Hi everybody,
Next questions are about modelling of drains and stone columns.
PVD Drains.
1. Improvement with PVD can be modelled as a material with a equivalent vertical permeability according to Chai et. al equation
or following the recommendations of Predicted and observed behaviour of soft clay foundations stabilised with vertical drains (Indraratna):
Questions are: is it necessary to draw the drains in the model when vertical equivalent permeability is used? what modelling is more appropiate to use?
Stone columns
1. In plain strain model we can simulate stone columns as a material which is drawing like walls?
Question is: how can I simulate vibro-sustitution/compaction in PLAXIS 2D? is it enough with give the stone colum material a high phi value? or should the stiffness and resistance parameters of the soils surrounding the stone columns modify (improved somehow)?
Thanks!
Ramón S.
Hi Ramón,
With respect to your question on drains ....
The idea drains is that the water flows horizontally towards the drain, and then inside the drains it vertically flows out of your soil layer. To model this there are 2 possibilities:
1) You don't model the drains, but you give the soil an equivalent VERTICAL permeability - so the water will flow out vertically through the soil to simulate the flow that in reality would go through the drains. This works in both 2D and 3D.
2) You do model the drains, but for instance at a larger distance then in reality (to avoid meshing problems, for instance) or in 2D where drains are surfaces while in reality they are line elements. In this case you need to give the soil an equivalent HORIZONTAL permeability to make sure that the water flows through the soil towards the drains the same way in your model as in reality.
About the stone columns:
It is possible to model stone columns in 2D like walls, but you have to make sure that the wall has the same (axial) stiffness as the columns per meter. This can be done by either reducing the E modulus of the stone column material, or by reducing the thickness of the "stone wall" in 2D. Both methods have their advantages and disadvantages.
Be aware that if the stone columns are also used for drainage, you may need to also adjust the permeability of the stone column material as well as the horizontal permeability of the surrounding soil (see my remark about drains).
Usually stone columns are used in clay and clay cannot really be compacted. When compacting the stone columns the surrounding clay will probably develop some excess pore pressures, but without really densifying. Hence, generally the soil parameters of the surrounding clay are not modified.
With kind regards,
Dennis Waterman
Dear Dennis,
Thanks for your replay. Just last question if it is possible:
When you mentioned: "give the soil an equivalent horizontal permeability", you actually mean I must take into consideration smear and well resistance effects around the drain, that's correct?
Many thanks for your support
Ramón Salazar
Dear Ramón,
The horizontal equivalent permeability is not so much for smear zone and well resistance effects, but to take into account that in 2D you're modelling infinitely long drain surfaces while in reality it's a 3D pattern of line (wick) drains. This 3D->2D conversion requires a horizontal equivalent permeability as described by Indraratna.
Perfect, totally understood.
Many thanks Dennis.