This cut represents the construction trench for tile drain 7196, which runs from the center of Room 5 northward (and sloping downward) through most of Room 4, before ending at tufo blocks 7197. It partially truncates an earlier layer that fills much of Room 4 (brown silt with a significant density of medium-sized basalt inclusions), probably a leveling layer related to this later imperial construction phase.
Construction cut for tile drain 7196
photo taken from north
photo taken from east
photo from south
Layer of brown silt west of SU 7241 in Room H4 and surrounding features.