膨胀土胀缩特性试验研究

以南(宁)友(谊关)膨胀土大量的室内试验为基础,研究了膨胀土胀缩时程曲线特征,寻找膨胀土膨胀和收缩曲线的异同点,探求了土的初始含水量、干密度与膨胀变形量的定量关系等。     

基于变湿应力概念的膨胀土初始开裂分析

含水率变化时膨胀土体积会产生吸水膨胀或失水收缩,这与温度变化时一般材料的反应相似。当土体的胀缩变形受到约束时就会产生应力,据此提出了变湿应力的概念。借用温度应力理论分析思路,基于弹性力学理论,采用变湿应力来计算土体的胀缩变形,建立了用含水率变化计算变湿应力的理论模型。揭示了裂隙产生的机理是不同埋深处土体水分丧失速率的不同,产生了不均匀的收缩变形。提出了膨胀土初始开裂的临界变湿判据,经比较与实测结果较一致。利用该模型,推导了地表蒸发条件下膨胀土的裂隙初始开展深度,并对其与土体变形模量、胀缩特征参数等的关系进行了讨论。     

膨胀土胀缩变形规律与灾害机制研究

采用普通固结仪和收缩仪分别进行蒙自重塑膨胀土浸水膨胀变形试验和膨胀土失水收缩变形试验,系统地研究了不同初始状态下膨胀土胀缩变形规律与致灾机制,并应用Does Response模型,定量模拟了膨胀土胀缩时程规律。研究表明,蒙自膨胀土胀缩变形差异较大,一般吸水膨胀率远大于失水收缩率,相似状态下试样膨胀系数越大,收缩系数亦越大,表现出较强的各向异性;膨胀土含有的大量细小黏土颗粒与较强的蒙脱石晶体矿物及显著的微结构特征,是其产生强烈胀缩变形灾害的内因与本质,而土中发育的微孔隙-裂隙结构及其初始状态,是发生胀缩变形灾害的外因。     

南水北调渠坡膨胀土胀缩特性及变形模型研究

南水北调工程膨胀土渠坡变形受大气降雨、蒸发等干湿循环作用较为显著,揭示渠顶膨胀土在干湿循环作用下的胀缩变形和垂直位移变形特征,可为渠坡的稳定性评价与处治方案提供依据。利用膨胀土室内胀缩特性试验结果,结合垂直位移量、含水率、降雨量、蒸发量和气温监测等现场观测数据,分析渠顶位移量和影响因子间关联度,建立了干湿循环作用下渠顶膨胀土的竖向变形模型。研究结果表明,以含水率、降雨量、蒸发量等构成的胀缩因子与渠顶垂直位移呈负相关关系,胀缩因子引起垂直位移分量较小;温度因子与渠顶垂直位移呈正相关关系,对渠顶垂直变形有较大影响,是影响渠顶变形的主要因素;渠坡开挖（时效因子）在施工初期对渠顶垂直变形有一定的影响,开挖完成后对渠顶垂直变形影响甚微。     

干湿循环条件下木质素改良膨胀土胀缩特性

实地调查发现河南省南阳市南阳盆地周边受持续降雨、干旱极端气候影响,出现了房屋破坏、边坡失稳等膨胀土灾害问题,危害人民财产生命安全。以南阳盆地弱膨胀土为研究对象,通过自由膨胀率试验,研究木质素磺酸钙改良弱膨胀土的最佳掺量问题,并在最佳掺量的基础上对试样进行无荷膨胀和收缩试验,进一步研究干湿循环条件下木质素磺酸钙对弱膨胀土胀缩特性的影响,结合试验过程中试样的裂隙发展规律,探究干湿循环条件木质素磺酸钙改良弱膨胀土胀缩特性的原因。试验结果表明对于弱膨胀土,木质素磺酸钙掺量在0.75%、养护天数14 d时改良效果最佳,此时改良土自由膨胀率最低。干湿循环条件下的无荷膨胀和收缩试验数据显示改良前后弱膨胀土绝对膨胀率最高由29%降至3%,绝对收缩率最高由17%降至2%,改良土相对膨胀率与相对收缩率基本不随干湿循环次数发生变化,说明木质素磺酸钙可以有效降低干湿循环过程中弱膨胀土的胀缩形变,保持干湿循环过程中弱膨胀土胀缩特性的稳定,且木质素磺酸钙的改良效果不受干湿循环次数的影响。对比未改良土与改良土裂隙发展规律,木质素磺酸钙在弱膨胀土中主要起到抑制裂隙发育,提高土体胶结能力的作用,使土体形成更致密的结构,...     

