不同加载速率下冻结黏土的强度及破坏特性

冻土在不同加载速率下的强度和变形特性,是改进冻土机械开挖方法和设备所需的重要参数。通过对冻结黏土进行单轴压缩试验,研究了不同温度和加载速率条件下冻结黏土的强度特性、模量特性和破坏特性。通过分析试验,得到以下结论:(1)冻结黏土的抗压强度和起始屈服应力均随温度的降低和加载速率的增大而增大,且起始屈服应力与抗压强度有很好的线性关系。(2)温度和加载速率对起始屈服模量和切线模量影响很大,不同温度条件下切线模量与加载速率存在对数函数关系。(3)在不同温度条件下,破坏应变和破坏时间均随加载速率的增大而减小。(4)在较大加载速率条件下,温度对冻结黏土的破坏应变和破坏时间影响不明显。     

冻结黏土动态力学特性的SHPB试验研究

通过使用分离式霍普金森压杆（Split Hopkinson Pressure Bar,简称SHPB）动力学试验,系统研究了不同应变率、不同温度和不同含水率条件下冻结黏土的动态力学特性,获得了冻结黏土的动态应力-应变关系以及相关冻土动力学参数的变化规律。试验结果表明：冻土具有明显温度效应和应变率效应,随着应变率的增加和温度的降低,冻土破坏过程从塑性破坏逐渐转变为脆性破坏。在冲击荷载作用下,冻土的动态应力-应变关系曲线随加载应变率的不同呈现3种典型曲线,冻土的最大应变与且仅与应变率呈线性递增规律;单一因素影响下,冻土的动强度、动弹性模量均随应变率的增大、温度的降低和未超过饱和状态的含水率的增大而增加。     

不同温度梯度冻结深部黏土偏应力演变规律研究

 采用先冻结后K0固结(FC)的传统冻土试验方法进行4种不同温度梯度冻结深部黏土的加、卸荷三轴试验,研究不同温度梯度冻结深部黏土在加、卸荷过程中的偏应力增长速率及偏应力的衰减规律。结果表明：不同温度梯度冻结深部黏土加、卸荷过程中偏应力增长速率与均匀温度下的试验结果基本相同,均可用变换的Duncan- Chang双曲线模型进行拟合,但温度梯度对冻结深部黏土偏应力增长速率衰减影响程度远大于温度;温度梯度减弱冻结黏土偏应力增长速率,同时抑制其衰减程度,这种弱化效应在加、卸荷初期最为显著;相同轴向变形对应的偏应力随温度梯度增长而衰减的规律可以通过衰减因子A加以描述,A与温度梯度的之间的关系符合广义双曲线模型;卸荷过程明显增加了加、卸荷初期冻结黏土偏应力增长速率的衰减,且随温度梯度增长,卸荷过程对冻结深部黏土偏应力衰减幅度和衰减速率影响均大于加荷过程;不考虑冻结壁(冻土墙)冻土温度的非均匀性以及卸荷路径的影响将高估冻土强度。     

超声波法测定冻土动弹性力学参数试验研究

运用UVM -2型声速测定仪 ,测定了不同含水率的冻结砂土、冻结黄土和冻结黏土在不同温度下的超声波波速 (纵波波速和横波波速 ) ;根据弹性理论 ,利用所测得的超声波波速 ,计算了被测冻土试样的动弹性力学参数 (动弹性模量E、动剪切模量G和泊松比 μ)。计算结果表明 :冻土的动弹性模量和动剪切模量随着温度的降低而增加 ,它们之间的规律可用一个统一的方程来描述 ;冻土的动泊松比随着温度的降低而减小 ;在低含水率范围内的冻结黄土 ,其动弹性模量和动剪切模量随含水率的增加而增加 ,在高含水率范围内的冻结黏土 ,它们则随含水率的增加而减少 ;冻土的动泊松比随含水率的增加而增加。     

不同小主应力和负温条件下冻结粉质黏土平面应变试验研究

针对冻结法施工中的平面应变问题,利用改进的安徽理工大学冻土真三轴仪进行平面应变试验,分析冻结粉质黏土在不同小主应力和负温条件下的强度和变形特性,建立基于Weibull分布和Drucker-Prager强度准则的冻结粉质黏土损伤本构模型。理论和试验结果表明：同一温度下,不同小主应力下的应力–应变曲线呈现不同程度的硬化特征。破坏强度随小主应力的增加表现出先增后减的规律,随温度的降低呈线性增长的趋势。建立的非线性莫尔–库仑强度准则可以近似描述冻结粉质黏土的强度随小主应力增大而呈现出的非线性变化特征。不同试验条件下小主应力方向的应变均为膨胀变形,体应变均呈现剪缩的特性。随着小主应力的增大,试样的变形模量呈现先增后减的趋势;当小主应力相同时,其值随温度的降低而增大。当温度相同时,破坏时的中主应力随小主应力的增大同样表现出先增后减的趋势。建立的损伤本构模型可以较好地反映复杂应力路径下小主应力和温度对冻结粉质黏土强度和变形特性的影响。     

