The code does not concern itself with the trench dimensions, only the minimum cover requirements. The minimum cover depends on a number of factors, such as raceway type and the use of the ground over the pipe. This is explained in Table 300.5
If that trench is only 24" deep and you need 24" of cover, you need to dig a little deeper (3" plus). The cover is measured from the top of the conduit, not the bottom of the trench.
steve