Swell pressure prediction by suction methods

Soil suction is the most relevant soil parameter for characterization of the swell behavior. An attempt was made to predict swell pressures from soil suction measurements. In this study, Na-bentonite was mixed with kaolinite in the ratios of 5, 10, 15, 20 and 25% of dry kaolinite weight to obtain soils in a wide range of plasticity indices (i.e. 30, 50, 68, 84 and 97%). Suction measurements using thermocouple psychrometer technique were made on statically compacted specimens. The dependence of soil suction on water content, dry density and bentonite content was examined. Soil suction was correlated to the soil properties, namely, water content, plasticity index, dry density, cation exchange capacity and specific surface area using multiple regression analyses. The correlations revealed a simple regression equation for a quick prediction of soil suctions from easily determined soil properties. In order to investigate soil suction versus swell pressure behavior, the results of standard constant volume swell tests (ASTM, 1990) performed on statically compacted samples of these clay mixtures were used. A linear relationship was established between the logarithm soil suction and the swell pressure. It was also found that an experimental relationship which would directly relate the initial soil suction to the swell pressure can be established.     

正常固结黏土中平板锚基础的吸力和抗拉力

平板锚是新近出现的一种系泊深海浮式结构的基础型式。当黏土地基中的平板锚承受上拔力时,平板上、下表面超静孔压差形成的吸力使其抗拉承载力显著增加,对于风浪等快速加载条件尤其如此。利用有限元软件ADINA建立有效应力形式的轴对称动力有限元模型,研究圆形平板锚在缓慢加载与快速加载时的超静孔压分布与地基破坏型式。快速加载时重黏土和高岭土两组典型正常固结土样所得极限承载力系数与塑性极限分析解一致。进而通过变动参数分析,讨论加载速率和埋深对吸力和总抗拉力极限值的影响,并给出排水和不排水加载条件对应的临界加载速率。结果表明,不排水加载条件的总抗拉力可能达到排水总抗拉力的3倍。     

不同压实度和基质吸力的粉煤灰三轴试验研究

粉煤灰的非饱和抗剪强度是准确分析非饱和状态灰坝的稳定性所必需的,但目前对其研究积累较少。通过控制不同压实度、不同基质吸力、不同净围压对粉煤灰进行一系列非饱和土三轴试验,探讨压实度和基质吸力对粉煤灰应力?应变关系曲线和强度参数的影响。研究结果表明,基质吸力较低时粉煤灰的应力?应变曲线没有峰值,呈硬化型;基质吸力逐渐增大时应力?应变曲线有明显的峰值,呈软化型;压实度越大,非饱和粉煤灰试样的应力?应变曲线的软化特征越明显,总黏聚力和内摩擦角均越大;非饱和粉煤灰的总黏聚力随着基质吸力的增大而逐渐增大,增大速率逐渐变缓,最后趋于稳定;不同基质吸力粉煤灰试样的内摩擦角变化不大,并且都近似等于饱和试样的内摩擦角;随着基质吸力的增大,吸力内摩擦角对粉煤灰试样的抗剪强度的贡献越来越小。研究得出的非饱和粉煤灰的抗剪强度特性对灰坝的设计和稳定性分析具有理论意义和工程实用价值。     

