Probabilistic analysis of tunnel face seismic stability in layered rock masses using Polynomial Chaos Kriging metamodel

Face stability is an essential issue in tunnel design and construction.Layered rock masses are typical and ubiquitous;uncertainties in rock properties always exist.In view of this,a comprehensive method,which combines the Upper bound Limit analysis of Tunnel face stability,the Polynomial Chaos Kriging,the Monte-Carlo Simulation and Analysis of Covariance method(ULT-PCK-MA),is proposed to investigate the seismic stability of tunnel faces.A two-dimensional analytical model of ULT is developed to evaluate the virtual support force based on the upper bound limit analysis.An efficient probabilistic analysis method PCK-MA based on the adaptive Polynomial Chaos Kriging metamodel is then implemented to investigate the parameter uncertainty effects.Ten input parameters,including geological strength indices,uniaxial compressive strengths and constants for three rock formations,and the horizontal seismic coefficients,are treated as random variables.The effects of these parameter uncertainties on the failure probability and sensitivity indices are discussed.In addition,the effects of weak layer position,the middle layer thickness and quality,the tunnel diameter,the parameters correlation,and the seismic loadings are investigated,respectively.The results show that the layer distributions significantly influence the tunnel face probabilistic stability,particularly when the weak rock is present in the bottom layer.The efficiency of the proposed ULT-PCK-MA is validated,which is expected to facilitate the engineering design and construction.

Investigation of anisotropic strength criteria for layered rock mass

Layered rock mass is a type of engineering rock mass with sound mechanical anisotropy,which is generally unfavorable to the stability of underground works.To investigate the strength anisotropy of layered rock,the Mohr-Coulomb and Hoek-Brown criteria are introduced to establish the two transverse isotropic strength criteria based on Jaeger's single weak plane theory and maximum axial strain theory,and parameter determination methods.Furthermore,the sensitivity of strength parameters(K 1,K 2,and K 3)that are used to characterize the anisotropy strength of non-sliding failure involved in the strength criteria and confining pressure are investigated.The results demonstrate that strength parameters K 1 and K 2 affect the strength of layered rock samples at all bedding angles except for the bedding angle of 90°and the angle range that can cause the shear sliding failure along the bedding plane.The strength of samples at any bedding angle decreases with increasing K 1,whereas the opposite is for K 2.Except for bedding angles of 0°and 90°and the bedding angle range that can cause the shear sliding along the bedding plane,K 3 has an impact on the strength of rock samples with other bedding angles that the specimens'strength increases with increase of K 3.In addition,the strength of the rock sample increases as confining pressure rises.Furthermore,the uniaxial and triaxial tests of chlorite schist samples were carried out to verify and evaluate the strength criteria proposed in the paper.It shows that the predicted strength is in good agreement with the experimental results.To test the applicability of the strength criterion,the strength data of several types of rock in the literature are compared.Finally,a comparison is made between the fitting effects of the two strength criteria and other available criteria for layered rocks.

A modified smoothed particle hydrodynamics method considering residual stress for simulating failure and its application in layered rock mass

Residual strength is an indispensable factor in evaluating rock fracture,yet the current Smoothed Particle Hydrodynamics(SPH)framework rarely considers its influence when simulating fracture.An improved cracking strategy considering residual stress in the base bond SPH method was proposed to simulate failures in layered rocks and slopes and verified by experimental results and other simulation methods(i.e.,the discrete element method).Modified Mohr–Coulomb failure criterion was applied to distinguish the mixed failure of tensile and shear.Bond fracture markψwas introduced to improve the kernel function after tensile damage,and the calculation of residual stress after the damage was derived after shear damage.Numerical simulations were carried out to evaluate its performance under different stress and scale conditions and to verify its effectiveness in realistically reproducing crack initiation and propagation and coalescence,even fracture and separation.The results indicate that the improved cracking strategy precisely captures the fracture and failure pattern in layered rocks and rock slopes.The residual stress of brittle tock is correctly captured by the improved SPH method.The improved SPH method that considers residual strength shows an approximately 13%improvement in accuracy for the safety factor of anti-dip layered slopes compared to the method that does not consider residual strength,as validated against analytical solutions.We infer that the improved SPH method is effective and shows promise for applications to continuous and discontinuous rock masses.

