Stability analysis of super-large-section tunnel in loess ground considering water infiltration caused by irrigation

Constructing a super-large-section loess tunnel under the irrigation area is a highly challenging task. One reason is that the strength of the loess decreases when it is soaked, which endangers the safety and stability of tunnels. In this paper, Majiazhuang tunnel is selected as a project case which provides in situ monitoring and measuring data, and Midas GTS NX software is used to build a tunnel structural calculation model under the coupled function of seepage and stress fields. Combined with the change law of physical and mechanical property for unsaturated loess, simulations are performed under different situations, which include the surface infiltration, the integral soaking, and partial soaking caused by concentrated infiltration. The results of in situ monitoring show that the complete structure of super-large-section tunnel buried at 50 m depth below the clayey loess was effected negligibly by irrigation and heavy rainfall, and it is stable during irrigation and heavy rainfall. The results of simulation show that when the integrity of the farmland topsoil is good and the weak structural planes, cracks, and fractures do not exist, the effective depth of irrigation water in Q₃ clayed loess is 4 m approximately; when the tunnel soil layer is integral soaked, the moisture content of the Q₂ silty clay loess increases from 16.6 to 20.5% and becomes saturated at the moisture content of 24.3%. The crown settlement, ground surface settlement, thickness and strain of the plastic zone significantly rise, and they slowly increase in the early stages while rapidly increase in the later stages. In addition, the rate of increase in the later stages is approximately 9.6 times faster than that in the early stages; when the crown, upper arch, and side wall are partial concentrated soaked, the thickness of plastic deformation zone at the crown has the most significant effect. The thickness of plastic deformation zone at the crown increased more than 100%, at the other two positions both have approximately 40% increasement compared with that without irrigation water. It indicates that the irrigation water infiltrates to the tunnel soil layer, the structural stability of the tunnel is affected by the surface irrigation, and the partial soaking at the upper crown has the greatest effect on the stability of the tunnel structure.