Recompaction of tilled layers, under the effect of rainfall or irrigation only (i.e. without any external loading), was called slumping by Mullins et al. (1990). It has been observed in various soil types with negative effects on plant production. Our objective was to characterise the dynamics of slumping at the ploughed layer scale in a sandy soil of North East Thailand. An experimental field was tilled to two depths (20 and 40 cm) with or without ridges and furrows and was submitted i) to natural rainfall during two months (214mm in June and July 2007) or ii) to experimental flood irrigation (100 or 200 mm over some hours). Changes in bulk density with time were observed, particularly under flooding and after heavy daily rainfall. Final bulk density of 1.60 Mg m 3 has been measured over 20 cm depth while initial bulk density after tillage was 1.25 Mg m 3. Bulk density profiles were often characterised with two maximum values, either in the top layer (0-5 cm) or at the bottom of the ploughed layer (15-20 or 35-40 cm). We demonstrated that several processes occurred simultaneously: i) a redistribution of sand particles from the top of ridges to the bottom of furrows that decreased soil roughness, ii) a 2 to 5 cm topsoil collapse when water infiltrated, iii) a soil collapse at greater depths due to overburden pressure. These phenomena agree with the theory of granular material and the decrease in capillary forces between sand grains during wetting. The specific changes in bulk density profiles induced by rainfall should allow the occurrence of slumping to be predicted or identified as a function of soil, climate and tillage conditions.