固化软土的抗剪强度测试与分析

利用三轴压缩实验,测试了硫铝酸盐复合固化剂作用下软土的力学性能,分析了复合固化剂的掺量和养护龄期等因素对软土三轴应力与应变关系的影响,同时计算固化软土的粘聚力和内摩擦角,表明合理的经济掺量条件下,硫铝酸盐复合固化剂对较土的抗剪强度有明显的改善。     

常州市浅层粘性土两种抗剪强度试验指标分析

对常州市区浅层Q3al粘性土不固结不排水三轴试验与直剪快剪试验抗剪强度指标粘聚力c、内摩擦角φ值进行了统计分析,建立两者间的相关关系方程,对该地区土工试验人员具有一定指导意义。     

粘土的抗剪强度指标试验对比分析

抗剪强度是土的重要力学性能之一,室内剪切试验主要有直剪试验、三轴试验、无侧限抗压强度试验。实际工程中常采用较为简便的室内直剪试验来确定土体抗剪强度中内摩擦角φ和黏聚力c。试验表明,对同样性质土样进行直剪试验和三轴压缩试验得出的内摩擦角φ和黏聚力c相差较大。说明了两者之间的差异。     

土的抗剪强度的再认识

对土的各项物理力学指标进行了阐述,通过对土样数据的整体分析与验算,研究了不同土体的固结快剪、直接快剪、三轴剪的粘聚力和内摩擦角与含水量的相关关系,并根据所建立的回归方程,得出了土的c,ψ值。     

用强度折减法分析土坡稳定参数敏感性

介绍了有限差分法和强度折减法在土坡稳定性分析中的优势和应用,采用标准的算例分析了土坡稳定性计算中土的剪胀角、粘聚力和内摩擦角对土坡稳定性的影响。分析结果表明：安全系数随着剪胀角、粘聚力和内摩擦角的增大而增大,塑性区和滑动面也相应地发生移动,分析结果对滑坡设计具有很好的指导作用。     

粘性土不同剪切方法所求指标对比

通过理论与经验综合分析,将不同剪切方法所得抗剪强度指标大小进行对比,得出若干结论,主要有三轴试验的内摩擦角大小关系:固结排水>固结不排水>不固结不排水;三轴试验的粘聚力大小关系:不固结不排水>固结不排水≥固结排水。从而对工程试验所得数据的合理性有一个大致判断,使抗剪强度指标在工程应用中更加合理安全。     

对于c,φ值的讨论和认识

粘聚力c和内摩擦角φ是岩土体的两个非常重要的参数,在工程设计中得到了广泛的应用。c,φ分别代表着岩土体的摩擦强度和粘结强度,虽然这两个参数是由试验得出的曲线人为定义的,但土的粘结强度和摩擦强度是真实存在的,因而这两个参数具有其实际意义。从c,φ的含义、确定方法、影响因素和两者之间的联系对土体的粘聚力和内摩擦角进行了讨论。     

非饱和土抗剪强度与含水率的关系研究

针对非饱和土抗剪强度与含水率的关系进行了试验研究,得出了在相同的密度状态下非饱和土的强度与含水率密切相关,随着含水率的增加,非饱和土的粘聚力和内摩擦角总体均呈现递减的趋势,并且含水率对粘聚力的影响要比对内摩擦角的影响大。     

考虑中间主应力的路堤临界填土高度统一解及比较

 临界填土高度是软土路堤沉降和稳定控制的重要指标,首先推导3种真三轴强度准则即广义M-N准则、广义L-D准则和统一强度理论的平面应变统一表达式,进而合理考虑中间主应力影响和土体实际侧压力系数,建立地基临界荷载与路堤临界填土高度的统一解,给出公式适用条件并进行验证,最后探讨路堤临界填土高度的强度理论效应,并得出土体黏聚力和内摩擦角对路堤临界填土高度的影响规律。研究结果表明：提出的统一解具有很好的可比性,且其强度理论效应即中间主应力效应明显;M-C准则的结果过于保守,应优先选用广义M-N准则、广义L-D准则或参数b = 1/2时统一强度理论;侧压力系数、黏聚力及内摩擦角对路堤临界填土高度的影响均很显著,应原位测试确定地基土的真实侧压力系数,并充分考虑土体抗剪强度参数的变异性。     

岩石三轴压缩峰后曲线与抗剪强度参数关系探讨

对岩样在常规三轴压缩下应力应变曲线特别是峰后强度变化曲线进行深入分析,通过分段函数和双曲函数建立峰后强度参数随软化参数变化的规律,给出了花岗岩常规三轴状态下峰后应力应变关系式的具体求法,并发现岩石粘聚力和摩擦角可用岩石峰值强度和残余强度结合破坏角近似描述,因此,提出一种较简便的计算岩石黏聚力和摩擦角的近似方法。对多组花岗岩试样进行三轴压缩试验,利用不同围压下应力应变曲线,求得内摩擦角φ和黏聚力c,将后者结果与前者对比可知:(1)该函数可近似的描述岩石峰后的应力应变关系和软化规律;(2)在围压的许可范围内,利用花岗岩三轴压缩强度峰后曲线求得的粘聚力和摩擦角可作为岩石的抗剪强度参数的近似值。上述方法可在一定程度上反映岩石力学三轴试验中常出现剪切破坏的物理意义,并对实际工程和试验提供借鉴。     

等向强化–软化Drucker-Prager材料强度参数演化

Drucker-Prager屈服准则中压力相关系数α和黏聚剪切强度k的实测结果少有报道。根据同一组致密砂岩三轴压缩试验,同时测试Mohr-Coulomb准则中的内摩擦角φ和黏聚力c以及Drucker-Prager准则中的压力相关系数α和黏聚剪切强度k。测试结果显示,描述岩石内摩擦性质的参数φ和α随着塑性变形的增长而增长,但是增长速率逐渐减小;描述岩石胶结性质的参数c和k在岩石初始屈服后会先上升至峰值,然后随着塑性变形的增长从峰值逐渐下降。同时,从5种常用的Mohr-Coulomb准则与Drucker-Prager准则转换关系中,通过同一组三轴压缩试验结果寻找到吻合程度最高的换算关系。Drucker-Prager材料强度参数的变化将来可应用到岩土类材料的数值计算中来模拟材料的强化和软化性质,同时也可以为Mohr-Coulomb准则参数与Drucker-Prager准则参数转换提供一定参考。     

Shear strength response of some geological materials in triaxial compression

The shear strength of geological materials, like soils and rocks, is often represented either by Coulomb or Mohr–Coulomb theory. It is assumed to vary linearly with the applied stress through two shear strength parameters commonly known as the cohesion intercept and the angle of shearing resistance/friction. In reality, whenever these materials are tested to high stresses, a non-linear shear strength response is observed. The effective cohesion intercept continues to increase while the effective angle of shearing resistance decreases with the increase of effective stress. These strength parameters, therefore, cannot be considered as constants in the analysis of soils and rocks. A more realistic and responsive shear strength criterion, involving only two different strength parameters, that remain practically constant with the applied stress, is advocated to represent non-linear response that minimizes the drudgery by conducting a number of triaxial tests on the material. This criterion has been verified with the experimental data of 41 different soils from clay to rockfill and with the data of a number of intact rocks, jointed rocks and rock-like materials tested in the axisymmetric triaxial compression exhibiting either brittle or ductile response. One of the strength parameters enables to define the brittle–ductile boundary. A boundary between rock and soil has been suggested on the basis of modulus ratio and also on the basis of a weakness factor in a jointed rock, named joint factor, representing the combined influence of joint frequency, inclination of the sliding joint and the strength on the sliding joint. When the joint factor is applied to the compressive strength or the modulus of an intact specimen, it provides the corresponding values of the jointed mass in uniaxial compression and also enables to estimate the two strength parameters, for use in the strength criterion applicable to the mass. The modulus ratio is also linked to the failure axial strain of jointed rocks when tested in uniaxial compression.     

筋土接触面直剪试验研究

为研究筋材与砂土接触面剪切特性,在改造的直剪仪上进行了筋材与砂土界面的直剪试验。结果表明:筋土接触面摩擦关系曲线符合摩尔-库仑强度准则。在本次试验的含水率范围内,筋土接触面的粘聚力随着含水率的增加而增加,摩擦角随着含水率的增加而减小。含水率为4%,筋土接触面的粘聚力和摩擦角随着砂土干密度的增大而增大。     

考虑中主应力影响倾斜填土面朗肯土压力参数的敏感性分析

挡土墙后填土面为倾斜时,朗肯土压力理论应用很少。经典的各种土压力理论是基于Mohr-Coulomb强度准则得出的,没有考虑中主应力的影响。首先推导了填土面倾斜时的朗肯主动和被动土压力的计算公式;根据平面应变假设确定了中主应力σ2=ν(σ1+σ3),在此基础上推导了统一强度理论下的材料强度指标计算公式,利用该公式可将Mohr-Coulomb强度准则下的材料强度指标转化为考虑中主应力效应的统一强度理论下的材料强度指标;最后通过一算例采用正交试验设计对反映中主应力影响的中间主剪应力系数、泊松比、填土面倾角、黏聚力、内摩擦角、重度6种因素进行了敏感性分析。结果表明:影响朗肯主动土压力强度的6种因素从大到小分别为:重度、中间主剪应力系数、黏聚力、内摩擦角、倾角、泊松比;影响朗肯被动土压力强度的6种因素从大到小分别为:内摩擦角、中间主剪应力系数、泊松比、黏聚力、倾角、重度;中主应力对朗肯土压力计算结果的影响是一个不可忽视的重要因素。     

基于单轴剪切破坏的岩石M–C准则参数反演分析

基于岩石单轴压缩试验中出现的不同剪切破坏形式,应用极限分析法推导出岩石单轴抗压强度与黏聚力及内摩擦角的关系式。结合矿区岩石单轴、三轴试验结果,验证以岩石M–C准则参数计算单轴抗压强度关系式的正确性,据此关系式获得的结果与试验结果差值在6%~10%之间。采用实例进行岩石黏聚力和内摩擦角的反演分析,以岩石实际单轴剪切破坏形式和试验结果为依据,两两结合联立计算得到岩石的黏聚力和内摩擦角。实例表明:基于单轴剪切破坏形式和单轴抗压强度值计算岩石黏聚力和内摩擦角的方法准确,具有实用价值,该关系式也揭示了岩石试样破坏类型与岩石力学参数间的内在关系。     

An approximate nonlinear modified Mohr-Coulomb shear strength criterion with critical state for intact rocks

In this paper, the Mohr-Coulomb shear strength criterion is modified by mobilising the cohesion and internal friction angle with normal stress, in order to capture the nonlinearity and critical state concept for intact rocks reported in the literature. The mathematical expression for the strength is the same as the classical form, but the terms of cohesion and internal friction angle depend on the normal stress now, leading to a nonlinear relationship between the strength and normal stress. It covers both the tension and compression regions with different expressions for cohesion and internal friction angle. The strengths from the two regions join continuously at the transition of zero normal stress. The part in the compression region approximately satisfies the conditions of critical state, where the maximum shear strength is reached. Due to the nonlinearity, the classical simple relationship between the parameters of cohesion, internal friction angle and uniaxial compressive strength from the linear Mohr-Coulomb criterion does not hold anymore. The equation for determining one of the three parameters in terms of the other two is supplied. This equation is nonlinear and thus a nonlinear equation solver is needed. For simplicity, the classical linear relationship is used as a local approximation. The approximate modified Mohr-Coulomb criterion has been implemented in a fracture mechanics based numerical code FRACOD, and an example case of deep tunnel failure is presented to demonstrate the difference between the original and modified Mohr-Coulomb criteria. It is shown that the nonlinear modified Mohr-Coulomb criterion predicts somewhat deeper and more intensive fracturing regions in the surrounding rock mass than the original linear Mohr-Coulomb criterion. A more comprehensive piecewise nonlinear shear strength criterion is also included in Appendix B for those readers who are interested. It covers the tensile, compressive, brittle-ductile behaviour transition and the critical state, and gives smooth transitions.     

