George Tierney was the first person to die working on a large construction project. Thirteen years later, his son son Patrick was the last person to die working on the same project. What is the name of the project and what was the shared date of their deaths?

John George Tierney and his son Patrick both died on December 20th whilst doing work related to the Hoover Dam.

George Tierney died in a flash flood on December 20, 1921 when his survey party were caught in a flash flood.

On Dec. 20, 1921, a crew surveying locations for the dam got caught in a flash flood, and a man named John Gregory Tierney was lost forever in the raging Colorado River, one of the first casualties of the project.

14 years later, on December 20, 1935, his son Patrick fell to his death off an intake tower and became the last man to die during the dam's construction.

Then on Dec. 20, 1935, 14 years later to the day, the job site suffered its last fatal accident, when a worker fell to his death from one of the two intake towers on the Arizona side of Black Canyon. That man was Patrick William Tierney, J.G. Tierney's only son.

https://www.reviewjournal.com/local/local-las-vegas/father-and-son-died-on-the-same-day-14-years-apart-while-working-on-hoover-dam/

(Note the George Tierney is often listed as the first man to die during the dam's construction; however he died before work actually began on the dam itself.)