ground conditions. In clayey soils, clogging of the working tools is among the main hazards. Attempts to reduce this risk by adding anti-clogging chemicals do not always produce the desired effect. In this paper, the limited efficiency of the existing conditioning chemicals is quantified and explained, and an enhanced interaction mechanism is proposed. This mechanism is based on sealing the clay aggregates against the penetration of water; it requires the addition of polyamine chemicals, as a newly patented application. These chemicals have been shown to dramatically reduce the clogging potential of different clay pastes over a broad range of water contents, using novel approaches for quantifying the clogging behaviour of clays in general. These methods also establish important correlations between empirical stickiness and more fundamental soil parameters, and in particular the ratio between adhesion and strength. Together with microscopic investigations and adsorption measurements, this provides insight into the working mechanism of the new chemicals at the particle level; it also highlights the differences between this mechanism and that of commercially used foams and polymers.

Earth pressure balance tunnelling in rock masses: Laboratory feasibility study of the conditioning process

Earth pressure balance (EPB) shield tunnelling is considered to be an effective tunnelling method when surface settlements must be avoided by controlling face stability and underground water inflow. EPB technology is applied increasingly to the conditioning of rock masses in which the presence of polluting material is foreseen and must be controlled, or when explosive gases from the rock mass must be prevented from entering in the machine. The use of an EPB machine in ‘closed mode’ can prevent the dispersion of dangerous materials in the underground environment. This study was performed to evaluate the feasibility of using EPB technology in rock masses and the effectiveness of this tunnelling process in terms of soil conditioning, conditioned soil permeability, pressure transmission on the tunnel face, and the ability to extract the material with a screw conveyor. The research was developed with a set of laboratory tests carried out on three different types of rock masses where tunnelling with EPB machines are planned to be constructed in Italy but the results can be easily applied to similar rock mass types. The tests have been carried out both with slump tests and with a specially constructed screw conveyor extraction device. In the paper the most important results are presented and discussed.

Calculation of surface settlements caused by EPBM tunneling using artificial neural network, SVM, and Gaussian processes

Increasing demand on infrastructures increases attention to shallow soft ground tunneling methods in urbanized areas. Especially in metro tunnel excavations, due to their large diameters, it is important to control the surface settlements observed before and after excavation, which may cause damage to surface structures. In order to solve this problem, earth pressure balance machines (EPBM) and slurry balance machines have been widely used throughout the world. There are numerous empirical, analytical, and numerical analysis methods that can be used to predict surface settlements. But substantially fewer approaches have been developed for artificial neural network-based prediction methods especially in EPBM tunneling. In this study, 18 different parameters have been collected by municipal authorities from field studies pertaining to EPBM operation factors, tunnel geometric properties, and ground properties. The data source has a preprocess phase for the selection of the most effective parameters for surface settlement prediction. This paper focuses on surface settlement prediction using three different methods: artificial neural network (ANN), support vector machines (SVM), and Gaussian processes (GP). The success of the study has decreased the error rate to 13, 12.8, and 9, respectively, which is relatively better than contemporary research.

Interaction of longitudinal surface settlements for twin tunnels in shallow and soft soils: the case of Istanbul Metro

Increasing demands on infrastructures increases the attention on shallow soft ground tunneling methods in urbanized areas. Especially, in metro tunnel excavations, it is important to control the surface settlements observed before and after excavation, which may cause damage to surface structures. To solve this problem, earth pressure balance machines (EPBMs) have widely been used throughout the world. This study focuses on surface settlement measurements, the interaction of twin tunnel surface settlement, and the relationship between shield parameters and surface settlement for parallel tunnels using EPBM shields in clay and sand soils. In this study, the tunnels were excavated using two EPBMs. The tunnels were 6.5 m in diameter, as twin tubes with a 14 m distance from center to center. The EPBM in the first tube followed about 100 m behind the other tube. Segmental lining with 1.4 m of length was employed as a final support. The results from this study showed that (1) the most important parameters for the maximum surface settlements are the face pressure and backfill; (2) in twin tunnel excavation with EPBM for longitudinal profile, the settlement rate reached its peak value when the shield came to the monitoring section and this peak value continued until the shield passed the monitoring section; (3) every shield affected the other tunnel’s longitudinal surface settlement profile by approximately 35–36.8 %; (4) SA, SB and SC values were found to be 38.0, 35.8 and 26.2 %, respectively for an EPBM, and (5) ensuring good construction quality is a very effective way to control face stability and minimize surface settlement.

