Effect of Freeze-Thaw Cycles on Mechanical Properties and Permeability of Red Sandstone under Triaxial Compression

Geological disasters will happen in cold regions because of the effects of freeze-thaw cycles on rocks or soils, so studying the effects of these cycles on the mechanical characteristics and permeability properties of rocks is very important. In this study, red sandstone samples were frozen and thawed with o, 4, 8 and 12 cycles, each cycle including 12 h of freezing and 12 h of thawing. The P-wave velocities of these samples were measured, and the mechanical properties and evolution of the steady-state permeabilities were investigated in a series of uniaxial and triaxial compression tests. Experimental results show that, with the increasing of cyclic freeze-thaw times, the P-wave velocity of the red sandstone decreases. The number of freeze-thaw cycles has a significant influence on the uniaxial compressive strength, elastic modulus, cohesion, and angle of internal friction. The evolution of permeability of the rock samples after cycles of freeze-thaw in a complete stress-strain process under triaxial compression is closely related to the variation of the microstructure in the rock. There is a highly corresponding relationship between volumetric strain and permeability with axial strain in all stages of the stress-strain behaviour.

Shear behavior of ultrafine magnetite tailings subjected to freeze-thaw cycles

To study the shear behavior of the ultrafine magnetite tailings subjected to freeze-thaw cycles,unconsolidated-undrained shear tests were conducted on ultrafine-grained tailings that were subjected to 1-11 cycles of freeze-thaw and defined as a type of clayey silt under confining pressures of 100,200,and 300 kPa.Taking the number of freeze-thaw cycles,cooling temperature,initial dry density,and moisture content as the four main influencing factors of shear behavior of the tailings samples,the shear stress-strain curve,compression modulus,failure strength,cohesion,and internal friction angle were measured.The results show that the freeze-thaw cycle has an obvious weakening effect on the shear behavior of the tailings material,and the shear mechanical parameters are affected by a combination of confining pressure,freeze-thaw cycle condition,and initial physical-mechanical properties of the tailings samples.Through the microstructural analysis of the tailings samples subjected to freeze-thaw cycles,it shows that the freeze-thaw cycle mainly affects the porosity,bound water,and arrangement of the tailings particles.Subsequently,the macroscopic changes in shear strength indexes emerge,and then the stability of the tailings dam will decrease.

Damage mechanism of soil-rock mixture after freeze-thaw cycles

As a widely distributed geological and engineering material,the soil-rock mixture always undergoes frequentative and short-term freeze-thaw cycles in some regions.Its internal structure is destroyed seriously,but the damage mechanism is not clear.Based on the damage factor,the damage research of properties of soil-rock mixture after different times of freeze-thaw cycles is investigated.Firstly,the size-distributed subgrade gravelly soil samples are prepared and undergo different times of freeze-thaw cycles periodically(0,3,6,10),and indoor large-scale triaxial tests are completed.Secondly,the degradation degree of elastic modulus is considered as a damage factor,and applied to macro damage analysis of soil-rock mixture.Finally,the mesoscopic simulation of the experiments is achieved by PFC3D,and the influence on strength between soil-rock particles caused by freeze-thaw cycles is analyzed.The results show that freeze-thaw cycles cause internal damage of samples by weakening the strength between mesoscopic soil-rock particles,and ultimately affect the macro properties.After freeze-thaw cycles,on the macro-scale,elastic modulus and shear strength of soil-rock mixture both decrease,and the decreasing degree is related to the times of cycles with the mathmatical quadratic form;on the meso-scale,freeze-thaw cycles mainly cause the degradation of the strength between soil-rock particles whose properties are different significantly.

