Jointed Basalt Research Articles

Evaluation of the source and mechanisms of groundwater recharge for the southern sections of the western Afar rift margin and associated rift floor

Study areaSouthern Sections of Western Afar rift margin and associated rift floor. Study focusThe purpose of this study is to identify the source and mechanisms of groundwater recharge based on geological, hydrochemical, and environmental isotope studies. New hydrogeological insightsThe investigation illustrate that the columnar jointed basalt was extended from the plateau to the rift margin. In addition, transverse structures trending NW-SE and NE-SW connect the plateau to the rift margin and the marginal grabens to the rift floor. The hydrochemical interpretations with EC and TDS shows that the groundwater from the plateau has evolved from Ca-dominated and slightly mineralized to Na-dominated and highly mineralized to the rift floor. Isotopically, the deep groundwater systems are depleted in the entire physiography in contrast to the shallow groundwater systems. Radon measurements are higher at the outlets of the marginal grabens and between the rift margin and the rift floor, suggesting groundwater feeds the river. A comprehensive analysis of the aforementioned results suggests that the deep circulating groundwater is recharged in the plateau and escarpment and chemically altered with increasing depth and along its preferential flow path into the rift floor. Therefore, the plateau area is the main source of recharge for the western Afar rift margin and associated rift floor, due to the presence of preferential pathways, mainly the columnar jointed basalts and cross-structures.

The Energy Characteristics of Columnar Jointed Basalt Size Effect Considering the Occurrence Sequence and Magnitude of Micro-cracks

The Energy Characteristics of Columnar Jointed Basalt Size Effect Considering the Occurrence Sequence and Magnitude of Micro-cracks

Experimental study on damage evolution process, seepage characteristics, and energy response of columnar jointed basalt under true triaxial cyclic loading

The physical response of Columnar Jointed Basalt (CJB) under a stress–seepage coupling field is of great significance for dam safety and oil and gas reservoir evaluation. A true triaxial experimental machine was used for investigating the deformation, damage evolution characteristics, seepage properties, and energy response of the CJB. Before the start of the test, a series of uniaxial compression tests were used to determine the ratio of similar materials, and the cyclic loading test results indicate that: The strength characteristics of CJB decreases slowly and then increases rapidly as the dip angle β varies from 0° to 90° under two loading conditions of the tiered cyclic loading (T–C) and the constant amplitude cyclic loading (CA–C), showing an obvious U–shaped trend, and four different loading stages can be distinguished by characteristic stress. In the yield stage, the stress–strain curve shows an obvious plastic hysteresis loop, there is a strong correlation between the failure modes of the CJB and β of the specimen, the joint surface gradually deteriorates owing to loading and unloading conditions, and structural control failure occurs widely in specimens. The damage evolution process is discussed in combination with the Load–Unloading Response Ratio, and the relative damage amount (Df) is defined by cycle index. The permeability–axial stress relationship of the specimen conforms to the power exponential relationship, and the permeability evolution characteristics are strongly correlated with the degree of damage. The energy response of the CJB were investigated by tress–strain curve, under two loading modes, T–C and CA–C, the cumulative curves of dissipation energy Ed can be fitted by an exponential function and a logarithmic function, respectively. The findings of this research hold immense importance for clarifying the mechanical and seepage properties of CJB under a stress–seepage coupling field.

Investigation on the failure mechanism of the collapse of the columnar jointed basalt in underground cavern

Columnar jointed basalt (CJB) is a kind of jointed rock with a polygonal cylinder mosaic structure that has complex mechanical properties such as discontinuity and heterogeneity. The typical geological structure of the CJB is the intercolumnar joint plane and the implicit joint plane, which obviously affect the mechanical properties of the rock mass. Controlling the unloading relaxation of the CJB is a key problem during the construction of underground engineering. In this paper, in-situ acoustic wave and panoramic borehole camera measurements were carried out in the cavern of the Baihetan project to understand the failure mechanism of the collapse of the CJB. It was quite clear that the evolution of the excavation damage zone (EDZ) of the CJB depends on the time and spatial effects. The closer to the collapse zone, the greater the degree of relaxation failure of the columnar joint rock mass; the further away from the cavern perimeter, the more stable the surrounding rock. The correction between wave velocity and cracks in the rock mass was also discussed. This field test and theoretical analysis can provide a reference for studying the failure mechanism and control measures of CJB in underground caverns under high geostress.

