Tread for heavy goods vehicle tire

Tread for a tire of a heavy goods vehicle having a tread surface intended to come into contact with a roadway, this tread having a total thickness E and a total volume V, this tread having:

• at least two wearing layers I, II in the thickness of the tread, each layer having a thickness at most equal to 75% of the total thickness E of the tread,
• a total voids volume Vc at least equal to 7% and at most equal to 12% of a volume equal to the sum of the total volume V of the tread and of the total voids volume,
• at least one continuous groove comprising a plurality of external cavities opening onto the tread surface in the new state and a plurality of internal cavities positioned radially and entirely on the inside of the tread surface in the new state, the first wearing layer extending into the thickness of the tread as far as the radially innermost points of the external cavities, each other wearing layer extending into the depth of the tread as far as the innermost points of the internal cavities of the wearing layer in question, the external cavities having a mean depth P1 at most equal to 75% of the total thickness E of the tread, a mean length L1 and a cross section of area S1, the internal cavities having a mean height P2 at most equal to 75% of the thickness of the tread, a mean length L2 and a cross section of area S2, each internal cavity being connected to an external cavity by a connecting channel having two ends, said ends having cross sections of areas equal to the areas S1, S2 of the cross sections of the internal and external cavities connected by this connecting channel,
• this tread furthermore having an effective voids volume Ve for each wearing layer, this effective voids volume Ve contributing to draining the contact patch in which the tread is in contact with a roadway, which is less than the total voids volume Vc of the tread, this effective voids volume satisfying the following equation:
• 0.4St<Ve<0.8St,
• in which 0.4 and 0.8 are heights expressed in millimetres and St is the surface delimited by the external contour of the contact patch of the tread measured under static conditions under nominal load and pressure, the tyre being mounted on its recommended rim,
• wherein at least the first wearing layer I is made up of a rubber compound based on at least one material having a dynamic Tg—measured in accordance with standard ASTM D 5992-96, higher than −40° C. and predominantly containing an SBR copolymer of Tg>−65° C.