Experimental study of dynamic resilient modulus of subgrade soils under coupling of freeze–thaw cycles and dynamic load

Although the dynamic properties of subgrade soils in seasonally frozen areas have already been studied, few researchers have considered the influence of shallow groundwater during the freeze–thaw(F–T) cycles. So a multifunctional F–T cycle system was developed to imitate the groundwater recharge in the subgrade during the freezing process and a large number of dynamic triaxial experiments were conducted after the F–T cycles. Some significant factors including the F–T cycle number, compaction degree, confining pressure, cyclic deviator stress, loading frequency, and water content were investigated for the resilient modulus of soils. The experimental results indicated that the dynamic resilient modulus of the subgrade was negatively correlated with the cyclic deviator stress, F–T cycle number, and initial water content, whereas the degree of compaction, confining pressure, and loading frequency could enhance the resilient modulus. Furthermore, a modified model considering the F–T cycle number and stress state was established to predict the dynamic resilient modulus. The calculated results of this modified model were very close to the experimental results. Consequently, calculation of the resilient modulus for F–T cycles considering the dynamic load was appropriate. This study provides reference for research focusing on F–T cycles with groundwater supply and the dynamic resilient moduli of subgrade soils in seasonally frozen areas.

Creep damage properties of sandstone under dry-wet cycles

Rock creep properties can be used to predict the long-term stability in rock engineering.In reservoir bank slopes,sandstones which are frequently used in the bank slope undergoing longterm effects of dry-wet(DW) cycles due to periodic water inundation and drainage may gradually accumulate creep deformation,resulting in rock structure’s damage or even geological hazards such as landslides.To fully investigate the effect of DW cycles on the creep damage properties of sandstone,triaxial creep tests were conducted on saturated sandstone with different DW cycles by using a triaxial rheometer apparatus.The experimental results show that both the instantaneous strain and the stabilized strain increase with the DW cycles.In addition,using the Burgers model,four kinds of functions including an exponentially decreasing function,a linearly decreasing function,a linearly increasing function and an exponentially increasing function were proposed to express the relationships between the shear modulus,viscoelastic parameters of the Burgers model and the deviatoric stress under different DW cycles.Through comparative analysis,it is found that the theoretical curves generated using proposed four kinds of functions are in good agreement with the experimental data.Furthermore,macromorphological and microstructural observations were performed on specimens after various triaxial rheological tests.For samples with small number of DW cycles,approximately X-shaped fracture surfaces were observed in shear failure zones,whereas several shear fractures including obvious axial and horizontal tensile cracks,and flaws were found for samples with relatively large DW cycles due to long-term propagation and evolution of micro-fissures and micro-pores.Furthermore,as the DW cycles increases,the variation in micro-structure of samples after creep failure was summarized into three stages,namely,a stage with good and dense structure,a stage with pore and fissure propagation,and a stage with extensive increase of pores,fissures and loose particles.It is concluded that the combination effect of permeation of water molecules through pores and fissures within sandstone,and the propagation of preexisting pores and fissures owing to the dissolution of mineral particles leads to further deterioration of the mechanical properties of sandstone as the number of DW cycles increases.This study provides a fundamental basis for evaluating the long-term stability of reservoir bank slopes under cyclic fluctuations of water level.

Dynamic mechanical characteristics of frozen subgrade soil subjected to freeze-thaw cycles

As a widely-applied engineering material in cold regions, the frozen subgrade soils are usually subjected to seismic loading, which are also dramatically influenced by the freeze-thaw(F-T)cycles due to the varying temperature. A series of dynamic cyclic triaxial experiments were conducted through a cryogenic triaxial apparatus for exploring the influences of F-T cycles on the dynamic mechanical properties of frozen subgrade clay.According to the experimental results of frozen clay at the temperature of-10℃, the dynamic responses and microstructure variation at different times of F-T cycles(0, 1, 5, and 20 cycles) were explored in detail.It is experimentally demonstrated that the dynamic stress-strain curves and dynamic volumetric strain curves of frozen clay are significantly sparse after 20F-T cycles. Meanwhile, the cyclic number at failure(Nf) of the frozen specimen reduces by 89% after 20freeze-thaw cycles at a low ratio of the dynamic stress amplitude. In addition, with the increasing F-T cycles,the axial accumulative strain, residual deformation,and the value of damage variable of frozen clay increase, while the dynamic resilient modulus and dynamic strength decrease. Finally, the influence of the F-T cycles on the failure mechanisms of frozen clay was discussed in terms of the microstructure variation. These studies contribute to a better understanding of the fundamental changes in the dynamic mechanical of frozen soils exposed to F-T cycles in cold and seismic regions.