南通地区冻结黏土蠕变特性试验研究

冻结法施工是地铁开挖的常用手段,因此典型土层的冻土蠕变特性可为工程提供安全资料。针对南通地区地铁开挖过程中典型土层人工冻结黏土在不同温度水平下进行单轴抗压和蠕变试验,获得在-5℃,-10℃,-15℃温度下冻土应力应变和蠕变的特性规律。在此基础上,引入模糊数优化传统的蠕变模型,利用大量实验数据回归改进后的模型,优化后的蠕变模型能更好的拟合试验值,更能反映实际地下工程的不确定性。     

冻结重塑黏土损伤特性及影响因素分析

1max一般介于0.6～0.8之间;②温度对冻结重塑黏土损伤特性影响较明显,降低温度可以明显强化冻土结构,减少冻土结构的损伤及延缓冻土损伤的发展,而含水率和围压对其影响不明显;③割线模量可以较好地表征冻土在加载过程中裂缝的发展特点和结构的损伤演化规律,损伤初始点轴向应变和割线模量与温度之间存在较好的线性增长关系,且冻结重塑黏土损伤应变门槛值]]>     

冻结粉质黏土声学特性与物理力学性质试验研究

 为分析冻结粉质黏土的声波参数与其物理力学性质之间的关系,进行了冻结粉质黏土的超声波测试,同时测定了其在不同温度下的抗拉、抗压强度。基于试验分析了温度、含水率和干密度等对冻土波速的影响以及波速与强度的关系。结果表明：土样在－1 ℃到－7 ℃的温度范围内,波速变化尤为剧烈;存在一个临界含水率16.03%,使波速随含水率的增加呈先增加后减少的趋势;在含水率相同的情况下,波速随干密度的增加呈线性增加;通过波速计算得到冻结粉质黏土的三个动弹性力学参数,并分析这些参数随温度的变化趋势;冻土声波波速与强度呈现出良好的相关性,冻土的强度越大,声波波速越高。因此,可利用声波测试技术对冻土进行无损检测,为冻土强度的预测奠定基础。     

人工冻结黏土可钻性试验与分析

以冻结黏土为试样土质,进行了-3,-5,-7,-10,-12,-15,-17,-19℃等温度的可钻性试验,测定了冻土的凿碎比功,对试验结果进行回归分析,较好地建立了冻土可钻性、凿入深度与温度的关系。试验表明,凿入深度随着温度的降低而减小,凿碎比功随着温度的降低而逐渐增大,在同一温度下凿碎比功在开始钻进10周增加较大,后10周增加不大,说明随着温度的降低,冻结黏土的硬度越来越大,钻进深度越来越浅,钻进程度越来越难,抵抗钻具的能力越来越强。     

主应力轴旋转条件下冻结黏土累积塑性应变与临界动应力特性研究

冻土作为寒区工程的基础和人工冻结工程的支护壁，经常承受动荷载的扰动，研究其在动荷载作用下累积塑性应变和临界动应力特性，可为寒区工程和人工冻结工程变形控制和稳定评价提供重要参考。为揭示主应力轴旋转对冻结黏土累积塑性应变和临界动应力特性影响，采用冻土空心圆柱仪进行了一系列考虑围压影响的动三轴试验和纯主应力轴旋转试验，分析了冻结黏土累积塑性应变、累积塑性应变率和临界动应力特征变化。研究表明，冻结黏土试样轴向累积塑性应变随着循环次数的增多而增大，围压的增大则会抑制冻结黏土轴向累积塑性应变发展速度，而主应力轴旋转效应会加快冻结黏土轴向累积塑性应变发展速度。冻结黏土轴向累积塑性应变率呈现3种不同变化趋势，提出了基于累积塑性应变速率判别的冻结黏土塑性变形行为划分准则，并建立了冻结黏土塑性安定和塑性蠕变临界动应力表达式，证实了主应力轴旋转条件下冻结黏土临界动应力显著降低。研究结果对冻土工程的设计、施工、稳定评价和寒区资源开发都具有重要的指导意义。     

Stress-Strain Relationships and Nonlinear Mohr Strength Criteria of Frozen Sandy Clay

