干湿循环后膨胀土力学特性的真三轴试验研究

为了研究原状膨胀土经过反复干湿循环稳定后的力学性状,利用非饱和土真三轴仪对经历不同干湿循环后的膨胀土进行了一系列脱湿-吸湿试验、常吸力等向固结试验和常吸力真三轴剪切试验,研究干湿循环对膨胀土土-水曲线、胀缩特性、压缩特性和强度特性的影响。研究结果表明:随着干湿循环次数的增加,脱湿曲线和吸湿曲线形成的滞回圈越来越小,甚至发生了逆向回滞;脱湿速率和吸湿速率随着循环次数的增加而增大。脱湿—吸湿过程中体变性状皆呈明显的屈服特性,验证了膨胀土本构模型中SI和SD屈服包线的存在,脱湿屈服吸力均大于吸湿屈服吸力,且二者皆随着循环次数的增加而减小。在低围压和低吸力条件下,干湿循环对膨胀土的压缩特性影响较大,而在高围压和高吸力条件下影响较小。干湿循环使膨胀土的黏聚力和内摩擦角降低,干湿循环后膨胀土的黏聚力随着吸力和中主应力的增大而增大,内摩擦角随着中主应力的增大而减小。在低吸力范围内(s≤200 kPa),干湿循环后的膨胀土的吸力摩擦角φ_b是一个变量。经历3次干湿循环后膨胀土的强度逐渐趋于稳定,其吸力摩擦角可看作是一个常量,其值随着b值的增大非线性增大。     

粉煤灰及电石渣改良膨胀土干湿循环特性研究

向膨胀土中加入火山灰类添加剂可有效改善其胀缩特性,但连续干湿循环气候可能改变改良土的胀缩稳定性,并导致其强度降低。通过系统的室内试验对粉煤灰、电石渣改良膨胀土进行了干湿循环试验,分析其微观结构变化特征。试验结果表明,随着干湿循环次数的增加,改良土的膨胀性增大,无侧限抗压强度降低;与未经干湿循环的试样相比,经过干湿循环的试样内部大孔隙增多,颗粒排列状态被扰动。     

干湿循环与荷载耦合作用下红黏土胀缩特性

为获取符合工程设计的实际参数,对常规固结仪进行了改造,提出了一种模拟土体在荷载作用下的干湿循环试验方法,对红黏土进行了有荷载条件模拟干湿循环过程的试验研究,得到其胀缩变形的变化规律。结果表明:该方法可以实现不拆卸试样干湿循环试验,克服了以前荷载为0的土体干湿循环试验的缺陷,为工程设计提供更加合理的技术指标;干湿循环下红黏土的胀缩变形不可完全恢复,上覆荷载可以抑制其膨胀性,增强其收缩性;荷载小于50 kPa时,试样膨胀,且压实度越大和初始含水率越小,膨胀性越强;荷载大于100 kPa时,试样收缩,压实度越小和初始含水率越大,收缩性越强;第1次干湿循环对红黏土胀缩性能影响最为显著,随着干湿循环次数的增加,胀缩率曲线越来越平缓,胀缩性能趋于稳定状态。     

非饱和重塑膨胀土各向等压加载试验研究

为分析裂隙和胀缩对非饱和膨胀土屈服和水量变化特性的影响,首先对重塑膨胀土试样进行常温条件的干湿循环,使其形成不同程度的裂隙和孔洞;其次以多功能土工三轴仪为研究工具,对已产生结构损伤的重塑土试样进行控制吸力为常数、净平均应力增大的各向等压加载试验。试验结果表明:随着干湿循环次数的增加,膨胀土试样裂隙和孔洞发育愈加明显,排水能力相应提高;同一吸力条件下,屈服应力随着干湿循环次数的增大而减小;同一干湿循环次数条件下,试样所受吸力越大,屈服应力则相应增大;干湿循环次数增多时,膨胀土的压缩指数在发生屈服前后呈现不同规律,前者一直增大,而后者可看作一常数。     

干湿循环效应对南宁外环膨胀土抗剪强度的影响

针对膨胀土边坡滑坍多呈浅层破坏的现状,选取南宁外环膨胀土原状样与重塑样开展对比试验,精心设计试验方案,改进常规直剪试样制备条件和试验方法,使模拟土体季节性干缩湿胀过程更接近实际,探究了2种土样经历多次干湿循环作用后抗剪强度的衰减规律。结果表明：试样制备中有上覆荷载作用,实测土样经历同样干湿循环次数后的强度衰减幅度比无荷条件下的小,再次验证荷载对抑制强度衰减作用明显；不管原状还是重塑样经干湿循环后的强度衰减主要是c值大幅降低,其值虽也减小但降幅都不大；此外,计入边坡浅层破坏时滑面的低应力条件开展试验并分析测试结果,得到做边坡浅层稳定性分析时合理的抗剪强度参数,研究结果可供工程设计参考。     

