The Justice Department has accused an Arlington contracting firm of fixing bids on a $1.8 million dredging construction project for the Army in a South Carolina harbor.

Department lawyers alleged, in filings this week in U.S. District Court in Columbia, S.C., that J. A. LaPorte Inc. conspired with others to "submit collusive, noncompetitive and rigged bids" in connection with a 1981 contract for the dredging of the Georgetown Harbor. A Justice Department official said that government attorneys have agreed to a plea-bargain arrangement with the defendants, although he refused to provide details.

A secretary who answered the telephone in LaPorte's Arlington headquarters said company executives had no comment.

The allegation was the second case resulting from a continuing federal investigation into alleged antitrust violations in the dredging industry in the southeast United States, government officials said. In October, Atkinson Dredging Co. of Chesapeake, Va., pleaded guilty in federal court in North Carolina to charges of rigging bids on a dredging project in Wilmington.

The investigation is being carried out by Justice's Antitrust Division, with assistance from the Pentagon's Defense Criminal Investigative Service, according to a Justice Department statement released yesterday.

Justice Department officials refused to identify other companies under investigation. They said, however, that the dredging probe is now part of a larger investigation the department's antitrust division is conducting into bid-rigging in federal procurement, in particular defense contracting.

The investigation into dredging operations involves projects authorized by the Army Corps of Engineers. The Corps, as part of its responsibility for maintaining federally authorized waterways, awards contracts for port dredging, according to Norman D. Moebs, a civil engineer with the corps in Charleston, S.C.

Moebs said three or four companies usually bid for a given project in his area, in a process the Justice Department said is designed to be competitive so that the "lowest responsible bidders" win the contracts.

Justice Department attorneys, however, alleged that LaPorte and other unnamed co-conspirators worked together to subvert this process in order to enable the company to win the contract for the harbor project.

As part of this conspiracy, the department alleges, the executives discussed the submission of prospective bids for Georgetown Harbor and decided to designate LaPorte to be the successful low bidder. The other conspirators submitted intentionally high bids and proposals "containing false, fictitious and fraudulent statements and entries," the Justice Department said.