Restraint effect of partition wall on the tunnel floor heave in layered rock mass

The presence of horizontal layered rocks in tunnel engineering significantly impacts the stability and strength of the surrounding rock mass,leading to floor heave in the tunnel.This study focused on preparing layered specimens of rock-like material with varying thickness to investigate the failure behaviors of tunnel floors.The results indicate that thin-layered rock mass exhibits weak interlayer bonding,causing rock layers near the surface to buckle and break upwards when subjected to horizontal squeezing.With an increase in the layer thickness,a transition in failure mode occurs from upward buckling to shear failure along the plane,leading to a noticeable reduction in floor heave deformation.The primary cause of significant deformation in floor heave is upward buckling failure.To address this issue,the study proposes the installation of a partition wall in the middle of the floor to mitigate heave deformation of the rock layers.The results demonstrate that the partition wall has a considerable stabilizing effect on the floor,reducing the zone of buckling failure and minimizing floor heave deformation.It is crucial for the partition wall to be sufficiently high to prevent buckling failure and ensure stability.Through simulation calculations on an engineering example,it is confirmed that implementing a partition wall can effectively reduce floor heave and enhance the stability of tunnel floor.

Physical model test and numerical simulation on the failure mechanism of the roadway in layered soft rocks

To explore the failure mechanism of roadway in layered soft rocks,a physical model with the physically finite elemental slab assemblage(PFESA)method was established.Infrared thermography and a video camera were employed to capture thermal responses and deformation.The model results showed that layered soft roadway suffered from large deformation.A three-dimensional distinct element code(3 DEC)model with tetrahedral blocks was built to capture the characteristics of roadway deformation,stress,and cracks.The results showed two failure patterns,layer bending fracture and layer slipping after excavation.The layer bending fracture occurred at positions where the normal direction of layers pointed to the inside of the roadway and the layer slipping occurred in the ribs.Six schemes were proposed to investigate the effects of layered soft rocks.The results showed that the deformation of ribs was obviously larger than that of the roof and floor when the roadway passed through three types of strata.When the roadway was completely in a coal seam,the change of deformation in ribs was not obvious,while the deformation in the roof and floor increased obviously.These results can provide guidance for excavation and support design of roadways in layered soft rocks.

Face stability analysis of circular tunnels in layered rock masses using the upper bound theorem

An analysis of tunnel face stability generally assumes a single homogeneous rock mass.However,most rock tunnel projects are excavated in stratified rock masses.This paper presents a two-dimensional(2D)analytical model for estimating the face stability of a rock tunnel in the presence of rock mass stratification.The model uses the kinematical limit analysis approach combined with the block calculation technique.A virtual support force is applied to the tunnel face,and then solved using an optimization method based on the upper limit theorem of limit analysis and the nonlinear Hoek-Brown yield criterion.Several design charts are provided to analyze the effects of rock layer thickness on tunnel face stability,tunnel diameter,the arrangement sequence of weak and strong rock layers,and the variation in rock layer parameters at different positions.The results indicate that the thickness of the rock layer,tunnel diameter,and arrangement sequence of weak and strong rock layers significantly affect the tunnel face stability.Variations in the parameters of the lower layer of the tunnel face have a greater effect on tunnel stability than those of the upper layer.

Modification of rock mass rating system:Interbedding of strong and weak rock layers

Rock mass classification systems are the very important part for underground projects and rock mass rating(RMR) is one of the most commonly applied classification systems in numerous civil and mining projects. The type of rock mass consisting of an interbedding of strong and weak layers poses difficulties and uncertainties for determining the RMR. For this, the present paper uses the concept of rock bolt supporting factor(RSF) for modification of RMR system to be used in such rock mass types. The proposed method also demonstrates the importance of rock bolting practice in such rock masses. The geological parameters of the Shemshak Formation of the Alborz Tunnel in Iran are used as case examples for development of the theoretical approach.

