Excavations and Groundwater

Excavation Below Water Table

When an excavation is added that extends below the water table (defined as a Piezometric Line), the water table is locally dropped to the bottom of the excavation. This is to ensure that the excavation is dry and not full of water. Query points inside the excavation will see a drop in pore water pressure. Query points outside of the excavation are unaffected by the drop in the water table.

This approach tends to overestimate the settlement (it gives a conservative answer), compared with what would be observed for a true three-dimensional groundwater analysis, since this approach does not account for horizontal water flow from areas of higher pressure (outside of the excavation) to areas of lower pressure (under the excavation).

The following series of images illustrates the effect of an excavation on defined piezo lines.

Excavations with PWP Functions

When the PWP Function option is selected in the Groundwater tab of the Project Settings, it is up to the user to define the pore water pressure profile for all stages of the analysis. No water table drop is approximated when the PWP function option is used. Query points below the excavation will have the same pore water pressure as before the excavation. It is up to the user to define the drop in pore water pressure associated with the excavation, and to assign it in the appropriate stage(s). Query points outside of the excavation are also unaffected by the excavation.