Effect of suction change on water content and total volume of an expansive clay

A laboratory study was carried out on both natural and compacted specimens to investigate the complex soil-water interaction in an unsaturated expansive clay. The laboratory study includes the measurement of soil-water characteristic curves, 1D free swelling tests, measurement of swelling pressure and shrinkage tests. The test results revealed that the air-entry value of the natural specimen was quite low due to cracks and fissures present. The hydraulic hysteresis of the natural specimen was relatively insignificant as compared with the compacted specimen. Within a suction range 0 to 500 kPa, a bilinear relationship between free swelling strain (or swelling pressure) and initial soil suction was observed for both the natural and compacted specimens. As a result of over-consolidation and secondary structures such as cementation and cracks, the natural specimens exhibited significant lower swelling (or swelling pressure) than the compacted specimen. The change of matric suction exerts a more significant effect on the water phase than on the soil skeleton for this expansive clay.

Swelling pressure evolution characterization of strong expansive soil considering the influence of reserved expansion deformation

Numerous engineering cases have demonstrated that the expansive soil channel slope remains susceptible to damage with the implementation of a rigid or closed protective structure. It is common for the protective structure to experience bulging failure due to excessive swelling pressure. To investigate the swelling pressure properties of expansive soil, the constant volume test was employed to study the influence of water content and reserved expansion deformation on the characteristics of swelling pressure in strong expansive soils, and also to explore the evolution mechanism of the swelling pressure. The findings demonstrate that the swelling pressure-time curve can be classified into swelling pressure-time softening and swelling pressure-time stability type. The swelling pressuretime curve of the specimen with low water content is the swelling pressure-time softening type, and the softening level will be weakened with increasing reserved expansion deformation. Besides, the maximum swelling pressure Psmax decreases with increasing water content and reserved expansion deformation, especially for expansion ratio η from 24% to 37%. The reserved deformation has little effect on reducing Psmax when it is beyond 7% of the expansion rate. The specimen with low water content has a more homogeneous structure due to the significant expansion-filling effect, and the fracture and reorganization of the aggregates in the specimens with low water content cause the swelling pressure-time softening behavior. In addition, the proposed swelling pressure-time curve prediction model has a good prediction on the test results. If necessary, a deformation space of about 7% expansion rate is recommended to be reserved in the engineering to reduce the swelling pressure except for keeping a stable water content.

Numerical Simulation of Moisture Movement in Unsaturated Expansive Soil Slope Suffering Permeation

This study develops a way of analyzing moisture movement in unsaturated expansive soil slope. The basic equations and the integrated finite difference method for moisture movement in unsaturated soils are briefly described, and the calculation code MFUS2 has been developed. The moisture movements in unsaturated expansive soil slopes suffering precipitation were simulated numerically. The simulation results show that expansion or contraction must be taken into account in an analysis model. A simplified equivalent model for calculating rainwater infiltration into expansive soil slopes has been developed. The simplified equivalent model divides the soil slope into two layers according to the extent of weathering of the soil mass at depth. Layer Ⅰ is intensively weathered and moisture can be fully evaporated or rapidly absorbed. The moisture movement parameters take into account the greater soil permeability caused by fissures. Layer Ⅱ is unweathered and the soil is basically undisturbed. The moisture movement parameters of homogeneous soils are applicable. The moisture movements in unsaturated ex- pansive soil slopes suffering precipitation were simulated numerically using the simplified equivalent model. The simulation results show that the moisture movement in the expansive soil slope under rainfall permeation mainly takes place in the extensively weathered layer Ⅰ which closely simulates the real situation.

Theory and practice concerning classification for expansive soils using standard moisture absorption water content

A series of tests on physical, mechanical and chemical properties of expansive soils have shown that the standard moisture absorption water content (SMA) of expansive soils has a good linear relationship with montmorillonite content, cation exchange capacity, specific surface, and it also presents a linear relationship tendency to plastic index and free swell ratio. All this indicates that the above SMA is of clear significance in physics. The plastic index can better reflect the interaction extent between composition and dispersing character of grains, cation and clay minerals. Consequently, SMA can reflect the basic essential properties of expansive soils. On the basis of the classification results that have been published in literature and in consideration of the succession to the classification results for the free swell ratio, this paper puts forward three indexes, i.e. the SMA, plastic index and free swell ratio to classify expansive soils. The tests proposed for the classification indexes are easy to operate, simple in testing device and last a short time. Compared with the latest issued “Classification Standard Code for Rocks and Soils of Railway Projects” (TB10077-2001), the method proposed in this paper is more convenient and reasonable, and can be popularized, when environment conditions ripen, for the application in field of exploration and design of highway in the areas surrounded by expansive soils.

