Pore water pressure condition in the staged construction of tailings dams

Question

Dear Plaxis, 
 I am running the fully coupled stress defromation analysis of a staged construction tailings dam. I have questions regarding the initial water table condition in simulating this staged construction . 
 Basically, I used to different ways to set the initial water level 
 1) draw differnt water level and then use different global water levels at the top surface of each added tailings layer, and chose "phreatic" in the consolidation calculation.

In this case, as phreatic is used, does it mean only the newlyed added tailings layer is hydrostatic pressure, and the excess pore pressure in the previously deposited tailings will still be kept? 
 2) Set only one global water table at the top surface of the first tailings layer, and chose "use pressure from the previous phase" for the consolidation calculation of the subsequent newly added tailings layers.

In this case, I do not understand why the waste rocks (started from the second layer) are always dry and no water is entering into the waste rock dykes during the calculation. I also want to see the flow between the tailings and waste rock. how can we set that in plaxis2d? 
 The drainage type is "undrained A" for tailings and "drained" for wsate rock. In the Groundwater section, I used user defiend-saturated model for both tailings and waste rock, is this okay? 
 
 Thank you,

Stefanos Papavasileiou · Accepted Answer

Dear Jian Zheng, 
 With the information I have from the few screenshots I am not sure if I can fully answer the question. I would need to see the full model and the staged construction phases to see what exactly you are modelling. 
 Nevertheless, I can comment on the points below: 
 The Fully coupled flow-deformation analysis is available with the Ultimate license (or for legacy the PlaxFlow module). If you do not own the appropriate license then you cannot see all the options. More information here: https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/42955/plaxis-connect-edition-licenses 
 1) Correct. With Consolidation there is no groundwater flow, only dissipation of excess pore pressures (and building up based on staged construction changes). 
 In general, yes, you could use the WaterCondition property for a cluster (polygon) and assign a Saturation degree value. Note that this is only an initial condition. If flow occurs this naturally gets updated based on the boundary conditions assigned and permeabilities of the soil

2) Excess pore pressures and saturation of a cluster are two different things. The excess pore pressure can build up since you do staged construction changes, e.g. activating clusters. Then, the Consolidation phase is used to dissipate with time these excess pore pressures. 
 The saturation is based on the settings you have defined and the water conditions of the previous phases. That I cannot see in the screenshots, which makes it difficult to understand what I see in this phase. 
 Should it have water in the new clusters tailings above the phreatic level?: Is there any water table at that elevation of these clusters? If not, the new clusters would have to be dry. Do you have any groundwater flow analysis? If not, then there is no water flowing towards the new clusters. 
 When you use the option "Use pressure from the previous phase" we copy the information about water from the previous phase, thus nothing is changed on the steady-state pore pressures. Therefore, you should check what is happening in the previous phase you refer to. 
 Again, without the model it is tricky to evaluate what is happening. Please submit a service request. Then, one of our support engineers can help you in detail: https://apps.bentley.com/srmanager/ProductSupport

Stefanos Papavasileiou · Answer

Dear Jian Zheng, 
 As you provided questions on many various aspects, I hope that I can give a sufficient answer in the Forum. Otherwise, we can check it in more detail in a Service Request: https://apps.bentley.com/srmanager/ProductSupport 
 First of all, I would like to comment on the fact that you refer to running a Fully coupled flow-deformation analysis but in your questions you mention Consolidation with pore pressure calculation type set to Phreatic . Those are two different calculation options and should not be mixed when it comes to the configuration one can have as it usually leads to different results. 
 To your questions now: 
 1) To any newly activated polygons/clusters (tailings layers) the following are considered: - the pore pressure distribution set to Phreatic means that the materials will follow a hydrostatic distribution if the water level is higher than the materials' elevation; - the suction/saturation is according to the configuration in the Groundwater tab, specifically the water retention curve, also known as Soil-Water Characteristic Curve - in short SWCC. In general, every time you run a Consolidation analysis, the excess pore pressures will be computed and given the time and permeabilities, possibly dissipate, too. 
 2) The option " Use pressure from the previous phase " as it describes it does not compute steady-state pore pressures, but uses exactly the steady-state pore pressures generated in the previous phase, here based on the Phreatic setting in the previous phase. When you activate the new clusters the following happens: the new clusters have no water as the water table is specified to a lower elevation and there is no water flow in the case of a Consolidation analysis. Note that the rest are fully saturated as specified before via the SWCC. 
 Also, be aware that the phase setting to " Force fully drained behaviour of newly activated clusters " in one of these cases, as it is by default activated. You can read more about it in the Reference manual. 
 In general, you can see water flow if you run a Fully coupled flow-deformation analysis (not Consolidation). 
 Regarding the configuration of the tailings and rock, the drainage type is ignored in the case of Consolidation or Fully coupled flow-deformation analysis (read this: https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45958/after-a-consolidation-analysis-i-have-excess-pore-pressures-in-drained-material-sets-why ). Lastly, setting the model to "Saturated" means that PLAXIS will continuously use the saturated permeabilities.

Stefanos Papavasileiou · Answer

Dear Jian Zheng, 
 I see you already posted this here: communities.bentley.com/.../

Pore water pressure condition in the staged construction of tailings dams

Top Replies