Petrogenesis and tectonic setting of the mafic rocks from the Mfengou-Manki area, Central Cameroon Shear Zone: constraints from petrology and bulk-rock geochemistry

In the Central Cameroon Shear Zone, several studies were focused on granitoids and very few on mafic rocks. Here we report the petrography, geochemistry and mineralogy of the Mfengou-Manki mafic rocks in order to constrain their petrogenesis and tectonic settings and the role of lithospheric and asthenospheric mantle sources in their genesis. The studied mafic rocks are subdivided into columnar jointed basalts and mafic dykes. Clinopyroxene thermobarometry indicates that the mafic dykes crystallized at a temperature of 1071 to 1193 °C and a pressure of 4 to 12 kbar while the columnar jointed basalts emplaced at a temperature of 1064 to 1152 °C and 2 to 13 kbar pressure. The mafic dykes and columnar jointed basalts present high La/Sm, Sm/Yb, Nb/Yb and Th/Yb ratios, indicating garnet to spinel transition zone mantle source. The multi-element diagram of the mafic dykes display enrichment in Nb, Ta, Pb and Ti and depletion in Th, U, Ce and Zr compared to that of the columnar jointed basalts (slight depletion in Nb and Ta and pronounced depletion in U, Pb and Zr and enrichment in Cs, Ba and Rb) indicating the little involvement of the sub-continental lithospheric mantle to the formation of the columnar jointed basalts. The Nb/La ratio > 1 for the mafic dykes and < 1 for the columnar jointed basalts also suggest the derivation of the mafic dykes from the asthenospheric mantle and the columnar jointed basalts from the mixed lithospheric-asthenospheric mantle due to the sub-continental lithospheric mantle delamination under the Central Cameroon Shear Zone.

Investigation into the Multistage Mechanical Damage Behavior of Columnar Jointed Basalts with Different Meso-Constitutive Relations and Model Sizes

Abstract The mechanical characteristics of columnar jointed basalts (CJBs) are generally considered to be subject to size effects. They display complex mechanical behaviors under lateral pressure, which may be related to the variation in the rock's mesoproperties. In this study, many nonuniform CJB models with various rock meso-constitutive relations and model sizes were established, and their influences on the strength and deformation properties, multistage mechanical damage behavior, and acoustic emission characteristics of CJBs were investigated. The results show that, as the residual strength coefficient increases, the compressive strength of CJBs rises, and the equivalent deformation modulus of CJBs decreases or increases slightly; with an increase in the model size, the compressive strength slightly decreases at first and then varies moderately, and the equivalent deformation modulus shows a fluctuating trend. Furthermore, the macro stress–strain relationships can be divided into different stages according to the corresponding damage behaviors. The strains and energy accumulations before peak stress was reached were further analyzed. The results greatly improve our understanding of the collapse process of CJBs and the instability precursors of related structures.

Improved polygonal constitutive model for columnar jointed basalt and its application in tunnel stability analysis

