Impacts of karst phenomena on engineering properties of limestone foundation bed,Ar Riyadh,Saudi Arabia

Evaluation of mechanical and petrophysical properties of the karst limestone became essential to avoid future risks in the construction of new urban cities built on limestones. Therefore, this study aims to evaluate the impact of karsts phenomena on engineering properties of limestone foundation bed at Ar Riyadh in Saudi Arabia. Three hundred core plugs were obtained by rotary drilling at depths ranging from the ground surface to 20 m collected from 24 boreholes in two sites: (1) karst limestone (KL) at Al Aziziyah district and (2) massive hard limestone (HL) at Hittin district in Ar Riyadh city, Saudi Arabia. Petrographic, SEM, EDX, and XRD analyses are used to identify the mineralogical composition and microstructures of limestone samples. The petrophysical properties included the ovendry density, P-wave, and porosity where the mechanical properties covered the uniaxial compressive strength (UCS), point load strength index (PLI), and rock quality designation (RQD) for the karst and hard limestone samples. KL is characterized by 17.11% total porosity, 14.71% water absorption, 32.1 MPa UCS, 1.70 g/cm³ ovendry density (γ dry), 51% weathered RQD, 5.49 MPa medium shear strength, and low modulus of deformation of the plate loading test. HL showed 11.63% total porosity, 9.45% water absorption 43.1 MPa UCS, 2.50 g/cm³ ovendry density (γ dry), 78% hard to fresh rock affinity RQD, and 9.93 MPa high strength and high modulus of deformation of the plate loading test. For the water absorption (%), KL at Al Aziziyah district showed a range of 12.85–17.80% averaged 14.71%. HL at Hittin district varied between 7.04 and 11.29% with an average of 9.45%. KL proved to be dense with ovendry density (γ dry) averaged at 1.70 g/cm³ while HL showed very dense affinity of 2.50 g/cm³. KL clarified a UCS range from 22.5 to 40.1 MPa and an average of 32.1 MPa while HL showed a range from 35.4 t o 48.1 MPa with an average of 43.1 MPa. KL is moderately weathered with RQD average of 51% while HL showed a hard to fresh rock affinity of 78%. Point load test clarified a medium shear strength with 5.49 MPa for KL and high strength of 9.93 MPa for HL. Plate loading tests indicated low and high modulus of deformation for KL and HL, respectively. Results of petrographical analyses and XED of limestone samples showed that the strength parameters of samples mostly composed of micrite (mudstone/wackestone) and dolomite in hard limestone of Hittin district. In Al Aziziyah district (KL), the samples mostly consist of foraminifera and high amount of calcite as in karst limestone (wackestone/packstone). Rock mechanical tests with combination of fabric analyses have shown that strength parameters depend not only on the amount of karst but also on the amount of allochem. Major geomechanical differences between the two types of limestone provide the proper base for prioritizing areas to alleviate future risks and sustainable urban planning for decision makers. The karstic limestone, therefore, is considered as an acceptable foundation bed for light engineering structures. However, for heavy structures and buildings, improving the foundation bed strength by grouting, cement injection, and mat foundations is necessary to avoid future failure risks.     

Predicting mechanical properties and ultimate shear strength of gypsum,limestone and sandstone rocks using Vipulanandan models

ABSTRACT

In this study, over 1000 data from the literature was used to characterize and compare the density, strengths, modulus, fracture toughness, porosity and the ultimate shear strengths of the gypsum, limestone and sandstone rocks. The compressive modulus and Mode-1 fracture toughness of the gypsum rock, limestone rock and sandstone rocks varied from 0.7 GPa to 70 GPa, and from 0.03 MPa.m^0.5 to 2.6 MPa.m^0.5  respectively. Vipulanandan correlation model was effective in relating the modulus of elasticity, fracture toughness with the relevant strengths of the rocks. A new nonlinear Vipulanandan failure criterion was developed to quantify the tensile strength, pure shear (cohesion) strength and to predict the maximum shear strength limit with applied normal stress on the gypsum, limestone and sandstone rocks. The Vipulanandan failure model predicts the maximum shear strength limit was, as the Mohr-Coulomb failure model does not have a limit on the maximum shear strength. With the Vipulanandan failure model based on the available data, the maximum shear strengths predicted for the gypsum, limestone and sandstone rocks were 64 MPa, 114 MPa and 410 MPa respectively.     

