For undergroundwater level, the internal force of the tunnel does not conform to the common sense

Question

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 str atum of width 100 m and height 80 m ( the top elevation of the model is 0 m ). The soil is modeled with the H SS M ALL constit utive model , and the structure is modeled with a linear elastic model . The groundwater levels are set to 0 m , - 10 m , and - 20 m 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 0 m , - 10 m , and - 20 m respectively , the maximum positive bending moments are 66 kn m / m , 163 kn m / m , and 346 kn m / 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 .

Berk Demir · Accepted Answer

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.

Hung Ho Manh · Answer

Hello Liu, It is related to a specific model configuration, so, we recommend to check outputs to understand how it behaves. The outputs can be found in the link that our colleague has showed below. 
 communities.bentley.com/.../727841 
 If you need us to take a look at the model, please submit a SR using the link below too: https://apps.bentley.com/srmanager/ProductSupport

Stefanos Papavasileiou · Answer

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