Two soil experts testified yesterday that dirt in a trench in which two young construction workers were killed two years ago near Annadale was the type that required certain safety precautions under federal regulations.

A Fairfax County soil scientist and a private engineer and samples taken from the trench and the surrounding area showed the soil to be the "sandy silty" type. Occupational Safety and Healthy Administration (OSHA) regulations require that trenches dug in such soil be either braced or sloped to prevent cave-ins.

Two top officials of the S.O. Jennings construction Corp. of Fairfax are being tried in U.S. District Court in Alexandria on there counts of violating those regulations. S.O. Jennings and his son, Bruce Jennings, are charged with failing to brace or shore the sides of the trench and with slopping it improperly.

Robert Baker and Michael DeGroot, both 25, were buried alive when the trench in which they were laying sewer pipe collapsed July 18, 1978.

The defendants have said they are contesting the charges partly on grounds that federal safety regulations are too vague. The regulations stipulate, however, that in sandy soil trenches must, in the absence of bracing or other protective devices, be sufficiently sloped away from the bottom so the walls do not collapse.

They specifically require that sandy soil trenches more than 5 feet deep be sloped so that the width at the top measures at least twice the trench's depth. s

The defendants have acknowledged they made no attempt to brace the sides of the trench that collapsed and killed Baker and DeGroot. The Jenningses maintain the 19-foot-deep ditch was sloped enough to be safe.

Various slopes for the trench have been described in the three-day trial. An OSHA investigator testified Monday that on the day of the accident he measured the distance between the two sides of the trench at the top at 18 feet across.

On Tuesday, the forman on the job disputed that, saying he estimated it to be at least 30 feet wide. Larry Cossaro, the foreman, said the trench was "safe" and that it was dug in "good soil."

Yesterday, however, a Fairfax County homicide detective testified that Cosaro had told him the trench was wide as the backhole they were using to dig it, plus three feet on each side.

The detective said measurements later showed that added up to 18 feet.

Federal prosecutors are seeking the convictions of the Jenningses and their company as part of what they say is an initial effort to crack down on on alleged safety violations at Northern Virginia construction sites.

They are attempting to show that the Jenningses "willfully and unlawfully" neglected to insure the safety of their employes.

In a move prosecutors say they had not anticipated, defense lawyers have disputed the location of the accident site, which could jepardize the government's soil analyses.

Foreman Cosaro placed the accident's location nearly 100 feet from the spot where OSHA investigator Edward Allen said it occurred. Cosaro also has testified that he regularly falsified his progress reports, which might otherwise indicate how much sewer pipe had been laid up to the day of the cave-in.

If convicted, S.O. Jennings and Bruce Jennings could receive maximum sentences of six months in prison and $10,000 fines on each of the three counts against them. Their company would face more than $24,000 in additional OSHA fines.