Xiyu conglomerate is a significant and extensively distributed geological formation in western China.A clear understanding of its properties and the establishment of a classification system are essential for selecting...Xiyu conglomerate is a significant and extensively distributed geological formation in western China.A clear understanding of its properties and the establishment of a classification system are essential for selecting appropriate research methods to investigate its engineering mechanical behavior.Based on geological data from eight typical Xiyu conglomerate geological belts and seven hydropower projects,this study summarizes the main engineering geological characteristics,and analyzes the fabric characteristics of various components of the conglomerate through laboratory tests and statistical analysis.A comprehensive classification system is proposed for Xiyu conglomerate based on two key criteria:(1)grain size distribution,quantified by the d_(50)(median grain diameter),and(2)cementation type,identified via mineralogical and geochemical analysis.This system divides Xiyu conglomerate into nine distinct categories,each defined by specific engineering geological and petrofabric properties.The results reveal that,even within the same region,the grain size composition and distribution of Xiyu conglomerate are highly heterogeneous.While the chemical composition of the cementing materials is generally consistent,notable differences in cement properties arise primarily from variations in mineral content,particularly the proportion of calcareous material(dolomite,calcite,and quartz).Conglomerates with gray or grayish-blue matrices typically exhibit higher calcareous content,whereas those with earth-yellow or khaki matrices contain less calcareous material and are predominantly argillaceous-cemented.Additionally,Xiyu conglomerate shows higher porosity compared to conventional rocks.The proposed classification method based on engineering geological and fabric characteristics offers a geological basis for further determining the engineering mechanical properties of various Xiyu conglomerate types.This approach holds potential for addressing the challenges related to unclear classification and difficulty in accurately defining mechanical parameters for Xiyu conglomerate across different regions.展开更多
The deteriorated bearing capacity and nonlinear expansion deformation of weakly cemented Xiyu conglomerate under complex water environments and stress disturbances pose significant risks to the safety of stratum engin...The deteriorated bearing capacity and nonlinear expansion deformation of weakly cemented Xiyu conglomerate under complex water environments and stress disturbances pose significant risks to the safety of stratum engineering construction.In this study,to precisely comprehend the influences of pore pressure(P_(w))and stress path on the deformation characteristics,dilation behavior,and damage evolution of Xiyu conglomerate,a series of triaxial monotonic loading and cyclic loading-unloading tests were conducted on saturated Xiyu conglomerate with varied confining pressures(σ_(3))and pore pressures.The results indicate that as P_(w)increases,the secant modulus,unloading modulus,and loading modulus decrease,but increase with risingσ_(3).Additionally,P_(w)accelerates the onset of dilatancy,whereasσ_(3)delays it.Asσ_(3)increases,the peak stress,crack damage stress,and residual strength increase,while these parameters decrease with increasing P_(w).A positive correlation exists between the effective confining pressure and the effective axial stress.Furthermore,an increase in P_(w)results in a greater maximum dilation angle,which decreases with increasingσ_(3).The failure mode is mainly a tensile-shear mixed failure mode.The high pore pressure and cyclic loading stress path aggravate the deterioration of strength and failure mode of the weakly cemented Xiyu conglomerate.Finally,a new damage variable and conceptual model are proposed and discussed.The findings provide insight into the damage and failure mechanism of the Xiyu conglomerate under pore pressure and cyclic disturbance,offering a crucial experimental foundation for the design and construction of hydropower projects in the Xiyu conglomerate layer.展开更多
The complex and special mechanical properties of Xiyu conglomerate are of great significance to the construction of water conservancy and hydropower engineering.The crack characteristic stress,dilatancy behavior,and f...The complex and special mechanical properties of Xiyu conglomerate are of great significance to the construction of water conservancy and hydropower engineering.The crack characteristic stress,dilatancy behavior,and failure mechanism of Xiyu conglomerate collected from Momoke Water Control Project,southwestern China,were analyzed and discussed based on the experimental results of triaxial compression test and 3D X-ray computed tomography test.The results show that with increasing confining pressure,the deformation characteristics and all characteristic stresses increase monotonically,while the dilation angle and dilatancy index decrease,and exponential function model can accurately describe the evolution rule of dilatancy index with confining pressure.While the porosity is negatively correlated with confining pressure.The failure modes of Xiyu conglomerate include axial tensile cracks,shear cracks,local cross cracks and cracks around gravel.With increasing confining pressure,the failure modes transform from tension cracks to shear cracks.A non-associated micromechanical damage model considering pressure dependent matrix presenting tension-compression asymmetry is proposed and applied to Xiyu conglomerate with pores and a large number of gravels.By comparing numerical calculations and experimental results,the proposed micromechanical plastic damage model is able to describe the mechanical behavior of Xiyu conglomerate.展开更多