The law of strength damage and deterioration of jointed sandstone after dry-wet cycles

Under the periodic rise and fall of the water level in the Three Gorges Reservoir in China,the rock mass in the ebb and flow zone of the slope is always in a state of a dry-wet cycle.In order to explore the influence of dry-wet cycle on mechanical properties of jointed sandstone,the triaxial and uniaxial compression tests of dry-wet cycle of jointed sandstone were carried out.For the experiment,four groups of samples with different numbers of joints were set up,and the jointed rock samples were subjected to 20 dry-wet cycles.Using both the triaxial compression test and the Mohr-Coulomb(M-C)rock fracture criterion,the strength envelope of the sandstone samples was fitted,and their strength degradation was further analyzed and studied.The results show that:(1)The peak intensity and elastic modulus of the sandstone samples decrease with increased number of dry-wet cycles.(2)The total deterioration of mechanical properties of intact rock samples is bigger than that of jointed sandstone samples as the number of dry-wet cycles increases.(3)With the increase of confining pressure,the peak intensity of intact sandstone samples increases much more than that of jointed sandstone samples,which indicates that joints and their numbers have obvious influence.(4)Joints and their numbers play an important role in guiding the damage effects of sandstone samples,which weaken the damage caused by dry-wet cycles.Therefore,the envelope of the M-C strength criterion of intact sandstone samples moves more than that of jointed sandstone samples.

干湿循环作用下掺铁尾矿砂水泥土耐久性研究

为了探究掺铁尾矿砂水泥土在特殊环境条件下的耐久性,对掺铁尾矿砂水泥土进行了干湿循环试验,并与相同条件下素水泥土进行了对比。通过干湿循环试验研究发现,高掺量水泥强度损失较低。铁尾矿砂的掺入会延缓水泥土受干湿循环次数增加所产生的强度损失,但随着干湿循环次数增加其强度损失与素水泥土逐渐接近,干湿循环导致的铁尾矿砂水泥土强度降低主要表现在黏聚力大幅下降,内摩擦角变化较小。利用极限偏应力(σ_(1)-σ_(3))_(max)、c,φ值与孔隙率η、胶结物体积含量Gv等参数的关系,建立了干湿循环循环作用下铁尾矿砂水泥土强度损伤模型,通过计算值与试验值对比,该损伤模型可以有效地预测铁尾矿砂水泥土在干湿循环下的极限偏应力(σ_(1)-σ_(3))_(max)及c,φ值。

冻融循环条件下含根系土的力学特性

植物根系对土体有力学加筋作用,可以有效提高土体的强度,防止浅层滑坡和地表侵蚀,因而广泛应用于边坡防护。然而在中西部的季节冻土区,植被边坡的土体反复冻结融化,在植物根系力学加筋和冻融损伤的双重作用下,其力学性质也会发生变化,对边坡稳定性产生不确定因素。因此,为探究冻融和根系分布对土体的复合影响,采用贡嘎山海螺沟流域土壤及花楸根系进行三轴固结不排水试验,研究了冻融循环作用下,含根系土体的强度变化。考虑不同冻融循环次数(0、1、5次)和根系分布方式(竖向均匀分布:三层各一根;水平均匀分布:中间一层三根),绘制应力应变曲线和孔压应变曲线,对比分析两种试样的应力应变特性,并通过抗剪强度包线获得总应力抗剪强度指标和有效应力抗剪强度指标。结果表明:(1)随冻融循环次数的增加,三层各一根试样和中间三根试样的峰值强度均逐渐降低,黏聚力和内摩擦力也逐渐减小,在低围压(25 kPa)条件下冻融循环作用对含根系土强度减弱作用较为明显,但在高围压条件下,植物根系对土的强度的增强作用仍不可忽视;(2)即使发生了冻融循环,根系仍在一定程度上提高了土体的强度,其增强效果受到分布方式的影响,三层各一根的排列方式增强作用最显著;(3)在相同围压、相同冻融循环次数条件下,根系沿竖向分布的试样峰值强度和黏聚力均较高,此外,相较于不含根的素土,根系分布方式主要影响土的黏聚力,对内摩擦角的影响不明显。研究成果可为寒区植被护坡的工程实践提供理论依据。

