Coupled consolidation model for negative skin friction on piles in clay layers

Expansive soils swell on absorbing water and shrink on evaporation thereof. Because of this alternate swelling and shrinkage, civil engineering structures founded in them are severely damaged. For counteracting the problems of expansive soils, different innovative techniques were suggested. Stabilization of expansive clays with various additives has also met with considerable success. This paper presents, by comparison, the effect of lime and fly ash on free swell index (FSI), swell potential, swelling pressure, coefficient of consolidation, compression index, secondary consolidation characteristics and shear strength. Lime content was varied as 0%, 2%, 4%?and 6%?and fly ash content as 0%, 10%?and 20%. A fly ash content of 20%?showed significant reduction in swell potential, swelling pressure, compression index and secondary consolidation characteristics and resulted in increase in maximum dry density and shear strength. Swell potential and swelling pressure decreased with increase in lime content also. Further, consolidation characteristics improved. Compaction characteristics and unconfined compression strength improved at 4%?lime and reduced at 6%?lime.     

粉煤灰改良膨胀土试验研究

研究掺粉煤灰对合肥膨胀土的物理性质指标以及胀缩性指标等的影响,探讨利用粉煤灰改良膨胀土的措施与效果。试验研究结果表明,在膨胀土中掺入适量的粉煤灰可有效降低膨胀土的塑性指数、降低膨胀势、减小线缩率与降低活性。在膨胀土中掺入粉煤灰还可改变膨胀土的击实特性,一定击实功作用下,随着掺灰率的增加,土体的最优含水率与最大干密度均减小,膨胀土中掺入粉煤灰后,膨胀土可在较小的含水率下通过击实或压实达到稳定。掺灰膨胀土的膨胀量与膨胀力随养护龄期的增长而减小;没有经过养护的掺灰土,其无侧限抗压强度随掺灰率的变化几乎没有变化,经过7 d养护后,土的无侧限抗压强度有所增长,并且存在一个峰值点,合肥膨胀土的无侧限抗压强度所对应的最佳掺粉煤灰率约为15 %～20 %。     

Influence of wetting–drying cycles on swell–shrink behaviour of GPA-reinforced expansive clay beds

ABSTRACT

Granular pile-anchor (GPA) technique is an innovative tension-resistant foundation technique which can effectively ward off the dual swell–shrink problem posed by expansive clays. The other tension-resistant foundation techniques are drilled piers, belled piers and under-reamed piles. Laboratory scale model studies and field scale experiments on GPAs revealed that swelling of expansive clay beds could be effectively controlled by GPA technique. This paper presents results obtained from laboratory scale model studies on GPA-reinforced expansive clay beds subjected to alternate cycles of wetting and drying. Swelling and shrinkage of the clay beds were monitored for three wetting–drying cycles (N) spanning a time period of 300 days. The clay beds were reinforced with varying number of GPAs (n = 0, 1, 2 and 3). Swelling (mm) and shrinkage (mm) of the clay beds in a given wetting–drying cycle decreased with increasing number of GPAs. Further, swelling (mm) and shrinkage (mm) significantly decreased with increasing number of wetting–drying cycles (N) also. For a given number of GPAs (n), swelling and shrinkage decreased with increase in depth from the top of the clay bed too.     

粉煤灰与生石灰加固软土的室内试验研究

粉煤灰是一种工业废弃物,与生石灰一样均可用来加固软土地基。笔者进行了粉煤灰与生石灰加固土的室内配方试验,分别将水泥或生石灰与粉煤灰按照不同的比例干拌均匀后,再与不同含水量的软土人工拌和均匀,测定了不同龄期的无侧限抗压强度;分析了石灰加固土强度与生石灰掺入比和龄期的关系及其压缩系数、压缩模量等的变化。并进一步分析了粉煤灰的加固机理,旨在为其进一步的研究和工程应用提供参考。     

消石灰对膨胀土团粒化作用的研究

用消石灰作为外掺剂,进行了两种不同的膨胀土的对比试验。研究了不同灰剂量和龄期对膨胀土“团粒化”作用效果的影响。对于消石灰改性膨胀土而言,不同性质膨胀土界限含水率的改性效果不尽相同。强膨胀土的改性效果相对较为明显,而弱膨胀土的改性效果相对较差,其改性效果存在一个下限。在膨胀土路基的施工中,二次掺灰工艺的闷料周期可以相对缩短。这样不仅有利于加快施工进度,同时对提高石灰改良土碾压质量和加固效果十分有利。     

