Unsaturated expansive soil fissure characteristics combined with engineering behaviors

The relationship among the surface fissure ratio, moisture content, seepage coefficient and deformation modulus of field unsaturated expansive soil in Nanning, Guangxi Province, China, was obtained by a direct or indirect method. Digital images of expansive soil of the surface fissure with different moisture contents were analyzed with the binarization statistic method. In addition, the fissure fractal dimension was computed with a self-compiled program. Combined with in situ seepage and loading plate tests, the relationship among the surface fissure ratio, moisture content, seepage coefficient and deformation modulus was initially established. The surface fissure ratio and moisture content show a linear relation, ＂y=-0.019 1x＋1.028 5＂ for rufous expansive soil and ＂y=-0.07 1x＋2.610 5＂ for grey expansive soil. Soil initial seepage coefficient and surface fissure ratio show a power function relation, ＂y=1× 10^-9exp（15.472x）＂ for rufous expansive soil and ＂y=5× 10^-7exp（4.209 6x）＂ for grey expansive soil. Grey expansive soil deformation modulus and surface fissure ratio show a power fimction relation of ＂y=3.935 7exp（0.993 6x）＂. Based on the binarization and fractal dimension methods, the results show that the surface fissure statistics can depict the fissure distribution in the view of two dimensions. And the evolvement behaviors of permeability and the deformation modulus can indirectly describe the developing state of the fissure. The analysis reflects that the engineering behaviors of unsaturated expansive soil are objectively influenced by fissure.

Relationship of Resistivity with Water Content and Fissures of Unsaturated Expansive Soils

The development of fissures in expansive soils has a great effect on the stability of slope. Of the three phases of soils,the gas phase and solid phase are relatively insulated,so the average resistivity of soils can be calculated from the resistivity of the liquid phase. On this basis,the two-part model of resistivity changing with the water saturation of the expansive soil can be deduced. A 2-D resistance grid model is established based on simulating the resistance of ver-tically developed fissures. Variation in measured resistance of vertically developed fissures at different positions can be calculated from this model. Fissure development can be inversely determined from the variation in the measured resis-tance. Finally,the model is verified by an indoor resistivity test for remolded soil samples,indicating that the test result agrees well with that of the model established.

Using Sorghum Stalk as a Partial Replacement of Lime in the Stabilization of Red Clay Soil for Road Sub-Grade Construction

This research aimed at testing the viability of using Sorghum Stalk Ash (SSA) as a partial replacement of lime in the stabilization of red clay soils for road subgrade construction. Red clay soils have been identified as highly expansive soils, which are affected by both climatic conditions and loading patterns. The consideration of both traffic loading patterns and climatic effects on these soils has been taken into account. A penetration test of 2.5 mm has been used on both pure red soils and stabilized soils at 10% and 15% partial replacement of lime with SSA and showed an improvement in the CBR of stabilized red clay soils up to 11.6%. Again, the PI of stabilized soils at 15% partial replacement of lime reduced up to 11.2%. The results obtained on both CBR and PI of these red clay soils are within the recommended values for the effective subgrade required for laying both permanent and flexible pavements. As a result, a recommendation of making use of SSA to lower the quantities of lime and its costs used in the stabilization of highly expansive soils have been tested through this research. However, further research on a more percentage partial replacement of lime to improve the PI of these soils to below 10% while keeping the CBR levels within the road construction regulations is welcomed.

Soil Stabilization Using Waste Plastic Materials

Expansive clay soils are the types of soils whose volume changes with the change in water content. They have a behavior of swelling and shrinking that is a serious hazard to structures built over them. Expansive soils are abundantly existing soil types in Ethiopia, particularly Addis Ababa. This paper shows the outcomes of an attempt to reinforce and stabilize expansive clay soil with plastic bottle strips. The plastic strips were prepared and added at three different mixing ratios (0.5%, 1% and 2%) by weight and in three different aspect ratios (5 mm × 7.5 mm, 10 mm × 15 mm, 15 mm × 20 mm). The experimental results showed that there was a significant improvement in shear strength parameters. The swelling and desiccation cracking behavior of the soil were also expressively reduced. There was a substantial reduction in the optimum moisture content and slight increment in maximum dry density. The optimum plastic size (aspect ratio) and plastic content that results in optimum result can be selected based on the importance of the selection parameter for a specified engineering work. Stabilizing expansive clay soils with waste plastic bottles simultaneously solves the challenges of improper plastic waste recycling that is currently a teething problem in most developing countries. The results obtained from this study favorably suggest that inclusion of this material in expansive soils would be effective for ground improvement in geotechnical engineering.

