In chapter 8, explain Jack's reasoning for wanting to "get more of the biguns away from the conch."

In chapter 8 of Lord of the Flies Jack is utterly humiliated when no one votes for him to become chief. Crying, he slinks off to the jungle to lick his wounds, so to speak. By rejecting Jack as their chief, the boys have shown—in public, at least—that they're still committed to building a civilization based on rules and order.
As he broods and simmers in the jungle, Jack plans his next move. He's now literally outside the bounds of civilization; savagery is his realm. He sets about starting his own gang, his own little tribe of savages. If he's to achieve his goal of absolute power on the island, then he's going to need to get as many of the older boys—biguns—away from the conch shell as possible. The conch is the symbol of rules-based authority, and Jack figures that the further the biguns are away from the conch, the less they will be subject to Ralph's authority as chief.
Fortunately for Jack, virtually all the biguns end up joining him in the jungle. When they are subject to the authority of the conch shell and all that it represents, they are unwilling to vote for Jack to take over from Ralph as chief. But by the simple expedient of taking to the jungle, they've put themselves far beyond the reach of the conch, far beyond the rational system of law and civilization established by Ralph. With the biguns on his side, Jack is now in a prime position to take by force what he could never get by a democratic vote.