3D FEM analysis for layered rock considering anisotropy of shear strength

An empirical expression of cohesion （C） and friction angle （Ф） for layered rock was suggested. This expression was compared with a test result made by the former researchers. The constitutive relationship of a transversely isotropic medium and Mohr-Coulomb criterion in which C and Ф vary with directions were employed, and a relative 3D elasto-plastic FEM code was developed, in which the important thing was to adopt a search-trial method to find the orientation angle （p） of shear failure plane （or weakest shear plane） with respect to the major principal stress as well as the corresponding C and Ф Taking an underground opening as the calculation object, the numerical analyses were carried out by using the FEM code for two cases of transversely isotropic rock and isotropic rock, respectively, and the computation results were compared. The results show that when the rock is a transversely isotropic one, the distributions of displacements, plastic zones and stress contours in the surrounding rock will be non-axisymmetric along the tunnel＇s vertical axis, which is very different from that of isotropic rock. The stability of the tunnel in transversely isotropic rock is relatively low.

Time-dependent squeezing deformation mechanism of tunnels in layered soft-rock stratum under high geo-stress

Large squeezing deformation of layered soft rock tunnel under high geo-stress has a significant time-dependent deformation behavior.In this paper,we studied the deformation mechanism during the construction period of deep-buried softrock tunnel by means of a combination of field observations and a numerical method.First,a new classification criterion for large deformations based on the power exponent variation law between the deformation and the strength-stress ratio is proposed.Then,the initial damage tensor reflecting the bedding plane(joint)distribution and an equivalent damage evolution equation derived from the viscoplastic strain are introduced based on the geometric research method,i.e.,a new rheological damage model(RDL model)of layered soft rock is established consisting of elastic,viscous,viscoelastic,viscoplastic and plastic elements.A field test was conducted on the Maoxian tunnel in Sichuan province,southwestern China,which is in broken phyllite(layered soft rock)under high geo-stress.The tunnel has experienced large deformation due to serious squeezing pressure,thus we adopted double primary support method to overcome the supporting structure failure problems.The rheological parameters of phyllite in the Maoxian tunnel were recognized by using SA-PSO optimization,and the RDL model does a good job in describing the time-dependent deformation behavior of a layered soft-rock tunnel under high geo-stress.Thus,the RDL model was used to investigate the supporting effect and bearing mechanism of the double primary support method.Compared with the single primary support method,the surrounding rock pressure,secondary lining force,surrounding rock deformation,and the depth of the damage to the rock mass was reduced by 40%-60%after the double primary support method was used.

Tomography of the dynamic stress coefficient for stress wave prediction in sedimentary rock layer under the mining additional stress

In this study,the tomography of dynamic stress coefficient(TDSC)was established based on a mechanical model of stress wave propagation in bedding planes and a mathematical model of the stress wave attenuation in rock masses.The reliability of the TDSC was verified by a linear bedding plane model and field monitoring.Generally,the TDSC in the dynamic stress propagation of bedding planes increases with the following conditions:(1)the increase of the normal stiffness of the bedding plane,(2)the increase of the incident angle of the stress wave,(3)the decrease of the incident frequency of the stress wave,or(4)the growth of three ratios(the ratios of rock densities,elastic moduli,and the Poisson’s ratios)of rocks on either side of bedding planes.The additional stress weakens TDSC linearly and slowly during the stress wave propagation in bedding planes,and the weakening effect increases with the growth of the three ratios.Besides,the TDSC decreases exponentially in the rock mass as propagation distance increases.In a field case,the TDSC decreases significantly as vertical and horizontal distances increase and its wave range increases as vertical distance increases in the sedimentary rock layers.

Groundwater monitoring of an open-pit limestone quarry:Water-rock interaction and mixing estimation within the rock layers by geochemical and statistical analyses

