基于收缩试验的膨胀土地基变形预测方法

进行膨胀土的三相收缩试验,确定了广西膨胀土的体积收缩指数。对依托工程膨胀土进行了现场静力触探试验,根据比贯入阻力随深度的变化曲线特征,确定了膨胀土活动区深度,给出了膨胀土裂隙开展深度的理论解,膨胀土裂隙开展深度的计算值与静力触探试验确定的膨胀土活动区深度和现场观测的裂隙开展深度基本一致。最后介绍了基于三相收缩试验基础上的膨胀土地基变形计算方法,并在试验基础上给出了膨胀土地基变形计算模式和计算结果。以膨胀土地基变形量作为膨胀土地基的分类指标,对膨胀土地基进行了分类,对膨胀土基础选型具有重要意义。     

The impact of natural ornamental limestone dust on swelling characteristics of high expansive soils

Expansive soils are considered as a potential natural hazard if they are not adequately treated. Expansive soils have high potential for shrinking and swelling under changing moisture conditions and cause extensive damages to engineering infrastructures. This study is concerned with the suitability of natural ornamental limestone dust to reduce the swelling characteristics of high expansive soils. The results are revealed that the swelling pressure and percent of heave are greatly decreased with increasing the inserted core diameters and mixing percentages of limestone dust. The average reduction percent values of swelling pressure and percent of heave are increased from 2.21% to 43.09% and from 2.56% to 45.64%, respectively, for treated soil samples with increase in limestone dust core diameters from 5 mm to 20 mm (1% to 16% of total soil area). The average reduction percent values of swelling pressure and heave percent are increased from 10.29% to 70.73% and from 22.29% to 82.90%, respectively, for treated soil samples with increase in limestone dust mixing percentages from 10% to 30%. The results are revealed that the enhancement in swelling characteristics of high expansive soils that are treated by ornamental limestone dust is mostly attributed to the replacement of high expansive fine clay particles by non-expansive and non-plastic coarse limestone dust particles, and to the presence of a considerable amount of free lime in limestone dust (4.97% as [Ca(OH)₂]) that is responsible for converting the high expansive soils to less expansive soils by pozzolanic reaction. The mixing treatment method is more suitable for surficial and shallow foundation high expansive soil beds, while the compacted limestone dust piles “inserted limestone dust core method” are more suitable for deep foundation high expansive soil beds.     

基于两种收缩试验的膨胀土地基变形预测方法的对比研究

基于两种收缩试验的膨胀土地基变形预测方法进行了比较研究。分析表明,两种预测方法的计算结果存在较大差异,其原因在于其考虑的膨胀土地基的侧限状态不同,分别为无侧限状态和完全侧限状态下的预测结果。基于以上分析成果,并考虑膨胀土的裂隙性和实际地基的侧限状态,给出了膨胀土地基变形预测的经验公式,并将其应用于实际工程,为膨胀土地基变形的准确预测及合理处置方案的制定提供了可靠的依据。     

Heave Studies on Fly Ash-Stabilised Expansive Clay Liners

Clay liners or compacted earthen barriers are important barrier materials used for preventing contaminant transport through soils. A low hydraulic conductivity (k) is a significant parameter that governs the design and construction of clay liners. Compacted expansive clays, which are montmorillonite clays, also have a very low hydraulic conductivity (k). When expansive clays are blended with fly ash, an industrial waste, the hydraulic conductivity (k) further reduces as the ash-clay blends result in increased dry densities at increased fly ash contents. Hence, fly ash-stabilised expansive clay can also be proposed as a unique clay liner material. As expansive clays undergo heave when they come into contact with water, it is necessary to study the heave behaviour of fly ash-stabilised expansive clay liners. This paper presents heave studies on fly ash-stabilised expansive clay liners. Fly ash in different contents by dry weight of the expansive clay was added to the clay, and the ash-clay blend was compacted as a liner overlying a natural field soil layer. Compacted lateritic clay was used for simulating the natural field soil into which contaminants migrate. Calcium chloride (CaCl₂) solution of varying concentration (5, 10, 20, 50, 100 and 500 mM) was used as the permeating fluid in the heave studies. The rate of heave and the amount of heave of the fly ash-stabilised expansive clay liners were monitored. Deionised water (DIW) was also used as inundating fluid for comparative study. Heave (mm) decreased with increase in solute concentration for all fly ash contents. For a given solute concentration, heave decreased up to a fly ash content of 20 % and thereafter it increased when the fly ash content was increased to 30 %. Heave of the fly ash-stabilised expansive clay liners was correlated with their permeability, liquid limit (LL) and free swell index (FSI) pertaining to the respective fly ash content and CaCl₂ concentration.     

