The permeability in the natural clay layer is obviously anisotropic, and the flow of water in the pores often deviates from Darcian law. In order to analyze the effect of anisotropic non-Darcian flow on the two-dimensional consolidation of a saturated clay layer, the vertical and horizontal permeability laws of saturated clay were measured by the falling-head permeability test. It was found that the flow of water in both directions can be described by Hansbo's flow equation, and Hansbo's flow parameters in these two directions were obviously different. Then, the two-dimensional Terzaghi consolidation equations were modified considering the anisotropic Hansbo's flow and discretized into finite-element formulations. The validity of the numerical model was verified through comparison with the literature solutions. The effect of the anisotropic Hansbo's flow on the consolidation process of a two-dimensional saturated clay layer was analyzed under different lower boundary conditions. The numerical results indicated that in the initial stage of consolidation, the excess pore pressure is slightly concentrated in a specific area below the loading boundary. Moreover, variations in the lower boundary conditions have little effect on the distribution of excess pore pressure, and the influence of the different Hansbo's flow parameters in the vertical direction on the dissipation rate of excess pore pressure is not evident. However, in the middle and late stages of consolidation, the pore-water pressure with the permeable lower boundary condition is significantly lower compared to that with the impermeable lower boundary condition. Additionally, increasing the values of Hansbo's flow parameters in the vertical direction further impedes the dissipation rate of excess pore pressure, which in turn slows down the consolidation process of the clay layer.

This paper presents an analytical theoretical model for air-boosted vacuum preloading, focusing on the influence of air injection on soil consolidation. A simplified force model based on the equivalence of air-boosted pressure is proposed, and a new parameter called the horizontal permeability coefficient increase parameter eta is introduced to account for changes in soil permeability due to gas injection. Based on Barron equal vertical strain assumption and linear Darcy's law, the governing equation and consolidation analytical solution for air-boosted vacuum preloading are derived, considering factors such as radial and vertical seepage, prefabricated vertical drain smear effect, well resistance, and surcharge. Additionally, the theoretical formula for the equivalent air-boosted pressure p(t) is derived using elastic mechanics methods. Specific analytical solutions are provided for two cases: instantaneous air-boosted and linear air-boosted. An engineering case study is used to verify the rationality of the analytical solution and the mechanical equivalence method. Through the analysis of consolidation behavior, the following conclusions are drawn: the air injection boosting method increases the negative pore water pressure in the soil, facilitating faster drainage and consolidation; the eta parameter significantly affects the consolidation rate, and considering changes in the horizontal permeability coefficient during gas injection improves the accuracy of the analysis model; well resistance slows down the consolidation rate, and while pore pressure can dissipate completely during radial and vertical seepage, it may not dissipate completely during radial seepage alone.

The construction of airport runways or other infrastructures on soft soil might risk damage and create potential hazards if inappropriate foundation treatments have been conducted. Preloading on soft soil is a commonly used ground improvement method in airport runway construction, owing to the cost efficiency and simplicity of this method. However, the theoretical basis for the design parameter estimation of this method has not been fully understood, for example, preloading height calculation and preloading time determination. In this paper, calculation models of settlement characteristics for soft soil preloading treatment and influence factors of preloading for soft soil are proposed, in accordance with Terzaghi one-dimensional consolidation theory. Preloading is counted as a dynamic process in this theory, and the settlement calculation model is expressed as an integration formula. In addition, a simplified calculation model is proposed in this paper for a specific condition of preloading treatment. Reliability of the models was verified with in situ data from an airport runway construction site in China. Results from multiple analysis reveal that preloading treatment can significantly increase the total settlement and consolidation, and accelerate the settlement rate of soft soil. Sensitivity analysis of the theoretical model found that preloading height is the key parameter affecting the preloading efficiency when ignoring the material type of soft soil. Moreover, the thickness of soft soil is the critical input parameter, as analyzing the factors affects the preloading height, and preloading treatment is a continues process rather than an instantaneous process, as can be noted from analyzing the settlement characteristics of the two loading conditions. The proposed settlement characteristic model provides valuable information for the design of preloading parameters of similar projects. Preloading is a simple and widely used method for improving the strength of soft soil; the design of preloading parameters mainly relies on the construction of a trial site or engineering judgment. The theoretical basis of this method is still weak. Thus, this paper proposes a settlement characteristic model under preloading conditions, which was verified by utilizing in situ data from an airport runway construction site. In addition, the influences of design parameters on such settlement characteristics as preloading height and thickness of backfill soil were analyzed. It was found that preloading height is the key parameter affecting the settlement characteristics of preloading treatment. In addition, sensitivity analysis was conducted on the preloading height design using an analytical model, and the results show that the thickness of soft soil is the critical factor affecting the preloading height. In that case, accurate site investigation is significant for the preloading design of soft soil.