The answer to this question is not as transparent as you may think. Let´s review OSHA´s definition of trench excavation.
By OSHA´s definition, a trench excavation means;
• A narrow excavation (in relation to its length) made below the surface of the ground. In general, the depth is greater than the width, but the width of a trench (measured at the bottom) is not greater than 15 feet. If forms or other structures are installed or constructed in an excavation so as to reduce the dimension measured from the forms or structure to the side of the excavation to 15 feet or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.
After reading the definition, a trench excavation certainly seems to meet the confined space criteria.