I like how people are saying "Build up the defenses" as if that is a safer plan.
The quarry of walkers is attracting more walkers, the sound of building reinforcements would have drawn even more walkers. Just that crappy little fence they built was enough to draw walkers to them (and the Alexandrians were totally unable to deal with them)
Getting the Alexandrian workers so close to the quarry would have been a disaster. One slip up and they'd get everyone killed/release the horde.
And that's not to mention the fact that the cliff gave way anyway, which would have happened during the building process if they had gone that route.
Plus it is just inherently dangerous to have that many walkers so near to the safe zone, they could have been freed a myriad of ways, the truck falling (which it did), travellers coming across it, extreme weather (Zombnado anyone?)
Not to mention that they could have been weaponized to attack Alexandria