Problem description

When conducting a fully coupled flow-deformation analysis Plaxis will solve the full interaction between deformations, consolidation and groundwater flow simultaneously in the same phase. In this calculation, the calculation uses Staged Construction and transient groundwater flow combined.

Staged construction

In a (mechanical) staged construction calculation, any change in load or displacement will result in so called out-of-balance forces that during the Finite Element calculation will be solved in several calculation steps to reach equilibrium. In this Staged Construction loading type, the ΣMstage multiplier indicates the amount of out-of-balance that is solved. Once ΣMstage reaches one [1], the full weight and loads are fully applied in the model.

In a Staged construction phase, the time and the ΣMstage parameter are linearly coupled, this implies that the out-of-balance solving is done linearly in time.

Transient groundwater flow

In a Fully coupled flow-deformation analysis the groundwater flow can change in time: the (total) active pore pressures will then change in time. In each time step (calculation step), equilibrium of pore pressures must be reached. This takes into account the amount of flow that may develop within this step which is based on the changing or fixed hydraulic conditions and the generation of pore pressures due to undrained behaviour of the soil.

Coupling staged construction and flow

When coupling the staged construction calculation approach with this transient groundwater flow, there is a discrepancy in the way the equilibrium is reached: on one hand the full out-of-balance for forces and stresses is solved evenly over the entire phase time interval (mechanical equilibrium) and on the other hand the equilibrium for the flow field is considered per calculation step. However, the influence of the change in pore pressures and (external) waterloads on the deformation is currently solved in the same manner as the mechanical out-of-balance forces.

Since (undrained) soil behaviour is highly nonlinear, the solution for rapid changes in the hydraulic conditions might deviate from the real soil behaviour.

Solution

This is now solved in PLAXIS 2D 2018 and PLAXIS 3D CONNECT Edition V20

For versions prior to PLAXIS 2D 2018 and PLAXIS 3D 2018 or earlier, when using a fully coupled flow-deformation analysis you are advised to calculate the situation split into several phases with sufficiently small changes in the (hydraulic) boundary conditions, instead of one phase with larger changes. This limits this discrepancy caused by the way the out-of-balance is solved.