Underground Singapore 2011 more concentrated near wall supports depending on soil type and wall type. These complications led Potts (1992) to comment that all simplified methods of analysis (such as stress field and limit analysis using upper and lower bounds of plasticity and equilibrium methods such as the Coulomb wedge analysis) would be flawed, and their application in the design of struts would be inappropriate unless it has been verified by field observations. The empirical approach of using apparent pressure diagrams by Tergazhi and Peck attempts to estimate the maximum strut load for excavations in different soil conditions based on actual field data. However, there are several limitations from the apparent pressure diagrams as these were developed from limited field data and primarily from excavations supported by flexible walls. Although Twine and Roscoe (1999) developed a distributed prop load method substantiated with more field data to improve on the limitations of the apparent pressure method, the original Peck’s method is still popular today as a simplified design check for the predictions from finite element analysis. In this paper, the strut monitoring data from the completed excavations in Circle Line was used to validate the use of the apparent pressure method. It was found that the back-analysed CCL data fit in with the original apparent pressure diagrams for flexible retaining walls in medium stiff to stiff clays, but the original apparent pressure diagrams were under-estimating the maximum strut loads for excavations supported using rigid diaphragm walls. This can be overcome by including a modification factor (based on empirical data) to the original apparent pressure diagrams. A trend of increasing apparent pressure for higher wall stiffness and greater strut distance to the hard stratum can also be interpreted from the CCL data. It is hoped that the compiled database of maximum strut forces recorded from the CCL excavations can become useful references for engineers undertaking future design of strut system for braced excavation in Singapore. REFERENCES Chang, J.D. and Wong, K.S. 1996. Apparent pressure diagram for braced excavations in soft clay with diaphragm wall, Proc. International of Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, London, 15-17 April 1996, pp.87-92. Peck, R.B. 1969. Deep excavations and tunnelling in soft ground. Proceedings of the 7th Int. Conf. on Soil Mechanics and Foundation Engineering, Mexico City, state-of-the-art volume, pp.225-290. Potts, D.M. 1992. The analysis of earth retaining structures. Proc. Conf. Retaining Structures, ICE, London, pp.167-186. Terzaghi, K. and Peck, R.B. 1967. Soil Mechanics in Engineering Practice, 2nd Ed., John Wiley & Sons. Twine, D. and Roscoe, H. 1999. Temporary propping of deep excavations – guidance on design, CIRIA Publication C517, London.
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