The influence of binder/aggregate ratio on the pore properties and strength of repair mortars

Repair mortars with different B/Ag (binder/aggregate) ratios and natural stone were studied in order to understand the influence of limestone aggregate on the properties of mortar and to check the compatibility of repair mortar with a porous limestone. Three different types of mortars with three different aggregate contents were evaluated by using mercury intrusion porosimetry (MIP) and water saturation method (WSM). Changes in density and uniaxial compressive strength as a function of aggregate content were also recorded. For comparison the properties of the porous limestone were also tested, since it is used in monuments and also as aggregate in the mortar. Tested mortars had average porosities of 27.0 % (±2.0), while oolitic limestone has higher average porosity of 32.2 % (±1.9). The limestone has mainly medium and large pore radii (1–100 μm) while the repair mortars even with high aggregate content has predominantly smaller pores. All but one pure repair mortars have higher uniaxial compressive strength than that of the oolitic limestone. The strength decreases while porosity increases with increasing aggregate content.     

Numerical Investigation of the Dynamic Compressive Behaviour of Rock Materials at High Strain Rate

The dynamic compressive strength of rock materials increases with the strain rate. They are usually obtained by conducting laboratory tests such as split Hopkinson pressure bar (SHPB) test or drop-weight test. It is commonly agreed now that the dynamic increase factor (DIF) obtained from impact test is affected by lateral inertia confinement, friction confinement between the specimen and impact materials and the specimen sizes and geometries. Therefore, those derived directly from testing data do not necessarily reflect the true dynamic material properties. The influences of these parameters, however, are not straightforward to be quantified in laboratory tests. Therefore, the empirical DIF relations of rock materials obtained directly from impact tests consist of contributions from lateral inertia and end friction confinements, which need be eliminated to reflect the true dynamic material properties. Moreover, different rocks, such as granite, limestone and tuff have different material parameters, e.g., equation of state (EOS) and strength, which may also affect the DIF of materials but are not explicitly studied in the open literature. In the present study, numerical models of granite, limestone and tuff materials with different EOS and strength under impact loads are developed to simulate SHPB tests and to study the influences of EOS and strength, lateral inertia confinement and end friction confinement effects on their respective DIFs in the strain rate range between 1 and 1,000 s^?1. The commercial software AUTODYN with user-provided subroutines is used to perform the numerical simulations of SHPB tests. Numerical simulation results indicate that the lateral inertia confinement, friction confinement and specimen aspect (L/D) ratio significantly influence DIF obtained from impact tests and the inertia confinement effect is different for different rocks. Based on the numerical results, quantifications on the relative contributions from the lateral inertia confinement and the material strain rate effect on DIF of granite, limestone and tuff material compressive strength are made. The effects of friction coefficient, L/D ratio and rock type on DIF are discussed. Empirical relations of DIF with strain rate for the three rock materials representing the true material strain rate effect are also proposed.     

Stability assessment of marble statuaries of the Schlossbrücke (Berlin, Germany) based on rock strength measurements and ultrasonic wave velocities

The degree of weathering in natural stones on buildings and sculptures has been determined for many years in numerous cases by means of ultrasonic measurements. Conclusions concerning the strength of the rock and the type of weathering can thus be drawn. This relationship has not been established for all rock types. Most of the progress utilizing this method has been made in the analysis of marbles, where an increasing degree of weathering shows lower ultrasonic velocities. In the present study, four Carrara marble samples showing similar rock fabrics, but with respect to weathering exhibit considerable differences are investigated. Porosity varies between 0.2 vol. % and ca. 2.4 vol. %, whereby with increasing porosity the pore radii changes as well. Parallel to this the ultrasonic velocities change in dry samples from about 5.5 to 1.6 km/s, respectively. Model calculations reveal that the velocity reduction is caused by cracks with an extremely small aspect ratio of about 0.005 or even less. After a specific loss of strength, however, solution processes can become active, which modify the microcracks and generate an opposite trend. In the process a strong porosity increase correlates to a relatively small velocity reduction. With the presence of water the V _p porosity weathering relationship experiences a considerable modification. Parallel to the reduction of the ultrasonic velocities, it was determined that the mechanical strength (compressive strength, flexural strength, etc.) as well as the static Young’s modulus is reduced almost equally by a progressive advancement of the weathering front. In one case study dealing with tensile strengths, it was clearly documented how tensile cracks develop and propagate in dependence of the rock fabric. The rock mechanical and ultrasonic velocity data were used for stability assessments applied to the marble statuaries from the Schlossbrücke in Berlin. Stability assessments of the sculpture group 4 reveal that some critical parts must be replaced due to safety reasons.     

