International Conference on Tunnel Boring Machines in Difficult Grounds (TBM DiGs) Singapore, 18–20 November 2015
THE DEVELOPMENT AND APPLICATION OF GUIDANCE DOCUMENTS FOR GROUND CONTROL FOR SLURRY AND EPB TBM TUNNELLING IN HONG KONG Nick Shirlaw1, David Salisbury2, Patrick Chau3, P.L.R. Pang4 1
Golder Associates (Singapore) Pte Ltd., Singapore. Email:
[email protected] Mass Transit Railway Corporation, Hong Kong. Email:
[email protected] 3 Geotechnical Engineering Office, Hong Kong. Email:
[email protected] 4 Geotechnical Engineering Office, Hong Kong. Email:
[email protected] 2
ABSTRACT: The Geotechnical Engineering Office (GEO) of the Civil Engineering and Development Department in Hong Kong published GEO Report No. 249, on ground control for slurry TBM tunnelling in 2009. The companion report GEO Report No. 298, for EPB TBM tunnelling, was published in 2014. These reports serve as a basis for the review of geotechnical control submissions made by practitioners on slurry and Earth Pressure Balance (EPB) TBM tunnelling proposals, for consultation with the Hong Kong Special Administrative Region Government (HKSARG). The reports provide a framework for the calculation of the face and tail void grouting pressures required to guard against Ultimate and Serviceability Limit States, under the ground and groundwater conditions commonly encountered in Hong Kong. They also provide guidance for the calculation of compressed air pressures for TBM interventions. The risk of localized settlement (potentially leading ultimately to a sinkhole) and heave (potentially leading to a blow-out) is considered, with guidance on the risk factors involved in such events. The genesis of the reports, and what was included and excluded in terms of content is discussed. The methods outlined in the reports have been applied on a number of completed or ongoing TBM tunnel projects. The methods have generally proven robust, but appear to be slightly conservative, in practice. The experience of working to the reports will be outlined, with comments on issues arising from that experience. KEYWORDS: TBM; Slurry; EPB; Saprolite; Weathering; Sinkhole; Intervention; ULS; SLS
1. INTRODUCTION The Geotechnical Engineering Office (GEO) of the Civil Engineering and Development Department (CEDD) in Hong Kong exercises geotechnical control on tunnel works in Hong Kong to protect public safety. In order to facilitate the control, the GEO published GEO Report No. 249, on ground control for slurry TBM tunnelling in 2009 (Pang, 2011). The companion report GEO Report No. 298, for EPB TBM tunnelling, was published in 2014. The core of these reports provides guidance on the calculation of the necessary support pressures in order to design against occurrence of ultimate limit states and serviceability limit states. In order to meet SLS requirements, the appropriate minimum support pressure needs to be applied at the face, along the skin and at the tail void (by grouting) of the TBM, during TBM driving and during interventions. This paper will discuss the genesis of these reports, how they were developed, and present some experience with the application of the guidelines outlined in them.
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2. GENESIS OF REPORTS 2.1 Report No. 249 on Slurry TBM Tunnelling Hong Kong has a population of over 7 million and a land area of about 1,100 km2. About 12% of the land area is used for buildings, and another 12% for transportation; much of the remaining land is hilly, with a large proportion designated as Country Parks. As a result, Hong Kong includes some of the most densely populated areas on earth. Because of this dense urbanization it is important to develop and utilize underground space. Extensive use of tunnelling is necessary, primarily to provide the utility and transport infrastructure required to supply and to connect the various urban centres. A key issue in urban tunnelling is the need to control the ground movements associated with the tunneling. If the ground movements are not adequately controlled this could lead to excessive settlement, with possible damage to buildings and other structures and utilities. Ultimately, inadequate ground control can lead to a sinkhole (Figures 1 and 2). A sinkhole in an urban area poses a serious risk to life.
