one good form of personal assurance and insurance, is to perform the necessary math specific to the crane and determine the maximum force that would be exerted on one outrigger, and once that load is determined, you can determine the necessary pad area to be within standardized load ratings for typical subterranean systems such as tanks, detention/retention systems as well as duct banks and utility tunnels. The general go to standard in the USA is aashto H20 load rating which is what your typical manhole cover and frame/catch basin and grate are required to withstand. the H20 load rating is with respect to a 32,000lb axle load, or 16,000lb load per tire/set of duals. The latest standardized testing/certification method for these structures is loading a 9x9 pad in the center of the structure to 40k lbs deriving a 2.5x safety factor. that 16,000lb load over 9x9 surface area (81 in-sq) would give you a ground pressure of 197.53PSI. Work your loads on your crane backwards to determine what pad area you will need to fall at that or below (for an even greater safety factor). from there you have a leg to stand on when it comes to recognizing the pertinent loads expressed by an outrigger pad onto the ground and anything beyond that would be from defective, non disclosed, or improperly installed structures.
On another note, for the construction company I am an engineer for, I have handled the construction of many crane pads, where it typically involves the excavation of any organic and fill materials down to virgin structural bearing soils and then filled with structural fill in uniform (typically 1' or less) lifts compacted and depending on the spec written for the project compaction tests are performed on each lift to assure everything is done properly.
-Steve
BTW-feels good to be back on the buzz, been away for too long guys!
