Shear strength response of reinforced pond ash

Reinforced pond ash is a composite material, which can be used as an alternative construction material in the field of geotechnical engineering. To study the shear strength response of reinforced pond ash, a series of unconsolidated undrained (UU) triaxial test has been conducted on both unreinforced and reinforced pond ash. In the present investigation the effects of confining pressure (σ₃), number of geotextile layers (N), and types of geotextiles on shear strength response of pond ash are studied. The results demonstrate that normal stress at failure (σ_1f) increases with increase in confining pressure. The rate of increase of normal stress at failure (σ_1f) is maximum for three layers of reinforcement, while the corresponding percentage increase in σ_1f is around (103%), when the number of geotextile layers increases from two layers to three layers of reinforcement. With increase in confining pressure the increment in normal stress at failure, Δσ increases and attains a peak value at a certain confining pressure (threshold value) after that Δσ becomes more or less constant. The threshold value of confining pressure depends on N, dry unit weight (γ_d) of pond ash, type of geotextile, and also type of pond ash.     

Deformation and failure characteristics and fracture evolution of cryptocrystalline basalt

Cryptocrystalline basalt is one of the two major types of rocks exposed in the super large-scale underground powerhouse in Baihetan hydropower station in China.The rock of this type shows various sitespecific mechanical responses(e.g.fragmentation,fracturing,and relaxation) during excavation.Using conventional triaxial testing facility MTS 815.03,we obtained the stress-strain curves,macroscopic failure characteristics,and strength characteristics of cryptocrystalline basalt.On this basis,evolution of crack initiation and propagation was explored using the finite-discrete element method(FDEM) to understand the failure mechanism of cryptocrystalline basalt.The test results showed that:(1) under different confining stresses,almost all the pre-peak stress-strain curves of cryptocrystalline basalt were linear and the post-peak stresses decreased rapidly;(2) the cryptocrystalline basalt showed a failure mode in a form of fragmentation under low and medium confining stresses while fragmentation-shear coupling failure dominated at high confining stresses;and(3) the initial strength ratio(σ_(ci)/σ_f,where σ_(ci)and σ_f are the crack initiation strength and peak strength,respectively) ranged from 0.45 to 0.55 and the damage strength ratio(σ_(cd)/σ_f,where σ_(cd) is the crack damage strength) exceeded 0.9.The stress—strain curve characteristics and failure modes of cryptocrystalline basalt could be reflected numerically.For this,FDEM simulation was employed to reveal the characteristics of cryptocrystalline basalt,including high σ_(cd)/σ_f values and rapid failure after σ_(cd),with respect to the microscopic characteristics of mineral structures.The results showed that the fragmentation characteristics of cryptocrystalline basalt were closely related to the development of tensile cracks in rock samples prior to failure.Moreover,the decrease in degree of fragmentation with increasing confining stress was also correlated with the dominant effect of confining stress on the tensile cracks.     

Measuring low permeabilities of gas-sands and shales using a pressure transmission technique

Liquid and gas permeability measurements for tight gas-sand and shales were done using a pressure transmission technique in specially designed apparatus in which confining pressure, pore pressure, and temperature are independently controlled. Downstream pressure changes were measured after increasing and maintaining upstream pressure constant. The initial pressure difference changes only after the pressure pulse propagates across the sample. For low permeability samples, the downstream pressure increase is delayed but the measurement senses a greater sample volume. On the other hand, conventional pulse decay techniques provide a more rapid response but are sensitive to local sample permeability heterogeneity. Permeability measured for the rocks studied varies from 1.18×10⁻¹⁵ to 3.95×10⁻²¹ m². The measured permeability anisotropy ratio in gas shale varies from 20% to 31%. The magnitudes of permeability anisotropy remain almost constant, but the absolute permeability values decrease by a factor of 10 with a 29.79 MPa effective pressure. All samples showed a nonlinear reduction in permeability with increasing effective pressure. The rate of reduction is markedly different from sample to sample and with flow direction. This reduction can be described by a cubic k–σ law and explained by preferential flow through microcracks.     