植被根系含量对膨胀土持水和渗透特性的影响

为探究植被根系含量对膨胀土持水和渗透特性的影响规律,选取南宁市膨胀土地区的膨胀土,掺入狗牙根根系制备不同含根率的试样,进行室内压力板仪试验和变水头渗透试验,采用Van Genuchten-Mualem模型预测不同含根率的膨胀土非饱和渗透系数。结果表明:掺入根系后,膨胀土持水能力减弱,根系掺入增加膨胀土内部大孔隙的比例,含根率越高,大孔隙体积占总孔隙体积比例越大,持水能力相较于纯土下降的幅度越大;饱和状态下,膨胀土渗透系数随含根率增大而增大;非饱和状态下,低吸力阶段(0~25 kPa)的膨胀土渗透性随含根率的增大而增大,随着基质吸力增加到高吸力阶段(200~1 000 kPa),根系优势流效果减弱,含根膨胀土渗透性逐渐趋近低于纯土;细观结果表明根系的掺入使膨胀土内部产生贯通裂缝,是影响膨胀土持水和渗透能力的关键因素。研究进一步揭示了植被根系对膨胀土增渗作用的机理和规律,为综合评价植被防护膨胀土边坡效果提供参考。

不同初始含水率下广西膨胀土膨胀变形规律及劣化机理研究

为研究初始含水状态对膨胀土吸水膨胀变形及强度劣化的影响,构建考虑初始含水率影响的膨胀土膨胀变形公式,选取广西区境内膨胀土为研究对象,开展了不同初始含水率条件下膨胀土的膨胀试验及力学特性试验,借助细观测试分析技术,研究了膨胀土的膨胀变形特征、结构及强度劣化规律,揭示了膨胀土膨胀软化机理。结果表明,土体膨胀过程可分为快速膨胀、减速膨胀及膨胀稳定3个阶段;土样膨胀稳定时最大膨胀率随初始含水率的增加逐渐变小;土体吸水膨胀系数随初始含水率增加呈指数减小,并最终趋于稳定;考虑初始含水率影响的膨胀变形公式可以更好地模拟不同初始含水状态下土样的膨胀过程;土体膨胀吸水量随初始含水率的增加呈现出先增后减的趋势;吸水膨胀导致土中孔隙从以小、中孔隙为主向以微、小孔隙为主转变,孔隙面积增加,强度降低;土体中亲水性矿物吸水后,晶胞变大,晶胞与晶胞之间距离变大,叠聚体数量变小,土颗粒之间的结合方式改变,影响了土体的细观组合结构形态,导致土体吸水膨胀,强度劣化。可见非饱和膨胀土宏观上的吸水膨胀软化,不仅取决于土体内部黏土矿物含量,还受控于土体初始含水率、黏土矿物与非黏土矿物之间的结合形式,即细观的空间结构形态。

