Geotechnical Engineering Journal of the SEAGS & AGSSEA ISSN 0046-5828
Vol. 47 No. 3 September 2016
Singapore Case Histories on Omission of Strut by Observation Approach For Circle Line and Down Town Line Projects
By David Ng C. C. and Simon Low Y. H.
ABSTRACT: This paper will describe in details the issues and challenges involved in the procedures for strut omission by observational approach for two case histories from two different projects – Circle Line Contract C824 and Down Town Line Stage 3 Contract C922. The case history of Overrun Tunnel (ORT) of C922 is basically an underground facility building functions as both Railway Facility (Operation Control Centre) and Electrical Substation (ESS) which is to be built next the Expo Station. ORT is located in old alluvium (OA). The proposed underground overrun tunnel is a box structure with dimensions of approximately 23m wide, 25m deep and approximately 440m long. The proposed diaphragm wall function as the earth retaining system (ERSS), it designed for both temporary loading conditions during excavation and permanent load conditions in accordance with LTA Civil Design Criteria. Bottom-up construction sequence is adopted where lateral supports using four (S3 to S6) or six (S1 to S6) layers of steel strutting were installed as excavation progresses downward. The most challenging part is the omission of the last layer of strut S6 for the whole ORT by using observational approach. The case history of C824 Nicoll Highway Station demonstrates that Jet Mechanical Mixing (JMM), if properly installed, has major benefits in controlling the stability and movements induced by deep excavations in soft ground. The reasons can be attributed to the fact that the inner soil column is comprehensively mixed, combined with the attributes of the outer jet grouted column with sufficient overlapping. The whole process undergoes tight quality control and rigorous testing to ensure a continuous and comprehensive slab. In addition to the JMM slab, there is the major benefit of the discrete soil mixing columns formed above the JMM slab during the withdrawal of the auger. This case history also shows that with observational approach, if used appropriately, the design of temporary works can be effectively streamlined to achieve a more economical and yet safe design, This is illustrated by the approach to omit the intermediate layer of strut in the original design after observing the better than expected performance of the JMM. Based on the limited usage to date it is difficult to suggest what parameters should be used for future design. The approach to the back analyses and forward analyses in the observational approach is presented in this paper. This paper will also discuss the design and construction considerations by focusing on the challenge of strut omission by observation approach. The instrumentation monitoring results will also be presented as evaluation of the performance of the ERSS. The site observation and instrumentation result is in line with the forward analysis prediction for the omission of strut. This proposal has helped to expedite the project with a more economical design. With the implementation of observational approach, we will be able to achieve a more sustainable development of underground infrastructure projects.
KEYWORDS: Deep excavation, Strut omission, Back analyses, Forward analyses, Observational approach
