UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO

TENDER OFFER STATEMENT UNDER SECTION 14(d)(1) OR 13(e)(1)

OF THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 3)

Great Lakes Dredge & Dock Corporation

(Name of Subject Company (Issuer))

Huron MergeCo., Inc.

(Name of Filing Persons (Offeror))

Saltchuk Resources, Inc.

(Name of Filing Persons (Parent of Offeror))

Common Stock, par value $0.0001 per share

(Title of Class of Securities)

390607109

(CUSIP Number of Class of Securities)

Jerald W. Richards

c/o Saltchuk Resources, Inc.

450 Alaskan Way South, Suite 708

Seattle, Washington 98104

(206) 652-1111

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications on Behalf of Filing Persons)

Copies to:

Philip Richter

Ryan Messier

Fried, Frank, Harris, Shriver & Jacobson LLP

One New York Plaza

New York, New York, 10004

(212) 859-8000

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer: ☒

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

This Amendment No. 3 to the Tender Offer Statement on Schedule TO (this “Amendment”) amends and supplements the Tender Offer Statement on Schedule TO filed with the U.S. Securities and Exchange Commission on March 4, 2026 (as it may be further amended or supplemented from time to time, the “Schedule TO”), with respect to the cash tender offer (the “Offer”) made by Huron MergeCo., Inc., a Delaware corporation (“Purchaser”) and a wholly owned subsidiary of Saltchuk Resources, Inc., a Washington corporation (“Parent”), to purchase all of the issued and outstanding shares of Common Stock, par value $0.0001 per share (the “Shares”), of Great Lakes Dredge & Dock Corporation (“GLDD”), pursuant to the Agreement and Plan of Merger, dated as of February 10, 2026, by and among Parent, Purchaser, and GLDD (as it may be amended or supplemented from time to time, the “Merger Agreement”), for $17.00 per Share, net to the seller thereof in cash, without interest, subject to any required tax withholdings (such consideration as it may be amended from time to time pursuant to the terms of the Merger Agreement), upon the terms and subject to the conditions set forth in the Offer to Purchase, dated March 4, 2026 (together with any amendments or supplements thereto, the “Offer to Purchase”), and in the related Letter of Transmittal (together with any amendments or supplements thereto, the “Letter of Transmittal”), copies of which are attached to the Schedule TO as Exhibits (a)(1)(A) and (a)(1)(B), respectively, as each may be amended or supplemented from time to time.

Except as otherwise set forth in this Amendment, the information set forth in the Schedule TO remains unchanged. This Amendment is being filed to reflect certain updates as reflected below. Capitalized terms used but not otherwise defined herein have the meanings ascribed thereto in the Offer to Purchase or the Schedule TO, as applicable. You should read this Amendment together with the Schedule TO and the Offer to Purchase.

ITEMS 1 THROUGH 11.

The information set forth in the Offer to Purchase and Items 1 through 11 of the Schedule TO, to the extent such Items incorporate by reference the information contained in the Offer to Purchase, are hereby amended and supplemented by adding the following three paragraphs:

“The Offer and withdrawal rights expired one minute past 11:59 p.m. New York City time on March 31, 2026 (the “Expiration Time”). Based on the final information provided by the Depositary and Paying Agent as of the Expiration Time, 53,738,558 Shares were validly tendered in the Offer and not validly withdrawn, representing approximately 79.88% of all outstanding Shares. As a result, the Minimum Tender Condition has been satisfied. As the Minimum Tender Condition and each of the other Offer Conditions have been satisfied, Purchaser has accepted for payment all Shares that were validly tendered and not validly withdrawn pursuant to the Offer.

Pursuant to a Certificate of Merger filed with the Secretary of State of the State of Delaware on April 1, 2026, Purchaser was merged with and into GLDD through a merger under Section 251(h) of the DGCL, with GLDD surviving the Merger and continuing as a wholly-owned subsidiary of Parent. In the Merger, each Share issued and outstanding immediately prior to the Effective Time other than Shares (i) held by GLDD in treasury or owned of record by GLDD or any subsidiary of GLDD and Shares owned of record by Parent, Purchaser (including Shares irrevocably accepted for payment by Purchaser in the Offer) or any of their respective wholly-owned subsidiaries (in each case, other than those held on behalf of any third party), which shall be canceled and cease to exist, with no payment being made with respect thereto and (ii) held by any GLDD Stockholders who have properly demanded appraisal rights of such Shares in all respects under, and who comply with, Section 262 of the DGCL and have not validly revoked such demand, was converted into the right to receive the Merger Consideration, without interest, and subject to any required tax withholdings. All Shares converted into the right to receive the Merger Consideration ceased to be outstanding and were automatically cancelled and ceased to exist. Following the Merger, the Shares will be delisted and cease to be traded on Nasdaq.

