Zombies see them as a block and will run around them to seek the cheapest pathway. Just like if you put a short wall up that they could easily run around they would do that instead of pound through it, they will run past spikes once there is a pathway through. By digging a trench they will see the spikes as just ground to run across and not avoid them. Even better, rather than run down and then back up through the trench once some spikes are destroyed, they will seek to run straight across so they will go to other places with spikes and choose to cross your trench there and die rather than choosing the open pathways. See? They still dumb.