Experimental verification of parameter m in Hoek–Brown failure criterion considering the effects of natural fractures

Hoek–Brown failure criterion is one of the widely used rock strength criteria in rock mechanics and mining engineering. Based on the theoretical expression of Hoek–Brown parameter m of an intact rock, the parameter m has been modified by crack parameters for fractured rocks. In this paper, the theoretical value range and theoretical expression form of the parameter m in Hoek–Brown failure criterion were discussed. A critical crack parameter B was defined to describe the influence of the critical crack when the stress was at the peak, while a parameter b was introduced to represent the distribution of the average initial fractures. The parameter m of a fractured rock contained the influences of critical crack (B), confining pressure (σ₃) and initial fractures (b). Then the triaxial test on naturally fractured limestones was conducted to verify the modification of the parameter m. From the ultrasonic test and loading test results of limestones, the parameter m can be obtained, which indicated that the confining pressure at a high level reduced the differences of m among all the specimens. The confining pressure σ₃ had an exponential impact on m, while the critical crack parameter B had a negative correlation with m. Then the expression of m for a naturally fractured limestone was also proposed.     

Poroelastic contribution to the reservoir stress path

Pore pressure/stress coupling is the change in the smaller horizontal stress σ_h associated with changes in pore pressure P, and has been measured in numerous reservoirs worldwide. These measurements suggest that the change in minimum horizontal stress Δσ_h is on average ca. 64% of the change in the reservoir pore pressure ΔP, but can be as low as 34% and as high as 118%. Conventionally it is assumed that the total vertical stress σ_v, given by the overburden, is not affected by changes in pore pressure, in contrast to the horizontal stresses σ_H and σ_h. We investigate analytically and numerically the spatio-temporal pore pressure and stress evolution in poroelastic media for continuous fluid injection at a point source, and calculate from the numerical modelling results the ratio Δσ/ΔP. Analytically, we show that the measured average of Δσ_h/ΔP can mathematically be deduced from the long-term limit of the spatio-temporal evolution of pore pressure and horizontal stress caused by fluid injection at a point source. We compare our numerical results to the analytical solution for continuous point injection into homogeneous poroelastic media as well as to Δσ_h/ΔP values measured in the field, and show that all stress components change with a variation in P. We use the concept of poroelasticity to explain the observed coupling between pore pressure and stress in reservoirs, and we consider different measurement locations and measurement times as one possible reason for the measured variation in Δσ_h/ΔP in different oil fields worldwide.     

Some aspects of unsaturated flow in jointed rock

The applicability of Darcy's Law to two-phase flow has been discussed. Specialised triaxial equipment has been employed to separately inject two pore fluid components (air and water) into fractured rock specimens, so that two-phase flow behaviour can be studied at high axial and confining stresses. Improvements to recently developed two-phase high-pressure triaxial apparatus have enabled the authors to continue their study of air–water (i.e. unsaturated) flow in intact and fractured rock specimens under a wide range of stress conditions, similar to those encountered in underground mining operations. In this paper, a simplified stratified two-phase flow model is also presented that satisfactorily predicts flow behaviour in an inclined rock fracture over a range of linear laminar flow for particular capillary pressure relationships. The mathematical model is based upon the principles of conservation of mass and momentum, and relates the fracture aperture (e_t) to phase permeability (k_i) using Poiseuille's law and the proposed ‘phase height’, h_i(t), for water and air phases. The experimental approach used to verify the model predictions is described and the predicted results compared with the measurements. The experimental data confirmed the relationship between relative permeability and flow rate, with respect to two-phase flow conditions.     

