This kind of weathering is called spheroidal weathering. Chemical weathering:. The chemical decomposition of the rock is called chemical weathering which is nothing but chemical reaction between gases of the atmosphere and minerals of the rocks. The chemical changes invariably take place in the presence of water generally rainwater - in which are dissolved many active gases from the atmosphere like C02, nitrogen, Hydrogen etc.
These conditions are defined primarily by chemical composition of the rocks humidity and the environmental surrounding the rock under attack. Chemical weathering is essentially a process of chemical reactions between gases of the atmosphere and the surface rocks. For example:. Engineering importance of rock weathering:. As engineer is directly or indirectly interested in rock weathering specially when he has to select a suitable quarry for the extraction of stones for structural and decorative purposes.
The process of weathering always causes a lose in the strength of the rocks or soil. For the construction engineer it is always necessary to see that:.
To what extent the area under consideration for a proposed project has been affected by weathering and. What may be possible effects of weathering processes typical of the area on the construction materials.
Developed by Therithal info, Chennai. Toggle navigation BrainKart. Thus, the diagram in Figure 11 b relates the mean value for each of the samples with the weathering grade of the individual sample. As shown in this diagram, the mean reflection values of the samples increase and hence the gray-toned colors become lighter as the weathering of the dacite samples increases. As a result of X-ray diffraction analyses realized within the scope of this work, clay minerals such as mixed-layered clay minerals, kaolin, and vermiculite are observed in relation with the increase in the grade of weathering.
When all of these data are taken into account, it appears that the newly formed clay minerals play an important role in lightening the color of the samples lighter as the weathering grade increases. In the study area, different types of slope instability are present [ 42 ]. An engineering geological documentation map for the study area shows that failures of the slopes may be strongly controlled by the distributed strength reduction within the slopes due to weathering. This distribution may also affect the failure mechanism as failures were concentrated in the weathered dacites from Grades III to VI see Figure 2.
Translational and discontinuity controlled failures such as plane and wedge originate in these moderately and highly weathered dacites. The translational failures are deep seated and larger. Toppling failures and columnar collapses occur in Grade II rocks. Toppling, wedge, and plane failures are restricted to a single bench. Kaolin and smectite infilled discontinuities form the rear failure planes of the translational failures and failure planes of the discontinuity controlled failures.
Weathering resulting from drying-wetting cycles leads to the formation of fine debris resting on the foot of the slopes Figure 12 a , corresponding to the weathering Grade IV. On a large scale, the effects of weathering due to these cycles are observed as deterioration-related debris slides Figure 12 b in Grade IV dacites, especially in the upper parts of the mine slopes.
Frequent slope undercutting to create access roads exposes weathered dacites with corestones Grade V , creating conditions for corestones to detach and fall. Weathering Grades V and VI are associated with soil slips with rotational mechanisms, and Grade I rocks are not generally associated with failures in the study area. In order to provide accurate descriptions of the weathering stages developed in dacites, a rock mass weathering classification system for engineering purposes is proposed under the guidance of previous studies [ 21 — 23 , 29 , 43 — 49 ]; Table 5.
The system includes six weathering grades that are defined by observations and supplemented by index tests. Mass characteristics and material features are incorporated in the descriptions. The equation obtained from the relationship between the Schmidt hammer rebound values and weathering grades see Figure 3 was used to define the boundaries of the weathering grades in the proposed rock mass classification system.
Some conclusions can be drawn from the analyses presented in the current paper. An advance in weathering from slightly to highly weathered is generally accompanied by an increase in sericite, clay, hematite, and carbonate as well as a decrease in plagioclase microlites in the dacites.
In respect to clay mineralogy, the amount of clay minerals as a product of weathering increases significantly from Grades II to VI. The color of the samples becomes lighter with an increase in weathering due to newly formed clay minerals. Mixed-layer clay minerals, kaolinite, hematite, smectite, and vermiculite develop as the degree of weathering increases from Grade II to higher grade levels. Occurrences of these clay minerals may have been favored by impeded hydrological conditions.
The transformation of illite and chlorite may lead to a reduction in MgO and K 2 O with increased weathering. The decomposition of feldspar may result in a loss of Na 2 O and CaO which may indicate the existence of secondary products such as clay minerals produced by chemical weathering. The LOI reflects the content of weathered minerals and may be a good indicator of chemical weathering for the dacites. The results also indicate an increasing trend in porosity with progressive weathering which may be due to the increase in the formation of micro cracks as well as a decrease in the initial major elemental concentrations that are mainly controlled by the decomposition of plagioclase grains.