基于PIV技术的非饱和膨胀土径向劈裂试验研究

非饱和膨胀土的径向劈裂强度与裂隙扩展之间存在密切联系,分析不同含水率下非饱和膨胀土的裂隙演化机制,对南水北调中线工程膨胀土渠段的工程灾害防治具有重要的理论意义和工程实用价值。联合运用WP4C仪、压力板和PIV(Particle image velocimetry,PIV)劈裂试验系统,对南水北调中线工程邓州段某高填方渠堤非饱和膨胀土进行了一系列的持水特性试验和劈裂试验,结合持水特征曲线,揭示了不同含水率膨胀土的劈裂强度、裂隙演化过程、位移矢量场的变化规律及其内在机理。试验表明:(1)膨胀土的含水率和饱和度均随吸力的增大而减小。在低吸力范围内膨胀土试样含水率和饱和度随吸力的增加而减小趋势缓慢;在高吸力范围内,含水率和饱和度与吸力基本呈线性关系,随吸力的增加,试样的含水率和饱和度显著减小。(2)含水率对膨胀土的劈裂强度具有显著影响。随含水率的增加,试样的劈裂峰值荷载呈先增加后减小的变化规律,劈裂峰值荷载在含水率为3%~12%范围内持续增加,而在12%~24%范围内持续减小;上述试验现象与不同含水率膨胀土试样具有不同的颗粒间液桥作用有密切的内在联系。(3)不同含水率下膨胀土劈裂试验荷载-位移曲线均可分为荷载接触阶段、荷载线性增加阶段、张拉破坏阶段和残余阶段。膨胀土劈裂试验不同阶段的裂隙、位移矢量场的变化规律与荷载-位移曲线变化之间存在着一一对应关系。

土工袋抑制膨胀土冻胀性能试验及机制探讨

寒区膨胀土输水渠道普遍存在严重的冻胀破坏问题,严重损害工程效益。为探究不同含水率、不同干密度下土工袋抑制膨胀土冻胀性能演化规律及其物理机制,开展了土工袋制样标准率定试验和有无袋体约束下膨胀土的冻胀变形试验。试验结果表明:土工袋试样的压实高度与其干密度存在一一对应关系,可以通过控制土工袋的压实高度来精确控制土工袋的制样干密度;无论是否有袋体约束,试样的冻胀变形与时间关系曲线均可划分为冷缩、快速冻结和冻结稳定3个阶段;对于初始状态相同的膨胀土试样,袋体约束作用下其冻胀量显著减小,也即土工袋可有效地抑制膨胀土冻胀;随着含水率的增加、干密度的增大,试样的冻胀率均显著增大;土工袋的冻胀抑制率随着干密度的增加而显著增大,但含水率增加后土工袋的冻胀抑制率并未显著提升;通过分凝势理论和加筋约束原理,揭示了土工袋抑制膨胀土冻胀的内在机制;基于压汞试验获取的膨胀土孔径分布曲线,提出了袋体约束作用下冻结膨胀土的骨架结构概念模型,较好地解释了含水率和干密度对土工袋冻胀抑制率演化规律的影响差异。

宁夏衬砌渠道渠基冻胀率水热响应试验研究

衬砌渠道周围的水分与温度场分布存在明显差异,此范围内渠基土在水热两场耦合作用下冻胀有明显不同。基于这种差异,开展渠基土在不同水分、温度条件下冻胀率试验研究。结果表明:在不同温度条件下,土体冻胀率随温度的变化过程可分为2个阶段,当温度从0℃降至-5℃左右时,在此阶段产生的冻胀率约占整个冻胀过程的50%~80%,冻胀率与温度线性关系明显。随着温度继续降低,2者非线性关系明显。在无补水情况下,相同温度条件下的土体,初始含水率越大,所产生的冻胀率越大;初始含水率25%和35%的土体产生的冻胀率均为初始含水率15%土体的1倍到2倍以上。当土体在冻胀过程中有水分补充时,其冻胀率与水分变化线性关系明显,初始含水率越大的土体产生的冻胀率增量越大,初始含水率25%、35%的土体冻胀率增量均为15%土体的2倍以上。提出的水热耦合条件与渠基土冻胀率的多元非线性关系与试验数据吻合较好。建立数值模拟模型,对衬砌渠道不同位置处冻胀作用分布与变形进行计算与分析,结果表明:温度越低、水分越大,衬砌渠道所受到的切向冻胀力与法向冻胀力越大。水热变化差异越大时,土体冻胀导致的衬砌渠道冻胀变形越显著。

