Summary reader response draft 2

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 jawcrushers positioned in front of the intake screen to further breakdown boulders and stones into conveyable size pieces (Herrenknecht, 2024). According to Herrenknecht and Rehm (2003), the TBM can excavate and build a ring around the machine continuously. 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 Herrenknecht Mixshield emphasises on safety and efficiency. It is established as an incredible tool for boring, laying the foundation for seamless project completion and ensuring a streamlined and secure tunnelling process. (be more specific and include points like clogging/ air pressure...) 


The Herrenknecht Mixshield prioritizes safety by using air pressure. (one way is by using air pressure) According to Herrenknecht (2024), there is a wall inside the excavation chamber that divides it into two parts. The front part is filled with a special fluid to fully support the tunnel's face during digging. But towards the back, the fluid level is lower, just above the machine's axis. To keep the tunnel stable, they use a cushion of compressed air above. Both parts of the chamber can communicate through an opening in the bottom of the wall. This feature enhances safety by providing stability. By fully supporting the tunnel face with suspension fluid in the front section, the risk of collapsing is minimized. This ensures the safety of workers and prevents accidents underground. Also with controlled pressure, the use of a compressible air cushion in the upper section allows for precise control of pressure at the tunnel face. This helps maintain stability and prevents excessive pressure build-up, reducing the likelihood of accidents or structural damage.


The Herrenknecht Mixshield is an efficient TBM because it has a feature against clogging to keep things moving smoothly. Herrenknecht (2024) stated that the cutting wheels have a design that lets material flow freely through the centre, reducing the chances of blockages. Herrenknecht improved how the fluid used in tunnelling moves through the system in areas where blockages are more likely, making sure it flows well. The separate systems of the machine have a system to handle materials. The user can adjust the flow with nozzles in the cutting wheel arms, the walls, the crusher, and the intake screen. Lastly, the chamber design helps keep things moving. Features like smooth edges and surfaces, coatings on parts of the machine, fewer openings in the walls, or optional buckets at the back all help stop blockages. These measures make sure the tunnelling process can keep on track, with fewer interruptions caused by blockages. It also helps the equipment last longer.


The Herrenknecht Mixshield ensures smooth project completion and a secure tunnelling process by improving efficiency and safety. According to Tunneling complete for Singapore’s Sewage Superhighway. (2023, October 31), Herrenknecht TBMs performed well in the Deep Tunnel Sewerage System (DTSS) phase 1, leading decision-makers to choose the same Herrenknecht TBM for DTSS phase 2. Thomas (2023) reported that PUB, Singapore’s National Water Agency, successfully completed tunnelling works for the second phase of the  DTSS, demonstrating the effectiveness of the Herrenknecht Mixshield.


Despite its numerous advantages, the Herrenknecht Mixshield is expensive. According to Lee, the total cost of the DTSS will be approximately S$10 billion, with the project costing over 200 million SGD per kilometre. In comparison, the Central Catchment Nature Reserve project, which utilizes a large-diameter slurry Tunnel Boring Machine (TBM), costs 151 million SGD per kilometre (Tjoe, 2022).


In summary, the Herrenknecht Mixshield prioritises safety and efficiency through features like controlled air pressure and innovative chamber designs where it ensure worker safety and structural stability underground. 


Successful projects such as Singapore's DTSS attest to its effectiveness. However, its high cost warrants careful consideration. Nevertheless, Herrenknecht Mixshield benefits, including enhanced safety and seamless project timelines, justify the cost. All in all, the Herrenknecht Mixshield offers a safe and efficient way to dig tunnels all around the world.



References

Harding, D. (2017, August 1). What’s in a name? Mixshield, crossover, hybrid and more. Robbins. https://www.robbinstbm.com/mixshield-crossover-hybrid/ 


Herrenknecht, M., & Rehm, U. (2024). Mixshield technology. http://inside.mines.edu/UserFiles/File/earthMechanics/seminar/03_soft_ground_tbms_epb.pdf 


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


Tunneling complete for Singapore’s Sewage Superhighway. (2023, October 31)  TBM: Tunnel Business Magazine. https://tunnelingonline.com/tunneling-complete-for-singapores-sewage-superhighway/ 


Thomas, T. (2023, August 23). PUB Completes Tunnelling On DTSS Phase 2. The Tunnelling Journal. https://tunnellingjournal.com/pub-completes-tunnelling-dtss-phase-2/ 


Tjoe, L. N. (2022, December 5). LTA awards $758M contract to build MRT tunnel that will go under Central Catchment Nature Reserve. The Straits Times. https://www.straitstimes.com/singapore/transport/lta-awards-758m-contract-to-build-mrt-tunnel-that-will-go-under-central-catchment-nature-reserve 

Lee, C. (2023, August 21). Tunnelling works completed for phase 2 of Singapore’s sewage “superhighway.” CNA. 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. 













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