Geotechnical characterization of geomaterial blends with zeolitic tuffs for use as landfill liners

Mixtures of volcanic and zeolitic tuffs from Jordan with other natural materials such as marl, sand, kaolinite and bentonite were prepared and tested for Proctor compaction in order to determine the best blend to achieve the highest dry density for use in a landfill liner. Compaction tests indicate that volcanic tuffs have higher dry densities than the zeolitic tuffs. The presence of zeolitic tuff in a mix decreases the dry density and increases the optimum moisture content. The study indicated that two mixtures of zeolitic tuff, marl, bentonite, kaolinite and sand gave the highest dry densities which, combined with the high cation exchange capacity of zeolitic tuff and bentonite, provide mixtures suitable for potential low cost landfill liners, if the shrinkage tests confirm that they are appropriate. The liquid limits measured using the cone penetration method of the volcanic tuffs were higher than those obtained using the Casagrande method except for the Rmah Tuff (RT) which showed the reverse. This needs further investigation.     

聂庆科;王英辉;田鹏程;梁书奇;

通过击实试验和击实后土样的压缩和剪切试验研究红黏土的变形和强度特性。研究表明:重塑土的最大干密度比原状土的最大干密度大,而重塑土的最优含水率则比原状土的最优含水率小;随击实功增大,土样的干密度增大,而最优含水率减小;含水率稍高的击实土样的水稳定性要比含水率稍低的击实土样的水稳定性好;当含水率较大时,击实土样的相对固结变形量较大,固结稳定并浸水再固结后,原来含水率较小土样的变形增量反而稍微大些。     

回填土干密度的测定在工程实践中的应用

介绍了工程特点，对压实功、土的含水量和每层铺土厚度等影响回填土压实质量的因素进行了分析，并介绍了环刀法和灌砂法两种回填土干密度的检测方法，以确保回填土的压实质量。     

混合型缓冲回填材料抗剪强度特性研究

高放废物深地质处置中,缓冲回填材料的抗剪性能对处置库的安全设计、施工及运营有重要的影响。本文从工程应用的角度出发,测试不同掺砂率时膨润土-砂混合物标准击实样在最优含水率条件下的抗剪强度指标。研究结果表明:随着掺砂率的增大,膨润土-砂混合物的抗剪强度指标略有下降;纯膨润土的抗剪强度指标随着干密度的增大而增大,随着含水率的增大而减小。通过测试和分析不同干密度和含水率时纯膨润土的抗剪强度指标,引入有效粘土密度和有效含水率的概念,对最优含水率条件下不同标准击实样的抗剪强度指标进行分析。     

Pullout tests conducted on clay reinforced with geogrid encapsulated in thin layers of sand

The interaction between reinforcement and backfill materials is a significant factor for analysis and design of reinforced earth structures which is simplified as pullout or direct shear resistance. This paper presents the results of pullout tests aimed at studying the interaction of clays reinforced with geogrids embedded in thin layers of sand. Pullout tests were conducted after modification of the large direct shear apparatus. Samples were prepared at optimum moisture content and maximum dry densities obtained from standard Proctor compaction tests. Tests were conducted on clay-geogrid, sand-geogrid and clay-sand-geogrid samples. A unidirectional geogrid with sand layer thicknesses of 6, 10 and 14 mm were used. Results revealed that encapsulating geogrids in thin layers of sand under pullout conditions enhances pullout resistance of reinforced clay. For the clay-sand-geogrid samples an optimum sand layer thickness of 10 mm was determined, resulting in maximum pullout resistance which increased with increasing confining pressure. The optimum sand layer thickness was the same for all the normal pressures investigated. For sandy soils the passive earth pressure offered the most pullout resistance, whereas for clayey soils, it was replaced by frictional resistance. It is anticipated that provision of thin sand layers will provide horizontal drainage preventing pore pressure built up in clay backfills on saturation.     

