Characterization of groundwater dynamics in landslides in varved clays

Abstract

Abstract. Groundwater dynamics may play a significant role in landslides. A detailed model is developed of the groundwater dynamics in landslides in varved clays in the Trièves area in the French Alps. The varved clays consist of a sequence of alternating silt and clay layers, covered by a colluvium layer and intersected by fissures. The hydraulic conductivity of the clay layers is negligible compared to the silt layers. It is conceptualized that fissures form a hydraulic connection between the colluvium and the varved clays. Groundwater recharge flows through the colluvium into the fissures, where water is exchanged horizontally between the fissure and the silt layers of the varved clays. Groundwater flow in the colluvium is simulated with the Boussinesq equation, while flow in the silt layers of the varved clays is simulated with the Richards equation. Longitudinal outflow from the fissure is simulated with a linear-reservoir model. Scattered data of relatively short monitoring periods is available for several landslides in the region. A good similarity between observed and simulated heads is obtained, especially when considering the lack of important physical parameters such as the fissure width and the distance between the monitoring point and the fissure. A simulation for the period 1959-2004 showed some correlation between peaks in the simulated heads and the recorded occurrence of landslides, while the bottom of the varved clays remained saturated during the entire simulation period.

A shift from drought to extreme rainfall drives a stable landslide to catastrophic failure

“…These large stress changes resulted from extreme rainfall that followed a period of historic drought and may have been enhanced by increased pore-fluid pressures from shear-induced compaction and longitudinal compression of the upslope material due to differential slip 3,4,17 . Our pore-fluid pressure model does not account for multi-dimensional pore-fluid pressure changes 46,47 or for mechanical-hydrologic feedbacks such as shear-induced dilatancy or compaction 4,45 , however this simple model has been widely used to describe the first-order changes in pore-fluid pressure that drive acceleration of similar deep-seated landslides around the world 1,2,48,49 and indicates that the Mud Creek landslide experienced unusually high pore-fluid pressures prior to catastrophic failure. Furthermore, we hypothesize that the preceding drought and extension near the headscarp likely increased rapid infiltration pathways such as tension cracks that can facilitate strong pore-fluid pressure changes 46 .…”

Widespread Initiation, Reactivation, and Acceleration of Landslides in the Northern California Coast Ranges due to Extreme Rainfall

“…However, they found that the soil swelling pressure exerted along the landslide's lateral shear zones increased landslide stability by as much as 6%, which contributes to reducing the potential for catastrophic failure. In addition, we hypothesize that landslide drainage networks are important for reducing pore‐water pressures and preventing runaway acceleration (e.g., Coe et al, ; Handwerger et al, ; Krzeminska et al, ; Van der Spek et al, ). These landslides tend to have well‐developed surface and possibly subsurface, drainage networks that can efficiently transfer water to the river network and reduce pore‐fluid pressures such that the landslide groundwater system typically maintains a narrow range of pore‐water pressures that are sufficient to drive motion but is also susceptible to changes during years of extreme precipitation or drought.…”

Quantification of the influence of preferential flow on slope stability using a numerical modeling approach

“…The dual-permeability model is a useful tool to simulate subsurface stormflow and solute transport in a forested hillslope when the parameterization is able to capture Introduction the hydraulic characteristics of each domain (Laine-Kaulio, 2011;Laine-Kaulio et al, 2014). As the dual-permeability model describes the subsurface as a continuum of two linked domains, it is suitable for heterogeneous slopes with a high density of preferential flow paths and not for slopes with only a few large fissures or cracks (van der Spek et al, 2013).…”

“…In the dual-permeability model, the pore water pressure of the matrix and the preferential flow domains are different and water flows from the domain with a higher pressure to the domain with a lower pressure. van der Spek et al (2013) show that in the case of varved clays with a low hydraulic conductivity of the soil matrix and a low density of fissures, the time delay between water entering the fissure network and an increase in pressure in the matrix is relatively large. This study concerns a system with a very high density of macropores and consequently the numerical simulations show only a small time delay for the pressure propagation from the preferential flow domain to the matrix domain.…”