Periodic recurrence and scale-expansion mechanism of loess landslides caused by groundwater seepage and erosion

Abstract

Due to perennial irrigation, massive loess landslides occur frequently in Heifangtai, Gansu Province in China. Because of the distinctive distribution feature of strata in this area, most of the infiltrated water is discharged slowly from the interface between the loess and the underlying clay layer at the border of the terrace, which forms numerous springs on the cliff-like steep slope around the loess platform. Loess landslides in Heifangtai mostly are triggered by local seepage and erosion due to extensive irrigation, which often occur periodically at the same site, and their scales tend to expand constantly. To account for these phenomena, a field test was conducted near a landslide edge at the eastern margin of Heifangtai, in which the groundwater flow direction was measured by a colloidal borescope, and the equipotential line was determined by the hydraulic head. The results show that the concave topography formed by past landslides can deflect groundwater, causing it to flow to the center of the landslide site. Besides, the failure mechanism of loess landslides were analyzed and discussed, considering the action of groundwater seepage and surface runoff. Then, based on the results of numerical simulation, the changing characteristics of the loess landslides in Heifangtai were summarized briefly in four stages, i.e., (1) seepage erosion at the toe of the slope, (2) small-scale landslides and the convergence of groundwater flow, (3) transportation of deposits and periodic recurrence of slope failure at the same site, and (4) the potential recurrence of a larger landslide in this area.