The Pioneer Corps were groups of soldiers that were detailed to perform engineer tasks such as field fortifications, obstacles, improvement of fording sites, improvement of roads, and destruction and construction of infrastructure. Cumming's Brigade, Stevenson's Division, Army of Tennessee had about 30 soldiers detailed for this task in the spring of 1864.

An account of what these guys did can be found in "This War So Horrible: The Civil War Diary of Hiram Smith Williams" edited by Lewis N. Wynne and Robert A. Taylor, published by The University of Alabama Press, 1993 who was originally assigned to the 40th Alabama but served with the "corps." The Pioneer Corps was called a "corps" in name only and was not of typical "corps" size (i.e. 30,000 men). Cumming's Brigade had 26 men detailed in December 1863, 15 men detailed for "Division Pioneers" in March 1864. By March 1865, Stevenson's Division "Pioneer party" was only 23 men and the were sent back to their companies after the reorganization.

I hope this helps.

Respectfully,

Gerald D. Hodge, Jr.
War Between the States Historian
Historian: 39th Georgia Volunteer Infantry Regiment
http://39thgavolinfrgt.homestead.com/39thHomepage.html