From La Visaille/Plan Lognan, 1670 m., pass the bridge and take the closed car road, that goes up gently for almost 3 km. until the panoramic Plan Combal, 1990 m. (1h - tiny bar/restaurant - a little deviations brings you in 5 minutes to the glacial Lac Miage). To the left of the bar you’ll see a path marked with a little “Bivacco Rainetto” sign. The path goes almost horizontally above the marshy plain (fantastic in June when is full of white and jellow flowers), then turn left and begin to climb with many turns over the massive southern rampart of the Petit Mt. Blanc. Around 2340 m., the path goes thorugh an area with many boulders, then enter the large and very evident couloir above.
This couloir is normally extremely easy with dry conditions. However, if there’s too much unstable snow (a not rare occurrence at the beginning of season), it can become dangerous or even impossible to climb. In doubt, renounce!
At the top of the couloir, reach the flat Col du Combal (2833 m.) to the left, then right for a while, then straight up scrambling above a easy barrier of excellent rock (beware in descent if wet!), following the many traces and stone cairns that mark the path. Then for a often snowy slope, you reach the bivouac (huge cross nearby).
3 hours from Plan Combal, 4 from Plan Lognan