原状和重塑软土力学特性的比较研究

为了研究结构性对土的力学特性的影响,对上海淤泥质软黏土的原状样和重塑样进行了等向压缩、排水剪切试验。试验结果表明:在固结压力相同时,原状样比重塑样具有更大的孔隙比、压缩指数Cc和膨胀指数Cs;原状样的应力—应变—体变曲线与剪切围压密切相关,而重塑样的应力—应变—体变曲线却不受剪切围压影响;相同剪切围压时,得到的重塑样强度高于相应原状样的强度,这是由于原状样的孔隙比大于重塑样的孔隙比造成的。     

相同剪切波速的原状与重塑饱和松散粉土的抗液化强度

采用剪切波速与振动三轴联合试验装置,研究了具有松散结构的原状和重塑饱和粉土抗液化强度之间的相关性。结果表明,具有松散结构的原状与重塑饱和土样的振动应变随振动次数的变化趋势是一致的。如果把重塑饱和土样的初始剪切波速恢复到原状饱和土样的初始剪切波速,重塑土样的抗液化强度也被恢复到原状土样的抗液化强度。此时,原状与重塑土样的抗液化强度与抗液化强度曲线之间的相关系数大于0.89,而且这种良好的相关性与确定破坏振次的液化判别标准无关。这里的研究结果支持饱和松散试验土样的初始剪切波速可以作为控制其抗液化强度的一个关键参数。     

结构性软土力学特性的试验研究

对扰动程度不同的原状样和重塑样分别进行单向固结试验和固结排水及不排水三轴剪切试验,研究结构性对上海软土的变形、强度特性的影响。压缩试验结果表明:结构性强的原状样具有明显的结构屈服应力,结构性一般的原状样,结构屈服应力不明显;原状样比重塑样具有更大的压缩指数Cc和膨胀指数Cs。剪切试验结果表明:相同固结压力下,结构性强的土样强度不仅低于结构性弱的土样强度,而且低于重塑样的强度。这是由剪切时土样的孔隙比差异造成的。若消除孔隙比的影响,则结构性将使原状样具有更高的强度,且结构性越强,土体强度越高。     

原状膨胀岩剪切性状的直剪试验研究

介绍了利用新研制的直剪试验仪所测得的膨胀岩试验结果,着重介绍了原状膨胀岩的剪切性状.试验结果表明膨胀岩原状样与重塑样在剪切性状上表现出很大的差别,前者剪切面呈现凸凹起伏的性状,凸凹起伏的剪切面相当于增大了有效剪切面积,这对试验结果有一定的影响;试验结果也同时表现出原状膨胀岩的结构性对抗剪强度起着重要作用.因此,室内重塑样剪切试验很难获得准确的膨胀岩土体的抗剪强度指标,工程上应尽量做原状或原位试验.     

天然沉积结构性黏土的不排水强度性状

通过对福州天然沉积黏土原状样及不同初始含水率重塑样进行三轴固结不排水剪切试验,探讨土结构性对天然沉积黏土强度性状的影响规律及作用机理。研究结果表明：结构性对天然沉积土固结不排水抗剪强度的作用程度受固结压力大小影响可以分为3个阶段：固结压力小于屈服压力阶段,不排水抗剪强度主要由土结构性控制,与应力水平基本无关;土结构逐渐屈服阶段,固结压力大于屈服压力,不排水抗剪强度随固结有效应力的增大逐渐趋于重塑样的强度线;土结构性影响消失阶段,原状样的不排水抗剪强度随固结有效应力的增长规律与重塑样相同。     

直剪试验下保山地区黏土抗剪强度分析

为研究保山地区黏土原状条件与重塑条件下,土体的抗剪强度指标,运用ZJ型应变控制式直剪仪进行试验,试验结果表明原状土样的抗剪强度具有不稳定的特点,同一区间含水率条件下原状土样的抗剪强度变化离散性大;重塑土抗剪强度中,粘聚力C对抗剪强度的影响占主导地位。     

