Tread for civil engineering machine tire comprising improved ventilation cavities

Tread (1) having tread surface (10) axially bounded by edge parts (11), and provided with cuts (3, 4) delimiting material parts forming raised elements (2), each raised element (2) comprising contact face (20), there being formed, in a plurality of raised elements (2), a ventilation cavity (5) having a depth at least equal to 70% of the thickness of the tread material to be worn away, each ventilation cavity (5) being delimited by wall surface (50) ending at bottom surface (530). Each ventilation cavity (5) comprises first cavity part (51) continued into the depth by second cavity part (52). The first cavity part (51) is situated between the tread surface and intermediate depth level H1 situated at between 30% and 70% of maximum depth H of ventilation cavity (5), the mean relief angle A of the wall is at least equal to 20 degrees. In the second cavity part (52), mean relief angle B of the wall is at most equal to 15 degrees.