Figure 1. Sinkhole above TBM exiting grout block, Canton Road, October 2006
Figure 2. Sinkhole above TBM passing from rock to soft ground, Salisbury Road, June 2007
Hong Kong is hilly, with the urban centres separated by, and partially built on, hills and ridges. The top of Victoria Peak on Hong Kong Island is at 552m, while the top of Tai Mo Shan is at 957m above sea level. The hills are largely formed of granitic and volcanic rock, although some sedimentary and metamorphic rocks are also found, principally in the New Territories. Deep tropical weathering has produced a mantle of saprolite over the less weathered rock; it is not uncommon to find this mantle to be in excess of 60m in depth, and much deeper locally. Stream channels have been eroded locally into the saprolite. These stream channels have been partially infilled with alluvial and/or marine deposits. There has been extensive reclamation at littoral margins. The groundwater pressure in many of the most densely populated areas is typically hydrostatic, with the groundwater level about 1 to 3m below ground level. A significant proportion of the tunnelling in Hong Kong has consisted of rock tunnelling, for road and rail connections through the hills, for surface water drainage, and for a deep sewer system. However, much of the urban tunnelling has been through soil or mixed ground conditions, comprising soil and rock. While a majority of the soft and mixed ground tunneling has been in saprolite and weathered rock, alluvial and marine soils have also been encountered, as well as reclamation fill. Successive phases of reclamation have commonly resulted in a series of old sea walls that have been buried by subsequent reclamation. Sea walls in Hong Kong are typically formed of large blocks of granite over a sand key; the sea walls have posed a significant issue for tunnelling and shaft construction. The early phases of the Mass Transit Railway (MTR) system were built in the 1970s and 1980s, and involved a large proportion of soft ground and mixed ground tunnelling. These early phases comprised the Modified Initial System (MIS), the Tsuen Wan Extension (TWE) and the Island line (ISL). Much of the tunnelling on the early phases involved the use of open face shields with pre-cast concrete linings. Tunnelling in soft and mixed ground was also carried out using sequential excavation methods with sprayed concrete linings. The tunnelling was under compressed air (mainly), or in ground treated by grouting or dewatering, to help control the stability of the soft ground below the phreatic surface. The compressed air
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pressure used for this tunnelling was based on trials (Haswell et al., 1980) and determined by observation, rather than by calculation. Earth Pressure Balance (EPB) TBMs were used to drive tunnels for part of the West Rail line (completed in 2003), and for the Lok Ma Chau Spur line tunnels (completed in 2007). Slurry TBMs were used for the Kai Tak drainage tunnel (completed in 2004) and part of the Kowloon Southern Link (KSL), completed in 2008. Detailed calculations of operating pressure were prepared in advance of the slurry shield tunnelling for the KSL. Advice on the basis for these calculations was provided by an expert from the US employed by the contractor, and review, with independent calculations, was carried out by another international expert employed by the GEO. The approaches used for the calculations and checking were based on published papers and experience from outside Hong Kong. The West Rail and Kowloon Southern Link involved a limited length of pressurized TBM tunnelling. However, under the Railway Development Strategy 2000 the HKSARG outlined a number of new railway lines. These were subsequently implemented as five major railway projects: West Island Line, South Island Line (East), Shatin to Central Link, Express Rail Link and Kwun Tong Line Extension. In addition to these railway projects, major drainage and road projects were also being planned. Extensive tunnelling would be required for many of the projects. These projects are either still under construction or recently opened (as of mid-2015). As the planning of the projects developed, it became apparent that there would be a major increase in the use of pressurized TBMs in Hong Kong. There had been two major, but localized, incidents of loss of ground during the construction of the KSL tunnels using a large diameter slurry TBM (Pang, 2011; Pang, 2014). Localized, but large, settlements and sinkholes had been documented over EPB tunnels in Singapore (Shirlaw et al., 2003) and over slurry and EPB shields elsewhere (Shirlaw et al., 2003; BTS/ICE, 2005). The need for a basis for detailed assessment of the appropriate operating support pressures and procedures for pressurized TBMs, as a risk control measure for such incidents, was evident. For the design and construction of deep cut and cover excavations, there was extensive experience within Hong Kong, and relevant standards and guidance documents were available. In comparison, at the time, there was limited experience in Hong Kong with pressurized TBM tunnelling, particularly