Application |
PLAXIS 3D |

Version |
PLAXIS 3D |

Date created |
03 September 2012 |

Date modified |
03 September 2012 |

Wells are modelled as line elements in PLAXIS 3D. Along these lines a specified discharge is extracted from or infiltrated into the soil. The discharge capacity along the length of the line/well (m^{3}/m/day) is a function of the total specified discharge capacity (m^{3}/day), the saturated permeability of the intersected soil layers and the intersected depth. This means for example that when a line/well crosses a clay layer (low permeability) and a sand layer (high permeability) most of the discharge capacity will be available in the sand layer. Note that the discharge capacity along the line/well is set/determined prior to the calculation and is a fixed value during the calculation.

Now also realise that when for example half of the line/well is deactivated (because the corresponding volume is deactivated) or only half of the line/well is submerged, only a part of the specified maximum total discharge capacity is available (and depending on the initial distribution of the discharge capacity along the line/well, this could be a small or a large part of the total capacity).

In a lot of practical situations however you will only need to model the “filter” part of the well at a certain depth beneath the surface level. The “filter” being the open or perforated part of the tube or well where water can actually flow in (or out, in case of an infiltration well). So there is no need to model the part of the well between surface level and filter. An example for a deepwell is presented below.

Note that in addition to the maximum total discharge capacity you need to specify the minimum possible head value in the well (h_{min}). When the groundwater head drops below this value no further extraction will occur. Here you should choose a value corresponding to the type of well you are using. An example for a deepwell is presented below. Here the h_{min} is chosen in such a way that when part of the pump becomes dry the extraction stops.

You can find the actual calculated discharges in the stresses menu in Output (*Stresses > Wells*). The total discharge is presented for each (activated) well. Note that in the table the total discharge is repeated for each node of the well. The total achieved discharge per well however is just one time this value.

Note that in the first implementation of wells in the 3D2012 versions the total discharge specified was always equally distributed over the length of the line/well (so not depending on the permeability of soil layers). So when specifying a maximum total discharge of 50 m^{3}/day and a line/well length of 10 m, the actual maximum discharge capacity is 5 m^{3}/m/day. This might result in unexpected results when the line/well crossed multiple soil layers (with different permeability). So in this case it is recommended you apply the line/well in one soil layer only.