Men in Pioneer Corps also included skilled tradesmen.
From The Long Long Trail:
Infantry Pioneer and Labour or Works Battalions
An early solution to the vast demand for labour was to create in each infantry Division a battalion that would be trained and capable of fighting as infantry, but that would normally be engaged on labouring work. They were given the name of Pioneers. They differed from normal infantry in that they would be composed of a mixture of men who were experienced with picks and shovels (i.e. miners, road men, etc) and some who had skilled trades (smiths, carpenters, joiners, bricklayers, masons, tinsmiths, engine drivers and fitters). A Pioneer battalion would also carry a range of technical stores that infantry would not. This type of battalion came into being with an Army Order in December 1914. In early 1916, a number of infantry battalions composed of men who were medically graded unfit for the fighting were formed for labouring work. They had only 2 officers per battalion. Twelve such battalions existed in June 1916.