单轴条件下毕节红黏土的强度破坏试验研究

基于红黏土的成因及结构性特征,对毕节红黏土原状土与扰动重塑进行无侧限抗压强度试验,整理分析试验结果。结果表明:单轴条件下原状土与重塑土的应力应变关系曲线有较大差异,原状样多为应变软化型、重塑样为硬化型;原状样的峰值强度高于重塑样,且由于干缩裂缝的产生,在低于某含水量时,原状试样强度突降,重塑样强度随着含水量增加而降低。     

武汉地区老黏性土的结构性试验研究

武汉老黏性土是一种结构性强、超固结、裂隙节理发育的硬塑黏土。从武汉市汉阳某基坑取得原状土样,进行一维固结试验、无侧限抗压试验和常规三轴不排水剪等试验,然后制备重塑土样做对比测试,以揭示老黏性土的结构性。对比相同条件下的三轴试验结果发现:重塑土的黏聚力明显低于原状土,剪切过程中原状土试样普遍表现为应变软化而重塑试样则为应变硬化;原状样比重塑样更容易出现剪胀现象,并且原状土样在较低围压下固结时,其剪切后期均出现负孔隙水压力,表现出明显的剪胀现象。     

弱胶结结构性软黏土力学特性的试验研究

对取自不同地区的两种弱胶结结构性软黏土原状(undisturbed)样、重塑(remolded)样和泥浆(reconstituted)样进行了单向压缩和三轴剪切试验,分别得到土样的压缩曲线和应力–应变曲线。试验结果表明:原状样的压缩曲线为陡降型曲线,而不同制样土样的压缩曲线存在明显的差异;由于孔隙比和孔径分布对土体抗剪强度的综合影响,不仅导致相同围压下三轴剪切时孔隙比不同的重塑样和原状样强度差异较大,且孔隙比相近的不同土样的强度也存在不同程度的差异;若同一孔隙比下,两种软黏土的不同制样土样的强度关系均为原状样的强度最高,重塑样的强度最低,并可通过相近孔隙比下孔径大于0.2μm的孔隙体积量和孔径分布均匀性可合理地解释3种制样土样强度高低的关系。由于不同制样土样的孔径分布的差异不会随固结压力的增大而消失,用参考孔隙比e_(10)~*,简单表示土的孔隙比和孔径分布(即组构)参数,对压缩和剪切试验结果进行归一化整理后,发现不同土样的试验结果可归一化为相关度高的e/e_(10)~*-σv曲线和e_f/e_(10)~*-qf曲线,证明结构屈服应力后,不同土样变形和强度差异主要是由孔隙比及孔径分布(即组构)的不同引起的,用参考孔隙比e_(10)~*简单表示土的组构参数是有效的。     

黄土塬地区非饱和结构性黄土的强度特性研究

为了揭示黄土塬地区非饱和原状及重塑黄土的结构性强度特性的有关规律,完善非饱和土的非线性模型,获得关于黄土塬地区非饱和黄土的变形和强度特性及相关参数,以陇东Q3原状及重塑黄土为对象开展试验研究,对不同初始吸力及不同净围压下的非饱和黄土进行了三轴固结排水剪切试验,研究了黄土塬地区非饱和原状及重塑黄土的变形特性、临界状态、强度参数及吸力变化特性。研究结果表明:原状黄土的剪切破坏表现出明显的剪缩现象,重塑土试样的变形出现剪胀和应变软化现象,剪切变形会出现明显的剪切带现象;不同初始吸力下,原状土偏应力与体应变临界状态线均成线性关系,而非饱和重塑黄土体应变受干密度的影响较大,试验过程中,较大干密度的试样出现了先剪缩后剪胀的现象;非饱和黄土的抗剪强度受基质吸力的影响,非饱和原状黄土的黏聚力随吸力的增大而增大,有效内摩擦角受吸力影响较小,可近似为一常数,非饱和重塑黄土的强度受基质吸力和干密度的影响较大,但对内摩擦角的影响较小;试验得出黄土塬地区非饱和原状及重塑黄土的强度参数,且求得的原状土参数K比重塑土参数K受吸力的影响更大。     

