基于非牛顿指数描述的非达西渗流一维固结分析

基于非牛顿指数描述的非达西渗流定律,同时考虑地基内部竖向附加应力随深度线性变化以及变荷载的影响,建立了一维固结控制方程并应用有限差分法进行数值求解,同时对不同参数单级加荷下的固结性状进行分析。结果表明：基于非达西渗流比达西渗流下固结速率要慢,且渗流模型中非牛顿指数越大,土层的固结速率越慢;土层厚度越厚,固结速率越慢,因此,传统固结理论中室内土样与地基土层之间的相似关系不再成立;作用于土层的平均附加应力越大,土层的固结速率越快;在单面排水情况下,附加应力分布对土层固结速率有较大影响;相反,双面排水条件下土层固结速率与附加应力的分布是无关的;荷载的加荷速率越快,则土层的固结速率越快。最后,讨论了达西渗流计算固结变形的适用范围。     

考虑非达西流的弱透水层固结计算

由于成岩作用的影响,深部弱透水层中的黏性土较为坚硬、致密,在这种黏土层中的孔隙渗流规律有偏离达西定律的现象,而在现有的地面沉降研究中,对深部弱透水层的固结变形计算始终沿用以达西定律为基础的固结方程进行,这显然是与实际情况不相符的。鉴于此,以孔隙渗流为非达西流为基础,对太沙基一维固结方程进行了重新推导,并采用半解析法对非达西流-太沙基一维固结方程进行了求解。分别采用达西流-太沙基固结方程和非达西流-太沙基固结方程对同一固结问题进行了计算,结果表明,考虑非达西流后土层达到某一固结度所需的固结时间较之不考虑非达西流时明显增加了,而且固结时间增长的趋势与实测的增长趋势较为一致。     

考虑非Darcy渗流时循环荷载下饱和黏土一维固结分析

考虑到土体处于超固结状态下的压缩性一般要比正常固结状态下的低,引入描述非Darcy渗流的Hansbo方程,修正了Terzaghi饱和黏土一维固结方程,并将其推广到低频循环荷载作用的情况。采用有限体积法对该方程进行了求解,并讨论了非Darcy渗流参数、循环荷载周期以及超固结状态下土体压缩性对固结进程的影响。计算结果表明,在矩形波载作用下,按孔压定义和按变形定义的固结度都随时间增长而震荡增加,且按孔压定义的固结度的震荡幅度明显大于按变形定义的固结度。另外,非Darcy渗流减缓了地基的沉降速率,且循环荷载周期越短,或超固结状态下压缩性越高,地基的沉降速率就越慢。     

Modeling fractures as interfaces: a model for Forchheimer fractures

In this paper we are concerned with modeling single phase flow in a medium with known fractures. In particular we are interested in the case in which the flow rate in the fractures is large enough to make it appropriate to use Forchheimer’s law for modeling the flow in the fractures even though the flow in the surrounding domain is such that Darcy’s law is adequate. We describe a model in which the fractures are treated as interfaces. We also consider the case of intersecting fractures and the case of nonconforming meshes.     

起始水力梯度对饱和黏土一维固结的影响

采用考虑起始水力梯度的非Darcy渗流方程,修正了Terzaghi饱和黏土一维固结理论,给出了相应的有限差分格式,并据此探讨了起始水力梯度对渗流前锋、孔隙水压力和土层固结度的影响。计算结果表明,如果起始水力梯度大于0,离排水面较远土层的渗流就存在滞后现象,这样土层的固结要慢于Terzaghi固结理论。此时土层的最终平均固结度小于1,这可归结为土层中存在一定的残余孔隙水压力无法完全消散的缘故。     

指数形式渗流定律下软土非线性大变形固结分析

考虑土中指数形式渗流定律以及土体的非线性固结特性,以超静孔隙水压力为变量在拉格朗日坐标系内建立了软土一维大变形固结问题的控制方程及其求解条件,并运用有限差分法获取其数值解答。在指数形式渗流定律退化为达西定律下,通过将差分解与已有的半解析解进行对比,验证了数值计算的可靠性。最后对指数形式渗流定律下软土一维非线性大变形固结性状进行计算分析,结果表明： 1时,软土的非线性大变形固结速率会随外载增大而减慢; 1时,软土的非线性大变形固结速率会随着外荷载的增加而加快;软土非线性大变形固结速率要比非线性小变形固结速率快,且差别会随荷载增大而加剧;此外,大变形固结理论的最终沉降值要小于小变形固结理论,且差别会随着荷载的增大而加剧。     