A series of triaxial compressive tests were performed on frozen sandy clay at -4 and -6°C under confining pressures from 0 to 18 MPa. The experimental results indicate that the stress-strain curves show strain softening and hardening phenomena when the confining pressures are below and above 3.0 MPa, respectively. Since the generally hyperbolic model can not describe the strain hardening behavior very well and the Duncan-Chang model can not ideally describe the strain softening behavior of the frozen sandy clay, an improved Duncan-Chang model is proposed. This model can describe not only the strain softening behavior but also the strain hardening behavior of the frozen sandy clay, and the calculated results are rather coincident with the corresponding experimental data. In addition, it is also suitable for frozen silty clay with a high precision. Due to pressure melting, the shear strength of the frozen sandy clay changes nonlinearly with increasing confining pressures. In order to solve the problem that the linear Mohr-Coulomb criteria can not exactly reflect the shear strength of the frozen sandy clay, a nonlinear Mohr criteria of the frozen sandy clay is presented. The calculated results illustrate that it has higher precision and can describe the shear strength of frozen sandy soils more accurately than the linear Mohr-Coulomb criteria does.     

三维胶结结构性土UH模型

为反映胶结对结构性土剪切最终应力比以及剪胀规律的影响,将p–q坐标中静态的临界状态线(CSL)扩展为与CSL平行并随结构性衰减而从左侧移向CSL的动态临界状态线(MCSL),构造与MCSL匹配的屈服面和剪胀方程,从而将以考虑加载体积垮塌为主的结构性土统一硬化(UH)模型扩展为能考虑胶结影响的胶结结构性土UH模型。在此基础上,应用变换应力三维化方法,将所提模型应用到三维应力空间。相对于结构性土UH模型,三维胶结结构性土UH模型增加了1个模型参数描述初始胶结程度。该参数可由无侧限压缩试验近似确定。通过4种结构性土的试验结果与模型预测对照表明,三维胶结结构性土UH模型能够较合理地反映受胶结影响的结构性土等向压缩、常规三轴剪切与真三轴剪切等特性。     

Investigation of unsaturated frozen soil behavior: Phase transformation state,post-peak strength,and dilatancy

In this study, a database of triaxial compressive tests on unsaturated frozen soils is compiled to investigate the mechanical behavior that has not been considered in previous studies. The results for the stress-strain volume changes are presented first. Then, the physical mechanisms that might control the deformation of unsaturated frozen soils, namely, volumetric compression and frictional sliding, are used to interpret the changes in volume and deviator stress during the tests. The relationship between the compression rate due to the mean stress and the dilation rate due to shearing determines the changes in sample volume and shear stress with an increasing axial strain. The test results indicate that confining pressure and temperature significantly affect the phase transformation state, the post-peak strength, and the maximum dilation ratio. A higher post-peak strength ratio is observed in tests performed at higher confining pressure or under a higher temperature. As the confining pressure is increased, the shear stress at the phase transformation state initially increases and then decreases or stabilizes. However, the maximum dilation ratio decreases considerably and tends to reach zero after a certain confining pressure is reached. Both the deviator stress at the phase transformation state and the maximum dilation ratio are higher under a lower temperature given constant confining pressure. Ice cementation and pressure melting are attributed to the specific features of frozen soils compared to those of unfrozen soils. This paper provides new insights into the mechanical behavior of frozen soils.     

超固结非饱和土的本构关系

Alonso等提出的巴塞罗那(Barcelona)弹塑性本构模型是非饱和土本构模型中的代表,将巴塞罗那本构模型与姚仰平等所提出的超固结土UH本构模型相结合,使之适用于超固结非饱和土。该模型在吸力等于零的时候就退化成饱和土的UH本构模型;在吸力不为零且无超固结的情况下,就退化成巴塞罗那本构模型;该模型不仅使超固结状态下的湿化模拟更为合理,而且也能够反映超固结非饱和土的硬化、软化、剪缩、剪胀特性和不同应力路径对超固结非饱和土变形特性的影响,同时能够反映湿化使超固结程度降低甚至使超固结消失的特性。与巴塞罗那模型相比,本文所提出的模型没有增加任何新的材料参数,且与已有的试验结果对比分析,表明该模型能够较为合理地描述超固结非饱和土的基本力学特性。     

A one-dimensional creep model for frozen soils taking temperature as an independent variable

In this paper, a one-dimensional creep model that takes temperature as an independent variable was proposed for frozen soils. A series of K₀ compression tests was conducted under different constant surcharge loads with stepped increases in temperature. An analysis of the test results indicated that the characteristics of creep strain, developing due to the stepped increases in temperature, matched well with the parallel lines postulate of the isotache model for unfrozen soils. The independent variable (stress) in the original model was replaced with a reciprocal of the temperature’s absolute value, and a novel model for directly describing the effect of the increase in temperature on the creep behavior of frozen soils was established. It was verified by the test results in this study and in previous research work that the tendency of the stepped development of creep strain, due to the stepped increases in temperature, can be reasonably captured by the model. Based on further analysis of the test results, a simplified parameter-obtaining method was recommended and its applicability was verified.     