Behavior and characteristics of compacted expansive unsaturated bentonite-sand mixture

Limited studies dealt with the expansive unsaturated soils in the case of large-scale model close to the field conditions and therefore, there is much more room for improvement. In this study, expansive （bentonite–sand （B–S） mixture） and non-expansive （kaolin） soils were tested in different water contents and dry unit weights chosen from the compaction curve to examine the effect of water content change on soil properties （swelling pressure, expansion indices, shear strength （soil cohesion） and soil suction） for the small soil samples. Large-scale model was also used to show the effect of water content change on different relations （swelling and suction with elapsed time）. The study reveals that the initial soil conditions （water content and dry unit weight） affect the soil cohesion, suction and swelling, where all these parameters slightly decrease with the increase in soil water content especially on the wet side of optimum water content. The settlement of each soil at failure increases with the increase in soil degrees of saturation since the matric suction reduces the soil ability to deform. The settlement observed in B–S mixture is higher than that in kaolin due to the effect of higher swelling observed in B–S mixture and the huge amount of water absorbed which transformed the soil to highly compressible soil. The matric suction seems to decrease with elapsed time from top to bottom of tensiometers due to the effect of water flowing from top of the specimen. The tensiometer reading at first of the saturation process is lower than that at later period of saturation （for soil sample B–S3, the tensiometer #1 took 3 d to drop from 93 kPa to 80 kPa at early stage, while the same tensiometer took 2 d to drop from 60 kPa to 20 kPa）.

Laboratory investigations of the thermal strain of frozen soils,using fiber-optic strain gauges based on Bragg gratings

Measurements of the thermal deformations of frozen soil samples were performed in the cold laboratory in temperature range from 0°C to-12°C.Fiber Bragg Gratings strain and temperature sensors were used to measure the deformation and temperature inside the samples.A number of tests with the samples prepared from Kaolin and Cambrian clay saturated with fresh water,and prepared from fine and silt sand saturated with fresh or saline water,are performed.Thermal deformations of the samples are analyzed depending on the cyclic changes of their temperature.

Study and Comparison of Swelling and Compressibility Characteristics of Crumb Marl, Flaky Marl with Attapulgite and Sandy Clay from the Diamniadio Urban Pole at the Oedometer

In Senegal, the Diamniadio, Sebikhotane and Bargny sector contains clay soils that are problematic for construction. In order to have more information on the behavior of the clay soils of Diamniadio, free swelling tests followed by load-discharge cycles were carried out according to standard NF P 94-090-1. These tests were carried out using an Oedometric device on the three samples from the study site (sandy clays with calcareous concretion, marls with crumbs and laminated marls with attapulgite) to apprehend their swelling aspects in saturated conditions. For the free swelling test, a determination of the different swelling phases will be carried out followed by a comparison of the rate of evolution of the phases for the three samples from the site. In the same vein, the compressibility characteristics of the samples will also be provided from load-unload Oedometric tests. Thereafter, we proceed to a comparison of the void index at the initial state of the samples after two charge-discharge cycles and the influence of the cycles on the reorganization of the internal structure of the samples. These studies will provide more information on the swelling behavior of Diamniadio soils in the presence of water.

滋阳山地区地裂缝分布规律及成因

为了解济宁市滋阳山地区地裂缝的成因机制,通过资料收集、地质测量、钻探、物探、遥感解译、山地工程、室内试验、水位动态监测、工程测量、综合分析等研究该地区的地裂缝现状及分布规律。结果表明:该地区主要出现6处地裂缝出露区;裂缝有直线型、交叉型和不规则塌坑型3种类型;地裂缝全部发育于林木种植地区,且该地区由农耕区变更为大面积种植林木之前,未见有村民反映的地裂缝相关报导,可推断地裂缝的出现和林木的种植关联性很强;经调查林木移植与地裂缝的产生有相关性;地裂缝在地表显现的方向与林木种植的行向方向一致;据推测地裂缝的垂向切割深度可能至膨胀土层底部。可见研究区控制地裂缝活动的首要条件是膨胀土的分布,其次是林木移植程度、林木种植以及水动力活动迁移条件。