Efficacy of Cement-stabilized Fly Ash Cushion in Arresting Heave of Expansive Soils

Expansive clays swell and shrink seasonally when subjected to changes in the moisture regime causing substantial distress to the structures built in them. Techniques like sand cushion and cohesive non-swelling soil (CNS) layer have been tried to arrest heave and consequent damages to structures. Sand cushion has been proved to be counter-productive. Studies have indicated that even though CNS layer was effective initially, it became less effective after the first cycle of swelling and shrinkage. Research carried out by the authors, using cement-stabilized fly ash as a cushioning material, has shown that it was quite effective in arresting heave. Fly ash cushion, stabilized with 10% cement with thickness equal to that of the expansive soil bed reduces heave by about 75% in the first instance. With subsequent swell-shrink cycles, the performance further improves, unlike in the case of a black cotton soil provided with a CNS cushion. At the end of fourth cycle of swelling, the reduction in the amount of heave is as high as 99.1%.     

饱和粉土冻结过程中的水分迁移试验研究

改变边界温度和土样高度,对饱和粉土进行了冻结试验,研究其水分迁移、水分重分布、冻胀和冰透镜体的发展规律。试验结果表明：当温度稳定时,水分持续迁移到冻结锋面附近,含水率急剧增大,易形成冰透镜体。饱和粉土冰透镜体几何形态较为规则,无枝状交叉结构,已冻土为整体状且无网状裂隙。冻结过程中存在起始冻胀时间,在起始冻胀时间之前,土中水分被排出,冻胀发生之后水分补给到冻土中,且补给水分产生的冻胀量和总冻胀量数值接近。土样高度影响水分迁移量和冻胀量,土样越高,冻胀量越小,冻土含水率增量越小,但水分增量分布区域越分散,且起始冻胀时间越长。     

植被作用下膨胀土渗透和力学特性及堑坡防护机制

以广西膨胀土为研究对象,考虑植被的影响,进行了现场渗透和植被根系加筋作用力学试验,并在试验结果的基础上,对膨胀土堑坡的植被防护作用进行数值模拟。研究结果表明,植被根系的加筋作用和植被的蒸发、蒸腾作用是从两个不同的方面维持边坡稳定的,并且在降雨过程中的发挥过程有着先后的顺序。根系的加筋作用增强了土体的强度,植被的蒸发、蒸腾作用阻止了土体强度的进一步衰减。此外,植被覆盖下土体的吸湿与脱湿过程应采用不同的渗透曲线,由于植被防护在施工后未长成的一段时间内十分脆弱,所以对生长期内的植被根系力学特性进行研究具有重要意义。项目研究成果可为膨胀土地区堑坡植被防护设计和施工的发展和应用提供有益参考。     

Assessment of Sulfate Concentrations in Water Used During Chemical Stabilization and Its Potential Impact on Sulfate Induced Heave

The occurrence of sulfate-induced heave of roadways that were chemically stabilized with either lime or cement can require expensive road repairs. Previous research attributed the heave to the formation of an expansive mineral named ettringite. However, not all chemically stabilized soils will exhibit heave. The overall goal of this research was to determine if the sulfate concentration in water can contribute to, or even cause, sulfate induced heave. Two soils, one with a soluble sulfate level below 3000 mg/kg and one with >8000 mg/kg sulfate, were stabilized with either lime or cement and subjected to a capillary soak with distilled water or saturated sulfate water. The low sulfate soils did not swell above the accepted limit of 1.5 %. The high sulfate soils swelled significantly (p < 0.05) above accepted level regardless of the stabilizer used. Overall, stabilized soils subjected to a capillary soak with saturated sulfate water swelled more than soils soaked with distilled water. The results found in this study demonstrated that cement will increase the axial load capacity of the soil, but the soil will still have the potential to heave excessively if sulfate and aluminum are present above the stoichiometric requirements to from ettringite.     