The columnar jointed basalt (CJB) in the dam site of the Baihetan hydropower station exhibits noticeable geometric anisotropy. The current constitutive relationship based on a quadrangular columnar structure fails to truly represent the natural characteristics of the CJB. To address the stability problem of the diversion tunnel at the dam foundation, the mechanics of composite materials and Goodman superposition principle were employed to establish improved three-dimensional constitutive relations for pentagonal and hexagonal CJBs. The constitutive relations of three types, including the quadrangular prism CJB, were optimized by considering the impact of transverse joints and internal staggered zones. The mechanical parameters of the Baihetan CJB were appropriately evaluated using the three constitutive relations, and the achieved results demonstrated that the determined elastic moduli of the three improved constitutive models were consistent with field results. Subsequently, the three anisotropic constitutive models were employed for numerical simulations of the diversion tunnel excavation in the Baihetan CJB stratum. Strain field tests were conducted at different locations within the tunnel, and an empirical method was proposed that combined statistical data on the shape of CJBs at the site, which can effectively predict the deformation of the surrounding rock. The theoretical and numerical results provide crucial reference values for engineering optimization design.

Size effect and lateral pressure effect on the mechanical resistance of columnar jointed basalt

To reveal the size effect and lateral pressure effect of columnar jointed basalts (CJBs), the meso-damage mechanics, statistical strength theory, continuum mechanics and digital image correlation are combined, and a series of heterogeneous numerical models of CJBs orthogonal and parallel to column axis are established. The elastic modulus, Poisson's ratio, uniaxial compressive strength, friction angle, residual strength coefficient, heterogeneity index of basalts 60 GPa, 0.2, 120 MPa, 56.15°, 0.1 and 5, respectively. The gradual fracture processes and acoustic emission characteristics of CJBs suffering various lateral pressures are numerically simulated under the axial loading rate of 0.05 mm/min, and the influence of model size on the anisotropy and lateral pressure effect of CJBs is analyzed. The results show that: for the direction I/Ⅱ orthogonal to column axis, when the lateral pressure is 2 MPa and 6 MPa, the critical value of size effect is 4 m and 6 m, respectively; for the direction parallel to column axis, the compressive strength of specimen can be obviously improved by increasing lateral pressure for the certain sizes; when the lateral pressure is 6 MPa and the distance ratio of the secondary joint set is 0% or 50%, the compression strengths of the 3 m and 6 m specimens change in a U-shape and a V-shape with the column dip angle increasing, respectively. The results can contribute to understanding the non-linear deformation and failure behaviors of CJBs.

Case Study on the Method of Rock Mass Overburden Consolidation Grouting for Dam Foundation

Conventional grouting methods present challenges in achieving effective surface rock grouting and avoiding interference with concrete grouting and construction processes. The proposed approach includes reserving rock overburden, controlling grouting pressure and concentration, using a segmented grouting method, and selecting appropriate materials to develop a key method for consolidation grouting. This method not only overcomes the problems associated with the conventional consolidation grouting method but also effectively protects the thin layer of breccia lava in the Baihetan Dam foundation and solves the problem of unloading and relaxation of columnar jointed basalt caused by exposed consolidation grouting. The results show that the post grouting inspection hole permeability qualification rate is 99.3%, with the proportion of high-speed measuring points of various rock types ranging from 91.8% to 93.3%, and the proportion of low-speed measuring points ranging from 2.0% to 2.9%. The drop rate of the grout take of sequence I and sequence II is between 30%–60%, except for some dam sections. These results indicate that the proposed method for consolidation grouting meets the design requirements. The findings have significant implications for the design and construction of consolidation grouting in similar projects.

Study on Shear Strength Characteristics of Columnar Jointed Basalt Based on in-situ Direct Shear Test at Baihetan Hydropower Station

Study on Shear Strength Characteristics of Columnar Jointed Basalt Based on in-situ Direct Shear Test at Baihetan Hydropower Station

Progressive Fracture Behavior and Acoustic Emission Release of CJBs Affected by Joint Distance Ratio

The progressive collapse behavior and energy release of columnar jointed basalts (CJBs) can be greatly influenced by different joint distance ratios. By adopting the digital image correlation, a series of heterogeneous CJB models are established. The continuous fracture process and acoustic emissions (AEs) are captured numerically under varying lateral pressures. The load curves under different joint distance ratios and model boundaries are analyzed. Meanwhile, the strength, deformation modulus and AE rule are discussed. The data indicate that under plane strain, the troughs of compression strength appear at the column dip angle β = 30°, 150°, 210° or 330°; the equivalent deformation modulus changes in an elliptical way with β increasing; the compression strength and equivalent deformation modulus are higher than the case between plane stress and plane strain under different joint distance ratios. When β = 30°, the accumulation of AE energy corresponding to the stress peak under plane strain are higher than the case between plane stress and plane strain but becomes lower when β increases to 60°, which implies the critical transformation of the AE energy-related failure precursor affected by column dip angle. These achievements will contribute to the design, construction and support of slopes and tunnels encountering CJBs.

