Check to see if the slope of the excavation matches that of the OSHA requirements for soil classification. A stiff clay should either be a class B or most likely a class C. For a class C excavation, I believe the steepest non-shored slope can be a 1:1. If your cut doesnt fit with that cut slope, you will need temp shoring.
For example, you only have a 10' wide work area to lay pipe. Assuming a 2' wide pipe bed, you would only be able to dig a 4' deep trench.
Being from Colorado, most soil types here are a class C. Class A soils are essentially bedrock and can be vertically cut, class B are weathered/weakened bedrock, and class C is everything else.
