Uhm, it doesn't take much of an impact to damage any hull. Being made of aluminum definitely doesn't help, but keep in mind, these boats weren't designed to have a go at concrete.
It is the pilot or mule operator's responsibility to ensure a safe a transit.
Three ships to go through since the new expansion was completed have sustained damage...
Here's one with a steel hull...
http://gcaptain.com/containership-damaged-in-panama-canals-new-locks/The operators of the canal are failing in their duties to keep the ships safe...