the calculation of the critical support pressures required. There were no standards for such calculations. Although there were published papers outlining the basis for the calculations, these were scattered in a variety of publications, and there was no common basis on how they should be applied in practice. It was considered that, with guidance and training, GEO staff would be able to undertake the review of operating support pressure calculations, carried out by contractors and consultants, for slurry and EPB TBMs. A guidance document was therefore prepared; which was initially intended for internal use within GEO. The first draft of the guidance document was produced by the end of 2007, largely based on expert advice and review material produced for the KSL project and the general success of the TBM tunnelling undertaken on that project. However, during the review of the draft, a question was raised on the use of the document. If it was for GEO internal use only, contractors and consultants would be making submissions to GEO without being aware of the basis on which their submissions were being reviewed. Rather than keeping the document for internal use only, it was considered appropriate to make it available to the industry. This, in turn, meant that it was necessary to have the document peer reviewed by a group of experienced professionals, representing a cross-section of the industry, prior to issue. After peer review and further editing, the document was finally issued in early 2009, as GEO Report No. 249 Ground Control for Slurry TBM Tunnelling in Hong Kong, with a soft copy version that could be downloaded for free from the CEDD website. There are numerous other geotechnical guidance documents and reports available for downloading on the CEDD website, and the guidance document became the next report in the numerical sequence. 2.2 Report No. 298 on EPB TBM Tunnelling
Report No. 249 was prepared following the experience of the KSL project, and was specific to slurry TBM tunnelling. However, this immediately raised the question as to what extent the document could be applied to EPB TBM tunnelling. The Working Group on Cavern and Tunnel Engineering of the Hong Kong Institution of Engineers’ Geotechnical Division, which is made up of industry and government representatives, took it upon themselves to prepare a draft guidance document. The draft document went through a similar peer review process as Report No. 249, and the final document, GEO Report No. 298 Ground Control for EPB TBM Tunnelling in Hong Kong, was issued in 2014.
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3. TECHNICAL CONTENT 3.1 Report No. 249 The section titles and respective number of pages for Report No. 249 are given in Table 1. Table 1. Summary of the sections in Report No. 249 Section title Scope and objectives Symbols and glossary Slurry pressure assessment Tail void grouting Excavation Management System (volume measurement) High Risk Activities References, Tables, Figures, Plates
Number of pages 1 1 22 2 2 2 19
As can be seen in Table 1, the primary focus of the report is on assessment of the appropriate slurry pressure required to provide support to the ground. Most of the references, figures and plates are related to this topic. However, tail void grouting, measurement of the volume of ground excavated, and risk identification and control are also important topics related to ground control. These items are discussed briefly to supplement the main section on assessment of the appropriate slurry pressure. 3.1.1 Slurry pressure calculations Report No. 249 was prepared for use in Hong Kong, and consideration was given to the typical ground and groundwater conditions encountered, although the karstic conditions encountered in the North West New Territories were specifically excluded. As discussed above, much of the soft and mixed ground tunnelling in Hong Kong involves tunnelling through saprolite and weathered rock, with relatively high piezometric pressures. The Grade V (completely decomposed) rocks of granitic and volcanic origin have a permeability similar to silty sand, and will flow or erode/ravel rapidly when subjected to seepage. Effective stress calculations are therefore appropriate. For tunnels in granular soils below the phreatic surface, the piezometric pressure is the dominant component in the face support (confinement) pressure required. Shirlaw (2012) provided worked examples where the piezometric pressure represented over 80% of the target pressure for tunnelling. Where there is a high groundwater table in granular soils, a high proportion of the face pressure balances the groundwater pressure, and only a small proportion of the pressure is required to support the soil skeleton. In Hong Kong, a tunnel drive can experience rapidly changing piezometric pressure. Factors in changing piezometric conditions can be the steep topography under and near the hills and the tidal changes for tunnels near the sea. In a heavily urbanized environment, with many tall buildings, the magnitude of the surcharge can also vary significantly. In order to allow for the potential variation in groundwater pressures and