Effects of aggregate size distribution and carbon nanotubes on the mechanical properties of cemented gangue backfill samples under true triaxial compression

The mechanical behavior of cemented gangue backfill materials(CGBMs)is closely related to particle size distribution(PSD)of aggregates and properties of cementitious materials.Consequently,the true triaxial compression tests,CT scanning,SEM,and EDS tests were conducted on cemented gangue backfill samples(CGBSs)with various carbon nanotube concentrations(P_(CNT))that satisfied fractal theory for the PSD of aggregates.The mechanical properties,energy dissipations,and failure mechanisms of the CGBSs under true triaxial compression were systematically analyzed.The results indicate that appropriate carbon nanotubes(CNTs)effectively enhance the mechanical properties and energy dissipations of CGBSs through micropore filling and microcrack bridging,and the optimal effect appears at P_(CNT)of 0.08wt%.Taking PSD fractal dimension(D)of 2.500 as an example,compared to that of CGBS without CNT,the peak strength(σ_(p)),axial peak strain(ε_(1,p)),elastic strain energy(Ue),and dissipated energy(U_(d))increased by 12.76%,29.60%,19.05%,and90.39%,respectively.However,excessive CNTs can reduce the mechanical properties of CGBSs due to CNT agglomeration,manifesting a decrease inρ_(p),ε_(1,p),and the volumetric strain increment(Δε_(v))when P_(CNT)increases from 0.08wt%to 0.12wt%.Moreover,the addition of CNTs improved the integrity of CGBS after macroscopic failure,and crack extension in CGBSs appeared in two modes:detour and pass through the aggregates.Theσ_(p)and U_(d)firstly increase and then decrease with increasing D,and porosity shows the opposite trend.Theε_(1,p)andΔε_(v)are negatively correlated with D,and CGBS with D=2.150 has the maximum deformation parameters(ε_(1,p)=0.05079,Δε_(v)=0.01990)due to the frictional slip effect caused by coarse aggregates.With increasing D,the failure modes of CGBSs are sequentially manifested as oblique shear failure,"Y-shaped"shear failure,and conjugate shear failure.

Stress-strain behavior of plastic concrete using monotonic triaxial compression tests

The mechanical behavior of plastic concrete used in the cut-off walls of earth dams has been studied. Triaxial compression tests on the specimens in various ages and mix designs under different confining pressures have been done and the stress-strain behavior of such materials and their strength parameter changes have been experimentally investigated. It has been observed that increasing the confining pressures applied on the specimens causes the material behavior to be alike the more ductile materials and the compressive strength increases considerably as well. Moreover, a parametric study has been carded out to investigate the influence of essential parameters on the shear strength parameters of these materials. According to the research, increasing the coarse to fine aggregates ratio leads to the increase of compressive strength of the specimens as well as the increase of the cohesion and internal friction angle of the materials. Furthermore, the bentonite content decrease and the cement factor increase result in an increase of the cohesion parameter of plastic concretes and decrease of the internal friction angle of such materials.

Temperature-controlled triaxial compression/creep test device for thermodynamic properties of soft sedimentary rock and corresponding theoretical prediction

In deep geological disposal of high-level nuclear waste,one of the most important subjects is to estimate long-term stability and strength of host rock under high temperature conditions caused by radioactive decay of the waste.In this paper,some experimental researches on the thermo-mechanical characteristics of soft sedimentary rock have been presented.For this reason,a new temperature-controlled triaxial compression and creep test device,operated automatically by a computer-controlled system,whose control software has been developed by the authors,was developed to conduct the thermo-mechanical tests in different thermal loading paths,including an isothermal path.The new device is proved to be able to conduct typical thermo-mechanical element tests for soft rock.The test device and the related testing method were introduced in detail.Finally,some test results have been simulated with a thermo-elasto-viscoplastic model that was also developed by the authors.

