The Fate of Chromium in Weathered Ultramafic Rocks and Their Derivitative Soils in Cuba: Clues from Spectroscopic Studies

In its cationic,trivalent form,Chromium(Cr)it is a micronutrient,and exhibits low environmental mobility.In hexavalent form,however,it is a human carcinogen and also highly mobile.Climate is a key environmental factor controlling weathering rates and stability of primary and secondary Cr-bearing minerals.Knowledge of Cr oxidation state and mineral residence is therefore essential to estimating the risk posed by Cr in serpentinites,chromite mine wastes,and soils developed on these parent materials.X-ray absorption spectroscopy(XAS)is currently the best available technique for determination of the relative abundance of Cr(III)and Cr(VI)in situ(that is,without digestion of solid phases).A brief review of relevant XAS studies of is presented below,focusing on studies in tropical climates1,as they will be most relevant to eastern Cuba’s extensively serpentinized ophiolite belt.Cr(III)-bearing spinels are usually the dominant and most refractory Cr host in ultramafic rocks.Previous XAS studies2 indicate that in tropical climates,Cr-spinels weather rapidly to form Cr(III)-bearing secondary Fe(III)(hydr)oxides(goethite,hematite).Manganese(Mn)is also enriched in ultramafic rocks2;as Mn(IV),it can also co-precipitate with Fe(III)(hydr)oxides,or form its own secondary(hydr)oxides.A previous study found up to 20%Cr(VI)in in a tropical,serpentine soil that contained substantial Mn,and a strong correlation between the*amounts of Cr(VI)and Mn(IV)in the soil profile2.Theresults of several XAS studies suggest that a close association of Mn(IV)and Cr(III)in secondary Fe(hydr)oxides is necessary for oxidation of Cr(III)to Cr(VI)via electron transfer reactions with Mn(IV);however,additional XAS studies have shown that organic matter3and Cr-bearing aluminosilicates4 may also be important sources of Cr(III)to the environment under specific conditions.The stability and fate of Cr has not been studied in detail for these two host phase types,to the best of our knowledge.Access to XAS facilities to perform Cr geochemical experiments is limited and will only become more so in the future.We are working to develop and apply(micro)Raman spectroscopy to evaluate Cr oxidation state and mineral residence(in crystalline and amorphous materials).In addition to standard Raman scattering,we are employing resonance Raman(785 nm laser)to enhance signal from Cr(VI)-bearing phases and laser-stimulated photoluminescence to identify Cr(III)associated with Al-rich alteration products

Slope structures and formation of rock–soil aggregate landslides in deeply incised valleys

Rock–soil aggregate landslides(RSALs) are a common geological hazard in deeply incised valleys in southwestern China. Large-scale RSALs are widely distributed in the upper reaches of the Dadu River, Danba County, Sichuan Province, and are influenced by slope structure, which can be divided into open, lock, strip, and dumbbell types, as well as soil type and meso-structure, which can be classified as layered rock–soil aggregate, block-soil, and grainsoil. In this study, the evolution of four types of structures, such as layered-dumbbell, block-soil lock, banded block-soil, and block-soil open types, were analyzed by field surveys, surface and deep displacement monitoring, and Flac3 D. It was found that the Danba reach of the Dadu River showed incised valley through the evolution from wide to slow valley affected by internal and external geological processes since the Quaternary Glaciation. In the layered-dumbbell rock–soil aggregate, the main sliding pattern is multi-stage sliding at different depths. Circular sliding in the trailing edge and plane sliding along the bedrock in the front edge body occurin the block-soil-lock type aggregate. Large-scale multi-level and circular sliding over long distances occur in the banded block-soil aggregate. The blocksoil open type is stable, with only circular sliding occurring in local and shallow surfaces of the body. The monitoring and numerical simulation results further show that slope structure and regularity have diversified with RSALs. The results provide a basis for analyzing the stability mechanism of RSALs and preventing RSALs in deeply incised valleys.

Mechanical characteristics of soil-rock mixtures containing macropore structure based on 3D modeling technology

Soil-rock mixtures containing macropore(SRMCM)is a kind of geological material with special mechanical properties.Located in the project area of Lenggu hydropower station on the Yalong River,Sichuan Province,China,there is an extremely unstable Mahe talus slide with a total volume of nearly160 million cubic meters,which is mainly composed of SRMCM.The study on the mechanical properties of SRMCM is of great significance for the engineering construction and safe operation.In this paper,laboratory tests and discrete element numerical tests based on three-dimensional scanning technology were conducted to study the influence of stone content,stone size,and the angle of the macropore structure on shear characteristics of SRMCM.The failure mechanism of SRMCM was discussed from a microscopic perspective.This work explains the internal mechanism of the influence of stone content,stone size,and the angle of the macropore structure on the strength of SRMCM through the microscopic level of stone rotation,force chain distribution,and crack propagation.As the macropore structure that intersects with the preset shear plane at a large angle could act as a skeleton-like support to resist the shear force,the fracture of the weak cemented surface of soil and stone in the macropore structure is an important cause of SRMCM destruction.