UH模型在双层地基受力变形分析中的应用

基于姚仰平等提出的 UH 模型，将土层的初始超固结比 OCR 引入到 UH 模型的有限元程序中，通过设置不同土层的 OCR 来控制土层的软硬程度，采用 UH 模型的有限元程序对上硬下软双层地基进行了有限元分析。首先将 UH 模型与简化双层地基应力系数法、 Boussinesq 弹性理论计算的附加应力进行了比较，从而证明 UH 模型在分析上硬下软双层地基应力特性方面的优越性，它能够反映上硬下软地基的应力扩散现象；然后用 UH 模型有限元程序计算了土层为不同 OCR 的上硬下软双层地基的附加应力，研究了土层 OCR 对双层地基应力状态的影响；最后对上硬下软双层地基的固结沉降进行了有限元分析，研究了地基沉降、土体应力应变特性和孔压变化规律。     

冻土原位旁压蠕变试验粘弹性模型分析

研究高温高含冰量冻土的蠕变特性,采用Menard旁压仪在青藏高原多年冻土区开展了大量的旁压蠕变试验,利用Merchant粘弹性模型对蠕变曲线进行回归分析并得到模型参数。结果表明,温度对高温冻土力学性质的影响要大于含水量。随着压力的增加,每级压力下冻土的瞬时应变在总应变中所占的比例逐渐减小。回归分析发现,瞬时剪切模量与负温的绝对值呈线性关系,而延迟剪切模量和粘滞系数与负温的绝对值之间为幂函数关系;当冻土的含水量达到46%时,各参数均出现峰值,而后缓慢减小,此结果与低温冻土有所差别。     

考虑时间效应的K_0各向异性UH模型

将所建立的考虑时间效应的超固结土统一硬化模型(时间UH模型)推广为能够考虑K0各向异性的弹黏塑性模型。基于广义非线性强度准则的变换应力三维化方法,将新模型扩展到三维应力状态。使用新模型预测K0固结土的一维和三轴等应变率加载试验,预测结果表明新模型能够合理反映应变率对K0固结土前期固结压力和应力应变曲线的影响。根据模型的基本方程,推导三轴压缩条件下不排水抗剪强度的理论公式,并将该公式的计算结果与试验结果进行对比,发现该公式能够反映不排水抗剪强度随超固结度和应变率的增大而提高的性质。     

Revising the unified hardening model by using a smoothed Hvorslev envelope

The original unified hardening (UH) model, in which a straight Hvorslev envelope was employed to determine the potential peak stress ratio of overconsolidated soils, is revised using a smoothed Hvorslev envelope (Hermite-Hvorslev envelope). The strength at the intersection between the straight Hvorslev envelope and the critical state surface (i.e. Mohr-Coulomb envelope) can be undefined due to the discontinuous change in the slope of the two linear strength envelopes mentioned above. A smoothed Hvorslev envelope is derived through Hermite interpolation to ensure a smooth change between the proposed Hvorslev envelope and the zero-tension surface as well as a smoothed transition between the proposed Hvorslev envelope and the critical state surface. The Hermite-Hvorslev envelope is then integrated into the original UH model, and then the UH models with four different functions of the Hvorslev envelope are compared with each other. The UH model revised by the Hermite-Hvorslev envelope can well predict the mechanical behaviors of normally consolidated and overconsolidated soils in drained and undrained conditions with the same parameters in the modified Cam-Clay model.     

长时高压K_0固结再冻结黏土的卸围压强度特性

深厚表土层中立井井筒建设普遍采用冻结法,而深部冻土的原位力学特性是影响冻结壁力学特性及其安全稳定性的关键;现有的浅部冻土的试验方法,由于忽略了深、浅部土体固结、应力环境及形成工况的差异,已难以可靠地获得深部土的力学参数。基于"长时高压K_0固结—冻结—恒轴压卸围压"试验模式,通过三轴试验,研究了深部土重塑人工冻结黏土的强度与变形特征,以及固结时间、固结应力的影响规律。结果表明:卸围压路径下冻土试样呈现为黏–弹塑性破坏,固结时间为1~7 d时,其卸围压强度随固结时间的延长增长显著,而单位降温引起的强度增长速率受固结时间的影响不明显;随着固结时间延长至28 d,其卸围压强度受固结时间的影响不明显,但单位降温引起的强度增长速率增加显著;单位降温引起的冻土卸围压强度增长速率不受固结应力影响。