有荷干湿循环条件下不同膨胀土抗剪强度基本特性

针对膨胀土分布区公路边坡常浅层坍滑现状,选取南、北方3种典型膨胀土样,模拟干湿循环并计及边坡破坏时滑面的上覆压力,改变常规试样吸湿及施剪条件开展试验研究,以获得较符合实际的膨胀土抗剪强度及其衰减规律。结果表明：不同上覆压力对膨胀土抗剪强度的影响为荷载越大其强度绝对衰减率越小;用双直线能较好表征其抗剪强度线特征,强度指标c、? 宜按高、低应力段分别获取;随干湿循环次数增加,c值呈指数函数衰减,是造成边坡浅层坍滑的主要原因;? 值随作用次数增加虽有减小趋势,但降幅不大;通过控制基本相同的试验条件,实测得3种膨胀土的最终抗剪强度均降至某一幅值范围。研究结论为解读膨胀土坡为何多浅层坍滑提供了有价值的信息。     

Swelling behaviour of GPA-reinforced expansive clay beds subjected to swell-shrink cycles

Granular pile-anchor (GPA) technique has been found to be an innovative foundation technique for expansive clays posing the dual problem of swelling and shrinkage. Swelling occurs during absorption of water and shrinkage during evaporation of water. Generally, in field expansive clay beds, swelling takes place during rainy seasons and shrinkage during summers. GPA is a recent innovative foundation technique devised to ameliorate the dual swell-shrink problem of structures founded on expansive clay beds. The other innovative techniques are drilled piers, belled piers and under-reamed piles. Laboratory scale model studies and field scale experiments on GPAs yielded useful results and revealed that swelling of expansive clay beds was effectively controlled by GPA technique. Studies on swell-shrink behaviour of GPA-reinforced clay beds have not been performed so far. This paper presents results obtained from laboratory scale model studies on GPA-reinforced expansive clay beds subjected to alternate cycles of swelling and shrinkage. The data presented in this paper pertain to the swelling of test clay beds under the influence of three swell-shrink cycles (N) spanning a time period of 300 days. The test clay beds were reinforced with varying number of GPAs (n = 0, 1, 2 and 3). Heave (mm) in a given swell-shrink cycle decreased with increasing number of GPAs. Further, for a given number of GPAs (n), heave (mm) also decreased with increase in depth from the top of the clay bed. It was found that the resultant thickness of the clay bed (H_r) for swelling increased with increasing number of cycles. However, the percentage heave (ΔH/H_r) decreased as the number of swell-shrink cycles (N) increased.     

引江济汉工程渠坡膨胀土分级评价量化模型研究

膨胀土的膨胀潜势由于受到诸多因素的控制,其量化分级长期以来都是一个未能得到很好解决的难题。本文以南水北调中线引江济汉工程渠坡膨胀土为研究对象,通过对试验数据的分析,给出了膨胀土分类的建议取值;依据该分类标准,综合人工神经网络、Fisher判别分析以及粗糙集理论3种数学方法,对40组土样的膨胀等级进行了综合性指标的判别分析,通过比较研究,确定了最终的分级结果;根据每种数学方法的判别结果与操作方法,基于理论相对简单、操作简便、易于为广大科研工作者接受的原则,建议优先考虑Fisher判别方法对膨胀土进行分级评判;结合样品的蒙脱石含量,给出了分级标准的建议值,验证了膨胀土分级数学方法的可靠性,从而为引江济汉工程后续开挖渠道段膨胀土的分类及判别提供了理论依据。     

Swell-compressibility characteristics of lime-blended and cement-blended expansive clays – A comparative study

Chemical stabilisation of expansive soils has been quite efficacious in reducing swelling characteristics, namely, swell potential (S%) and swelling pressure (p_s). When chemicals such as lime and cement are added to an expansive clay, flocculation and cementation take place. Flocculation, which is an immediate reaction, is instrumental in reducing plasticity and swell potential significantly. It also reduces the time required for equilibrium heave. This paper presents experimental data on lime-blended and cement-blended expansive clay specimens. Free swell index (FSI), heave, rate of heave and swelling pressure were studied. FSI, heave and rate of heave decreased with increasing lime content and cement content in the blends. But, during a 3-day inundation (a period, generally allowed for the sample to attain to equilibrium heave), cementitious products developed and resisted the applied compressive loads stiffly, resulting in high swelling pressures in the case of lime-blended specimens. Swelling pressure could not be determined in the case of cement-blended specimens. Hence, short inundation tests (inundating the specimens only for 15 minutes) were performed. But, even from these tests, swelling pressure could not be determined for cement-blended specimens. This indicated the development of strong cementitious products in them. It was interesting to find that, in both long and short duration, the lime- and cement-blended specimens attained to equilibrium heave in the same time period. FSI decreased from 185% to 63.63% when lime content was increased from 0% to 4%, and from 185% to 110% when cement content was increased from 0% to 20%. Swell potential reduced by 42.5% at 4% lime and by 46.4% at 20% cement. Swelling pressure increased from 210 kPa to 320 kPa when lime content was increased from 0% to 4%. Linear shrinkage of the specimens also decreased with increasing additive content.     