压实系数对粗粒土剪切特性的影响

压实系数是控制填方体工程质量的重要指标,但目前关于压实系数对填料剪切特性的影响研究还不够深入。采集重庆江北机场扩建工程回填夯实的粗粒土制备不同压实系数的重塑试样,通过室内大型直剪试验,研究了粗粒土在不同压实系数下的剪切特性,结果表明:高法向应力,低压实系数时,粗粒土表现为应变硬化和剪缩现象;低法向应力,高压实系数时,则表现为应变软化和剪胀现象;粗粒土在剪切过程中表现出“结构性”和“摩擦性”,“结构性”发挥在先,“摩擦性”在后;压实系数对剪切强度有增大效应,但法向应力对这种增大效应有削弱作用;压实系数对粘聚力有增大效应,但对内摩擦角的影响不显著;剪切破坏面上遇到粗颗粒时所表现出的“饶石”、“穿石”、“夹石”现象会随粗颗粒的强度、尺寸以及法向应力等因素的变化而互相转换。     

干湿循环效应对南宁外环膨胀土抗剪强度的影响

针对膨胀土边坡滑坍多呈浅层破坏的现状,选取南宁外环膨胀土原状样与重塑样开展对比试验,精心设计试验方案,改进常规直剪试样制备条件和试验方法,使模拟土体季节性干缩湿胀过程更接近实际,探究了2种土样经历多次干湿循环作用后抗剪强度的衰减规律。结果表明：试样制备中有上覆荷载作用,实测土样经历同样干湿循环次数后的强度衰减幅度比无荷条件下的小,再次验证荷载对抑制强度衰减作用明显；不管原状还是重塑样经干湿循环后的强度衰减主要是c值大幅降低,其值虽也减小但降幅都不大；此外,计入边坡浅层破坏时滑面的低应力条件开展试验并分析测试结果,得到做边坡浅层稳定性分析时合理的抗剪强度参数,研究结果可供工程设计参考。     

含水率和颗粒级配对稍密碎石土抗剪性能及地基承载力的影响

碎石土因特殊的工程特性而被科研人员关注,但对大粒径、粉质粘土填充、稍密状态下碎石土的力学性能涉足较少。根据该类型碎石土特点及大量勘察结果配置出两种比较典型级配的稍密碎石土,通过大型直剪试验仪测定了不同含水率状态下碎石土试样的抗剪强度,绘制剪应力与水平位移的关系图,通过一元回归得出稍密碎石的抗剪强度指标c、φ值,分析了不同含水率、不同级配对稍密碎石土抗剪性能的影响。结果表明:随着含水率的增大,碎石土粘聚力总体呈减小趋势,但变化趋势具有显著的阶段性且和粉质粘土的含量有关,含水率变化对碎石土内摩擦角的影响较小。用规范推荐方法计算了该类型碎石土地基承载力,并与地方规范经验取值和勘察工程实践取值进行了对比,结果表明:含水率对地基承载力影响较大,且与碎石土填充物的粉质粘土含量有关,粉质粘土含量越高,承载力影响越大。     

混凝土底板与碎石垫层室内水平抗滑试验研究

本文介绍了用于测定混凝土底板与碎石或土之间的摩擦滑动或剪切滑移特性的室内模型试验。通过对不同垫层填料在不同密度下的多组试验， 对几种确定水平剪应力选值方法进行了对比分析， 认为以应变控制法确定抗剪强度是合理而实用的。     

Study on the shear strength of soil–rock mixture by large scale direct shear test

Soil–rock mixtures (S–RM) which formed in the quaternary period are a type of extremely inhomogeneous and loose geomaterial with a certain percentage of rock blocks. They are composed of rock blocks with various sizes and high strength, fine grained soil and pores. The meso-failure mechanism and macro-physical and mechanical characteristics of S–RM are largely controlled by its rock block proportion and the granular distribution. As we know, when the rock blocks in the S–RM are larger, it is difficult to take an in-situ sample for an on-site test. In addition, it is difficult to obtain the granular distribution of rock blocks in S–RM by traditional sieving tests. This paper uses a new method called digital image processing (DIP) in which the rock blocks in S–RM samples are separated from the soil matrix, and the proportion and distribution of the rock blocks is obtained quantitatively. The results are used for the sample preparation of the large scale direct shear tests which provide a new method for the test study of S–RM. According to the results of large scale direct shear tests the rock block size proportion controls the deformation and fracture mechanism of the S–RM. The shape of the shear stress vs horizontal displacement curve and the vertical displacement vs horizontal displacement curve of the S–RM samples are different from that of general “soil” and “rock”. With the increment of the rock block proportion the shear band of the S–RM increases. When the rock block proportion lies in the range of 25–70%, the increment of the internal friction angle linearly increases with the increment of the rock block proportion. The cohesion of the S–RM decreases compared with that of the soil. When the rock block proportion is larger than 30%, however, there is only a little decrease in the cohesion with the increment of the rock block proportion.     

