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.

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.

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.

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.

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。

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。

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。

冻融裂隙砂岩三轴压缩破坏特征宏细观试验研究

裂隙岩体在冻融作用影响下在其内部产生损伤,为研究不同裂隙砂岩冻融作用下的力学特性、破坏模式及细观孔隙结构,采用三轴压缩试验和核磁共振试验开展冻融裂隙砂岩三轴压缩的宏细观损伤破坏规律的研究。结果表明:冻融后砂岩的峰值强度、弹性模量降低,泊松比呈倒“V”形,峰值强度随着裂隙倾角的增大呈现出逐步递减的趋势,裂隙倾角为90°时为最不利情况;破坏后岩样内部中孔升高,大孔大幅提升,宏观上表现为砂岩在三轴压缩过程中出现明显裂缝,且裂隙砂岩在冻融作用后的破坏模式趋于复杂;核磁共振谱面积随裂隙倾角的增大而增大,破坏后谱面积进一步增大,T_(2)谱出现新的第三峰,大孔隙占比增大。冻融作用下寒区裂隙岩体的宏细观破坏规律的研究对寒区岩体工程稳定性评价具有重要意义。

干湿循环和动载作用下粉砂土累积变形研究

研究目的:粉砂土分布地区路基在干湿循环及行车荷载往复作用下累积变形特性更加复杂,影响路基在服役期间的耐久性。本文通过室内GDS动三轴试验测试围压、含水率、动应力幅值、压实度以及干湿循环次数对粉砂土在动载作用下的累积变形特性规律,并基于Monismith模型对试验结果进行回归,分析粉砂土在不同因素影响下累积塑性应变模型的参数值变化规律。研究结论:(1)粉砂土累积塑性应变与荷载振动次数呈现非线性关系,均表现出先快速增长后缓慢增长,最终趋于稳定;(2)累积塑性应变的大小与压实度、围压呈负相关性,而与动应力幅值、干湿循环次数呈正相关性,随着含水率的增长,累积塑性应变先减小后增大;(3)试样经过1次干湿循环后,其累积塑性应变增长幅度最为显著;(4)根据Monismith模型得到的预测值与试验值拟合度较高,模型参数a/c值大小在一定程度上可以反映不同因素影响下粉砂土累积塑性应变变化规律;(5)本研究结论可为粉砂土地区路基在动载和干湿循环作用下的动力特性研究提供参考和借鉴。

冻融循环下水泥改良风积沙力学性能研究

为研究吉林省双辽地区水泥改良风积沙路基稳定性,室内试验分析了冻融循环下力学参数变化规律.以吉林省双辽地区公路侧面风积沙为研究对象,采用TSZ全自动三轴仪进行室内试验和试验分析.试验结果表明,随着水泥掺量的增加,改良土的峰值偏应力、粘聚力及内摩擦角都呈现增加的趋势.同一水泥掺量下,随冻融循环次数的增加,由于土中水分冰与水之间形态的变化,导致其体积发生变化,改良土的峰值偏应力、粘聚力及内摩擦角都呈现减小的趋势;水泥的掺入可以明显的改善风积沙的强度和变形特征,获得良好的工程路基性能,在吉林省双辽路基工程中具有良好的应用前景.

干湿循环导致粉砂质泥岩力学特性变化规律研究

受亲水矿物成分影响,水侵作用下粉砂质泥岩力学特性易劣化.开展多种干湿条件下粉砂质泥岩的单/三轴压缩试验,研究试样在饱水作用下的强度和刚度弱化,分析干湿循环对试样力学特性的影响规律.试验结果表明,粉砂质泥岩强度和刚度均随饱和度增加而明显降低,在中期变形阶段刚度下降程度明显大于早期变形阶段.粉砂质泥岩强度随围压增大而线性提高,其强度特性可采用Drucker-Prager准则描述,且试样开裂角符合剪切滑移破坏特征.干湿循环试验结果表明,粉砂质泥岩单轴抗压强度和割线模量随循环次数增大而近似线性衰减,并且其刚度衰减快于强度衰减.

三峡库区砂质黏性紫色土强度劣化规律试验研究

为揭示三峡库区周期性干湿交替作用对紫色土强度的影响机制,研究干湿循环次数、细颗粒(d<0.075 mm)含量以及质量含水率三因素对紫色土三轴抗剪强度特性的影响,通过试验数据回归分析,提出干湿循环作用下砂质黏性紫色土黏聚力劣化程度的定量表达式.试验结果表明:①干湿循环作用对黏聚力的影响远大于摩擦角,其中,第7次循环黏聚力下降24.87%,内摩擦角下降幅度为11.17%.在干湿循环达5次以后,强度参数基本保持不变,土体结构也基本趋于稳定.②随着质量含水率从18%降低至6%,试样黏聚力增加38.81%,内摩擦角增加32.85%,证明质量含水率对试样抗剪强度参数有重要影响.细颗粒含量对强度参数的影响较小,随着细颗粒含量的增加,黏聚力也逐渐增加,对摩擦角影响无明显变化规律,但总体呈下降趋势.③根据二次多元回归拟合及方差分析,三因素对抗剪强度以及参数均有影响作用,根据p值大小进行显著性分析,含水率的强度参数的影响高于另外两因素的影响,且三因素对试样黏聚力的影响高于摩擦角;该研究结果对揭示土体抗剪强度的劣化机制有重要参考意义.