石灰改良土干湿循环下胀缩性研究

为获取改良土工程设计参数,对李渡红土加入掺合料石灰后,进行了有荷载条件模拟干湿循环过程的试验研究,得到了其胀缩变形变化规律,指出有荷条件下,不同配比的试样随干湿循环次数增多其相对和绝对胀缩率都在逐渐减小;石灰量在一定范围内,土样的绝对和相对胀缩率随土料中含石灰量增多而减小。     

击实膨胀土的循环膨胀特性研究

主要研究了干湿循环对击实膨胀土胀缩特性的影响。研究结果表明,击实膨胀土的胀缩变形并不是完全可逆的,随干湿循环的发展,膨胀土的膨胀速率加快,绝对膨胀率总是增大而相对膨胀率则降低。这种变化在第Ⅱ、Ⅲ级循环时最明显,第Ⅲ级循环后便趋于稳定。这些变化特性主要是粘粒集聚、微结构改变的结果。     

干湿循环对有无上覆荷载作用镇江下蜀土抗剪强度的影响

针对下蜀土边坡常呈浅层滑动破坏的状况,选取镇江市某边坡的下蜀土为研究对象,通过干湿循环直剪试验,对制备试样时有、无上覆荷载,分别开展不同干湿循环次数下下蜀土抗剪强度劣化规律研究。结果表明:随着干湿循环次数的增加,无荷及有荷下蜀土的剪应力峰值、黏聚力及割线模量都逐渐降低,下蜀土黏聚力的衰减可用分段函数描述,当循环次数小于6时可用直线描述,而后趋近于一稳定值,而内摩擦角则保持相对稳定;法向应力及干湿循环次数相同,制样时有上覆荷载的下蜀土抗剪强度的衰减率明显小于无上覆荷载的衰减率,并且两种情况下的抗剪强度都随干湿循环次数的增加而不断衰减;相同干湿循环次数下,有上覆荷载下蜀土的黏聚力明显大于无上覆荷载的黏聚力,并且随着干湿循环次数的增加,其差距的增长率呈增大趋势。     

干湿循环过程中膨胀土的胀缩变形特征

为了了解干湿循环过程中膨胀土的胀缩变形特征,分别开展了两组干湿循环试验。在控制吸力干湿循环试验中,吸力控制范围为 0.4 ～ 262 MPa ,采用了两种吸力控制方法,分别为渗析法（吸力＜ 4 MPa ）和蒸汽平衡法（吸力＞ 4 MPa ）,当每一级吸力达到平衡时,测量试样对应的含水率和体积;在常规干湿循环试验中, 采取了两种干缩路径,分别为全干燥和部分干燥,并测量试样在每次干湿循环过程中的轴向变形及循环结束后的含水率。结果表明： 在脱湿和吸湿过程中,试样孔隙比随吸力变化可分 3 个典型阶段：大幅变化阶段（ 0.4 ～ 9 MPa ）、过渡阶段（ 9 ～ 82 MPa ）和平缓阶段（ 82 ～ 262 MPa ）; 当吸力大于 113 MPa 时,试样的胀缩变形基本可逆,而当吸力小于 113 MPa 时,试样的胀缩变形 表现出明显的不可逆性,且不可逆程度随吸力的减小而增加。试样在常规干湿循环过程中的胀缩变形随循环次数的增加逐渐趋于稳定;胀缩特征受干缩路径的影响非常明显,全干缩路径中测得的膨胀率高于部分干缩路径,膨胀速率随干湿循环次数的增加而增加;试样在干湿循环过程中的膨胀率大小在一定程度上取决于吸湿能力。     

红层泥质岩干湿循环下的变形特性试验研究

泥质岩地区高速铁路开挖的路堑和隧道底部路基,许多出现了长时间持续性上拱变形现象,致使高速铁路降速运行。为研究泥质岩路基持续性上拱变形的原因,文章以成都至重庆高速铁路客运专线某中间站开挖路基持续变形现象为典型,对四川红层泥质岩原状样在侧限约束条件下的浸水膨胀与失水收缩构成的胀缩变形特性进行了室内干湿循环试验。试验结果表明:红层泥质岩在干湿循环条件下具有浸水膨胀失水收缩变形特性,循环胀缩变形量值不同,逐次产生了不可逆的变形累积,致使泥质岩在干湿循环过程最终产生了较大的净膨胀变形量;在每个浸水膨胀变形与失水收缩变形循环初期,泥质岩变形速率均是由大逐渐减小最后向零趋近,各单循环累积变形量先增加最后趋于稳定;泥质岩干湿循环中产生的累积变形量均随浸水与失水循环次数的增加而增大;累积变形量与泥质岩初始含水率相关,随着初始含水率的增加而增大。     

