A discussion of the Prandlt calculation formula for anti-uplift stability of the bottom of a foundation pit wall in deep soft soil areas

The Prandlt formula is often used to calculate the anti-uplift stability of foundation pits in deep soft soil areas, which often fails to meet the specification requirements and brings great confusion to the design of foundation pit support. Based on the analysis of the difference in mechanical boundary conditions between the foundation pit excavation and the Prandlt formula for calculating the foundation bearing capacity, this paper points out the shortcomings of the improved calculation methods such as the Prandlt formula, critical width method, and takes into account the shear strength of the soil inside the foundation pit method, and puts forward an improved calculation formula considering the shear effect of the soil inside and outside the foundation pit support. Through sensitivity analysis of each parameter of the four calculation formulas, it is found that the internal friction angle is the most important factor affecting the stability of foundation pit wall bottom against uplift. The excavation depth and the insertion depth of the supporting body are the main influencing factors, the cohesion of soil body is the secondary influencing factor, and the influence of soil weight can be ignored. The friction angle of soft soil is small. Under certain excavation depth, the stability against uplift of foundation pit wall bottom can be guaranteed only by increasing the insertion depth. Therefore, considering the favorable effect of shear strength of soil on the inner and outer sides of foundation pit support, the foundation pit design can be reasonably optimized. This paper also verifies the rationality of taking into account the shear strength effect of soil on the inner and outer sides of the foundation pit support through engineering case studies. At the same time, according to the statistical analyses of wall bottom stability calculation of many projects, it is pointed out that the current standard is too high and unreasonable in soft soil areas. It is suggested that the standard limited value of the standard calculation method should be appropriately reduced on the basis of accumulated engineering experience.