Adnan Aydin

Re-assessment of chemical weathering indices: case study on pyroclastic rocks of Hong Kong

Abstract Quantitative and objective characterization of weathered states of rock by means of chemical weathering indices has significant potential in ground investigation practice. This paper reviews more than thirty different chemical indices and re-assesses their variations along a weathered profile developed over pyroclastic rocks under subtropical conditions. Chemical weathering indices are principally based on the basic assumption that distributions of chemical elements as well as loss on ignition (LOI) content along the profile are mainly regulated by the degree of weathering. However, this study clearly demonstrates that the behavior of chemical elements and LOI during weathering depend not only on the degree of weathering, but also on microenvironmental conditions that control the type and abundance of clay minerals and sesquioxides and thus determine the level of heterogeneity of the profile. It is suggested that signatures of chemical weathering should be used to establish a framework for the interpretation of the heterogeneity level and the prediction of variability in engineering properties rather than for quick classifications using chemical weathering indices.

ISRM Suggested method for determination of the Schmidt hammer rebound hardness: Revised version ☆

With its portable, simple and affordable attributes, the Schmidt hammer (SH) is an ideal index apparatus, which underlies its increasing popularity and expanding range of applications. The SH rebound hardness value (R) is perhaps the most frequently used index in rock mechanics practice for estimating the uniaxial compressive strength (UCS) and the modulus of elasticity (E) of intact rock both in laboratory conditions and in situ. The SH is also widely used for estimating the (UCS) of discontinuity walls and assessing the workability, excavatability and boreability of rocks by mechanical means (cutting, polishing, milling, crushing and fragmentation processes in quarrying, drilling and tunneling).

Distribution of clay minerals along a weathered pyroclastic profile, Hong Kong

Abstract The distribution of clay minerals along a weathered profile developed over crystal-vitric tuffs are examined by X-ray diffractometry (XRD) and scanning electron microscopy (SEM) techniques. Chlorite and illite are the most abundant clay minerals in fresh and moderately decomposed rocks, respectively. Chlorite is limited to fresh samples, whereas illite and trace amounts of interstratified illite–smectite are ubiquitous along the profile. Halloysite and kaolinite are the dominant clay minerals in highly to completely decomposed rocks. Halloysite clusters in moderately decomposed samples provide clear evidence of crystallization from solution, whereas vermiform kaolinite books reveal evidence of transformation from illite in most samples. In general, as the intensity of weathering increases: (a) kaolin-to-illite and kaolinite-to-halloysite ratios increase; and (b) the morphology of halloysite varies from spheriodal clusters to tubular aggregates. It is concluded that the nature, type and abundance of clay minerals along the profile result from complex combinations of several different processes controlled mainly by three factors: the pre-weathering alteration history of parent rock, the degree of weathering, and microenvironmental conditions. The relative role of each factor can vary significantly within the same profile. As evidenced by mineralogical and petrographical characteristics, the nature and type of clay minerals in fresh and moderately decomposed rocks can be attributed to pre-weathering hydrothermal and/or deuteric alterations. On the other hand, the degree of weathering regulates the nature, type and abundance of clay minerals, particularly kaolin group minerals, in highly to completely decomposed rocks. Sesquioxides are sensitive to microenvironmetal conditions and can modify the distribution of clay minerals regardless of the degree of weathering of the rock.

Behaviour of chemical elements during weathering of pyroclastic rocks, Hong Kong

The behaviour of whole-rock major, trace and rare earth elements (REE) during weathering under subtropical conditions is examined along a profile developed over crystal--vitric tuffs with eutaxitic texture. The intensity of weathering within the profile varies erratically, indicating weathering processes operate over different scales. Quartz, K-feldspar, plagioclase and biotite are the main primary minerals, whereas clays, sesquioxides, sericite and chlorite are the alteration products. Kaolinite, halloysite and illite-mica are the dominant clay minerals present in significantly varying proportions. Two competing processes, namely leaching and fixation, are the main regulators of variations in mostly major and some trace element concentrations along the profile. In general, as the intensity of weathering increases, Ca, Na, K, Sr +/- Si decrease, while Fe, Ti, Al and loss of ignition (LOI) increase. Likewise, the intensity of negative Eu-anomaly decreases while the intensity of negative Ce-anomaly and the La/Lu and Sm/Nd ratios increases. In detail, however, the behaviour of chemical elements cannot be solely explained in terms of the degree of weathering. This study makes it clearly evident that the type and abundance of sesquioxides and clay minerals can significantly modify the geochemical signatures of weathering processes.