In-situ Rock Spalling Strength near Excavation Boundaries

It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch’s solution (with approximated circular tunnel geometry and hence σ _max = 3σ ₁ ?σ ₃) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ _max at the excavation boundary. The ratio of σ _max /UCS is related to the observed depth of failure and failure initiation occurs when σ _max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the “actual” in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic “as-built” excavation boundary condition is honored, the “actual” in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an “apparent” rock strength.     

Back analysis of a large landslide in a flysch rock mass

Flysch is a sedimentary rock consisting of a rhythmic alternation of hard (limestone, sandstone, siltstone) and weak (marl, mudstone, claystone) layers. Because of the presence of layers with different physical properties, the mechanical characterization of heterogeneous rock masses such as flysch is a real challenge. Different methods have been proposed in the literature to characterize flysch, combining empirical classification indexes with laboratory tests. Most of these methods, however, were specifically designed for tunneling and underground excavations, and their applicability to slope stability problems is not yet fully investigated. In this study, we analyze a large landslide in a cretaceous flysch rock in order to compare the mobilized strength at failure with those predicted by the modified GSI method (Marinos and Hoek, 2001). The landslide occurred in the Savena River basin (Northern Apennines of Italy) on April 6, 2013, with a volume of about 3 million m³. Soon after the failure, geological, geotechnical, and geophysical investigations were carried out to detect the failure mechanism and define the landslide geometry. Back analyses of the failed slope were performed using both limit equilibrium and finite difference methods to estimate the in situ strength of the flysch. The results show that the mobilized rock mass cohesion is very low (c ^'?≈?20?÷?40 kPa) and that the modified GSI method can predict the in situ strength only assuming a disturbance factor D = 1. Moreover, the analysis shows that the linearization criteria proposed in literature to compute the equivalent Mohr-Coulomb parameters remarkably overestimate the rock mass strength.     

Prediction of Engineering Properties of Basalt Rock in Jordan Using Ultrasonic Pulse Velocity Test

Basalt is an extrusive igneous rock derived from Lava and spread over different localities in Jordan. It can be used in industrial applications, and as construction materials. Before using basalt, it is essential to determine its dry density, porosity, uniaxial compressive strength, and Brazilian tensile strength. The testing procedure and sample preparation used to determine the engineering properties are time-consuming and need expertise. Hence, the ultrasonic pulse velocity (UPV) test, a quick and non-destructive evaluation method, was used to determine the engineering properties. Empirical relationships for determination of dry density, porosity, uniaxial compressive strength, Brazilian tensile strength, and tangent modulus of elasticity were deduced. Good correlation coefficients (R²?=?0.832–0.929) were obtained between UPV and dry density, porosity, uniaxial compressive strength, Brazilian tensile strength and modulus of elasticity. These correlations were limited to intact basalt with UPV?≥?4000 m/s.     

A Study on Crack Damage Stress Thresholds of Different Rock Types Based on Uniaxial Compression Tests

When rock samples are loaded until macroscopic fractures develop, the failure process can be divided into several stages based on axial and lateral strain responses or the acoustic emission sequence during uniaxial compression tests. Several stress thresholds may be identified: the crack closure stress σ _cc, crack initiation stress σ _ci, crack damage stress σ _cd, and uniaxial compressive strength σ _ucs; these may be used as a warning indicator for rock rupture. We investigated the crack damage stress σ _cd, its threshold, and a possible relationship between σ _cd and the uniaxial compressive strength. The σ _cd of different rock types were compiled from previous studies based on uniaxial compression tests. The results showed that the overall averages and standard deviations of σ _cd /σ _ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.11), ~0.85 (±0.11), and ~0.73 (±0.18), respectively. There were no significant differences in σ _cd /σ _ucs between the different rock types, except that the sedimentary rock had a slightly larger standard deviation attributed to the variation of porosity in the samples, while the metamorphic rock had higher average σ _cd /σ _ucs resulting from the small statistical sample size. By excluding the higher-porosity (>10 %) rock samples, the averages and standard deviations of σ _cd /σ _ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.09), ~0.85 (±0.09), and ~0.78 (±0.11), respectively. The results imply that the rock origin process (i.e., igneous, metamorphic, and sedimentary) has a minimal effect on σ _cd /σ _ucs. The ratio σ _cd/σ _ucs could be an essential intrinsic property for low-porosity rocks, which could be used in rock engineering for predicting the failure process.     

