engleski

Shear strength properties of soil materials from the Grohovo Landslide

The shear strength characteristics of soil materials have the main role in determination of a slope behaviour. Examination of soil strength characteristics is in most situations based on laboratory test conducted on soil samples taken from slip surface zone using different shearing devices. In this paper we will present an overview of laboratory shear strength test results carried out on samples from the Grohovo Landslide soil and their impact on slope behaviour. The Grohovo landslide is situated in the central part of the Rječina River Valley near the city of Rijeka. A lot of instabilities on the slopes of the Rječina River Valley were recorded during the 19th and 20th century and the last landslide was reactivated on the north eastern slope near the Grohovo Village in December 1996. The Grohovo Landslide was chosen as pilot site of the Croatian–Japanese Bilateral Project Risk identification and Land-Use Planning for Disaster Mitigation of Landslides and Floods in Croatia”, and a comprehensive integrated real-time monitoring system, consisted of geodetic and geotechnical equipment, was installed in 2011. Measured displacements inside the slope profile clearly indicated position of slip surface that is identified at the contact of slope superficial deposits and flysch bedrock. The flysch bedrock mainly consists of siltstones while slope superficial deposits are mostly a mixture of clayey silts and limestone’s fragments and blocks. Soil testing was conducted on samples taken from boreholes in the portable ring shear apparatus ICL-1. This apparatus was designed for testing the residual shear resistance mobilized along the sliding surface at large shear displacements under static and/or dynamic local conditions. Undrained speed control tests were conducted under constant shear speed until steady state condition was obtained. From this test basic material properties, mobilized and apparent friction angle and mobilized cohesion, are gained. Obtained test results are in accordance with results obtained in previous laboratory tests, and conducted slope stability back analyses. The previous laboratory tests, conducted in direct shear apparatus, didn’t give reliable undrained strength parameters because it was impossible to prevent localized drainage away from the shear plane. It wasn’t possible to measure pore pressures in the direct shear box ; and consequently, it wasn’t possible to determine effective stresses. Determination of effective stresses during shearing in ring shear apparatus is enabled by measuring pore water pressure using pore transducers installed near the shear plane. Previous 2D slope stability analyses were based on direct shear laboratory results and resulted safety factors pointed on too high shear strength parameters. Conducted slope stability back analysis resulted by lower effective shear strength parameters. The forms, shapes and geometries of landslide bodies in this complex landslide consisted of 13 different landslide bodies, point on using of 3D stability analysis, where the resistance on landslide’s flanks have significant impact on resulting factor of safety. Comparison of shear strength parameters obtained from direct shear and ring shear tests, so as from 2D and 3D slope stability back analyses point on average mobilized shear strength parameters of soil material on slip surface during occurrence of the Grohovo Landslide.

Ispitivanje posmične čvrstoće; Klizište Grohovo

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