DRAIN FIELD FOR WASHING MACHINE

IVE RECENTLY DUG UP PORTIONS OF THE DRAIN FIELD FOR MY WASHING MACHINE, SINCE IT WAS NOT DRAINING AND BACKING UP. THE SMALL SEPTIC TANK WAS FINE, BUT THE PIPE IS CLOGGED WITH GUNK.(ITS PROBABLY ABOUT 20 YEARS OLD, WHICH IS THE AGE OF MY HOUSE). IM ASSUMING MY ONLY OPTION IS TO DIG A NEW DRAIN FIELD. MY QUESTION HAS TO DO WITH THE SPECIFICS. HOW WIDE SHOOLD I DIG THE TRENCH ? HOW LONG ? (ITS ABOUT 50 NOW) HOW MUCH GRAVEL SHOULD I USE ? WHAT TYPE OF PIPE SHOULD I USE ? SHOULD I USE A SLEEVE OVER THE PIPE ? WHAT DOES THE SLOPE NEED TO BE ? THANKS FOR YOUR HELP