ORAL PAPER PROCEEDINGS

Transcription

1 ITA - AITES WORLD TUNNEL CONGRESS April 2018 Dubai International Convention & Exhibition Centre, UAE ORAL PAPER PROCEEDINGS

2 Shatin Central Link First Variable Density Tunnel Boring Machine in Hong Kong for a Shallow Tunnel Drive Neil Ng1, Didier Jacques2, James Reilly3 and Michael Cheung4 1 MTR Corporation Limited, Hong Kong, Hong Kong NGNEIL@mtr.com.hk 2 Dragages-Bouygues Joint Venture, Hong Kong, Hong Kong didier.jacques@dragageshk.com 3 Dragages-Bouygues Joint Venture, Hong Kong, Hong Kong james.reilly@dragageshk.com 4 Dragages-Bouygues Joint Venture, Hong Kong, Hong Kong michael. cheung@dragageshk.com ABSTRACT The Shatin Central link (SCL) is one of the railway projects set out in the Railway Development Strategy (RDS 2000) by the HKSAR Government and is a new rail corridor providing a direct link between the New Territories and Hong Kong Island. SCL Contract 1128 involves the construction of two sections of twin tunnels between Causeway Bay Typhoon Shelter and Admiralty MTR Station, boring underneath the densely urbanized district of Wan Chai. Challenging and variable ground conditions were specifically expected throughout the shallow alignment of the Eastern Down Track tunnel, with the geology ranging from hard rock to marine deposits and reclaimed fill. As a result, a tunnel boring machine (TBM) was required that could cope with such heterogeneous ground conditions. The main constraints for the choice were the shallow ground cover of less than one tunnel diameter giving high risk of ground settlement, slurry blow outs to the surface and the potential encounter obstructions in the reclaimed and developed areas. A Mixshield TBM was deemed less suitable and the preliminary consideration of an Earth Pressure Balance (EPB) TBM was considered to come with a certain degree of residual risk. At the Tender stage, DBJV, drawing on experience in similar densely urbanized areas, proposed a TBM capable of operating in-between slurry and EPB modes, mitigating the range of risks anticipated. As a result, the implementation of Hong Kong s first Variable Density (VaD) TBM, with an ability to use both low and high density slurries, was chosen. In partnership with Herrenknecht and MTR Corporation (MTR), DBJV refined and improved the concept that had been used during the construction of the Klang Valley MRT project in Kuala Lumpur, where the first ever Variable Density TBM was deployed. The TBM selection and its development processes for SCL Contract 1128 were specifically designed to suit the needs of the anticipated ground conditions and to manage the project s inherit risks associated with one of the tunnel drives. Key Words: TBM, Variable Density, Shallow Cover 1. INTRODUCTION The Shatin to Central Link (SCL) comprises two sections; the Tai Wai to Hung Hom section and the Hung Hom to Admiralty section. The Hung Hom to Admiralty section is an approximate 6km extension of the East Rail Line from Hung Hom station in Kowloon - crossing Victoria Harbour with a new station at the Exhibition Centre - to an interchange at Admiralty Station on Hong Kong Island. The 1128 works contract - South Ventilation Building (SOV) to Admiralty Tunnels which forms part of the hung Hom to Admiralty section - was awarded to the Dragages Hong Kong and Bouygues Travaux Public joint venture (DBJV). It involves the construction of 2.3 km twin tunnels (ø 7.45 m excavated diameter with lengths between 470 m and 678 m), ventilation buildings and ancillary works. The stacked eastern bored tunnels run between the SOV building and future Exhibition Station; with the western bored tunnels running from the Fenwick Pier Emergency Egress Point (opposite side of the Exhibition Centre) to Admiralty Station. During the tender stage, it was understood that the Eastern Down Track (EDT) tunnel would present a significant challenge to the project team. Thus, DBJV proposed an alternative solution to the client, MTR, to limit the risks due to shallow ground cover and complex geological conditions by making use of a Variable Density (VaD) TBM, the first in Hong Kong. 2. ALIGNMENT AND GEOLOGY 2.1. Alignment The alignment of the Eastern Down Track tunnel passes through the very dense urban district of Wan Chai on Hong Kong Island in the east to west direction. The tunnel excavation path is exclusively within a reclamation area, constructed in various stages during the 1960 s and 1970 s. Figure 1 below shows the progression of reclaimed land from 1945 to 2009, with the red line indicating the future SCL alignment and new MTR Exhibition Centre station. Figure 1. Progression of reclaimed land along the SCL alignment from 1945 to 2009 (MTR, 2014) Due to the MTR program of development, requiring a cross platform interchange at Exhibition Centre Station for the future North Island Line, the two eastern SCL 1128 tunnels are stacked, requiring the Down Track tunnel to mine through the fully reclaimed land. The two excavated tunnels pass underneath extremely critical road structures, services and sensitive areas which could not be disturbed during construction. These include Gloucester Road, Cross Harbour Tunnel slip roads, various highway flyovers and foot bridges, the Wan Chai Sports Ground (WCSG) (under normal operation), sewers and drains. Figure 2 shows the alignment with the various sensitive areas and structures colour coded.

