Pit-in-pit excavation is becoming more and more popular

and concerns are increasing on the influence exerted by the interior pit. However

the calculating depth in design remains a puzzle

taking the exterior pit excavation depth is a risk

while the interior is conservative. This paper raises a concept of equivalent depth and two parameters called equivalent depth coefficient and equivalent impact angle based on analysis of various relative position of interior and exterior pit

width and depth of the former

whose feasibility is proved. The equivalent depth converts the pit-in-pit into normal excavation model

and the equivalent depth factor quantifies this kind of conversion.