Effect of Soil Conditioners on the Pressure and Rate-Dependent Shear Strength of Different Clays

Tunneling using the earth pressure balance machines method in clayey soils requires the addition of conditioning chemicals to reduce the torque moment of the machine and to guarantee a safe and economical excavation process. Injection of foams and polymers at the cutter head of the tunnel boring machine with subsequent remolding in the pressure chamber transforms the excavated material into a deformable soil paste with preferably low strength and adhesion. The soil becomes more compressible and can develop viscous properties, such that the pressure and rate dependency of the shear strength of the mixtures are of major importance for the proper design and analysis of the tunneling processes. In this paper, the results of vane shear strength measurements on various conditioned clays and clay-sand mixtures at various shearing velocities and applied pressures are presented and analyzed. The results clearly show the effects of the chemicals used and also point out the differences in their efficiency depending on the clay mineralogy. This allows for an improved understanding of the working mechanisms of the chemicals.

Stickiness and adhesion of conditioned clay pastes

One of the main hazard scenarios for tunnelling with Earth Pressure Balance Machines (EPBMs) in clay soils is the clogging of the shield openings and, in the worst case, plugging of the machine. Application of soil conditioners, particularly foams and dispersing polymers, leads to decreased stickiness and clogging tendency and to improved soil–machine interaction in the excavation process. Due to the lack of standard procedures, an attempt is made to propose new methods for stickiness and adhesion quantification. The first one is a simple procedure which enables the determination of an empirical stickiness ratio by weighing the soil adhering to a mixing tool after the soil preparation process. The second method uses a newly developed device, which allows measuring of the tangential adhesion and sliding resistance between a steel plate and soft soil pastes at different applied pressures, as they occur in the pressure chamber of an EPBM. The stickiness ratio was found to correlate well with the tangential adhesion and sliding resistance following a logarithmic trend for all tested mixtures, exhibiting a higher effect of chemicals on stickiness reduction at soft soil consistencies but a lower effect at stiffer consistencies. The results clearly showed a decreased sliding resistance due to chemical application with a strong dependency on the clay mineral encountered. The mixture water content is of major importance for chemical efficiency, diminishing adhesion reduction at lower water contents.

THE 2009 BENJAMIN FRANKLIN MEDAL IN ENGINEERING PRESENTED TO RICHARD J. ROBBINS

Fifteen years ago construction started on phase one of the Mass Rapid Transit tunnels in Singapore. Since that time tunnelling technology has advanced considerably. Earth Pressure Balance Machines (EPBMs) are far more commonly used than ever before on tunnelling projects in the Far East, Asia and Europe. For the first time in Singapore full-face tunnelling is being carried out in the Old Alluvium for the construction of North East Line Project (NELP). The TBMs selected for the NELP are, in the main, EPBMs. This paper compares the types of EPBMs chosen for each Contract, highlighting the particular design features of each. The paper also provides an update on the progress of the EPBM tunnelling operations currently being undertaken on the NELP.

Surface subsidence induced by twin subway tunnelling in soft ground conditions in Istanbul

Unlike the symmetrical surface settlement trough of a single tunnel which can be described using the Gaussian function, surface settlement over twin tunnels can be symmetric with respect to the mid-point between two tunnels or asymmetric. The paper reports the settlement troughs which developed when earth pressure balance (EPB) machines were used to excavate twin tunnels at shallow depth in the soft ground conditions beneath a developed part of Istanbul. An attempt is made to evaluate the effects of different factors on the surface subsidence. Detailed monitoring was undertaken when one tunnel was advanced ahead of the other and when only one tunnel was being driven. It was found that the shapes of the subsidence troughs over the two tunnels were different and varied with the excavation of the second/subsequent tunnel. It is concluded that changes in the subsidence trough are related to disturbance in the geo-material when an excavation is advanced ahead, as well as the nature and thickness of the overburden.

Discrete element simulation of mechanical characteristic of conditioned sands in earth pressure balance shield tunneling

The discrete element method (DEM) was used to simulate the flow characteristic and strength characteristic of the conditioned sands in the earth pressure balance (EPB) tunneling. In the laboratory the conditioned sands were reproduced and the slump test and the direct shear test of the conditioned sands were implemented. A DEM equivalent model that can simulate the macro mechanical characteristic of the conditioned sands was proposed, and the corresponding numerical models of the slump test and the shear test were established. By selecting proper DEM model parameters, the errors of the slump values between the simulation results and the test results are in the range of 10.3%−14.3%, and the error of the curves between the shear displacement and the shear stress calculated with the DEM simulation is 4.68%−16.5% compared with that of the laboratory direct shear test. This illustrates that the proposed DEM equivalent model can approximately simulate the mechanical characteristics of the conditioned sands, which provides the basis for further simulation of the interaction between the conditioned soil and the chamber pressure system of the EPB machine.