3 Legend: Public Road Crossings Road Structures WCSG Running Track Buildings Underground Drainage Figure 2. Alignment of the Eastern Tunnels with various sensitive structures identify (Auvergne, 2017) With a number of structures supported on piles that conflicted with the TBM alignment, advance works were required to remove the obstructions. These works prevented interruption to TBM operations and avoided the need to undertake a number of potentially dangerous compressed air interventions (CAI). For some existing flyovers on piles, temporary steel structures and new piles were constructed to redefine the foundation support, allowing the existing piles that clashed with the tunnel alignment to be removed prior to TBM arrival. It was imperative that these construction works did not interrupt any road traffic. Potential settlements on structures during TBM excavation were counteracted with hydraulic jacks installed on temporary steel structures. Such arrangements limited movement that would otherwise affect the permanent structure. More detail regarding obstruction removal and re-provision can be found in the paper by Kwok (2017) Geology From the site investigations conducted, the Eastern Down Track alignment was found to have a complex geological profile. The solid geology consists of Kowloon Granite/Completely Decomposed Granite (CDG), comprising of fine, fine-medium and medium grained varieties, found roughly m below surface level. Above this stratum, sandy to clayey alluviums (ALL) and sedimentary marine deposits (MD) of varying thickness can be found before reaching the top layer of fill. The fill layer was expected to contain reinforced concrete, bricks, plastics, wood and steel bars as well as rubble mounds that had been previously formed as parts of temporary or permanent sea walls. The alignment itself can be roughly divided into three general geological sections. First, 120 m was excavated mainly in a rock, followed by a 90 m section of transition from rock and CDG mix to softer alluviums. The remaining 470 m of the drive was completed in soft open ground conditions, containing alluviums, clayey and sandy marine deposits and reclaimed areas of fill. However, the last 90 m of the excavation contained corestone sized rubble embedded in silty fine coarse sands. At the shallowest point, the excavation took place with cover of less than one TBM diameter and at some locations, the TBM passed close to existing live structures with less than 1 m of very soft soil in between. Figure 3 highlights one of the significantly more challenging areas in terms of ground cover and potential obstructions in various geological layers under heavy traffic roads above. Figure 3. Section of Down Track Tunnel with a number of constrains and challenges 2.3. Risk Management The inherit risks on this project had been identified at the design stage where more than several events had an initial rating of high. The nature of the risks included tunnel face collapse, compressed air/slurry blow out, over excavation, TBM encountering unforeseen obstruction particularly in reclaimed ground and also failure to remove known and unforeseen obstructions. Action at design stage had to be taken together to recommended mitigation measures required during construction to reduce the risk rating of these risks. The live risk register was carried forward into the construction stage where each risk was further reviewed and additional risks also identified. Mitigation measures agreed between MTR and DBJV were carried out in advanced of the TBM tunnel excavation to reduce the risk to an acceptable level. The mitigation measures included extracting and removal of left-in steel obstructions, advanced ground treatment works, using experienced TBM operators and a rigorous TBM selection and design process. Emergency Action Plans catering for the various scenarios were also fully developed and coordinated with local Authorities. 3. TBM SELECTION AND EXPLANATION 3.1. Choice of TBM During the project s tender stage, the three deepest tunnels were specified to be excavated by using Mixshield TBMs. However, the shallow cover East Down Track was initially specified by the client to be completed by an EPB TBM, avoiding the potential risk of slurry blowouts that could occur using a Mixshield TBM. DBJV highlighted the key concerns in using an EPB TBM with risk of collapse and high settlement due to inefficiencies in managing the confinement support because of the complex geology, limited cover and possibility of encountering unforeseen obstruction in reclaimed ground. Further risks using the EPB TBM included foam blowouts, a more complex process for completing compressed air interventions and, given the geology, concern of the moisture content for spoil disposal (to be below 25%) not being reached.

