Elsevier

Engineering Geology

Volume 108, Issues 1–2, 14 September 2009, Pages 36-42
Engineering Geology

Relative contribution of various climatic processes in disintegration of clay-bearing rocks

https://doi.org/10.1016/j.enggeo.2009.06.002Get rights and content

Abstract

The climatic processes of heating and cooling, wetting and drying, and freezing and thawing affect the disintegration characteristics of clay-bearing rocks (shales, claystones, mudstones, and siltstones) to varying degrees. Although heating and cooling, wetting and drying, and freezing and thawing are known to be the main processes responsible for physical disintegration of rocks under natural conditions, most of the previous investigators have used methods based only on water content variations (e.g., jar slake, slake index, and slake durability index tests) to assess the disintegration of clay-bearing rocks. Such assessments may not be adequate to explain the field behaviour of clay-bearing rocks subjected to a full range of climatic processes. In order to evaluate the combined effects as well as relative contributions of various climatic processes on the disintegration behaviour, samples of selected clay-bearing rocks, consisting of 5–6 particles, each weighing 85–150 g, were subjected to multiple cycles of heating and cooling, wetting and drying, and freezing and thawing. These treatments resulted in fragmentation of samples with fragments ranging from 50 to 2 mm and finer in dimensions. A new approach, referred to as the disintegration ratio, and defined as the area under the grain size distribution curve of the disintegrated material to the total area encompassing all grain size distribution curves of the samples, was used to account for fragmentation into varying sizes. Statistical analyses were performed to investigate the relationship between fragmentation, mineralogical composition, and physical properties.

Introduction

The clay-bearing rocks such as shales, claystones, mudstones, and siltstones are generally defined as fine to very fine-grained silici-clastic sedimentary rocks. These rocks are extensively exposed throughout the world and are the most frequently encountered rocks in engineering projects. They account for more than 60% of the sedimentary rocks, constitute approximately two-thirds of the stratigraphic column (Blatt, 1982), and cover one-third of the total land surface area (Franklin, 1983). The clay-bearing rocks are well known for their sensitivity to atmospheric processes. Dick et al. (1994) indicated that many clay-bearing rocks deteriorate rapidly when subjected to changes in moisture content, and this nondurable behaviour of clay-bearing rocks is responsible for numerous slope stability and underground excavation problems. The most significant and well known atmospheric processes responsible for rock breakdown are “heating and cooling”, “wetting and drying” and “freezing and thawing” (Hale and Shakoor, 2003). Although these three atmospheric processes are considered as important agents of physical breakdown under field conditions, most of the previous investigators (Deo, 1972, Franklin and Chandra, 1972, Wood and Deo, 1975, Cripps and Taylor, 1981, Taylor, 1988, Moon and Beattie, 1995, Koncagul and Santi, 1999, Dick et al., 1994) have only used laboratory methods based on water content variations to numerically determine the disintegration characteristics of the clay-bearing rocks. The most commonly known of these methods are jar slake, slake index, and slake durability index tests. Since only water-content variations are considered in these tests, it can be stated that such an evaluation of disintegration of clay-bearing rocks is not adequate to predict their field behaviour against climatic variations. It may be because of this deficiency in laboratory evaluation that Gamble (1971) emphasized the importance of studying the in-situ behaviour of clay-bearing rocks and concluded that more work was necessary to correlate laboratory results with field behaviour for fully understanding the disintegration behaviour of such rocks. During the last three decades, the slake durability index test (Franklin and Chandra, 1972) has been used to assess the durability of clay-bearing rocks by many investigators. However, in addition to the limitation related to mechanical disintegration, this test method only considers the wetting and drying process and, therefore, does not completely simulate the disintegration behaviour of clay-bearing rocks in the field.

In addition to the wetting and drying process, the heating and cooling and freezing and thawing processes may also be effective in causing deterioration of clay-bearing rocks. These atmospheric processes may act individually or in combination depending on the regional climatic conditions. For example, heating and cooling is dominant in some regions such as Kingdom of Saudi Arabia, Kuwait, Qatar, and Yemen where desert-like climatic conditions prevail with substantial difference in day and night temperatures. In some other regions, such as Turkey and U.S.A., continental climatic conditions are the dominating agents in rock fragmentation. Naturally, in these regions the winters are cold with frequent snowfall and freezing temperatures, springs are dominated by rainfall, and summers are characteristically hot. Therefore, when all of these seasons are considered, it is obvious that generally all three physical weathering processes (heating and cooling, wetting and drying, and freezing and thawing) influence the disintegration behaviour of clay-bearing rocks. Thus, the combined effects of these processes need to be considered in assessing the disintegration characteristics of clay-bearing rocks.

In order to evaluate the relative contribution of the various climatic processes, a research program was conducted with the following specific objectives:

  • 1.

    Investigate the effect of a selected number of heating and cooling, wetting and drying, and freezing and thawing cycles on the disintegration behaviour of four different types of clay-bearing rocks: shales, claystones, mudstones, and siltstones.

  • 2.

    Evaluate the relative contribution of each treatment cycle in the overall disintegration of the four types of clay-bearing rocks.

  • 3.

    Interpret the results of different treatment cycles in light of clay mineralogy, dry density, and absorption values of clay-bearing rocks.

