I have the following three questions about the input of the plate's unit weight.
（1） If the groundwater level is -0.5m, the model has two layers of soil, the depth of the first layer is 1m, γunsat=17, γsat=18, and the depth of the second layer is 2m, γunsat=18, γsat=19. The actual unit weight of the plate γ (concrete) =25. Based on the above parameters, what is the value of γ I should input into plaxis3D? γ=25-(17*0.5+18*0.5+19*2)/3， Is that correct?
（2） Will PLAXIS 3D automatically consider the buoyancy effect on the plate element in version V22? If not, and I want to consider the buoyancy effect, still take my above example as an example, how should I calculate γ?
（3） In the case of foundation pit excavation, the plate element is fully buried in the soil in the first phase, but in the second phase, one side of the plate is exposed to air, while the other side is still buried in the soil, so do I need to set the different values of γ in the two phases? , for example, phase 1: γ=γconcrete-γsoil, phase 2: γ=γconcrete-0.5* γsoil （Suppose there is only one layer of soil）
Dear Stefanos,
Thank you very much for your answer！
In fact, I have carefully read the two links you recommended to me, but I am still not sure about the specific calculation method of plate unit weight. Can you give me an example of plate unit weight calculation with two layers of soil and groundwater level is -0.5m? And the link only mentions "the actual weight of the plate, w, is, therefore, the difference between the weight of the plate and the soil occupying its true volume". However, for the same plate element , It completely occupies the soil in the first phase and partially occupies the soil in the second phase. For this situation, should I choose γ = γconcrete-γsoil or γ = γconcrete-0.5 * γsoil ? In short, do I need to assign different γ to the plate before and after excavation？
Dear Xin,
Please check our Wiki article about this: Considerations for modelling a thick slab as a plate
And another relevant Forum post on the topic: Buoyancy effect on plates for PLAXIS 3D
I would be grateful if any geotechnical engineer could answer this question for me！