Underground works in soils and soft rock tunnnelling doc

Break-in/Break-out transition zones need to be introduced in these areas; these should serve five main purposes: 1 Ground support in the direction perpendicular to the opening; 2 Face re

UNDERGROUND WORKS IN SOILS AND SOFT ROCK

of advances in computer technologies The scope of increasing difficult project conditions to be addressed requires that the best use be made of these technologies, as well as of lessons gained from past experience and current observational records

Growing needs for modern transportation and utility networks have given rise to an increased demand for a more extensive and elaborate use of underground space Some of these projects are related to urban development, which requires the construction of more metro systems, underground water mains, gas pipes, telecommunication and electric power networks, as well as underground parking facilities Other applications of underground construction include the crossing of natural barriers such as rivers and mountains that are found across the alignment of major road, motorway or railway link projects

Many of these structures have to be constructed in difficult ground conditions, including soft clays and bearing sands, as well as soft rocks with particular behavioral features such as creep, weathering and swelling Additional difficulties may arise because of the occurrence of a variety of heterogeneous ground conditions, with strong contrasts in the characteristics of materials encountered within the same run, which may require frequent adjustments to be made in the course of tunneling

water-These projects have brought new challenges to the tunneling engineer, and have triggered many technological and scientific advances over the past thirty years Reviews of the geotechnical aspects of soft ground tunneling have been provided by Peck (1969), Cording and Hansmire (1975), Clough and Schmidt (1981), Ward and Pender (1981), O’Reilly and New (1982), Schlosser et al (1985), Attewell et al (1986), Konda (1987), Rankin (1988), Uriel and Sagaseta (1989), Clough and Leca (1989), Fujita (1989), Cording (1991), Fujita (1994), Mair and Taylor (1997) and Mair (1998) Some recent developments are also discussed in Leca and Guilloux (1999), and will be reviewed herewith

The present report addresses the main aspects of underground works in soil and soft rock, and reviews some

of the more recent advances in this field The following features of soft ground tunneling are considered: construction techniques, ground investigations, design methods, instrumentation and monitoring practices Some comments on major advances accomplished in recent years, as well as trends for future developments are provided in the conclusion

1

Eric Leca, SCETAUROUTE/DTTS, Groupe EGIS, Les Pléiades n° 35, Park Nord Annecy, 74373 Pringy Cedex, France

2 Yann Leblais, EEG SIMECSOL, Consulting Engineers, 18 rue Troyon, 92316 Sèvres Cedex, France

3 Karl Kuhnhenn, BUNG GmbH, Englerstra 8 e 4, Postfach 101420, 69004 Heidelberg, Germany

2.0 CONSTRUCTION ASPECTS

Soft ground tunneling is often challenging because of the occurrence of soft water-bearing soils and environmental constraints that require strict ground motion control Tunneling in such conditions has been made possible due to significant technological advances that were achieved over the past twenty to thirty years These include the development of shield tunneling, as well as major improvements in the more conventional methods

of tunneling or in ground conditioning schemes employed in underground construction

Cutter driving motor

Agitator

Erector motor Tail seal

Shield jacks Segments

Erector

Cutter face

Slurry supply Slurry return

Figure 1: Principle of the slurry shield machine (after Fujita, 1989)

This technique, which makes use of a bentonite slurry to stabilize the working face of the tunnel, was introduced in the early 1960s in UK, and in then Japan The EPB shield was developed a decade later The principle used with this latter technique is described in Figure 2 (after Fujita, 1989): in this case, face support is obtained by retaining the spoils in the working chamber so that sufficient confining pressure is reached Compressed air has also been used successfully in some projects to support the working face of the shield, but this technique is essentially limited to the less pervious categories of soils

Additional improvements have been made to the shield tunneling technique over the most recent years, particularly in terms of machine size and ground motion control Large Tunnel Boring Machines (TBM) are now common, and shields with diameters up to 14 m and over have been manufactured for projects such as the Trans-Tokyo Bay Highway (14.14 m diameter) in Japan (JSCE, 1996; Asakura and Matsuoka, 1997) and the 4thcrossing of the Elbe River (14.20 m diameter) in Hamburg, Germany (Bielecki and Zell, 1999) A 14.87m diameter TBM is currently being manufactured for the construction of the Groene Hart High Speed Rail tunnel

in the Netherlands These advances have allowed the shield technique to be extended to a larger scope of project conditions, including motorway tunnels that currently require openings in the order of 12 m to be excavated Such advances have been accompanied with significant technological improvements that allow a more appropriate management of adverse conditions to be obtained, when tunneling in difficult grounds (Herrenknecht, 1998 & 2000) The introduction of foams in EPB shields allows a more appropriate control to be achieved of ground deformations at the working face and, in turn, of tunneling induced settlements Similarly,

large diameter TBMs can be equipped with a secondary internal cutting wheel to help excavate through sticky clays The introduction of advanced back-filling processes at the shield tailpiece has also strongly contributed to significantly reduce the potential for tunneling induced settlements in providing a means for limiting the amount

of ground movement into the tail gap

From a more general standpoint, several developments have been devoted to the design of “mixed-shields” (Herrencknecht, 2000) that would be capable of handling a variety of heterogeneous materials, which are often found in urban areas and usually result in major tunneling difficulties Examples of such difficulties were reported during the construction, in the mid-1980s, of one section of the Washington Metro, where large settlements were recorded at several locations with an EPB shield These excessive ground movements were mainly attributed to the strong contrast in ground properties found at the face, with a mixture of soft water-bearing sands and gravel in the crown overlying stiff to hard clays (Clough and Leca, 1993)

Cutter driving motor

Screw conveyor Tail seal

Screw conveyor driving motor

Belt conveyor

Gate jack

jacks Bulkhead

Cutter frame Cutter face

Figure 2 : Principle of the Earth Pressure Balance (EPB) shield (after Fujita, 1989)

Even though the “universal machine” is yet to be invented, concepts such as the “mixed-shield” can help in adjusting to the variety of grounds encountered along a tunnel alignment, particularly at shallow depth An extension of this concept was been used recently in the design of the TBM that is being manufactured for the SOCATOP project (underground section of the second ring of Paris Beltway, in the city’s western suburb) This 11.57 m diameter machine will have the capability of being operated in an open mode or as a slurry or an EPB shield, depending on the grounds found at the face (Carmes and Athenoux, 1998)

Construction would, however, need to be halted several days to allow modifications to be made on the machine, which means that alternating tunneling modes would only be possible if they occurred a limited number of times along the project Moreover, sufficient knowledge of ground conditions should be available so that the location of changeovers could be identified and planned ahead of time

Advances in the shield technology have allowed significant improvements to be made in terms of ground motion control, and tunneling induced settlements can now be kept under relatively low values (Mair and Taylor, 1997) in comparison with previous records (New and O’Reilly, 1991) Significant experience has also been gained over the past years in shield operation know-how As a result, and provided an appropriate machine is selected and skillful workmanship is available, high performances should be expected for most shield tunneling jobs, with limited impact on the environment (Richards et al., 1997)

Ground collapse may, however, be experienced - even when using the most elaborate machines - in situations where unexpected conditions are encountered, or when face pressures fail to be maintained at the design level Some attempts have been made to anticipate and prevent localized face collapse through a more systematic real time use of shield parameters recorded during tunneling (Aristaghes and Blanchet, 1998) The system, termed CATSBY, was installed and tested on a 10.8 m diameter slurry shield used for the construction of the Sydney metro in Australia, and lead to promising results, in extremely heterogeneous grounds - ranging from hard rock

to soft marine clays and dune sands - below the water table

This system allows all recorded data to be either stored, or used to estimate some pre-established key parameters that could, in turn, provide some indication of the ground-structure interactions associated with the tunneling process These include pressures in the muck chamber, as well as characteristics of the thrust resultant acting on the tunnel face Measurements are taken at regular intervals (typically every 3 minutes), and each parameter is characterized in terms of mean value and standard deviation These data can be used by the shield operator to check that mean values remain within acceptable levels and that no sharp changes occur in the time response of pre-established key parameters The concept can be applied to a variety of project conditions, and is designed with sufficient flexibility to allow adjustments to be made as required in the course of the project One particular difficulty to be emphasized with shield tunneling is the operation of the machine through the entrance and exit shafts, as these junctions usually result in reduced confining pressures in the surrounding ground, which could lead to critical conditions in grounds such as water-bearing soils with low cohesion Break-in/Break-out transition zones need to be introduced in these areas; these should serve five main purposes:

(1) Ground support in the direction perpendicular to the opening;

(2) Face reinforcement to ensure ground stability ahead of the shield, using a confining pressure limited to the thrust reaction capacity;

(3) Ground support in the vault to limit decompression effects, so that settlements can be controlled despite reduced confining pressures;

(4) Control of water pressures and water ingress, so that blow-in and flood can be prevented;

(5) Guidance to the TBM along the first meters of drive, to prevent sinking of the machine to occur

Several treatment solutions have been made available to cope with these difficulties (Richards et al., 1996); in particular, techniques based on partial ground substitution have proven to be fully efficient in soft water-bearing grounds

2.2 Conventional Methods

Considerable progress has also been achieved in the more conventional methods of tunneling, mostly in relation with an extensive use of ground reinforcement and improvement techniques Recent advances in this area include: the development of pre-lining techniques; improvements in tunnel support systems, the introduction of advanced ground conditioning methods, and new developments in compensation grouting

2.2.1 Pre-lining techniques

Using a similar approach to fore-poling, several techniques have been developed over the most recent years, that consist in placing some reinforcement over the tunnel face so as to obtain some kind of structural support at the front prior to proceeding with ground excavation This support can be made of an “umbrella” of peripheral bolts or jet grouted columns, or a concreted vault The latter is usually referred to as the “precutting” method This technique was first introduced during the construction of the Paris metro (Bougard et al., 1977; Péra et Bougard, 1978) in weak rock and then extended to softer materials

When used in soft ground, the method is completed in three stages (Figure3): (1) excavation of a curved shaped cut over the tunnel face, using a large excavator; (2) concreting inside the cut so as to obtain a vault, termed “pre-vault”, ahead of the tunnel face; (3) ground excavation underneath the “pre-vault”

a Longitudinal Section b Cross-Section

Figure 3 : Mechanical precutting with “pre-vault”

Some face bolting may be required to help stabilize the face when large tunnel sections have to be excavated

in soft materials This can be accomplished by means of fiberglass bolts that have the capability of offering sufficient tensile resistance without impeding the excavation process (Lunardi, 1993) This process, which has been used in Italy since the late 1980s, allows the excavation of large tunnel sections (over 100 square meters) to

be performed, without recurring to any partition of the face This approach tends to be preferred to the more conventional top-and-heading method, as it is perceived to be more efficient, both in terms of construction time and ground motion control

Full-face tunneling, with combined fiberglass bolting and pre-vault support, has been extensively used in France for the excavation of large tunnel openings, since the construction of the La Galaure High Speed Train (TGV) tunnel, which was successfully completed in molasses, with a 150 square meter profile (EMCC, 1993) Recent experience in this field includes the construction of the Pech Brunet motorway tunnel in southwestern France, which required the excavation of a 155 square meter profile in marls (Gaudin et al., 1999)

Combined use of pre-lining and face bolting has also been developed, during the same period of time, in conjunction with the “umbrella” vault technique, with primary application to large transportation tunnel projects

in Italy A typical layout for the “umbrella” technique is shown in Figure 4 It refers to the San Vitale tunnel, which was constructed as part of the Caserta-Foggia railway line project in Italy (Lunardi, 1998) This 4.2 km long tunnel was excavated under 150 m of ground cover, in soils consisting of sands, silty clays, clay-marls and limestone

The reinforcing system for this project included: 18 m long fiberglass bolts installed in the face; peripheral bolts sealed under high pressure, to form an “umbrella” arch over the tunnel face; and ground drainage from the face Another important feature of this project was the introduction of a reinforced concrete invert right behind the tunnel face, which contributed to achieve adequate ground motion control This concept was proposed after several unsuccessful attempts had been made to excavate the tunnel with staged excavation at the face

Figure 4 : Peripheral and Face Bolting (after Lunardi, 1998)

Design of the reinforcement system was achieved by means of a combination of experimental work (extrusion laboratory tests and pullout tests) and numerical studies (three-dimensional Finite Element analyses) Construction with the “umbrella” arch technique is usually accompanied with extensive tunnel instrumentation,

to check for the adequacy of bolt design Instrumentation includes settlement markers, convergence pins, and pressure cells to monitor the overall ground response to tunneling In addition, borehole extensometers are installed at the front, so that ground deformations ahead of the tunnel face can be better anticipated, and the amount of bolting adjusted accordingly

2.2.2 Tunnel support systems

Some progress has also been made in conventional methods, with the development of more flexible tunnel support systems This includes the increased use of shotcrete in “hand-mined” tunnels A comprehensive review

on sprayed concrete liners for tunnels has been produced recently by the Institution of Civil Engineers (ICE) in

UK (ICE, 1996) The use of sprayed concrete as primary liner, particularly when reinforced with steel fibers, allows early support to be applied to the tunnel walls (and/or face) after excavation, which contributes to achieving reduced construction time and improved ground motion control The amount of support can also be

Pre-vaults

Bolts

modified as required in view of the observed tunnel response, and ancillary reinforcements, consisting of radial bolting or steel ribs can be incorporated when necessary, as excavation proceeds

Shotcrete has been extensively used in the completion of tunnel support systems in conjunction with the New Austrian Tunneling Method (NATM) This approach (Rabcewicz, 1964), which was originally introduced for the construction of rock tunnels in the Alps in the 1950s, has been more recently extended to softer materials It

is based on the principle that much of the tunnel stability comes from the self-supporting capability of the surrounding ground, and that some optimization of tunnel liners can be achieved by continuously adjusting the type and amount of support to that required to enhance the ground’s ability to reach equilibrium around the opening

The deformations of the tunnel walls are continuously monitored during construction, and adjustments made

on the basis of the observed ground response to tunneling When used in soils and softer rock, where early installation of support systems is necessary, this approach results in making extensive use of shotcrete with steel fiber and/or lattice girder reinforcement, in combination with radial ground bolting when required, in view of the observed tunnel response

For instances such as shallow soft ground tunnels in urban areas, real time optimization of the tunnel support system becomes hardly possible because of major concerns for preventing any potential damage to existing structures In such cases, shotcrete liners would be used without formally recurring to the NATM, and this technique could be preferably be referred to as Sprayed Concrete Lining (SCL) rather than NATM, as proposed

by the ICE (1996)

Another major advantage of sprayed concrete is its ability to adjust to variable tunnel geometries, which can save from complex form-works and contribute to more cost-effective design, particularly when large size openings have to be built A typical example for such application was provided by the Chauderon railway station

in Lausanne, Switzerland, which required the construction of a funnel shaped opening where railway tunnels merged into the station Advanced shotcrete specifications had to be used for this project, to allow high short-term mechanical characteristics with durable strengths to be obtained (Tappy et al., 1994)

Recent improvements in shotcrete characteristics have also allowed a more extensive use to be made of this material in tunneling, including for the long-term stability of underground structures (Leca et al., 2000) These developments, which should eventually result in reduced steel reinforcement and improved cost-efficiency for tunnel projects, tend to be counterbalanced by current trends to systematically recur to fully reinforced concrete liners