武汉地铁三号线土层盾构开挖引起的地表沉降研究

为了研究红黏土的变形特性,使用常规固结仪和自制收缩仪,对压实桂林红黏土进行了无荷载条件下4次膨胀和收缩试验。结果表明:红黏土的胀缩变形过程是不可逆的;随着干湿循环次数增加,膨胀试验试样高度变化较小,收缩试验试样高度逐渐增大,膨胀变形和收缩变形均呈逐渐减小趋势;稳定后膨胀率和收缩率均逐渐减小,收缩系数不断减小,最终趋于一个较稳定数值;采用绝对膨胀率、绝对收缩率、相对膨胀率、相对收缩率等参数可以定量分析干湿循环作用下压实红黏土的胀缩变形程度及其变化规律。     

荆门弱膨胀土的胀缩与渗透特性试验研究

以荆门弱膨胀土为研究对象,开展了完整的胀缩与渗透特性试验,获得了5种不同压实度下膨胀土及其石灰改良土的胀缩与渗透特征。结果表明：膨胀土的膨胀力-干密度关系可用幂函数表达,而无荷膨胀率、有荷膨胀率、体缩率与干密度均非单调关系;由于压实与膨胀效应的耦合作用,95%压实度下膨胀土的无荷膨胀率、有荷膨胀率和体缩率均较小,若直接利用膨胀土进行路基与地基填筑,该压实度下土样不仅具备较大的刚度与强度且胀缩变形较小。经石灰改性后,膨胀力与湿胀变形基本消除,干缩变形大幅降低,胀缩总率仅为0.7%。膨胀土与石灰土渗透系数均很小,且都随干密度的增大而降低,其与干密度的关系仍可用幂函数描述;石灰土的渗透系数大于相应压实度下的膨胀土;而当二者干密度相近时,渗透系数接近。     

基于集对分析的膨胀土胀缩性Vague集评价模型

膨胀土胀缩性等级评价是膨胀土地区工程治理中的首要问题。本文探讨了基于集对分析的膨胀土胀缩性等级评价模型,以克服膨胀土胀缩性评价指标的不确定性和复杂性。模型以Vague集描述膨胀土胀缩性等级的确定性和不确定性,并应用集对分析构造Vague集的隶属函数。通过实例应用及与其他方法评价结果的对比分析,表明该模型应用于膨胀土胀缩性等级评价是有效可行的,且具有判断结果准确、易于操作等优点。     

陕南膨胀土加灰改良的试验研究

陕南膨胀土分布地区由于膨胀土地基的胀缩变形，常常导致建筑物开裂和毁坏。如何改良膨胀土，消除其胀缩性，加固膨胀土地基，确保工程建筑物的安全和稳定，是膨胀土分布地区工程建设实践中急待解决的重要课题。结合陕南三种类型的膨胀土，我们选取有代表性的试样进行了加灰（石灰、粉煤灰）处理的试验研究，其效果较好，消除或减弱了膨胀土的胀缩性，达到了改良膨胀土不良工程地质性质的目的。     

不均匀收缩方式对膨胀土工程性状影响试验研究

通过控制恒温恒湿箱的温湿度工作参数,改变膨胀土试样的蒸发路径,实现饱和膨胀土在不同脱湿速率下的不均匀收缩,结合有荷膨胀率试验和直接剪切试验,研究膨胀土不均匀收缩方式下胀缩特性和强度特性。试验结果表明,脱湿速率越小,膨胀土的胀缩性越大,剪切强度越高;对比不同收缩方式,均匀收缩下膨胀土的收缩与膨胀变形和剪切强度均大于不均匀收缩下的收缩与膨胀变形和剪切强度。造成这些差异的主要原因是不均匀收缩试验中水分蒸发的路径不同,沿深度方向的含水率不同,基质吸力不同,引起的应变也不同,这种不均匀性改变了原有膨胀土的均一性,限制了膨胀土胀缩特性和强度特性的发挥。     