软黏土带压冻融循环下不固结力学特性试验研究

人工冻结法和季节性冻土地区土体通常在一定围压下经历冻融循环作用。受到冻融围压和不排水条件影响,融化后土体残存超孔压,并非完全固结状态,开展此工况下土体不排水力学性能研究具有重要工程和科学意义。通过自主改进的温控三轴仪,对重塑软黏土开展冻融后未完全固结不排水三轴剪切和微观试验,分析了冻融围压、次数和冻结温度对冻融后软黏土力学特性影响。结果表明:与零围压相比,带围压冻融后土体中絮凝结构和孔隙数量增大,加剧了冻融对土体强度和模量的弱化作用;随着冻结温度降低和冻融次数增大,土体力学性能弱化效应越明显。

The dual cycle bridge detection of piezoresistive triaxial accelerometer based on MEMS technology

A cycle bridge detection method, which uses a piezoresistive triaxial accelerometer, has been described innovatively. This method just uses eight resistors to form a cycle detection bridge, which can detect the signal of the three directions for real time. It breaks the law of the ordinary independent Wheatstone bridge detection method, which uses at least 12 resistors and each four resistors connected as a Wheatstone bridge to detect the output signal from a specific direction. In order to verify the feasibility of this method, the modeling and simulating of the sensor structure have been conducted by ANSYS, then the dual cycle bridge detection method and independent Wheatstone bridge detection method are compared, the result shows that the former method can improve the sensitivity of the sensor effectively. The sensitivity of the x, y-axis used in the former method is two times that of the sensor used in the latter method, and the sensitivity of the z-axis is four times. At the same time, it can also reduce the cross-axis coupling degree of the sensor used in the dual cycle bridge detection method. In addition, a signal amplifier circuit and adder circuit have been provided, Finally, the test result of the ＂eight-beams/mass＂ triaxial accelerometer, which is based on the dual cycle bridge detection method and the related circuits, have been provided. The results of the test and the theoretical analysis are consistent, on the whole.

不同冻融循环作用下分散土力学性质及微观结构

通过探究不同冻融周期条件下分散土的微观孔隙结构及力学特性,分析冻融环境对土体强度的影响。利用三轴试验及扫描电镜分别研究冻融循环下分散土力学性质变化规律及微观结构,并通过图像处理软件IPP定量获取土样的微观孔隙参数,包括孔隙率、平均孔径及平均形状系数。研究结果表明,土体结构内部孔隙数量及孔径大小均为呈现增加趋势,土体颗粒间的黏结效果减弱;分散土的破坏强度主要受土体孔隙的数量、大小及孔隙形状的影响,且与平均孔径、孔隙率及平均形状系数均呈负相关。由此得出分散土在不同冻融循环次数条件下微观结构会不断变化,进而导致土体的力学特性发生变化。

冻融循环下罗布泊盐渍土的累积变形特性研究

土体的累积变形特征对于地下结构设计与路基长期动力稳定性能保持具有关键作用。在盐渍土地区由于冻融循环作用影响,盐渍土中赋存的易溶盐溶液与结晶盐颗粒,随着温差变化与水分迁移产生的结晶-溶解、积盐-脱盐过程,可引起土体颗粒结构与物理性质变化,从而影响盐渍土动力学性质。为研究铁路动荷载作用下新疆罗布泊地区盐渍土的累积变形特性,在室内冻融循环试验的基础上开展不同应力水平的动三轴试验,提出盐渍土塑性行为判定准则,得到盐渍土累积塑性应变预测模型及模型参数。结果表明:盐渍土的累积塑性应变随冻融循环的次数增加和动应力幅值的增大而增大,在模拟铁路振动荷载条件下,研究区盐渍土的累积塑性应变行为主要为塑性安定型和塑性蠕变型,其累积塑性应变速率εp与循环振次N之间呈幂函数关系。在此基础上,建立了考虑冻融循环次数和动应力的盐渍土累积塑性应变预测模型。研究成果可为盐渍土路基在冻融循环和循环动荷载下的沉降预测和动力稳定性评估提供参考。

