Proposal of a reasonable method for analyzing shoring system when soft ground has been improved

Abstract

At the four stations constructed using the cut-and-cover method for the extension of the Hanzomon subway line (TRTA) in Tokyo (between Suitengumae and Oshiage), quicklime piles were placed into very soft clayey soil layers to ensure trafficability during construction and to improve passive earth pressure. This article proposes a lateral pressure model and an equation for estimating lateral pressure in which the effects of quicklime expansion are considered. It also proposes a reasonable design method that reflects the total effect of quicklime piles by defining the horizontal subgrade reaction coefficient in such a way that the calculated displacement matches the measured displacement during ground excavation. Method of analysis - Based on case studies involving various lateral pressures, the author proposes a lateral pressure beam model, with the beam on an elastic foundation slab best matching the measured displacement of the walls before and after the injection of quicklime piles. Further, considering that the strength of the improved ground was higher than the designed value, we focused our attention on cohesion being increased by quicklime. Parameters were defined so that the measured displacement of the shoring system would come close to the calculated displacement during the final ground excavation. This was done by redefining the spring value while changing the coefficient of horizontal subgrade reaction. Results of analysis - Using a lateral pressure estimation equation consisting of such functions as the influence coefficient, ground strength, and improvement ratio and a lateral pressure model using the load working range as a parameter, the author studied the extent to which measured values matched the calculated values. We found that they matched best when the influence coefficient in the lateral pressure estimation equation was set at a PS 9:3 and the load that is a combination of uniform and triangular distributions was used. Further, it was found that, by varying the cohesion and making the coefficient of horizontal subgrade reaction approximately 200 times the ground cohesion, the calculated and measured values of displacement matched in their mode, peak position and maximum displacement in almost all steps of excavation work. Based on the results outlined above, the article proposes a reasonable method of analysis of shoring systems used when the soft ground has been improved. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.