The Virginia Department of Labor and Industry, critizied for not aggressively enforcing worker safety standards, yesterday pledged a crackdown on construction site violations involving trenches.

Charles G. Wicker, director of the department's construction safety division, said all 15 of the division's inspectors would concentrate on finding, citing and correcting trenching and excavation violations, which he said had led to an outbreak of fatal accidents over the past year.

Wicker said five construction workers, three of them from Northern Virginia, had been killed during cave-ins during the last 13 months. The latest death occurred last week in Ettrick, just south of Richmond.

"We're going to use every means at our disposal to find these problems, and when we find violations, we're going to take strong action," said Wicker.

The division has been under fire since a fatal trench collapse in Annandale in July 1978 in which two Fairfax County men were killed. The county's board of supervisors has complained that the division, which has three inspectors for the Northern Virginia area, is undermanned and not aggressive in finding and pursuing violations.

The supervisors voted to empower county building inspectors to shut down unsafe construction sites, a move opposed by the department, which says the county lacks legal authority to enforce safety laws.