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We present results of ring shear frictional resistance for mudstone granules of different size obtained from a landslide shear zone. Little rate dependency of shear resistance was observed in sand-sized granules in any wet or dry test, while saturated gravel-sized granules exhibited significant and abrupt reversible rate-weakening (from =0.6 to 0.05) at about 2mm/s. Repeating resistance variations occurred also under constant shear displacement rate. Mudstone granules generate mud as they are crushed and softened. Shear-thinning and thixotropic behavior of the mud can explain the observed behavior: with the viscosity decreasing, the mud can flow through the coarser soil pores and migrate out from the shear zone. This brings new granules into contact which produces new mud. Thus, the process can start over. Similarities between experimental shear zones and those of some landslides in mudstone suggest that the observed behavior may play a role in some landslide kinematics.

Plain Language Summary Shear strength and resistance are important parameters influencing the occurrence and behavior of landslides. They depend on several characteristics, among which are the mineral composition of the soil or rock, the properties of the fluids which fill the pores, and the speed at which shearing takes place. Landslides that move along layers of mudstone, a widespread, often weak sedimentary rock, rich in clay minerals, can become very rapid, with catastrophic consequences for people and infrastructure. The behavior of such landslides suggests that the shear resistance can decrease to very low values as the shear speed increases, resulting in dramatic acceleration. Why this happens is not fully clear yet, and various mechanisms have been proposed. The weak mudstone is crushed, broken, and softened by the actions of shear strain and pore water. The mudstone rock fragments are reduced to a fine powder, which becomes mud in contact with water. In our work, we show that the mud becomes more fluid as the shear speed increases, and the same happens when it is sheared for a long time or over a long distance at the same speed. These two behaviors are termed shear thinning and thixotropy.