Whereas these trends are primarily governed by concerns for concrete cracking and liner water-tightness, they probably also result from most concrete codes being mainly intended for aboveground structures This emphasizes the need for more exchange between geotechnical and structural engineers to be organized, so that our standards could appropriately reflect the experience gained by practicing engineers An attempt in that respect has been made by the AFTES (French Tunneling Society), with the preparation of recommendations for the use of plain concrete in tunnel liner design (Colombet et al., 1998)

2.3 Ground conditioning methods

Major progress has been achieved over the past years in the applications of ground conditioning techniques to tunneling projects A general review of recent advances in this field can be found in the works published by the Soil Improvement and Geo-textiles (SIG) Committee of the American Society of Civil Engineers (ASCE, 1997a

& 1997b) Using the same classification, the criteria summarized in Table 1 can be proposed as for the potential impact of each technique in terms of improvement in mechanical and hydrological ground properties

Table 1 : Effects of ground conditioning schemes on ground properties TREATMENT REINFORCEMENT IMPROVEMENT

Fracture grouting ➋ ➌ Jet grouting ➊ ➋ ➍

Permeation grouting ➊ ➌ ➍ Soil mixing ➊ ➍

Effect on stiffness ➊

Effect on displacement ➋ Effect on permeability Effect on strength ➍➌ Effect on water level ➎

Additional insight into the ranges of application of grouting products was provided by the European Standard

Committee (CEN, 1998), as reproduced in Figure 5 and Table 2

principle grouting

method

injection (impregnation)

with ground without ground

Figure 5 : Grouting principles and methods (after CEN, 1998)

Other conditioning techniques include drainage and ground freezing Pumping, with generalized water

draw-down, tends to be avoided or limited because of the potential for consolidation settlements to take place in soft

cohesive soils Conversely, some localized water draw-down, with drains installed at the front of open-face

advancing tunnels, is often used to help stabilize the ground and limit water inflows during construction

Table 2 : Types of grouts applicable for grouting different types of ground (after CEN, 1998)

GROUTING

CONTACT GROUTING

BULK FILLING

Gravel, coarse sand

and sandy gravel

k> 5*10-3 m/s

Pure cement suspensions, Cement based suspensions Granular soil Sand

5*10 -5 <k< 5*10 -3 m/s

Microfine suspensions, Solutions

suspensions, Mortar Medium to fine sand,

5*10-6 <k< 1*10-4 m/s

Microfine suspensions, Solutions,

Special chemicals Faults, cracks, karst

c > 100 mm

Cement based mortars, Cement based suspensions (clay filler)

Mortars, Cement based suspensions with short setting time,

Expansive polyurethane, Other water reactive products

Microfissures

c < 0.1 mm

Silicate gels, Special chemicals

Cement based suspensions with short setting time,

Expansive polyurethane, Other water reactive products

(c = fissure width; k = ground permeability)

Gonze (1989) discussed the applications of ground freezing in underground projects This technique is rarely considered in practice, because of the expenses involved in its implementation in the field, but can prove reliable and cost-effective when used appropriately One recent application of this technique was related to the construction of the northern section of the Lyons beltway in France This section includes a twin tube tunnel, with cross-passages installed at regular intervals between each tube, for safety purposes Ground freezing was used on this project to excavate one of the cross-passages, which had to be hand-mined in an urban environment, through grounds consisting of mixed molasse and water-bearing pervious soils underlain with granite, with 25 m

of water head This technique was found appropriate in view of the strong contrast in mechanical and hydraulic properties between the two ground formations, and allowed the cross-passage to be safely executed

Recent advances in grouting techniques have been mostly associated with the introduction of extremely fine grained components (ultra-fine cement or mineral based chemicals with low viscosity) in injection products so that better groutability could be achieved in finer soils These products allow significant and durable increases in the mechanical characteristics of grouted soils to be obtained, which was practically impossible with conventional products

An interesting application case of mineral based grout, of the SilacsolTM type, was provided by contract D3M10 of the Paris metro extension project (Joho and Morand, 1995; Gouvenot et al., 1994) These works were completed in a densely inhabited area, and included the construction of large span openings (15,60 m) in coarse Seine alluvium, under 5.50 m of ground cover

Pressuremeter and plate tests, performed on the site during the ground improvement works, allowed to evidence a sharp increase in the mechanical properties within the grouted soil, with cohesions in excess of

200kPa and Young’s modulus values in the order of 350 MPa (i.e seven times higher than before grouting) Construction could proceed safely, with no noticeable settlement at the ground surface; excavation took place in

a concrete-like material, which allowed open face tunneling to be used with a perfectly stable 5 m high front Significant advances have also been achieved in the field of jet grouting, with applications in both ground improvement and seepage control An example of extensive use of jet grouting in underground projects was provided by the construction of two major railway stations, the Magenta (Fauvel, 1997) and Condorcet (Vignat, 1998) stations, as part of the EOLE subsurface rapid rail transit system in Paris Each station comprised three vaults, with an overall span of 53 m, and were excavated in the central part of Paris with limited ground cover Heterogeneous ground conditions were present on both sites, with fill and alluvium in the crown and sands or limestone in the invert, underlain with fine water-bearing sands

The project included the construction of four pillar and side galleries, which were used to stabilize the ground

by means of a network of vertical and inclined jet grouted columns, prior to proceeding with the excavation of the three main openings The design and construction procedure associated with the completion of the jet grouting works was adjusted on the basis of three real scale in situ tests (Fredet and Leblais, 1997) These confirmed that a significant increase in cohesion and furthermore ground modulus would be achieved through the jet grouting process, with improved ground characteristics typically 2 to 5 times higher than initially measured This ground conditioning work was essential in achieving adequate surface settlement control during construction

Ground conditioning schemes may also be used to assist in the completion of shield driven tunnels in difficult ground conditions Campo et al (1997) reported on various grouting and jet grouting works being completed in fine water-bearing soils, during the construction of Line 2 of the Cairo metro, with a slurry shield This case history emphasizes the requirement for a thorough examination of all specific situations that may arise along the completion of a tunneling project, even when the most sophisticated fully mechanized techniques are used From a more general standpoint, grouting and jet grouting schemes can be appropriately used, as remedial or preventive techniques, where weaker materials are found along the tunnel alignment Such applications include occurrences of weak water-bearing grounds in rock tunneling projects An example of ground conditioning works associated with rock tunneling was provided by the Freudenstein tunnel, which was constructed in Gypsum Keuper, as part of the Mannheim-Stuttgart railway line project in Germany (Kirschke et al., 1991b) Gypsum Keuper, as found in Baden-Württemberg, Germany, is composed of two layers with distinct rock attributes, separated by the gypsum upper limit The rock is leached above the gypsum level, because of its sulfatic components being dissolved and washed away through progressive decomposition It tends to be split into jointed masses and partially disassembled, and usually looses its original competence Some overstressing and fracturing is also produced in the overlying grounds, as a result of stress rearrangements associated with this process

The “active leaching” zone is subject to high water pressures, and shows no or short term stability when exposed to excavation works Conversely, the underlying gypsum rock can be described as compact and nearly watertight The main part of the eastern drive of the Freudenstein tunnel had to pass through the leached gray Estherien layers, with gypsum level only a few meters below the tunnel invert Along a 450 m stretch of the tunnel, a 1-3 m thick layer of weak water-bearing ground was present at various levels at the face (from crown to invert) Extensive grouting works were used to cut through this area

Grouting was performed using a pilot adit, and targeted so as to form a sealed zone in the tunnel area (Figure6) Some 38000 m of grout-holes were drilled and more than 2000 tons of cement were injected The procedure allowed the tunnel to be successfully excavated Water inflows were reduced and durably controlled Nevertheless, water-flows in the order of 60 l/s had to be pumped during construction, along the 2 km long stretch that linked the treated zone to the closest portal

