Mechanism of the slow-moving landslides in Jurassic red-strata in the Three Gorges Reservoir, China

Abstract

Landslides in Jurassic red-strata make up a great part of geohazards in the Three Gorges Reservoir (TGR) in China. Most of them begin to move slowly with the accumulated displacement increasing stepwise, which corresponds to seasonal rainfall and 30 meters of reservoir water level fluctuation (145 m to 175 m on elevation). We analyzed the movement of 21 slow moving landslides in Jurassic red-strata in TGR, and found that all these landslides involved in two differing processes; one is the sliding process with different shear speeds of soils within the sliding zone (landslide activity), and the other one is in steady state with different durations (dormant state). This means that the soil within the sliding surface may experience shearing at different shear rates and recovery in shear strength during the dormant period. To clarify the mechanism of this kind of movement, we took soil samples from the sliding surface of Xiangshanlu landslide, which occurred on August 30, 2008 in Jurassic red-strata in TGR, and examined the shear rate dependency and recovery of shear resistance by means of ring shear tests. The results of tests at different shear rates show that the shear strength is positively dependent on the shear rate, and can be recovered within short consolidation duration after the shearing ceased. By increasing the pore-water pressure (PWP) from the upper layer of the sample, we also examined the 2 initiation of shearing which can simulate the restart of landsliding due to the fluctuation of groundwater level caused by rainfall or changes in reservoir water level. The monitored PWP near the sliding surface revealed that there was delayed response of PWP near the sliding surface to the applied one. This kind of delayed response in pore-water pressure may provide help for the prediction of landslide occurrence due to rainfall or fluctuation of reservoir water level.

Simulating the process of reservoir-impoundment-induced landslide using the extended DDA method

“…The results suggested that rainfall and the reservoir water level were the principal factors influencing the deformation of the rock mass (Xia et al, 2013). Other similar works corroborate these results: an increase in water pore pressure increases the likelihood of a landslide (Zangerl et al, 2010;Zhu et al, 2012;Wei et al, 2013;Miao et al, 2014).…”

Time Series Analysis of Very Slow Landslides in the Three Gorges Region through Small Baseline SAR Offset Tracking

“…The As is well known [31,32], the ground water table or pore-water pressure within the soil layers of the landslide body are affected by the reservoir surface fluctuation and local precipitation, which decreases the effective normal stress leading to a decrease in shear strength of the soils. In previous studies, the fluctuation of the reservoir water level and seasonal rainfall are found as the two main triggering factors for landslides along the Yangtze River banks [31,[33][34][35][36].…”

Wide-Area Landslide Deformation Mapping with Multi-Path ALOS PALSAR Data Stacks: A Case Study of Three Gorges Area, China

“…The change in displacement rate between these two periods might be triggered by the consecutive rainfall in the summer of 2008. Water level fluctuations and rainfall are the two most important impact factors of slope stabilities in the Three Gorges region for slopes near the river [16]. The upstream water level of Three Gorges and rainfall records from Badong and Fengjie were used to investigate the relationship between displacements and water level fluctuation/rainfall.…”

“…As we can see from Figure 11c, the landslides located at P4 experienced stepwise displacements. The landslides in the Three Gorges areas with stepwise displacement progression mainly undergo two main processes: the sliding process and the dormant process [16]. During the sliding process, the landslide moves at inconsistent rates, whereas during the dormant stage, shear strength has been recovered and stability is achieved for some duration of time.…”

“…Periods I, III and IV could be caused by the joint effect of water level decline and rainfall. During these three periods, the rainfall was relatively low at first while the fast decline of the reservoir water level triggered the movement by changing the pressure difference between the groundwater inside the slope and the Water level fluctuations and rainfall are the two most important impact factors of slope stabilities in the Three Gorges region for slopes near the river [16]. The upstream water level of Three Gorges and rainfall records from Badong and Fengjie were used to investigate the relationship between displacements and water level fluctuation/rainfall.…”