"Grass" is a short and pretty compact poem. You won't find a lot of alliteration or assonance hereabouts. Still, when you read it aloud, you definitely get the sense of momentum building. All of the repetition of "pile the bodies high" and "shovel them under" makes us really feel the burdens of war building up and building up. Even if the grass always has the same, disaffected response to the horrors of work—"let me work"—we the readers feel the weight of Sandburg's words.
For a real treat, head over here and listen to Sandburg read "Grass." His rendition of the poem is actually quite musical; when Sandburg reads it, you really get the sense of horror building as the sonic repetitions—and the bodies—pile up one atop another.