Figure 6 : Freudenstein tunnel - Grouting works completed in unleached gypsum

Additional difficulties were found on this project with the excavation of an intermediate shaft, which had to

be introduced for ventilation purposes during construction Because anhydrite was present in this area, with the potential for swelling to occur in this formation, if exposed to water, the shaft had to perfectly sealed The sealing works were successfully completed using jet grouting columns driven from the bottom of the advancing shaft excavation

2.3.1 Compensation grouting

Advances have also been made in the field of ground improvement applied to tunneling, and particularly with the compensation grouting approach (Mair and Hight, 1994) This technique was introduced in the early 1980s,

in the form of compaction grouting, to assist in controlling tunneling induced settlements in dense sands (Baker

at al., 1983) The same principle was used more recently for shallow tunnels excavated underneath sensitive buildings, as part of the construction of the Vienna metro (Pototschnik, 1992) and the Jubilee Line Extension in London (Harris et al., 1996) In both cases, tunneling was completed in clays, using fracture grouting with fluid grout to limit the impact of settlements on existing structures Extensive monitoring was used to adjust the amount of grouting to observed ground deformations

The main construction features are illustrated in Figure 7, which refers to the construction of a 10 m diameter shallow tunnel, underneath a masonry building in London (Osborne et al., 1997; Mair, 1998) The tunneling works were performed in a layer of London clay, overlain with water-bearing gravel Prior to construction, a shaft was excavated next to the building, and used to install a network of horizontal pipes within the clay layer,

at some depth between the building foundations and the tunnel crown The building was equipped with shallow and deep settlement markers to allow ground movements to be monitored during each construction stage Construction involved two excavation steps: a 5.75 m diameter pilot gallery was first excavated and lined, and then enlarged to 10 m

Because of the large size of the opening in comparison to its depth, it was anticipated that large settlements could take place during the enlargement stage As a result, settlements were continuously monitored during

construction, and grouting activated through the already installed pipes, so as to counteract observed movements Grouting was performed using the “tube-à-manchette”, technique to allow accurate grout placement above the tunnel crown to be achieved This procedure allowed the tunneling works to proceed successfully, with deep settlements being kept lower than 20 mm, whereas up to 90 mm of vertical displacements had been recorded above the tunnel crown

Figure 7 : Compensation grouting scheme (after Obsborne et al., 1997)

This overview of ground conditioning cases allows some appreciation to be made of the broad scope of application of these techniques in underground construction Based on observed practices, the following points should be emphasized in view of achieving appropriate design specifications, as well as satisfactory field performances:

(1) Obtain sufficient knowledge of the geotechnical and environmental conditions, as well as of the accessibility of ground improvement equipment to the site;

(2) Identify correctly the ultimate purpose of the conditioning works: mechanical improvement and/or tightness;

water-(3) Account for local technical practices and capabilities, as the success in such work largely depends on the contractor’s skills;

(4) Adjust design to local practices and establish the cost/time schedule accordingly;

(5) Adapt the contract type in view of the actual ground conditioning purposes

Geological and geotechnical investigations provide early information on the tunnel feasibility and on the ground characteristics to be used for design Some general guidelines for planning and performing geotechnical investigations for tunneling projects were presented by Parker (1999), who emphasized the need for increased geotechnical investigations at the early stage of a project, in view of the amount of construction cost overrun attributed to unexpected ground conditions

Practical guidelines for the preparation of geotechnical reports for contract documents related to underground construction projects were produced by the Underground Technology Research Council of the ASCE (1997c) Additional insights into the parameters to be collected at the different stages of a tunneling project were provided

by the AFTES (Guillaume et al., 1994 & 1999) The information on the ground conditions to be expected along

a tunneling project can also be improved by conducting additional investigations in the course of construction Modern technologies have allowed advances to be made both in terms of processing the information collected during the different stages of investigations and of the capability for performing specific ground investigations during construction Some of these advances are commented in the present section They mainly refer to the developments of geological models and the increased use of geophysical methods in underground projects

Deep settlement indicators

3.1 Geological Models

The potential use of Geotechnical Information Systems for tunneling projects was discussed by Maurenbrecher and Herbscheb (1994), who presented a model developed in the late 1980s in Amsterdam, Netherlands, to store the information retrieved from geotechnical investigations completed in a district of the city A database, termed INGEOBASE, was designed to store data from borings, static penetrometer tests and hydro-geological investigations performed over a 4 square kilometer area Computerized data storage of available geotechnical information allowed some mapping to be performed of this area, according to specific pre-established geotechnical parameters

The authors mentioned the potential use of this model for a planned metro rail project in the same district of Amsterdam Such device should help identify geological difficulties along a tunneling project, and provide early information for performing risk analyses and making decisions in terms of project relocation or ancillary construction measures to be introduced to cope with any major anticipated difficulty

In a similar effort, Gaudin et al (1997) commented on the advances allowed in geotechnical investigations by three-dimensional geological modeling, on the basis of the experience gained over the past years, with the software, EARTH-VISION The primary purpose of this software is to allow all data obtained from geological and geotechnical investigations to be collected and stored within the same electronic base These include surface geology, photo-geology, thermographical data, numerical earth models, satellite views, boring logs, geophysical records In a second stage of analysis, this information is processed to prepare a three-dimensional geological model of the site

One essential feature of this system is its capability to be updated in view of additional data that are collected

in the course of the project The model can be used to prepare different graphical representations of the expected geology, check for the consistency of all available data, and identify areas where additional investigations should

be conducted A typical three-dimensional output obtained from this model is shown in Figure 8 (after Gaudin et al., 1997) It refers to one section of the Hurtières motorway tunnel, in the French Alps, where a collapse was experienced during construction

Figure 8 : Three-dimensional modeling of the glacial gorge found on the Hurtières tunnel (Gaudin et al., 1997)

The tunnel consists of two tubes, and was excavated using conventional methods in rock Failure occurred at one particular location where a glacial gorge, filled with weak water-bearing materials was encountered This

geological accident, which had not been identified at the design stage, caused construction to be halted several months, to allow additional investigations to be undertaken and remedial action to be taken (Hingant, 1999) The three-dimensional output presented in Figure 8 was obtained by combining data collected from 63 investigation borings and 104 additional bore-holes, that were completed as part of the grouting remedial works This model allowed a more accurate evaluation to be made of the extent of encountered weak zone, and helped

in determining the ancillary works that had to be performed before construction could resume Additional experience of three-dimensional geological modeling was reported by Houlding (1995), with reference to works planned in heterogeneous variable grounds, as part of the Tạpei metro extension project

These few examples of recent developments related to ground modeling illustrate some of the potentials offered by these techniques, which should probably be used on a more regular basis to assist in planning and conducting tunneling projects in the future

One major difficulty in tunnel construction relates to the length of the projects, which requires that geological conditions be interpolated on the basis of relatively limited information Improvements could be expected in this respect by both increasing the amount of geotechnical investigations and making the best possible use of all available information The development of decision aid systems, based on the statistical evaluation of geological data (Sinfield and Einstein, 1996) could assist the engineer in achieving a more accurate appreciation of construction hazards related to geological uncertainties

A software, termed DAT (Decision Aids for Tunneling), was developed by Einstein et al (1992) to provide some evaluation of the implications, in terms of tunnel construction cost and duration, of geological hazards This model makes use of probabilistic methods to account for geological and construction uncertainties in the simulation of some anticipated project characteristics and construction sequences It can help identify additional investigations that would be required to reduce cost uncertainties, and can be updated in the course of the project, and the cost and duration estimates adjusted accordingly

This model has been used for the St-Gothard and Lưtschberg major railway tunnel projects in Switzerland, to assist in the cost and duration evaluation process One contribution of the model was to demonstrate that a drastic reduction in the cost uncertainties related to the construction of the St-Gothard tunnel project could be anticipated, after a more accurate knowledge of the geology found in the difficult Piora area had been established The model also allowed to confirm, on the basis of all available information obtained after the required complementary investigations had been completed, that cost uncertainties would remain relatively low (in the order of ± 7%) for both projects and that the error on construction duration estimates would be less than one year (Dudt and Descoeudres, 1998)