线性吸力下非饱和土条形地基临界荷载三剪统一解

基于非饱和土抗剪强度三剪统一解,该解充分考虑了中间主应力效应、材料拉压不等特性和强度准则差异对非饱和土抗剪强度的影响,严密推导了基质吸力线性分布下条形地基临界荷载三剪统一解,讨论了中间主应力、有效黏聚力、有效内摩擦角和强度非线性等参数对临界荷载的影响。研究结果表明:在基质吸力均匀分布和线性分布下,临界荷载随着中间主应力的增大呈现线性增大,中间主应力效应对临界荷载具有显著影响;临界荷载随着有效黏聚力的增大呈现线性增大;临界荷载随着有效内摩擦角的增大呈现非线性增大;处理非饱和土强度非线性采用双曲线吸力角的方法二精确但复杂,高基质吸力下基质吸力对强度提高的影响大于对吸力角使强度降低的影响。非饱和土临界荷载三剪统一解包含了多种屈服准则下的计算公式,具有较广泛的适用性,可为实际工程提供一定的参考。     

岩石黏结摩擦特性及非线性强度的指数准则描述

 岩石具有黏结和摩擦特性,但两者在同一位置并不同时存在。裂隙摩擦力随围压增加,达到邻近完整岩石黏结力后将不再滑移引起材料破坏,影响试样强度的裂隙倾角范围随之减小,引起强度非线性增加。主控裂隙引起强度在低围压下线性变化,但不能以Coulomb准则直接确定岩石的内摩擦角。砂岩内存在多种倾角的自然裂隙,引起单轴压缩及低围压的强度离散,围压增高裂隙影响减小,强度随围压规则变化而以指数准则描述;轴向压缩塑性变形引起大理岩黏结力由低向高逐步丧失,而热力损伤引起黏结力整体降低;冻结使岩石黏结力提高而内摩擦角不变。若岩石具有宏观各向同性特征,则常规三轴强度可用指数准则描述,据其确定的初始围压影响系数可估计裂隙摩擦系数;进而理解裂隙对试样强度影响的非线性特征,评价岩石材料的真实黏结力和损伤。     

Moisture content effect on the ring shear characteristics of slip zone loess at high shearing rates

The residual strength of slip zone loess is strongly affected by the moisture content. To explore such an effect, slip zone loess obtained from Baqiao landslide was used in ring shear tests under moisture content ranging from 6% to 29%. Laboratory tests were utilized for the determination of the stress–displacement relationship and residual strength parameters for the effect of moisture content in loess. The experimental results showed that the residual strength generally decreases with increasing moisture content. In addition, for a given shearing rate, the residual strength at any moisture content is approximately proportional to the normal stress. The stress fluctuation of shear stress was found to be greater with high shearing rates. Furthermore, the effect of moisture content on the residual strength parameters (cohesion and friction angle) was presented: the residual cohesion of slip zone loess was observed to increase with moisture content to a certain limit that approaches the plastic limit, above which it decreases. Also, the friction angle of slip zone loess was found to decrease with increasing moisture content. Additionally, due to the increase of the moisture content, the scratch color on the shear surface changed from the original color yellow to brown and the thickness of the shearing band of samples reduced. Finally, the mechanism of the influence of moisture content on the macroscopic morphology of the shear surface was also analyzed and illustrated. The study may provide a basis for understanding the shear behavior of slip zone loess under different moisture contents, as well as guidelines for loess landslide stability prevention and prediction.