Study on the Constitutive Model of Marble Based on the Conventional Triaxial Compression Test

The RMB-150B rock mechanics test system was employed to obtain the complete stress-strain test curves under confining pressures of 0-30MPa for marble samples from Ya'an ,Sichuan province. On the basis of former study and the convention triaxial pressure test results ,the complete procedures curves which described the relationships between yielding strength、 peak strength、 residual strength and confining pressure was obtained. Taking the strain softening of rock into account, the bilinear elastic-linear softening-residual perfect plasticity four-linear model was put forward in this paper on the basis of the test results and theory of plasticity. This model was adopted to describe the behaviors of marble in different phases under triaxial compression with the constitutive equation of strain softening phase as focus. The results indicated that the theoretic model fitted in well with the test results.

Experiments and 3D DEM of Triaxial Compression Tests under Special Consideration of Particle Stiffness

Discrete element modelling is commonly used for particle-scale modelling of granular or particulate materials. Developing a DEM model requires the determination of a number of micro-structural parameters, including the particle contact stiffness and the particle-particle friction. These parameters cannot easily be measured in the laboratory or directly related to measurable, physical material parameters. Therefore, a calibration process is typically used to determine the values for use in simulations of physical systems. This paper focuses on how to define the particle stiffness for the discrete element modelling in order to perform realistic simulations of granular materials in the case of linear contact model. For that, laboratory tests and numerical discrete element modelling of triaxial compression tests have been carried out on two different non-cohesive soils i.e. poorly graded fine sand and gap graded coarse sand. The results of experimental tests are used to calibrate the numerical model. It is found that the numerical results are qualitatively and quantitatively in good agreement with the laboratory tests results. Moreover, the results show that the stress dependent of soil behaviour can be reproduced well by assigning the particle stiffness as a function of the particle size particularly for gap graded soil.

Three dimensional discrete element modelling of the conventional compression behavior of gas hydrate bearing coal

To analyze the relationship between macro and meso parameters of the gas hydrate bearing coal(GHBC)and to calibrate the meso-parameters,the numerical tests were conducted to simulate the laboratory triaxial compression tests by PFC3D,with the parallel bond model employed as the particle contact constitutive model.First,twenty simulation tests were conducted to quantify the relationship between the macro–meso parameters.Then,nine orthogonal simulation tests were performed using four meso-mechanical parameters in a three-level to evaluate the sensitivity of the meso-mechanical parameters.Furthermore,the calibration method of the meso-parameters were then proposed.Finally,the contact force chain,the contact force and the contact number were examined to investigate the saturation effect on the meso-mechanical behavior of GHBC.The results show that:(1)The elastic modulus linearly increases with the bonding stiffness ratio and the friction coefficient while exponentially increasing with the normal bonding strength and the bonding radius coefficient.The failure strength increases exponentially with the increase of the friction coefficient,the normal bonding strength and the bonding radius coefficient,and remains constant with the increase of bond stiffness ratio;(2)The friction coefficient and the bond radius coefficient are most sensitive to the elastic modulus and the failure strength;(3)The number of the force chains,the contact force,and the bond strength between particles will increase with the increase of the hydrate saturation,which leads to the larger failure strength.

Triaxial compression strength for artificial frozen clay with thermal gradient

A series of triaxial compression tests for frozen clay were performed by KoDCGF （freezing with non-uniform temperature under loading after K0 consolidation） method and GFC （freezing with non-uniform temperature without experiencing Ko consolidation） method at various confining pressures and thermal gradients. The experimental results indicate that the triaxial compression strength for frozen clay in KoDCGF test increases with the increase of confining pressure, but it decreases as the confining pressure increases further in GFC test. In other words, the compression strength for frozen clay with identical confining pressure decreases with the increase in thermal gradient both in KoDCGF test and GFC test. The strength of frozen clay in KoDCGF test is dependent of pore ice strength, soil particle strength and interaction between soil skeleton and pore ice. The decrease of water content and distance between soil particles leads to the decrease of pore size and the increase of contact area between particles in KoDCGF test, which further results in a higher compression strength than that in GFC test. The compression strength for frozen clay with thermal gradient can be descried by strength for frozen clay with a uniform temperature identical to the temperature at the height of specimen where the maximum tensile stress appears.

Strength of Lightweight Concrete Under Triaxial Compression

The strength of lightweight concrete under triaxial compressive stress is studied experimentally with the concrete triaxial apparatus designed by the authors, and is compared with that of normal concrete under the same stress state. Ninety-five 100 mm cubes under twenty stress ratios are tested. As compared with normal concrete, it is found that not only the multiaxial compressive strength of lightweight concrete is small, but also the ratio of the multiaxial compressive strength to the uniaxial compressive strength is small. The influence of the intermediate principal stress on the multiaxial strength of lightweight concrete is discussed. The strength criteria which are expressed in the principal stresses and the octahedral stresses respectively are proposed.

Triaxial creep tests of weak sandstone from fracture zone of high dam foundation

The lithology of fracture zone which was developed at the dam foundation of a hydropower station is weak sandstone with poor integrity and pore cementation contact.Its creep properties have a significant impact on the deformation and stability of the dam.Based on the characteristics of loose organizational structure,high moisture content and poor mechanical properties,the triaxial compression tests and creep tests were carried out,respectively.The results show significant non-linear,low strength and no obvious strength peaks.Both axial and lateral strains are achieved more than 3%when the tests are failed.The weak sandstone has a significant creep property,but only transient and steady state appear under low stress.Increased stress causes creep intensified and lateral strain gradually exceeds axial strain.In the failure stage,it has characteristics of large axial plastic deformation,obvious volumetric ductility dilation and large steady creep rate.The accelerated creep appears shortly after transient loading under confining of pressures 1.0 MPa and 1.5 MPa.Therefore,an improved Burgers creep model considering the non-linear characteristics of weak sandstone is built based on hyperbolic equation and the creep parameters are identified.This model can well describe the creep properties of weak sandstone.

An investigation into the effects of lime on compressive and shear strength characteristics of fiber-reinforced clays

To meet the ever-increasing construction demands around the world during recent years,reinforcement and stabilization methods have been widely used by geotechnical engineers to improve the performances and behavior of fine-grained soils.Although lime stabilization increases the compressive strength of soils,it reduces the soil ductility at the same time.Recent research shows that random fiber inclusion modifies the brittleness of soils.In the current research,the effects of lime and polypropylene(PP)fiber additions on such characteristics as compressive and shear strengths,failure strain,secant modulus of elasticity(E50)and shear strength parameters of mixtures were investigated.Kaolinite was treated with 1%,3% and 5% lime by dry weight of soil and reinforced with 0.1% monovalent PP fibers with the length of 6 mm.Samples were prepared at optimum conditions and cured at 35℃ for 1 d,7 d and 28 d at 90% relative humidity and subsequently subjected to uniaxial and triaxial compression tests(UCT and TCT)under cell pressures of 25 kPa,50 kPa and 100 kPa.Results showed that inclusion of random PP fibers to clay-lime mixtures increases both compressive and shear strengths as well as the ductility.Lime content and curing period were found to be the most influential factors.Scanning electron microscopy(SEM)analysis showed that lime addition and the formation of cementitious compounds bind soil particles and increase soil/fiber interactions at interface,leading to enhanced shear strength.The more ductile the stabilized and reinforced composition,the less the cracks in roads and waste landfill covers.

