Sections of Interstate 95 in Prince William and Stafford counties are blowing up and other parts are sinking into the ground.

The experts advance many reasons: the pounding the road takes from an ever increasing number of trucks, inadequate road design, inferior construction materials, ordinary wear and tear after almost 13 years of use, or a combination of all these factors.

But no one is certain why the extraordinarily busy road linking Washington with Richmond and points south has deteriorated so alarmingly. Homer L. Chryssikos, the engineer in charge of the repairs said, "We haven't been able to put our finger on the source of the problem."

But whatever the causes, repairs must go on. All through the night, in the illumination of giant floodlamps, construction crews slice away slabs of concrete with diamond saws, put in an improved base, and lay new, quick-setting concrete mixes.

The repairs, which began last spring and will continue into the fall, have detoured thousands of nighttime travelers into the preturnpike age. From 7 at night to 7 in the morning, traffic in one direction is diverted to Rte. 1 in Stafford. When the detour began, local residents stood on the curb in the county seat to marvel at the caravans of trucks that came through town. The detour is one of the biggest events in Stafford since I-95 was built.

In those days, the traffic volume in the Stafford area was about 3,000 to 4,000 vehicles a day. Today, according to Chryssikos, the daily volume can go up to 37,000 vehicles. On an average day, he said, there are 5,600 four-axle trucks, each weighing up to 80,000 pounds, and another 4,000 smaller trucks.

"The road has received quite a pounding, far heavier than was expected when it was built," said F. L. Burroughs, chief construction engineer in the Virginia Highway Department.

While highway officials say that no one forsaw today's traffic volume, especially so many and so heavy trucks, the American Trucking Association disagrees. "The interstate system was designed to be adequate for the loads it is carrying at this time," said Richard A. Lill, highway engineer at the association. "I feel pretty confident there is no reason to single out trucks as a problem."

According to trucking association economist William Mertz, the federal act that got the interstate system started set a maximum truck weight of 73,280 pounds. In 1975, the maximum weight was raised to 80,000 pounds. "That's not a substantial increase," Mertz said.

Lill said the Baltimore-Washington Parkway developed many of the same problems of I-95, even though no truck traffic has ever been permitted on it.

One of the chief problems is what highway engineers call "blowups," when concrete expanding under intense heat (110 to 115-degree surface temperatures) buckles at the seams. Blowups, said Chryssikos, can send the ends of concrete slabs shooting into the air 6, 8 or 10 inches.

At first, Chryssikos said, the highway department tried using 4-inch-wide rubber joints in the hope they would absorb the expansion of the concrete. But still, he said, there were blowups. So now the broken areas are replaced with bituminous concrete, which is a mixture of stone and liquid asphalt.

Another solution, Chryssikos said, would be to lay a continous strip of concrete, whose seasonal expansion theoretically results in small cracks dispersed along the length of the road instead of the damaging blowups that occur at the seams of 30-to 50-foot strips of old, conventional highways. This, however, would involve the closing of 22 miles of the road for some time.

Besides blowing up, I-95 has settled along some strips. The problem here may be improper drainage and what Chryssikos says is "just a matter of the concrete pavement giving way to the tremendous volume and weight of traffic."

There have been reports that the gravel used as a supporting bed beneath the concrete was so inferior that water couldn't drain away. Whatever the cause of the mushy subsurface, 9 inch-thick strips of concrete settled into the gravel under the weight of the traffic.

"I guess (the gravel) might have been a problem," one former highway department official said. "Local materials were used. Better materials could have been shipped in at great expense."

But the former official said that the gravel, obtained from a quarry in the Fredericksburg area, met state load-bearing standards at the time.

A present highway department official said the gravel passed specifications, but it "should have been laid to the ditch in the side of the road." It wasn't, he said, and "moisture got locked in underneath the concrete."

The cost of the repairs in Stafford will amount to about $2 million. The repairs this year in Prince William, which are limited to 12 miles of the southbound lanes won't be made until next year.