Polyaxial strength criterion and closed-form solution for squeezing rock conditions

In this paper,a nonlinear strength criterion is proposed using the average of intermediate(σ_2) and minor(σ_3) principal stresses in place of σ_3 in Ramamurthy(1994)'s strength criterion.The proposed criterion has the main advantages of negligible variation of strength parameters with confining stress and ability to link with conventional strength parameters.Additionally,a new closed-form solution based on the proposed criterion is derived and validated for Chhibro Khodri tunnel.Further,analytical solutions including Singh's elastoplastic theory,Scussel's approach,and closed-form solutions based on conventional and modified Ramamurthy(2007) criteria are compared with the results of proposed approach.It is shown that the in situ squeezing pressure predictions made by the proposed approach are more accurate.Also,a parametric study of the present analytical solution is carried out,which displays explicit dependency of tunnel stability on internal support pressure and tunnel depth.The influence of tunnel geometry is observed to be dependent on the applied support pressure.     

两种有效应力下花岗岩和充填体渗透率变化规律研究

干热岩是地热资源的主要载体。干热岩体花岗岩受地质构造运动影响产生裂隙,熔融岩浆侵入到花岗岩裂隙中,形成含有充填体的花岗岩体。故采用压力脉冲法,以花岗岩母岩和充填体为研究对象,研究改变围压或孔隙压力两种路径下有效应力对花岗岩母岩和充填体渗透率的影响规律。研究表明:在孔隙压力一定情况下,随着围压增大,花岗岩母岩和充填体的渗透率都有一个快速大幅下降阶段和缓慢小幅降低的阶段;通过孔隙压力不变时卸载围压的路径减小有效应力,可以有效恢复花岗岩的渗透率。但通过围压不变时提高孔隙压力的路径减少有效应力达到恢复渗透率的目的时,存在一个“失效围压阈值”,当围压低于该阈值时,提高孔隙压力可以使渗透率得到有效恢复,高于该阈值,提高孔隙压力对渗透率不会有太大的提高;采用孔隙压力一定时降低围压和围压一定时(低于“失效围压阈值”)增大孔隙压力两种路径测试同一试样渗透率时,若两种路径下有效应力相同,则试样渗透率相差不大;通过波速对比和偏光镜图像对比分析了试样在试验前后的孔隙裂隙的压密和塑性变形,从宏观和微观角度证明了“失效围压阈值”存在的合理性。     

Use of polyethylene terephthalate fibres for mitigating the liquefaction-induced failures

The presence of non-biodegradable plastic waste is a serious concern for the health of endangered species. The present study is based on the sustainable utilisation of polyethylene terephthalate (PET) fibres obtained from waste plastic bottles to enhance the liquefaction resistance of fine sand. After performing a series of stress-controlled cyclic triaxial tests, the cyclic behaviour of PET-fibre reinforced sand has been investigated. The application of PET fibres was found to be more satisfactory in medium dense sand than that in loose sand as observed by residual excess pore water curves. In medium dense sand with 0.6% PET-fibres, the number of cycles to reach liquefaction was about 4 times that of the unreinforced sand. Using the dynamic shear modulus (G), the degradation index was calculated for both reinforced and unreinforced soils to assess stiffness characteristics. After nearly 50 loading cycles, the value of G/G_max increased 2.55 times with the addition of 0.4% PET fibres in unreinforced sand. Based on the results obtained, a regression model has been developed for the calculation of number of liquefaction failure cycles (N_cyc,L) in correlation with several parameters, namely, relative density (D_r), fibre content (FC) and σ_d/σ_c′ (σ_d = deviator stress, σ_c′ = effective confining stress).     

