Rheological properties of clayey soils originating from flow-like landslides

Abstract

Flow-like landslides in clayey soils represent serious threats for populations and infrastructures and have been the subject of numerous studies in the past decade. However, despite the rising need for landslide mitigation with growing urbanization, the transient mechanisms involved in the solid-fluid transition are still poorly understood. One way of characterizing the solid-fluid transition is to carry out rheometrical tests on clayey soil samples to assess the evolution of viscosity with the shear stress. In this study, we carried out geotechnical and rheometrical tests on clayey samples collected from six flow-like landslides in order to assess if these clayey soils exhibit similar characteristics when they fluidize (solid-fluid transition). The results show that (1) all tested soils except one exhibit a yield-stress fluid behavior that can be associated with a bifurcation in viscosity (described by the critical shear rate $\dot{\gamma_c}$) and in shear modulus G; (2) the larger the amplitude of the viscosity bifurcation, the larger the associated drop in G; and (3) the water content (w) deviation from the Atterberg liquid limit (LL) seem a key parameter controlling a common mechanical behavior of these soils at the solid-fluid transition. We propose exponential laws describing the evolution of the critical shear stress τ_c, the critical shear rate $\dot{\gamma_c}$, and the shear modulus G as a function of the deviation w-LL.

Appendices

Appendix 1

Table 3 Mineralogical content of the soil samples

Appendix 2. Back-analysis of Pont-Bourquin debris flow event

The Bing software (Imran et al. 2001) solves a 1D shallow-flow model relying on a viscoplastic rheology (Bingham or Herschel Bulkley) to simulate the propagation of debris flows. It has been initially developed for submarine debris flows, but can be adapted to subaerial flows by using an ambient fluid density equivalent to the air (1 kg m⁻³). The computation of the flow starts from a semi-parabolic mass defined by a length and a thickness. The flow then propagates on an altimetric profile which is provided by the user.

Similar to Malet et al. (2004), we used this software to carry out a back-analysis of a debris flow event that occurred on the PO landslide in summer 2007 and was documented by Jaboyedoff et al. (2009). After heavy rainfalls, a flowing volume of about 11,000 m³ cut the road at the toe of the landslide. A detailed geomorphological analysis provided the necessary information regarding the localization and size of the initial unstable mass. Accordingly, a maximum thickness of 3 m and a length of 100 m were used as initial conditions in Bing. Field observations after the event indicated a length of propagation of 130 to 160 m, and a front thickness of approximately 2 m on the road. In agreement with recommendations of Imran et al. (2001), numerical parameters were taken as follows: artificial viscosity = 10⁻⁴; number of nodes in domain = 21; time step = 10⁻⁵ min. A Herschel-Bulkley viscoplastic law with an exponent n = 0.25 was used. In agreement with the common practice for muddy debris flows (Coussot et al. 1998; Rickenmann et al. 2006), the ratio τ_c/K between the critical stress τ_c and consistency K the material was considered constant, with values in the range 3–5 s⁻ⁿ. Figure 8 shows the evolution of flow runout as a function of the considered critical shear stress and ratio τ_c/K. All the simulated-flows shown in Fig. 8 present front thicknesses between 1 and 2 m, in agreement with field values. It is observed that the simulated runout significantly decreases with the ratio τ_c/K. Depending on the value of this ratio, values of critical shear stress τ_c lying between 550 and 1300 Pa are necessary to reproduce the runout of the 2007 event.

Fig. 8

Back-analysis of the 2007 flow event at Pont-Bourquin (PO) landslide with Bing software using a Herschel Bulkley viscoplastic law