Water-rock interaction and groundwater mixing are important phenomena in understanding hydrogeological systems and the stability of rock slopes especially those consisting largely of moderately watersoluble minerals like calcite. In this study, the hydrogeological and geochemical evolutions of groundwater in a limestone quarry composed of three strata: limestone layer(covering), interbedded layer under the covering layer, and slaty greenstone layer(basement) were investigated. Water-rock interaction in the open-pit limestone quarry was evaluated using PHREEQC, while hierarchical cluster analysis(HCA)and principal component analysis(PCA) were used to classify and identify water sources responsible for possible groundwater mixing within rock layers. In addition, Geochemist's Workbench was applied to estimate the mixing fractions to clarify sensitive zones that may affect rock slope stability. The results showed that the changes in Ca2+and HCO3àconcentrations of several groundwater samples along the interbedded layer could be attributed to mixing groundwater from the limestone layer and that from slaty greenstone layer. Based on the HCA and PCA results, groundwaters were classified into several types depending on their origin:(1) groundwater from the limestone layer(LO),(2) mixed groundwater flowing along the interbedded layer(e.g., groundwater samples L-7, L-11, S-3 and S-4), and(3) groundwater originating from the slaty greenstone layer(SO). The mixing fractions of 41% LO: 59% SO, 64% LO: 36% SO, 43%LO: 57% SOand 25% LO: 75% SOon the normal days corresponded to groundwaters L-7, L-11, S-3 and S-4,respectively, while the mixing fractions of groundwaters L-7 and L-11(61% LO: 39% SOand 93% LO: 7% SO,respectively) on rainy days became the majority of groundwater originating from the limestone layer.These indicate that groundwater along the interbedded layer significantly affected the stability of rock slopes by enlarging multi-breaking zones in the layer through calcite dissolution and inducing high water pressure, tension cracks and potential sliding plane along this layer particularly during intense rainfall episodes.

Numerical simulation and experimental study on dissolving characteristics of layered salt rocks

Underground salt cavern reservoirs are ideal spaces for energy storage. China is rich in salt rock resources with layered lacustrine sedimentary structures. However, the dissolution mechanism of layered salt rocks remains poorly understood, resulting in significant differences between the actual measurements and the designed indices for the layered salt rock water-soluble cavity-making cycle and the cavity shape. In this work, the dissolution rates of 600 groups of layered salt rocks in China under different conditions were determined experimentally.Thus, the established artificial neural network prediction model was used to assess the effects of the contents of NaCl, Na2 SO4, and CaSO4 in the salt rocks, concentrations, dissolution angles, and flow rates on their dissolution rates by performing ANOVA and F-test. The results provide a theoretical basis for evaluating the dissolution rate of layered salt rocks under different conditions and for the numerical simulation of the layered salt rock water-soluble cavity-making process.

Mechanical properties and associated seismic isolation effects of foamed concrete layer in rock tunnel

Foamed concrete has a good energy absorption capability and can be used as seismic isolation material for tunnels. This study aims to investigate the mechanical properties and associated seismic isolation effects of foamed concrete layer in rock tunnel. For this, a series of uniaxial/triaxial compression tests was conducted to understand the effects of concrete density, confining stress and strain rate on the mechanical properties of foamed concrete. The direct shear tests were also performed to investigate the effects of concrete density and normal stress on the nonlinear behaviors of foamed concrete layer-lining interface. The test results showed that the mechanical properties of foamed concrete are significantly influenced by the concrete density. The foamed concrete also has high volumetric compressibility and strain-rate dependence. The peak stress. residual stress. shear stiffness and residual friction coefficient of the foamed concrete layer-lining interface are influenced by the foamed concrete density and normal stress applied. Then, a crushable foam constitutive model was constructed using ABAQUS software and a composite exponential model was also established to study the relationship between shear stress and shear displacement of the interface, in which their parameters were fitted based on the experimental results. Finally, a parametric analysis using the finite element method(FEM) was conducted to understand the influence of foamed concrete layer properties on the seismic isolation effect, including the density and thickness of the layer as well as the shear stiffness and residual friction coefficient of the interface. It was revealed that lower density and greater thickness in addition to smaller shear stiffness or residual friction coefficient of the foamed concrete layer could yield better seismic isolation effect, and the influences of the first two tend to be more significant.