Numerical Investigation on Anisotropy and Shape Effect of Mechanical Properties of Columnar Jointed Basalts Containing Transverse Joints

We studied the non-linear mechanical response and failure mechanism of columnar jointed basalts (CJBs) with transverse joints by modeling meso-mechanics, statistical damage theory and continuum mechanics. The anisotropy and shape effect of CJBs with transverse joints were captured under different lateral pressures. The digital images were transformed into heterogeneous element meshes, and the gradual fracturing process and various failure modes of CJBs were reproduced. The compressive strength (CS) and equivalent deformation modulus (EDM) of CJBs parallel and perpendicular to the column axis were studied. The results show that the U-shaped CS curve of CJB appears as the column dip angle increases, and the CS is obviously improved as the lateral pressure increases when the column dip angle is 0°–90°. When the shape of CJB changes from 6 m × 3 m, 3 m × 3 m to 1.5 m × 3 m, the CS continues to increase. Meanwhile, the transverse joints are proven to be critical for determining the mechanical properties of CJBs at the certain dip angles of columns. However, the high lateral pressure can reduce the CS difference between the CJBs with and without the transverse joints. Besides, as the elastic modulus of joints rises, the CS will grow up, and the EDM will increase first and then almost remain at the same level. The coefficient of rock residual strength has a great influence on the CS at the certain dip angles of columns. Additionally, the model boundary significantly affects the anisotropy and shape effect of mechanical properties of CJBs under compression. These conclusions will improve our knowledge of the failure mechanisms and failure patterns of CJBs containing transverse joints.

AE energy evolution during CJB fracture affected by rock heterogeneity and column irregularity under lateral pressure

Considering that the mechanical mechanism and energy release law of gradual failure of columnar jointed basalt (CJB) are affected by lateral pressure, rock heterogeneity and column irregularity, etc., the numerical CJB models with various column dip angles, irregularity degrees of column and lateral pressures were established through the digital image correlation. The progressive failure process and released energy of CJBs were simulated under the loading rate of 0.05 mm/step based the continuum mechanics and statistical damage theory. The influences of rock inhomogeneity, column irregularity degree, and model boundary on the nonlinear deformation and failure of CJB were comprehensively investigated. The results demonstrate that the effect of the inhomogeneity index depends on column dip angle, and the equivalent deformation modulus of CJBs is more sensitive to the inhomogeneity index than the lateral pressure. Moreover, the compressive strength along the direction perpendicular to the column axis basically decreases with column irregularity degree increasing when the meso strength of rock is 120 MPa. Besides, the mechanical properties and elastic energy of irregular CJBs are greatly affected by the model boundaries. These achievements will contribute to understanding the inner failure mechanism and energy evolution of CJBs.

Characterisation and Evaluation of Columnar Basalt Geoheriatge in Thailand: Implication for Geotourism Management in Post-Quarrying Area

Abstract Late Cenozoic basaltic rocks in Nang Rong Columnar Jointed Basalts in southern Khorat Plateau are among the greatest columnar basalts in Thailand as famous geoheritage sites. This work aims to characterize and evaluate the post-quarrying area by field investigation, petrography, geochemistry, and geological engineering. A petrographical study determines these basalts as olivine basalts, presenting porphyritic texture with olivine microphenocryst. Geochemical characteristics indicate these basalts were alkaline basalt generated from alkaline sodic magma series within the continental plates. In addition, columnar jointed basalt presents compressive strength and point load strength index higher than platy basalt with a higher percentage of alteration. Local authorities enhance people in the area to develop and promote their georesources through touristic activities. This geoheritage demonstrates high geology and tourism management values; thus, it could be the prototype of the post-quarrying geosite of Thailand.

