605 ILCS 5/9-104
In grading highways corner stones marking sectional or other corners shall not be disturbed, except to lower such stones so that they will not rise above the surface of the highway. If a corner stone is covered to a depth greater than 12 inches or is covered with a highway surfacing material other than road oil, the location of the corner stone shall be preserved by setting a suitable monument over the stone which shall be level with the highway surface or by setting at least 3 offset monuments in locations where they will not be disturbed. When any corner stone is lowered or when a monument is set over a stone or when offset monuments are set it shall be done in the presence of and under the supervision of a Registered Illinois Land Surveyor who shall record the type and location of the reference monuments with respect to the corner stone in the office of the recorder in the county in which such stone is located.
605 ILCS 5/9-104