Characteristics of moisture content variation of loess under seepage and its influence on tunnel engineering

The increase of moisture content after excavation of water-rich loess tunnel has a great influence on the construction of tunnel. Many discriminant standards of engineering measures based on loess moisture content were established by predecessors, but there is a lack of systematic researches on the causes and temporal and spatial characteristics of the change in loess moisture content. The variation characteristics of moisture content under different working conditions of the Yima No.1 Tunnel along the Xi'an-Yinchuan High-Speed Railway show that the average moisture content of loess in the tunnel body under the state of natural seepage is 25.9%, with some part being soft plastic. Under the influence of seepage and water gusher, the loess moisture content rises to 31.3% on the average, and the moisture content in the lower arch waist rises to 32.2%, which causes problems such as tunnel bottom softening, slide and instability of the tunnel face, and poor stability of surrounding rock. After the adoption of surface precipitation, the moisture content of loess decreases to 25.4%, which improves the physical properties of loess and ensures the safety and progress of tunnel construction. After the groundwater level is restored, the loess moisture content rises to 29.4% on the average. The loess moisture content of the arch roof and upper arch waist changes little, and that of the lower arch waist reaches 37.2%. It is believed that the variation of groundwater seepage will make the moisture content of loess range from 15% to 33%, and the safe construction of tunnel can be achieved by controlling groundwater seepage.