4 The project team was aware of the experience from the Klang Valley MRT project in Kuala Lumpur, where a Variable Density (VaD) TBM was used for the first time, to overcome the complex Karstic limestone geology where problems (slurry blow-outs) had previously occurred using Mixshield TBM s (Schaub, 2014; Bäppler, 2016). The machine was developed to give the advantage of both a Mixshield TBM and an EBP TBM in providing an adequate face support medium (Bappler, 2016). The Variable Density TBM can make use of a thicker, denser slurry inside the excavation chamber instead of a thin slurry or earth paste. By using a higher density at the excavated face than a normal Mixshield TBM, the VaD TBM was more suited to the shallow cover Down Track east tunnel excavation on The proposal to use a VaD TBM for the first time in Hong Kong was accepted by the client Variable Density Concept The applied concept of this VaD TBM is the ability to work with a range of slurry densities to support and stabilise the excavated face while preventing surface blow-outs. Similar to a Mixshield TBM, the confinement pressure is regulated accurately due to a pressurised air bubble. With a standard Mix-shield TBM using low density slurry at the designed confinement pressure, combined with the shallow cover found along the alignment, there was an increased risk of blow-outs. The slurry is of a low viscosity (very fluid) and can be forced to the surface with such a confinement pressure more easily. Alternatively, having higher density slurry with a higher viscosity in the excavation chamber can minimize these risks due to the heavier particles in the mud not being able to reach the surface at the designed confinement pressure. Figure 4 below illustrates a simplified description of the concept, indicating the difference between using low and high density slurries at the excavated face. For the same pressure applied at the axis of the TBM in the excavation chamber, given the difference in density and viscosity, the blow-out risk is controlled by applying a higher density medium (Straesser, 2012; Straesser 2016). On this project, the VaD TBM used two different modes of excavation; a Low Density mode where slurry is supplied similarly to a Mixshield TBM (roughly T/m3) through the slurry circuit, and a High Density mode that took the TBM to the lower boundary of an EPB TBM where the density in the excavation chamber was limited to 1.50 T/m3, as described by Schaub (2014). Above this density, a full switch to EPB mode is required including foam conditioning and installation of a conveyor belt for spoil mucking out. 3.3 Recommended Density To provide a recommend density for the TBM operators to follow and limit risk of slurry blow-outs, a basic calculation was computed based on an open path between the axis of the TBM and the surface. The calculation assumed the design confinement pressure to be applied at the TBM axis for relevant sections across the drive. This was compared to the depth of TBM excavation giving the density to be applied for each location along the drive, controlling the blow-out risk. The calculation is provided below (equation 1) with 9.81 representing the effect of gravity on the slurry/soil column. Variable Density Specific Components Figure 5 identifies the key components of the variable density system with Schaub (2014) also identifying these to enable hydraulic transportation. The VaD type shares similarities between Mixshield and EPB TBM s, with a number of common major components. However, additional new components are required to enable the high and low density modes of operation. Figure 5. VaD TMB system with specific components labelled (Straesser, 2012; Schaub 2014) Figure 4. Blow out risk control using low or high density for the same confinement pressure (Straesser, 2016) With a VaD TBM, material is removed from the pressurised excavation chamber through a screw conveyor up to the slurryfier box. The main difference with an EPB is that the screw is not required to regulate the front pressure as the slurryfier box remains pressurised with the slurry circuit.