Experimental Anelastic Strain Recovery Compliance of Three Typical Rocks

The experimental determination of anelastic strain recovery (ASR) compliances for three types of rocks (granite, marble, and sandstone) was performed in the laboratory. Preloading of specimens for uniaxial compression creep tests was at 50 % of the uniaxial compressive strength (UCS) for each rock type. We obtained the shear mode Jas(t) and volumetric mode Jav(t) ASR compliances and calculated the ratio of Jas(t) to Jav(t). The Kelvin model for rock rheology was then applied in numerical simulations and the results were in good agreement with the measured data for Jas(t) and Jav(t). These results showed that both the magnitude and rate of increase of the ASR compliances are strongly dependent on the rock type, and the values of the Jas(t)/Jav(t) ratio for a loading of 50 % of the UCS showed a trend leading to different constants for each of the three rock types. Further experimental and numerical analyses showed approximate power-law relationships between the ASR compliances at 50 % of UCS, and both the UCS and the tangential Young’s modulus at 50 % of UCS (E _t50). These relationships may be useful for the preliminary estimation of ASR compliances.     

Petrophysical properties, composition and deterioration of the Calatorao biogenic stone: case of the sculptures masonry of the Valley of the Fallen (Madrid, Spain)

The huge sculptures placed outdoors in the Valley of the Fallen Memorial Park (El Escorial, Madrid) made with blocks of Black-Limestone from Calatorao-Zaragoza, Spain (BLCZ) and disposed on a concrete core exhibit weathering traces, flaking, saline efflorescence and falling fragments, currently represent a danger for visitors. Frost action is important in the Valley of the Fallen by the large number of freeze–thaw cycles produced during Sculptures‘live under a temperate Mediterranean climate with severe seasonality. The formation of fissures facilitates the water transport within the rock and the salt- and ice-induced deterioration. Temperate climates with frequent freezing and thawing cycles can be the most effective drivers of the visible physical weathering. In order to propose a suitable weathering model, collected black-limestones from sculptures and Calatorao quarries were analyzed by optical microscopy, environmental scanning electron microscopy with energy dispersive spectrometry (ESEM-EDS), inductively coupled mass spectrometry (ICP-MS) and X-ray diffraction. Mercury intrusion porosimetry (MIP), nitrogen absorption and helium pycnometry techniques were used for pore analyses of the BLCZ micro-blocks (10 × 10 × 10 cm) described in terms of pore size distribution, pore volume and specific surface area. The appreciable amount of organic matter was isolated by solvent extraction, acid treatment, flotation and perborate degradation followed by Gas Chromatography–Mass Spectrometry (GC–MS), Analytical Pyrolysis (Py-GC/MS), Fourier Transformed Infrared Spectroscopy and Raman techniques. Both weathered and fresh BLCZ samples contained more than 90 % calcite shells with circa 10 % of pyrite (fresh samples) or iron hydroxides (weathered samples), quartz grains, claystone and fossil organic matter consisting of a condensed matrix with polyalkyl chains and polycyclic methoxyl-lacking aromatic structures. The petrophysical analyses revealed volumes of pores, sized <0.025 μm obtained by N₂ adsorption, of 3.18 × 10^?3 cm³ g^?1 while the measured porosity by MIP in the pore range from 0.005 to 200 μm was 3.30 × 10^?3 cm³ g^?1. These data could be explained by the existence of clay minerals and organic matter in the pore system less than 50 nm of diameter. Concerning BLCZ deterioration it was found that the porous framework of BLCZ was filled with sulphates formed from artificial cement observed in the sculptures inside trough a testing hole and from its intrinsic pyrite. The results suggested that although biological processes were not major agents in rock deterioration, there was also weak compatibility between sculptures‘constituents, (limestone, concrete and oxidized iron clamps) which under, continental Mediterranean conditions, were continuously releasing weathering compounds accelerating disruption of the cut-stone sculptures.     

