制样方法对非饱和土力学特性的影响

以一种非膨胀性黏土为试验材料,对孔隙比相近的非饱和压实试样和预固结试样进行一系列等吸力控制下的等向压缩试验和三轴剪切试验,研究了制样方法对非饱和土的压缩特性和剪切特性的影响。由等吸力等向压缩试验结果表明:初始孔隙比相近的预固结样和压实样在等向净应力20 k Pa条件下进行等吸力平衡,平衡过程中预固结试样的孔隙比明显减小而压实样的变化不大;其原因是两种试样的初始吸力和土结构存在差异。此外,预固结样和压实样在吸力150 k Pa条件下,预固结样的含水率随着等向净应力的增大而减小,而压实样的含水率几乎不变。上述变形特性可用SFG弹塑性模型说明。在净围压和吸力相同条件下对剪切前孔隙比相同的两种试样进行了三轴剪切试验,试验结果表明:在剪切前的密度、剪切时的吸力和净围压相同条件下,预固结样的偏应力–应变曲线和强度明显高于压实试样的曲线和强度。其原因可用两种试样的孔隙尺寸分布不同来解释。     

Experimental evaluation of the effect of the incidence angle and consolidation pressure on the hydraulic resistance capacity of clayey soils

The primary aim of this study is to examine the variations in erosion and hydraulic resistance capacity of clayey soils with a changing water flow under variable flow conditions. Clayey soils were artificially prepared with various kaolin-sand ratios and consolidation pressures. A laboratory flume apparatus, called an erosion function apparatus (EFA), is improved to measure the scour rate and critical shear stress of samples with changing water flow directions. A series of tests that use the improved EFA was performed for artificial clayey soils considering various directional changes in the water flow at incidence angles of 0°, 90°, 135°, and 180°. Based on the test results, the undrained shear strength indicated the highest correlation with the critical shear stress. Furthermore, the critical shear stress decreased and the scour rate increased with an increasing incidence angle. Finally, the characteristics of the hydraulic resistance capacity, the variations in critical shear stress, and the scour rate changed with the magnitude of consolidation pressure of the clayey soils.     

酸性溶液对花岗岩残积土物理力学特性的影响

花岗岩残积土是华南地区进行工程建设时的常见地层之一,它的工程性质易受到周围环境的影响。采用人工方法制备了4种不同浓度的硫酸溶液浸泡后的试样,同时与原状试样进行对比,系统研究了不同浓度的硫酸溶液、不同的酸液溶蚀时间对花岗岩残积土试样物理、力学特性的影响。试验结果表明:(1)随着酸液浓度及酸液溶蚀时间的增大,试样的土粒比重减小、孔隙比增大、颗粒粒径级配变差;(2)试样的剪应力-剪切位移关系曲线类型会随着法向压力及酸液溶蚀作用而发生变化;(3)硫酸溶液浸泡后,试样的抗剪强度参数粘聚力和内摩擦角都降低了,但粘聚力的降低效果更明显。     

荆门膨胀土土–水特征曲线特征参数分析与非饱和抗剪强度预测

 为探讨荆门原状、压实、石灰改良膨胀土的非饱和抗剪强度与土–水特征曲线(SWCC)的相关关系,利用Fredlund-Xing模型对3种膨胀土的SWCC试验数据进行非线性拟合,获得相应模型参数;利用Fredlund公式(1996年)对3种膨胀土控制吸力下的固结排水剪三轴压缩试验抗剪强度成果进行相关分析。分析结果表明：(1) 使用Fredlund-Xing模型拟合3种膨胀土SWCC试验数据的效果均较好。(2) 荆门原状、压实、石灰改良膨胀土的进气值分别为210,68和18 kPa;石灰改良膨胀土的持水能力(水稳定性)最强,压实膨胀土次之,原状膨胀土最弱;3种膨胀土的残余吸力可均取为3 000 kPa。(3) 3种膨胀土的有效内摩擦角(j´)和与基质吸力相应的内摩擦角(jb)均非常数;j´与基质吸力相关,jb与基质吸力和应力状态均相关,这意味着Mohr-Coulomb破坏包面是双向弯曲的。(4) 使用Fredlund公式,能够在较大的吸力范围内较好地预测荆门膨胀土的非饱和抗剪强度,原状、压实、石灰改良膨胀土的土性参数κ分别为2.4,2.7及3.4。(5) 预测值与实测值间存在差别的主要原因是Fredlund公式本身未考虑到jb与应力状态相关,若要考虑该因素,可将应力状态对SWCC的影响引入到Fredlund公式中。     

