A C172 Instructor Under Training was told by his CFI Instructor to clear the runway for traffic by taxiing onto wet ground where a wheel sank and the propeller was damaged after it contacted the ground.

Narrative

As a CFI candidate; I was in the right seat of the airplane and the CFI instructor was in the left seat. I was operating the controls and the instructor was observing and providing verbal instructions. After proper run-up at the sole and narrow taxiway of this small and private airport we hear that another airplane was on base leg for landing. After it landed; my instructor tells me to taxi across the runway to give way to the landed aircraft to exit. After such; my intention was to taxi to position for takeoff but another aircraft calls on traffic and my instructor tells me to return to the taxiway and make a 180 degree turn to position us again over the taxiway. For such maneuver she says to use a portion of the adjacent grass area to the narrow taxiway. She may have imagined that I would use the area to her left but; as I looked forward I saw one runway light ahead on her left side and elected to use the area on my right; as my position would permit a better; although reduced visibility. As I was finishing the portion of the maneuver over the grass; unknown to me; the right wheel sunk into the wet and soft grounds and the airplane stops. I applied more power; causing the wheel to sink even deeper; still unknown and unaware of such fact and then the propeller touched the soft ground under power; without another engine indication or vibration. As the airplane was not moving forward under the power; I stopped the engine manually. Moved the airplane out of the grass area and we observed the propeller bend. The aircraft was towed to the hangar. Lesson learned: never leave a paved surface on unknown terrain; even if such demands longer taxi times.