5 The slurryfier box s primary function is to dilute dense material removed from the front to a suitable density allowing hydraulic transportation to the Slurry Treatment Plant (STP) on surface. The 6 m3 box was constantly fed with slurry by various feed lines providing adequate flushing and preventing clogging. A ø500 mm jaw type crusher permitted the TBM to pass through the initial granite geology. The diluted product was then pumped through the return line. Bubble communication to the excavation chamber is not possible through a submerged wall, as found in Mix-shield TBM s. Instead, pressure is transmitted via a communication pipe, joining the working and excavation chambers at crown. The air bubble is regulated by a Samson system, forcing slurry up through the pipe, helping to regulate the design confinement pressure at the face. Figure 6 explains its function, comparing the bubble pressure delivery at crown instead of through the submerged wall. A minimum flow of slurry must be injected inside the working chamber so there is constant flow through the pipe. A hydraulic propeller is present at the top of the pipe to minimise backflow of large particles into the working chamber. The ratio is adjusted by the operator depending on the density of slurry measured in the excavation chamber. When the density is too low in the excavation chamber, the flow of slurry must be reduced and vice versa, if the density it too high, the flow of slurry must be increased. Part of slurry injected to the front is sent to the working chamber where it is pushed to the excavation chamber through the communication pipe. The remaining slurry is directed through the face injection lines located on the cutter head which work the same as the foam injection lines found on an EPB TBM. A 2 m3 tank supplies slurry to the 6 individually controlled pumps with the facility to add a polymer to aid dispersion in the excavation chamber if necessary. In semi-automatic mode, the PLC system regulates the flow through the face injection lines to make up the total slurry reaching the set BIR. In manual mode, the operator has control of the pumps which can be adjusted independently. 4. TBM OPTIMISATION From the original Kuala Lumpur VaD concept, several changes have been developed with Herrenknecht during the design stage to upgrade the TBM processes for working in Hong Kong ground conditions, specifically the transitions and areas with rubble mounds, corestones and boulders. Below is a list of some items specifically redesigned for the S989 VaD TBM: Figure 6. Comparison of chamber confinement pressure delivery (Straesser, 2012) To measure density inside the excavation chamber, sampling of slurry from the crown is completed via a small pump, where it is recorded by a gamma density meter. This information is provided to the TBM operator, who can adjust TBM parameters to reach the recommended density. The VaD TBM has the option of injecting a high density mix (HDM) directly to the excavation chamber. Its main purpose for this project was to fill voids encountered at the excavated face temporarily or refill the excavation chamber after compressed air interventions. It was not used to control the face density. The circuit on the TBM was made up of a 5 m3 tank with a pump delivering HDM to front. The mix itself was manufactured on surface with a density of 1.5 T/m Bentonite Injection Ratio (BIR) To enable the higher density slurry mode in the excavation chamber, regulation of slurry injected to the excavated face is necessary. The Bentonite Injection Ratio (BIR) controlled this injection. The BIR is the flow of slurry sent to the excavation chamber divided by the flow of material physically excavated, with the basic formula shown below in equation 2. l The main drive size and power was increased from 8 motors on a Mixshield TBM to 12 on the VaD TBM l The crusher needing to operate in hard rock conditions required a jaw type crusher, impacting the size and shape of the slurryfier box while still providing access to the front shield l Most of the interfaces between the slurryfier box and working chamber were redesigned l Independent injection lines to the cutter head to regulate the BIR for maintaining the face density l Creating several automatic and semi-automatic functions for the slurry circuit operation, simplifying the interfaces on a touch screen for the operators 5 EXCAVATION ANALYSIS 5.1 TBM Operation The VaD TBM requires more active manipulation than a Mixshield TBM due to the increased number of parameters the operator must be aware of when excavating. In Low Density mode, the machine operates similar to a Mixshield, where little focus is placed on the density. There is roughly a split between the flow sent to the excavated face and the slurryfier box for flushing in the crusher area. The main change in operation is the use of the screw conveyor to remove the spoil from excavation chamber, adjusting the revolution speed as the geology changes.

