Excessive use of detergent based products i.e. dishwasher tablets, washing powders etc. will lead to eventual failure of the soakaway.  This is caused by a process called sodium binding where the sodium present in detergents cause any silt or clay particles in the sub soil to bind together to form am impervious layer.  The same effect occurs if a water softener is present as the softened water will have relatively high sodium content.