surcharges, the report recommends that one calculation section is carried out for every 10m to 50m along the tunnel, i.e. 20 to 100 calculation sections per kilometer of tunnelling. Given the quantity of calculations required, the report is based on simple, chart-based methods of analysis, so that numerous calculations can be presented in a single spreadsheet. These spreadsheets can then be easily input into the programmable logic control (PLC) systems of modern TBMs. The use of numerical analysis is not excluded, but is seen as being used to supplement, or for the refinement of, the chart-based methods. Much of the soft and mixed ground tunnelling in Hong Kong is likely to encounter saprolite, or alluvial or marine sand. For these ground conditions, the charts published by Anagnostou and Kovari (1996) are referenced in the report. The assessment of effect of surcharge is not covered in the Anagnostou and Kovari paper, so a further reference is made to Atkinson and Mair (1981) with respect to surcharges. Reference is made to alternative calculation methods to those given in Anagnostou and Kovari: specifically, reference is made to papers by Jancsecz and Steiner (1994), and to Broere (1998) and (2001). For tunnelling in clay, undrained analysis is recommended. Reference is made to the results of the testing of model tunnels in a geotechnical centrifuge, published by Kimura and Mair (1981). Calculations for both Ultimate Limit State (ULS) and Serviceability Limit State (SLS) are required. The methods proposed by Anagnostou and Kovari are strictly for ULS, and make no reference to settlement control. As a result guidance had to be given in Report No. 249 on how to assess SLS in granular soils. The
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calculations for the KSL tunnels had addressed this by making the analogy to the design soil pressures on a stiff tunnel lining. If the face pressures were of similar magnitude, it was implicit that ground movements would be of small magnitude. The design soil pressures on a stiff lining could be obtained by reference to an old, but still commonly used, manual written by Proctor and White (1977). By the time Report No. 249 was finalised, the KSL tunnelling was complete, and it was apparent that the pressures derived on this basis had generally resulted in effective control of surface settlement. The calculation methods outlined by Anagnostou and Kovari and Kimura and Mair provide the magnitude of the soil and groundwater pressures that need to be balanced during tunnelling. However, there were a number of issues on how to apply these in practice. Recommendations on these issues are provided in Report No. 249. Four issues addressed were: •Surcharge •Distribution of pressure over the face •The fluctuation in face pressures during TBM driving •Excess pore water pressures generated during slurry TBM advance The assessment of surcharge is not covered in the Anagnostou and Kovari paper, so a further reference is made in Report No. 249 to Atkinson and Mair (1981) with respect to the effect of a uniform surcharge at the ground surface. For undrained analysis, a uniform surcharge is considered in the work of Kimura and Mair. The confinement pressure provided by a slurry TBM increases between tunnel crown and invert, due to the self-weight of the slurry. The calculated face pressure to be resisted increases, for effective stress calculations, based on the unit weight of water. As the unit weight of the slurry is greater than that of water, it is recommended that the initial calculation of the target face pressure is at tunnel crown. This pressure is then increased to obtain the target face pressure at the level of the pressure sensor. As there are always several pressure sensors on a TBM, it is important to define which sensor the calculations relate to. Pressures will fluctuate during TBM driving. As the ULS pressure is the minimum necessary to achieve the required Factor of Safety, it is necessary to allow for the fluctuation, to ensure that the pressure does not fall below the minimum value. A typical value used for slurry shields is +/-0.2 bar. Under stringent control by the TBM operator, the pressure fluctuation can be kept below +/-0.05 bar, but this will require close monitoring and verification in each case. As discussed in Broere (2005), slurry TBM tunnelling can cause an increase in the pore water pressure close to the TBM. This is because the filter cake on the face is constantly being destroyed and reformed when the TBM is advancing. In order to form the filter cake, water must be expelled from the slurry by a TBM excavation chamber pressure (the confinement pressure) that is greater than the external groundwater pressure. Under normal conditions, and with a reasonable quality of slurry, the effects are small. It is not generally necessary to consider the possible increase in the pore water pressure and the fluctuation in groundwater pressure in combination. However, where the tunnelling encounters a relatively permeable layer of soil that is a confined aquifer, the increase in pore water pressures may exceed the fluctuation in the slurry pressure. In order