压实膨胀土的膨胀变形规律与计算模式

以陕西安康压实膨胀土为对象,通过不同初始干重度、初始含水率和上覆压力的一系列膨胀量试验,得到了人工压实膨胀土的膨胀变形随以上3种因素变化的规律,并总结提出了考虑初始含水率、初始干重度和上覆压力3种因素的耦合变化,满足工程精度要求的膨胀量计算模式,并对该计算模式进行了验证,证实了其正确性。结果表明:在实际工程中,可以通过有限的几组膨胀量试验得到计算模式的参数,用文中提出的计算模式直接计算任意初始干重度、初始含水率的土体在任意压力下的膨胀变形量,具有重要的实用价值。     

剪切波速-相对密度联合试验与经验公式验证

汶川Ms8.0地震发生的数百公里大规模砾性土液化震害,引起了国内外学者对剪切波速液化判别技术适用性的高度重视。为合理评价波速测试手段应用于高含砾量、宽级配粗粒土液化判别的可靠性,发挥该技术在室内外土动力参数关联方面的独有优势,以相对密度与抗液化强度相关性为基础,通过自主研制大型波速量测系统,开展饱和砂土与砾性土剪切波速-相对密度联合室内试验,分析砾性土相对密度与波速指标内在关联性,对已有经验公式进行验证。研究结果表明:砾性土波速与相对密度呈良好线性关系,颗粒组排结构的不同是导致砾性土与砂土整体波速差异及数据离散度差别的主要原因;砾性土剪切波速对孔隙比变化灵敏度高于角砾砂,所提修正经验公式更适于描述砾性土波速随相对密度变化关系。     

A study of the weathering of the Bukit Timah granite Part B: Field and laboratory investigations

Field investigations consisting of surface seismic refraction and electrical resistivity surveys, drilling and downhole geophysical logging of the rock mass and of the residual soil indicate that the weathering of the Bukit Timah granite has been rapid and is primarily caused by chemical decomposition due to the tropical climate in Singapore. The granite is usually overlain by a thick layer of residual soil. Laboratory tests indicated a large reductions of the material strength, the modulus of elasticity, the rock mass quality and other mechanical properties of the weathered granite. Apart from the mechanical properties of the residual material and/or of the weathered granite rock, physical properties including density, water content, Atterberg's limits, grain size distribution, permeability, sonic velocity and electrical resistivity were also studied in order to assess the weathering grade and the weathering processes.     

碱液加固黄土的电阻率特征试验及其效果评价

为建立快速无损的评价碱液处理黄土场地体系,探寻电测无损检测结果与场地工程力学特性之间的联系,在室内开展了不同浓度碱液处理原状黄土试样的电阻率测试及直剪试验,得到了碱液处理后黄土试样的剪应力-应变曲线变化趋势; 进一步分析碱液处理黄土场地的抗剪强度特性和电阻率特征,得到了指标间的定量联系,并以碱液浓度为关键变量对抗剪强度指标与电阻率、液限、塑限等物理力学参数进行关联分析。基于硬化型曲线的数学模型,给出了考虑碱液处理浓度和电阻率影响的剪应力-剪应变预测关系。最终根据抗剪强度、最大干密度、塑性指数等工程力学参数的变化情况,提出了基于电阻率变化的碱液处理黄土工程力学性能变化的量化评价。结果表明:随着碱液浓度的增加,液限、塑限均出现增大的趋势,最大干密度随着碱液浓度的增加而减小; 处理后的黄土试样剪应力-剪应变曲线呈现硬化型曲线的特征,且随着碱液浓度的增大,抗剪强度逐渐增加,电阻率值逐渐减小。     

