Can humans or animals walk on the liner?

Both the regular strength and extra strength liners are strong enough to handle some traffic as long as there's nothing sharp. That being said, it's always a good idea to protect the liner if you can, especially if the liner will come in contact with claws, sharp rocks, roots or digging animals.  A popular way of protecting a liner is to put down a protective layer of geotextile fabric followed by some gravel, rock, brick or other material to prevent direct contact with the liner.