225 ILCS 710/45.23

All abandoned shafts, temporarily out of use, or shafts used only as airways, shall be securely covered or fenced and shall be so maintained. All mill holes, glory holes, or cavernous stopes opening to the surface shall be securely fenced and shall be so maintained if such mill holes, glory holes, or cavernous stopes are within 300 feet of a highway or thoroughfare. All other abandoned excavations whereof the side slopes more than 40 degrees from the horizontal, and whereof the depth is more than 10 feet, shall be securely fenced, but such fencing need be erected only at those places where such slope is in excess of 40 degrees, and all such fencing shall be maintained in good condition.