柔性与硬性地工格网在砂土与风化泥岩围压下之力学特性

地工格网（以下称格网）用於加劲土壤时，除考虑无围压下的张力行为之外，围压下之力学性质更是设计考量的重点。实际工程应用而言，基於经济考虑，期以现地土壤作为回填材料。本研究分别以拉出、围压抗张与直剪三种试验来探讨格网放土壤中之力学行为；并利用凝聚性泥岩与非凝聚性细砂作为回填材料，评估两种回填材料对加劲成效之影响。结果显示，柔性格网之肋条在拉出过程中易扭曲，造成主应力面旋转的现象，以致拉出阻抗大放硬性格网；围压下格网抗张的应力-应变行为可分为三阶段，即束制阻抗期、张力发展期与破坏期。束制阻抗期大都於3％应变内即已完成；在低围压情况拉出阻抗达20％～60％之拉出强度（相同应变），在高围压下达150％。由直接剪力试验结果可以预测：（a）格网／泥岩加劲结构-低围压时，剪力破坏面应通过格网／泥岩之界面；而高围压时，剪力破坏面应通过泥岩上体。（b）格网／细砂加劲结构-低围压与高围压下剪力破坏面应通过格网／细砂之界面。     

沉入粘性土中桩的挤土效应探讨

在沉入粘性土中不同挤土密度的桩的现场试验和模型试验的基础上,本文阐述了桩的挤土效应——桩侧摩阻力随着桩的挤土密度ρ的增大而增大。 本文把桩的沉放过程假定为圆柱形孔洞扩张的平面应变问题,计算出桩周土体的应力变化与模型试验实测到的应力变化相比较,得到了较满意的结果。 本文根据桩的挤土效应,提出了大直径开口管桩极限承载力的计算方法,可供工程设计人员参考。     

基于优质块状土样的孔压静探测试结果解释

实践证明，即使与75mm的薄壁活塞土样相比，直径为250mm的块状士样的质量也是非常好的。因此，优质块状土样的三轴和固结室内试验结果为孔压原位静探测试结果的相关性分析提供了最佳的对比参数。以软粘土为研究对象，提出了孔压静探参数与块状土样不排水抗剪强度、先期固结压力及超固结比之间的新一代相关关系。     

有效自重压力下预固结的三轴不排水试验

在我国的一些有关基坑工程的规范中,对于饱和软黏土地基的抗剪强度,规定采用"有效自重压力下预固结后的三轴不排水试验"确定,原位地基土处于静止土压力状态(K0状态),而常规三轴试验是各向等压固结,这种各向等压固结压力与原位的K0状态固结压力如何对应,以使强度指标一致,是本文讨论的问题。文中根据剑桥模型的物态边界面的概念和模型的加卸载准则,得出了二者的平均主应力应当相等的结论,这对于在设计中正确确定和选用强度指标是十分重要的。     

Evaluation of Undrained Shear Strength of Soils from Field Penetration Tests

The evaluation of undrained shear strength of soils is necessary in determining the possibility of occurrence of flow deformation during earthquakes. The present study is aimed at examining the evaluation of undrained shear strength of silty sands from field with Swedish weight sounding tests and cone penetration tests. Based on the outcome of the previous studies on laboratory triaxial tests, the undrained shear strength ratio is defined as the undrained shear strength divided by the initial effective major principal stress. The undrained shear strength ratio is then formulated with respect to the relative density. The penetration resistances of Swedish weight sounding and cone penetration tests are then formulated with respect to the effective overburden stress and relative density, based on laboratory calibration chamber tests. By combining these formulations, the correlations of the undrained shear strength with Swedish penetration resistance and cone tip resistance are established. The range of values of penetration resistances indicative of soil layers susceptible to flow deformation is discussed. The correlations of the undrained shear strength with field penetration resistances thus derived are then examined from case history studies. Two case history studies are carried out with Swedish weight sounding tests at the sites of flow failures induced during the recent earthquakes. A series of case history studies are reexamined, which were carried out with Dutch cone penetration tests in the past studies.     