Polarized electronic absorption spectra of colourless chalcocyanite,CuSO4, with a survey on crystal fields in Cu2+ minerals

Polarized electronic absorption spectra of colourless chalcocyanite, CuSO₄, have been measured using microscope-spectrometric techniques. The spectra are characterized by a structured and clearly polarized band system in the near-infrared spectral range with components centred at 11,720, 10,545, 9,100, and 7,320 cm^?1, which have been assigned to crystal field d–d transitions of Cu²⁺ cations in pseudo-tetragonally elongated CuO₆ polyhedra with point symmetry C _i ( \(\bar{1}\) ). The polarization behaviour is interpreted based on a D ₂(C ₂″) pseudo-symmetry. Crystal field calculations were performed for the actual triclinic point symmetry by applying the Superposition Model of crystal fields, as well as in terms of a ‘classic’ pseudo-tetragonal crystal field approach yielding the parameters Dq _(eq) = 910, Dt = 395, and Ds = 1,336 cm^?1, corresponding to a cubically averaged Dq _cub = 679 cm^?1. A comparative survey on crystal fields in Cu²⁺ minerals shows that the low overall crystal field strength in chalcocyanite, combined with a comparatively weak pseudo-tetragonal splitting of energy levels, is responsible for its unique colourless appearance among oxygen-based Cu²⁺ minerals. The weak crystal field in CuSO₄ can be related to the lower position of the SO₄ ^2? anion compared to, e.g. the H₂O molecule in the spectrochemical series of ligands.     

Application of statistical methods for predicting uniaxial compressive strength of limestone rocks using nondestructive tests

Uniaxial compressive strength (UCS) of an intact rock is an important geotechnical parameter for engineering applications. Using standard laboratory tests to determine UCS is a difficult, expensive and time-consuming task. The main purpose of this study is to develop a general model for predicting UCS of limestone samples and to investigate the relationships among UCS, Schmidt hammer rebound and P-wave velocity (V _P). For this reason, some samples of limestone rocks were collected from the southwestern Iran. In order to evaluate a correlation, the measured and predicted values were examined utilizing simple and multivariate regression techniques. In order to check the performance of the proposed equation, coefficient of determination (R ²), root-mean-square error, mean absolute percentage error, variance accounts for (VAF %), Akaike Information Criterion and performance index were determined. The results showed that the proposed equation by multivariate regression could be applied effectively to predict UCS from its combinations, i.e., ultrasonic pulse velocity and Schmidt hammer hardness. The results also showed that considering high prediction performance of the models developed, they can be used to perform preliminary stages of rock engineering assessments. It was evident that such prediction studies not only provide some practical tools but also contribute to better understanding of the main controlling index parameters of UCS of rocks.     

The role of microalgae and their carbonic anhydrase on the biological dissolution of limestone

This study aims to investigate the effect of microalgae and their carbonic anhydrase (CAex) on limestone dissolution. The dynamics of Ca²⁺ and Mg²⁺ release, the model for the amount of Mg²⁺ released and biological cumulative effect time by the microalgae Chlamydomonas reinhardtii (CR) and Chlorella pyrenoedosa (CP), and the algal stable carbon isotopic composition (δ¹³C) in the presence and absence of the membrane-impermeable CAex inhibitor acetazolamide (AZ) were compared in a medium containing limestone. The amount of Mg²⁺ released from the limestone in the treatment without AZ was more than that with AZ during the logarithmic phase. The amounts of Mg²⁺ release unit algal biomass and unit time in CR and CP were 3.37 × 10^?4 and 2.44 × 10^?4 mg/μg days in the treatment without AZ, respectively, and only 1.99 × 10^?4 and 2.19 × 10^?4 mg/μg days in the treatment with AZ, respectively. The biological dissolution of the algae increased with increasing algal CAex activity. The variation of Ca²⁺ was influenced by reprecipitation, and the algal limestone dissolution cannot be shown distinctly. The CAex of the microalgae may be beneficial for CaCO₃ reprecipitation, and the δ¹³C values of the algal cells with AZ were lower than those without AZ. Therefore, AZ not only can inhibit limestone dissolution by inhibiting microalgal growth, but also can reduce limestone dissolution by decreasing CAex catalysis. The results suggest the important influence of microalgae and their CAex on the biokarst process.     