to calculate the pressure required to prevent an ULS, it is necessary to apply the appropriate Factors of Safety. Historic European practice is to apply a partial factor of 1.05 to 1.1 to the assessed water pressure, and factors of 1.3 to 1.5 on the derived earth pressure. These factors were applied in the calculations for the tunnelling on the KSL project. However, no separate ULS and SLS calculations were carried out; satisfying ULS was also considered as satisfying the SLS requirement of 1% volume loss. For Report No. 249, it was decided to recommend that the calculations follow common Hong Kong geotechnical practice, as used for the design of deep excavations, with: • Separate ULS and SLS calculations • A partial factor of 1.0 on water pressure; the water pressure to be assessed as the highest credible value applicable during tunnelling, at the relevant calculation section • Partial factors on soil shear strength and surcharge as outlined in Table 2
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Table 2. Partial factors recommended in Report No. 249 Factor on tan φ’ c’
su Unit weight Surcharge (unfavourable) Surcharge (favourable)
ULS Partial Factor 1.2 1.2 1.5 1.0 1.5 0
SLS Partial Factor 1.0 1.0 1.0 1.0 1.0 1.0
A calibration exercise, based on comparison with the operating pressures used for the KSL project, showed that similar pressures could be obtained, using the revised approach, if the fluctuation in the face pressure was included in the SLS calculation. The tunnelling for the KSL project was in saprolite, mixed ground and rock grades of weathered granite, and effective stress calculations were applicable in this case. If face pressure is required to control ground movement during tunnel advance, face pressure is also required when the TBM is halted to allow tool changes and other maintenance. Generally, tool changes require personnel to enter the TBM excavation chamber (intervention). The confinement pressure during an intervention is commonly, but not always, provided by compressed air. Most interventions are carried out with the excavation chamber emptied to tunnel axis level (half face interventions), with the cutterhead being rotated to allow access to all of the tools. However, some interventions involve fully emptying the excavation chamber (full face interventions). Although it is good practice to plan for regular interventions, particularly in abrasive ground, interventions are often required at short notice due to measured or suspected tool wear/damage, or other issues during tunnelling. Compressed air pressures therefore need to be defined for both half and full face interventions before the start of tunnelling, as this is an integral part of pressurized TBM tunnelling. As the distribution of pressure using compressed air confinement is different to that of slurry, the pressure provided by compressed air being constant over the height of the exposed face, the pressures derived for slurry tunnelling cannot be directly applied for compressed air. A separate set of calculations is needed. Extensive compressed air tunnelling was carried out during the early phases of the construction of the MTR. It was found that, for tunnelling in saprolite, it was necessary to balance the groundwater pressure close to the base of the exposed face to control raveling or erosion of the face. Report No. 249 suggests setting the compressed air pressure to balance the groundwater pressure at a level 1m above the base of the exposed face. For a half face intervention in saprolite, the compressed air pressure is typically significantly lower than the target face pressure for slurry operation at tunnel axis level. However, the compressed air pressure can be significantly higher than the target face pressure at the crown, depending on the size of the tunnel. The safety against a blow-out during an intervention is a design case that needs to be verified as part of the calculations. For shallow tunnels, ensuring safety against a blow-out may be a critical consideration in deciding the tunnel alignment. 3.1.2 Tail void grouting It is essential to carry out effective grouting at the tail void; the space around the tunnel lining rings. This grouting has a number of functions, including limiting ground movement due to tunnelling (Shirlaw et al., 2004). Grouting for modern pressurized TBMs is generally simultaneous with TBM advance, using grout pipes installed along the tailskin of the TBM. The tail void grouting can be supplemented by back-grouting through the rings. 3.1.3 Risk and risk control There have been numerous reports of sinkhole incidents as a result of pressurized TBM tunnelling projects, worldwide. This suggests that the likelihood of a major loss of ground during slurry (and EPB) TBM tunnelling has tended to be underestimated in risk assessments prepared prior to tunnelling. The likelihood of a sinkhole, and consequent risk, can only be properly controlled if a realistic assessment is made of the likelihood of a loss of ground, and measures taken to reduce that likelihood. Guidance is given in Report No. 249 on the tunnelling activities which have, historically, been particularly associated with a loss of ground, and on possible risk control measures. These activities include: break-in and –out, tunnelling in mixed ground, interventions and extended flushing. Consideration also has to be given to the risk of encountering unexpected ground or groundwater conditions, based on a critical evaluation of the site investigation