A Comparative Study of Particle Size Analyses by Sieve-Hydrometer and Laser Diffraction Methods

Combined sieve-hydrometer method (SHM) is one of the two conventional methods commonly used in research and practice in all branches of science and engineering dealing with soils. SHM, however, has some inborn defects, which lead to systematical errors. The paper discusses the major causes of such errors in the analysis with examples from saprolitic soils derived from granites and volcanics. It is concluded that the deviations may be explained by three factors: differences in definitions and measurements of particle sizes; variations in densities of constituent grains; and turbulence in flow pattern of suspension. Particle size distribution (PSD) curves produced by the laser diffraction method (LDM) generally agree with those of SHM. However, some discrepancies between the results of these methods become evident upon closer inspection, and vary in extent according to the soil type and specific size fraction. For the same soil sample and a given fraction, such discrepancies can be mainly attributed to variations in density, shape, and mineralogy of particles. LDM with its many advantages over SHM should be adopted as the standard in geoengineering and geoenvironmental research, particularly involving problematic soils.

Fate of heavy metal contaminants in road dusts and gully sediments in Guangzhou, SE China: A chemical and mineralogical assessment

ABSTRACT The urban environment of Guangzhou, the largest industrialized center in SE China, has elevated levels of heavy metals. In places, Cu, Pb, and Zn contents exceed 490, 920, and 1,820 mg/kg, respectively. The accumulation of these contaminants is likely to accelerate as a consequence of rapid economic and industrial growth in the region. Understanding of the possible fate of the contaminants is therefore imperative in order to assess their potential long-term ecological impacts. This article documents the results of a sequential extraction procedure involving five operationally defined fractions to determine the chemical partitioning of Cu, Pb, and Zn in the urban deposits represented by road dusts and corresponding gully sediments. Special emphasis was given to the mineralogical characteristics of the urban deposits. Road dusts were mainly composed of quartz, K-Feldspar, plagioclase, and calcite, and contained minor amounts of mica and clay minerals. The corresponding gully sediments, however, typically contained minor amounts of calcite, mica, and clay minerals, and were dominated by quartz and K-feldspar. The road dusts and gully sediments exhibited comparable chemical partitioning patterns of Cu, Pb, and Zn, despite significant differences in the relative abundances of minerals, especially of calcite. Lead and Zn occurred mainly in the operationally defined carbonate/ specifically adsorbed (Pb: 48%; Zn: 50%) and Fe-Mn (Pb: 36%; Zn: 27%) phases, whereas Cu was largely associated with the organic (70%) and residual (15%) phases. In general, the residual phases of the heavy metal contaminants were equal or less than 15%, suggesting their dominantly anthropogenic origin. The relative mobility and bioavailability of the heavy metals in the urban deposits of Guangzhou was: Pb ∼ Zn > Cu. The ever-increasing accumulations of heavy metals may pose a threat in the region both to the environment and to human health.

Chemical Heterogeneities of Weathered IgneousProfiles: Implications for Chemical Indices