荆门非饱和膨胀土的变形与强度特性试验研究

通过压力板试验,对比分析了荆门原状膨胀土与石灰改良土持水特征的差异性;应用非饱和土三轴仪,开展了原状膨胀土、石灰改良膨胀土与重塑膨胀土的变形及强度特性试验。结果表明：①石灰改良膨胀土与原状膨胀土相比,其进气值有显著降低,残余含水率显著升高,土-水特征曲线两个特征点的斜率较为平缓,说明其水稳定性较好,土体性状更为稳定;②原状膨胀土和石灰改良膨胀土在非饱和与饱和状态的应力-应变关系曲线均呈应变软化型,随净围压的增大,应变软化的程度趋缓;随含水率的减小,峰值应力增大,应力峰值点有所提前,应变软化现象更加显著。饱和重塑膨胀土的应力-应变关系呈应变强化型,非饱和重塑膨胀土则呈现为应变软化型,但其应变软化的程度较前两类土大为趋缓;③经石灰改性后,膨胀土强度参数值有大幅度提高,即使在湿化饱和后,石灰改良膨胀土仍保持了相对稳定的力学性质和较高的抗剪强度参数值。相比之下,无论是重塑膨胀土,还是原状膨胀土,对湿化作用均十分敏感,其强度参数值或波动较大,或整体水平较低。     

循环荷载下粉质中液限黏土和粉煤灰土的动剪强度研究

通过对路基填料粉质中液限黏土和粉煤灰土的动力循环荷载试验,分析了粉质中液限黏土和粉煤灰土1: 2的动力特性,得出了动剪强度与循环荷载次数间的变化规律以及动模量与动应变的关系表达式。研究结果表明：粉煤灰土1: 2动剪强度优于粉质中液限黏土,可以作为路基填料。     

非饱和红粘土和膨胀土抗剪强度的比较研究

红粘土是对环境湿热变化敏感的塑性粘土,具有一般膨胀土吸水膨胀失水收缩的特性。与普通粘性土相比,红粘土与膨胀土的强度特性更为复杂。它既是土体抵抗剪切破坏能力的表征,也是计算路堑、渠道、路堤、土坝等斜坡稳定性以及支挡构筑物土压力的重要参数。通过试验研究讨论了红粘土与膨胀土的强度特性以及与一般粘性土的差别及其各种影响因素,并探讨了非饱和红粘土与膨胀土的抗剪强度指标与含水量之间的相关关系。试验结果表明,红粘土与一般膨胀土的吸水膨胀规律完全相同。其试验结果可为红粘土与膨胀土地区工程设计与建设提供参考依据。     

基于工程包边法的膨胀土抗剪强度干湿循环效应试验研究

根据包边法施工中填芯重塑膨胀土和包边石灰改性膨胀土的实际工程状态,设计了反映其运营状态的干湿循环过程,对6次干湿循环前、后膨胀土的强度特性进行了较为系统地试验研究。结果表明,在压实度为90%~96%时,干湿循环前重塑膨胀土和石灰改性膨胀土慢剪强度及强度参数均随干密度单调增加,而干湿循环后其黏聚力c随干密度单调增加,干密度对内摩擦角φ的影响则明显变小;重塑膨胀土和石灰改性膨胀土干湿循环后的残余强度受干密度制约性不大,但干湿循环前、后重塑膨胀土和石灰改性膨胀土的残余强度参数存在差异,且干湿循环幅度对膨胀土强度参数也有一定的影响;在分析干湿循环前、后反复剪切试验结果及膨胀土边坡长期破坏机制的基础上,认为对于膨胀土路堤,在进行强度参数选取时宜适当考虑干湿循环及其幅度对于残余强度参数的影响;利用石灰改性膨胀土包边处理填筑膨胀土路基较为适宜。     

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。     

粉煤灰土冻融循环后的动力特性试验研究

为研究粉煤灰土作为路基填料的可行性，把易液化的粉煤灰和易冻胀的粉质黏土按干重1：2混合，通过冻融循环后的动三轴试验，研究了粉煤灰土的动力特性，得出了动强度和循环荷载次数及冻融循环次数的数量关系，以及动模量与动应变及冻融循环次数的关系曲线。研究表明，粉煤灰土经过3次冻融后的动力性能指标优于粉质黏土，具有良好的抗冻性及稳定性，可以作为季冻区路基填料。     

