I am studying the effect of groundwater level on the internal force of the tunnel. I have established a numerical model of tunnel excavation with a uniform stratum of width 100 m and height 80 m (the top elevation of the model is 0 m). The soil is modeled with the HSSMALL constitutive model, and the structure is modeled with a linear elastic model. The groundwater levels are set to 0m, -10m, and -20m respectively, and the boundary conditions are set to default and the undrainage behavior is ignored. The calculation results show that when the groundwater levels are 0m, -10m, and -20m respectively, the maximum positive bending moments are 66 knm/m, 163 knm/m, and 346 knm/m. The higher the groundwater level, the smaller the internal force of the structure, which is contrary to our usual understanding. I don't know where the problem is and I hope to get your help.
Hi, please revisit your expectation. Groundwater is an all-around pressure. Does it create bending moment? If you go to Muir Wood etc., you will see that water pressure creates axial force. It creates a little bending moment due to change of water pressure with depth.
Why do you have higher bending moment with lower groundwater? Let's think together: If you lower the groundwater, effective stress at the level of tunnel increases. This increases the lateral pressure since lat_pressure = K x effective_pressure. Bending moment is created by the difference of forces.
So, your results seem logical. This is why usually we do a dry check for NATM tunnels, because NATM is usually a draining tunnelling method. But for TBM, it is not done since it is not draining the water.
Note: This outcomes might change with undrained response if you use undrained soil and short term analysis. But I hope you get the logic.
Answer Verified By: G. G Liu