Determining Groundwater Table in Slopes With Horizontal Drains and Design Framework for Length and Spacing

ABSTRACT Horizontal drains (HDs) are commonly implemented in slope stabilization to reduce the pore water pressure (PWP); however, they also cause complex three-dimensional (3D) variations in the groundwater table (GWT), which require intricate 3D flow models. To address this challenge, we propose a novel semi-empirical method based on a series of numerical simulations validated through numerical and field studies to determine the GWT of a slope with HDs. Subsequently, the proposed method was extended to calculate the average PWP (U) acting on a predefined slip surface via linear integration. The decrease of U (ΔU) resulting from HDs can be used to evaluate the performance of HDs. To create design charts that consider the impact of length and spacing of HDs on ΔU for a specific slope, we developed a Python-based computer program. Two case studies were conducted, which showed that ΔU increases with longer HDs and shorter spacing. The results also indicated that extending the HDs beyond a particular length does not significantly affect ΔU; it is highly sensitive to the spacing in short HDs and not sensitive in long HDs. Furthermore, we found that the total length of HDs required to achieve a target ΔU is less in wide spacings than in short spacings. In conclusion, long HDs with wide spacings are more effective and economical. Owing to the unique nature of each slope stability problem, this study offers a practical tool for analyzing the effectiveness of HDs instead of providing a general guide.