Design Approach Implementation - EC7 A3

Question

Hello, I would appreciate you answering the following inquiry: When running a slope stability model on PLAXIS 2D while implementing manually partial safety factors, in my case the ones stated for Approach 3 of EC7, I'm having trouble deciding the basis on which each Construction Phase is considered passing or not. For instance, according to A3 in EC7, when using the right reduction factors for soil and structures' strengths, the Phase is considered verified in case the Safety Factor is above 1.00. 
 For comparison, other slope stability software that are based on LEM, the SF is calculated based on Bishop's method. 
 Therefore in the case of PLAXIS's FEM, what is the factor that can be representative of the SF which should be compared to 1.00? Is it the Mstage of each construction phase (where in that case, a phase is considered verified if it finishes the calculation properly)? Or are we obliged to add a Safety type phase for each Phase to extract the Msafety of each one based on c-phi reduction? 
 Your help is much appreciated.

Vasileios Basas · Answer

Hi Mohamad, 
 Since staged construction is performed using the Load advancement ultimate level procedure, it is controlled by a total multiplier (&Sigma;Mstage). This multiplier starts at zero and is expected to reach the ultimate level of 1.0 at the end of the calculation phase. Therefore, a "passing" construction phase means the stage was calculated successfully (no errors occurred during calculation), and the full out-of-balance forces were solved. 
 When you have applied the partial factors on actions, and soil materials (e.g. either manually or via the Design Approaches tool) and the staged construction calculation was completed successfully, it usually means that no failure mechanism has developed in your model. However, you will have to evaluate the results in Output. 
 A successful calculation suggests a FoS > 1.0 but doesn't give the exact value. You could run a Safety analysis after the phase where the partial factors were applied to roughly evaluate the actual FoS. Please note that in a safety calculation, you are successively reducing the material shear strength parameters (tan&phi;, c and &sigma;t) until failure of the structure occurs. However, there is no change in the loads unless you do it manually. 
 Although similar, a Design Approach (DA) is not equivalent to a Safety calculation. In the former case, a partial factor is first applied to the material properties (&phi; &ndash; c) before calculations. In contrast, the material strength parameters are progressively reduced until failure in the latter case. Also, the Apply Strength Reduction applies only for Safety calculations (phi/c reduction) to all materials that have this option activated, regardless of whether those materials have already had their material properties reduced by a partial factor. 
 Finally, when using DA with advanced soil models, these models will continue to behave as advanced models, maintaining all their features, such as stress-dependent stiffness behaviour and hardening effects. However, that is not the case when using a Safety analysis with advanced models (see Chapter 7.3.5 of PLAXIS 2D &ndash; Reference manual) since advanced models lose their advanced features and switch to Mohr-Coulomb. Therefore, when comparing a Safety analysis to a target value of &Sigma;Msf with a Design approaches analysis using the same partial factor for c and tan(&phi;), it should be realised that the results could be different because of this reason. 
 You can find some more information on the use of DA in the link below, as well as: The Use of Design Approaches with PLAXIS