膨胀土卸荷蠕变特性及其非线性蠕变模型

鉴于膨胀土滑坡往往表现为长期性、渐进性等与时间相关的特性,利用GDS应力路径三轴仪对膨胀土进行了三轴卸荷蠕变试验。试验结果表明：当偏应力较小时,膨胀土的蠕变曲线仅出现瞬时变形和衰减蠕变;当偏应力达到一定值时,其蠕变曲线也呈现衰减蠕变、稳态蠕变和加速蠕变3个阶段,但其加速蠕变阶段的特征与一般岩土体不同,其蠕变速度近乎常数。膨胀土的应力?应变等时曲线显示,膨胀土卸荷蠕变具有非线性特征,且其非线性程度与蠕变时间和应力水平相关,蠕变时间越长、应力水平越高,其非线性程度越高。基于非线性流变力学理论,提出了一种非线性四元件蠕变模型,将标准线性体与一个非线性黏壶串联,该模型可描述等围压三轴压缩应力状态下膨胀土轴向应变随时间的演变规律。根据膨胀土卸荷蠕变试验结果,采用曲线拟合法对三维非线性模型的参数进行反演识别。拟合曲线和试验曲线对比显示,两者吻合良好,说明该模型可以很好地描述膨胀土的蠕变特性。此外,基于该蠕变模型获取了膨胀土的临界破坏应力,其与常规剪切破坏应力的比值随着固结压力的减小而减小,表明越接近坡面的土层越容易发生蠕变破坏。     

考虑抗剪强度衰减特性的膨胀土边坡稳定性分析

针对传统膨胀土边坡稳定性分析中无法考虑膨胀土在降雨入渗过程中抗剪强度动态衰减的问题,本文开展了室内直剪试验,系统研究了干密度和含水量变化对膨胀土抗剪强度指标的影响;同时,以试验抗剪强度结果为参数,开展了基于强度折减法的膨胀土边坡稳定性分析,获得了抗剪强度动态衰减过程中边坡稳定性的变化规律。结果表明,含水量的增加和干密度降低会造成膨胀土黏聚力和内摩擦角的衰减,黏聚力衰减显著,内摩擦角衰减较少;膨胀土边坡稳定性主要受风化层土体含水量控制,随着膨胀土含水量的增加,膨胀土边坡逐渐由稳定状态演变为欠稳定状态;干密度对膨胀土边坡稳定性的影响则相对较小。     

Determination of Swelling and Shrinkage Properties of Undisturbed Expansive Soils

Expansive soils exhibit large volume changes when their water content changes. Alternate heave and settlement due to seasonal climatic variations result in distress and damage in civil infrastructure systems. This research focuses on the understanding of swelling and shrinkage phenomenon in the surface layer of expansive soils. Undisturbed field samples were used to capture the effect of in situ conditions (geologically induced fissuring and environmentally caused saturation) on volume change properties of Regina clay. Based on laboratory investigations, the swelling potential and swelling pressure of the native clay at S = 82% were found to be 1.5% and 3.5 kPa, respectively. The swell-shrink path during progressive soil drying followed an S-shaped curve comprising of an initial low structural shrinkage followed by a sharp decline during normal shrinkage and then by a low decrease during residual shrinkage. The soil microstructure correlated well with the observed volume change behaviour as well as with the consistency limits. The presence of fissures in field samples at various degrees of saturation confirmed that the investigated deposit is at an equilibrium condition with respect to the swell-shrink phenomenon. The swelling properties at any initial saturation state were estimated using the free swelling test and the swell-shrink test data in conjunction. The swelling potential increased 12 times (from 2 to 24%) and the swelling pressure increased by two orders of magnitude (from 27 to 2500 kPa) with a change in the degree of saturation from 80% (at the plastic limit) to 60% (at the shrinkage limit).     