Application of wave equation theory to improve dynamic cone penetration test for shallow soil characterisation

Among the geotechnical in situ tests,the dynamic penetration test(DPT)is commonly used around the world.However,DPT remains a rough technique and provides only one failure parameter:blow count or cone resistance.This paper presents an improvement of the dynamic cone penetration test(DCPT)for soil characterisation based on the wave equation theory.Implemented on an instrumented lightweight dynamic penetrometer driving with variable energy,the main process of the test involves the separation and reconstruction of the waves propagating in the rods after each blow and provides a dynamic cone load-penetration(DCLT)curve.An analytical methodology is used to analyse this curve and to estimate additional strength and deformation parameters of the soil:dynamic and pseudo-static cone resistances,deformation modulus and wave velocity.Tests carried out in the laboratory on different specimens(wood,concrete,sand and clay)in an experimental sand pit and in the field demonstrated that the resulting DCLT curve is reproducible,sensitive and reliable to the test conditions(rod length,driving energy,etc.)as well as to the soil properties(nature,density,etc.).Obtained results also showed that the method based on shock polar analysis makes it possible to evaluate mechanical impedance and wave velocity of soils,as demonstrated by the comparisons with cone penetration test(CPT)and shear wave velocity measurements made in the field.This technique improves the method and interpretation of DPT and provides reliable data for shallow foundation design.     

Experimental study of the effect of fines content on dynamic compaction grouting in completely decomposed granite of Hong Kong

This paper addresses the effects of fines contents on the mechanical behavior of CDG (completely decomposed granite) during dynamic compaction grouting. The first set of tests involved adding different amounts of kaolin clay into CDG to study the effects of fines contents on the compaction characteristics, permeability, consolidation behavior and shear strength of soils. It is found that, as the fines content increases, the dry density decreases, however the optimum water content increases. In addition, the permeability decreases due to reduction of void ratio, and the coefficient of consolidation increases with higher fines content. Furthermore, with increasing fines contents, the shear stress tends to be constant after it reaches the peak strength. However, when reaching the peak value, the shear stress reduces with further adding of fines content. In the second set of tests, the dynamic compaction grouting tests are carried out to study the effect of fines content on the compaction efficiency of Hong Kong CDG. It is found that, the compaction efficiency increases with increasing fines content, and reaches the peak compaction efficiency when the fines content reaches 6%, and then the compaction efficiency drops abruptly from a peak value of 0.55–0.25 when the fines content increases from 6% to 8%. However, when the fines content increases from 8% to 41%, the compaction efficiency decreases very slowly.     

瑞利波检测技术在地基强夯加固中的应用

利用瑞利波检测技术,对厦门某海域吹填砂强夯加固地基进行了强夯前后的现场测试,并与动力触探试验成果进行了对比。结果表明:瑞利测试得到的是场地分层剪切波速,土层介质力学参数多与剪切波速有关;瑞利波波速与地基承载力有一定的相关性,可大致估算强夯地基的承载力;地基强夯前后的试验结果与动力触探成果相符,说明瑞利波可以有效地应用于地基强夯效果的检测中;对瑞利波检测技术在地基强夯加固效果评价中的应用提出了合理化建议。     

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.     

Applicability of CPT for Construction Control of Seawall on Soft Clay Improved by Sand Drain Method

A case study is presented of an application of the cone penetration test for construction control of a seawall construction on soft ground improved by sand drain. Effective construction control of such a soil structure requires a systematic way to collect accurate information about the progress of consolidation and gain in soil strength. An empirical correlation is proposed concerning cone resistance measured by a cone penetration test and the undrained shear strength of the ground. The relationship between cone resistance and the undrained shear strength is not affected by the state of consolidation of the ground. Based on the empirical correlation found in this study, gain in undrained shear strength during construction can be accurately evaluated by the cone penetration test. This paper also demonstrates the usefulness of cone penetration test to monitor the degree of consolidation of the improved ground. Also estimated by the cone penetration test are aging effect, classification of clay layer and strength increment ratio in normally consolidated state. Aspects geotechnical behavior of the ground are discussed such as the process of strength increment, aging effect, sequence of various shear tests to examine change in strength and considerations on shear strength of improved ground after deformation. The results of this study clearly show that the cone penetration test is a very useful investigation method and has high applicability for construction control.     

