what role does the conch play? How does it represent a civilizing force?

The conch held great significance. It was a symbol of civilization; it meant democracy, order and decency. This was all the more vital to Piggy who continually cradles it amidst the destruction and chaos on the island. Boy's like Piggy need some semblance of order to exist. As the conch's symbolic meaning for "democracy" erodes, so does any chance of survival or redemption for the boys. It is apt that Golding destroys the conch at the same time as Piggy. The item that recalled civilization for the boys is destroyed along with the smart and gentle Piggy.