Detailed comparison of nine intact rock failure criteria using polyaxial intact coal strength data obtained through PFC<Superscript>3D</Superscript> simulations

Study of intact rock failure criteria is an important topic in rock mechanics. In this study, applicability of nine different intact rock failure criteria is investigated for intact coal strength data. PFC^3D modeling was used to simulate the laboratory polyaxial tests for cubic intact coal blocks of side dimension 110 mm under different confining stress combinations. A modified grid search procedure is proposed and used to find the best-fitting parameter values and to calculate the coefficient of determination (R ²) values for each criterion. Detailed comparisons of the nine criteria are made using the following aspects: R ² values, σ ₁ ? σ ₂ plots for different σ ₃, shapes on the deviatoric plane, linearity or nonlinearity on the meridian planes. Through the comparisons of R ² values, σ ₁ ? σ ₂ plots and meridian lines, the modified Wiebols–Cook and modified Lade criteria were found to fit the intact coal strength data best. The nine failure criteria are categorized into three types based on the appearances on the deviatoric plane.     

Is there Link between the Type of the Volumetric Strain Curve and Elastic Constants, Porosity, Stress and Strain Characteristics ?

The stress [crack damage stress (σ _cd) and uniaxial compressive strength (σ _c)] and strain characteristics [maximum total volumetric strain (ε _cd), axial failure strain (ε _af)], porosity (n) and elastic constants [elastic modulus (E) and Poisson’s ratio (ν)] and their ratios were coordinated with the existence of two different types (type 1 and type 2) of volumetric strain curve. Type 1 volumetric strain curve has a reversal point and, therefore, σ _cd is less than the uniaxial compressive strength (σ _c). Type 2 has no reversal point, and the bulk volume of rock decreases until its failure occurs (i.e., σ _cd = σ _c). It is confirmed that the ratio between the elastic modulus (E) and the parameter λ = n/ε _cd strongly affects the crack damage stress (σ _cd) for both type 1 and type 2 volumetric strain curves. It is revealed that heterogeneous carbonate rock samples exhibit different types of the volumetric strain curve even within the same rock formation, and the range of σ _cd/σ _c = 0.54–1 for carbonate rocks is wider than the range (0.71 < σ _cd/σ _c < 0.84) obtained by other researchers for granites, sandstones and quartzite. It is established that there is no connection between the type of the volumetric strain curve and values of n, E, σ _cd, ν, E/(1 ? 2ν), M _R = E/σ _c and E/λ. On the other hand, the type of volumetric strain curve is connected with the values of λ and the ratio between the axial failure strain (ε _af) and the maximum total volumetric strain (ε _cd). It is argued that in case of small ε _af/ε _cd–small λ, volumetric strain curve follows the type 2.     

Geochemical alterations in surface waters of Govind Ballabh Pant Sagar,Northern Coalfield,India

Major ions showed high concentrations, ionic strength and chemical activity in the surface waters of Govind Ballabh Pant Sagar reservoir. Various geochemical ratios showed the dominance of silicate over carbonate weathering and major ions such as Na⁺ + K⁺ account for about 52 % of the cation budget. The high Na⁺ and K⁺ showed sedimentation of rock/coal particles consisting of highly weathered silicate minerals contributed by the discharge of mine water, fly ash mixing during transportation, etc. Further, Ca²⁺ + Mg²⁺/Na⁺ + K⁺ ratio was <1 (0.92) indicating the occurrence of silicate weathering in the reservoir catchment. The comparative assessment showed that the proportion of Ca²⁺ + Mg²⁺/Na⁺ + K⁺ tends to be lower along the coal mining belts compared to non-coal mining regions in the world. The Ca²⁺/SO₄ ^2? ratio <1 revealed not only H₂CO₃ but H₂SO₄ also acting as a source of protons for rock weathering. The cause underlying these differences can be related directly to geological substrate and anthropogenic activities.     