基于有效固结应力法确定结构性黏性土不排水抗剪强度

由于结构性的存在使得很多天然黏性土的强度和变形特性不同于重塑土和非结构性土。首先回顾了适用于确定重塑土和非结构性土不排水抗剪强度的有效固结应力法,并推导了相应的方程。在此基础上,对于结构性黏性土,采用两段不同斜率（内摩擦角正切）和截距（黏聚力）的直线模拟其抗剪强度包络线,建立了确定其不排水抗剪强度的有效固结应力法方程;当有效应力小于结构屈服应力时,有效固结应力方程中有效应力部分需乘以0.8的修正系数;只要已知剪切前的有效应力,利用相应的有效固结应力法公式,可确定结构性土体的不排水抗剪强度。利用连云港结构性软黏土的等压固结三轴试验数据,验证了有效固结应力法的适用性。分析表明：对于连云港软黏土的不排水抗剪强度,有效固结应力法的计算结果与试验结果吻合较好;对于结构屈服应力,有效固结应力法的预测结果与试验数据点的拟合结果有一定偏差,但并不明显。     

粉煤灰增强回填碱渣工程特性的试验研究

碱渣与饱和卤水混合制成浆体回填到盐矿废弃盐腔可同时解决碱渣处理问题和地下废弃盐腔存在的地质隐患。回填碱渣强度是影响充填效果的重要因素。因此,为了提高回填碱渣强度,采用掺入粉煤灰制成复合碱渣对其强度特性进行改良。针对不同粉煤灰掺合比的碱渣开展了组成、力学和细观试验。研究结果表明：（1）掺入粉煤灰能明显改善碱渣的强度,使其黏聚力、内摩擦角都大幅提高,抗剪强度大幅增加;（2）粉煤灰掺合比越大,增强效果越明显,但强度并非随掺合比呈线性变化,对黏聚力而言,在0～20%内的掺合比下增加速度最快,而对内摩擦角则在20%～30%的掺合比区间增加最快,对抗剪强度而言,0～20%的掺合比内增加最明显;（3）粉煤灰掺入还可显著改善碱渣的压缩固结特性,使其固结系数大幅提高,从而提高碱渣固结速度,缩短充填工期,其中在0～10%的掺合比内对压缩固结特性改善最显著;（4）矿物组成分析表明,粉煤灰掺入改变了矿物组成,使得亲水性矿物含量急剧锐减,进而改变了其沉积特性。而细观分析则表明,粉煤灰掺入使碱渣从絮凝团细观结构变成了粉煤灰充当骨架的充填结构,且粒间支撑和拉联效应明显。从增强效果提高、压缩固结特性增强、控制成本和工期综合分析表明,最优掺合比为20%左右,建议工程中以不高于20%的掺合比作为实用掺合比即可取得较为理想的充填增强效果。该研究为揭示碱渣增强机制及废弃盐腔碱渣充填工艺优化提供了有益参考。     

大掺量粉煤灰淤泥固化土的强度与耐久性研究

基于传统水泥和石灰固化处理方法,提出了利用大掺量低钙粉煤灰、水泥和石灰固化剂进行淤泥固化处理的方法,以期改善淤泥的强度和耐久性,达到淤泥和粉煤灰双重资源化利用的目的。通过一系列室内试验,探讨了该方法处理后固化淤泥的击实特征、强度特性、水稳性和耐久性。试验结果表明：淤泥固化后最优含水率有所降低、最大干密度则略有增加;弹性模量、无侧限抗压强度和抗拉强度均有不同程度的增加,水泥掺量越大,养护时间越长,强度和弹性模量越大;固化后淤泥水稳性得到明显改善;浸水软化和冻融循环导致固化土抗压强度显著劣化,冻胀融缩导致设计混合料的无侧限抗压强度减小约22%。     

粉煤灰处理含氟废水的正交试验研究

对粉煤灰处理含氟废水进行了正交试验研究。结果表明各因素对F-去除率的影响顺序为:pH>V/M>T>CF-,极差R分别为49.6、9.1、4.9和3.6;在pH=5、V/M=10、T=3h时,粉煤灰对于CF-<500 mg/L的废水具有较好的去除效果;粉煤灰吸附F-的行为符合Langmuir等温方程,方程为Ce/qe=0.251 8Ce+6.087 3。     

