In John Steinbeck's story, the turtle's shell represents safety.
In Chapter 3 of The Grapes of Wrath, the author presents a turtle walking across a road very slowly. While doing this, the turtle hides inside its shell for safety; however, when a truck hits the turtle intentionally, the shell flips over. After that, the turtle is able to get back on track but the writer uses the shell as a symbol. The shell represents protection against a dangerous and harsh world and the cruelty of its inhabitants. In that way, the turtle also functions as a symbol: it is used to refer to the Joads family, who live in a merciless society.