An assessment of the loading efficiency on the process of consolidation in unsaturated soils

Due to the uneven distribution of water resources during wet and dry seasons in Taiwan, the extraction of groundwater has become one of the most rapid and convenient supply options. In addition, the lack of a concept for national land conservation has led to overpumping of groundwater, thus causing land subsidence. In recent years, because of the development of the transport infrastructure, the impact of the loading of surface structures on land subsidence regions has become a crucial issue. The Taiwan High Speed Rail Corporation has been monitoring the vertical displacement of the railroad pier every year since 2003. The latest monitoring results show that land subsidence in Changhua and Yunlin counties is respectively around 3 to 5 centimeters every year, which threatens the safety of the high speed rail. Thus, it remains a necessity to develop a rational approach to solving these engineering problems based on theoretical analysis. To date, the most widely-used theories of consolidation are those of Terzaghi (1925) and Biot (1941). However, these two theories are different in many aspects, especially in terms of developing initial pore fluid pressure. In the current study, we apply the consolidation theory of poroelasticity developed by Lo et al. (2014) to illustrate the effect of loading efficiency on one-dimensional consolidation in unsaturated soils. Based on Lo et al. (2014), closed-form analytical solutions describing the excess pore air and water pressures along with the total settlement in response to time-invariant external loading under three boundary drainage conditions are formulated by employing the Laplace transform. In Terzaghi’s (1943) theory, at the instant the external loading is applied, it is assumed to be sustained entirely by pore fluid, while in Biot’s (1941) theory, the external loading is partially sustained by pore fluid according to loading efficiency. As illustrated through the use of samples, three initial water saturations (i.e., 0.7, 0.8, and 0.9) with respect to various elapsed time periods (i.e., 1 min, 1 hr, and 1 day) are selected for (i.e., clay) soils. Our numerical results show that, in the early stage of consolidation (T = 1 min), excess pore water pressure with loading efficiency is well represented and is less than that by ignoring loading efficiency. For a longer elapsed time (T = 1 day), the differences between excess pore water pressures induced with or without loading efficiency included are quite small. We also demonstrate that the total settlement is significantly different between these situations in the early stage of consolidation. As the process of consolidation undergoes one day, the total settlement achieves the same magnitude with respect to these two situations. Thus, in the early stage of soil consolidation, if loading efficiency is not well represented, initial total settlement will be underestimated.