分层土中裙式吸力基础吸力沉贯特性模型试验研究

针对一种新型海上风电基础形式-裙式吸力基础,开展模型试验,研究其在分层土地基中的沉贯特性。讨论了土层分布形式（砂土、黏性土、上层砂土及下层黏性土（简称上砂下黏）、上层黏性土及下层砂土（简称上黏下砂））、沉贯方式的影响。研究表明：裙式吸力基础在分层土中具有良好的沉贯性能。与传统吸力基础相比,裙式吸力基础在砂土、黏性土、上砂下黏和上黏下砂地基中最终沉贯深度较传统吸力基础分别增加10.0%、2.3%、3.0%和9.6%,沉贯最大吸力值分别增加0.9%、14.4%、66.2%和92.2%。黏性土层位置和厚度对基础沉贯特性影响显著。上黏下砂地层中,基础最大吸力值出现在土层分界面处,最大吸力值随土层分布系数t（上层土厚度与土体总厚度的比值）的增加而逐渐增大,最终沉贯深度随土层分布系数增加而逐渐减小。上砂下黏地层中,裙式吸力基础最大吸力值出现在最大沉贯深度处,吸力最大值随土层分布系数的增加而逐渐减小,最终沉贯深度受土层分布系数影响较小。此外,同时抽吸主桶和裙结构内水体进行沉贯,最终沉贯深度大于只抽主桶情况。在砂土、黏性土、上黏下砂（ 0.4）和上砂下黏（ 0.4）等4种地基中,裙式吸力基础采用同时抽主桶和裙结构的沉贯方式,最终沉贯深度较只抽主桶情况分别增大了13.8%、3.4%、16.4%和4.6%。研究成果为进一步阐明吸力基础在分层土中沉贯机制及指导工程实践,具有借鉴意义。     

控制基质吸力的非饱和粉质粘土固结试验研究

针对胶济客运专线非饱和原状粉质粘土,在控制基质吸力条件下进行了K0固结试验和常基质吸力固结试验研究,得到了不同含水量土体的侧压力系数,并在此基础上进行常基质吸力固结试验,通过分级施加围压和上覆应力对现场应力状态进行模拟,得到原状粉质粘土的水土特征曲线,进而探讨了非饱和粉质粘土的水土特性及体变规律。试验结果表明:不同含水量的非饱和土体侧压力系数不同,并且侧压力系数随含水量的增加而增加;非饱和原状粉质粘土的进气值约25kPa;土体饱和度对固结时间的影响显著,非饱和粉质粘土的固结速度随着饱和度的增加而减小。     

砂土中吸力式沉箱基础的最佳荷载作用点位置模型试验研究

通过一系列模型试验研究了倾斜荷载作用下吸力式沉箱基础的最佳作用点位置,试验中考虑了荷载作用点、荷载作用角度和长径比3个重要因素。基于模型试验得到的荷载-位移曲线,采用合适的破坏标准确定了各工况下吸力式沉箱基础的抗拔承载力,根据承载力的变化规律分析确定了吸力式沉箱基础的最佳作用点位置。结果表明,荷载作用角度对吸力式沉箱基础承载力的影响显著,当荷载水平作用于基础时,其承载力最大。荷载作用点位置对吸力式沉箱基础承载力的影响取决于荷载作用角度的大小,当荷载作用角度较小时,荷载作用点位置的影响显著,且最佳作用点位置位于沉箱高度的2/3～3/4处;当荷载作用角度较大时,荷载作用点位置的影响很小,可以忽略。结合荷载与吸力式沉箱基础的转角关系,分析了荷载水平作用于最佳作用点下吸力式沉箱基础的破坏模式。     

双边非对称裂纹平板试样拉伸试验研究

采用自制的压–拉转换装置,配合RMT 150C岩石力学试验系统及数字散斑相关方法,对双边非对称裂纹类岩石平板试样进行直接拉伸试验,得到类岩石试样的拉应力–应变曲线、试样表面应变场演化过程和裂纹扩展模式。研究发现,类岩石试样直接拉伸的拉应力–应变曲线大致可以分为4个阶段：（1）近似线性阶段,预制裂纹基本不起作用,应力随应变增加较快,试样表面应变场的分布主要受试样内部的孔隙及颗粒的影响;（2）整体缓慢增加阶段,两预制裂纹和试样内部的孔隙及颗粒共同影响试样表面应变场的分布,整体上应力随应变呈增加趋势;（3）短暂峰值过渡阶段,试样中某个预制裂纹对试样表面应变场的分布起决定性作用;（4）破坏阶段,裂纹起裂位置在应变场相对集中区域,并扩展导致试样破坏。对于直接拉伸条件下的双边非对称裂纹平板试样,其中某条预制裂纹会率先扩展,先向远离前方裂纹的方向扩展,再向靠近前方裂纹的方向扩展,对采用数值模拟方法研究张拉应力状态下裂纹相互作用扩展规律具有重要意义。     