冻融循环下压实度对粉质黏土力学性质影响的试验研究

 冻融循环是冻土地区路基填料性能劣化的主要因素之一。以压实度和冻融循环次数为主要变量,对青藏高原粉质黏土力学性质的变化规律进行三轴试验研究。试验结果表明：初始压实度对粉质黏土力学性质的冻融效应具有显著影响。不同压实度试样的应力–应变曲线形式随冻融次数的增加趋于接近,并由应变软化型向硬化型过渡。封闭系统中试样的水分迁移会引起含水率的增减分区分布,低压实度有利于增大水分迁移量和含水率增高区的分布范围。冻融过程对高压实试样的破坏强度以降低为主,对低压实度试样则相反。在最优含水率附近,土体抗剪强度随含水率呈非线性变化规律,因此试样内部水分重分布也可能会导致强度的改变,且其作用效果受压实度影响具有不确定性,压实度较高时会导致试样黏聚力减小,压实度较低时则相反;不同压实度下内摩擦角均呈现增大的趋势,且压实度越低,变化幅度越大。冻融过程中,土体干密度和含水率变化对力学性质的影响是同时存在的,由于初始压实度和冻融次数的不同,对强度变化起主导作用的因素也不同。水分重分布是不同压实度土体力学性质冻融循环效应的整体趋势和具体过程呈现多样化的原因之一。     

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。     

土工格栅–土体界面特性大型直剪试验研究

土工格栅与土体的界面特性直接影响了加筋土工程的安全和稳定性,土工格栅两侧为不同材料的界面特性研究还较少。采用双向土工格栅为加筋材料,对其两侧分别为不同含水率粉质黏土及不同粒径石英砂的界面特性开展一系列的大型室内直剪试验,分析法向应力、粉质黏土含水率、剪切速率、石英砂粒径及粉质黏土压实系数等因素对土工格栅–土体界面抗剪强度的影响。结果表明:土工格栅–土体界面抗剪强度与法向应力呈线性相关,符合莫尔–库仑理论;粉质黏土含水率的变化对土工格栅–土体界面抗剪强度有较大的影响,在最优含水率时其界面抗剪强度指标最高;剪切速率的大小和石英砂的粒径变化对土工格栅–土体界面的抗剪强度有一定的影响,其影响范围分别在±10%和±7%内;粉质黏土压实度的增加能有效增加界面抗剪强度,压实系数越高,其提高幅度越大。这些影响应在工程应用中适当考虑。     

环剪试验条件下不同地区黄土的残余强度研究

利用HJ-1型环剪仪,以泾阳、子洲、兰州3个地区的黄土为研究对象,分析了剪切速率、含水率、竖向应力及干密度4个影响因素对黄土残余强度的影响。结果表明:竖向应力对黄土的残余强度具有最明显的影响,其次是含水率、剪切速率、干密度;分析比较3个地区黄土的残余强度,得出对于粉粒含量较多的兰州黄土,其含水率为19%时对应的残余强度较大;对于粗粒含量较多的子洲黄土,含水率小于最优含水率,同时干密度为1.63 g/cm~3时对应的残余强度较大;对于黏粒含量较多的泾阳黄土,含水率较小,同时干密度较小时对应的残余强度较大。探讨不同地区黄土残余强度的变化规律,对于边坡的稳定性分析及滑坡带的防治具有重要的学术及工程意义。     

砂土最大最小孔隙比测定及其影响因素分析

相对密实度是影响砂土力学性质的重要指标,密实的砂土呈现强度软化,松散的砂土却呈现强度硬化,而测定最大、最小孔隙比是计算相对密实度的前提。砂土的最小及最大孔隙比是通过直接测定的相应最大、最小干密度换算得到的,但目前常忽略了试验方法对其试验结果的影响,也忽略了黏粒含量对砂样密实度的影响。现取细、中、粗砂3种砂样,进行了干密度测试试验并测定了不同黏粒及黏粒掺量下砂样的最大、最小孔隙比。研究结果表明:采用量筒慢转法测量砂土最小干密度较为合理;采用振动锤击法测定砂土最大干密度时,建议细砂采用容积为250 m L击实筒,中、粗砂采用1000 m L击实筒;掺入粉粒、黏粒后砂样的最小孔隙比均随黏粒掺量(≤30%)增加而减小,且两者之间存在一定的线性关系;砂样最大孔隙比随粉粒、黏粒掺量增加逐渐减小,而随高岭土黏粒掺量增加呈缓慢增大趋势。     