Diagenesis of the Sedimentary Rocks EnclosingCoaly Layers in Gavatha Area, Lesvos Island,Based on Silica Polymorph's Transformations

A Tertiary non-marine stratigraphic sequence composed of carbonates (limestone),siliceous carbonates, coaly layers overlain by pyroclastic rocks and lavas, outcrops in the Ga-vatha area of northwestern Lesvos Island. Pure carbonates consist almoot completely of calcite,the siliceous carbonate sediments of quartz, opal-CT and calcite, the shales of quartz, opal-CT,K-feldspar, smectite-illite and calcite, and the coaly layers of organic matter, quartz, opal-CT,feldspars and pyrite. Geochemical data indicate that smectite-illite, feldspars and associated ele-ments (La, Zr, Y, Ba, Ce) are the preducts of alteration of volcanic rocks in a subtropicalarea.A combination of sources is suggested for the formation of silica polymorphs: (a) biogenicor non-biogenic silica (opal-A) that was originally present in the form of diatom frustules or inthe form of inorganically precipitated silica; (b) transformation of opal-A to opal-CT and quartzopal-C from alteration of volcanic glass of intercalated tuffites and overlying volcanics; and (c)opal-CT deposited primarily from hydrothermal solutions.

大庆油田裸眼井测井技术进展与展望

为了提高大庆油田裸眼井测井技术支撑能力和研究成果领先水平,全面回顾了大庆油田测井采集系列及解释技术的创新发展历程,系统总结了特高含水期剩余油解释、火山岩等复杂岩性测井评价、碎屑岩储量参数解释、非常规油气“甜点”分类、缝洞型碳酸盐岩储层测井评价等油田勘探开发测井评价技术。在客观分析大庆油田勘探开发测井解释评价需求和面临瓶颈问题的基础上,结合当前油田测井评价对象规模小、物性差、埋藏深、地层结构复杂、非均质性强的特点。指明了测井解释评价核心技术主攻方向。围绕新阶段测井采集及解释评价技术体系完善与建立,对高分辨率和成像测井采集、后油藏时期和非常规测井解释评价、新一代智能解释技术体系等未来发展进行了展望。

层状岩石不排气三轴压缩力学特性试验研究

为研究地下隧洞开挖过程中围压和孔隙气压对层状千枚岩力学特性及变形参数的影响,以氩气作为渗透介质,开展层状千枚岩不排气三轴压缩试验,分析层状千枚岩在不同围压和孔压下应力-应变关系、峰值强度、变形特性及破坏形式随层理倾角的演化特征。引入基于层理倾角的各向异性度公式,探讨围压和孔压对层状千枚岩各向异性的影响。结果表明:不排气条件下,当孔压减小或围压增大时,层状千枚岩峰值强度逐渐增大;岩石峰值强度σc、弹性模量及变形模量随层理倾角的增大呈“U”型变化;层理倾角在30°~60°之间,层状千枚岩存在层理弱面,破坏形式主要为沿层理面剪切滑移破坏;层状千枚岩在平行层理方向受围压、孔压影响较大,呈现明显各向异性特征。

中国石油深层-超深层碎屑岩油气勘探进展与潜力

近期中国石油天然气股份有限公司在深层-超深层碎屑岩勘探领域不断获得多项重大油气发现并落实一批规模储量,展现出该领域巨大的勘探潜力。基于勘探进展与取得的地质认识,结合我国陆上含油气盆地的特点,将深层-超深层碎屑岩领域划分为前陆盆地、断陷盆地和坳陷盆地3种类型,揭示了深层具有有效烃源岩规模分布、富油更富气,以及4种成储机制控制深层优质储层发育,深层成藏要素配置关系好等特征。结合深层碎屑岩的基础地质认识、生烃及资源潜力认识、有效储层分布规律和控藏因素等新认识优选有利区带,提出未来四大攻关方向,并基于剩余资源分布特点和勘探认识,优选3类盆地的深层-超深层碎屑岩领域的十大重点区带作为下一步重点勘探目标。