干旱和局部渗水对膨胀土渠堤稳定性影响研究

为探究膨胀土渠堤表面风干、地下水位升高和侧坡渗水等因素对其稳定性的影响,防控由此引发的现场灾害,开展了填方渠坡室内模型试验,研究了南水北调渠堤膨胀土含水量波动对渠坡变形和裂缝发育规律的影响,分析了边坡变形值、裂缝长度宽度与含水率、时间和渗水位置的关系,揭示了膨胀土含水量波动的致灾过程和时空发育规律。试验结果表明:(1)旱季坡面水分蒸发导致渠坡表层200 mm深度内首先产生垂直坡表面的“V”形簇状纵向裂缝,后逐渐水平发育,坡面最大沉降值为4 mm, 15 d后沉降趋于稳定;(2)雨季渠堤下部地基水位抬升使渠堤底部膨胀土发生显著膨胀,渠堤整体抬升3.32 mm;(3)在渠堤侧坡渗水中心点附近土体含水量上升8%以上,坝堤内自由水沿纵横向不均匀扩散而引起土体各向不均匀膨胀变形,并在右坡坡脚处产生长400 mm、宽14 mm的宽大裂缝;(4)基于膨胀系数的地基变形计算公式可以较好地预测渠坡浸水后各位置的变形量,有助于依据土体含水量判别各点的变形状态,为渠堤稳定性分析提供参考。

梅雨气候区膨胀土填芯路基设计研究

将膨胀土作为路基填料,可有效利用膨胀土路堑开挖产生的弃土。以铜陵江北港铁路专用线膨胀土路基工程为依托,提出了梅雨气候区膨胀土填芯路基的设计方法。首先,进行膨胀土的膨胀力试验;然后,研究梅雨季节膨胀土填芯路基内部体积含水量与包边黏性土种类、包边厚度、路基压实系数的关系,分析梅雨季节填芯膨胀土膨胀力发育情况;最后,确定了膨胀土填芯路基设计方案,并将膨胀力引入折线型滑动面安全系数计算公式进行稳定性验算。结果表明,铜陵江北港铁路专用线膨胀土填芯路基采用亚黏土包边,包边厚度取1 m,压实系数取0.92,不仅能减少浪费,安全系数也满足规范要求。

基于含水率改变的抗剪强度指标折减方法在膨胀土基坑支护设计计算中的应用

通过膨胀土室内试验,得到了膨胀土含水率与抗剪强度指标的定量关系。试验研究表明,抗剪强度指标随含水率的增加呈逐渐降低的趋势,与内摩擦角φ相比,黏聚力c对含水率的变化更加敏感,含水率每增加5%,黏聚力c降低7.5kPa,内摩擦角φ降低3°。在此基础上,提出了适用于膨胀土基坑支护设计计算的基于含水率改变的抗剪强度指标折减方法,并通过工程实例计算与现场监测验证了该方法在膨胀土基坑支护设计中的适用性。

延边地区膨胀土冻胀特性及起始冻胀含水率试验研究

季冻区膨胀土可能产生多种工程病害隐患,以吉林省延边地区膨胀土为研究对象,在封闭系统条件下对膨胀土进行不同因素(含水率、冻结温度及压实度)的冻胀试验,研究膨胀土的冻胀特性及起始冻胀含水率的影响因素,并对试验结果进行分析。试验表明:不同含水率试样冻胀量随时间呈现“冻缩型”“先冻胀后冻缩型”“先冻缩后冻胀型”这3种冻结类型。低含水率试样冻胀量为负值,高含水率试样冻胀量为正值,冻结温度越低冻胀量越大、冻缩量越小;试样冻胀和冻缩变形量随压实度增大逐渐减小。起始冻胀含水率随冻结温度的降低和压实度的增大而呈线性减小,并提出了不同冻结温度和压实度条件下膨胀土起始冻胀含水率的经验公式。

石灰改性膨胀土工程性质的试验研究

通过对济南、淄博地区膨胀土的室内物理力学性质、石灰改性的系列试验分析,确定了膨胀土的等级、改性后土的胀缩性、强度与剂量的关系及掺入石灰的最佳配比。

雨水入渗与膨胀性土边坡稳定性试验研究

采用离心模型试验方法,完成了2组不同坡度的膨胀性土开挖边坡土工离心模型试验,比较了边坡坡度大小对其稳定性的影响,并尝试通过控制注水浸泡时间来模拟短期和长期的雨水入渗,从而再现了降雨条件下膨胀性土开挖边坡的破坏特征和破坏机制。结果分析表明膨胀性开挖边坡表层土体因雨水入渗膨胀软化,强度降低,严重削弱了边坡稳定性;鉴于膨胀性土路堑在雨水入渗后发生浅层滑动破坏,因此,单纯放缓边坡坡度并不是防治这种破坏的最有效措施,重要的是做好隔水防水等处治工作。

