Weathering Indices Used in Evaluation of the Weathering State of Rock Material

Weathering Indices Used in Evaluation of the Weathering State of Rock Material

Sener Ceryan (Balikesir University, Turkey)
DOI: 10.4018/978-1-5225-2709-1.ch004
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Abstract

There are various definition of weathering and differences between authors seem to steam in part from the different viewpoints of pedolog, geomorpholog, geolog, geochemists and geology engineer. In this study, weathering is handled from various aspects such as time, form and phases of progress, studies it is majored and research scale. The engineering behavior of rock materials depends not only on stress state and stress history but also on the physical, mineralogical and chemical change of the rock materials due to weathering. Weathering indices are used to define these changes due to weathering. Several weathering indices have been devised for quantifying the changes in the intrinsic properties of rocks from different points of view, some of which can be related to the engineering properties of weathered rocks. The most commonly used methods can be broadly categorized as chemical, mineralogical-petrographical, petro-chemical and engineering indices. In this study, the brief literature review for weathering indices used to evaluate of the effects for weathering of rock materials.
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1. Introduction

The term ‘weathering’ has been in use for a very long time but it has come to mean different things to different people and hence, as scientific short-hand, it no longer functions (Hall et al. 2012). Weathering studies cover a quite large variety of topics, often accomplishedby applying multi- or trans-disciplinary approaches, in turn involving multi-scale and multi-analytical procedures (Scarciglia et al. 2016).

It is known that the weathering processes result the change of the physical, mineralogical, chemical and mechanical properties of the rock materials. The changes in these properties of a rock during the weathering process allow calibrating the degree of weathering. Based on those properties, various weathering indices can be used as tools for this purpose (Udagedar et al. 2016). The most commonly used methods in setting weathering definition can be broadly categorized as chemical, mineralogical-petrographical, petro-chemical and engineering indices.

Decomposition of rocks is one of the fundamental processes that modify the earth’s surface (Lee et al. 2008). Decomposition due to weathering and hydrothermal alteration is calculated in different ways, including using the normalized values of elements (or oxides) by using their parent rock concentrations or the immobile element concentrations in the samples (Krauskopf 1967), employing standard cell calculation (Colman 1982), calculating the ratio of elements to immobile elements (Chesworth et al. 1981, Colman 1982, Guan et al. 2001), the gamma-ray spectrometric study (Chen and Chan 2002), determining cation exchange capacity (Arikan et al. 2007), using an EC/pH meter (Shalkowski et al. 2009) and reflectance spectroscopy (Hyun and Park 2011). Alternative methods of calculating the change in weight or volume of rock materials due to chemical weathering include using immobile elements (Huston 1993), modelling compositional changes (Eynatten et al. 2003), using the k-value (Ceryan et al. 2008b, Ceryan 2011, Ceryan 2015) and applying chemical weathering indices (Price and Velbel 2003, Ceryan 2008, Ceryan et al. 2008a-b, Ceryan 2012, Gong et al 2013, Ceryan 2015).

The engineering behavior of rock materials depends not only on stress state and stress history but also on the physical, mineralogical and chemical change of the rock materials due to weathering. Weathering causes rock material to become more porous, individual mineral grains to be weakened and bonding between grains to be lost (Ceryan et al 2008a-b). Generally spoken, rock will lose strength and become more deformable and its permeability may change depending upon the nature of the rock, the presence and type of weathering products and the stage of weathering (Ceryan et al 2008a-b). The engineering weathering indices are based on these changes in the physical and mechanical properties of rock materials

In this study, the weathering concept was discussed from the different viewpoints and especially focused on chemical weathering indices and their usage. In addition, engineering weathering indices was given in detail.

Key Terms in this Chapter

Rock Material: The component of rock mass. Rock mass implies the all volume of the in-situ rock affected by engineering technical initiative. The in-situ rock, or rock mass, is comprised of intact blocks of rock separated by discontinuities such as joints, bedding planes, folds, sheared zones and faults. Rock materials is continuum of polycrystalline solid between the discontinuities consisting of an aggregate of minerals or grains. Its physical properties are reflecting chemical, mineralogical and petrographic characteristics. The engineering behavior of rock materials depends not only on stress state and stress history but also on the physical and chemical change of the rock materials due to weathering.

Durability: Defined as inverse of the alterability of rocks. The alterability of rocks is as a dynamic concept which refers to the ability of a rock to alter with time. Under weathering, this time is said to be geologic while when referring to durability, it is said to be engineering time, in other words, referring to all phenomena that take place simultaneously though the life of rock monuments and engineering constructions containing stone. Durability is an important characteristic for controlling not only the stability of slopes, surface and underground excavations but also that of a great number of stone structures, buildings and monuments. The durability of a building stone can be defined as the measure of the ability of natural building stone to endure and maintain its essential and distinctive characteristics of strength, resistance to decay and appearance in relation to a specific manner, purpose and environment of use.

Weathering: Disintegration or decomposition of rock in its natural or original position at or near the Earth’s surface through physical, chemical, and biological processes. Physical weathering involves the breakdown of rocks and soils through direct contact. Chemical weathering is the process including leaching and forming weathering product and is one of the vital processes in geochemical distribution of elements. During the chemical weathering process, some alkaline and alkali-earth elements are easily leached from rocks and the residue reconstituted with components from the atmosphere form new minerals that are in a stable or metastable equilibrium with the environment.

Strength and Deformation: The strength of a material may be broadly defined as the ability of the material to resist imposed forces. If is often measured as the maximum stress the material can sustain under specified loading and boundary conditions. For example, the compressive strength is then defined as the maximum load applied to crush the specimen divided by the cross-sectional area. In order to obtain the uniaxial or unconfined compressive strength of rock materials, a standard circular cylinder of the material is compressed between the platens of a testing. Application of stress causes a body of rock to yield or deform. The amount of deformation is called strain. Strain is defined as any change in original shape or size of an object in response to stress acting on the object. The basic deformation characteristics of rock materials are Young's Modulus and Poisson’s Ratio. Other Elastic Parameters can be Found from these Properties: Young's Modulus can be experimentally determined from the slope of a stress-strain curve obtained during compressional or tensile tests conducted on a rock sample.

Chemical Weathering Index: Formulation of the behavior of chemical elements controlled solely by the degree of weathering. Chemical weathering indices are widely used for evaluating elemental mobility during weathering, characterizing modern and ancient in situ weathering profiles and their interpretation, demonstrating the impact of climate on bedrock weathering, quantifying the engineering properties of regolith, categorizing soil and evaluating soil weathering, the evaluation of chemical overprinting of magmatism by weathering, assessing the deterioration mechanisms of monuments. Recently, weathering indices have been used as a palaeo-climatic indicators and for palaeo-environmental implications, for characterizing alterations associated with neotectonic delineations and the subsequent implications for provenance studies. e et al., 2006). In addition, chemical weathering indices provide input for prediction models to assess the engineering behaviors of rocks and predicting the durability and the weathering state of building stones

Weathering Indices: Used to identify physical, mineralogical and chemical change of the rock materials due to weathering. These indices based on index test, mineralogical, petrographical, chemical analysis and simple mechanical tests.

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