致密岩石气体渗流滑脱效应试验研究

由于致密结构和低的渗透率,气体在孔隙喉道小的致密岩石中流动会受到滑脱效应的影响。以湖南某试验场地致密砂岩为研究对象,对岩样进行了微观结构的SEM分析,通过一系列围压和孔隙压力作用下的砂岩气体流量和渗透率测试,研究气体在致密岩石中渗流特征,证明了致密砂岩气体流动存在滑脱效应现象,其渗流不符合Darcy定律。分析了孔压对滑脱效应的影响、滑脱效应对气测渗透率的影响以及滑脱因子与绝对渗透率的函数关系。研究结果表明,滑脱效应对气测渗透率的影响随着围压和气体孔隙压力的变化有所不同。同等围压下,孔隙压力越小,滑脱效应越明显,导致气测渗透率大于砂岩绝对渗透率。同等孔压下,当围压达到某一值后,其对滑脱效应的影响有限,同时也说明围压对岩石的压密是有限的。砂岩的平均气体孔隙压力与气测渗透率关系更加符合二次项曲线方程。计算获得的克努森数Kn说明了在相对高的围压和低的孔隙压力条件下,气体渗流过程位于滑脱流和过渡流之间,传统的N-S方程可能不再适用,应用Knudsen扩散方程更加合理,特别是当克努森数Kn比较高时。     

Dynamic characterization of EPS geofoam

This paper attempts to describe the dynamic behavior of expanded polystyrene EPS geofoam, and shows the dependence of shear modulus, G, and damping ratio, λ, on shear strain, γ, density, ρ, and confining stress, σ₃, through the results of a series of resonant column and strain- and stress-controlled cyclic compression tests. Shear modulus and damping ratio versus shear strain curves were obtained and a series of equations were developed to model the dynamic behavior of EPS. From stress-controlled cyclic compression tests the effect of the number of cyclic load applications, N, on the maximum axial strain ?_max (for a specific static deviator stress, σ_e, plus the amplitude of the loading cyclic stress, σ_c) and on the dynamic modulus of elasticity E_dyn was evaluated as a function of the EPS density, confining stress, and the applied cyclic stress amplitude σ_c.     

Consolidation behavior of dredged ultra-soft soil improved with prefabricated vertical drain at the Mae Moh mine,Thailand

The effectiveness of the prefabricated vertical drains (PVDs) in the consolidation of ultra-soft dredged soil with various soil water contents (W) in Mae Moh mine, Lampang, Thailand was researched via a series of large-scale model tests and numerical analysis. Large settlements with the delay of excess pore pressures is a distinct behavior of ultra-soft soil. The PVD dimensions were found to have a significant effect on the rate of consolidation and the delay of excess pore pressure at low total vertical stress (σ_v). The smaller PVD dimension resulted in the smaller rate of consolidation and longer delay of excess pore pressure. The undrained shear strength (S_u) of ultra-soft clay at various degrees of consolidation could be approximated by the vertical effective stress (σ′_v) based on the SHANSEP where the σ′_v was determined from the Asaoka's observational method. The finite element analysis with axisymmetric and plane strain models showed that the axisymmetric model produced an excellent settlement prediction. However, the excess pore pressures were not well predicted by the axisymmetric model, due to the delay of excess pore pressures at the early stages of consolidation. In practice, the plane strain models proposed by Chai et al. and Indraratna and Redana's methods are suggested to predict the consolidation settlement of the Mae Moh dredged soil improved with PVD. The outcome of this research will facilitate the geotechnical design of reclamation of ultra-soft dredged soil in Mae Moh mine and other similar soils.     

低渗透裂缝性煤层气储层压力敏感性研究

近年来,随着煤层气开采技术的进一步发展,国内煤层气开发已具有一定规模。由于煤层气储层具有低渗透裂缝性的特征,压力敏感性强,导致钻井过程中容易造成储层损害。因此,开展低渗透裂缝性煤层气储层的压力敏感性研究具有重要的意义。结合山西沁水盆地某区块的煤心,分别开展了煤岩受围压和孔隙压力条件下的压力敏感性实验研究,分析了煤岩渗透率随围压和孔隙压力的变化规律,探讨了孔隙压力和裂缝宽度的内在联系,认为围压增大和孔隙压力降低都会导致煤岩裂缝宽度变小,造成煤层气储层渗透率降低。     

