A southbound stretch of Interstate 81 in Montgomery County is closed because of an unstable slope. The Virginia Department of Transportation says the slope is in the truck climbing lane work zone between mile markers 120 and 125. Emergency repairs are being made. Traffic is being detoured at the Dixie Caverns exit onto U.S. 11/460 and then back onto the interstate at exit 118 in Christiansburg. VDOT said Wednesday that it doesn’t have an estimated time for reopening the soutbound lanes.