Influence of particle breakage on the isotropic compressibility of sands

Particle breakage is a theme of great focus on affecting the behavior of granular soil.This paper presents an experimental investigation on the influence of particle breakage on the isotropic compressibility of the precrushed sands using a number of drained isotropic consolidation tests.Particle breakage resulted in movement of the compression lines followed by the rebound lines towards a decrease in the void ratio,implying that particle breakage caused a more contractive soil.Particle breakage impaired the bulk deformation modulus by increasing the compression coefficient for the compression behavior of the precrushed sands,but showed a complex effect on the bulk deformation modulus and rebound coefficient for the rebound behavior of the precrushed sands.However,particle breakage caused an increase in the compression indexes of the precrushed sands but showed a complex effect on the rebound indexes of the precrushed sands.In the e-p’plane and the e-logp’plane,the compression lines and rebound lines of the precrushed sands were curved.A generalized model was proposed to straighten the compression and rebound lines of the precrushed sands in the e-(p’p_(a))^(α) plane.Particle breakage resulted in a general rotation and translation of the linear compression and rebound lines of the precrushed sands in the e-(p’p_(a))^(α)plane.The critical state line and isotropic consolidation line on the loosest state of silica sand no.5 were curved in the e-logp’plane but straightened in the e-p’^(α=0.7) plane.In the e-p’^(α=0.7 )plane,a reasonable linear critical state line of silica sand no.5 was proposed by adjusting it to match the isotropic consolidation line on the loosest state.

Development and application of multi-field coupled high-pressure triaxial apparatus for soil

The increasing severity of ground subsidence,ground fissure and other disasters caused by the excessive exploitation of deep underground resources has highlighted the pressing need for effective management.A significant contributing factor to the challenges faced is the inadequacy of existing soil mechanics experimental instruments in providing effective indicators,creating a bottleneck in comprehensively understanding the mechanisms of land subsidence.It is urgent to develop a multi-field and multi-functional soil mechanics experimental system to address this issue.Based soil mechanics theories,the existing manufacturing capabilities of triaxial apparatus and the practical demands of the test system,a set of multi-field coupled high-pressure triaxial system is developed tailored for testing deep soils(at depths of approximately 3000 m)and soft rock.This system incorporates specialized design elements such as high-pressure chamber and horizontal deformation testing devices.In addition to the conventional triaxial tester functions,its distinctive feature encompass a horizontal deformation tracking measuring device,a water release testing device and temperature control device for the sample.This ensemble facilitates testing of horizontal and vertical deformation water release and other parameters of samples under a specified stress conditions,at constant or varying temperature ranging from-40℃–90℃.The accuracy of the tested parameters meets the requirements of relevant current specifications.The test system not only provides scientifically robust data for revealing the deformation and failure mechanism of soil subjected to extreme temperature,but also offers critical data support for major engineering projects,deep exploration and mitigation efforts related to soil deformation-induced disaster.

动力打桩桩周泥岩损伤特性与损伤模型

针对泥岩地基同施工场地条件下动力打入桩的承载力差异性大且部分打入桩承载力不足等异常现象,文章开展了桩周0.2m处打桩前后的桩周泥岩单轴抗压强度对比试验,和打桩后的桩端中风化泥岩三轴压缩试验,明确打桩后桩周泥岩的损伤特性,基于统计损伤理论建立考虑受打桩影响的桩周泥岩损伤本构模型,并对泥岩地基打入桩的承载性能进行探讨。研究表明:打桩后桩周0.2m处泥岩强度平均损伤28.2%,弹性模量平均损伤41.4%;桩端中风化泥岩损伤后抗剪强度参数平均值为c=217.2kPa、φ=21.6°,φ仅是同场地原状强风化和全风化泥岩的49.1%和51.4%;打桩后桩周泥岩应力-应变曲线的损伤模型计算值的与实测值吻合较好,验证模型的合理性;泥岩地基打入桩承载力异常特征为桩端受力小,但桩端沉降显著,静载破坏时桩端阻力的平均占比为5.25%;动力打桩对桩周泥岩的损伤是泥岩地基打入桩承载力异常的重要原因;泥岩地基打入桩的设计计算与数值模拟采用原状泥岩力学参数不合理,泥岩损伤后不易恢复,建议按损伤特性赋予参数。研究结果对泥岩地基打入桩的设计、施工、承载力评价有较强的参考价值。