Criteria of Acceptance for Constant Rate of Strain Consolidation Test for Tropical Cohesive Soil

In this study, the rapid consolidation equipment (RACE) was developed as an alternative device to the conventional consolidation test using Oedometer, consuming merely a few hours for the whole precedure to determine the consolidation characteristics of cohesive soil. RACE operates based on the constant rate of strain (CRS) consolidation theory, which is a continuous loading method of testing, requiring a good estimation of the loading rate such that it is ideal for the achievement of steady state condition during testing. The steady state condition is achieved when the c _v values from drained and undrained face of CRS converged with the c_v from Oedometer test. A slightly modification has been made on the normal constant rate of strain (CRS) test by proposing a direct back pressure system to the specimen using a tube to saturate the soil sample. This research has produced a set of criteria for determining the suitable rate for the rapid consolidation test based on the ratio of normalized strain rate, β, and proposed a new coefficient in terms of a ratio of β to clay fraction (CF), as a part of new criteria for testing a fine soil. Four types of sample were tested with different rates of strain using the RACE and their results were compared with those conducted using the Oedometer on the same soil type, from which fairly good agreements were evident in many specimens. It was found from the study that the minimum value of normalized strain rate, β, for the CRS test is 0.005 and for the u _a /σ _v ratio is suggested as 0.01. Also, the maximum β/CF for soils with clay friction lower and higher than 50 % are 0.008 and 0.001, respectively. The minimum β/CF value for both conditions is 0.0001.     

Upscaling of Navier–Stokes equations in porous media: Theoretical, numerical and experimental approach

The accurate estimation of hydraulic conductivity is important for many geotechnical engineering applications, as the presence of fluids affects all aspects of soil behaviour, including its strength. Darcy’s law is the key experimental (or phenomenological) equation employed to model ground water flow. Yet, this phenomenological equation can be linked to a more fundamental microscale model of flow through the pore spaces of the porous material. This paper provides an experimental verification of the relationships between Darcy’s law (macroscale) and the Navier–Stokes equations (microscale) for actual complex pore geometries of a granular material. The pore geometries are experimentally obtained through state-of-the-art X-ray computer assisted micro-tomography. From the numerical modelling of the microscale flow based on actual pore geometries, it is possible to quantify and visualize the development of pore-scale fluid preferential flow-paths through the porous material, and to assess the importance of pore connectivity in soil transport properties.     

Experimental and numerical studies of rock salt strain hardening

The work presented in this paper comes as part of a research program dealing with the thermomechanical behaviour of rock salt. It aims to study laboratory and in-situ long-term behaviour by means of creep tests with deviator and temperature changes. The laboratory results, using a triaxial multi-stages creep tests, highlighted the strain hardening character of rock salt. Furthermore, the in-situ results, using a borehole dilatometer multi-step creep test, have shown that the drilling is carried out in a weakly stressed pillar. The interpretation of the laboratory results, using the J.LEMAITRE law, did not indicate full agreement with all the test results. As a result a ‘double’ J.LEMAITRE model, which takes into account a double strain hardening variable, has been put forward. The validation of this model on the laboratory creep tests is very satisfactory. Furthermore, the activation energy seems satisfactory to represent the influence of the temperature. The in-situ behaviour modelling is clearly more complex than the modelization based on laboratory tests. In fact, it seems that if the rock salt behaviour is maintained by J.LEMAITRE law, it is necessary to vary with the stress, at least, one of the parameters assumed constant in the basic law.     

破碎砂岩渗透特性与孔隙率关系的试验研究

利用一种专利装置与MTS815.02型岩石力学试验系统,进行了破碎砂岩的稳态渗透试验,得到了不同孔隙率下破碎砂岩的渗透率和非Darcy流 因子。通过线性回归得到了渗透率、非Darcy流 因子与孔隙率之间的关系。认为破碎岩石的渗透特性主要是由孔隙率决定的,孔隙率不仅与当前的应力有关,更取决于加载历史。研究表明,水在破碎砂岩中的渗流一般不服从Darcy定律,而服从Forchheimer关系,特别是在小孔隙率下,非Darcy性更为突出;渗透率、非Darcy流 因子与孔隙率之间的关系可用幂函数拟合。     

