Failure models of a loess stacked dam: a case study in the Ansai Area (China)

A loess dam is not fully compacted, so there may be a high permeability area in a loess dam. The seepage channel appears inside the dam body, and osmosis damage occurs; these events cause the dam body to be unstable and destroyed. In this paper, the loess dam in the Ansai Area, Yan’an, Shaanxi Province, China, is used as an example monitoring target, and the whole life cycle of the dam is monitored by measuring cracks, liquid level and pore pressure. The timeline regarding multiple instances of damage during the period is used as the basis for subdividing the whole life cycle, and dam damage types are distinguished according to the internal factors and external conditions. By analysing the on-site monitoring data, the dam experienced three obvious types of damage from the onset of monitoring until the dam broke. The triggering factors of the three damage types differ. One factor type is the difference in liquid levels on the two sides of the dam and the seepage channel that is then formed inside the dam. The fine-grained soil in the dam is removed from the seepage channel under the action of hydrodynamic force, the soil at the foot of the dam slope tends to be saturated under the action of osmotic water and the shear strength of the dam slope is reduced. When the soil weight of the upper soil is greater than the shear strength of the soil at the foot of the dam, the first damage type occurs; therefore, it is called “internal erosion damage”. The second type of damage is attributed to the softening of the dam feet under immersion by rainwater, resulting in the dam collapse; it is called “slumping damage”. The third type of damage is due to the infiltration of rainwater through a crack at the top of the dam. The soil at the top of the dam slides along the rainwater intrusion surface, and the width of the dam top is continuously decreased until it completely disappears; therefore, it is called “slippery damage”. On that basis, the loess dam failure modes are proposed due to the difference in hydrodynamic forces on the two sides of the dam and formation of a seepage channel. The fine-grained soil is removed from the seepage channel under the action of hydrodynamic force. The seepage channel is continuously expanded, the soil at the foot of the dam is saturated under the action of seepage water, the soil shear strength is reduced, the dam body becomes unstable and the width of the dam crest is decreased. Under the joint action of infiltration and rainfall, the dam begins the next round of infiltration and erosion and the width of the dam roof continues to decrease. The cycle continues as the width of the dam roof is continuously reduced until overtopping occurs.