Experimental study in gaseous helium at low temperature (4 K) of thermal convection up to a Rayleigh number $R={10}^{11}$. Three regimes are observed, a chaotic state up to $R=2.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$, a soft-turbulence state up to $R=4\ifmmode\times\else\texttimes\fi{}{10}^{7}$, and then a hard-turbulence state. We associate those three regimes to the boundary-layer formation and dynamics.