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information. It is now common practice in Hong Kong to have an emergency/contingency plan in place prior to the commencement of the tunnel drive. 3.1.4 Issues not covered or covered by reference Report No. 249 is intentionally focused on the calculations for the face pressures needed for slurry tunnelling, and on associated risk control measures. Issues that are not covered included: • Detailed operation or arrangement of a slurry shield • Basis of selection of a slurry shield • Slurry preparation, cleaning or testing These issues are covered elsewhere, and references are made in the report to selected documents. 3.2
Report No. 298
The section titles and respective number of pages for Report No. 298 are given in Table 3. Table 3. Summary of the sections in Report No. 298 Section title
Number of pages
Scope and Objectives
1
Face Pressure Assessment and Control
35
Screw Conveyors
6
Conditioning Agents
3
Tail Void Grouting
4
Excavation Management Control System
3
High Risk Activities
9
References
3
Glossary of Terms and Symbols
4
3.2.1 Major additions and changes from Report No. 249 When drafting Report No. 298, a conscious decision was made to maintain the format and, as far as possible, the content of Report No. 249. This was done to allow the reader of either report to be immediately familiar with much of the content of the other. The permission of the original author of Report No. 249 was sought and obtained. One format change was to include the figures within the text at the relevant reference rather than as a list at the end, making it easier to understand them in context to the text. A number of changes, deletions and additions were required to make Report No. 298 compatible and relevant for use with EPB TBM's: 1. All references to slurry TBM tunnelling were reviewed, removed and replaced where necessary with references to EPB TBM tunnelling except where reference to both methods was pertinent to the narrative. 2. The main section on Face Pressure Assessment and Control was largely re-drafted, while maintaining the same sequence of considerations, and the contained equations modified to suit EPB TBM tunnelling, mainly by the use of revised symbols and nomenclature. Effective stress methods are recommended with total stress calculations used as a check to ensure that the governing case was identified. The method of EPB TBM excavation is considered, including the three common modes of EPB operation. 3. Two new sections which are relevant to EPB TBMs, one on the function and types of screw conveyors, and a second on conditioning agents, are introduced into Report No. 298. Examples of common types of screw conveyors are provided with some reasoning behind their selection. However, no detailed assessment is provided. Conditioning agents are discussed in relation to the purpose for use and the various categories of conditioning agents available.
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3.2.2 Alternative operating modes The use of EPB TBM in Semi-Open mode, where compressed air is used in the upper half of the TBM excavation chamber is discussed. This mode of operation has been used on limited stretches of EPB TBM tunnelling in Hong Kong and Singapore, generally in mixed faces of rock and soil. However, there are significant risks and concerns over the use of this mode of operation, which are highlighted in the report. The ongoing development of more complex variable density TBMs, which combine the advantages of both EPB and slurry TBMs, brings yet more variations in the area of confinement pressure assessment and control. As this new type of hybrid TBM was still in its early stage of development, it was not covered in either Report No. 249 or Report No. 298. 3.2.3 Conditioning agents Section 5 of Report No. 298 discusses the major types of conditioning agents currently available. They are broken down into four categories, viz. foaming agents, polymers, anti-clogging agents and anti-abrasion agents. Many of the products available on the market are designed to achieve a combination of these effects to achieve a suitable soil paste for EPB TBM operation, or to reduce the operating cost of the TBM. Foaming polymers are currently the most commonly used type of conditioning agents for EPB TBM tunnelling, providing a range of beneficial effects in commonly encountered ground conditions. 4. EXPERIENCE WITH REPORTS 249 AND 298 4.1 General A number of tunnel drives have been carried out in Hong Kong since Report No. 249 was issued. Many of these are either recently completed or still in progress, and a systematic review of the data has not been carried out. However, a summary of some of the relevant projects and the results of the tunnelling are presented below. In this review, reference is made to the ‘overpressure’ applied during tunneling. Overpressure is the difference between the confinement pressure and the groundwater pressure, as discussed in Aristaghes and Autori (2003). The value of the overpressure provides a simple basis for comparing tunnels driven at different levels below ground and groundwater pressures. For slurry TBMs, the value of the overpressure is also a major factor in filter cake formation. 