糯米浆灰土无侧限压缩和三轴剪切离散元分析

糯米浆灰土是一种常见的遗址保护建筑材料,为研究糯米浆灰土压缩加载过程中的糯米-石灰胶结破坏和接触组构演化情况,采用离散单元法开展了糯米浆灰土无侧限压缩和三轴剪切离散元模拟。首先基于软胶结模型考虑糯米浆-石灰的强度特征,制备了糯米浆灰土离散元试样;然后通过参数敏感性分析为试样赋予合理的接触模型等效模量和胶结强度参数,最后对试样开展无侧限和三轴压缩模拟。结果表明:离散元模拟能再现糯米浆灰土加载试验的主要特征;加载过程中胶结破坏数量先缓后快增加,最终趋于平缓,胶结破坏呈现一定的聚集效应;无侧限压缩下胶结接触主要发生拉伸破坏,随着围压增加,剪切破坏接触数量增加;试样偏组构快速增加段为胶结破坏诱发。

基于胞元模型的土体应变局部化试验与机理研究

土体的碎散性和天然性使其变形和强度特性具有明显的尺度效应,所以研究土体应变局部化过程,不能忽略级配颗粒的分布和比例关系,须从多个尺度层面表征宏观力学特性。基于胞元模型理论,将土体视作由加强颗粒和基质组成的散体材料,通过三轴压缩试验研究不同加强颗粒粒径试样的内禀尺度规律,并通过数值模拟方法研究了细观层面剪切带的形成过程和启动机理;引入平均应变能释放系数,量化了土体在到达峰值应力之后试样整体的应变能转化情况,再现了多带的启动和渐进竞争过程。结果表明:不同尺度的加强颗粒强度试验说明颗粒的非连续性使土体具有尺寸效应,内禀尺度与粒径之比随加强颗粒尺度增大而减小。在应力峰值前非弹性耗散能增加,引发应变局部化;峰值后,剪切带内应变急速增大,带外发生回弹。

三向应力下板岩力学特性试验研究

以川西某隧道开挖为研究背景,针对板岩在不同应力下的损伤力学特性,在MTS815伺服试验系统上开展了常规三轴压缩试验。结果表明:(1)板岩的起裂应力比相对于闭合应力比与损伤应力比受围压的影响较小,且随着围压增大,板岩表现出脆性降低、延性增强的状态;(2)随着围压的升高,扩容应力和抗压强度均增大,体积扩容点前移,两者的比值先降低后稳定在84%左右;(3)泊松比、变形模量和弹性模量与围压的增大成正比,变形模量普遍大于弹性模量,且后者受围压的影响更加显著,通过m-c包络线法和σ_(1)-σ_(3)最佳关系曲线法求得板岩黏聚力为35.04 MPa,内摩擦角为40.42°;(4)板岩破坏过程中加载总能量与耗散能均增大,两者的差值呈“月牙状”;弹性能先升高后降低,在峰值应力处达到存储极限;峰值应力前的损伤演化可分为损伤起始、损伤稳定累计和损伤加速三个阶段,且在相同损伤程度下,高围压环境中的试样会产生更大的应力变形。

基于统计损伤修正的冻结砂—黏混合土改进Duncan-Chang模型

为描述冻结砂—黏混合土的偏应力—应变关系,进行不同含砂量冻结砂—黏混合土的冻土三轴压缩试验。基于改进的Duncan-Chang模型(D-C模型),引入统计损伤理论,拟合三轴压缩试验结果,分别建立以Mises强度准则和应变发展为微元强度变量的统计损伤修正模型,对比计算结果与试验数据。结果表明:冻结砂—黏混合土偏应力峰值随含砂量的增加而增加,偏应力—应变曲线由应变硬化型逐渐过渡到应变软化型的临界含砂量为50%。当含砂量介于20%~60%时,基于统计损伤理论的修正改进D-C模型能有效拟合应变硬化型曲线和具有轻微应变软化现象的偏应力—应变曲线。当含砂量为90%时,基于应变发展的统计损伤修正模型的偏应力峰值与试验结果相差3%,具有较强适用性。该结果为不同微元强度变量的统计损伤修正模型提供参考。

