In relation to Chapter 36, during World War II the Russians used a "scorched earth" policy, which meant that while they were retreating they destroyed anything the Nazis could've used against them, such as food sources, train lines, roads, etc. This is what the humans are doing by destroying Boulder and Fort Irwin, all the resources the zombies would have to use there (namely, the humans to eat or turn), have been destroyed. Just what I think it's referring to.