The Cenozoic uplift of the Central Asia Tian Shan Mountains has driven significant subsidence in the foreland basins along its northern and southern flanks,leading to the extensive deposition of the Late Cenozoic allu...The Cenozoic uplift of the Central Asia Tian Shan Mountains has driven significant subsidence in the foreland basins along its northern and southern flanks,leading to the extensive deposition of the Late Cenozoic alluvial-gravel deposits at its piedmonts known as the Xiyu Conglomerates.At the base of these conglomerates,localized gravel deposition replaces sandstones vertically over tens of meters with a sharp increase in median grain size(D50)by c.100-fold.However,the origin of the transition remains a subject of controversy,with multiple potential factors intricately linked to regional tectonics and climatic variations.To address this question,we investigated the grain size variations of modern riverbed sediments along six rivers and the Xiyu Conglomerates in two sections within the northern foreland area of Tian Shan Mountains.We observed a rapid gravel-sand transition(GST)along the present-day rivers,20-50 km downstream from the outlet,as well as a sharp conglomerate-sandstone transition at the base of the Xiyu Conglomerates,both of which exhibit similar fining rates.Furthermore,a provenance investigation of the Jingou River basin,using heavy mineral assemblages and detrital zircon U-Pb ages,indicates consistent sources for both the Xiyu Conglomerates and modern riverbed sediments.The combined results suggest that the striking grain size changes observed in both the Xiyu Conglomerates and along these modern rivers from similar internal hydraulic processes within the piedmont rivers,specifically size-selective sorting controlled by the bimodal grain size distribution of sediments.This implies that the emergence of sharp grain size transitions in the vertical successions was a result of the continuous northward progradation of the GST in the basin,driven by the long-term northward thrusting of the Tian Shan Mountains,independent of sharp and specific changes in climatic or tectonic forcing events.The average northward migration rate of the GSTs is calculated to be3.9±0.2 mm/yr since c.7.5 Ma along 85°30′E,and 7.6±2.1 mm/yr since c.2.1 Ma along 86°30′E.These rates closely reflect the long-term crustal shortening rates across the northern Tian Shan Mountains,and its increase may denote an acceleration of the shortening post-Miocene.展开更多
基金Xinjiang Water Science and Technology Special Project,Grant/Award Numbers:XSKJ-2022-05,XSKJ-2023-30State Grid Co.,LTD.Technology Project,Grant/Award Number:5108-202218280A-2-301-XG。
文摘Xiyu conglomerate is a significant and extensively distributed geological formation in western China.A clear understanding of its properties and the establishment of a classification system are essential for selecting appropriate research methods to investigate its engineering mechanical behavior.Based on geological data from eight typical Xiyu conglomerate geological belts and seven hydropower projects,this study summarizes the main engineering geological characteristics,and analyzes the fabric characteristics of various components of the conglomerate through laboratory tests and statistical analysis.A comprehensive classification system is proposed for Xiyu conglomerate based on two key criteria:(1)grain size distribution,quantified by the d_(50)(median grain diameter),and(2)cementation type,identified via mineralogical and geochemical analysis.This system divides Xiyu conglomerate into nine distinct categories,each defined by specific engineering geological and petrofabric properties.The results reveal that,even within the same region,the grain size composition and distribution of Xiyu conglomerate are highly heterogeneous.While the chemical composition of the cementing materials is generally consistent,notable differences in cement properties arise primarily from variations in mineral content,particularly the proportion of calcareous material(dolomite,calcite,and quartz).Conglomerates with gray or grayish-blue matrices typically exhibit higher calcareous content,whereas those with earth-yellow or khaki matrices contain less calcareous material and are predominantly argillaceous-cemented.Additionally,Xiyu conglomerate shows higher porosity compared to conventional rocks.The proposed classification method based on engineering geological and fabric characteristics offers a geological basis for further determining the engineering mechanical properties of various Xiyu conglomerate types.This approach holds potential for addressing the challenges related to unclear classification and difficulty in accurately defining mechanical parameters for Xiyu conglomerate across different regions.
基金This work was supported by the Central University Basic Research Fund of China(B230201059and No.B240201155)the water science and technology special fund of Xinjiang Uygur Autonomous Region(No.XSKJ-2023-30)funded by China Power Construction Group research project(Grant No.DJ-HXGG-2023-16).