The clay-bearing rock samples collected for a previous study by Hajdarwish (2005) were used to accomplish the above stated objectives. Because mineralogical and physical properties of these samples had been determined previously by Hajdarwish (2005), only the effect of heating and cooling, wetting and drying, and freezing and thawing cycles was investigated in this study. After these cyclic treatments, the clay-bearing rocks showed fragmentation ranging from 50 to 2 mm and finer in dimensions. A set of sieves was used to describe the disintegration characteristics of the rocks after each five cycles. Furthermore, a new parameter, referred to as the “disintegration ratio”, was used to assess the nature of fragmentation and investigate the relationship between fragmentation, mineralogical composition, and physical properties.

Section snippets

Physical properties and geological classification of selected samples

The locations of the samples selected from Hajdarwish's (2005) collection are given in Fig. 1. Hajdarwish (2005) used the classification system proposed in Potter et al. (1980) to classify his clay-bearing rocks. This classification system is based on percentage of silt and clay, presence or absence of laminations, and degree of metamorphism. The samples selected include 5 shales (SN: 6, 22, 31, 32, and 45), 5 claystones (SN: 10, 14, 17, 24, and 29), 5 mudstones (SN: 1, 3, 13, 18, and 28), and

Durability assessment

Shales, claystones, mudstones, and siltstones show rapid disintegration when exposed to atmospheric processes. The disintegration characteristics of these rocks depend on their physical and mineralogical properties such as the amount and type of clay minerals, dry density, absorption, porosity, presence of laminations, and geological age. A series of durability assessment methods (Gamble, 1971, Wood and Deo, 1975, Dick et al., 1994, Santi, 1998) have been suggested to classify the degree of

Laboratory investigations

Heating and cooling, wetting and drying, and freezing and thawing tests were performed on all 20 samples of clay-bearing rocks to simulate the effect of atmospheric processes in the laboratory. The wetting and drying and freezing and thawing tests were carried out in accordance with the ASTM, 1993a, ASTM, 1993b procedures D 5313 and D 5312, respectively. According to ASTM, 1993a, ASTM, 1993b specifications, the samples for these tests should consist of at least 5 pieces with each rock piece

Relationship between disintegration ratio and physical properties

Statistical analyses were performed comparing the disintegration ratio and the number of wetting and drying and freezing and thawing cycles. In these analyses, the number of cycles was taken as the independent variable, and disintegration ratio (DR) values for each treatment interval was considered as the dependent variable. The results of statistical analyses are summarized in Table 4, whereas Fig. 8 shows the variation of disintegration ratio with respect to the number of treatment cycles for

Conclusions

The following conclusions can be drawn from the above described research:

  • 1.

    Due to atmospheric processes, clay-bearing rocks disintegrate into fragments of different dimensions, ranging from 50 mm to less than 2 mm. Because of this wide range, disintegration behaviour of such rocks cannot be explained adequately by using only one sieve size.

  • 2.

    Compared to wetting and drying and freezing and thawing processes, the heating and cooling process causes negligible disintegration of clay-bearing rocks.

Acknowledgements

The authors would like to express their appreciation to Guzide Kalyoncu Erguler, mining engineer, General Directorate of Mineral Research and Exploration (MTA), Turkey, for her help in laboratory investigations and data analysis. Special thanks are due to Prof. Dr. Resat Ulusay of Hacettepe University, Turkey, for his valuable comments throughout this study. Thanks are also due to Dr. Paul Santi and an anonymous reviewer whose critical comments greatly improved the quality of this paper.

References (28)

  • KoncagulE.C. et al.

    Predicting the unconfined compressive strength of the Breathitt shale using slake durability, shore hardness and rock structural properties

    International Journal of Rock Mechanics and Mining Sciences

    (1999)
  • ASTM, 1993a. Standard test method for evaluation of durability of rock erosion control under wetting and drying...
  • ASTM, 1993b. Standard test method for evaluation of durability of rock erosion control under freezing and thawing...
  • ASTM

    Annual Book of ASTM Standards, Soil and Rock, Construction: v. 8, Section 4, West Conshohocken, PA

    (1996)
  • BirkelandP.W.

    Soil and Geomorphology

    (1984)
  • BlackwelderE.

    The insolation hypothesis of rock weathering

    American Journal Science

    (1933)
  • BlattH.

    Sedimentary Petrology

    (1982)
  • BloomA.L.

    Geomorphology

    (1997)
  • CrippsJ.C. et al.

    The engineering properties of mudrocks

    Quarterly Journal of Engineering Geology

    (1981)
  • Deo, P., 1972. Shales as embankment materials. PhD Thesis, Purdue University, West...
  • DickJ.C. et al.

    A geological approach toward developing a mudrock-durability classification system

    Canadian Geotechnical Journal

    (1994)
  • FranklinJ.A.

    Evaluation of shales for construction projects: an Ontario shale rating system

  • FranklinJ.A. et al.

    The slake-durability test

    International Journal of Rock Mechanics and Mining Sciences

    (1972)
  • Gamble, J.C., 1971. Durability–plasticity classification of shales and other argillaceous rocks, PhD Thesis, Geology,...
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