On the rigidity of the North American plate

Recently reported Very Long Baseline Interferometer (VLBI) observations between Green Bank, West Virginia, Haystack, Massachusetts and Owens Valley, California, show that the linear distance between these sites has not changed for at least three years (i.e., smooth rate of change less than 1 cm/yr). This level of stability is logically consistent with the hypothesis that the continental U.S. North American plate is geodetically rigid at least on the 2 cm level. Model studies show that this result is in accord with present knowledge of the intra-plate stress field and the elastic properties of typical continental lithosphere.     

High pressure equilibrium of nickel and cobalt between metal and mantle minerals

In order to test the effect of very high pressures on the siderophile behaviour of two elements, Ni and Co, we have carried out diamond anvil cell experiments on Ni- and Co-bearing systems, up to 70 GPa. Observation of recovered samples by analytical transmission electron microscopy shows that Ni and Co remain siderophile at least up to 70 GPa, but that their siderophile character decreases with pressure, as already observed in previous studies at lower pressures. Our results also suggest that the abundances of Ni and Co observed in the Earth’s upper mantle cannot be explained by very high pressure equilibrium between silicate perovskite, magnesiowüstite, and metal.     

On the mechanics of dynamic penetration tests

This paper describes a numerical model developed to simulate the wave propagation in an elastic media that is applied to in situ dynamic penetration test devices currently used for site characterization. In the model, dynamic equilibrium equations are solved by finite difference analysis in the time domain to produce the discretization of a penetration system – including hammer, rod, penetrometer (or sampler) and soil. In standard penetration tests numerical simulations are shown to agree well with energy measurements derived from force and acceleration signals produced by the impact of a hammer. A parametric study allowed the identification of the relevant factors affecting penetration by demonstrating that the energy effectively delivered to the soil is a function of hammer height of fall and weight of both hammer and rods, as well as the permanent penetration of the penetrometer into the ground produced by a single stroke. Based on these evidences, an approach is suggested to compare results from different dynamic penetration tests without the need to rely on empirical correlations, which is achieved by demonstrating that different equipments should yield the same normalized energy once the influence of both the hammer and rod potential energies are properly considered.     

On the connection between tidal friction and plate tectonics

The tectonic plate motion is considered as resulting from the interaction of lunar and solar tides with the non-linear elastic behavior of the mantle in the regions of phase transitions. The cumulative motion is generated by asymmetries of stress and strain at plate boundaries.     

冻土/岩的液相吸力与固相冰压作用机制

为研究冻土/岩内液相吸力和固相冰压在冻结过程中的作用机制,首先根据热力学基本原理,统一了不同边界条件下的理论冰压方程;然后,联立广义Clapeyron方程和Gibbs-Thomson方程,在阐释液相吸力和固相冰压物理意义基础上,给出了弯曲界面液相水的冻结温度方程;最后,将冻结温度方程引入冻结毛细管模型,将液相吸力方程代入达西定律,分别对固相冰压和液相吸力作用机制进行验证。研究表明：（1）平衡状态下的理论冰压仅与温度线性相关,与边界条件无关;（2）液相吸力来源于理论吸力与固相冰压抵消因子之差,为水分迁移统一驱动力,当固相冰压趋近于理论冰压时,液相吸力趋近于0;(3)固相冰压为绝对压力,可抵消液相吸力,而液相吸力为相对吸力,不可抵消固相冰压;（4）孔隙内的总压力仅为固相冰压,在毛细管冻结时,可达到平衡状态下的理论冰压,故遵循冰压法分凝冰形成机制。研究结果揭示了冻土/岩的液相吸力与固相冰压作用机制,对完善现有冻胀理论具有较高的理论价值和科学意义。     

