Open pit mine strategy: Seepage Nodes vs. Fluid-Transfer BC

Hi,
A 3-dimensional flow model was created to simulate the groundwater flow regime due to open pit mining. The mined-out materials in the model were represented by assigning a low hydraulic conductivity (e.g. K=1X10-8 m/s). Two types of boundary conditions (i.e. "Seepage Nodes" and "Fluid-Transfer BC") were tried separately to mimic the pit excavation. (1) For "Seepage Nodes", they were assigned along the pit wall as well as the bottom of the pit excavation. (2) For "Fluid-Transfer BC", they were assigned along the pit wall only with a very high rate of "Transfer-Out". The same convergence critera were used for two separate transient runs. The model run with "Seepage Nodes" was very slow; and the model run with "Fluid-Transfer BC" was fast. So, I am wondering is it a right approach to use ""Fluid-Transfer BC" to simulate open pit mine or what may cause the model run with "Seepage Nodes" so slow? Your help is really appreciated!

Thanks!
b
       

My guess is that a seepage face is more slow to simulate because it is a constrained bc, while a fluid-transfer bc is a an unconstrained one. A constrain is an extra numerical load. However, are you able to achieve the same results with both approaches ?

The hydraulic heads simulated look similar. The purpose of the modeling is not to estimate how much groundwater will flow into the pit once it is excavated. It is just used as a boundary condtion and see how it is going to affect the groundwater flow regime of the interested area.

thanks
b 

If the focus of your model is the larger groundwater system and not the pit then 1st kind boundary nodes set below the bottom of the pit should be ok.  Apply a constraint so that the nodes don't "inject" water into the system.  Using seepage faces implies that your focus is on the pit and its walls rather than on the much larger groundwater system.

The amount of water entering the pit is very important even if it is not an objective of the modeling because the inflow rate determines in large part the effect on the groundwater system.  1st kind bc with constraints will work fine.

It would be better to put a larger value of K in the pit rather than a small one because the boundary condition you specify will be influenced by that "pit" K.  I understand the the K of air is essentially zero, but that air is not part of the groundwater system.  For pit dewatering, I leave the bedrock K in place or would use an even larger K because the K of free water is essentially infinitely large (lakes are often modeled with very large K).  If the dewatering is insufficient to keep the pit dry, then either you need more wells, deeper wells or both.

If you have to simulate the active dewatering, I'd start with 4th-kind BC (wells) extracting water at rates that are either specified by mine operators or at a rate needed to keep the pit dry.  If the pit is dry, there won't be much in the way of actual seepage unless you are working in a very wet or very high-k area (in which case either a seepage face or a constrained 1st kind bc should be used).  I don't recommend using 3rd type bc.

Pete