The main contractors for Georgia Power’s Plant Vogtle expansion project are suing the utility and the project’s co-owners over additional expenses stemming from backfilling two excavation sites.

Westinghouse and a Shaw Group subsidiary are suing Georgia Power and the group of municipal and cooperative utilities for an amount originally set at $58 million. Georgia Power and the other owners have paid $29.3 million of that, the lawsuit says.

“When compared to the project whole, the disputed amount is not large,” Georgia Power said in a statement. “We will continue to negotiate with Shaw on this very issue.”

The contractors have accused the utilities of giving inaccurate information about how much soil was available to backfill the excavation sites for two new nuclear reactors.

Backfill is material excavated from a site and reused for filling. The material must meet standards set by federal nuclear safety regulators.

According to the lawsuit, “there was significantly less suitable material than required and expressly represented by the owners.”

As a result, the contractors had to “excavate and stockpile a significant amount of additional material and haul it further than anticipated, which significantly increased the contractor’s costs and expense of the work,” the document said.

Shaw’s subsidiary, Stone & Webster, has paid the subcontractors for the additional work, the lawsuit says. The company then billed the utilities for that amount.

A contract between Shaw, Westinghouse and the utilities maps out how disputes are handled for the $14 billion project. According to the lawsuit, the contract states if there is a billing dispute, the owners are required to pay half of that disputed amount. The owners also have to tell the contractors within 15 days whether an invoice is being disputed.

The breach-of-contract suit was filed in U.S. District Court last week.

Separately, the utilities are disputing an estimated $800 million claim, made by Shaw and Westinghouse. The vendors say Georgia Power is responsible for $400 million of that commercial claim. The utility continues to negotiate with Shaw.