3.2 Geophysical Investigations

Improved knowledge of geological conditions can also be obtained from the combined use of geotechnical and geophysical means of investigations Significant progress has been made over the past years in the applications of the latter approach to tunneling projects, both at the design stage and during construction Corbetta and Lantier (1994) reported on an experimental use of the “electrical cylinder” method, as part of the geological investigations completed for the construction of a water main in Bordeaux, France This tunnel had to

be excavated with an EPB shield in karstic grounds

The “electrical cylinder” technique consists in measuring the ground resistivity from a pre-drilled borehole It was used on this site to assist in locating existing voids or pockets of weaker materials within the ground before proceeding with excavation The method is designed to identify contrasts in ground properties in a 5 m deep area around the borehole, and can be implemented from the soil surface or through sub-horizontal boreholes driven from the front during construction

Other similar attempts have been made more recently to use different geophysical techniques at the face of advancing tunnels, including seismic investigation techniques (Bousquet-Jacq and de Sloovere, 1999) and geological radar surveying In particular, an extensive radar surveying program was performed during the construction of the EOLE urban train tunnel in Paris, using boreholes driven through the face of a slurry shield (Pierson d’Autrey et al., 1995) This tunnel was excavated in heterogeneous materials, and data obtained from the radar investigations used to identify areas of weak grounds where ancillary action had to be taken

These experiments were completed as part of a nationwide research program, termed EUPALINOS-2000, which aimed at improving technologies for managing shield tunneling in heterogeneous grounds The feasibility

of conducting radar investigations through the face of the machine was clearly demonstrated, but questions remained as to the possibility of extending the method to various ground conditions and the time efficiency of the process, as construction would need to be halted for the tests to be performed

Further work is underway to analyze potential improvements in that sense, and investigate the benefits of combining different geophysical methods at the tunnel face Another concept, consisting in using vibrations emitted by the TBM during construction, was also tested, as part of this research program and lead to promising results Such approach could contribute to reducing investigation times, as it would require no construction stoppages to be performed

An experiment using a similar principle was completed during the construction of the second Heinenoord tunnel, south of Rotterdam, Netherlands Swinnen et al (1999) This tunnel consists of a double tube excavated with a TBM, at shallow depth in soft soils The experiment was intended to analyze the possible use of shear waves emitted by the TBM to improve the knowledge of ground conditions to be found at the face It was demonstrated that the source for the shear waves would be located at the hydraulic jacks used to advance the shield, and that these waves could be recorded at the ground surface A second experiment was carried out after completion of the first tube, to check for possible uses of shear waves emitted by the TBM in ground investigation

Geophones were installed along the liner of the second tube under construction, in an attempt to record shear waves emitted during the drive that would be reflected by the already built tube The experiment showed clear evidence of such reflections, which tended to confirm the potential use of vibrations induced by the TBM to better check for anomalies in the surrounding ground Further work would however be required before this technique could be introduced for operational use These should primarily aim at improving the characterization

of the uncontrolled source waves, and locating receivers in such a way that geological conditions could be investigated in the ground ahead of the tunnel

Another approach based on the analysis of seismic signals was proposed by Neil et al (1999) who developed

a three-dimensional ground mapping system, termed ROCKVISION3D, for detecting ground anomalies (voids, hard inclusions,…) The system makes use of seismic tomographies to identify anomalies or geological boundaries that would be expected to act as reflectors to the seismic waves Data recorded during the investigations are processed to generate a ground model; this allows comparisons to be made between the modeled seismic response and the recorded travel times The model parameters (wave velocity, distance to anomaly) are then varied until the predicted response matches that recorded in the field

The authors discussed the potential application of this technique for investigating the ground ahead of TBM driven tunnels in hard rock (Figure 9) In that case the disk cutters would be used as seismic sources and arrays

of receivers would be installed at the TBM tailpiece to record the source signals and reflected waves This system would have the capability of producing real time mapping of the ground ahead of the tunnel face Questions would however seem to remain as to the required time for installing the receivers behind the TBM and the generalized use of the system for the variety of ground conditions that could be encountered when tunneling

in difficult ground

TopRock drillhole Probe – 2-in dia

Disc cutter

Direct waves generated

By disc cutter Geological

anomaly

Figure 9 : Ground mapping ahead of a TBM driven tunnel in hard rock (after Neil et al., 1999)

Seismic techniques, with calibrated signals, have also been developed to investigate the ground at the tunnel face, using sources installed along the tunnel walls (Sattel et al., 1996) or within the head of the TBM The latter was implemented on the shield designed for the 4th Elbe crossing in Hamburg, with a theoretical investigation range of 50 m ahead of the tunnel face (Herrencknecht, 2000)

Other tunneling applications of geophysical methods include the increased use of these techniques as part of preliminary investigations New fields of application could also be envisaged in this area, in association with recent developments in directional drilling technologies Directional drilling techniques, which were originally issued from the petroleum industry, have been increasingly used over the past ten years for trenchless pipe installations and could contribute to the development of geological investigation methodologies for tunneling (Mermet et al., 1997)

This is of particular relevance for long mountain tunnels, with limited access to the site, and where long vertical boreholes would otherwise be required to investigate existing ground conditions at the planned tunnel level Such investigations are currently underway, as part of the preliminary studies of the Lyons-Turin Rail Link, between France and Italy, in the Alps

In his state-of-the-art report on deep excavation and tunneling in soft ground, presented at the 7th International Conference on Soil Mechanics and Foundation Engineering, Peck (1969) introduced three main issues to be addressed for the design of soft ground tunnels:

- stability of the opening during construction, with particular attention to tunnel face stability;

- evaluation of the ground movements induced by tunneling and of the incidence of shallow underground works on surface settlements;

- design of the tunnel liner system to be installed to ensure the short and long term stability of the structure These three aspects are reviewed in the following sections, with reference to the concepts developed by Peck(1969) and to theoretical and practical contributions published over the past thirty years in this area

Design principles for bored tunnels -with particular attention to urban tunneling - were recently discussed in a comprehensive review by Mair and Taylor (1997), which should be referred to for detailed background on some

of the issues presented hereafter Additional insights into recent developments related to modeling and prediction techniques are also provided in papers presented at the International Symposium on the Geotechnical Aspects of Underground Construction in Soft Ground (Leca, 1996)

4.1 Tunnel Face Stability

Several approaches have been developed, over the past thirty years, to analyze the face stability of tunnels constructed in soft ground Most methods refer to the case of a circular tunnel constructed in homogeneous soil (Figure 10) The main parameters involved in the face stability are:

- the tunnel diameter, D and depth to axis, H (or depth of cover C = H-D/2);

- the soil unit weight, γ and pressure of overburden (or existing building), σS;

- the support pressure to be applied at the tunnel face (if required), σT;

- the soil shear strength

The shear strength can be characterized by the soil cohesion, c’ and friction angle, ϕ’ or, for tunnels in clays and other non-pervious soils, the undrained shear strength, su

Figure 10 : Model for tunnel face stability analysis

4.1.1 Undrained Stability in Cohesive Ground

Early works on face stability were concerned with tunnels constructed in clays (Broms and Bennermark, 1967; Peck, 1969) They allowed a stability criterion to be established, on the basis of the consideration of an overload factor, N,

u

T S

This result is illustrated in Figure 11 (after Mair, 1979; Kimura and Mair, 1981) where design curves for N*, derived from centrifuge model tests, are plotted as a function of the depth of cover to diameter ratio, C/D This Figure also introduces the influence of the length of unlined tunnel, P behind the face, often found when tunneling with conventional methods The plots obtained for different values of the ratio, P/D provide an indication on how face stability can deteriorate when long stretches of tunnel are left unlined behind the heading