桶形基础液压下沉过程的耦合欧拉-拉格朗日有限元法分析

由于结构物贯入时网格变形过大而产生扭曲畸变等问题,常会造成岩土工程下沉贯入领域的数值分析收敛困难甚至计算结果失真。采用合适的数值方法分析此类问题颇具挑战性。传统的有限元模拟方法往往会出现收敛困难、作出不合理的假设以及需要依赖用户的专业网格重划分和插值程序等问题。耦合的欧拉-拉格朗日(CEL)分析方法结合了拉格朗日网格与欧拉网格的优点,可以有效地解决有关大变形和材料破坏等诸多问题。通过位移控制法和力控制法两种下沉方式,进行桶形基础室内液压下沉模型试验,得出不同强度的黏土中桶形基础下沉阻力和下沉深度的关系及土塞高度。应用CEL有限元法进行模拟,计算结果与试验结果较为符合。采用的CEL有限元模拟方法不仅可对桶形基础自重下沉和液压下沉进行预测,也可为其他海洋基础结构的贯入模拟提供有益参考。     

Study on the action of the active earth pressure by variational limit equilibrium method

Within the framework of limiting equilibrium approach, the problem of active earth pressure on rigid retaining wall is formulated in terms of the calculus of variations by means of Lagrange multipliers. It is transcribed as the functional of extreme‐value problem by two undetermined function arguments, and is further transformed into determining the minimax solution of restrained functions incorporating the geometrical relations of the problem. The function of (fmincon) in the optimization toolbox of MATLAB 6.1 can be used to find the minimax solution. Computation results show there exist two kinds of modes of failure sliding along plane surface and rotating around log‐spiral cylinder surface when the soil behind the walls reaches the critical active state. The magnitude of active earth pressure in the case of translational mode is less than that in the case of rotational mode. The location of action point of earth pressure in the case of translational mode is at or below $\frac{1}{3}$ height of the wall, and in the case of rotational mode, is above $\frac{1}{3}$ height of the wall. Preliminary study indicates a pair of numbers by two theoretical modes can be regarded as an interval estimation of active pressure. Copyright © 2009 John Wiley & Sons, Ltd.     

关于基质吸力及几个相关问题的一些思考

基质吸力是非饱和土力学中一个非常重要的概念,也是初学者学习中的一个难点,要把它理解透彻并不容易。本文从表面物理化学的基本公式,亦即Young-Laplace公式和Kelvin公式出发,对基质吸力概念和与基质吸力有关的若干问题进行了一些思考和初步探讨。研究表明,基质吸力的大小受土的孔径大小的影响;在环境条件下,基质吸力的大小受外界平衡相对深度的影响;土的孔径分布决定了土水特征曲线的特征。     

Analysis of simple shear tests with cell pressure confinement

The high cost of offshore infrastructure provides continuous encouragement for optimisation of design practices. Development of a more rational method to interpret results from simple shear tests with cell pressure confinement can reduce costs and improve reliability of offshore infrastructure. This paper addresses a commonly overlooked issue affecting design parameter selection: specimen shape varies from right cylinder to oblique cylinder after loading along a single shearing direction. Thus, horizontal stresses are not always equal to the cell pressure and their magnitude varies throughout the specimen’s lateral surface. An analysis is proposed that accounts for changing specimen geometry and lateral surface area during shearing and for the actual effect of cell pressure during testing. The analysis also describes how the intermediate principal stress can be assessed. Test results for medium dense silica sand are interpreted following de Josselin de Jong’s alternative shearing mechanism hypothesis. Conventional interpretation methods yield conservative design parameters for this soil. Failure states develop when the intermediate principal effective stress is halfway between major and minor principal effective stresses. Typical results for the soil tested show equipment performance meets standard direct simple shear requirements for shear strain rate, vertical stress and specimen height control.     

用快剪试验推求先期固结压力

土体先期固结压力是判断土体应力历史的关键性指标。依据Wroth在一维固结条件下提出的不排水强度与超固结比（OCR）的关系式,利用快速直剪的剪切强度计算临界状态孔隙水压力系数Λ₀,从而计算出OCR以及先期固结压力。对南京某黏土的重塑饱和黏土的试验分析结果表明,快剪试验确定的超固结比与设计超固结比基本一致,因此可作为求解土体先期固结压力的一个途径。