Piggy tells Ralph to blow the conch long and loud as they arrive at Castle Rock. It is an ineffective action because both boys know that non of the painted savages will follow Ralph anymore whether he has the conch or not. The use of the conch at this point is simply as a show of leadership and power. Piggy and Ralph recognize that they've lost leadership on this island simply because they no longer have followers. The conch, however, is the one thing that Jack hasn't been able to take away from Ralph. Even though it is just a symbol, it is the one symbol that all of the boys recognize and relate to their first attempt at creating an adult world. Piggy is hoping that piping one long tone on the conch will trigger a pang of jealousy and guilt in Jack, which is all Piggy and Ralph can hope for to get back at Jack. The best case scenario for Piggy and Ralph was that some of the boys would realize how far they've come from civilization and revert back to "normal" behavior, but it was wishful thinking.
The symbolism of the conch is carried out in this chapter through the death of Piggy. Before Golding shows us Piggy dying, we see the conch explode into a thousand pieces and cease to exist. It isn't the physical non-existence of the conch that should worry the reader; it's the symbolic and metaphorical existence. The conch represented civilized behavior and order; without it, the savagery is complete.