Negative Backfill

Negative back fills result when the subsurface layers associated with a Over cut area rise above the proposed surface elevation.

One cause of this error is calculating with too low a resolution. Try calculating with a higher resolution.

Another cause is a modeling error regarding topsoil re spread. For example consider a 6" slab with a surface 3" above grade.

The subsurface layers, if extended to the Over cut, will only be 3" below grade. Therefore, a 6" topsoil re spread covering the Over cut area will cause a negative backfill. In this case, the topsoil re spread can not include the Over cut. Copy the Over cut area as a topsoil re spread exclusion to eliminate the message.

Another cause is a modeling error where the proposed surface falls away too steeply near the area, "exposing" the subsurface layers.

In this case, contour lines and point elevations may have to be revised to keep the subsurface layers "buried."