Technical Note Reactive silica and strength of fly ashes

The use of fly ash in geotechnical engineering depends greatly on its pozzolanic reactivity. Though many factors influence the reactivity of fly ash it is well recognized that reactive silica and lime content play a major role. A new, accurate and reliable method for the determination of reactive silica content of fly ash has been established. The reactive silica content, obtained as acid soluble silica in about 2 to 3 N hydrochloric acid, is found to correlate well with unconfined compressive strength of fly ashes. The reactive silica content of fly ash is also important in the stabilization of soils using fly ash. © Rapid Science Ltd. 1998     

Effect of some additives on synthesis of zeolite from coal fly ash

Hydrothermal conversion of fly ash into zeolites was conducted and the effects of the addition of sodium halide and waste solutions produced after zeolitization of fly ash, as well as the adjustment of the Si/Al ratio prior to synthesis process on the formation and cation exchange capacity (CEC) of zeolite product were evaluated. Both the addition of NaCl and NaF ameliorated the crystallinity and CEC of synthesized zeolite, but NaF had a better improvement effect. Na⁺ was considered to enhance the crystallization of zeolite, while F⁻ favored the dissolution of fly ash. The type of zeolite formed depended on the Si/Al ratio of the starting material prior to the nucleation and crystallization of zeolite. The adjustment of the Si/Al ratio of fly ash by addition of Na₂SiO₄ and Al(OH)₃ changed the type and CEC of zeolite. Waste solutions contained large amount of Si and little Al due to the formation of a zeolite named NaP1 in zeolite terminology with the Joint Committee of Powder Diffraction Standard (JCPDS) code of 39-0219. The alkalinity decreased largely. As a result, the CEC value of zeolite products synthesized with waste solution as alkali source decreased. The supplementation of new alkali to adjust the alkalinity of waste solution could enhance the CEC of synthesized product. It was concluded that: (1) addition of sodium halide and adjustment of the Si/Al ratio prior to synthesis can improve the quality of zeolite; (2) waste solutions produced following the zeolitization of fly ash can be reused as an alkali source in the activation of fly ash; zero-emission of waste solution in the synthesis of zeolite from fly ash is possible.     

结构性对膨胀土收缩特性影响的试验研究

膨胀土的收缩性明显,容易引发边坡与地基开裂,但有关结构性对收缩特性影响的认识甚少。采用收缩自动试验装置,在恒湿恒温条件下对原状膨胀土和重塑膨胀土开展了收缩对比试验和扫描电镜（scanning electron microscope,简称SEM）测试分析,结果表明：与原状土相比,重塑土在土中水流动阶段的蒸发速率较小,蒸汽扩散阶段收缩稳定速率较慢,最终体积收缩应变量更大;重塑土体积收缩−含水率关系曲线的线性段较长,斜率较大,直线段与稳定段之间的过渡不明显,而原状土则反之;重塑土和原状土的收缩特征曲线（soil shrinkage characteristic curves,简称SSC）在较高含水率段基本重合,随着含水率下降,重塑土的SSC下降更快,对应的含水率范围更宽,最后进入残余−零收缩阶段时,孔隙比明显较小;Chertkov收缩模型适用于原状膨胀土,但不适用于重塑膨胀土。SEM测试结果表明,原状膨胀土较重塑膨胀土具有更强的原生结构性,初始密度与湿度相同情况下,两者颗粒排列、接触方式、胶结状态、孔隙大小与分布特征等微观结构上差异明显,导致蒸发过程中重塑土的水分迁移速率较小、基质吸力较大,是重塑土收缩更剧烈的内在原因。研究结果可为膨胀土边坡的坡面工程防护设计提供参考依据。     

粉煤灰、粘土、膨润土等对Zn2+的吸附试验研究

研究了粉煤灰、粘土、膨润土等从溶液中去除有毒金属离子Zn2+的吸附过程。动态试验显示吸附过程是快速的。吸附试验结果表明,粉煤灰、膨润土对Zn2+的吸附能力相当,但远大于粘土、粉质粘土。平衡吸附模型充分说明,在高浓度下Zn2+在粉煤灰、粘土、粉质粘土上的吸附符合Langmuir等温线。试验结果亦表明,随着吸附剂中Zn2+含量的增加,粉煤灰等吸附剂对Zn2+吸附的百分率均呈减小的趋势。