A PERTURBATION BOUNDARY INTEGRAL EQUATION METHOD FOR ANALYSIS OF INTERACTION BETWEEN FOUNDATION AND NONLINEAR ROCK AND SOIL MEDIUM

A scheme is developed for analysing the interaction between afoundation and a nonlin- ear rock and soil medium, in which thefoundation is considered as a linear elastic body and a typicalboundary integral equation method (BIEM) is employed. On the basis oftaking the nonlinear proper- ties of the medium into account, aperturbation BIEM is developed. The fundamental equations for thenonlinear coupling analysis are formulated, and typical problems aresolved and discussed by the pre- sent method.

RIGID FINITE ELEMENT AND ITS APPLICATIONS IN ENGINEERING

Rigid Finite Element Method (RFEM) was proposed to simulate the mechanical behavior of discontinuous structures such as rock and soil structures. The authors' work on the theory and applications of RFEM is summarized in this paper. Based on the theory of RFEM, the Elastic Body-Seams Model (EBSM) is proposed to take the deformation and damage of rock masses into account.

DOUBLE-MEDIUM CONSTITUTIVE MODEL OF GEOLOGICAL MATERIAL IN UNIAXIAL TENSION AND COMPRESSION

Based on elasto-plasticity and damage mechanics, a double-medium constitutive model of geological material under uniaxial tension and compression was presented, on the assumption that rock and soil materials are the pore-fracture double-medium, and porous medium has no damage occurring, while fracture medium has damage occurring with load. To the implicit equation of the model, iterative method was adopted to obtain the complete stress-strain curve of the material. The result shows that many different distributions （uniform distribution, concentrated distribution and random distribution） of fractures in rock and soil material are the essential reasons of the daedal constitutive relations. By the reason that the double-medium constitutive model separates the material to be porous medium part, which is the main body of elasticity, and fracture medium part, which is the main body of damage, it is of important practical values and theoretical meanings to the study on failure of rock and soil or materials containing damage.

Influence of water and rock particle contents on the shear behaviour of a SRM

The contents of both water and rock particles are important factors affecting the mechanical strength of a soil–rock mixture(SRM)filled subgrade in the western mountainous area of China.Therefore,the purpose of this paper is to study the mechanisms of reconstituted landslide deposit samples with different water and rock particle contents by analysing the characteristics of shear strength,volumetric strain and‘jumping’phenomenon via large-scale direct shear tests.The results show that the influence of water content on shear strength is greater than the influence of rock particle content under a lower normal stress,and the results are reversed in the case of a higher normal stress.The effect of water content on the equivalent cohesion is bigger,especially for the sample with a high rock particle content.The friction angle of the specimen with same water content increases with the increasing rock particle content,but when the number of rock particles increases to a certain extent,there is a little effect on the friction angle.However,the friction angle decreases with increasing water content at the same rock particle content.Specimens with the same rock particle content change from dilation to compression with increasing water content.Finally,the continuous stage of the‘intense jumping’at different water content has been analysed.The‘jumping’phenomenon of samples with low water and rock particle content will first strengthen and then weaken the samples with increasing normal stress.

Practical configured microtremor array measurements(MAMs)for the geological investigation of underground space

Underground space utilization is becoming increasingly essential for modern metropolitan cities such as Singapore.Mapping a soil/rock interface using traditional borehole investigation methods is expensive and difficult,owing to the numerous physical constraints within a built-up city.Boreholes are often far apart,resulting in many unforeseen ground conditions during subsequent excavation.Geophysical methods are sometimes employed as possible alternatives for fast,economical,and efficient bedrock surveys.The goal of this study is to investigate the practical details of applying microtremor array measurement(MAM)as a non-invasive surface wave survey for mapping soil/rock interfaces in Singapore.Critical configurations in field data acquisition are examined,and practical recommendations for array construction are provided.In addition,30 in situ MAM tests are carried out for two major geological formations in Singapore.From the results,a standard shear wave velocity(V_(s))of 500 m/s is found to be suitable for interpreting the soil/rock interface,for the Bukit Timah Granite and Jurong formations.However,the method does not predict well when soft Kallang formation deposits are present.Other limitations are also discussed in the later parts of this paper.Conclusions and practical recommendations are discussed,providing constructive guidance to the industry.The proposed Vs-based method and associated guidelines and limitations can be used to create a digital geological database and are especially useful for rock profiling in an urban environment.