Find here below a workflow to get excavation quantities for the foundations of a building. 

As input, we have a terrain (LIDAR point cloud) and got the ifc of a building.

First steps requires extract or multi-extract of the bottom faces of the pillars, footings and slab that will define the contour of the excavation.

Then, thanks to Component Based Design, we apply in one operation an excavation template to all those surfaces.
This template is driven by parameters defining the downward and side offset for each excavation, and the slope. A platform earthwork is computed in the template on the black bottom contour.

If necessary different parameter values can be used for footings, pillars and slab in the CBD panel.

When all those excavation volumes have been generated, we can compute the total excavation of the terrain using a Civil Set and multiselect as input all the volumes.