改良膨胀土的干湿循环特性试验研究

干湿循环作用对改良膨胀土的工程性质存在重要的影响,而目前这方面的研究成果很少。以掺石灰和粉煤灰改良的典型合肥膨胀土为研究对象,通过系统的室内试验,研究在干湿循环作用下,改良膨胀土的膨胀性、界限含水量、颗粒分布以及无侧限抗压强度等方面的变化规律,深入探讨干湿循环作用对改良膨胀土工程性质的影响。结果表明,改良膨胀土的膨胀量、液限、塑性指数、黏粒含量随干湿循环次数的增加而增大,而塑限、粉粒含量以及无侧限抗压强度则随干湿循环次数的增加而减小。     

Field study on swelling-shrinkage response of an expansive soil foundation under high-speed railway embankment loads

This paper presents a comprehensive field investigation of the swelling-shrinkage behavior of an expansive soil ground under high-speed railway embankment loads. In this study, a test site close to the Kunming-Nanning high-speed railway (KNHR) was chosen for the construction of four full-scale field test facilities for artificially soaking the expansive soil ground. Three of the facilities consist of embankments of three different heights, while the fourth facility is for a series of plate load swelling tests. All the test embankments were fully instrumented to monitor the ground deformation and the changes in volumetric water content profiles of the foundations. The full-scale field tests were complemented by a detailed site investigation comprised of cone penetration tests (CPTs), standard penetration tests (SPTs) and a comprehensive laboratory characterization of intact expansive soil samples retrieved from the test site. The results obtained from the laboratory and field tests show that the swelling behavior of the expansive soil ground mainly depends on the embankment load. By properly designing the embankment height and considering the maximum swelling pressure the expansive ground could induce, the heave of the embankment could be controlled efficiently. The measured displacements at the ground surface are well correlated with the evolution of measured volumetric water contents within a ground depth of around 4.5?m. The majority of these displacements occurred when the ground was approaching saturation along both wetting and drying paths. Finally, a simple method based on one-dimensional test results was proposed, and a good performance was shown in predicting the heave or settlement of embankments over an expansive soil ground upon wetting and drying.     

重塑膨胀土三轴增湿变形及力学特性的试验研究

通过三轴增湿变形试验和直接剪切试验,对膨胀土的变形和抗剪强度规律进行了研究.结果表明,膨胀土的极限剪应力、凝聚力和内摩擦角都随着含水率的增大而减小,试样含水率越接近饱和含水率,试样的抗剪强度减小的越快;膨胀土的应力随应变增加呈衰减趋势,并且在增湿变形中以体积变形为主,轴向变形较小;在同一应力状态下,随着含水率的增大,体应变逐渐减小,土样表现为膨胀,在含水率相同的条件下,应力越小,膨胀土在浸水过程中的体积变化就越大.     

Soil element assessment of cyclic-load-induced settlement considering combination of vertical,horizontal, and shear stresses in cohesive soil

Grounds subjected to cyclic loading, either continuous or fragmentary, widely exist in natural deposits and at reclaimed sites, e.g., induced by traffic loads, earthquakes, and wave-induced loading environments. Assessing the ground deformation under cyclic loading conditions is necessary for proper design and maintenance planning for traffic services. Accordingly, a testing program, in which vertical, horizontal, and shear stresses are automatically controlled, were newly developed in this study to reproduce the actual cyclic traffic load conditions using a hollow-cylindrical torsional shear apparatus.Estimations based on the results obtained from cyclic triaxial stress conditions were seen to underestimate the ground settlement compared to actual stresses, with combined vertical, horizontal, and shear stresses. Two kinds of vertical strain arose after a certain number of cycles: incremental collapse, in which unstable deformation (failure) is caused, and plastic creep, in which deformation continues to accumulate with the dampened rate (per cycle). The number of cycles of combined vertical, horizontal, and shear stresses necessary to reach a certain vertical strain increased linearly on a logarithmic scale when the magnitude of loading decreased. Moreover, the relationship between vertical strain and excess pore water pressure presented the same trend independent of the magnitude of loading. Practical uses of the obtained results were proposed and discussed.     