This study investigates the behavior of chemical elements and the performance of chemical indices within and between weathered pyroclastic and granitic profiles. The profiles were developed under humid, relatively well-drained conditions in Hong Kong. Levels of chemical heterogeneity of the profiles are assessed, and their adverse effects on the selection and use of chemical indices are discussed. Distribution of the chemical elements varies significantly within and between the profiles. At the early stages of weathering, behavior of the elements is fairly predictable and largely controlled by the degree of weathering. At the advanced stages, however, behavior of the elements can be erratic and appears to be governed by micro-environmental conditions and the degree of weathering. Based on the nature of the chemical elements displaying unpredictable distribution along the weathered profiles, two groups of elements are identified. The first group includes K and Al together with loss-on-ignition content. Their variation can be explained in terms of the type and abundance of clay minerals. The second group comprises Fe and Mn, and their unexpected behavior can be correlated with the irregular distribution of sesquioxide-rich zones. Micro-environmental conditions, which govern the type and abundance of clay minerals and sesquioxides, ultimately determine the level of chemical heterogeneity of the profiles. Parent-normalized major element variation diagrams are useful tools for assessing the level of chemical heterogeneity of the profiles. Overall, the pyroclastic profile displays a higher level of heterogeneity than the granitic profile. Performances of chemical indices are particularly poor where a high level of chemical heterogeneity exists. Similarly, chemical indices based on the elements displaying similar geochemical behavior or the degree of decomposition of a single mineral (e.g., plagioclase) will not remain valid over the entire course of weathering. Given that each profile has its own way of responding to weathering processes, the search for a universally accepted ‘best chemical index’ is likely to disappoint. Recognizing the significance of the processes and products of weathering in controlling behavior of the chemical elements, however, and thus in generating chemical heterogeneities helps to avoid random or subjective selection of chemical indices and to interpret unforeseen deviations.

Microstructural study of a natural slip zone: quantification and deformation history

Abstract Microstructure of a natural slip zone was comprehensively examined using a combination of images captured systematically by optical microscopy (OPM) and backscattered electron microscopy (BEM) techniques. Microstructural features identified on these images were processed and evaluated using an advanced image analysis system, which proved that quantitative analyses could considerably enhance the understanding of shear behavior of slip zones. It was found that variations of porosity, abundance of platy clay particles and alignments of particles are significant indicators revealing nature of deformation processes. These indicators show that global mechanical behavior of the investigated slip zone can be conceptualized as that of normally consolidated clayey soils under drained conditions. The geometric patterns of the microstructure of the slip zone are similar to the S–C fabrics seen in tectonic shear zones. It is suggested that combined progressive bulk simple shear and pure shear modes enable to realistically reconstruct the kinematic history of the slip zone, through which particle movements and microstructural evolution were accomplished via various types of particulate flows. The results of this study show that clay mineralogy plays a more important role in the development of the slip zone than abundance of clay-size particles, while both clay mineralogy and relative proportions of each particle size fraction control the response of particles to shear deformation. Among the fractions present in the slip zone, fine silts are the strongest indicator of global shear stress characterized by their highest degree of alignment, whereas clay particles are the weakest. Highest degree of shape preferred orientation is also found within fine silt domains.

Deformation History of a Landslide Slip Zone in Light of Soil Microstructure

The microstructure of a slip zone from a recent landslide was investigated by combined optical microscopy (OPM) and backscattered electron microscopy (BEM) techniques. Along the toe of the slope, the slip zone consisted of two distinct segments, formed a) along a pre-existing slip zone and b) through completely decomposed granite (CDG), both of which comprise composite soils. The microstructural features were systematically captured, quantified, and analyzed using an advanced image-analysis system. It is shown that variations in density, porosity, presence of matching fragments of coarse particles, abundance of tabular aggregates and plates of kaolinite and particle alignment are significant indicators of the deformation history of the slip zone. The results indicate that multiple processes concurred in the course of deformation: compaction and dilation, cataclasis and comminution, and particle rearrangement. Such deformation features of the natural composite soils are fundamentally different from the relatively simple soils commonly used in laboratory studies. The extent of each process depends on the particle-size distribution and stress level that prevailed at time of failure. The particle movement and microstructural evolution of the slip zone could be described as particulate flow through simultaneous bulk simple shear and pure shear. Depending on their relative abundance, fine particles play an important but variable role in the development of the slip zone and the mobilization of its shear resistance to deformation. Reactivation and/or prolonged deformation evidently change the deformation behavior of the pre-existing slip zone. In general, shape-preferred orientation of particles occurs where they are obviously aligned.

Upgraded ISRM Suggested Method for Determining Sound Velocity by Ultrasonic Pulse Transmission Technique

Please send any written comments on this ISRM Suggested Method to Prof. Resat Ulusay, President of the ISRM Commission on Testing Methods, Hacettepe University, Department of Geological Engineering, 06800 Beytepe, Ankara, Turkey.