低矮路堤下膨胀土地基渗流与变形耦合研究

二维渗流与变形耦合控制方程是基于流体质量守恒、Darcy定律及膨胀土弹性本构方程得出的。结合云桂高速铁路低矮路堤下膨胀土地基现场浸水试验,并采用考虑耦合和不考虑耦合两种数值模拟方法,分析了低矮路堤下膨胀土地基的膨胀变形特性。研究结果表明,耦合情况下地基表面膨胀变形呈双曲线变化,非耦合情况下地基表面膨胀变形随时间的增加而逐渐增大,对同一时间而言,未考虑耦合效应数值模拟得到的膨胀变形落后于考虑耦合影响的膨胀变形;耦合与非耦合情况下地基的相对膨胀率沿深度方向呈衰减变化。数值计算结果与现场试验结果比较表明,不考虑耦合影响的数值模拟方法难以描述人工浸水条件下膨胀土地基的膨胀变形规律,而考虑耦合影响的数值模拟方法与现场实测值吻合较好。     

甘肃省公路沿线典型地段含盐量对冻胀盐胀特性影响的试验研究

季节冻土区含盐土公路路基在季节性冻胀、盐胀和融沉作用下,发生大量的道路病害,给道路的安全运营带来了严重的隐患。在考察大量现场道路病害的基础上,针对甘肃省季节冻土区公路沿线盐渍化道路病害比较严重的地段,选取几种典型的盐渍土进行室内冻融循环试验来研究它们在周期波动温度条件下冻胀、盐胀和融沉特性,进一步探讨盐渍土地区道路病害产生的机制。试验结果发现,含盐量对路基土冻胀、融沉和盐胀等变形过程有明显的影响,不同的含盐量路基土膨胀机制不同。含盐量较高的土体,变形主要由盐胀引起,没有明显的融沉变形;含盐量较低的路基土,变形主要由冻胀和融沉引起,可能存在盐胀;对于无盐但冻胀敏感性路基土,其变形主要由冻胀和融沉引起。另外,开放系统下盐渍土反复冻融循环后含水率重新分布,含水率普遍增加,且形成了两端含水率高、中间低的现象。     

季节冻土区高铁路基冻胀研究进展及展望

随着我国高速铁路建设的快速发展,穿越广阔季节冻土区的高速铁路越来越多,工程面临的冻胀问题已成为研究和工程人员关注的焦点,并取得了许多研究成果。基于前人研究,总结了我国季节冻土区高速铁路路基冻胀特点和分布规律,探讨了高铁路基粗颗粒填料的冻胀特性及其影响因素,分析了路基在冻融过程中水热变化情况,讨论了现有高铁路基防冻害措施以及其适用性。在此基础上,提出季节冻土区高铁路基冻胀研究面临的主要问题及展望,为季节冻土区高铁路基冻胀及其防治工作研究提供新思路。     

膨胀土的现场抗剪试验研究

由于膨胀土的裂隙性和超固结特性,膨胀土的抗剪强度不仅是膨胀土地区边坡设计的重要依据,而且一直是膨胀土边坡研究领域的重要课题。本文以南阳膨胀土为例,对膨胀土现场抗剪试验的试样制备及试验方法进行探讨,通过对该地区典型滑坡反算证明,文中的现场抗剪试验方法及抗剪强度的计算公式是合理的。     

南宁膨胀土非线性流变模型研究

对南宁非饱和膨胀土进行一系列的加载-卸载固结蠕变试验,得到了不同含水率的膨胀土在不同应力水平下应变-时间曲线和不同时刻的应力-应变等时曲线簇。通过对膨胀土各阶段蠕变变形的分析,得到了膨胀土的固结蠕变是包含有黏弹性变形和黏塑性变形成分的非线性蠕变。采用模型流变与经验流变理论相结合的方法,将膨胀土的蠕变变形分成线性和非线性两部分进行分析,蠕变柔量和线性黏弹性模量随时间减小,而线性黏塑性模量随时间增大。结合膨胀土的线性黏塑性模型和非线性黏塑性经验模型,建立能够描述膨胀土非线性流变的黏塑性本构方程。根据试验所得数据,通过非线性拟合方法得到各模型参数,用该模型进行数值分析得到的理论与蠕变试验结果对比非常吻合。研究成果为实际工程提供了流变变形计算分析的可靠依据。