AS THE TWO hikers approached, their mouths dropped and their shoulders perked under heavy pack frames. Here on a lonely side trail off the Appalachian Trail they'd come upon a traffic jam of a dozen-and-a-half grunting, sweating construction workers.

It was, one of the workers quickly explained, a Potamac Appalachian Trail Club work committee, charged this winter's day with restoring washed-out sections of the Corbin Cabin Trail and shoring up a main timber of the bridge that leads to the cabin.

Surprisingly, while pick-and-shovel wielding backpackers seem the logical explanation, few who use the Shenandoah National Park's system of trails are aware of how the paths come into existence and how they are maintained.

The National Park Service is the caretaker for wilderness trails, but with but two men assigned to the entire 2,000-mile Appalachian Trial, the NPS relies heavily on regional organizations such as the PATC.

Trail systems are not unlike over interstate highway system. The same natural forces that pot-hole street, erode highly shoulders and the freeze bone-jarring welts across macadam reap similar treachery on our wilderness paths. And man isn't guiltless either.

Two hikers startled momentarily by the returbished efforts will doubtless during their hike violate the paths that they depend. By discarding trash or cutting across swithbacks (sections of trail that reduce steep grade to more manageable inclines, they unwittingly contribute to undermining trails.

And no legions of state-paid workers and equipment stand ready to patch the holes and mend the way. In fact, up until a year ago there was no program of trail rehabilitation in the area at all. That was before Tom Floyd becamep PATC supervisor of trails.

"Traditionally, the club has been involved in building new trails," Floyd Said. "Only recently have we moved to achieve and an active program designed to rehabilitate existing trails."

Just after he took over, the need for such a program was brought home in an unpleasant fashion while he performed one of his new duties.

"On an inspection trip last winter, while packing down into Nicholson Hollow along the Corban Carbin Trail, such erosion and occured from water washing down the path that my entire hike was spent stumbling from one ditch to the next, avoiding endless yards of rocks washed out by seasons of rain.

From that day forward, tapping the kind of determination that puts him among only three people to have back-packed the Grand Canyon's entire length, Floyd has worked on a system to ensure well maintained trails.

Floyd describes his program as a "package arrangement" of new trail development and trail rehabilitation. The program starts with a master trail plan, then a plan to implement it through construction or reconstruction, blazing, mapping, maintenance, a trail patrol, and inclusion of new and rebuilt trails and guide books.

With over 800 miles of trails under PATC jurisdiction, including a 230-mile stretch of the Appalachian Trail, it's no small chore maintaining on effective watch over the system.

There are six assistant trail supervisors, each in charge of 30 trail overseers. Each overseer, in turn, is responsible for a three-mile section of trails. Floyd feels reasonably certain that when a problem occurs it will be brought to his attention swiftly.

So it was that Jack Elis, overseer for the Cabin Trail, and Floyd decided to open the rehabilitation program with work on the 1.48-miles path that connects the At with Corbin and Nicholson Hollow. Floyd was aware of the erosion problem on the upper trail and a recent inspection trip by Ellis revealed that a main timber in the footbridge across the Hughes River, which bisects the hollow, was rotten. The bridge angled precariously toward the water.

The PATC monthly bulletin carried the call for help and 17 members responded in a predawn rendervous in the parking lot of the Maple Avenue Shopping Center in Vienna, Va. Two four-wheel drive vehicles and an indefatigable ancient Volvo roared mountainward on the promise of clear skies and mild temperatures.

An hour and a half later, Floyd stood abreast a waterbar installed on the first rehabilitation trip a month before. He detailed work parties to their tasks. It wasn't long before an eight of a mile of the trail was under seige and Floyd was busy explaining what was going on to the wide-eyed hikers.

"We install two types of waterbars, one rock-lined," he explained, gesturing to the trench he straddled, "and the other long-lined."

"By cutting diagonally across the path, we are able to divert the water away from the trail and downhill, preventing the trails surface from being washed away."

Ideally, waterbar trenches are established every 75 feet. Once two waterbars are completed, the intervening trails is resurfaced, first with a layer of twigs and branches, then with leaves and earth removed from the waterbar excavation. That provides a cushioning path, eliminating ankle wrenching rock fields bared by the rain.

After lunch, weighed down by block and tackle and mealtime indescretions, Ellis and his crew descended the trails to shore up the damaged footbridge. As the last timber was shouldered into position the weary trail workers begin trickling down into the hollow once known as "freestate," for the lawlessness that abounded when "revenooers" and mountain distillers clashed. The area was later named Nicholson Hollow after a prominent family there when the Park Service acquired the land.

Saturday night was spent in Corbin Cabin, a stones throw from the rushing river. After a rib-stricking dinner of "mountain gruel" (chili with cheese and generous quantities of available spirits), half the group dozed before a warming blaze while the others admired rising Orion from the porch of the cabin.

Sunday morning dawned clear and crisp, 24 degree outsides. Sausages and steming scrambled eggs laced with wild onion from the river's edge were prepared. The fireplace roared.

At the long communal table, Floyd and few of the group outlined the day's work. Aired sleeping bags were jammed into stuff sacks and pack frames were readied for the steep climb out of the hollow. Before day's end and back to the Vienna parking lot there would be more waterbars, yards of resurfaced trail and fresh blue paint blazes on trees along the path's entire length, products of the PATC enthusiasm and concern for the wilderness trail.