DCB溶液作用下膨胀土裂隙面力学特性试验研究

为了探究水化学作用下膨胀土裂隙面力学特性的变化,选取引江济淮工程菜子湖C003标段膨胀土,通过特定的制样方式赋予试样预设裂隙面,采用连二亚硫酸钠-柠檬酸钠-碳酸氢钠(DCB)溶液对含裂隙土样进行淋滤,并对淋滤后的试样进行抗剪强度试验研究,探究裂隙面强度变化与淋滤时间的关系,揭示淋滤后膨胀土裂隙面强度变化的微观机理。试验结果表明:DCB溶液沿着预设裂隙面和试样内部淋滤不同天数后,试样形貌特征出现了不同的变化;裂隙面抗剪强度降低速率随着淋滤时间的增加先变快后变慢,在淋滤28 d后裂隙面抗剪强度指标下降明显,黏聚力和内摩擦角分别下降51.5%和45.9%;对淋滤后试样表面灰绿土样和直剪试验后剪切面土样进行微细观研究发现,淋滤后灰绿土和剪切面Fe_(2)O_(3)含量下降明显,且黏土矿物衍射峰强度有一定程度的升高。微观机理分析可知,DCB溶液通过氧化还原作用对土中游离氧化铁进行去除,使试样外观形貌出现较大变化,破坏了土中大的团聚体与颗粒之间的黏结力,从而导致土体力学特性劣化。

膨胀土裂隙对渠坡渗透性及工程效应影响研究

膨胀土是一种特殊的非饱和土,也是南水北调中线渠首段土体的主要成分。与一般黏性土相比,膨胀土具有裂隙性,裂隙的存在会对其渗透性产生影响,进而影响到膨胀土渠坡的实际排水效果。为了探究膨胀土渗透性与其裂隙和含水量之间的关系,采用自制的渗流装置对相关地区的非饱和膨胀土进行渗流试验。试验结果表明,不同初始条件的土样在饱和环境中渗流时其渗透系数都会随着渗流的进行而逐渐减小。在同等初始含水率情况下,土样的裂隙率越大,渗透系数越大,渗透系数到达稳定状态所需的时间就越长;在相同初始裂隙率情况下,初始含水率低的土样,渗透性小但基质吸力大,吸水后会迅速膨胀导致渗透性越来越差。通过分析土体从不同初始条件开始渗流至饱和状态期间土体渗透系数的变化过程,可以发现在渗流开始的2 h以内,渠坡排水设施的排水效果最好,在8 h内渠坡排水设施都具有一定效果,超过8 h后排水效果有限。

单排微型桩加固缓倾长大裂隙土边坡的力学机制

预制混凝土微型桩广泛应用于南水北调工程含长大裂隙膨胀土边坡抢险加固工程,研究其加固力学机制,对膨胀土边坡安全防护具有重要意义。在相似模型试验的基础上,建立含裂隙面边坡滑体及微型单排桩模型;以平行裂隙面方向施加推阻力,开展了相似模型试验及数值模拟。结果表明:微型单排桩加固表现出较好的阻滑效果,将坡体系统的抗滑力维持在较高水平,边坡抵抗破坏的韧性增加,边坡稳定性显著提高。边坡抗滑阻力随桩间距增加而减小,单根微型桩的影响范围有限,随着桩间距的增大,土拱效应逐渐减弱,当桩间距为12倍桩直径时,桩之间的相互作用可以忽略,建议桩间距取8倍桩直径;布置在边坡中部的微型桩加固效果较布置在边坡上1/3位置的微型桩加固效果更好。