饱和粉土静态液化性能试验研究

通过固结不排水三轴压缩试验,分析了围压、固结比和干密度等因素对饱和粉土静态液化特性的影响。试验结果表明,在干密度较小时,饱和粉土的偏应力-应变曲线呈现明显的硬变软化型,随轴向应变增大超静孔隙水压力增加、有效应力减小而发生静态液化,当干密度达到1.58 g· cm-3时,饱和粉土的偏应力-应变曲线表现出硬变硬化现象,超静孔隙水压力为负值或接近0,饱和粉土不再发生静态液化,即饱和粉土存在静态液化的干密度临界值;其他条件不变,随着围压、固结比或干密度的增大,偏应力峰值和残余强度均增大,静态液化势降低;根据有效应力路径建立了流滑面以作为饱和粉土稳定区与非稳定区的分界面。     

三轴冲击荷载作用下淤泥力学响应研究

利用SPAX-2000（改进型）静动真三轴系统,针对淤泥类超软土进行静动力排水固结试验,研究包括较高频率在内的各冲击荷载频率（1、8、16 Hz）与不同围压（200、250、300 kPa）作用下淤泥的力学响应;此外,结合三轴剪切试验研究不同固结条件对淤泥不排水强度的影响。试验结果表明：（1）在相同冲击频率作用下,试样抗剪强度随着围压增大而提高;围压增大,使得球应力增大,体应变随着增大,有效地促进试样排水固结。（2）相同围压作用下,存在着一个冲击荷载频率阈值,当冲击频率低于这一阈值时,随着冲击荷载频率的提高,轴向应变量逐渐变小,反之逐渐变大。（3）冲击瞬间,体应变为负值,表现为体胀,这与原位试验夯击瞬间的孔隙水压力变化曲线一致,从而验证了高含水率淤泥在冲击作用下的孔压负增长现象。     

Experimental study on water-oil migration and accumulation in a 2D micro-model of carbonate fracture media

Many achievements have been made in experimental studies of hydrocarbon migration in the clastic reservoir. On the other hand, few migration experiments have been reported in the carbonate reservoir simulation realm. This article is a tentative experimental study on hydrocarbon migration and accumulation in the carbonate reservoir, which is a complex media that includes a pore system and fracture system. This microcosmic experiment simulates oil-water displacement using a real core model. Plentiful seepage phenomena were observed in the microcosmic experiments. Three kinds of pathways were found in the plane: parallel pathway, oblique-cross pathway and network pathway. Three types of flow were found: continuous flow, sectioning flow and their combination. Three driving fronts were found in the experiment: piston front, encircle front and impulse front. All these vary with many factors such as fluid pressure, oil saturation, fissure configuration and wettability, and these factors affect each other. The results show that the relation between the fluid pressure and volume of flow is a complex segmenting correlation, instead of a simple positive correlation like the pipe flow that follows Darcy’s law. The relation between fluid pressure and the speed of flow also follows the same correlation. Speed of flow relates to the angle and width of the fissure. Speed of flow in the wide fissure that has an acute angle with the pressure gradient is faster than that in a narrow fissure with a high angle with the pressure gradient. Translated from Geological Review, 2006, 52(4): 570–576 [译自: 地质 论评]     

广州饱和软土固结过程微孔隙变化的试验分析

软土的孔隙特征及其变化规律对软土的压缩性和渗透性有重大的影响,软土孔隙特征随荷载变化规律的试验研究为揭示软土排水固结过程与变形规律,建立基于孔隙压缩规律和渗流机制的排水固结模型提供有力依据。利用美国全自动压汞仪,对不同压力下固结的广州番禺淤泥土样的孔隙及其尺度分布进行测试,根据测试结果对土样孔隙尺度分布特征及其随固结压力的变化规律给出了定量分析。试验结果表明：淤泥土中介于0.4～2.5 μm范围的颗粒间及团粒内的小孔隙所占比例最大,而大于10 μm的大孔隙和小于30 nm的超微孔隙所占比例均很小;较大的孔隙更易于被压缩而湮灭或被分裂成微小孔隙;固结压力将显著改变淤泥土的孔隙尺度及其分布特征,以致改变土体的压缩性和渗透性。在固结前期( 200 kPa)孔隙尺度较大,压缩系数和渗透系数较大并随固结压力增加而快速减小;在固结后期( 200 kPa)孔隙尺度小,压缩系数和渗透系数小,且随固结压力增加的变化趋于平缓。     

