This paper presents a numerical study of the gas-solid flow in a bed by a Combined Continuum and Discrete Model (CCDM). Numerical experiments are carried out to simulate the motion of 10,000 spherical particles of 4 mm diameter caused by lateral gas blasting into a bed with its thickness equal to the diameter of particles. It is shown that, depending on the gas velocity, the bed can transform from a fixed bed to a fluidised bed or vice versa. Two zones can be identified in such a bed: a stagnant zone in which particles remain in their initial positions, and a mobile zone in which particles can move in various flow patterns. If the gas velocity is in a Certain range, the mobile zone is confined in front of the gas inlet, forming the so-called raceway in which particles can circulate. If the gas velocity is higher than a critical value, fluidisation results, with the mobile zone growing by the combined effect of bubble penetration and shearing between moving and static particles until a stable state where the boundary separating the mobile and stagnant zones is unchanged. The dependence of raceway and fluidisation phenomena on gas velocity has been examined in terms of the size and shape of the mobile zone, gas-solid flow patterns and forces acting on individual particles. It is found that large interparticle forces occur along the boundary between the mobile and stagnant zones, whereas large fluid drag forces occur at the roof of a raceway or bubble. The predictions of transition between the static and dynamic states, and the complicated hysteretic behaviour in terms of either bed pressure drop or raceway size are in good agreement with the experimental observations.