Abstract: The stability of Huangtupo landslide in the Three Gorges Reservoir area has always received attentions from researchers as it exerts significant influence on the safe operation of the Three Gorges Project. Nevertheless, the mechanism of porosity and seepage of slip zone affecting the landslide stability is still unclear. In the present paper, the pore structure and seepage characteristics of the slip zone soil of Huangtupo landslide are investigated in consideration of groundwater seepage via collecting samples from Badong field test site and by using laboratory approaches. Results reveal that 1) the roundness and distribution pattern of gravels, the clear stratigraphic boundary of slip zone as well as the friction marks between gravel and bedrock surface all indicate that the slip zone soil is of evident sliding features. 2) Penetrating cracks at the upper bound (interface between the slip zone soil and landslide accumulation body) of slip zone and the lower bound (interface between slip zone soil and bedrock) are the concentrated seepage channels of groundwater. The slip zone soil is of apparent stratification structure; however, the soil is dense, and groundwater doesn't flow preferentially along the gap between soil layers under the condition of low groundwater head. 3) The saturated permeability coefficient of slip zone soil is lower than those of overlying unconsolidated accumulation body and strongly-weathered bedrock by 2-3 orders of magnitude, which results in the concentrated drainage of rainfall water through the upper bound and fissure water in bedrock through the lower bound. Under the action of matric suction, the groundwater slowly infiltrates into the slip zone soil from both upper and lower bounds, implying that slip zone soil notably controls the groundwater flow. 4) Under natural water head, the slip zone soil is of strong resistance against seepage destruction as the seepage gradient at failure exceeds 16, which suggests that groundwater mainly increases the moisture content and reduces the mechanical properties of slip zone soil rather than destructs it.

Key words: Huangtupo landslide, groundwater, slip zone, pore, seepage, Three Gorges Reservoir area