根直径及干湿循环对根土复合体抗剪强度影响的试验研究

探究植被根系对土体的加固效应及干湿循环对根土复合体强度的劣化机制,为边坡生态治理及灾害防治提供帮助。开展了不同根直径及干湿循环的根土复合体三轴试验,利用电子显微镜对根土复合体的微观结构进行分析。研究结果表明:新鲜根、饱和根和干燥根的抗拉强度随直径的增大呈幂律减小。干燥根的抗拉强度最大,其次是新鲜根和饱和根。根土复合体的应力—应变曲线表现为持续硬化。随着应变的增大,偏应力先迅速增大后缓慢增加,最终保持稳定。随着根直径的增大,根土复合体的抗剪强度先增大后减小。根直径过小或过大,根对土壤的加固效果减弱。根系显著改变土壤黏聚力,对内摩擦角影响较小。抗剪强度随围压的增加而增大,随干湿循环的增加而减小。随着干湿循环次数的增加,抗剪强度和黏聚力呈线性下降,土颗粒团聚体分解,土体裂缝和孔隙面积呈指数增长。根土复合体的破坏形式为剪胀破坏。考虑干湿循环,修正的根土复合体强度预测模型是有效的,预测结果与试验数据吻合较好。

高填方粗粒土干湿循环及蠕变特性三轴试验研究

白鹤滩水电站库区蓄水后,其高填方场地将长期经历水位升降,对于场地的变形和稳定来说十分不利。依托于白鹤滩水电站库区移民安置区消落带高填方场地稳定性研究项目,通过大型常规三轴剪切试验和蠕变试验分析了粗粒土在干湿循环作用和饱水条件下的应力应变关系、剪切形态特征、长期强度、变形规律等,得到主要结论如下:①粗粒土的强度随着干湿循环次数的增加逐渐降低;②干湿循环次数n≤3时,粗粒土试样先剪缩后剪胀,n>3时,粗粒土只有剪缩变形;③粗粒土的峰值偏应力随着干湿循环次数的增加逐渐降低,围压越高干湿循环作用对粗粒土抗剪强度损伤劣化的影响越大;④粗粒土的长期强度较剪切破坏强度有所降低,在低围压下降幅较小,一般情况下可达到剪切破坏强度的80%甚至更高,在高围压下由于颗粒破碎严重粗粒土的长期强度损伤较大。

干湿循环对压实红黏土动强度与动模量影响研究

采用自制一次成型模具制备压实度分别为90%、93%和96%的三轴试样,自制干湿循环试验装置对试样进行喷雾加湿和暖风干燥,定量模拟现场路基土含水率变化过程获得不同湿度和不同干湿循环次数的试样,通过常含水率动三轴试验测试其动强度与动模量变化规律。结果表明:累积应变随振次的变化分为塑性安定型、临界型和破坏型;临界动应力和最大动弹模随压实度的降低而减小,且减小幅度逐渐增大,随含水率的增大压实度的影响减弱;临界动应力和最大动弹模随含水率的增大而减小,临界动应力在含水率大于最优含水率时减小幅度相对较大;临界动应力和最大动弹模随循环次数的增多而降低,且降低幅度逐渐减小。

基于压实与干湿循环综合作用的粉土路堤稳定性演变规律研究

为研究压实和干湿循环综合作用下路堤边坡的长期稳定性,基于室内三轴试验与数值仿真分析,以某高速公路路堤段粉土为研究对象,开展了不同干湿循环次数、压实度下的室内三轴剪切试验,获取了不同干湿循环次数下的抗剪强度参数,并将参数导入至数值仿真模型中开展计算分析,获取了干湿循环对潜在滑移面的影响规律,探索了粉土路堤长期稳定性演变特征。研究表明,压实粉土的强度随干湿循环次数增加而减小,以K=0.93为例,经历干湿循环2、4、6、8次后,黏聚力分别下降24.1%、38.4%、44.1%、51.0%,内摩擦角分别下降11.4%、17.5%、21.9%、25.4%;在不同干湿循环次数下,粉土路堤边坡均存在明显的潜在滑移面,同时滑体上还出现了大量剪切裂隙;路堤边坡在填筑初期的安全系数可满足规范要求,但干湿循环会使路堤的稳定性会出现明显降低,K=0.96、0.93、0.90时,经历8次干湿循环后路堤的安全系数分别下降0.878、0.736、0.587,可见在路堤设计时留足安全储备十分必要,在极端气候偏多的环境下更是应该如此;为提升粉土路堤长期稳定性,提高抵抗病害的韧性,应当重视压实质量和排水。研究成果可为粉土路堤安全储备设计提供有益参考。