石灰-沸石粉改良膨胀土强度特性及微观机理研究

针对膨胀土胀缩性造成的工程病害问题及石灰改良膨胀土产生的污染问题,以河南南阳地区膨胀土为研究对象,制备素膨胀土及不同石灰和沸石粉掺量的改良土试样。对试样进行无侧限抗压试验、自由膨胀率试验、三轴试验、X射线衍射(XRD)分析和扫描电子显微镜(SEM)测试,研究石灰-沸石粉复合改良膨胀土的强度特性及微观加固机理。结果表明:石灰改良和石灰-沸石粉复合改良均能有效增强土体的强度并降低自由膨胀率,复合改良效果更好,石灰和沸石粉的最佳掺量分别为6%(质量分数)和12%(质量分数);石灰改良土和复合改良土的应力-应变曲线转化为应变软化型;石灰的加入显著提升了膨胀土的黏聚力,随着沸石粉掺量的递增,复合改良土三轴抗剪强度不断增大,内摩擦角呈先增大后减小的趋势;沸石粉与石灰发生火山灰反应,生成大量胶凝物质,填充土体孔隙并使微观结构更加密实,增强土颗粒间的摩阻力,宏观层面上土体强度增强。

隧道弃渣级配碎石细观力学参数研究

隧道弃渣级配碎石作为道路基层填料,能有效避免资源浪费与环境污染。有限元方法将其视为连续介质进行研究时存在一定局限性,而采用离散元对颗粒材料进行模拟时,细观参数对其宏观力学性质的影响并不明确。基于三轴压缩试验和数值模拟,通过正交控制参数变化分析,揭示了颗粒间细观参数与宏观力学参数之间的关联性,明确了细观参数对隧道弃渣级配碎石路用力学性能的影响。结果表明:细观参数中,切向黏结强度对黏聚力值的影响最大,而摩擦系数与内摩擦角值变化联系更密切;初始杨氏模量与刚度比呈负相关,其他参数对其影响不大;与泊松比相关的变化关系中,刚度比与切向黏结强度较其他参数对其影响更大。研究成果可为颗粒流模拟级配碎石的细观参数标定提供参考,也为级配碎石作为道路基层填料的力学性能研究提供了基础。

土工格室加筋正常固结粉质黏土应力应变响应

土工格室加筋正常固结粉质黏土对工程建设意义重大。基于土工格室与填土的相互作用机制和增量法,采用弹塑性理论、邓肯-张双曲线模型、修正剑桥屈服函数以及剪胀方程,推导了土工格室加筋正常固结黏土的应力应变响应计算模型,进行了土工格室加筋正常固结黏土的三轴固结排水剪切试验,实测不同围压下的应力应变关系,采用三轴试验结果验证了所提理论模型的正确性和可靠性。此外,采用所提理论模型进行了参数敏感分析,研究了填土的力学性质参数、格室的网格尺寸和力学性质参数对于加筋土应力应变响应的影响。研究结论表明,与砂砾料相似,土工格室约束作用对于正常固结黏土的内摩擦角影响较小,而黏聚力有所增加;随着轴向应变的增加或围压的减小,加筋效果逐渐明显;加筋正常固结黏土的强度和刚度随着非线性弹性常数k的增加、强度参数φ的增加和Rf的减小而增大,常数n对于加筋土应力应变响应影响较小。本研究成果可为工程建设提供理论借鉴。