Figure 11 : Critical overload factor, N* versus depth ratio, C/D (after Mair, 1979; Kimura and Mair, 1981) Works completed at Cambridge University also allowed to evidence the influence of the ratio, γD/su , which should be considered for characterizing the local stability of large diameter tunnels These conclusions were mainly derived from the results of model tests completed on the Cambridge centrifuge, as well as theoretical work based on the yield analysis principles (Salençon, 1990)

4.1.2 Stability in Sands and Other Frictional Materials

A similar approach was used more recently to analyze the case of tunnels excavated in sandy grounds, which had gained interest because of the number of tunnels constructed with pressurized shields in pervious water-bearing soils A method was proposed by Leca and Dormieux (1990) to estimate the support pressure, σT to be applied at the tunnel face, using a three dimensional failure mechanism, as described in Figure 12 This mechanism involved the rigid body motion of two conical blocks (labeled ① and ②, with velocities V1 and V2 in Figure 12) For dry cohesionless soils, the limiting support pressure, σT*, derived from this mechanism, can be expressed as,

DS

This approach was checked against model tests performed with Fontainebleau sand in the centrifuge of the Laboratoire Central des Ponts et Chaussées (LCPC) in Nantes, France (Chambon and Corté, 1994) and lead to values of the limiting support pressure in close agreement with experimental data Recent developments allowed this method to be extended to tunnels excavated in waterbearing soils (Leca et al., 1997) In that case, water

0 2 4 6 8 10

C/D

P/D = 2.0 P/D = 1.0 P/D = 0.5 P/D = 0

effects are introduced as additional loads, which magnitudes are derived from Finite Element seepage analyses (Atwa and Leca, 1994)

Figure 12 : Failure Mechanism considered by Leca and Dormieux (1990)

Similarly to the approach proposed by Davis et al (1980) for clays, this method presents the advantage of taking full account of the main factorrs involved in the front stability, and more importantly of the three-dimensional geometry found at the tunnel face This latter parameter should be viewed as an essential feature of the evaluation of the ground response at the tunnel face, as demonstrated by several analytical as well as numerical works (Davis et al., 1980; Chaffois, 1985) The same requirement applies to seepage analyses: three-dimensional numerical computations of seepage towards the tunnel face indicate a high concentration of hydraulic gradients around the tunnel face (Figure 13) which would not be properly evaluated with a two-dimensional model (Leca et al., 1993)

Figure 13 : Hydraulic gradients computed at the tunnel face

Another approach, taking full account of the three-dimensional geometry at the tunnel face, was proposed by Anagnostou and Kovári (1996a), using limit equilibrium principles This latter method is based on the consideration of a failure mechanism initially introduced by Horn (1961), and consisting of the rigid body motion of prismatic blocks (Figure 14) The stability analysis is run in effective stresses, with due account of water loads, and provides an estimate of the support pressure to be used at the face of a shield driven tunnel One key computational parameter is the magnitude of horizontal stresses acting upon the boundaries of the prismatic blocks, which needs to be empirically established

Both slurry and EPB shields can be considered, and charts are provided to allow for parameters such as head losses within the excavation chamber or slurry penetration into the face, to be accounted for A comparison was made between this method and that proposed by Leca and Dormieux (1990) on the basis of centrifuge model tests published by Chambon and Corté (1994), and lead to similar estimates of the limiting support pressure σT* (Anagnostou and Kovári, 1996b) for conditions used in the tests

One difficulty, when using most existing approaches for evaluating the stability of the tunnel face, is that they

were primarily developed for tunnels excavated in homogeneous materials Care must be taken when analyzing

heterogeneous conditions, as often found in the construction of shallow tunnels in soft grounds

An interesting case study in this respect was provided by the construction of section 1 of the Lille metro in France (Leblais et al., 1996) This tunnel was excavated using a 7.65m diameter EPB shield, in Flandres clay, with a depth of cover in the range 8.7-21.8 m At one location along the tunnel alignment, a face collapse occurred and a sinkhole, with an average diameter of 8 m, formed at the ground surface

A

F

KG

DC

JN

M

LH

E

B

ω

Figure 14 : Three-dimensional failure mechanism considered by Anagnostou and Kovári (1996a)

Figure 15 illustrates the soil profile found at this location, which consists of 5.8 m of silty and sandy loess, 4.8

m of clayey and sandy silts, and Flandres clay The upper one meter of the Flandres clay layer is weathered The Flandres clay is a stiff clay, with an undrained shear strength at this location in the order of 130 kPa When failure occurred, the shield was operated with a face pressure of 200 kPa, which means that the overload factor could be estimated to be in the order of N=2.5, i.e significantly lower than values classically associated with failure (N<5-7)

Theoritical failure mechanism

Figure 15 : Geotechnical profile found on section 1 of the Lille metro (after Leblais et al., 1996)

Table 3 summarizes the results of stability analyses completed in an attempt to explain the observed tunnel face collapse: the limiting overload factor, N* was estimated using Davis et al (1980) approach, with three different assumptions for the clay cover, C over the tunnel crown The first analysis assumed that the tunnel had been excavated in a homogeneous layer of clay (C/D=1.6) The two latter cases took account of the actual extent

of the clay layer, with (C/D=0.10) and without (C/D= 0.25) allowance for some weathering in the upper one meter In both cases the ground above the clay layer was conservatively assumed to act as a surcharge

Table 3 : Limiting overload factor estimates (after Leblais et al., 1996)

Clay layer limited to the extent of the Flandres clay 0.25 2.8

Clay layer limited to the extent of the unweathered Flandres clay 0.10 2.2

The results shown in Table 3 indicate that no tunnel face collapse could have been anticipated with the assumption that the tunnel would be driven in a homogeneous layer of clay Conversely the consideration of the actual extent of the clay layer lead to estimates of the limiting overload factor in the same order of that computed where failure occurred It is also worth mentioning that the extent of the sinkhole observed at the ground surface was similar to that derived from the theoretical failure mechanism as proposed by Davis et al (1980)

Another difficulty, when using existing methods for tunnel face stability analyses relates to the restrictive assumptions they imply, as these essentially apply to circular shield driven tunnels, where stability is achieved by means of the supporting action a pressure, σT applied at the front Such assumptions were found appropriate in view of the number of tunnels excavated in weak grounds with pressurized shields

With recent advances in conventional tunneling, large size tunnels can now be excavated full-face, in soft grounds In such cases, face support is not provided by a fluid or earth pressure but through the action of a reinforcing system, which primarily consists of fiberglass bolts Such configuration is not accounted for by design methods developed for shield driven tunnels Some attempts have been made to extend the use of existing solutions for shields to the face stability of tunnels excavated by means of conventional methods, with bolt support These usually introduce an equivalent support pressure, or equivalent soil cohesion (Grasso et al., 1991),

to account for the supporting action of the bolts

More recently, the approach proposed by Leca and Dormieux (1990) was extended to the case of conventional tunnels constructed in soft ground, with bolt reinforcement at the face (Leca et al., 1997) These developments also included the supporting action of a “pre-vault”, as used with the precutting method (Figure16) The “pre-vault” was modeled as a rigid boundary that restrained the possibility for failure to develop above the tunnel face Theoretical estimates derived from this approach showed that the “pre-vault” would have little impact on the face stability, and this was confirmed by model tests performed on the LCPC centrifuge (Skiker, 1995) The tests also allowed to evidence the restraining effect of the “pre-vault” with respect to ground deformations around the opening (Skiker et al., 1994)

γ V

ϕ '

Figure 16 : Failure Mechanism with a “pre-vault” associated with face bolting

Typical failure mechanisms observed in centrifuge tests, with and without a “pre-vault”, are illustrated in Figure 17 They show that, with no “pre-vault”, the failed area would tend to propagate towards the soil surface, whereas it was restricted to the ground at the front where a “pre-vault” of sufficient length was used As a result,

it was concluded that the main benefit of the “pre-vault” would be to help control ground movements around the face and, in turn, tunneling induced settlements

