渗流作用下黄土含水率变化特征及对隧道工程的影响

富水黄土隧道施工开挖后含水率增加对隧道工程施工影响较大，前人建立了多种以黄土含水率为指标的工程措施判别标准，但对于黄土含水率的变化原因及时空变化特征缺乏系统研究。银西高铁驿马一号隧道不同工况下含水率的变化特征表明，自然渗流状态下隧道洞身黄土含水率平均为25.9%，局部为软塑；施工排水阶段受渗涌水影响，黄土含水率平均上升到31.3%，下拱腰上升到32.2%，引起了隧底软化、掌子面滑塌失稳、围岩稳定性变差等问题；采取地表降水后，黄土含水率下降为25.4%，改善了黄土的物理性质，确保了隧道施工安全与进度；水位恢复后，黄土含水率平均上升到29.4%，拱顶与上拱腰变化较小，下拱腰达到了37.2%。研究认为地下水渗流变化将使得隧道洞身黄土含水率变幅达15%～33%，通过控制地下水渗流作用可以达到隧道安全施工的目的。

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.