砂土EPS颗粒混合轻质土的最优击实含水率

为了明确击实含水率对砂土EPS颗粒混合轻质土的密度和强度的影响,采用2因素(水泥添加量和EPS颗粒置换率)5水平均匀试验设计确定了5个不同配比,利用密度试验和无侧限抗压强度试验进行了研究。为了确定砂土EPS颗粒混合轻质土的最优击实含水率,引入了基于比强的最优击实含水率确定方法,并采用二分法(初始击实含水率设为6.5%和13%)寻求了每个配比的最优击实含水率,基于物理分析和试验数据给出了最优击实含水率的计算式。研究发现:击实含水率的增加有利于提高击实效果,对水泥的胶结作用则是先提高后降低;基于比强的最优击实含水率确定方法是适宜的;最优击实含水率受到EPS颗粒置换率和水泥添加量的影响。     

昆明新机场红黏土冲压地基处理实验研究

基于昆明机场原状红黏土工程特性以及冲压地基处理原理,采用冲击压实技术对原状红黏土地基进行冲击碾压处理实验,通过现场地基沉降量观测和密实度、轻型圆锥动力触探、静力触探测试以及连续冲压反应测试系统(CIR)监测,对冲击压实处理效果进行了分析和研究,得到了冲击压实方法对原状红黏土地基适宜,可以提高地基承载力和地基土密度的影响深度以及经处理后红黏土地基密实度和承载力等与冲击碾压遍数的关系,为昆明机场大面积红黏土地基处理提供设计依据和施工工艺参考。     

强夯法处理吹填细砂地基的试验研究

介绍了强夯法处理吹填细砂地基的设计及施工,并对孔隙水压力和夯坑沉降量的监测结果进行分析,通过室内土工试验、扁铲侧胀试验、静力触探试验、标准贯入试验及载荷试验对强夯加固效果进行了分析。     

重塑黄土抗剪强度研究

对延安新区重塑Q3黄土进行了控制不同含水率、不同干密度的直剪试验,研究了含水率、干密度对黄土抗剪强度的影响,试验结果表明:黄土的强度主要受含水率的影响;随着干密度的增大,黄土粘聚力呈指数增长,内摩擦角增大,抗剪强度总体增大;随着竖向荷载的增加,黄土的抗剪强度得到提高。     

土工直接拉伸试验装置的研制及应用

针对现有土工直接拉伸试验装置的不足,研制了一套卧式直接拉伸试验装置;装置由拉伸加载系统、数字液压伺服控制系统、数据采集分析系统3部分组成。试验装置可开展多种易成型材料的直接拉伸试验,能精确测试材料的抗拉强度并能给出拉应力-应变曲线;巧妙设计了加载夹具解决了试验材料与拉伸装置的连接难题;能精确连续控制试验拉力,并能获得材料抗拉强度峰后段的拉应力-应变曲线。使用新研制的试验装置开展了黏土、铁晶砂相似材料和水泥砂浆的直接拉伸试验,试验结果表明：在黏土直接拉伸试验所涉及的含水率范围内,当干密度保持不变时,黏土的抗拉强度随含水率的增加逐渐降低,而峰值拉应变则随含水率的增加逐渐增加;黏土达到峰值强度后,并不会突然断裂而是存在一个软化段,此时土体仍然具有一定的承载能力;铁晶砂相似材料的抗拉强度随着松香酒精溶液浓度的增加而提高;随灰砂比的提高,水泥砂浆的抗拉强度和拉伸弹性模量均相应提高。