膨胀岩隧道缓冲层复合支护体系让压机理

为构建缓冲层复合支护体系理论模型,分析缓冲层复合支护体系让压机理,弥补当前缓冲层让压机理的局限性。利用弹性力学知识,以单层厚壁圆筒模型为基础构建3层厚壁圆筒模型,推导出缓冲层复合支护体系中支护结构受力与外荷载之间函数关系,并用数值模拟的方法验证缓冲层复合支护结构受力解析解的正确性。基于此理论模型,分析了有无缓冲层复合支护体系中二衬和初支的受力特点,总结出缓冲层让压机理:在含有缓冲层的复合支护体系中,缓冲层材料被压缩,初支能够发生更大的向内变形量,进而吸收外部荷载作用,减小二衬的受力,从而整个结构能够实现让压效果,即缓冲层复合支护通过缓冲层压缩实现让压。通过改变缓冲层厚度对让压机理进行分析。结果表明:当缓冲层厚度为5 cm时,初支的形变量增加了1.34 cm,传递到二衬上的作用力也相应减小了71.6%;当缓冲层厚度增加至10 cm时,二衬的受力也进一步减小,对二衬结构更为有利。在让压支护体系中,随着缓冲层被压缩,初支也会发生更大的变形,初支内部应力也会增大,尤其是环向应力,即缓冲层的让压对初支受力是不利的,对二衬受力是有利的。因此,在复合让压支护体系中,当外部作用力一定时,对支护体系中缓冲层的设计要同时考虑初支和二衬的应力,即缓冲层的厚度存在一个最优值。

层状岩石逆倾向与顺倾向剪切破坏特征的差异性研究

层状岩石层理效应的研究对深部岩体稳定性分析具有重要意义,而天然层状岩石逆倾向与顺倾向剪切力学行为差异性仍认识不清。为此,开展了0°≤ψ≤180°(ψ为剪切面顺时针旋转至层理面的滑动倾向角)的页岩全角度剪切试验,详细地研究了逆倾向与顺倾向下剪切力学特性和破坏模式的差异性,并结合离散元模拟进行了补充分析与验证。研究结果表明:顺层面剪切时抗剪强度取得最小值,ψ=30°时取得最大值,90°与135°时取得局部极大值,逆倾向抗剪强度相对更高,ψ>30°时随滑动倾向角增加抗剪强度总体呈减小趋势;根据不同滑动倾向角下剪切力学行为的差异性,按滑动倾向角将层状岩石分为三组,即层面张拉与基质剪切组(15°~60°)、基质剪切组(75°~120°)、基质与层面剪切组(135°~180°);基质剪切组在剪切应力-位移曲线峰前均存在应力降现象,层面张拉与基质剪切组在峰后呈“阶梯”状应力降低;张拉破坏与剪切破坏同时存在且以剪切破坏为主;顺层面剪切时层面的剪切裂纹数目占优,ψ=90°时基质体的剪切裂纹数目最多,ψ=30°时层面的张拉裂纹数目最多,其次是基质体的剪切裂纹,ψ=150°时以层面、基质体的剪切裂纹为主。研究揭示了层状岩石逆倾向与顺倾向剪切的各向异性特征及差异性根源,可为各向异性力学模型完善、灾变机制及围岩稳定性分析提供科学依据。

多因素耦合效应对层状千枚岩动力特性的影响

为了研究不同长径比和倾角下应变率对层状千枚岩动力特性的影响,选用4种倾角(α=0°,30°,60°,90°)两种长度(L=25,50 mm)的层状千枚岩为研究对象,采用分离式霍普金森压杆(SHPB)装置进行不同冲击气压(p=0.150,0.175,0.200,0.225和0.250 MPa)的动态压缩试验.研究了两种长径比层状千枚岩不同应变率下动力特性的影响,分析了层状千枚岩层理倾角、长径比和应变率之间的耦合效应对层状千枚岩强度特征的影响关系.结果表明:动态压缩下两种不同长径比层状千枚岩动态抗压强度随层理倾角增加呈现先减小后增大趋势,动态峰值强度和峰值应变均随应变率增大而增大,层状千枚岩动态抗压强度与应变率呈幂函数关系;动态峰值强度关于倾角α和长径比L/D呈二元函数关系,在α=60°时,层状千枚岩动态抗压强度的长径比效应最为显著,在L/D=1时,倾角效应最为显著;不同倾角下,峰值应变在0°倾角下长径比效应最为显著,在90°倾角下长径比效应最弱;动态冲击下层状千枚岩动态抗压强度和峰值应变的应变率效应均强于长径比效应.