Stability Analysis of Deep Excavation

Question

Greetings, I conducted stability analysis of deep excavation at the final stage using Plaxis 2D analysis. The excavation depth is 15 m while the penetration depth is 12 m. When performing stability analysis, the plate element (diaphragm wall) has a large settlement at the y-direction, while the value is depending on the number step of the calculation. My question is, why the displacement of plate element at y-direction is larger than x-direction when performing stability analysis at divergence condition? Thank you.

Dennis Waterman · Accepted Answer

Dear Ari, 
 As you already say yourself, in a Safety analysis the displacements are unimportant as they have no physical meaning. Hence, there is no reason to want to reduce them. In fact, your suggestion to fixate the top point of the wall may very well lead to a larger factor of safety, which means that you're going to overestimate the strength. 
 With kind regards, 
 Dennis Waterman

Stefanos Papavasileiou · Answer

Dear Ari, 
 It is very difficult for me to understand what is happening without a screenshot to better see the way the model is. 
 Note that results at PLAXIS are stored per phase and per step. During the steps, part of the out-of-balance is solved (&Sigma;Mstage goes to 1.00), therefore there is stress redistribution and development of deformations. 
 However, indeed, I would not expect the vertical deformations to be larger than the horizontal. I would definitely double check on the material properties of the plate, bearing in mind that when considering the unit weight of a plate it is overlapping with the soil. In general, to calculate accurately the total weight of soil and structures in the model, the unit weight of the soil should be subtracted from the unit weight of the plate material. Specifically, for sheet-pile walls, the weight is generally provided by the manufacturer. This value can be adopted directly since sheet-pile walls usually occupy relatively little volume. 
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Stability Analysis of Deep Excavation

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