A hydromill wheel (5) for excavating a trench in hard rock includes a drum (4) arranged to be rotated about its axis (A, B). The wheel (4) further includes a plurality of single disc cutters (11) mounted on the periphery of the drum (5), the single disc cutter (11) having a rotatable single cutting disc (11) arranged to come in contact with and crush the rock during excavation. The spacing of the projection of at least some of the cutting discs (11) on the drum axis (A, B) is 20% to 70% of the cutting disc diameter.