The role of “fabric” and mineralogy in planar sliding processes in maly rocks: investigation using Scanning Electron Microscopy (SEM) and Environmental-SEM

 

Planar sliding phenomena are typical of the Piemonte’s Langhe region (northern Italy). They occur periodically in sedimentary turbiditic formations, and are triggered by heaxy rain events. The sliding of rock blocks occurs along bedding planes that have gentle inclinations (8° to 14°), and often coincide with joints between marl strata.

 

Planar sliding phenomena are typical of the Piemonte’s Langhe region (northern Italy). They occur periodically in sedimentary turbiditic formations, and are triggered by heaxy rain events. The sliding of rock blocks occurs along bedding planes that have gentle inclinations (8° to 14°), and often coincide with joints between marl strata.

Many studies have been carried out to model the planar sliding phenomena, and they all could explain the planar phenomena only by assuming that the sliding planes were already in the residual state, and that the residual shear strengths were lower than those evaluated by means of standard laboratory tests. Therefore, it has been necessary to perform further investigations in order to provide a better understanding on the possible processes of shear strength decay.

Accurate analyses have been carried out to investigate the swelling characteristics of the marls, by means of mechanical tests, mineralogical analyses, SEM and Environmental-SEM observations.

This paper describes the important results that have been obtained from the microscope observations. Scanning Electron Microscope (SEM) and Environmental-SEM (ESEM) observations of materials present on the sliding surfaces showed that the clay particles rearrange in a different way after water intake: i) illite particles become sub-parallel, thus favouring repulsion; and ii) smectite levels show an open "spongy" fabric, giving rise to volume increase and significant

swelling pressure; after repeated swelling/drying cycles a gel-like material without cohesion is formed. Both mechanisms can cause the shear strength, and hence the value ofthe safety factor, to decrease, thus initiating the sliding process.

Additional contributions to the mechanical decay of the marly lithotypes are: i) decalcitication of carbonate grains and their sulfatiou (formation of gypsurn); ii) oxidation of pyrite fractions and their transformation into jarosite (also observed during the wetting/drying treatments within the ESEM chamber). Both reactions are causing a significant volume increase and, consequently, destructuration of the rock fabric.


  Planar s …


AUTORI: Veniale G., Simeoni L., Rodriguez-Navarro C., Setti M.
RIG ANNO:2001 NUMERO:2
Numero di pagine: 23


Allegato: https://associazionegeotecnica.it/wp-content/uploads/2010/03/RIG_2000_2_23.pdf
Allegato:
Articolo completo: https://associazionegeotecnica.it/wp-content/uploads/2010/03/RIG_2000_2_23.pdf