Keywords

1 Introduction

Weathering profiles can vary widely in the area and in-depth in the same rock mass. The spatial variability and weathering intensity depend on the rock mineralogy and texture, the discontinuities, the topography, the climate, and the groundwater conditions. The description of such profiles gave rise to proposals for the classification of weathering grade classes based on colour, friability, modifications of mineralogical composition, and microfracturing. Several authors, institutions, and technical associations proposed their weathering grade classifications. However, the ISRM classification (1978, 1981, 2007) is universally used for rock masses and considers five weathering grade (W1–W5) and one grade corresponding to residual soil (W6). A similar description is found in GSE (1995) for intact rock, which is very useful for obtaining correlations with the laboratory mechanical tests.

The influence of weathering grade on petrographic, physical, and mechanical characteristics has been studied by several authors in different lithologies (e.g., Irfan and Dearman 1978; Viana da Fonseca 1996; Arel and Tuğrul 2001). According to Letto et al. (2018), the geomechanical and mineral-petrographic properties of the rocks decrease with the weathering grade increase. Therefore, the weathering profiles' mechanical characterisation must be made for the rock masses for geotechnical purposes. Knowledge of the index properties, as listed in González de Vallejo and Ferrer (2011), which can be evaluated in laboratory tests, enables the classification of intact rocks, while field tests are usually needed for the characterisation of rock masses according to various technical criteria.

For the Portuguese granitic rock masses, widely outcropping in the country, the hypothesis arose that radon gas could be used as a plotter for geological materials’ transformation processes, both due to weathering or when subjected to stresses (Koike et al. 2015). Pereira et al. (2017) showed a possible relationship between weathering grade in granitic rocks and exhaled gas concentration.

The Granite of Porto presents weathering profiles in which the entire range of weathering grades (W1–W5) is found and a variable and erratic thickness of residual soil (W6). The main objective of this work is to present the methodologies which are used in the research being carried out, aiming to establish correlations between the geological characteristics of Granite of Porto, namely, the weathering grade, mineralogy, geochemistry, porosity, and microstructure, with the hydraulic and geomechanical properties of the rock mass, namely, the parameters of stiffness and shear strength of the granite for the various weathering grade.

The present work aims to present the methodology of the work developed in the scope of the ongoing research project. Results are not available yet and are planned to be published soon.

2 Methodology

In order to thoroughly characterise the geological–geotechnical variability of the Porto granite weathering profiles, it is fundamental that the amount of data is sufficiently broad. Furthermore, it is necessary to conduct tests that consider the mineral and chemical composition and determine the physical and mechanical properties for better characterisation. Thus, in order to carry out this study, a strategy was defined: first, the selection of sites for sampling, followed by the careful sampling and preparation of specimens; finally, the laboratory and in situ testing. For the first step, it was essential to take advantage of new excavation works in Porto city due to its extensive urban network. Moreover, the lack of outcrops available, namely, the excavation for constructing a building in the D. João I block in the city centre (Fig. 1), complemented a former sampling during the site investigation works for Metro do Porto tunnels, underground stations, and other infrastructures.

Fig. 1
figure 1

Excavation works in Porto City: a Location of two experimental fields (adapted from COBA 2003); b Stone-wall in the D. João I city block

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In order to minimise the samples disturbance, among the several existing sampling techniques, the block sampling method was selected, using excavation and manual moulding. This method is the most recommended for residual soils, while for un-weathered rock, the ideal sampling is obtained by drilling with continuous coring.

2.1 Pedological Characterisation and Associated Trials

The pedological characterisation is based on the FAO soil classification system—World Reference Base for Soil Resources (FAO 2015), using the internationally accepted FAO guidelines for soil description—Guidelines for Soil Description (FAO 2006)—to describe soils and their characteristics (Table 1).

Table 1 Methods of pedological characterisation
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2.2 Mineralogical and Geochemical Properties

For evaluating the mineralogical and textural characteristics, the petrographic analysis of thin sections, X-ray diffraction, and scanning electron microscopy are used (Table 2).

Table 2 Methods for determining mineralogical and geochemical characteristics
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2.3 Physical Properties

The evaluation of physical properties includes water content, porosity, voids index, unit weight, and rock permeability. In addition, the size distribution and the consistency limits (Atterberg limits) are determined for the residual soils. Finally, in Table 3, the properties and respective standards, considering the need to apply the soil geotechnical classifications.

Table 3 Methods for determining physical properties
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2.4 Mechanical Properties

In order to characterise the residual soil and the rock material, various mechanical properties will be determined. These tests are listed in Table 4, and the equipment used is listed.

Table 4 Methods for determining mechanical properties
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2.5 Radiometric Methods

It is intended to use the radon accumulator method, which allows obtaining radon concentration in the samples to show the variations it presents according to the rock weathering grade. The method is based on measuring the exhalation of radon and is obtained using an Alphguard Pro2000 equipped with an ionisation chamber (University of Coimbra). Subsequently, the exhalation rate of radon is calculated.

3 Concluding Remarks

The proposed programme will contribute to the evolution of knowledge globally since, in most cases presented in the literature, the evaluation is based on a limited number of tests and samples. There is a wide variety of papers regarding laboratory tests due to the need to define the best method to reflect the variability of the geotechnical properties with the granitic rock masses' weathering grade profiles.

The methodology proposed above intends to obtain a complete characterisation to establish correlations between the rock mass's geological, hydrogeological, and geotechnical characteristics. The aim is to achieve comprehensively relevant results in granitic rock masses with varying weathering grade and in residual soils with similar properties to the materials studied in the present work.