Abstract: Aiming at the problems of ground settlement induced by the underground pipeline damage in water-rich sand layer, a set of visualization experimental device was designed. For these cases of 11 kinds of sand samples with particle size of skeleton d90=1.45?8.45 mm and 5 kinds of full pipe flow velocity, the law of ground settlement induced by seepage erosion was studied. Research show that: 1) There are three modes of seepage erosion induced by pipeline damage: only water inrush without settlement, soil arching formation with settlement and sand crushing with settlement; 2) The particle size of soil skeleton , damaged mouth size and thick-span ratio are the main factors to determine the seepage erosion mode of soil; 3) When the soil arching or sand crushing is formed in the soil above the damaged mouth of pipeline, the relationship between the particle size of skeleton d90 and the thick-span ratio r is that: when 8.0≥r≥4.2, the d90 decreases parabolically with the increase of r; when 12.5≥r≥8.0, d90 remains unchanged; 4) When the soil arching or sand crushing is formed in the soil above the damaged mouth of pipeline, the initial settlement radius and settlement depth with the flow velocity equal to 0 are determined by the ratio (D/d50) of the damaged mouth diameter D and the average particle size of soil d50; the settlement radius and depth increase linearly with the increase of full pipe flow velocity; when the soil arching is formed, the expansion velocity （VL、VH） of settlement radius and settlement depth with the increase of flow velocity is logarithmic to D/d50. When the sand crushing is formed, the expansion velocity VL of settlement radius with the increase of flow velocity is that, when 23.0≥D/d50≥6.0, it increases linearly with the increase of D/d50; when 42.0≥D/d50≥23.0, it decreases logarithmically with the increase of D/d50.

Key words: seepage erosion, settlement, thick-span ratio, flow velocity, particle size