皖中膨胀土的承载比(CBR)强度特性研究

针对皖中地区高速公路建设中遇到的膨胀土问题,选取合六叶高速公路典型土样开展了系统的承载比（CBR）特性试验研究,并在此基础上探讨了膨胀土作为路基填料的适用性。研究表明:（1）起始含水量对膨胀土CBR值影响显著,CBR最大值对应的含水量高于最佳含水量,且击实功越大,二者差值也越大;（2）CBR膨胀量随起始含水量增大而减小,起始含水量越低,CBR膨胀量就越大,路基的水稳性就越差;（3）当击实功较大时,膨胀土的最佳含水量较小,适合填筑的可变含水量范围较宽,建议现场施工控制采用重型击实标准;（4）在重型击实条件下,将弱膨胀土起始含水量控制在比最优含水量大2%4%范围内,能同时满足压实度和CBR值要求以用于填筑下路堤,中膨胀土作为路基填料时必须经过改性处理。研究结果对于在膨胀土地区进行公路建设具有参考意义。     

ISS对不同初始含水率膨胀土的改良效果研究

离子固化剂(ISS)对膨胀土改良后,膨胀土稳定性会改善。为了分析研究不同初始含水率的膨胀土经过ISS改良效果,进行了膨胀土改良前后的剪切试验、界限含水率试验和自由膨胀率试验。通过改良前、后土体的抗剪强度、塑限、液限、塑限指数、自由膨胀率的变化规律研究,得出膨胀土的初始含水率在11%～30%区间时,改良后土体的膨胀性得到了消除,抗剪强度得到了明显提高; 初始含水率不在11%～30%区间时土样改良后仍具有一定膨胀性,抗剪强度提高不明显。这是因为,当膨胀土烘干时,靠毛细吸力吸附的固化剂量不能与所有黏土颗粒进行反应; 当膨胀土饱和时,靠溶液渗透到膨胀土表面的固化剂也不能与所有黏土颗粒进行反应。该试验结果对膨胀土现场改良时含水率的确定具有重要的指导意义。     

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).     

Silica fume stabilization of an expansive clay subgrade and the effect of silica fume-stabilised soil cushion on its CBR

ABSTRACT

Expansive soil subgrades, which are subjected to dual swell-shrink problem consequent upon absorption and evaporation of water, need to be improved by chemical stabilization or compacted cushion or geosynthetic reinforcement in order that pavements constructed over them are even, stable and safe. This paper presents extensive experimental data on plasticity, free swell index (FSI) and compaction characteristics of a highly swelling expansive clay stabilized with varying silica fume contents. In another series of tests on a laterite soil to be used as a cushion over the expansive clay subgrade, plasticity properties, compaction characteristics and strength characteristics were determined at varied silica fume contents. Further, CBR of the expansive clay subgrade was determined in the laboratory stabilizing it with varied silica fume contents and providing a cushion of 50 mm thickness of silica fume-stabilized lateritic soil. Liquid limit (LL), plasticity index (PI) and free swell index (FSI) of the expansive clay decreased with increasing silica fume contents. The compaction and strength characteristics of both the soils improved with silica fume stabilization. The CBR of the expansive clay provided with silica fume-stabilized cushion improved significantly.     

蒙皂石含量与膨胀土膨胀势指标相关关系研究

对沿南水北调中线工程邯郸永年、南阳、平顶山等地区及襄樊、荆门钟祥地区采集的各类地表、近地表膨胀土样品5 6件 ,运用现代XRD定量方法和有机染料吸附方法测量了粘土矿物成分和蒙皂石含量 ,同时测定了它们的液限、塑限、自由膨胀率、比表面积、粘粒、胶粒含量等指标。蒙皂石含量与各种膨胀势判别指标研究表明 ,蒙皂石含量是各种膨胀势的主要控制因素。当然 ,各种测量误差 (主要是蒙皂石含量的测量误差 )也是影响膨胀势判别的因素。Seed膨胀势较本文采用的推荐方案划分结果高 1～ 2个级次。蒙皂石含量与各参量之间呈线性、对数、指数关系 ,与粘土矿物的诊断吸收特性一起 ,这为膨胀土遥感提供了基础.     

