General

When performing a groundwater flow time-dependent analysis, there might be an interest in evaluating the calculation results within a specific time interval. To achieve this, the flow control parameters of the calculation phase should be set in a way that fits the desired criteria.

This article explains which flow and numerical control parameters should be considered to obtain time-dependent groundwater flow analysis results at specific timestamps in a given time interval.
For demonstration purposes, the PLAXIS 2D Tutorial exercise Potato field moisture content will be used.

Note, that this is applicable to any other groundwater flow time-dependent analysis.

Overview of the exercise

For reference, the abovementioned tutorial exercise background and purpose are explained in this introduction:

This lesson demonstrates the applicability of PLAXIS to agricultural problems. The potato field lesson involves a loam layer on top of a sandy base. Regional conditions prescribe a water level at the position of the material interface. The water level in the ditches remains unchanged. The precipitation and evaporation may vary on a daily basis due to weather conditions. The calculation aims to predict the variation of the water content in the loam layer in time as a result of time-dependent boundary conditions.

Figure 1 – Saturation at day 15 of the Potato field exercise

By default, when adding a new phase involving a groundwater flow analysis at Staged construction, the default flow and numerical control parameters are shown in Figure 2.

Figure 2 – Default numerical and flow control parameters

If the calculation would be set to calculate with these default parameters, it would only be possible to check the calculation results of the last calculation step (day 15) in PLAXIS Output. This is because the Max number of steps stored = 1. Therefore, only the last calculation step is stored and accessible in the results inspection.

For demonstration purposes, let us assume that we would like to store and inspect the calculation results for every day of the 15-day analysis. Then, the Max number of steps stored should be set to a value ≥ 15.

But how do we know that each calculation step that is stored corresponds to a step of 1-day in the analysis?
For this, we need to make changes to some flow control parameters, such as: First time step, Min time step and Max time step.

To obtain the calculation results for every day of the groundwater flow analysis, these three flow control parameters should be set to 1 day, as shown below.

Figure 3 – Relevant numerical and flow control parameters for this example

Figure 4 – Calculation results per calculation step in PLAXIS Output

It is important to note that for this specific case, we have a discharge function in a table format where a certain value of discharge is specified per day (see Figure 5). Therefore, we know that each flow step in the calculation will follow the time step from our input signal.

Figure 5 – Discharge function in a table format

Be aware that if, for instance, the First, Min and Max time step at the flow control parameters would be set to 4 days, the calculation would skip relevant information of the input signal in the analysis.
Therefore, these parameters should be changed appropriately for each case input signal and general time interval.

For more details regarding the relationship between an input time series and PLAXIS calculation steps, please refer to the article Time step used in dynamic calculation as this is relevant for both groundwater flow and dynamic analysis.