Underground works in soils and soft rock tunnnelling doc

UNDERGROUND WORKS IN SOILS AND SOFT ROCK TUNNELING Eric Leca 1 , Yann Leblais 2 and Karl Kuhnhenn 3 ABSTRACT Growing needs for modern transportation and utility networks have increased the demand for a more extensive and elaborate use of underground space. As a result, more underground projects have to be completed in a variety of ground conditions, including weak water bearing soils and soft rocks. Significant technological advances have rendered these projects possible, but have also given rise to new challenges as many of these projects have to be completed in difficult conditions, with very strict environmental constraints, particularly in urban areas where the potential impact of tunneling on existing structures is a major concern. This report addresses the main aspects of tunneling and underground works performed in soils and soft rocks. A summary is presented of the main features related to construction techniques, ground investigations, design methods, and instrumentation and monitoring practices, as well as of some of the more recent advances in these fields. Significant progress has been made in the area of soft ground tunneling over the past thirty years, partly because 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. 1.0 INTRODUCTION 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 water- 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. 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, Englerstra8e 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. 2.1 Shield Tunneling Developments related to the shield tunneling technology have been reviewed in Clough (1981), Béjui and Guilloux (1989), Fujita (1989). A technique using a concept close to shield tunneling was used for the first time in the mid-1880s in UK, to build a pedestrian gallery underneath the Thames River. Major advances have been accomplished since then, particularly with the introduction of shields of the pressurized type, which allow tunnels to be constructed in all types of soils including sands under high water head. These include the slurry shield and the Earth Pressure Balance (EPB) shield. The main features of the slurry shield technique are presented in Figure 1 (after Fujita, 1989). 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 4 th crossing of the Elbe River (14.20 m diameter) in Hamburg, Germany (Bielecki and Zell, 1999). A 14.87 m 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 Erector Shield 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 (Figure 3): (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”. Concreted Pre-vaults Cut 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. a. Longitudinal Section b. Cross-Section 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 Dewatering ➎ Bolts ➊ ➍ Compaction grouting ➋ Fracture grouting ➋ ➌ Jet grouting ➊ ➋ ➍ Freezing ➊ ➌ ➍ Micro-piles ➋ ➍ Jet grouting ➊ ➌ ➍ Pre-vault ➊ Permeation grouting ➊ ➌ ➍ Soil mixing ➊ ➍ Effect on stiffness ➊ Effect on displacement ➋ Effect on permeability ➌ Effect on strength ➍ 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) grouting displacement displacement hydraulic compaction penetration bulk filling fissure/contact grouting bulk filling subprinciple fracturing permeation 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) HOST RANGE NON-DISPLACEMENT GROUTING DISPLACEMENT GROUTING MEDIUM PERMEATION FISSURE OR 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 Cement based 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 Fissured rock Cracks, fissures 0.1 mm < c < 100 mm Cement based suspensions, Microfine suspensions Microfissures c < 0.1 mm Microfine suspensions, Silicate gels, Special chemicals Cavity Large voids Cement based mortars, 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 Silacsol TM 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 200 kPa 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 (Figure 6). 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 water- tightness; (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. 3.0 GEOLOGICAL AND GEOTECHNICAL INVESTIGATIONS 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 Shaft Tubes à manchette Thames gravel London clay Tunnel Building [...]... characterizing the magnitude of ring forces and bending moments to be anticipated on tunnel liners, and have been increasingly used in tunneling practice, including for the design of segmental concrete liners used in shield tunneling Care should however be taken, with shield tunneling, of “external” loading conditions, as defined in Peck’s (1969) design principles These should include loads involved in handling... obtained with the Einstein and Schwarz (1977) simplified approach are plotted in the interaction diagram reproduced in Figure 27, together with envelopes of allowable combined thrust/moment loads for three categories of liners: 20 cm shotcrete, 40 cm plain concrete, and 40 cm reinforced concrete Also plotted in Figure 27 are liner loads obtained for tunnel sections in loosened rock, as well as handling... 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. .. Peck (1969) and recently reviewed by Mair and Taylor (1997) Liners should be designed to withstand the anticipated ring forces and bending moments resulting from the soil liner interaction In addition, provisions should be made to achieve adequate contact conditions between the ground and liner and to allow for “external” loading conditions, such as jacking loads associated with shield tunneling or stress... results in Figure 27 show that: (1) a 40 cm thick plain concrete liner would be able to withstand most ground load conditions, with the exception of areas in loosened rock; (2) ground loads in loosened rock and handling loads would produce the most severe conditions and require some reinforcement of the liner segments to be provisioned Steiner and Meier (1996) also commented on design constraints associated... 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... segments and shield jacking operations Guidelines for the design of concrete segmental liners used in shield tunneling were proposed by Guédon et al (1998) This issue was discussed by Steiner and Meier (1996), in relation with the construction of the Grauholz tunnel in Switzerland This 10.6 m internal diameter tunnel was constructed in mixed glacial grounds using a TBM, with the machine being operated... three-dimensional Finite Element model, using the computer code CESAR-LCPC, and this approach was found to provide a reasonable representation of experimental results (ElHallak, 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... 1979) can be used to introduce three-dimensional effects in 2D Finite Element models With this approach, termed the “progressive softening” method, tunnel construction is modeled in three stages: (1) evaluation of initial ground stresses; (2) “softening” of the ground in the excavation area; (3) activation of liner elements and application of excavation loads et the soil-liner interface In the second stage... 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 . UNDERGROUND WORKS IN SOILS AND SOFT ROCK TUNNELING Eric Leca 1 , Yann Leblais 2 and Karl Kuhnhenn 3 ABSTRACT Growing needs for modern transportation and utility networks. addresses the main aspects of tunneling and underground works performed in soils and soft rocks. A summary is presented of the main features related to construction techniques, ground investigations,. 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: