Unveiling the role of seepage forces in the acceleration of landslides creep

Abstract

In the context of global climate change, geological materials are increasingly destabilized by water flow and infiltration. We study the creeping dynamics of a densely monitored landslide in Western Norway to decipher the role of fluid flow in destabilizing this landslide. In Åknes, approximately 50 million cubic meter of rock mass continuously creeps over a shear zone made of rock fragments, with seasonal accelerations that strongly correlate with rainfall. In this natural laboratory for fluid-induced frictional creep, unprecedented monitoring equipment reveals low fluid pressure across the shear zone, thereby challenging the dominant theory of fluid-driven instability in landslides. Here, we show that a generic micromechanical model can disentangle the effects of fluid flow from those of fluid pressure, and demonstrate that seepage forces applied by channelized flow along the shear zone are the main driver of creep accelerations. We conclude by discussing the significance of seepage forces, the implications for hazard mitigation and the broader applicability of our model to various geological contexts governed by friction across saturated shear zones.

Figures (5)

• Figure 1: Geometry of a typical creeping landslide studied in this work. A rock mass of thickness $H$ slides downhill following a slip profile $u$ where most of the strain is localised within a shear zone of thickness $h$ made up of debris and rock particles crushed by frictional sliding. Due to its granular nature, the damaged shear zone has a larger porosity $\phi$ and hydraulic conductivity $K$ than the surrounding competent rock mass ($K_{wr}$,$\phi_{wr}$) and represents a favourable flow corridor for underground water. The inset details the effect of water at the grain scale of the shear zone, which is twofold: 1) a hydrostatic effect caused by the pore pressure $p$, which reduces the effective normal stress and 2) the seepage force exerted by fluid flowing at an average rate $v_{\rm w}$ on the solid particles, which increases the shear stress at the base of the landslide.
• Figure 2: The creeping landslide of Å knes in Norway is closely monitored for the risk of inducing a catastrophic tsunami in the Sunnylvs fjord. (a) Morphological map of the Å knes landslide with monitoring instruments. Borehole (white) and GNSS (yellow) arrows indicate mean velocities for the observation period (2020-2022), scaled by magnitude. Two sliding scenarios are outlined by the purple and orange polygons and are respectively associated with the upper and lower shear zones shown along the transect A–A$'$ in panel (b). (c) The structure of the upper shear zone revealed by the drill log of the borehole KH-02-17 at 69.7 m depth comprises clay-rich debris and crushed rock. (d) Cumulative slip profile (in log axis) for the period 2020-2022 was recorded at different depths of each borehole, where the upper and lower shear zones as well as the groundwater level are respectively marked by the horizontal purple, orange, and dashed blue lines. The water head coincides with the depth of the shear zones that represent favourable flow path of larger permeability.
• Figure 3: Marginal fluid pressure variations are measured in the shear zone of Å knes landslide. (a) Time series of different data recorded at the level of the shear zone in the borehole KH-02-18: water head relative to the depth of the shear zone (orange), cumulative creep displacement over the period of observation (black) and creep velocity (pink) featuring seasonal bursts that correlate well with the precipitation summed over the last 14 days shown in blue. The dashed grey box highlights the background creep rate that is reached after a dry period. (b) Sketch of the borehole instrumentation adapted from aspaas2024 and interpretation of the water head profile (dashed orange line). The borehole causes a draining effect, leading to local drop in the water head, which can explain the negative values shown in panel (a).
• Figure 4: The creep equation (\ref{['equ:creep_equ']}) and its parameters computed from the monitoring data allow for constructing a reliable prediction of the creep rate that is solely based on the accumulated precipitation over the last fourteen days. The model prediction (blue line) shows excellent correlation with the observed borehole KH-02-18 velocity (orange line), particularly during the creep burst events highlighted by the boxes a) and b). It represents a promising tool to decipher any changes in the mechanical behaviour of the shear zone in the future.
• Figure 5: A schematic illustration of water infiltration and its effects on subsurface water flow and water table height. Variations in hydraulic conductivity within the landslide shear zone create preferential flow paths, referred to as permeable channels. As illustrated in the comparison between dry and wet conditions, increased infiltration raises the height of water saturation $h_{\rm w}$ within the shear zone and, thereby, the average flow rate. According to the seepage force equation, higher infiltration leads to stronger seepage forces acting across the shear zone.