Grenoble only had two penalties from scrum half James Hart to their name at the break with the Blues holding a 15-6 lead at half-time.
The Blues replicated their great start immediately after the break when flanker Sam Warburton stretched over under the cross bar to score with winger Richard Smith, who impressed throughout with his strong running, secured the bonus point when he touched down out wide while Josh Turnbull completed a good day’s work with the Blues fifth try late on.
The Blues began with a flourish when fly half Gareth Davies sent a flat pass to Patchell who stepped inside and raced over for the try. He then converted his own try from wide out.
The Blues forged straight back downfield from the re-start with the forwards setting up a perfect platform for Williams to touch down under a pile of bodies.
Hart halted the one way traffic with a two well struck shots at goal before Patchell replied with one to send the Blues in 15-6 ahead at the break.
The Blues pack tore into Grenoble from the re-start with Warburton powering over from close range after Gavin Evans caused the damage after breaking free.
Hart and Patchell exchanged penalties before replacement hooker Kristian Dacey broke clear and passed to Patchell who fired a pass to Smith who dived over in the corner.
Dacey was again prominent for the Blues final try of the game when he tore into Grenoble’s defence to send Turnbull rampaging over.
Grenoble scored a late consolation as Louis Marrou went over in the corner.
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