Evolution of Pajonales landslide (Tirajana Depression, Gran Canaria): A case of advancing landslide

Abstract

We studied the evolution of different stages of Pajonales landslide (Tirajana Depression, Gran Canaria), based on the geotechnical investigations of both in situ and mobilized volcanic materials. The deposit extends over 560 ha and it has undergone successive reactivations, some during the 20th century. The landslide comprises four large bodies that have successively broken away from a single initial rock mass. The main scarp affects the lava flows with intercalations of pyroclastic materials of the Roque Nublo Group and later volcanic activity (5.5 Ma to present-day). The basal surface of the landslide developed in old rocks of the Mogan Group (14.0-13.3 Ma), which are rhyolitic and trachytic ignimbrites with hydrothermal alteration related to the infilling of the Tejeda caldera. This alteration caused silty-clay layers presenting a low friction angle, high plasticity and expansive behavior. Representative samples of pyroclasts and soils from landslides were collected and laboratory tests were performed to identify them, determine unit weight, grain-size, plasticity, and shear strength. The morphology of slope prior to sliding was reconstructed considering the location of the failure surfaces and scarps of first and second generation. Failure surfaces for each stage of sliding were identified using software of limit equilibrium analysis. These surfaces have been developed through clayey-silt levels that result from the alteration of pyroclastic materials, showing the most unfavorable geotechnical parameters (minimal or residual values). Moreover, the presence of water is a triggering factor, since total or partial saturation of the materials is required. Finally, the landslide is an advancing type: as successive reactivations occur, the sliding masses are broken down into smaller ones moving towards the Tirajana ravine.