脱湿状态对膨胀土膨胀变形和强度特性的影响

非饱和膨胀土的脱湿状态对土中水分散失速率影响较大,不同的脱湿速率又会使膨胀土呈现不同的工程性状。通过控制恒温恒湿箱的温湿度工作参数,设置不同的脱湿环境制备土样,进行土工试验,深入认识脱湿状态对膨胀土膨胀变形和抗剪强度的影响。将饱和土样脱湿到设定的不同含水率,进行有荷载膨胀率试验,结果表明:在相对湿度越大的环境下,脱湿速率越小,土体膨胀率越大;起始含水率和上部荷载越小,土体膨胀变形越大。将不同起始含水率的土样膨胀完全后放入直剪仪,进行固结直剪试验,结果表明:脱湿到不同含水率的试样,脱湿速率越小,土体越密实膨胀后的抗剪强度越大;上部荷载存在可提高土体的抗剪强度,起始含水率越小上部荷载越大土样的抗剪强度越高。研究结果可为大气作用下膨胀土边坡的防护设计提供参考,有助于对自然气候环境下膨胀土边坡的灾变进行预警,并可优化膨胀土边坡的施工方案。     

干湿循环作用下固化淤泥的抗剪强度变化规律

河、湖等的疏浚淤泥多采用固化方式进行处理。针对固化淤泥材料的干湿稳定性问题,系统开展了干湿循环作用下水泥固化疏浚淤泥的抗剪强度特性试验研究,揭示了固化淤泥在干湿循环作用下抗剪强度的变化机理,并对各影响因素进行了定量分析。结果表明:随着干湿循环次数的增加,固化淤泥的抗剪强度逐渐变化,且先快后慢,最后趋于稳定;干湿循环后,水泥掺量100kg/m~3固化淤泥试样的抗剪强度降低,而水泥掺量150、200kg/m~3试样干湿循环后的抗剪强度不降反增,说明干湿循环对固化淤泥的影响与水泥的掺量有关。较高的干燥温度促进了水泥水化,从而导致水化产物增加,固化淤泥的抗剪强度增大;同时,干湿循环过程中,微裂缝的发育导致固化淤泥的抗剪强度降低,干湿循环对固化淤泥抗剪强度的影响取决于二者的综合作用。     

土体干湿循环过程中的体积变形特性研究

本文通过室内试验,研究了干湿循环过程中,土体沿不同方向失水导致裂隙沿不同方向开展时,土体应变的变形的特征。采用高精度卡尺直接量测的方法测量了膨胀土膨胀收缩后的应变变化,获得了膨胀土膨胀收缩过程中的应变曲线,试验结果表明:在干湿循环三次后土体的裂隙开始大幅的开展,但强度不会明显降低,此时土体可以看成是由强度和体积均较大的骨架颗粒组成,土体体积的增大即为团聚体间空隙增大的结果,土体在宏观上仍为强度和体积均较大的骨架结构,从而解释为何土体经多次干湿循环后裂隙不断开展但强度趋于稳定。在干湿循环过程中,无论土样沿轴向或径向失水,土样的轴向裂隙一直有较大程度的开展,而在干湿循环三次以后,土样的径向裂隙才开始大幅开展。     

Hydraulic and volume change behaviors of compacted highly expansive soil under cyclic wetting and drying

The wide engineered application of compacted expansive soils necessitates understanding their behavior under field conditions. The results of this study demonstrate how seasonal climatic variation and stress and boundary conditions individually or collectively influence the hydraulic and volume change behavior of compacted highly expansive soils. The cyclic wetting and drying (CWD) process was applied for two boundary conditions, i.e. constant stress (CS) and constant volume (CV), and for a wide range of axial stress states. The adopted CWD process affected the hydraulic and volume change behaviors of expansive soils, with the first cycle of wetting and drying being the most effective. The CWD process under CS conditions resulted in shrinkage accumulation and reduction in saturated hydraulic conductivity (k_sat). On the other hand, CWD under CV conditions caused a reduction of swell pressure while has almost no impact on k_sat. An elastic response to CWD was achieved after the third cycle for saturated hydraulic conductivity (k_sat), the third to fourth cycle for the volume change potential under the CV conditions, and the fourth to fifth cycle for the volume change potential under the CS conditions. Finally, both swell pressure (σ_s) and saturated hydraulic conductivity (k_sat) are not fundamental parameters of the expansive soil but rather depend on stress, boundary and wetting conditions.