The bolting effect can be introduced in the stability analysis through additional loads provided by bolts intersected by the failure envelope (Leca et al., 1997) The contribution of face bolting to the stability of tunnel headings is well evidenced by the model Model tests performed in the LCPC centrifuge also allowed some experimental quantification of the bolting effect to be obtained (Al-Hallak, 1999)

The tests were analyzed by means of a three-dimensional Finite Element model, using the computer code CESAR-LCPC, and this approach was found to provide a reasonable representation of experimental results (El-Hallak, 1999) In this study, the individual action of every singular bolt was considered separately and allowed for in the model Another approach for introducing the bolting effect in Finite Element analyses consists in replacing the bolted ground by an equivalent homogeneous material Promising results have been obtained in applying this technique to the analysis of bolt reinforced tunnel sections (Greuell, 1993; Greuell et al., 1994)

Figure 17 : Failure mechanisms observed in centrifuge model tests

Advances in computer systems have allowed complex conditions to be accurately modeled by means of numerical methods, thus providing new approaches for characterizing the stability of shallow tunnels constructed

in soft ground Sloan and Assadi (1997) developed a Finite Element model for the evaluation of the dimensional stability of tunnel sections excavated in clays One interesting feature of this approach was to introduce the influence of an increase in undrained shear strength with depth, which can usually not be accounted for with analytical solutions

two-A similar approach was used by two-Antao (1997), on the basis of works performed over the past twenty-five years at the LCPC in France (Friaâ, 1978; Frémond and Friaâ, 1979; Guennouni and Le Tallec, 1982) It is based

on cinematic solutions derived from visco-plastic Finite Element analyses of the ground response to tunneling This numerical method was introduced by Jiang (1992), in a subroutine, termed LIMI, of the Finite Element program CESAR-LCPC; it was primarily intended to provide solutions for the stability of earth structures, and then extended to the analysis of shallow tunnels constructed in soft ground (Antao et al., 1997)

The model allows failure mechanisms to be automatically generated through an optimizing process of numerically obtained cinematic solutions It was applied to the stability of shallow tunnels and provided results

in close agreement with published analytical solutions for both the two-dimensional tunnel section and dimensional tunnel heading cases (Antao, 1997)

three-Unlike analytical methods, this approach does not require any pre-established failure mechanism to be considered It also presents the advantage of allowing a more accurate representation to be made of the actual geometry and geotechnical conditions found on the site Improvements would however be needed in terms of required computer time, to allow a more general use of such tools Work is currently underway at the LCPC to increase the numerical efficiency of the model and extend its application to a broader range of geotechnical conditions

4.2 Evaluation of Tunneling Induced Ground Movements

4.2.1 Surface Settlements

The construction of a tunnel produces some deformation of the surrounding ground, which could result in surface settlements when the excavation is performed at shallow depth In that case, a settlement trough is formed at the ground surface (Attewell et al., 1986), and tends to propagate together with the advance of the tunnel heading (Figure 18)

Based on observations made on several tunneling projects, Peck (1969) proposed that the settlement trough produced at the ground surface could be characterized by means of a reversed error function curve as shown in Figure 19

With this assumption, the surface settlement, s(x) observed at a distance, x from the tunnel center-plane can

be computed using the following mathematical expression:

( ) =   − 2 

2 max

2 exp

i

x s

x

s (3)

where smax is the maximum surface settlement produced above the tunnel crown, and i the distance to centerplane

of the inflexion point in the reversed error function curve

Extent of surface settlement trough

S max

z y

x

H

Figure 18 : Three-dimensional distribution of tunneling induced settlements (after Attewell et al., 1986)

Equation (3) allows an expression of the volume of settlement trough, Vs per unit length of tunnel to be obtained:

As a result, the settlement trough induced by tunneling can be entirely characterized by means of two parameters:

- the volume of settlement per unit length of tunnel, Vs and

- the distance to centerplane of the settlement curve’s inflexion point, i

Based on these considerations, semi-empirical methods were proposed by several authors to evaluate the magnitude and distribution of tunneling induced settlements; these include works by Peck (1969), Cording and Hansmire (1975), Clough and Schmidt (1981), Fujita (1981), O’Reilly and New (1982), Attewell et al (1986), Rankin (1988), Uriel and Sagaseta (1989)

The distribution parameter, i is mainly dependent on the depth to tunnel axis, H and nature of ground conditions A recent review of existing correlations by Mair and Taylor (1997) concluded that this parameter could be reasonably estimated, using the following expression:

H K

i= (5)

with the coefficient, K being a function of the ground type The settlement trough tends to be of broader extent in clays than in sands, with K values typically in the range 0.4-0.6 for tunnels in clays and 0.25-0.45 in sands It must be emphasized that these estimates were derived from observations made in mostly homogeneous grounds, which means that some adjustments would be required when estimating tunneling induced settlements in heterogeneous soils (Mair and Taylor, 1997)

The volume of settlement trough, Vs is generally more difficult to evaluate, as this parameter is highly dependent on construction methods as well as workmanship This parameter is usually compared to the volume

of ground loss produced at tunnel level, and expressed as a percentage, Vl of the theoretical volume of excavated ground For a circular tunnel of diameter, D, this leads to:

4

.

2

D V

Vs = l π

(6)

with Vl being expressed in percent

Sources for ground loss were analyzed by Cording and Hansmire (1981) for the shield driven tunnel case, which can be considered as the most complex as far as the ground response to tunneling is concerned

Four major contributions to ground loss can usually be identified (Figure 20):

(1) face intake due to stress relief associated with ground excavation;

(2) displacements along the shield: these included deformations induced by shear stresses along the shield, and inward displacements due to deviations of the machine, as well as actions taken to ease shield advance (overcutting, conical shaped skin);

(3) ground movement into the tail gap (that forms as a result of the shield outer diameter being larger than the lined tunnel section);

(4) liner deformations (usually of limited magnitude for reinforced concrete segments)

Depending on the characteristics of the soils at and over tunnel level, ground losses could propagate fully or partly towards the ground surface

Tunneling in dense dilating sands usually results in surface settlements, which are lower than the cumulated volume of ground losses produced at tunnel level Conversely surface settlements in loose or compressible soils could lead to larger amounts of settlements at the ground surface The immediate ground response to tunneling in saturated clays can be assumed to take place with no volume change, in which case ground losses produced at tunnel level tend to result in an equal amount of settlement at the ground surface Additional settlements could take place in the longer run with cohesive soils, because of consolidation effects associated with pore water pressure changes induced around the tunnel during construction

Some correlations have been proposed (Clough and Schmidt, 1981; Attewell and Yeates, 1984) to provide an order of magnitude of the amount of tunneling induced settlements Based on a semi-empirical approach, Clough and Schmidt (1981) established that the volume of settlement trough for tunnels in clays should be related to the overload factor N, with values typically in the range 0-1% of the excavated volume, for overload factors lower than 2, and up to 10% of the excavated volume, for overload factors in the range 2-4 Other correlations were proposed for tunnels in sands by Attewell and Yeates (1984)

Such correlations can provide an indication of the maximum amount of settlement that could be induced by tunneling works, but do not account for the potential for failure to take place due to accidental losses of face support during construction Conversely, advances in the tunneling industry, particularly in the field of shield tunneling, have allowed significant improvements to be made in terms of ground motion control, which should allow settlements to be kept under significantly lower levels than predicted by existing semi-empirical estimates

Face

intake

Movement into the tail gap

Surface settle ments

Displace ments along the shield

Shield Liner segments

Figure 20 : Contribution to ground losses around a shield driven tunnel

A review by Mair (1996) of recent case histories lead to the following conclusions (Mair and Taylor, 1997): (1) open face tunneling in stiff clays usually results in Vl values in the range 1-2 %;

