# Compound Instability Effect and Countermeasures of Pit-in-Pit in Collapsible Loess Strata
## 7 Conclusions  
Pit-in-pit engineering is a complicated engineering problem, especially in collapsible loess areas. In this paper, the combined instability effect of pit-in-pit in the collapse loess stratum is proposed, the influence of different geometric conditions and reinforcement factors on the safety of the foundation pit is studied, the numerical simulation calculation and analysis are carried out for a pit-in-pit project in Zhengzhou, Henan Province, and the corresponding reinforcement works and measures are proposed. The key findings are as follows:  
(1) The soil pressure distribution law of the pit in a collapsible loess formation is a complex function of soil parameters, the relationship between the position of the pit and that of the pit, and the time of immersion. Under the condition of water immersion, the complex stress system of the pit-in-pit in the loess stratum will be disturbed, and the relative position relationship between the critical shear failure angle and the inner foundation pit will change. We have calculated the critical moisture content of the foundation pit instability under each parameter.  
(2) The variation rules of the force and displacement of the inner and outer pits under different factors are obtained through a similar model. During the excavation process, the vertical displacement of adjacent measurement points increases significantly. However, the farther the distance between the inner and outer foundation pits, the smaller the influence of this factor on the vertical displacement in the far area. The height difference of the foundation pit has a relatively small influence on the displacement of the inner pit. Through the orthogonal test, it is found that the width and depth of the reinforced soil have the most significant effect on the horizontal displacement control of the external foundation pit enclosure unit and the internal foundation pit maintenance unit.  
(3) Taking a pit-in-pit project in Zhengzhou, Henan Province, as an example, the pit displacement monitoring during the construction period is carried out. The results show that the reinforcement measures proposed in this paper can effectively control the displacement of each measuring point of the pit during the excavation process and can provide reference for similar projects.  
J.X.Writing original draft, Investigation, Data curation, Conceptualization, Writing--review and editing. P.Y.: Investigation, Methodology. J.L.: Investigation, Data curation, Supervision. P.C.: Conceptualization, Supervision. X.S. ([PERSON]): Writing--review and editing, Methodology. X.S. ([PERSON]): Investigation, Data curation. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The National Natural Science Foundation of China (52278393, 52078421), the Key R&D Program of Shaanxi Province (2023-YBSF-511, 2024 SF-YBXM-635, 2023 KQJ-159), and the Innovation Capability Support Program of Shaanxi (2023-CX-TD- 35).  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** Author [PERSON] was employed by the company CCCC Second Highway Engineering Co., Ltd. [PERSON] was employed by the company of Changqing Engineering Design Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.