6 The concept behind the High Density mode is controlling the flow of injected slurry in front to produce a higher density mud by using the excavated ground to mix with the slurry in the excavation chamber, producing a suitable density support medium. To sustain the higher density, the screw speed and BIR must be manipulated while maintaining a stable pressure in the excavation chamber with a controlled slurry circuit. In general, if a higher density is required, the screw speed will be reduced to retain more material inside the excavation chamber while limiting the slurry directed to the face with a BIR reduction. If a lower density is required, the opposite is applied. a Mixshield TBM completed the drive in 86 days, excavating an average of 7.9 m/ day. The difference is largely a result of more rock and transition sections for the Mixshield drive, due to the deeper alignment, requiring more CAI interventions. Table 2 provides a comparison of average excavation rates for the various types of geology encountered. Table 2. Comparison of excavation rates When working in the High Density mode, the recommended densities calculated were reached with TBM operators manipulating the BIR and screw rotation speed to maintain the set value. Areas where quick changes of operation were required occurred when excavating back filled concrete piles and jet grouted ground. The developed BIR concept gave the TBM operator the means to react swiftly to control and regulate the face density and avoid clogging the cutter head. 5.2 Other Advantages With a hydraulic flap located on the communication pipe between the excavation and working chamber, there was the ability to isolate the working chamber from the excavation chamber. This allowed access to the working chamber at atmospheric pressure while maintaining the confinement pressure at the excavated face. Maintenance could therefore be completed inside the working chamber and also provide the option to prepare materials for compressed air interventions. Pressure was maintained in the excavation chamber via injections of HD mix or bentonite slurry. The crusher, located inside the slurryfier box, could also be accessed under non-hyperbaric conditions, the screw gate preventing any loss of pressure from the front. 5.3 General Comparison Between Mixshield and Variable Density TBM s used on SCL 1128 The main difference between the two types of TBM s is the extra length of the back-ups, impacted by the addition of the screw conveyor, the slurryfier box, and the extra equipment on the VaD TBM. In addition, the VaD had a more powerful main drive to cope with the thicker slurry in the excavation chamber, independent slurry injection lines to the face, HD Mix tank and injection lines among other items. Table 1 below provides some basic information for comparison Drive Analysis Despite the fact the drives were very short (~677 m each) and learning curves considered, the EDT drive, mined with the VaD TBM, was completed in 81 days with an average excavation of 8.3 m/day including stoppages for assembly, break in and false tunnel dismantling. In comparison, the East Up Track tunnel mined with The progress of the VaD TBM was impacted by the processing time to treat the excess slurry in the Marine Deposit area with up to 60% fine content. As a result, the processing time of the filter presses was restrictive to TBM progress but this was anticipated during the tender stage as insufficient space was available to increase the size of the STP for this very short drive. The same STP was used for each TBM. As the VaD TBM went through an initial long transition of rock/cdg and mixed face soil with boulders, several CAI s were required to maintain and replace cutters. For the remainder of the drive, due to the shallow cover and very unstable ground conditions, most of the CAI s were pre-planned and carried out as much as possible in defined, ground treated areas. This minimized risks to the public on