How to Model Ventilation Shaft Excavation After Regional Tunnel Drawdown in MODFLOW?

[김영인]

Hi, 
I am currently conducting a groundwater safety assessment using MODFLOW-2000/2005 with a regional groundwater flow model.
In 2018, a tunnel excavation was carried out, which caused continuous groundwater drawdown. Recently, the main tunnel excavation has been completed, but a ventilation shaft collapsed during construction, and now I need to update the groundwater safety evaluation.

Here is my main question:

At present, the groundwater table has already been lowered due to the main tunnel excavation (transient conditions). Now I only have the observed groundwater levels (after drawdown) and geotechnical parameters from current borehole investigations.

Should I (A) set up a new steady-state model using the current groundwater heads and hydraulic parameters, and then perform a transient simulation for the ventilation shaft excavation?
Or (B) should I reproduce the entire transient process starting from 2018 (main tunnel excavation) up to the present, and then add the ventilation shaft excavation scenario on top of that transient simulation?

In the transient state, the observed water level in 2018 was about 20 m lower than the predicted value. In this case, should calibration be carried out through transient adjustment? If calibration is required, please also provide the method.

Any advice on the most appropriate modeling workflow in MODFLOW-2005 for this situation would be greatly appreciated.

Chris,