A piezocone dissipation test interpretation method for hydraulic conductivity of soft clays

An alternative approach is developed in order to estimate the hydraulic conductivity of soft fine grained soils, based on numerical simulation of the full penetration and dissipation process for piezocones. Unlike previous methods of analysis, the process of penetration and dissipation has been explicitly simulated, thus eliminating several of the simplifications inherent in existing interpretation methods such as geometric approximations, predefined stress fields or neglecting material compressibility. The presented method is not established upon a particular set of data leading to limited applicability, but is rather developed using a more general approach and can be extended to other datasets if intended. Given the time to 50% consolidation and a number of influencing soil parameters, a single estimate of the soil horizontal permeability can be obtained via a single-run piezocone sounding using pore pressure measurements taken at the shoulder filter element (u₂) located immediately behind the cone.The proposed interpretation method embodies many of the key parameters (namely the soil shear strength, soil rigidity, and soil confining stresses) likely to influence the soil behaviour and thus the parameter to be interpreted. Numerical analyses demonstrated that the rate of dissipation increases as the soil rigidity or the soil confining pressure increases, which is a consequence of higher excess pore pressure gradient at higher depths or at larger rigidities. The method, which involves a new excess pore pressure normalisation technique, is applicable to both monotonic and dilative dissipation data. The proposed interpretation method is compared to a series of experimental data including two recent field tests. Although the method was calibrated against only a select few cases, its applicability to a wide range of clayey soils was verified.     

Unloading behavior of clays measured by CRS test

The unloading behavior of clays was studied by the Constant Rate of Strain (CRS) test, for three clays: two of them are reconstituted and the other was intact. In the conventional CRS test where the stress monotonically increases, the distribution of the pore water pressure in a specimen is assumed to be parabolic, the effective stress is calculated and then the compression behavior is evaluated. However, this assumption cannot be directly applied the unloading condition. In this study, the pore pressure distribution under unloading was simulated by a cubic polynomial under the assumption that hydraulic conductivity does not change in the unloading process. A unique relation in the e–log σ′_v relation was found, irrespective of both the magnitude of stress or strain and the compression index, C_c, at the unloading test, when the consolidation pressure is normalized by σ′_vmax, which is the consolidation pressure before the unloading test. In addition, the creep strain, which is gained by constant loading before the unloading test, was shown to have a great effect on the unloading behavior: that is, the soil behaves stiffly when subjected to a constant load for a prolonged period of time. A strain rate dependency in the unloading process was also observed particularly for heavily unloaded specimens. The unloading behavior was also investigated by the conventional constant load test. The test results show reasonable agreement with those obtained from the CRS test.     

Effects of principal stress rotation on stress–strain behaviors of saturated clay under traffic–load–induced stress path

A series of cyclic torsional shear tests using hollow cylinder apparatus (HCA) were performed to investigate the effect of principal stress rotation (PSR) on the stress–strain behaviors of saturated soft clay. The traffic–load–induced shear stress path was used in the cyclic test and the investigation mainly concerned the influence of PSR on the shear stiffness and non-coaxiality. It indicated that the effects of PSR substantially depends on the magnitude of deviatoric stress (q?=?{[(σ₁???σ₂)²?+?(σ₂???σ₃)²?+?(σ₁???σ₃)²]/2}^1/2) as well as the intermediate principal stress ratio (b?=?(σ₂???σ₃)/(σ₁???σ₃)). At low deviatoric stress, the trajectory envelope of deviatoric strain path translates with a nearly constant size, showing constant shear stiffness and strong non-coaxiality. However, at high deviatoric stress, the trajectory envelope of deviatoric strain rapidly expands towards instability, showing degenerating shear stiffness and weak non-coaxiality. Moreover, the excess pore water pressure increases and the shear stiffness decreases more rapidly as b value increases. The results can provide an experimental basis for constitutive modelling of clays under traffic–induced loadings.     