Geometrical characteristic investigation of the Baihetan irregular columnar jointed basalt and corresponding numerical reconstruction method

Geometrical characteristic investigation of the Baihetan irregular columnar jointed basalt and corresponding numerical reconstruction method

Numerical study on size effect and anisotropy of columnar jointed basalts under uniaxial compression

Under uniaxial loading, the heterogeneity of rock mass, the change of mechanical properties of joints, and the progressive fracture process of rock mass significantly affect the size effect and anisotropy of columnar jointed basalts (CJBs). To reveal the fracture features and failure mechanisms of CJBs influenced by the size effect and anisotropy, the digital image correlation (DIC), meso-damage mechanics, statistical strength theory, and continuum mechanics were combined (the DIC-improved RFPA), the digital images of CJB specimens were processed to establish the inhomogeneous numerical models, and a series of numerical tests were therefore conducted. The gradual fracture processes and macro failure patterns of CJBs orthogonal and parallel to column axis under uniaxial compression were studied, and the effects of various factors on the size effect and anisotropic mechanical properties of CJBs were further analyzed. The results show that model size, column dip angle, rock heterogeneity, column diameter, elastic modulus of joints, residual strength coefficient of joints, ratio of shift distance of joints, irregularity degrees of columns, and model boundaries have remarkable and complex effects on the mechanical behaviors of CJBs. The results greatly improve our understanding of the non-linear deformation and failure behaviors of CJBs and provide theorical basis for engineering construction in the areas of CJBs.

Numerical Investigation on Fracture Mechanisms and Energy Evolution Characteristics of Columnar Jointed Basalts With Different Model Boundaries and Confining Pressures

To reveal the mechanical mechanisms and energy release characteristics underlying progressive failure of columnar jointed basalts (CJBs) with various model boundaries and confining pressures, by combining the meso-damage mechanics, statistical strength theory, and continuum mechanics, inhomogeneous CJB models with different dip angles to the column axis are constructed. In the cases of plane stress, plane strain, and between plane stress and plane strain, the gradual fracture processes of CJBs are simulated under different confining pressures and the acoustic emission (AE) rules are obtained. The results show that: 1) in the case of plane stress, the fracture process of CJBs along direction I orthogonal to the column axis: at the initial stage of loading, the vertical joints and the transverse joints in the CJB specimen are damaged. Then, more columns in the upper middle part are cracked; 2) in the case between plane stress and plane strain, the fracture process of CJBs along the direction parallel to the column axis: at the initial stage of loading, the columnar joints are damaged. Then, the area of the damaged and broken columns at the top of the specimen increases and the crushing degree intensifies; 3) for the case between plane stress and plane strain, the AE energy accumulation before the peak stress is higher than the plane strain state along the direction orthogonal to the column axis. Meanwhile, along the direction parallel to the column axis, this value becomes larger when changing from the state between plane stress and plane strain to the plane strain state. These achievements will certainly improve our understanding of the fracture mechanism and energy evolution of CJBs and provide valuable insights into the instability precursor of CJBs.