碱渣改良膨胀土室内试验研究

通过室内试验,探讨利用碱渣作为添加剂对膨胀土改良的可行性和改良效果和碱渣改性土的基本物理力学性质和膨胀性。研究结果表明,随着掺渣率的增加,碱渣改性土的黏性成分的含量降低,粗颗粒含量增加,导致相对密度、液限、塑性指数、自由膨胀率、有荷膨胀量均呈明显减小趋势,这说明碱渣对膨胀土膨胀性的改良有显著效果;碱渣改性土可击实含水率范围较之膨胀土宽,这给碱渣改性土的施工带来很大方便;经过7 d养护后的土样无侧限抗压强度和抗剪强度都显著增加,并在掺渣率为30%时存在一个峰值点。抗剪强度增强主要体现在黏聚力显著增加,而内摩擦角变化不大。     

膨胀土判别与分类方法探讨

膨胀土的胀缩等级评判是进行膨胀土处治的首要任务,开展膨胀土的判别与分类方法探讨具有重要意义。对现有膨胀土判别与分类方法进行了评价,对反映和表征膨胀土胀缩机理和特性的指标进行了深入探讨,提出了以能充分反映和表征膨胀土胀缩机理和特性的液限、塑性指数、自由膨胀率、小于0.005 mm颗粒含量、胀缩总率等5个指标作为膨胀土的判别指标,建立了一种新的膨胀土判别与分类方法,并通过试验进行了验证。新的膨胀土判别与分类方法具有准确度高、易操作的优点。     

Three dimensional numerical simulation of residential building on shrink–swell soils in response to climatic conditions

Shrink–swell soils can cause distresses in buildings, and every year, the economic loss associated with this problem is huge. This paper presents a comprehensive system for simulating the soil–foundation–building system and its response to daily weather conditions. Weather data include rainfall, solar radiation, air temperature, relative humidity, and wind speed, all of which are readily available from a local weather station or the Internet. These data are used to determine simulation flux boundary conditions. Different methods are proposed to simulate different boundary conditions: bare soil, trees, and vegetation. A coupled hydro‐mechanical stress analysis is used to simulate the volume change of shrink–swell soils due to both mechanical stress and water content variations. Coupled hydro‐mechanical stress‐jointed elements are used to simulate the interaction between the soil and the slab, and general shell elements are used to simulate structural behavior. All the models are combined into one finite element program to predict the entire system's behavior. This paper first described the theory for the simulations. A site in Arlington, Texas, is then selected to demonstrate the application of the proposed system. Simulation results are shown, and a comparison between measured and predicted movements for four footings in Arlington, Texas, over a 2‐year period is presented. Finally, a three‐dimensional simulation is made for a virtual residential building on shrink–swell soils to identify the influence of various factors. Copyright © 2015 John Wiley & Sons, Ltd.     

Reduction of Expansive Index,Swelling and Compression Behavior of Kaolinite and Bentonite Clay with Sand and Class C Fly Ash

Swelling behavior of expansive soil has always created problems in the field of geotechnical engineering. Generally, the method used to assess the swelling potential of expansive soil from its plasticity index, shrinkage limit and colloidal content. Alternative way to evaluate swelling behavior is from its expansive index (EI) and swelling pressure value. The present study investigates the reduction of EI and swelling pressure for kaolinite and bentonite clay when mixed with various percentages of Ottawa sand and Class C fly ash. The percentages of Ottawa sand and Class C fly ash used were 0–50 % by weight. The results show that there is a significant reduction in the swelling properties of expansive soil with the addition of Ottawa sand and Class C fly ash. The reduction in EI ranged approximately from 10 to 50 and 4 to 49 % for kaolinite and bentonite clay, respectively. Also the maximum swelling pressure of kaolinite and bentonite clay decreased approximately 93 and 64 %, respectively with the addition of various percentages of Ottawa sand and Class C fly ash. Standard index properties test viz., liquid limit, plastic limit and linear shrinkage test were conducted to see the characteristics of expansive soil when mixed with less expansive sand and fly ash. Also, for these expansive soils one dimensional consolidation test have been conducted with sand and fly ash mixtures and the results were compared with pure kaolinite and bentonite clay.