干湿循环作用下膨胀土改性前后宏观裂隙性及微观结构演化特征分析

膨胀土具有典型的裂隙性,对工程性质影响巨大。研究了干湿循环作用下,素土和改性土的宏观裂隙性和微观结构演化特征,揭示了干湿循环作用下试样宏观裂隙和微观结构的发育规律,研究了离子土固化剂对膨胀土裂隙性的抑制作用,并简单探讨了其改性机理。研究结果表明:干湿循环作用下,膨胀土裂隙性呈现明显的缓慢增长、迅速增长和缓慢下降3个阶段;试样的微观结构从密实状态逐渐变到松散状态,黏土矿物片由卷曲片状逐渐变为单片状,卷曲程度降低,试样的结构变松散;经过离子土固化剂改性后,试样裂隙的峰值面积可降低20%,裂隙宽度降低值可达到30%;黏土矿物卷曲片状的卷曲程度降低,黏土矿物片开始出现“黏土片-片聚体-微聚体”的团聚现象,出现较大的团聚体。说明离子土固化剂能够改善膨胀土的黏聚性和水稳性,能够有效抑制膨胀土的裂隙发育,改善其工程性质。

论裂隙对膨胀土边坡稳定的影响

人们早就知道膨胀土具有多裂隙的特点,也知道它影响边坡的稳定;但怎样影响,严重程度如何,并未清楚认识,在膨胀土边坡设计施工的各环节中并没有充分考虑裂隙影响,以致膨胀土边坡失稳事故仍时有发生。对此作深入研究实有必要。笔者曾发表裂隙开展对强度指标影响的试验成果,提出考虑裂隙影响的边坡稳定分析方法和用土工膜限制裂隙发展的加固方法。在此基础上,本文进一步论述了膨胀土裂隙的产生机理与发展过程,结合膨胀土边坡失稳实例探讨了裂隙所起作用,解释了裂隙开展与膨胀土边坡失稳特征的联系,论述了换土加固方法之所以有效主要在于它限制裂隙开展等。上述各点,从不同的角度论证了多裂隙性是膨胀土边坡易于失稳的关键,是其不同于其它黏性土坡的根本;提出要在膨胀土边坡的分析、计算、施工和处理等各环节都考虑裂隙影响,形成系统理论。

膨胀土裂隙的量化手段与度量指标研究

为了对膨胀土的裂隙进行定量分析,利用远距光学显微镜对膨胀土试样进行观测,观测的图像经图像处理软件进行合适的灰度处理,计量裂隙所造成的暗色区域所占面积比例或灰度熵。通过对重塑膨胀土中的裂隙发展变化进行动态、定量的观测,认为裂隙图像的灰度熵可以较好地表征裂隙的发育发展程度,且土体处于裂隙不发育状态,其灰度熵越小,反之,土体裂隙越发育,其灰度熵越大。

反映裂隙影响的膨胀土边坡稳定性分析

膨胀土是多裂隙土,其边坡的稳定性在很大程度上受裂隙影响。裂隙开展后抗剪强度显著降低,裂隙开展深度将本来均一的土层划分为强度差异显著的不同土层,雨水进入裂隙中形成渗流增加了滑动力矩,裂隙还随时间而发展。这些都显著影响膨胀土边坡的稳定性。本文提出了一种以条分法为基础的近似反映裂隙影响的膨胀土边坡稳定性分析方法。将膨胀土坡划分成3个亚层,即裂隙充分发展层、裂隙发育不充分层和无裂隙层,分别取用不同的强度指标;给出了裂隙深度、各亚层界面、各层强度指标以及裂隙渗流浸润线的近似确定方法。所提出的方法反映了膨胀土边坡失稳机理,可体现其平缓性、浅层性、牵引性、长期性、季节性、方向性等特点。算例分析表明了它的合理性和实用性。

基于FBG技术的饱和膨胀土失水致裂过程试验研究

为了解饱和膨胀土在失水条件下,干缩裂隙发育过程中土体内的应变状态、分布以及变化规律,设计了一个长40cm、宽5cm、深3cm的一维模型槽,将3条FBG传感光栅串埋入饱和膨胀土试样中,对膨胀土不同位置的应变进行了测量,得到了土体失水干缩致裂过程中,各个FBG传感器的应变状态及其变化规律,并分析了首条干缩裂隙及其附近土体的应变分布特征及其时空演化规律。试验结果表明:随着土体含水率的降低,土体首先呈现出整体收缩,之后应变状态发生分异,呈现出拉张区和收缩区交替出现的分布规律;位于最大收缩区的边缘,当FBG传感器的应变由压缩状态转变为拉伸状态,并且拉应变及其变化速率相对较大的区域将产生首条裂隙。可见,本文所采用的高空间分辨率FBG传感技术为分析膨胀土裂隙发育全过程、揭示膨胀土失水致裂机理,以及膨胀土裂隙性特征的研究提供了新的思路和技术手段。