软土地层浅埋圆形隧道非达西渗流场解析研究

软土在低水力坡降下的渗流会偏离达西定律,即为非达西渗流模式。假设孔隙水渗透服从指数渗流模式,采用镜像法原理推导了浅埋单孔和双孔圆形隧道非达西渗流场的解析解。结合算例,对浅埋圆形隧道非达西渗流解析解与达西渗流解析解进行了对比分析与验证,并对非达西渗流指数、隧道周围土体与衬砌渗流系数比值对隧道渗流场的影响进行了讨论。结果表明：非达西渗流指数、渗流系数比值对隧道渗流量和周围土体孔压均有较大的影响;随着渗流指数逐渐增大,土体内水头损失加快,隧道周围土体孔压及渗流量逐渐减小;随着土体与衬砌渗流系数比值逐渐增大,衬砌排水能力增强,隧道渗流量逐渐增大,隧道周围土体孔压减小更大。     

冲击荷载下饱和软土动态响应特征的试验研究

基于现场动力排水固结法加固饱和软土地基工程实践和室内动力固结试验,结合土体压力分析了饱和软粘土在冲击荷载作用下的变形与孔压的发展变化规律。研究表明：冲击荷载引起土体变形和激发的孔压具有不同发展模式,土体变形与冲击击数是对数双曲线关系,而孔压与冲击击数之间仅是双曲线关系。     

A one-dimensional viscoelastic and viscoplastic constitutive approach to modeling the delayed behavior of clay and organic soils

Accurate modeling of the time-dependent behavior of geomaterials is of great importance in a number of engineering structures interacting with soft, highly compressible clay layers or with organic clays and peats. In this work, a uniaxial constitutive model, based on Perzyna’s overstress theory and directly extendible to multiaxial stress conditions, is formulated and validated. The proposed constitutive approach essentially has three innovative aspects. The first concerns the implementation of two viscoplastic mechanisms within Perzyna’s theory in order to distinguish between short-term (quasi-instantaneous) and long-term plastic responses. Similarly, elastic response is simulated by combining an instantaneous and a long-term viscous deformation mechanism. The second innovative aspect concerns the use of a bespoke logarithmic law for viscous effects, which has never been used before to simulate delayed soil behavior (as far as the authors are aware). The third concerns the model’s extensive validation by simulating a number of different laboratory test results, including conventional and unconventional oedometer tests with small and large load increments/decrements and wide and narrow loading/unloading cycles, constant rates of stress and strain tests, and oedometer tests performed in a Rowe consolidation cell with measurement of pore pressure dissipation.     

固结作用下粉土的持水性能与细观机制研究

针对传统仪器无法考虑岩土工程中温度和荷载等因素综合作用的缺陷,研制了一套多功能土-水特征曲线试验仪。以粉土为研究对象,开展了不同固结应力作用下粉土的土-水特征曲线试验,并测量其增减湿过程中土体的体积胀缩量,以便修正体积变化对其体积含水率的计算误差。结果表明,粉土在减湿过程中,土体的体积发生明显的收缩。其收缩量与固结应力水平有关,固结应力越小,则减湿引起的体缩量越大。但土体在增湿过程中的体积则基本保持不变。固结应力对粉土 土-水特征曲线的进气值、增减湿速率影响较大,固结压力越大,其进气值越大、减湿速率也越大。最后,为揭示固结应力对土的持水性能的影响机制,开展了不同固结应力作用下土体的细观试验。结果表明,固结应力主要改变了土体团粒间的大孔隙,而对黏土颗粒之间的孔隙影响较小。土体的持水性能与土体的孔隙大小和分布模式密切相关,大孔隙主要影响土体的进气值,而其孔隙分布模式则控制其增减湿湿速率。     

Stress-strain behaviour of reconstituted illitic clay at different temperatures

Tests on specimens of reconstituted illitic clay have examined the influence of temperature on the mechanical behaviour of clay soils. The program involved consolidation to effective confining pressures up to 1.5 MPa, heating to 100°C, and tests on normally consolidated and overconsolidated specimens with OCR = 2. The tests included isotropic consolidation, undrained triaxial compression with pore water pressure measurement, drained tests along controlled stress paths to investigate yielding behaviour, and undrained tests which involved heating and measurement of the resulting induced pore water pressures. The large strain strength envelope is independent of temperature. However, peak undrained strengths increase with temperature because smaller pore water pressures are generated during shearing. An important contribution from the study is a series of results for the yielding of illitic clay at three different temperatures. For the first time, there is clear evidence of yield loci decreasing in size with increasing temperature. An associated flow rule can be assumed without serious error. The results contribute to the confirmation of a thermal elastic-plastic soil model developed by the authors from cam clay following the addition of a small number of extra assumptions. Depending on the initial stress state, heating under undrained conditions may produce shear failure.