Experimental study on the shear performance of quasi-NPR steel bolted rock joints

Quasi-NPR (negative Poisson’s ratio) steel is a new type of super bolt material with high strength, high ductility, and a micro-negative Poisson’s effect. This material overcomes the contrasting characteristics of the high strength and high ductility of steel and it has significant energy-absorbing characteristics, which is of high value in deep rock and soil support engineering. However, research on the shear resistance of quasi-NPR steel has not been carried out. To study the shear performance of quasi-NPR steel bolted rock joints, indoor shear tests of bolted rock joints under different normal stress conditions were carried out. Q235 steel and #45 steel, two representative ordinary bolt steels, were set up as a control group for comparative tests to compare and analyze the shear strength, deformation and instability mode, shear energy absorption characteristics, and bolting contribution of different types of bolts. The results show that the jointed rock masses without bolt reinforcement undergo brittle failure under shear load, while the bolted jointed rock masses show obvious ductile failure characteristics. The shear deformation capacity of quasi-NPR steel is more than 3.5 times that of Q235 steel and #45 steel. No fracture occurs in the quasi-NPR steel during large shear deformation and it can provide stable shear resistance. However, the other two types of control bolts become fractured under the same conditions. Quasi-NPR steel has significant energy-absorbing characteristics under shear load and has obvious advantages in terms of absorbing the energy released by shear deformation of jointed rock masses as compared with ordinary steel. In particular, the shear force plays a major role in resisting the shear deformation of Q235 steel and #45 steel, therefore, fracture failure occurs under small bolt deformation. However, the axial force of quasi-NPR steel can be fully exerted when resisting joint shear deformation; the steel itself does not break when large shear deformation occurs, and the supporting effect of the jointed rock mass is effectively guaranteed.     

白鹤滩水电站层间错动带的剪切特性

基于白鹤滩水电站层间错动带的室内和现场剪切试验结果,对层间材料、层间材料/母岩接触面(土/岩接触面)的剪切力学特性进行研究。试验结果表明,层间材料和土/岩接触面都表现出无剪胀行为的理想弹塑性力学特性;层间材料的蠕变特性不明显;土/岩接触面的抗剪强度与接触面形貌密切相关,接触面越粗糙,其抗剪强度就越高。比较层间材料的室内和现场直剪试验的抗剪强度可知,试样具有强烈的不均匀性。试样扰动对其抗剪强度的影响要比材料变异性的影响小。尺寸效应使得室内试验获得的内摩擦角要比现场试验获得的大。黏粒含量是影响层间材料抗剪强度的重要因素:随着黏粒含量的增加,内摩擦角减少,黏聚力增大。层间材料的抗剪强度对初始饱和度的变化不敏感。室内试验和现场试验结果在黏粒含量和初始饱和度影响上表现出良好的一致性,说明试样的尺寸效应和变异性只是影响其抗剪强度的次要因素。     

Strength parameter identification and application of soil–rock mixture for steep-walled talus slopes in southwestern China