4.2 Slurry TBM Tunnelling for West Island Line Tsang, Salisbury and Yeung (2012) reported on the tunnelling for Contract C703 of the West Island line. There were two drives by the 6.35m diameter slurry TBM, with a total length of 1,260m. The drives were through Completely Decomposed Granite (CDG), mixed ground and rock, and passed under a number of high rise buildings. The target face pressures were initially calculated based on Report No. 249, and an SLS set at 1% volume loss, the default value for effective stress analysis in Report No. 249. Numerical analysis was carried out, with a target SLS for design of 1.5% volume loss. The numerical analysis indicated that higher confinement pressures were required to meet the SLS than found in the calculations using the simpler methods outlined in Report No. 249, despite the less stringent target for volume loss. The overpressure applied at tunnel axis level was typically 0.1 bar in rock, 0.25 bar in CDG and mixed ground, but up to 0.7 bar when the tunnel was directly under the heavy loads imposed by the foundations of high rise buildings. Numerical analysis was particularly useful in assessing the effects of the buildings on the tunneling (and vice versa) and the interaction with an existing tunnel. The reported actual volume loss was in the range of 0.24 to 0.44%, compared with the target value for design of 1.5%. Although 1.5% was set as the maximum target value for the tunnelling, a second check had been made to confirm that 3% volume loss would be acceptable in relation to the effect on buildings and utilities. Actual values of volume loss were therefore much lower than the highest value for which the buildings were assessed. Based on the site investigation, the CDG encountered at tunnel level had an SPT-N value that was generally 100+, but locally about 50.
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4.3 Slurry TBM Tunnelling for Guangzhou-Shenzhen-Hong Kong Express Rail Link (XRL), Contract 820, Mei Lai Road To Hoi Ting Road Tunnelling - Tunnel Sections from Tai Kok Tsui to Hoi Wang Road The XRL is a cross boundary infrastructure project which will provide high speed rail connection between Guangzhou and Hong Kong. Four of the construction contracts involve soft and mixed ground tunneling: XRL 820, 823A, 825 and 826. XRL 823A and 825 have used EPB shields, while 820 and 826 have used slurry TBMs. This sub-section will review data from XRL 820, Tai Kok Tsui to Hoi Wang Road. XRL820 comprises two sections of bored tunnels, driving north and south from the launching shaft at Nam Cheong. The route length of the tunnels is about 3.8km. The 9.33m diameter TBMs tunnels passed through saprolite, mixed ground, strong rock, with approximately 250m of alluvial and marine deposits. The tunnels were driven beneath two sensitive areas: Tai Kok Tsui and Hoi Wang Road, largely built in the 1960’s and 1990’s respectively, which were directly above or adjacent to the tunnels (Figures 3 and 4). Report No. 249 was adopted for assessing the required confinement pressures. For granular soils, ULS analysis was carried out using the method based on Anagnostou and Kovari (1996), following section 3.2.1 of Report No. 249. For clayey soils, ULS analysis was carried out based on the results outlined in Kimura and Mair (1981). In order to assess the effects of the existing foundation loads and the adequacy of applied face pressures, and to properly control ground settlement and building movements, numerical analysis using a finite element computer program was also carried out to supplement Report No. 249. The target volume loss for the design was 1.0% to 2.0%, depending on the presence and nature of sensitive receivers.
Figure 3. One of the TBMs used for the XRL 820 Figure 4. Jet grouting for a cross-passage and pile tunnels, at the end of the drive foundation protection along the running tunnels. Note the dense urban area with many high-rise buildings The TBM drives were completed in September 2014, with face stability adequately maintained during the course of construction. The design face pressures varied from 2.4 to 4.2 bars, depending on the ground conditions and the proximity of existing buildings in the vicinity of tunnels. The actual applied face pressures were generally higher than the design values by 0.1 bar to 0.2 bar. The overpressure applied at tunnel axis varied from 0 bar (in rock or jet grout zone) to 1.05 bar (in mixed ground underneath the existing buildings). Several arrays of instruments had been specified at designated sections in the Tai Kok Tsui (TKT) and Hoi Wang Road (HWR) areas, to capture the actual volume loss. The design and actual volume loss, geology and confinement pressures and TBM advance rates are tabulated in Tables 4, 5 and 6 respectively.
