X0306 4 2021-05-03 0 0001372020 Great Lakes Dredge & Dock CORP GLDD 0001384117 Simonelli David E C/O GREAT LAKES DREDGE & DOCK CORP. 9811 KATY FREEWAY, SUITE 1200 HOUSTON TX 77024 0 1 0 0 Chief Operating Officer Common Stock 2021-05-03 4 S 0 8566 16.1374 D 249439 D The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on November 16, 2020. Represents the weighted average price. The shares were sold in multiple transactions ranging from $16.015 to $16.215, inclusive. Upon request by the Commission staff, the issuer or security holder of the issuer, the reporting person will provide information regarding the number of shares disposed of at each separate price. /s/Katherine M. O'Halloran, by Power of Attorney 2021-05-05