考虑状态含水率和密度的膨胀土膨胀模型试验研究

以邯郸强膨胀土为研究对象,通过室内K0应力状态膨胀试验,研究了膨胀土的膨胀率与压实度、初始含水率、上覆荷载之间的关系,进而建立了考虑上述3种影响因素的膨胀率公式。同时,建立了终了含水率与初始含水率、压实度、上覆荷载之间的关系,从而得到了膨胀土线膨胀系数表达式。利用提出的膨胀模型可以得出任一含水率状态下膨胀土的膨胀本构关系。该试验操作简单,模型表达式简洁,具有较强的实用性,可为膨胀土工程的设计施工提供理论依据和参考。

荆门膨胀土的水稳定性及其力学效应

本文针对湖北襄荆高速公路膨胀土,在室内开展了原状土在脱湿吸湿过程的无侧限抗压强度演化特征与击实土的胀缩性状及力学效应试验研究,发现原状膨胀土的工程性状同时受含水率与裂隙性的耦合影响,耦合程度随裂隙发育程度和膨胀潜势等级而各异;膨胀土的CBR值随其含水率的变化规律类似于击实曲线,但CBR峰值含水率大于最佳含水率,并非压实度最大时CBR值也最大,CBR值取决于击实膨胀土浸水膨胀后的含水率与干密度以及结构所处状态;膨胀土路堤填筑除考虑压实度与CBR值要求外,尚需考虑胀缩总率的影响,为深入认识膨胀土的工程特性提供了帮助。

基于最优含水率的聚丙烯纤维增强膨胀土强度研究

将不同长度的聚丙烯纤维按照0.0%、0.1%、0.2%、0.3%和0.4%的重量比分别掺入膨胀土中配制试样,通过大量的室内试验研究了聚丙烯纤维对膨胀土强度的影响规律。试验结果表明:随着纤维的加入,膨胀土的抗剪强度得到了明显的增强,且无侧限抗压强度也随着纤维含量的增加而急剧增加,但当纤维含量超过最优加筋量0.3%时,加筋膨胀土的无侧限抗压强度和抗剪强度反而呈现降低趋势。相同纤维含量的情况下,膨胀土的强度也会随着纤维长度的增加而明显增加,同时聚丙烯纤维还可以增加纤维复合膨胀土的峰值强度,延缓土样的破坏,其研究结果为膨胀土性质的改良提供一种可借鉴的方法。

重塑膨胀岩土微变形条件下膨胀力试验研究

通过对南宁膨胀性泥岩重塑土样进行变形量分别为0,0.2,0.5和1 mm的微变形膨胀力试验,分析膨胀力的主要影响因素,建立膨胀力与干密度、初始含水率双因素之间关系,研究微变形条件下膨胀力变化机制。结果表明:(1)零变形时,膨胀力随干密度增大呈指数形式增大,并与初始含水率呈幂函数关系。(2)微变形时,膨胀力随变形增大而衰减明显,在干密度大、初始含水率低时衰减幅度较大,反之较小;在干密度及初始含水率一定时,微变形膨胀力随着变形率增大而呈指数衰减。(3)微变形与零变形膨胀力之比随变形率增大而呈指数减小,当变形率为1%,2.5%和5%时,微变形膨胀力与对应零变形膨胀力之比分别为75%,55%和35%。

南阳膨胀土变形与强度特性的三轴试验研究

对南阳膨胀土做了4种应力路径的三轴试验,即控制吸力的各向同性压缩试验、控制净平均应力的收缩试验、控制净平均应力的膨胀试验、同时控制净室压力和吸力的排水剪切试验,结果表明:膨胀土各向同性压缩屈服特性仍可用一般非饱和土的LC屈服线描述;不同围压下的收缩路径的屈服吸力随围压增大而减小,且屈服后的收缩指数接近各向同性压缩屈服后的压缩指数;非饱和状态下的内摩擦角近似等于饱和状态下的有效内摩擦角,总粘聚力随吸力增加而线性增大;在各向同性压缩试验和三轴剪切试验过程中,土样含水量随净平均应力的变化都近似为线性关系。