Reader Response Draft 1
According to Harding,(2017), Mixshield is a slurry tunnel boring machine (TBM) that utilizes an air bubble to manage and stabilize pressure at the tunnel face. The "tunnel face" refers to the foremost part of a tunnel excavation where the actual digging or cutting of the tunnel occurs. This is further explained by Herrenknecht. The TBM has an automatic air cushion that precisely controls the face support pressure in the excavation chamber (Herrenknecht,2024). It also offers an array of powerful cutting tools such as cutting knives and disc cutters. It is enhanced by jaw crushers positioned in front of the intake screen to further breakdown boulders and stones into conveyable size pieces (Herrenknecht, 2024). According to Herrenknecht and Rehm, the TBM can excavate and build a ring around the machine continuously (Herrenknecht and Rehm). This is supported by Herrenknecht’s claim that the TBM can do so with the help of its erector and thrust system. The erector is controlled by a remote and it positions the segments during ring building whereas the thrusters are positioned around the circumference which pushes the shield forward, leaving behind the previously built ring (Herrenknecht,2024). The machine can dig while building a solid structure around itself called the ring. The ring is a circular concrete structure that is used to hold a tunnel so that it will not collapse on itself.
The machine can achieve tremendous progress with its great efficiency as well as its adaptability to be used in mixed soil conditions. The greatest hurdle is the cost. It is very expensive to run a mixshield TBM.
Moving on to its great efficiency, according to Herrenknecht (2024), it has a system that allows itself to continuously dig and secure itself simultaneously. This is achieved by its erector and thrust system. It can be segmented into the front and back of the machine. The front segment will continuously dig into the rocks in front of it while the back of the machine will reinforce the surrounding structure which is called a ring. This allows the TBM to cover an approximate of 10m every 24 hours. (MetroTunnel, 2020). Besides it progressing at a fast rate due to this function. The TBM's cutting wheel arms are easily accessible for maintenance (Herrenknecht, 2024). This in turn reduces its downtime significantly. According to Tey (2019), the main reason a TBM project is delayed is due to the frequency the cutter head needs to be replaced whether planned or unplanned. By making it accessible to the cutter head's assembly, it drastically reduces the downtime of the TBM making it more efficient.
Besides being efficient, the adaptability of a Mixshield TBM makes it a relatively good choice to be used in Singapore. Singapore has a non-homogenous soil which makes it perfect for using a Mixshield TBM for her tunneling projects (Tey, 2019). Since the Mixshield TBM can dig through heterogeneous soil (Herrenknecht, 2024), Singapore's tunneling projects will only need to use one type of TBM. This, in turn, will bring down the cost of the whole project.
Although the TBM is relatively efficient and adaptable. The cost of running a TBM is expensive. In Singapore, it costs taxpayers $10 billion for Phase 2 of the Deep Tunnel Sewerage System, DTSS, to build (CNA, 2023). It is known that the Mixshield TBM is being used for the project (Herrenknecht, 2024). This means that it is almost $100 million per km to build the tunnel. This is in comparison to the $90 million per km of the Gotthard Base Tunnel (EcoTransit Sydney, 2011).
In conclusion, Mixshield TBM is still the ultimate choice in Singapore's context. It is efficient and adaptable which is in line with most of Singapore's agency requirements. Albeit it is on the expensive side, it is fully justified for the cost.
Reference List.
https://www.robbinstbm.com/mixshield-crossover-hybrid/
https://inside.mines.edu/UserFiles/File/earthMechanics/seminar/04_soft_ground_tbms_slurry.pdf
https://www.channelnewsasia.com/singapore/deep-tunnel-sewerage-system-used-water-superhighway-tunnelling-works-completed-phase-2-3708366#:~:text=Construction%20of%20the%20first%20phase,last%20for%20about%20100%20years
http://www.ecotransit.org.au/ets/files/Rail-Tunnelling-Factsheet.pdf
https://www.herrenknecht.com/en/references/referencesdetail/singapore-deep-tunnel-sewerage-system-phase-2/
https://www.linkedin.com/pulse/challenges-bored-tunneling-work-stanley-tey-1d
https://bigbuild.vic.gov.au/__data/assets/pdf_file/0008/390986/MT-Tunnelling-with-TBMs-factsheet-April-2020.pdf
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