With the threat of a strong El Niño now very real for much of the United States, not just Southern California, crane operators have to be alert for changing ground conditions. No matter the composition of the soil you're working on, frequent heavy rains and possible street flooding can cause the crane to become unsteady. The large stature of the crane and the heavy weight of the items it's meant to lift are a recipe for disaster even if other safety protocols are followed correctly.
It's already considered unsafe to operate a crane in heavy rain or muddy conditions because the electrical equipment and the stability of the crane would be compromised. But the problems with soil can remain long after the rain has stopped. Even if the ground appears to have dried, if the rain was very heavy or resulted in minor, localized flooding, enough of the ground could have been washed away to make the crane ever so slightly off-balance. Were a crane operator to try to lift something heavy with the crane, that slight off-balance issue could cause the crane to tip, and the weight of the heavy item could pull the crane down. It sounds like a very small problem, but given the size of a heavy lift crane, plus the weight of its cargo, you can't be too careful.
Not Letting Go
Another issue is whether the crane has sunk into the mud. A huge crane isn't going to sink down and disappear, but whatever the crane is sitting on -- wheels, tank treads or caterpillar tracks, or posts -- can sink a short bit into the mud, which would then dry around the treads or other parts touching the ground. If the crane has to move a bit to get into a better position, that again could throw the crane off-balance.
Be aware that using a platform doesn't necessarily make the situation any better. If the crane is on a hard surface, the soil underneath can still be affected by any water that seeps under the surface. That can throw the platform off-balance or cause cracks. The weight of the crane and anything it lifts would again pose a tip-over problem.
Moving out of Harm's Way
After heavy rains, local flooding, or other conditions leading to mud and potential erosion, check the ground under the crane and move the crane with the help of a rigging crew if necessary. Don't operate the crane unless the engineers in your crew are sure that the ground under the crane remains level enough and stable enough for operation. If you want more information on crane safety and moving the crane to another area, check with rigging companies, like Lockwood Brothers Inc, to see how quickly they'd be able to come out and move the equipment.