文摘The deteriorated bearing capacity and nonlinear expansion deformation of weakly cemented Xiyu conglomerate under complex water environments and stress disturbances pose significant risks to the safety of stratum engineering construction.In this study,to precisely comprehend the influences of pore pressure(P_(w))and stress path on the deformation characteristics,dilation behavior,and damage evolution of Xiyu conglomerate,a series of triaxial monotonic loading and cyclic loading-unloading tests were conducted on saturated Xiyu conglomerate with varied confining pressures(σ_(3))and pore pressures.The results indicate that as P_(w)increases,the secant modulus,unloading modulus,and loading modulus decrease,but increase with risingσ_(3).Additionally,P_(w)accelerates the onset of dilatancy,whereasσ_(3)delays it.Asσ_(3)increases,the peak stress,crack damage stress,and residual strength increase,while these parameters decrease with increasing P_(w).A positive correlation exists between the effective confining pressure and the effective axial stress.Furthermore,an increase in P_(w)results in a greater maximum dilation angle,which decreases with increasingσ_(3).The failure mode is mainly a tensile-shear mixed failure mode.The high pore pressure and cyclic loading stress path aggravate the deterioration of strength and failure mode of the weakly cemented Xiyu conglomerate.Finally,a new damage variable and conceptual model are proposed and discussed.The findings provide insight into the damage and failure mechanism of the Xiyu conglomerate under pore pressure and cyclic disturbance,offering a crucial experimental foundation for the design and construction of hydropower projects in the Xiyu conglomerate layer.
基金supported by National Natural Science Foundation of China(Nos.12102129 and 12072102)the Water Science and Technology Special Fund of Xinjiang Uygur Autonomous Region(No.XSKJ-2023-30)+1 种基金the Central University Basic Research Fund of China(Nos.B220202014 and B230201059)the Key Laboratory of Safe Mining of Deep Metal Mines,Ministry of Education(No.DM2022B01)。
文摘The complex and special mechanical properties of Xiyu conglomerate are of great significance to the construction of water conservancy and hydropower engineering.The crack characteristic stress,dilatancy behavior,and failure mechanism of Xiyu conglomerate collected from Momoke Water Control Project,southwestern China,were analyzed and discussed based on the experimental results of triaxial compression test and 3D X-ray computed tomography test.The results show that with increasing confining pressure,the deformation characteristics and all characteristic stresses increase monotonically,while the dilation angle and dilatancy index decrease,and exponential function model can accurately describe the evolution rule of dilatancy index with confining pressure.While the porosity is negatively correlated with confining pressure.The failure modes of Xiyu conglomerate include axial tensile cracks,shear cracks,local cross cracks and cracks around gravel.With increasing confining pressure,the failure modes transform from tension cracks to shear cracks.A non-associated micromechanical damage model considering pressure dependent matrix presenting tension-compression asymmetry is proposed and applied to Xiyu conglomerate with pores and a large number of gravels.By comparing numerical calculations and experimental results,the proposed micromechanical plastic damage model is able to describe the mechanical behavior of Xiyu conglomerate.
基金the financial support of the National Natural Science Foundation of China[42102250 and 42072153]the Second comprehensive scientific expedition to the Qinghai-Tibet Plateaus[2022QZKK0202]
文摘The Cenozoic uplift of the Central Asia Tian Shan Mountains has driven significant subsidence in the foreland basins along its northern and southern flanks,leading to the extensive deposition of the Late Cenozoic alluvial-gravel deposits at its piedmonts known as the Xiyu Conglomerates.At the base of these conglomerates,localized gravel deposition replaces sandstones vertically over tens of meters with a sharp increase in median grain size(D50)by c.100-fold.However,the origin of the transition remains a subject of controversy,with multiple potential factors intricately linked to regional tectonics and climatic variations.To address this question,we investigated the grain size variations of modern riverbed sediments along six rivers and the Xiyu Conglomerates in two sections within the northern foreland area of Tian Shan Mountains.We observed a rapid gravel-sand transition(GST)along the present-day rivers,20-50 km downstream from the outlet,as well as a sharp conglomerate-sandstone transition at the base of the Xiyu Conglomerates,both of which exhibit similar fining rates.Furthermore,a provenance investigation of the Jingou River basin,using heavy mineral assemblages and detrital zircon U-Pb ages,indicates consistent sources for both the Xiyu Conglomerates and modern riverbed sediments.The combined results suggest that the striking grain size changes observed in both the Xiyu Conglomerates and along these modern rivers from similar internal hydraulic processes within the piedmont rivers,specifically size-selective sorting controlled by the bimodal grain size distribution of sediments.This implies that the emergence of sharp grain size transitions in the vertical successions was a result of the continuous northward progradation of the GST in the basin,driven by the long-term northward thrusting of the Tian Shan Mountains,independent of sharp and specific changes in climatic or tectonic forcing events.The average northward migration rate of the GSTs is calculated to be3.9±0.2 mm/yr since c.7.5 Ma along 85°30′E,and 7.6±2.1 mm/yr since c.2.1 Ma along 86°30′E.These rates closely reflect the long-term crustal shortening rates across the northern Tian Shan Mountains,and its increase may denote an acceleration of the shortening post-Miocene.