the surface, as well as the workers working in compressed air. The TBM s design, combined with our in-house BOUYGUES MOBYDIC system which monitors the instrumented disc cutters during excavation, reduced the risk of failure and improper maintenance of the cutter head. Due to the complexity and challenges within the unknown geology of the DTE tunnel excavation within transitions and reclaimed areas, the MOBYDIC system provided the operator with very accurate information including the ground repartition at face, unforeseen obstructions, and corestones presence. As the TBM excavated through the reclamation area, many non-indigenous objects were encountered ranging from pieces of steel to blocks of wood, bricks and plastics which were expected based on the Geotechnical Baseline Report (MTR, 2014). The specific cutter head design as well as the jaw type crusher allowed the TBM to pass these objects, with approximately 5 tons of steel removed from the slurry circuit by the magnet at the STP. Due to the risk of encountering unexploded ordnances, specific training was conducted with the workers, with stoppages nec-

7 essary to clearly identify and check objects found on the magnet before CAI and allowing the restart of TBM excavations. The TBM performed within the expected settlement values, based on the designed confinement definition, with results of volume losses between 0.1 to 1% maximum. There was continual monitoring along the alignment with numerous ADMS systems measuring points on surface, roads, existing structures and services as the TBM progressed (Auvergne, 2017). No TBM stops or impacts to the surface (roads, footbridge) occurred as a result of either effects on the surface settlement or risk to the safety of the public. Straesser, M. (2012), Klang Valley Mass Rail Transit Kuala Lumpur, Malaysia, Final Report of the Predesign for the Variable Density TBM, Herrenknecht AG, Schwanau Straesser,M.; Klados, G.; Thewes, M.; Schoesser, B. Developments of LDSM and HDSM Concept for Variable Density TBM s. Tunnel, No. 7, 2016, pp CONCLUSIONS Overall, the application of the VaD TBM for the Eastern Down Track drive in these very shallow conditions was deemed a success, with no effects to existing structures or to the highly dense and congested roads on the surface, which continued to run as normal during tunnel excavation. With an initial learning curve, the TBM operated smoothly, having no significant breakdowns during operation and production up to 22.5 m (15 rings) per day and 108 m/week. By using a Variable Density TBM, it provides the contractor and the client the flexibility to operate in challenging and changing ground conditions, with the ability to switch modes when necessary. Although the full EPB mode was not used for this tunnel, it is clear that this new development of High Density mode can tackle some new challenging geological environments that would otherwise not be possible with more conventional machines. 7. ACKOWNLEDGEMENTS 8. REFERENCES Auvergne, S.; Gauffre, J.; Prost, A.; (2017) Extensive use of I&M to Enable Safe TBM Drive in Urban Dense Area, a Focus on SCL1128 Eastern Downtrack Drive, Proceeding of the HKIE Geotechnical Division Annual Seminar 2017, Hong Kong Bäppler, K.; New Developments in TBM tunnelling for changing grounds, Tunnelling and Underground Space Technology, Volume 57, 2016, Pages Kwok, K.; Bracq, G.; Barrett, T.; (2017) Hong Kong MTR Shatin to Central Link Contract1128 (Eastern Approach) Tunneling in Sensitive and Congested Urban Environment, Proceedings of the HKIE Geotechnical Division Annual Seminar 2017, Hong Kong MTR Geotechnical Base Line Report Deliverable No 3.6H, Works Contract 1128, Consultancy Agreement No. C1108 (2014), SCL Hong Kong Section Construction Scoping and Sequencing Schaub, W.; Duhme, R.; (2014) Multi-Mode and Variable Density TBMs Latest Trends in Development, Underground Singapore 2014, Singapore

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