Groundwater and ground displacement monitoring in the source area of the Montecchi earthflow (Northern Apennines, Italy)

The importance of pore water pressure regime for landslide activity is generally accepted. In the case of earthflows, generalized failures (reactivations) are rare and sustained slow movements can proceed for decades. The relationship between precipitation, pore water pressure responses and movement is not straightforward. We document rainfall, pore pressure regime and displacements in the source area of an active earthflow. Pressure heads at shallow depth are clearly related to infiltration from the surface and can be satisfactorily reproduced by a diffusive model while the hypothesis of gravity-dominated flow can be rejected based on the short delay between rainfall and pressure response. Displacement rates are very small to zero during the summer and increase a couple of months later than the onset of the precipitation of the wet season. Only late in the wet season, velocities attain peak values (up to 4 mm/day) and show a remarkable correlation to rainfall episodes. Higher displacement rates correspond to unexceptional pressure head values, we therefore believe that alternative mechanisms of water pressure build-up may exist. Fractures likely act as a preferential flow system and influence both the hydrological responses to rainfall and the deformation behavior of the landslide.