Lessons from the back analysis of Nicoll Highway incident

• Main Author: Foo, Yee Chuan
• Other Authors: Teh Cee Ing
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Civil engineering::Geotechnical
• Online Access: https://hdl.handle.net/10356/150023
• Institution: Nanyang Technological University

Nicoll Highway Incident is a major incident that caused fatalities and millions of dollars lost. This project studied this incident by analysing the 30m deep braced excavation for the MRT Circle Line adjacent to the Nicoll Highway. It was analysed by Finite Element Method via Plaxis 2D. Prior analyses during the Nicoll Highway project’s design stage have relied on Mohr-Coulomb Method A model, which was an unsafe model for the Nicoll Highway case. Mohr-Coulomb Method C model was studied, and it showed crucial results that the excavation was unsafe to construct. The use of Jet-grouted Piles (JGP) has gained popularity in Singapore when excavating in soft soil; it is a soil-improvement method that improves the soil characteristics. This study had reassessed the role that the JGP played in the Nicoll Highway Incident. Based on the finite element analysis, it was found that there may be a lack of deep understanding of how the sacrificial JGP could exert an undue influence on the stability of the entire temporary support system upon the sacrificial JGP’s removal, especially when there are large uncertainties on the physical properties of the JGP such as its modulus and strength. A separate case study on Singapore Post Centre was done to illustrate that the JGP could be used appropriately in an excavation work. Using Plaxis Method C as the basis of analysis, various adjustments to the design elements of the braced excavation system were explored. It was found that it was not possible to reach the final excavation level by merely increasing the thickness, strength, or stiffness of the JGP layers alone. Moreover, it has been established through the analysis that a greater wall embedment depth coupled with a stronger JGP was required to safely excavate at the Nicoll Highway Project. The locations of the JGP had been shown to control the lateral wall displacement, bending moment and strut forces, as well as the overall stability. The complete removal of a thick, stiff JGP layer would engender a major load transfer that was hitherto carried by the JGP to the whole strutting system. Such a sudden and large transfer of load from one design element to other design elements should not be undertaken lightly.