Effects of Active Axial Force Adjustment of Struts on Support System during Pit Excavation: Experimental Study

Given their high cost, servo steel struts typically are used only in specific sections of foundation pits in which nearby buildings are sensitive to excavation-induced deformation. Servo steel strut adjustments affect the mechanical behaviors of adjacent ordinary-steel-strut-supported areas. However, the plane strain assumption is used in current designs, ignoring this influence. To propose a more reasonable design, the effects of servo steel strut adjustments should be examined. In this study, a model support system with adjustable strut axial forces was constructed (scale: 1∶20, under 1g), and several groups of tests with different adjustment schemes were conducted. The lateral earth pressure, lateral wall deflection, and strut axial forces were monitored and analyzed. The results indicated the following: (1) the first level of struts is not suitable to be actively adjusted because this can increase the maximum lateral wall deflection; (2) increasing the strut axial forces can lead to redistribution of lateral earth pressure along the horizontal direction, which can be divided into two areas with different characteristics, and whereas the lateral wall deflection decreases in both areas, the lateral earth pressure increases in the area adjacent to the adjusted strut and decreases in the other area; and (3) a transition section can be designed between the servo-steel-strut-supported area and the ordinary-steel-strut-supported area, within which strut adjustment diminishes as one approaches the ordinary-steel-strut-supported area. However, the horizontal length of the transition section should exceed a threshold value, which is approximately 3 times the horizontal strut distance, to effectively mitigate the axial force reduction in ordinary steel struts.