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Table 4. Summary of Volume Loss and Settlement for XRL Project Contract 820 Cross Section/ Length (m) 1 (20m) 2 (10m) 3 (20m)
TKT TKT TKT
1.5 1.5 1.5
Actual Volume Loss (settlement in mm) (Downtrack Tunnel /Uptrack Tunnel) 1.3(22) / 0.65(11) 1.0(17) / 0.18(3) 0.65(11) / 0.71(12)
4 (50m) 5 (20m) 6 (60m) 7 (10m) 8 (40m) 9 (60m) 10 (50m)
TKT TKT TKT HWR HWR HWR HWR
1.0 1.0 1.0 2.0 2.0 2.0 1.0
0.24(4) / 0.12(2) 0.18(3) / 0.12(2) 0.24(4) / 0.24(4) 0.18(3) / 0.18(3) 0.18(3) / 0.29(5) Heave (-5) / 0.18(3) Heave (-3) / 0.18(3)
Area
Design Volume Loss (%)
Remarks Trial section Mixed ground Stiffer soil (refer to Table 5 for SPT ‘N’ values) Rock Rock Rock Local jet/TAM grout blocks Local jet grout blocks Local jet grout blocks Jet grout zone
Table 5. Summary of Confinement Pressures Tunnel Face Geology CDG Mixed ground, CDG & Rock CDG Mixed ground, CDG & Rock Rock Rock CDG CDG CDG /Alluvium CDG/Allu vium (With Jet Grouting) *Before grouting
Confinement Pressure/Hydrostatic Pressure at Tunnel Axis Level/ Overpressure (Bar) Design Actual Design Actual (Downtrack) (Downtrack) (Uptrack) (Uptrack) 4.0/3.2/0.8 4.1/3.2/0.9 4.0/3.2/ 0.8 4.0/3.2/0.8
Ground Cover (m)
SPT ‘N’ at Tunnel Crown
30
45
30
55
4.1/3.4/0.7
4.35/3.3/1.05
4.1/3.4/0.7
4.1/3.3/0.8
30
96
3.9/3.45/0.45
4.1/3.5/0.6
4.2/3.45/0.75
4.2/3.4/0.8
30
>200
3.7/3.5/0.2
3.73/3.5/0.23
4.24/3.5/0.74
4.24/3.44/0.8
31 31 30 27
N/A N/A 34* 32*
3.6/3.43/0.17 3.6/3.4/0.2 3.5/3.1/0.4 3.5/3.0/0.5
3.42/3.4/0.22 3.4/3.4/0 3.5/2.8/0.7 3.5/2.8/0.7
3.7/3.43/0.27 3.7/3.4/0.3 3.5/3.1/0.4 3.4/3.0/0.4
3.7/3.5/0.2 3.7/3.5/0.2 3.5/3.2/0.3 3.4/3.0/0.4
25
18*
3.3/2.8/0.5
3.3/2.6/0.5
3.3/2.8/0.5
3.3/2.74/0.56
21
12*
2.6/2.4/0.2
2.6/2.4/0.2
2.4/2.4/0
2.4/2.4/0
Table 6. Average TBM Advance Rates Cross Section 1 2 3 4 5 6 7 8 9 10
Average TBM Advance Rate (m/day) Downtrack Uptrack 16 16 4.5 4 13 9 8 9 11 9 9 7 16 30 28 34 22 25 10 15
298 of 691
The actual volume loss was generally significantly lower than the design target. However, the maximum measured volume loss, at 1.3%, was only a small margin lower than the design target value of 1.5%. If the design volume loss is treated as a ‘not to exceed’ value, then the result, overall, appears reasonable. Very low values of volume loss were recorded where the tunnel was in a full face of rock or a ground treatment zone. There also appears to be a general correlation between the SPT ‘N’ value of the CDG at tunnel crown and the measured volume loss, with a smaller value of volume loss in CDG with SPT>100, compared with the experience in SPT
