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