Effects of fine aggregate content on the mechanical properties of the compacted decomposed granitic soils

Decomposed granitic soil is widely used as the subgrade material in Korea. The mechanical behavior of the soil can differ depending on the fine aggregate content of the soil. Due to particle crushing during loading and compaction, the soil may behave as either cohesive or cohesionless. In order to evaluate the effects of fine aggregate content on the mechanical properties of the compacted decomposed granite soils, the direct shear test, the triaxial shear test as well as some fundamental property tests were made by using samples with the five different fine aggregate contents of 10, 20, 30, 40, and 50% by weight. Results of the triaxial tests showed that as the percentage of the fine aggregates decreased, the cohesion decreased as well as the internal friction angle increased. In the direct shear test, the soils containing lower amount of fine aggregates exhibited that the shear strength was considerably reduced after it reached the peak value. Comparably, soils with the high fine aggregates showed that the shear strength was quite constant after the peak strength. The critical state friction parameter showed that the parameter decreased with an increase of the fine aggregate content. Results of the triaxial and direct shear tests indicated also that the fine aggregate content of 30% could be one indicator for soil materials behaving either as sand or as clay materials. Thus fine aggregate content, less than 30% by weight, could be more obtainable for the subgrade materials when leading also to the increased stability of the roads. With the lower fine aggregate content the soils then also show similar and better characteristics of sands not clays. Further studies are recommended on understanding of the mechanical behavior of the decomposed granitic soils.     

Experimental study on stiffness degradation and monotonic response of reconstituted volcanic ash induced by internal erosion

Volcanic ash soils are a common geological body in earthquake prone regions, and they are widely used as construction materials. The present study discusses the contribution of initial density, stress state and seepage time on seepage-induced internal instability, stiffness degradation, and monotonic response of volcanic ash collected from Satozuka, Japan. Reconstituted specimens are tested using an erosion triaxial apparatus. The results exhibit that the rate of erosion is influenced by initial density, stress state and hydraulic gradient. The mercury intrusion porosimeter tests are conducted to explore the distribution of constriction sizes after erosion. Post-erosion stress-strain responses are also affected by the stress state during erosion, initial density and seepage time. Particularly, internal erosion affects the dilatancy response of relatively loose specimens and has an impact on the critical state line, brittleness, and peak strength. Further, the maximum shear modulus of eroded soil is found to be greater than that of non-eroded soil, mainly due to the particle rearrangement and removal of fines. However, a sharp reduction in stiffness during monotonic shearing is maybe evidence that temporary reinforced soil packing collapses at a large strain. This indicates that the elastic yield surface has expanded for eroded soils.     

基于离散元法的接触冲刷影响因素研究

成层土间容易发生接触冲刷破坏,对水工建筑物的安全运行有不利影响。为此探讨了粗层孔隙率、粗层颗粒形状和剪切位移对接触冲刷的影响,采用离散元法对成层无粘性土进行了接触冲刷数值模拟,得到以下结论:粗土层孔隙率的增加对土层抗渗流冲刷不利,对于D₁₀ /d₁₀比值为10的土层,当粗土层的孔隙率相对较大时,在一定的渗流压力作用下,试样的渗透流速在初始阶段会有一定减小,随着细颗粒流失比的不断增大,渗透流速也随之增大,土层结构发生较大改变,也会发生接触冲刷破坏,在工程实践中应该引起一定的重视;当粗层颗粒形状为非球形时,细土层颗粒流失比减小,土层抗冲刷破坏能力有所提高;剪切变形后试样中粗土层孔隙率会有一定增大,孔隙结构也会发生改变,细土层颗粒流失比增大,加剧了土层发生接触冲刷破坏的风险。     

