| Summary: | According to the lack of theoretical research on the spatial effect of the foundation pit, the concept of the reasonable arch curve (bending moment on the curve is zero everywhere) is introduced to establish a new three-dimensional instability failure theoretical model of the water-rich foundation pit. Taking the K6 + 650 ∼ K6 + 880 section of the GUI River Foundation Pit as an example, the active earth pressure acting on the supporting structure per unit length, the critical length–depth ratio, the main influence range, and the influence coefficient of spatial effect are calculated and compared with previous research results and field monitoring data, in order to explore the influence law of the length, depth, and moisture content of soil layers on the spatial effect. The results show the following: (1) When the length–depth ratio is greater than 1, the foundation pit presents the failure mode of the end arch curve and middle straight line, and when the length–depth ratio is greater than 3, the main influence range has a linear relationship with the depth and a negative correlation with the moisture content, but little relationship with the length. (2) The active earth pressure is approximately a quadratic function of the depth, and there is a more than 16% increase when the moisture content increases from 21% to 25%. (3) The change law of the horizontal displacement of the pile top monitored verifies the rationality and progressiveness of the new instability failure model. Through this study, the spatial effect revealed can provide the theoretical basis for pile support of the foundation pit.
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