Geoelectrical Survey for Evaluation of Groundwater Potential Within the Basaltic Terrain of Chikotra River Basin, Maharashtra (India)

Geoelectrical data was acquired using Wenner array over 23 sites with constant electrode separation of 70 m over Chikotra Basin, Dist. Kolhapur, Maharashtra (India). The spatial variation maps of resistivity at depths from 2 to 70 m were plotted using Inverse Distance Weighted (IDW) technique for interpolation in ArcGIS 10.5 to obtain a comprehensive subsurface hydrogeological representation of the study area. High resistivity (&gt;140 Ωm) up to 20m depth, indicative of massive basalts is deciphered towards the NE part of the study area, while the NW sector reveal low resistive (up to 40 Ωm) feature at shallow depths due to fractured basalts, thus conducive for groundwater exploration. Alluvium deposits and columnar jointed basalts in the central part depicts as EW trending conductive (&lt; 30 Ωm) feature suggesting prospective groundwater zone. Low resistivity (6-50 Ωm) from shallow to deeper depths (up to 70m), in the southern region can be identified as potential aquifer system. Longitudinal geoelectric cross-sections are generated over four profiles to identify the lateral and vertical variation in geology and groundwater potential zones. The western and central part of the northern profile (A-A') is highly resistive with resistivity of the order of 80-140 Ωm constituting compact basalts and thus devoid of water. Low resistive zone (30 Ωm) in the eastern part suggests groundwater at shallow depths. Low resistivity zones ranging from 10-50 Ωm is observed at different depth levels over the central profile (B-B') which can be tapped for groundwater exploitation. Several sites over profiles C-C' and the southern-most D-D' suggest promising aquifer zones. Because defining prospective groundwater zones in hard rock terrain is difficult, it’s crucial to look into a river basin’s hydrogeological arrangement early on in the planning process.

Technological innovations in construction of underground caverns in basaltic rocks at Baihetan Hydropower Station on Jinsha River

The two underground powerhouses on the left and right banks of Baihetan Hydropower Station (BHT-HS) are symmetrically arranged inside basaltic rocks in the alpine gorge reach of the lower Jinsha River. Each of them is equipped with 8 sets of 1,000 MW hydraulic turbine generators, with the largest unit capacity, total installed capacity, span of main powerhouse (i.e. 34 m), diameter of dome-shaped tail-regulating chamber (max diameter of 48 m) and scale of underground caverns around the world. Due to complex layout, interaction between caverns and high hollowing rate, the construction process of the underground cavern group is faced many challenges. For example, the cavern excavation is located in the high ground stress area dominated by horizontal tectonic stress; the underlying basalt fractures are well developed, and the rock is hard and brittle, with low crack initiation strength; columnar jointed basalts develop locally; a number of large and weak gently-sloped shear zones develop diagonally cutting the main powerhouse. Following the construction ideology of “understanding, utilization, protection, monitoring, and feedback analysis on the surrounding rock masses during the construction period”, and with the engineering construction procedure of “the excavation, analysis, prediction and inspection of each rock layer”, this paper proposes a set of innovation construction technologies including “advanced pre-control, thin vertical layer, small planar partition, short excavation progress, fine blasting, strong and quick support, short interval monitoring, quick feedback, precise dynamic optimization” to control the internal cracking of the hard and brittle basaltic rocks, discontinuous deformation of staggered zones and spatio-temporal deformation of underground caverns in high geostress. It has promoted the safe and orderly construction of BHT-HS, the technical progress of the construction of hydropower station projects in China, and may also be applicable to the design and construction of similar projects worldwide.

Estimation of unloading relaxation depth of Baihetan Arch Dam foundation using long-short term memory network

The unloading relaxation caused by excavation for construction of high arch dams is an important factor influencing the foundation's integrity and strength. To evaluate the degree of unloading relaxation, the long-short term memory (LSTM) network was used to estimate the depth of unloading relaxation zones on the left bank foundation of the Baihetan Arch Dam. Principal component analysis indicates that rock characteristics, the structural plane, the protection layer, lithology, and time are the main factors. The LSTM network results demonstrate the unloading relaxation characteristics of the left bank, and the relationships with the factors were also analyzed. The structural plane has the most significant influence on the distribution of unloading relaxation zones. Compared with massive basalt, the columnar jointed basalt experiences a more significant unloading relaxation phenomenon with a clear time effect, with the average unloading relaxation period being 50 d. The protection layer can effectively reduce the unloading relaxation depth by approximately 20%.