Soil–rock mixture (SRM) is a heterogeneous geomaterial which is widely used in geotechnical engineering projects. As a special engineering geological body, SRM is composed of many complex components and is a heterogeneous multiphase material with various structural characters, and, thus, exhibits complex mechanical characteristics. The mechanical and physical properties of SRM are major factors which lead to different developmental patterns and deformation characteristics for talus slides. The formation mechanism and mechanical parameters of SRM also play important roles in research regarding slope stability. Taking the Mahe talus slide of the Lenggu hydropower station located on the Yalong River in southwestern China as a study example, many methods, such as the analogy method used in engineering, as well as laboratory model tests, large in situ shear tests, the back analysis method and numerical experiments, are applied in the comprehensive analysis of SRM from a macroscopic–microscopic perspective. The SRM samples collected from the Mahe talus slide consist of various soil and rock contents. The parameters gained from the frontal methods are applied in the stability of the Mahe talus slide. The main contents of the study are as follows: (1) according to the special structure of SRM, ten groups of SRM samples collected from different slide parts are used to perform particle size analysis experiments. The grading combination of the ten groups of samples is analyzed and the gradation curves are obtained from laboratory tests; (2) based on the intensive considerations of different particle compositions, the ten SRM group samples collected from the talus slide are used to perform direct shear tests; (3) due to the fact that the samples containing large-sized particles cannot be simulated by means of indoor direct shear tests, large in situ SRM shear tests are performed in the field; (4) SRM containing large-size particles is used to carry out numerical experiments using the similarity ratio, which is determined by contrasting the results of the laboratory tests and numerical experiments for the same size samples containing the same particle combinations. The numerical experiments are then adopted to obtain the shear strength parameters of different large size samples containing different particle combinations from the perspectives of rock content, particle size, and particle graduation; (5) according to the terrain, geomorphology and stability of the talus slide, the shear strength parameters in the case of natural conditions and magnitude 6 earthquakes on the Richter Scale are obtained using the back analysis method from the perspective of the limit equilibrium of the talus slide; and (6) the shear strength parameters of the various methods listed above are contrast-analyzed. The general shear strength parameters of the SRM are attained properly by using the weighted superposition of the safety coefficients from the different calculation methods. The general strength parameters are used to calculate the stability factor of the Mahe talus slide.     

复杂层状结构岩石抗剪强度试验方法研究

岩石的抗剪强度是边坡稳定性、地基强度和稳定性评价中的一项非常重要的指标,不同类型的岩体,岩石的抗剪强度特性是不同的,其中复杂层状结构岩石的抗剪强度是岩土工程研究的重点内容之一.工程中经常采用的岩石剪切试验方法包括室内试验和现场原位试验,由于试验约束条件不同,试验结果存在一定的差异.本文以某核电工程为例,该项目分别采用了...     

Shear strength of a fibre-reinforced clay at large shear displacement when subjected to different stress histories

The topic of fibre-reinforced soil has been introduced and studied increasingly in the past few decades. However, the shear strength response of fibre-reinforced clay soils with different initial void ratio values when subjected to large shear displacement has not been explored in the literature. The purpose of this study is to evaluate the shear strength responses of fibre-reinforced clay soils when remoulded with relatively small and large initial void ratio and subjected to large shear displacement. In order to exclude the composition variability of the fibre-reinforced samples when subjected to various normal effective stresses, a series of multi-stage reverse drained direct shear test was undertaken with four reverse cycles of ±7 mm, ±7 mm, ±7 mm and +14 mm to achieve an accumulative horizontal shear displacement up to 56 mm that is 93% of the sample dimension. The first stage of the testing programme was carried out on soil samples consolidated at normal effective stress of 600 kPa and unloaded to 50 kPa, followed by 4 shear cycles at normal effective stresses of 50, 100 and 200 kPa, respectively. The results of these tests confirmed significant effective stress ratio improvement with fibre reinforcement, even at large shear displacement to the fourth cycle. However, the rate of improvement decreased with normal effective stress and initial void ratio. Based on the experiments carried out in this study, the optimum fibre content to increase the shear strength of the clay soil with initial void ratio of 0.64 was found to be 0.25% with 140%, 81% and 23% increase in the stress ratio over that of the unreinforced soil at normal effective stresses of 50, 100 and 200 kPa, respectively. The second stage of the testing programme was conducted on a set of samples consolidated and sheared at normal effective stresses of 50, 100 and 200 kPa, respectively. The optimum fibre content was found to be related only to the initial void ratio of the soil, irrespective of the stress history of the soil and the applied normal effective stress. The shear stress ratio of the fibre-reinforced clay soils at large shear displacement was found to be relatively independent of the stress history of the soil. For all soil samples tested in this study, the stress ratio at 200 kPa normal effective stress was found to remain between 0.45 and 0.60.