干湿冻融循环条件下重塑性黄土的强度劣化规律及非线性模型研究

新疆某地的输水明渠工程途经大面积湿陷性黄土区域,在间歇供水的运行工况及季节性温差变化的共同作用下,经常发生渠坡滑动破坏现象。为深入研究渠坡变形破坏机制,通过开展干湿冻融循环条件下黄土的三轴试验和扫描电镜试验,研究其变形规律及微观机制,最后基于黄土的变形规律对邓肯-张模型进行改进。三轴试验结果表明:随着干湿冻融循环次数的增加,黄土的应力-应变曲线下降,体变曲线上升,且变化幅度越来越小,当循环次数大于5次后,应力-应变曲线和体变曲线基本不变,黏聚力和内摩擦角均呈现出指数函数衰减的变化趋势,其中黏聚力受干湿冻融循环作用的影响较大。电镜扫描试验结果表明:通过扫描电镜进行微观分析,在干湿冻融循环作用下,大的颗粒和团聚体被分裂,颗粒间的胶结物质减少,面接触增加,大孔隙面积占比减少,小孔隙和中孔隙面积占比、三维孔隙率和孔隙圆形程度增加,分形维数下降;三维孔隙率和分形维数与黏聚力呈显著相关,大孔隙面积占比与内摩擦角呈显著相关。由于邓肯-张模型不能合理反映土体的软化现象,采用二次函数对邓肯-张模型进行修改,并通过修改公式对泊松比进行拟合,以此提出改进邓肯-张模型,改进后的模型对不同循环次数下黄土三轴试验结果拟合效果较好。随着循环次数的增加,应力-应变模型参数k和q呈指数函数下降的趋势,应力-应变模型参数o呈指数函数上升的趋势,体变模型参数j、l呈指数函数上升的趋势,体变模型参数t呈指数函数下降的趋势。

冻融循环作用下硫酸盐盐渍土物理力学特性试验研究

为研究硫酸盐盐渍土在冻融循环作用下的力学特性,结合京沈高速铁路朝阳段的工程实际,利用三轴不固结不排水试验对不同冻融循环次数(最多50次)、盐质量分数(0%、2%和5%)和围压(100 kPa、200 kPa、300 kPa和400 kPa)下的硫酸盐盐渍土进行研究,分析不同试验条件下硫酸盐盐渍土的抗剪强度特性及强度劣化机理。研究结果表明:随着冻融循环次数、盐质量分数和围压的逐渐增加,硫酸盐盐渍土的应力-应变曲线逐渐由应变软化向应变硬化转变;抗剪强度随冻融循环和盐质量分数的增加而逐渐降低;对于盐质量分数0%的试样,内摩擦角受冻融循环影响不明显,在30°~34°之间;在短期冻融循环(1~9次)作用下,黏聚力呈上升趋势,在冻融循环9~20次过程中,黏聚力急剧下降,当冻融循环大于20次后,黏聚力逐渐趋于稳定;对于盐质量分数2%和5%的试样,短期冻融循环(小于9次)对内摩擦角影响不显著,而在冻融循环20次后,内摩擦角急剧减小;在冻融循环1~20次过程中,试样的黏聚力逐渐减小,在冻融循环20次时,黏聚力达到最小值;土样在反复冻融循环作用后,土体颗粒大量分解,粒度逐渐变细,细颗粒占比逐渐增大。研究结果可为类似工程提供理论指导。

冻融循环作用对青藏粘土物理力学性质的影响

为研究冻融循环作用对土物理力学性质的影响,以青藏粘土为研究对象,通过对室内制备的粘土试样在经历0～21次完整冻融循环过程后,进行不同围压下三轴压缩试验,研究经多次冻融作用后,高度、试样含水量、应力–应变行为、破坏强度、弹性模量、抗剪强度指标等物理力学性质的变化。试验结果表明:冻融过程是使土体从不稳定态向动态稳定态的发展过程,反复的冻融循环改变了土体的性状,使得土体向新的动态稳定平衡状态发展。