Jack says that his father Harold was a member of the 23rd Engineers. He says that this was an almost all African-American unit, and that although Harold was only a private he was put in charge of the unit (I supposed because he was white - these were racist times). Here is information about the 23rd Engineers coming from The Association of the Third Armored Division Veterans http://www.3ad.org/23en/23en_wwi.htm
"The 23rd was constituted on 15 August 1917, in the National Army by the War Department General Order 108, as the 23d Engineer Regiment (Road). In spite of the problems faced by the War Department and the Corps of Engineers since the declaration of war in the previous April, it took just three weeks to gather a nucleus of men and activate the 23d Engineers on 5 September 1917 at Camp Meade, Maryland. There they began their training. Very little has been recorded of these early days of the 23d Engineers except the terse entries in Army Records.
The 23rd Engineers began deploying to France in November 1917, and served in support of various allied actions, earning for itself the title "The Road Builders of the AEF." Based upon the written histories of other units in the same sections, we know that the 23rd Engineers contended successfully with building and maintaining highways destroyed by shell fire, areas contaminated by chemical warfare that had to be rehabilitated in order to be used by the support troops, and as in any war, the enemy of all armies, mud. The degree of success of the unit is measured by their nickname and earning three campaign streamers: Lorraine, Saint Mihiel and the Meuse-Argonne.
From the day of surrender of the Germany Army until June 1919, the 23rd Engineers were engaged in the standard after action (post hostilities) missions of clean up and repair. In that month of June, the unit was returned to the United States."
My dad says that they dug trenches, buried bodies, and built bridges.
The picture was taken from a web site "Military Life - World War I, WWI African-American Soldiers." There are more pictures here.