On April 1, 2026, GLDD and Parent issued a joint press release announcing the successful completion of the Offer and the Merger. The full text of the press release is attached hereto as Exhibit (a)(5)(iv) and is incorporated herein by reference.”

ITEM 12. EXHIBITS.

Item 12 of the Schedule TO is hereby amended and supplemented by adding the following Exhibit:

SIGNATURE

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: April 1, 2026

Exhibit (a)(5)(iv)

Saltchuk Welcomes Great Lakes Dredge & Dock to its Family of Companies

Seattle, WA, Houston, TX- April 1, 2026 – Saltchuk Resources, Inc. (“Saltchuk”) today welcomed Great Lakes Dredge & Dock Corporation (“Great Lakes”) as its newest wholly owned subsidiary. The transaction closed this morning for a purchase price of $17.00 per share in cash, and an enterprise value of approximately $1.5 billion.

“We’re proud to welcome Great Lakes to the Saltchuk family of companies,” said Saltchuk Chairman Mark Tabbutt. “With Great Lakes, we have grown to nearly 10,000 team members united by a shared commitment: delivering safe, responsible, and reliable service to the communities we serve. We look forward to supporting Great Lakes’ reinvestment and growth ambitions for generations to come.”

Great Lakes joins Saltchuk as a stand-alone business unit, and it will continue to operate independently under its experienced leadership. The acquisition diversifies Saltchuk’s portfolio, adding dredging services to complement more than 30 other U.S. freight transportation, marine services, and energy distribution companies.

“Joining Saltchuk’s family of companies is a proud moment for us, as it is an organization that shares our deeply rooted culture and unwavering commitment to safety, to the communities we serve, our valued customers, and our dedicated employees. This partnership represents a natural alignment of values and vision, providing a strong foundation for continued collaboration and success,” said Lasse Petterson, Great Lakes’s President and Chief Executive Officer. “As we look ahead, we remain focused on executing our long-term growth strategy with discipline and purpose. At the same time, we are committed to maintaining and enhancing our leadership position in U.S. dredging, the global offshore energy sector, and continuing to deliver excellence and innovation across all aspects of our business.”

The transaction was previously announced on February 11, 2026, and the tender offer for all of the outstanding shares of common stock of Great Lakes for $17.00 per share, net to the seller in cash, without interest and subject to any required tax withholdings, by Huron MergeCo., Inc., a wholly owned subsidiary of Saltchuk, expired at one minute after 11:59 p.m., New York City Time, on March 31, 2026.

Broadridge Corporate Issuer Solutions, LLC, acting as joint depositary and paying agent for the tender offer, have advised that, as of the expiration of the tender offer, approximately 53,738,558 shares of Great Lakes common stock were validly tendered and not validly withdrawn pursuant to the tender offer, representing approximately 79.88% of the issued and outstanding shares of Great Lakes common stock.

As a result of the completion of the transaction, prior to the opening of trading on the NASDAQ on April 1, 2026, all shares of Great Lakes common stock will cease trading, and all shares of Great Lakes common stock will subsequently be delisted from NASDAQ and deregistered under the Securities Exchange Act of 1934, as amended.

Additional details regarding the previously announced debt tender offer will be provided in a subsequent press release. Great Lakes will share further information regarding the status of the debt tender offer at that time.

About Saltchuk Resources, Inc.

Saltchuk is a privately owned enterprise that has built a reputation over 40 years of being a multi-generational home for great companies. Headquartered in Seattle, additional information is available at www.saltchuk.com.

About Great Lakes Dredge & Dock

Great Lakes Dredge & Dock Corporation is the largest provider of dredging services in the United States, which is complemented with a long history of performing significant international projects. In addition, Great Lakes is fully engaged in expanding its core business into the offshore energy industry. Great Lakes employs experienced civil, ocean and mechanical engineering staff in its estimating, production, and project management functions. In its over 136-year history, Great Lakes has never failed to complete a marine project. Great Lakes owns and operates the largest and most diverse fleet in the U.S. dredging industry, comprised of approximately 200 specialized vessels. Great Lakes has a disciplined training program for engineers that ensures experience-based

performance as they advance through Great Lakes operations. Great Lakes’s Incident-and Injury-Free^® (IIF^®) safety management program is integrated into all aspects of the Great Lakes’s culture. Great Lakes’s commitment to the IIF^® culture promotes a work environment where employee safety is paramount.

Contact

Eric Birge

Vice President of Investor Relations,

313-220-3053

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