(2) closed face (EPP or slurry) shields allow settlements to be kept under relatively low levels, with Vl being usually lower than 0.5 % in sands and in the order of 1 to 2 % in soft clays;

(3) larger amounts of settlements could result from the use of shields, including of the pressurized EPB or slurry type, in mixed face conditions;

(4) conventional tunneling with sprayed concrete liner can provide satisfactory ground motion control (e.g

Vl values in the range 0.5-1.5 % were recorded on recent tunneling projects in London clay)

Among the case histories considered by Mair (1996), particular attention is due to the Cairo metro Line 2 project, in Egypt (Ata, 1996) which was excavated with a 9.48 m diameter slurry shield in the Nile alluvium, under the water-table Ground losses on this project were in the range 0.2-1.0 %, with an average of 0.5% This value should be compared to measurements taken during the construction of the Villejust High Speed Rail tunnel (TGV), in France (Leblais and Bochon, 1991), which was excavated by the same contractor as in Cairo, using a 9.25 m diameter shield in fine sands, with slurry pressure at the face

Ground losses on this latter project were kept in the same range as in the Cairo case (0.22-0.90 %) above the water table, but reached higher values, in the order of 0.77-1.32 %, in saturated areas, which could yet be considered promising at that time (New and O’Reilly, 1991) This comparison, of two case histories of slurry shield tunneling projects completed in similar conditions, is illustrative of the progress that has been accomplished in the past decade: in average, ground losses were reduced by 50 %, which is considerable given the already high standards achieved in the late 1980s

These improvements were mainly obtained because of the experience gained in controlling ground motion into the tailpiece gap Settlement values in the same order as those experienced in Cairo were reported in other recent projects such as the construction of the Lyons metro extension, in France, using a slurry shield in mixed alluvial deposits (Ollier, 1997; Kastner et al., 1996) Based on these observations, it is proposed that a typical ground loss of 0.5 % be used for settlement analyses in alluvial soils

The prediction of tunneling induced surface settlements is important in view of the potential damages they could produce to above ground structures These are usually associated with excessive amounts of absolute and furthermore differential settlements within the structure, which can be appreciated from the evaluation of the magnitude and lateral distribution of surface settlements (Figure 21, after Mair et al., 1996) Care should also be taken, in this respect, of structures located along the tunnel center-plane, that could experience some temporary differential settlements during construction, as a result of the settlement bowl progressing above the tunnel heading (Leblais et al., 1995 & 1999; Mair and Taylor, 1997)

4.2.2 Horizontal Surface Displacements

Damages to above ground structures could also result from horizontal ground deformations induced by tunneling It is usually assumed (Leblais et al., 1995 & 1999; Mair et al., 1996) that the horizontal surface displacement, sh (x) at a distance, x from the tunnel center-plane can be expressed as:

( ) s ( ) x H

x x

sh = (7)

where H is the depth to tunnel axis and s(x) the settlement at a distance, x from the tunnel center-plane This approximate expression, which is based on studies by Attewell (1978) and O’Reilly and New (1982) relative to tunnels in clays, has been found to provide reasonable estimates, for practical purposes, of horizontal deformations observed in different project conditions (Mair and Taylor, 1997)

hogging zone

sagging zone

building H

Figure 21 : Deformations induced to buildings located above a tunnel (after Mair et al., 1996)

Ground motion predictions should be used in relation with the sensitivity of above ground structures to vertical and horizontal deformations Three damage categories are usually be considered (Leblais et al., 1995 & 1999):

(1) architectural (essentially related to visual appearance);

(2) functional (disruptive to building usage);

(3) structural (could affect the building stability)

More elaborate characterizations of damages to buildings were proposed by Burland et al (1977) and Boscardin and Cording (1989), on the basis of a review of several case histories In this latter classification, damages were quantified in terms of the estimated tensile strain produced in the building (Table 4)

Table 4 : Damage categories (after Boscardin and Cording, 1989; Mair and Taylor, 1997)

Category of damage Normal degree of severity Limiting tensile strain (%)

0 Negligible 0-0.05

3 Moderate 0.15-0.3

Further works by Mair et al (1996) allowed a method for risk assessment of tunneling induced damage to buildings to be established This method is based on a correlation of the category of damage to the combined

effects of the maximum vertical relative deflection and horizontal strain produced in the structure (Mair and Taylor, 1997)

4.2.3 Subsurface Displacements

With the increasing use of soft ground tunneling in urban areas, situations more often arise where construction may affect existing underground structures, including other tunnels, buried pipes or piled building foundations As a result, it becomes necessary to be able to predicting, not only the magnitude and distribution of surface displacements, but the whole field of ground movements around the tunnel to be excavated This issue was addressed by Mair et al (1993) who proposed solutions for extending existing methods for surface settlement estimates to the prediction of tunneling induced subsurface settlements (Mair and Taylor, 1997) Specific studies were also completed on side by side tunnels, and provided some basis for comparison with the single tunnel case, in terms of modifications to surface settlements and/or liner loads induced on each interacting tunnel (Cording and Hansmire, 1975; Ghaboussi and Ranken, 1977; Leca, 1989)

Some of these studies made use of numerical methods These techniques, and particularly the Finite Element Method, have tremendously developed over the past thirty years, and provided other efficient means of characterizing the ground motion to tunneling Additional insights into the overall ground response to tunneling can now also obtained from physical modeling, primarily due to advances in centrifuge model testing

4.2.4 Numerical Models and their contribution to the prediction of tunneling induced ground movements

A review of Finite Element modeling applied to soft ground tunneling was completed by Clough and Leca (1989) who examined the contribution of numerical methods to the analysis of the complex soil-structure interaction phenomena associated with the construction of tunnels in soils The more recent advances related to these techniques were further analyzed by Mair and Taylor (1997) and Leca and Mestat (1999) Finite Element modeling can be considered as a powerful means of analysis, because it allows the main parameters involved in the tunneling process to be accurately accounted for These include:

- the actual geometry of the project (tunnel shape, size and depth, soil layering,…);

- the ground behavior (constitutive law associated with each soil layer);

- the construction sequence (a variety of loading conditions can be introduced to account for the different construction phases associated with each excavation technique)

Additional benefits can be found in the broad variety of outputs obtained from the analyses, which include the whole field of underground movements and surface settlements induced by tunneling, as well as liner loads to be accounted for in the design of the tunnel liner

Conversely, the ability for performing sophisticated numerical analyses should not elude limitations associated with these techniques, nor should it eliminate the need for conventional methods, which are founded and calibrated on the basis of experience and well documented evidence Numerical methods should not be viewed as a substitute to conventional methods, but rather as a means of filling gaps that may exist in the conventional approaches, as well as a tool for acquiring a better understanding of the complex ground response

to tunneling

The following limitations should be considered when using numerical methods for tunneling:

(1) the tunneling process involves a three-dimensional deformation pattern around the front, which should be accounted for in the analysis; the ground response to tunneling can be analyzed by means of three-dimensional Finite Element models, but this process is perceived costly and time consuming;

(2) the behavior of soils and soft rocks is complex and can hardly be accurately modeled, even with the most elaborate constitutive models; moreover, parameters such as the soil deformation modulus, which are essential to the output of numerical analyses, can hardly be estimated with sufficient accuracy with existing testing techniques;

(3) the construction techniques involve complex soil-structure interaction phenomena; these should be well appreciated and their effect accounted for in an appropriate manner in the model;

(4) the practical accuracy of existing monitoring methods can hardly provide the basis for validating some of the most elaborate numerical tools

Significant progress has been made over the past ten years in these fields, which has allowed some of these difficulties to be overcome In particular, a more general use can now be made of three-dimensional and non-linear analyses, due to advances in numerical methods and computer power Nevertheless, there are still hesitations to recur to such approaches because they are often perceived as time consuming in terms of data preparation and result processing