# Compound Instability Effect and Countermeasures of Pit-in-Pit in Collapsible Loess Strata
###### Abstract  
The construction of pit-in-pit has become increasingly challenging due to the bad geological conditions, particularly in collapsible loess strata. To understand its supporting characteristics and failure mode, it is necessary to study the composite instability mechanism in the loess strata. This research systematically investigates the interacting instability modes of pit-in-pit under a collapsible loess stratum, studies the effects of different reinforcement parameters through physical model tests, analyzes the significance level of each reinforcement factor, and monitors the displacement of the foundation pit during construction in a pit project in Zhengzhou. The result shows that the soil pressure distribution law of the pit in a collapsible loess formation is a complex function of soil parameters, the relative positional relationship between the inner and outer foundation pits, and the time of immersion. The model test shows that the width and depth of reinforced soil have the most significant influence. The reinforcement measures proposed in this paper can effectively control the displacement of each measuring point during the foundation pit excavation, which can provide a reference for similar projects. pit-in-pit; compound instability effect; collapsible loess strata; similar model test 202520257710  
soil flow. On the other hand, due to the coupling influence of outer pit and inner pit excavation and unloading [18], compared with conventional pit excavation, the stress state of the soil associated with the stress path becomes more complicated [19]. In the analysis of the pile foundation performance, there are complex soil stress changes in the transition zone between the outer pit and the inner pit, which makes it more difficult to analyze the stress-strain state of the soil located in this area [20]. In the research of foundation pit engineering, scholars at home and abroad mainly adopt the Finite Element method (FEM), the Finite Difference method (FDM), and the Discrete Element method (DEM) to simulate the influence of excavation on the retaining structure and the surrounding soil. The FEM precisely calculates the structural response through unit discretization, the FDM discretely processes boundary problems through differential equations, and the DEM is good at simulating the mechanical behavior of granular materials such as soil and rock. [PERSON] et al. [21] used the PLAXIS software to analyze the influence of the excavation depth of the inner pit and its relative position on the horizontal displacement of the retaining structure. It was concluded that the depth significantly impacted the displacement, and the horizontal displacement of the retaining structure decreased with the increase in the pit spacing. [PERSON] et al. [22] studied the influence of the area ratio and depth ratio on the equivalent depth coefficient of the outer pit wall through numerical model experiments. However, there are no calculation cases of collapsible loess areas at present. At present, the research on this type of foundation pit is mostly a simple description of the construction methods and support means, or a record of engineering cases. However, there are few in-depth theoretical studies. The common means of foundation pit reinforcement are also limited to enhancing the stiffness of the supporting structure. Groundwater level variation directly affects soil's effective stress and pore water pressure. Considering the influence of groundwater level variation in the numerical simulation process of foundation pit excavation is extremely important for analyzing the deformation of the retaining structure and soil and evaluating the stability of the foundation pit [23; 24]. Meanwhile, in loess areas, due to the existence of collapsibility, the soil in the foundation pit is highly prone to local instability under the action of water, resulting in a local stress concentration and excessive displacement in the support [23; 24; 25].