Elsevier

Geomorphology

Volume 87, Issue 3, 15 June 2007, Pages 158-175
Geomorphology

Weathering grade of rock masses as a predisposing factor to slope instabilities: Reconnaissance and control procedures

https://doi.org/10.1016/j.geomorph.2006.03.031Get rights and content

Abstract

Weathering of rock masses often assumes importance as a predisposing factor to slope instability and it is possible to map it at various scales depending on the different purposes. The effects of weathering processes are particularly intense on crystalline rocks (plutonic and metamorphic). These rocks are present in large areas of the globe and widespread in Calabria. The relationships between rock mass weathering grades and slope instabilities are analysed, with reference to sectors (1:50,000 scale) and areas (1:10,000 scale) where crystalline rocks are strongly affected by weathering. To this aim a reconnaissance procedure has been proposed to delimitate the zones with different weathering condition, three macro-classes at average scale (1:50,000) and six classes at detail scale (1:10,000). In this procedure first analysis of aerial photos and then field observations of representative situations have been used. The reconnaissance procedure has been verified in a selected study area (Acri), whose geological features are provided, by the comparison with weathering maps obtained by means of a control procedure. This last procedure consists of observations and index tests carried out in check points located in representative check sites (discolouration, sound when struck by geological hammer, effect of the point of geological pick, breaking with the hands, rebound of Schmidt Hammer, grain-size analysis). The results obtained confirm through quantitative data that the weathering of a rock mass can be assumed as a predisposing factor to slope instability. At average scale (1:50,000) the reconnaissance procedure is able to give weathering maps representative for this type of evaluation (the ratio between the landslides area in each weathering macro-class and the whole landslide area goes from 67% to 14% for the macro-class A and from 24% to 9% for the macro-class B); at detail scale (1:10,000) it is necessary to use a control procedure to obtain weathering maps indicative of predisposition to slope instabilities.

Introduction

Lithologic maps, even though they describe thoroughly the characteristics of outcropping rocks, provide only general information about the weathering grade of rock masses. This assumes particular relevance for crystalline rocks, because the weathering processes they undergo could present heterogeneous characteristics in outcrop and in depth, from a mineralogical, physical and mechanical point of view. The lack of information on weathering conditions could strongly limit the value of lithologic maps.

Rock weathering takes on great importance for many aspects: the evolution and development of landforms (Modenesi and Paulo, 1983, Ollier, 1984, Migon and Lindmar-Bergstrom, 2001, Pain et al., 2001); the possible presence in rock masses of parts with both hard-rock and soil-like behaviour (Dearman, 1976, I.S.R.M., 1978, Dearman and Irfan, 1978, I.A.E.G., 1981, G.C.O., 1984, G.C.O., 1988, Gullà and Matano, 1997); the relevance for the slope instabilities (Deere and Patton, 1971, Hencher et al., 1984, Brand, 1985, Nishida and Aoyama, 1985, Cascini et al., 1992, Cascini et al., 1994, Gullà and Nicoletti, 1996, Lacerda and Santos, 2000, Calcaterra et al., 2004, Gullà et al., 2004a, Lacerda, 2004, Terranova et al., 2004, Valley et al., 2004). Therefore, weathering survey can be useful for different purposes.

In the present paper weathering survey is aimed to individuate the relationships between rock weathering grade and slope instabilities. In particular, the reliability of a reconnaissance procedure of rock mass weathering grade (Gullà et al., 2004b) is tested at 1:50,000 and 1:10,000 scales. To this purpose, the methodology and the results of reconnaissance procedure are synthesized; geological features of the selected study area are provided to verify the reconnaissance procedure; the methodology and the results of the control procedure are illustrated; relationships between rock weathering grade and slope instabilities are analysed; the reconnaissance procedure map and the control procedure map are compared.

Section snippets

Weathering grade maps from reconnaissance procedure

In order to verify if the weathering grade can be assumed as a predisposing factor of rock masses to slope instability, weathering maps for large areas are necessary at different scales. To this aim a reconnaissance procedure has been pointed out by Gullà et al. (2004b) and Borrelli et al. (2004). In the present paper this reconnaissance procedure has been used at average scale (1:50,000) and at detail scale (1:10,000).

Geological contexts, that are representative of outcropping rocks, have been

Geology and slopes instability of the study area

The study area is located on the western side of the Sila Massif (Fig. 1) that, geologically, is composed of stacked nappes of several Paleozoic metamorphic and igneous terranes (Tortorici, 1982), emplaced during Oligocene–Lower Miocene time. Structurally the Sila Massif represents a horst bounded by Quaternary normal faults, having a N–S trend and forming fault steps declining to the W. Along this fault system the crystalline lithotypes are raised against the Plio-Pleistocene deposits filling

Methodology

A control procedure has been defined at a scale of 1:10,000 in order to verify the results obtained from the reconnaissance. The control procedure was divided in two stages.

In the first stage the main and secondary roads of the study area were traversed. Along these roads observations of the distinctive characters of rock mass, such as colour and presence of relict structures, were made. Photographic documentation of the weathering conditions was acquired, and possible representative check

Conclusions

The results obtained confirm through quantitative data that the weathering of rock masses can be assumed as a predisposing factor to slope instability. The verifications carried out through the control procedure show that at a scale of 1:50,000 the reconnaissance procedure is able to give representative weathering maps for this type of evaluation. In particular the ratio between the area of the landslides in each weathering macro-class and the whole landslide area is from 67% to 14% for the

Acknowledgments

The authors thank Prof. Domenico Calcaterra for his comments that helped clarify and improve the manuscript. Many thanks also to Prof. Serge Leroueil and the anonymous referee for their useful suggestions and critical comments.

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