All of the investigated index and mechanical properties of the dacites in the study are closely correlated with the weathering grade. The direct use of these properties is promising for determining the extent of weathering. The translational and discontinuity controlled failures such as planes and wedges developed along kaolinite and smectite-infilled discontinuity surfaces in the dacites from Grades III and IV. Columnar collapses and toppling failures occurring along these discontinuity surfaces are common in Grade II dacites.
The breakdown of the rock due to the smectite content brought about by weathering leads to the formation of fine debris and contributes to the development of deterioration-related debris flow, circular failures, and corestone falls in the dacites from Grades IV to VI.
The authors would like to express their gratitude to the Mineral Research and Exploration Company of Turkey and the Geological Engineering Department of Hacettepe University for their permission to use the laboratories. Special thanks go to Geo engineer Serap Durmaz for thoughtful and constructive help during the study.
This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Academic Editor: D. Received 14 Feb Accepted 29 Mar Published 29 May Abstract The purpose of this study is to investigate dacites of different weathering grades from the Cakmakkaya and Damar open-pit copper mines in northeastern Turkey based on their mineralogical, petrographical, and geomechanical characteristics.
Introduction Weathering is an essential process that affects the mechanical properties of rock material and mass at shallow depths and on the surface through chemical and physical weathering. Figure 1. Figure 2. Typical weathering profiles developed in the dacites at the Cakmakkaya and Damar open-pit mines.
Figure 3. Relationship between Schmidt rebound values and weathering grades for the dacites. Table 1. Figure 4. High persistent joints a , relict structure as a discontinuity surface b , typical corestones in the dacites c. Figure 5. RQD versus weathering grade in the dacites a , borehole cores showing weathering of the dacites below the surface b. Table 2. Intensity of plagioclase microlites in groundmass and phenocrystals with respect to weathering grade.
Figure 6. Fresh dacite with an unweathered plagioclase phenocrystal showing cooling microfractures and opaque inclusions, and with unstained plagioclase microlites in abundance a , slightly weathered dacite with a slightly sericitized plagioclase phenocrystal and lesser amount of plagioclase microlites b , moderately weathered dacite with a plagioclase phenocrystal showing an increasing amount of sericite around the edges and center c , and highly weathered dacite with a plagioclase phenocrystal extensively weathered to clay minerals d.
Figure 7. Fresh dacite consisting of a great amount of unweathered plagioclase microlites and trace amount of sericite a , slightly weathered dacite showing a reduction in the amount of microlites and increase in the amount of slightly weathered sericite b , moderately weathered dacite showing almost sericitized microlites c , highly weathered dacite with completely sericitized microlites and replacement of these grains into groundmass d. Figure 8. Figure 9. Sample No.
Table 3. Major element analyses of the dacites with different weathering grades. Numbers in the parentheses indicate the mean values. Table 4. Summary of statistics for the index and geomechanical parameters of the dacites in different weathering grades. Figure Relationship between the engineering parameters and weathering grades of the dacites. A view of the differently weathered dacite samples a and variations in their threshold levels with weathering grades b. Fine debris pile at slope foot a and deterioration-related debris flows b.
Grade Rock mass description I No visible sign of weathering or slight discoloration on discontinuity surfaces. Rock material is very strong and highly difficult to break with a hammer. Feldspar phenocrystals cannot be scratched with a nail. Contacts between phenocrytals and matrix are distinct. II Ligth and dark orange brown discoloration with no inward penetration on discontinuity surfaces. Rock material can be broken with a couple of hammer blows. Edges of the specimens cannot be broken by hand.
Feldspar phenocrystals preserve their original shape, but they can be scratched with a nail. Toppling and columnar collapses are common. III Weathering products such as clay and carbonate are observed on discontinuity surfaces. Weathering is caused due to frost, heat, rainfall, and frequent variation of temperature during days and nights.
Weathering is felt when the texture of an object is worn out. There are several causes of weathering, which can be summarized in the following four reasons:. Chemical weathering means, the weathering and erosion caused due to the chemical action. Sulfate attack is an example of chemical weathering to the concrete and masonry structures. Chloride attack near off-shore buildings is also an example of chemical weathering.
In chemical weathering, the chemical compounds do not exert any physical pressure on the structure, but rather the chemical compounds react with the ingredients of the concrete and masonry structure. The chemical change brought about by means of chemical reaction loosens the strength of the structure. In order to prevent chemical weathering of the concrete and masonry structures, specified types of cement and chemicals are used in making concrete and mortar material.
Water freezes at a temperature of zero degrees Celsius. On lowering down its temperature, the volume of water reduces. Find useful content for your engineering study here.
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