The impact of adverse geological conditions on the design and construction of the Thames Water Ring Main in Greater London, UK

Abstract The Thames Water Ring Main has been driven through the various Palaeogene deposits of London and the underlying Cretaceous Chalk. Much of the tunnel alignment was designed to keep within the London Clay Formation over the western half of the project, although discovery of a number of Quaternary scour hollow features, during the ground investigation, led to revisions in tunnel design. In addition, a scour feature was also intersected during tunnelling, necessitating changes to the tunnel lining and use of compressed air. Over much of the eastern half of the project, the tunnel alignment was unable to avoid the lower Palaeogene deposits of the Lambeth Group, and Thanet Sand Formation, as well as the underlying Chalk. These deposits were found to contain hazardous features, such as sand-filled channels, sub-artesian groundwater and hard ground, complicated further by geological faults along the route. Intersection with these features could have had serious implications for the health and safety and efficiency of tunnelling operations. These adverse geological conditions were avoided by changes to the alignment during design or by ground dewatering prior to construction. When intersection occurred, the open-faced machines used compressed air and lining changes to mitigate the risk. From 1990, the project used Earth Pressure Balance Machines (EPBMs) for the first time in the UK. These allowed better control of problematic ground and allowed the project to set an all-time productivity record.

Ground Information and Selection of TBM for the Thessaloniki Metro, Greece

The Thessaloniki Metropolitan Railway is currently at its primary construction stage. The railway is composed of two separate ∼6-m-diameter parallel tunnels, each of which is ∼8 km long and has 13 stations. The span between the tunnel axes is approximately 12 m, and the depth of the red line (track level) varies from 15 m to 30 m. The geology of the urban area of Thessaloniki is characterized by the presence of Neogene and Quaternary deposits. The base formation for the study area is a very stiff to hard red clay, dating to Upper Miocene–Pliocene time. On top of this formation, Quaternary sediments have been deposited, most of which consist of sand and/or gravel in a clay-silt–dominated matrix, covered in places by anthropogenic fill. Ground investigation campaigns have incorporated a significant number of sampling boreholes and in situ and laboratory testing. In this study, data from borehole loggings, particle-size analyses, natural moisture content, Atterberg limits, permeability tests, pressure-meter tests, cone penetration tests (CPT), and standard penetration tests (SPT) were analyzed in order to obtain a better geological understanding, a geotechnical zonation, and a classification of the ground with respect to mechanized tunneling. The characteristics and parameters of the soils and the hydrogeological regime indicate that an EPBM (Earth Pressure Balance Machine) rather than a slurry TBM (Tunnel Boring Machine) should be used for the tunnel construction.

Effect of building stiffness on tunnelling-induced ground movement

The progressive response of two–three storey masonry buildings to bored tunnelling on the Jubilee Line Extension in London is investigated in order to determine the effect of building stiffness on tunnelling-induced ground movement. The masonry buildings, at Moodkee Street and Keetons Estate, were affected by tunnelling with earth pressure balance machines on Contract 105 of the Jubilee Line Extension. Bending deformations and axial strain induced in these structures are compared to greenfield ground deformations and strain in order to infer the effect of building stiffness. The modifying influence of the stiffness of these buildings on ground movement is interpreted using the Potts and Addenbrooke [Potts D.M., Addenbrooke, T.I., 1997. A structure’s influence on tunnelling induced ground movements. Proc. Inst. Civ. Eng. Geotech. Eng. 125 (2), 109–125] method, which is based on a series of finite element analyses. The inferred building stiffness that can be used for design purposes with the Potts and Addenbrooke method is compared to an estimation for a Class A [Lambe, T.W., 1973. Predictions in geotechnical engineering. Geotechnique 23 (2), 149–202] prediction exercise by Mair and Taylor [Mair, R.J., Taylor, R.N., 2001. Settlement predictions for Neptune, Murdoch and Clegg Houses and adjacent masonry walls. Building response to tunnelling – case studies from construction of the Jubilee Line Extension, London. In: Burland, J.B., Standing, J.R., Jardine, F.M. (Eds.), Projects and Methods, vol. 1. CIRIA SP200, pp. 217–228 (CIRIA and Thomas Telford, 2001). ISBN: 0 7277 30177] using the same method. As a result an alternative approach is proposed for estimating the relative bending stiffness of masonry structures for future use with the Potts and Addenbrooke method when making simple predictions.