膨胀土干湿循环胀缩裂隙的定量分析

为研究膨胀土在降雨与蒸发过程中的胀缩裂隙演化规律,对原状膨胀土进行干湿循环的CT扫描试验。利用Matlab软件编写程序对土体CT图进行三维体重建以及裂隙信息的提取,从定性与定量的维度研究土体三维空间裂隙与干湿循环之间的关系。定性研究发现,裂隙发育开始于土体内部初始孔洞与微纹裂等薄弱处,并扩展延伸,形成裂隙面、裂隙体,最后形成复杂的三维裂隙网络;随着干湿循环的进行,裂隙面积和体积逐渐增加,最后趋于稳定。定量研究发现,裂隙面积沿试样轴向的分布具有明显的周期性规律,随着干湿循环的进行,周期逐渐减小,振幅A先增加后减小;裂隙开展分为3个阶段,即裂隙酝酿期、裂隙快速发育期和裂隙平稳发展期,而裂隙发育主要发生在裂隙快速发育期。得出了试样径向裂隙面积、轴向高度与累计干缩体变之间的近似关系,以裂隙体积定义了符合Logistic函数变化规律的扰动函数,可以预测裂隙在试样局部的发展及分布情况,为裂隙膨胀土的渗透特性以及整体与局部的应力、应变研究提供参考。

膨胀土裂隙的平面描述分析

通过室外数码成像,记录了不同含水量下原状膨胀土表面裂隙的开展状态,基于MATLAB软件用二值化像素统计的方法对膨胀土表面裂隙率与含水量的关系进行了量化,并且在二值化图像的基础上,基于AutoLISP程序对原状膨胀土表面裂隙的分形维数进行自动化统计。分析结果表明,膨胀土表面裂隙率和分形维数与含水量的关系基本呈线性关系,单位含水量变化引起裂隙率和分维的不同变化特征可对膨胀土胀缩性做出相应区分;二值化像素和分形维数统计的方法均可对原状膨胀土表面裂隙进行描述分析,为进一步研究膨胀土的裂隙性提供了有效途径。

膨胀土裂隙的工程特性研究

裂隙是膨胀土的重要特征之一,长期以来,人们常常把分布于膨胀土大气影响深度范围内受干湿循环影响而产生的干缩裂隙与膨胀土地层中特有的原生裂隙混为一谈。在对膨胀土边坡失稳机制的研究中,也忽视了原生裂隙对边坡稳定的重要影响。为研究南水北调中线工程膨胀土地段渠道破坏机制,在河南省南阳市郊建设了一条长为2.05 km试验渠道,通过渠道开挖期间的地质勘察、现场观测和取样试验,以及对现场滑坡体的取样分析,研究了膨胀土裂隙及周边土体的物质成分、密度、含水率和强度特性,结合现场观测资料分析了裂隙与渠道滑坡的关系。研究表明,膨胀土地层中的原生裂隙有两种类型:光滑蜡状裂隙和有一定厚度充填黏土的裂隙。裂隙面上的物质成分与两侧土略有不同,且天然密度低、含水率高,强度远低于相邻土体的强度,一旦地层与边坡倾向一致,将产生沿裂隙面的滑动。

膨胀土裂隙的量化指标与强度性质研究

利用远距光学显微镜对重塑膨胀土中在自然风干条件下的裂隙发展变化进行了动态的、定量的观测,对不同裂隙发育程度的膨胀土进行了不排水、不排气的非饱和三轴多级剪切试验。试验结果表明,裂隙图像的灰度熵可以很好地表征裂隙的发育发展程度,适合作为膨胀土裂隙发育程度的度量指标。环境温度较高、湿度较低有利于膨胀土中裂隙的发育发展。膨胀土中裂隙发展是一个长期的、渐进的过程,初期裂隙发展速度较快,后期大大减缓。裂隙膨胀土的强度指标与饱和度和裂隙度呈线性关系,土体裂隙不发育时,饱和度是影响强度的主导因素,而饱和度较低时,裂隙对膨胀土强度的影响占主导地位。