Log-derived petrophysical characteristics and oil potentiality of the Upper Qishn Clastic Member,Masila Basin,Yemen

The Lower Cretaceous Qishn Formation at Masila Basin, Yemen, contains substantial amounts of proven crude oil. It is divided into a lower shaly silisiclastic member and an upper carbonate member. In the present work, detailed formation evaluation of the Upper Qishn Clastic Member was carried out using the digitized well logging data from 12 wells, via computer-assisted analysis, to accomplish its reservoir characteristics and oil potentiality. Accordingly, the true resistivity (R _t), porosity (total φ _t and effective φ _e), shale content (V _sh), permeability (K), water saturation (S _w), hydrocarbon saturation (S _h) (movable “S _hm” and residual “S _hr”) and lithologic composition in each well were estimated, analyzed, and illustrated versus depth on litho-saturation cross-plots. Likewise, the areal distribution of such parameters alongside the member in the investigated area and their relations to geological setting were defined from the constructed isoparametric contour maps. Based on the obtained results, the components of Upper Qishn Clastic Member of Masila Basin are mainly shaly sandstone and minor calcareous sandstone with considerable amount of shale. Regionally, the Member was deposited on an inner neritic to shallow-marine platform setting. It is interpreted as a good-quality reservoir rock which has been established from high total porosity (15–22 %), good effective porosity (8–13 %), and variable shale volume (24–31 %). A number of high hydrocarbon saturation zones (exceeding 55 %) are detected through intervals having S _w?<?60 % and S _hm?>?20 % which have been considered as economic oil producers. The storage and pay capacities of the reservoir intervals have been resolved. The obtained results were confirmed from the available data of core analyses and production tests. The Upper Qishn Clastic Member reveals promising reservoir characteristics which should be taken into consideration during future development of the oilfields in the area.     

The Effect of High Temperature on Tensile Strength and Damage Characteristics of Limestone

In this paper, the limestone specimens are heated from room temperature 25 to 800 °C in a high temperature furnace and then are subjected to Brazilian test with the AG-I250 electronic precision material testing machine. The physical properties, mechanical properties, disc failure pattern, energy absorbed per unit area and damage characteristics of disc are comprehensively investigated. The results show that: with the increase of temperature, the changing trends of tensile strength, peak strain, tensile modulus and accumulated energy absorbed per unit area of disc are similar, they are first increases, then decrease, the energy consumption index is consistent with the macroscopic damage characteristics; the value of ε_s increase first and then reduce, reaches the maximum at 600 °C. The value of n is increasing and fluctuating, but the change trend of D_c is opposite, which is decreasing and fluctuating. The slope of the damage variable-strain curves decreases first and then increases, the minimum value at 600 °C. This study is of significance to the prediction and evaluation of the stability and safety of rock mass post-high temperature.     

胶西北寺庄金矿床红化蚀变过程及其对金成矿贡献

胶东是我国最重要的金矿集区,其内金矿床均赋存于沿NE-NNE向断裂带展布的大规模红化蚀变带中;然而对红化蚀变是钾长石化还是赤铁矿-金红石化,及其对金成矿的贡献尚存争议。寺庄超大型金矿床的红化蚀变沿NE-NNE向焦家断裂带及其次级断裂-裂隙系统发育,占已探明资源储量70%的Ⅲ号矿体群即赋存于红化蚀变带内,是研究红化蚀变与金成矿关系的理想对象。本文以该金矿床红化蚀变花岗岩为研究对象,通过对比新鲜花岗岩与强、弱红化蚀变岩内矿物组合和地球化学组成,探讨红化蚀变对于金成矿的贡献。矿物学研究表明,弱红化蚀变岩内的蚀变发生在斜长石核部,以钠长石化为主,同时形成绢云母和少量热液钾长石,且赤铁矿在此阶段沉淀;而弱红化蚀变岩进一步水岩反应成为强红化蚀变岩的过程中出现大量热液钾长石。质量平衡计算表明,红化蚀变过程中SiO_2、K_2O迁入,而Na_2O、CaO、Al_2O_3、FeO~T、MgO迁出;红化流体由早期富Na向后期富K转变。岩石地球化学与氢氧同位素综合示踪显示,红化流体为高温、高氧逸度、富K的玲珑岩浆期后热液,与胶东金矿床中-低温、还原性、富CO_2成矿流体性质相反,表明红化流体未直接参与成矿过程。综合研究揭示,流体交代斜长石后形成贯通性孔隙提高红化蚀变岩的渗透性;热液钾长石交代斜长石导致岩石体积膨胀而破裂,降低岩石抗压强度;这些为成矿期断裂活动以及成矿流体的运移和成矿物质的沉淀提供了极为有利的围岩条件,可能是巨量金聚集成矿的关键因素之一。