煤岩渗透率对孔隙压力变化响应规律的试验研究

 利用自主研发的含瓦斯煤热流固耦合三轴伺服渗流装置,开展不同有效围压条件下分别充CH4与CO2气体时原煤的渗透率与孔隙压力之间关系的试验研究,以探讨在低孔隙压力的环境下,煤岩渗透率对孔隙压力变化响应的敏感性。研究结果表明：(1) 在低孔隙压力的环境下,煤岩渗透率随孔隙压力的增加呈幂函数递减趋势,其过程可分为渗透率加速变化阶段和稳定变化阶段;(2) 相同孔隙压力、有效围压条件下,充CH4气体的煤岩渗透率高于充CO2时的煤岩渗透率;(3) 采用渗透率变化率Dp以及孔隙压力敏感性系数Cp评价渗透率对孔隙压力的敏感性,得出孔隙压力小于1.0 MPa时,煤岩渗透率对孔隙压力的响应程度较为显著,并基于Cp推导出煤岩渗透率与孔隙压力的函数关系式。     

Three-dimensional FDEM numerical simulation of failure processes observed in Opalinus Clay laboratory samples

This study presents the first step of a research project that aims at using a three-dimensional(3D) hybrid finite-discrete element method(FDEM) to investigate the development of an excavation damaged zone(EDZ) around tunnels in a clay shale formation known as Opalinus Clay. The 3D FDEM was first calibrated against standard laboratory experiments, including Brazilian disc test and uniaxial compression test. The effect of increasing confining pressure on the mechanical response and fracture propagation of the rock was quantified under triaxial compression tests. Polyaxial(or true triaxial) simulations highlighted the effect of the intermediate principal stress(s2) on fracture directions in the model: as the intermediate principal stress increased, fractures tended to align in the direction parallel to the plane defined by the major and intermediate principal stresses. The peak strength was also shown to vary with changing σ2.     

动三轴试验中饱和软黏土的孔压特性及其对有效应力路径的影响

 变围压动三轴试验能够同时施加循环变化的偏应力与循环变化的围压,可以模拟地震荷载下动剪应力与动正应力的耦合。通过GDS双向动三轴设备进行一系列饱和软黏土的变围压动三轴试验,系统研究循环偏应力和循环围压耦合对饱和软黏土孔压特性的影响。试验结果表明：在单纯循环围压条件下,饱和软黏土会产生相应的正的瞬时动孔压,但是并没有产生明显的负的瞬时动孔压;在循环偏应力与循环围压耦合情况下,饱和软黏土的孔压时程曲线表现出与常规动三轴试验不同的特性,即动孔压的振幅更大,并且最大动孔压和最小动孔压表现出不同的发展规律：最大动孔压持续增长,而最小动孔压在加载一定周数后趋于稳定。此外,对残余孔压的定义进行量化,并对循环偏应力和循环围压耦合对有效应力路径的影响进行研究。     

单幅值循环动载下饱和砾砂动力特性试验研究

为探究动应力比和围压对饱和砾砂动力特性的影响,通过一系列饱和砾砂动三轴试验,分析了动应力比和围压对饱和砾砂轴向累积应变、回弹模量、动孔隙水压力以及滞回曲线的影响。结果表明:在同一围压下,动应力比增加时,砾砂的轴向累积应变、回弹模量增大,而动孔隙水压力减小;不同围压下,饱和砾砂的轴向累积应变、回弹模量以及动孔隙水压力均随围压的增加而增大;围压的增大有利于提高土体能量耗散的能力,而振次的增加会使土体的耗能作用减小;滞回曲线的倾斜程度随动应力比、围压以及振次的增加而增大,表明土体的回弹模量增加、刚度增大;滞回曲线的演化规律能够更加形象地反映出土体形变、刚度和耗能作用的变化,对土体动力特性进行分析时应结合滞回曲线的演化规律进行综合分析。