桩-土-承台共同作用的模型试验研究

通过带承台单桩及双桩基础的模型试验,对低承台桩基桩间土变形发展及其与承台板板底应力、桩侧摩阻力及桩端阻力间的相互影响进行较为细致的研究。试验表明:在相同基础荷载作用下,桩数的增加使桩端刺入变形量占基础沉降的比例降低。双桩基础桩体的存在对板底应力体现出增强作用,在相同桩间土变形量下,双桩基础板底应力大于带承台单桩基础。桩土相对位移的发展从桩端部位开始,逐步向承台板扩展,同一部位基础外侧的桩土相对位移要大于基础内侧。靠基础内外,桩的不同侧面表现出不同的侧阻发挥过程及极限值。同样桩间土变形量下,带承台双桩基础在桩端平面上土体的竖向应力要大于带承台单桩基础,从而发挥出较大的桩端阻力。     

开挖条件下黏土中单桩竖向承载特性模型试验与分析

利用已有的离心机预固结加荷装置,开发了一套可以有效控制超固结比的模型桩竖向加荷系统,进行了开挖条件下饱和黏土中单桩竖向承载特性的室内模型试验。在模型试验用高岭土单元体试验的基础上,考虑开挖引起土体K0系数和不排水抗剪强度变化对桩侧极限摩阻力的影响,提出了开挖卸荷条件下饱和黏土中单桩竖向承载特性的非线性简化计算方法,并通过与模型试验结果进行对比验证了计算方法的正确性,算例分析表明:开挖将会使得竖向桩顶刚度和极限承载力的降低,合理的预测方法应该既要考虑开挖卸荷引起的桩侧土上覆压力的减少,又要考虑桩侧土体K0值和不排水抗剪强度的提高。     

初始干密度及掺砂比对膨胀土抗剪强度指标影响

研究了风化砂改良膨胀土对抗剪强度及其指标的影响。通过改变风化砂的掺量,研究了风化砂对膨胀土物理性质的影响。试验表明,随着掺砂比例的增加,最佳含水率逐渐下降,渗透系数逐渐增大,说明风化砂能有效降低膨胀土的塑性指数,增大膨胀土的透水性能,使之更适合用作公路路基填料。通过研究风化砂掺入比例及初始干密度对膨胀土抗剪强度指标c、φ值及抗剪强度的影响,可以得出:在掺砂比例一定时,改良膨胀土的粘聚力随着初始干密度的增大而增大,内摩擦角随着初始干密度的增大先增大后减小再增大,抗剪强度值总体逐渐增大;在初始干密度一定时,改良膨胀土的粘聚力随着掺砂比例的增大而减小,内摩擦角随着掺砂比例的增大先增大后减小,抗剪强度总体变化趋势减小。当初始干密度为2.0 g/cm³,掺砂比例为30%时,抗剪强度达到最大值。     

粗粒土大型直剪试验的二维离散元模拟

通过一组强风化玄武岩粗粒土的室内大型直剪试验,获取其强度、变形等宏观参数。并基于颗粒离散元软件PFC2D,采用圆盘模拟土颗粒建立了考虑级配的二维直剪模型,结果表明：黏结半径因子为0．39时,剪应力一剪位移模拟值与试验值相符较好,垂直位移一剪位移模拟值与试验值之间存在一定的偏差;通过参数校正发现圆盘间摩擦系数随着垂直压力增加而增大,其值介于0．15—0．60;剪切过程造成了接触压力的偏转和集中,剪切缝附近圆盘呈不规则的涡状细观运动。