Reader Response 3

 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 (Herrenknecht, 2024) stating 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, 2003). 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 Mixshield machine can achieve tremendous progress with its great efficiency as well as its adaptability to be used in mixed soil conditions but the cost to operate a TBM is steep compared to using a drill and blast method in Singapore.

A point to consider is the efficiency of the TBM, 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. Due to the accessibility to the TBM's cutter head's assembly, the downtime of the TBM is drastically reduced making the TBM 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. Focusing on the project by PUB to build a deep tunnel sewerage system along the central and western coast of Singapore. The soil is relatively made up of an array of soil profiles. It is made up of reddish mudstone, prey fossil-rich mudstone and shale, fossil-rich limestone, siltstone, sandstone, conglomerate (Cai, 2012). This makes it perfect for the PUB engineers to consider the use of the Mixshield TBM.

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). Using a rough estimation from the website, Civils.ai, it cost an approximation of $40 million to construct the tunnel using a TBM. As compared to $32 million when using the drill and blast method. Since the tunnel is 98km long, this brings the total to $3.924 billion for the TBM method and $3.136 billion for the drill and blast method. By using the drill and blast method, $784 million could have been better used in other projects by PUB. Although the drill and blast method is much cheaper. The choice of using TBM is much more efficient than the drill and blast method.

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

Harding, D. (2017, August 1). Redefining the Industry. Https://www.Robbinstbm.com/Mixshield-Crossover-Hybrid/.

Herrenknecht, M., Dr. (2003, March 28). Mixshield Technology. Https://emi.Mines.edu/Wp-Content/Uploads/Sites/162/2023/01/04_soft_ground_tbms_slurry.pd

Herrenknecht (2024) Mixshield. https://www.herrenknecht.com/en/products/productdetail/mixshield/

CNA (2023, August 21) Tunneling Works Completed for Phase 2 of Singapore’s Sewage ‘superhighway’ . 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

Herrenknecht (2024) SUPERHIGHWAY FOR SEWAGE TREATMENT https://www.herrenknecht.com/en/references/referencesdetail/singapore-deep-tunnel-sewerage-system-phase-2/

Tey. S (2019, June 12) Challenges of bored tunneling work https://www.linkedin.com/pulse/challenges-bored-tunneling-work-stanley-tey-1d

MetroTunnel (2020, April) Tunnelling with tunnel boring machines https://bigbuild.vic.gov.au/__data/assets/pdf_file/0008/390986/MT-Tunnelling-with-TBMs-factsheet-April-2020.pdf

Cai J.G. (2012, July) Geology of Singapore.

https://www.srmeg.org.sg/docs/N13072012_2.pdf