I am simulating a shallow tunnel in plaxis 2D using the stress reduction method.The Tunnel diameter is 6m with an overburden of 7.6m.what Stress reduction value can be applied?
I assume that by that you want to simulate the 3D effect of the excavation and its impact on the deformation occurring. It would be helpful to have a look at the following paper as it describes in detail different methods that can be applied in order to simulate the 3D effect.
http://nicholasvlachopoulos.ca/uploads/3/5/5/4/3554643/2014_-_journal_-_gege_-_2d_paper.pdf
One thing that you could do would be to apply the Vlachopoulos and Diederichs LDP method in order to make an estimate of the expected radial displacement at every stage and then calculate the required pressure that needs to be applied on the tunnel circumference in order to achieve the required amount of deformation.
TBM or classical NATM (SCL) tunneling method?
Below you can find link to the paper were analyses were performed in Plaxis 2D with beta-method.
https://www.researchgate.net/publication/285322101_Convergence_Predictions_and_Primary_Support_Optimization_of_the_Tunnel_Progon
Conference Paper Convergence Predictions and Primary Support Optimization of ...
1. For control calculations with a two dimensional Finite Element Method (FEM), the release factor at the tunnel face was varied between 0.3 and 0.7. If ground deformations must be limited, the small factor of 0.3 is used.
2. Draw the GRC curve
I feel it depends on the type of soil and to my knowledge stress reduction factor can use 2/3rd of 'c' or 2/3rd of phi value.
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