Statistical analyses of the effect of a drainage tunnel on landslide hydrogeological characteristics

A high groundwater level is highly relevant to the slope instability. Drainage tunnel is an effective method for groundwater level control, but its effect on landslide hydrogeological characteristics is rarely discussed. This study analysed the changes of the landslide hydrogeological characteristics under the effect of a drainage tunnel by real-time monitoring of rainfall, groundwater level, and surface displacement. The trend and mutation of groundwater level are analysed by the Mann-Kendall test and the Mann-Kendall mutation test. The memory effect of groundwater in the landslide area was analysed using autocorrelation analysis. The response characteristics of groundwater level to rainfall were evaluated using cross-correlation analysis and mutual information theory. Variations of groundwater levels were further investigated based on hydrograph analysis. Results showed that the groundwater level had a downward trend from 2016 to 2017. The significant downward trend of groundwater levels began in August 2016. The memory effect of groundwater levels was longer under the effect of the drainage tunnel. Before the construction of the drainage tunnel, the response time of groundwater to rainfall was less than 3 hr and rainfall can generate dramatic groundwater level variations. After the drainage tunnel was completed, time lags can be observed in the groundwater response, and the variation of groundwater levels was smaller than before. A strong correlation was found between groundwater levels and the landslide movement. This study demonstrated that the drainage tunnel had effectively controlled the groundwater level in the landslide and ensured the stability of the landslide.