Charges are pending against an Ottumwa man after a Tuesday (7/23) collision between a semi tractor/trailer and a farm tractor.  The Mahaska County Sheriff’s Office says the accident happened around 8:25 am at the 3000 block of Highway 23 in rural Mahaska County.  A farm tractor driven by 58-year-old Bruce Caves of Fremont and a semi driven by 54-year-old Douglas Young of Ottumwa were both going west on Highway 23.  Young told investigators he had fallen asleep at the wheel and rear-ended the farm tractor.  Caves was taken to Mahaska Health Partnership with back injuries.  Young was not injured.  The investigation of this accident is continuing.