改进超固结状态参量次加载面模型数值实现与应用

为合理描述超固结土复杂的弹塑性力学行为,对现有Hashiguchi次加载面模型中的超固结状态参量R进行修正,在硬化方程中,考虑塑性体应变与塑性剪应变的综合作用,提出了修正超固结状态参量的次加载面模型。同时,着重介绍了该模型的隐式积分算法及数值实现过程,编制了对应的接口子程序,实现了该模型在有限元软件ABAQUS中的应用。通过不同工况和加载方式下的数值模拟验证了程序的合理性,最后应用模型研究了Fujinomori黏土的三轴压缩力学特性并与UH模型的模拟结果、室内试验研究进行对比。结果表明,子程序具有较高的计算精度和可靠性,模型能够准确地模拟黏土的超固结特性。     

Review of methods for predicting in situ volume change movement of expansive soil over time

The soil movement information over time is required for the design of foundations placed in expansive soils. This information is also helpful for the assessment of pre-wetting and controlled wetting mitigation alternatives for expansive soils. Several researchers during the past fifteen years have proposed different methods for the prediction of the soil movements over time. The available methods can be categorized into(i) consolidation theory-based methods,(ii) water content-based methods, and(iii) suction-based methods. In this paper, a state-of-the-art of the prediction methods is succinctly summarized. The methods are critically reviewed in terms of their predictive capacity along with their strengths and limitations. The review highlights the need for prediction methods that are conceptually simple yet efficient for use in conventional engineering practice for different types of expansive soils.     

Determining osmotic suction through electrical conductivity for unsaturated low-plasticity soils

Determining osmotic suction from the electrical conductivity (EC) of soil pore water was widely reported in the literature. However, while dealing with unsaturated soils, they do not have enough soil pore water to be extracted for a reliable measurement of EC. In this paper, the chilled-mirror dew-point hygrometer and contact filter paper method were used to determine the total and matric suctions for low-plasticity soils with different salinities (0.05‰, 2.1‰, and 6.76‰). A new piecewise function was proposed to calculate the osmotic suction, with the piecewise point corresponding to the first occurrence of precipitated salt in mixed salt solutions (synthetic seawater). EC, ion and salt concentrations used for osmotic suction calculation were transformed from the established relationships of mixed salt solution instead of experimental measurement. The calculated osmotic suction by the proposed equation and the equations in the literature was compared with the indirectly measured one (the difference between the measured total and matric suctions). Results showed that the calculated osmotic suction, especially the one calculated using the proposed function, was in fair agreement with the indirectly measured data (especially for specimens with higher salinity of 6.76‰), suggesting that the transformation of EC and concentrations from the established relationship is a good alternative to direct measurement for low-plasticity soil. In particular, the proposed method could be applied to unsaturated low-plasticity soils which do not have enough soil pore water for a proper EC measurement.     

Creep model for unsaturated soils in sliding zone of Qianjiangping landslide

The mechanical behavior of sliding zone soils plays a significant role in landslide. In general, the sliding zone soils are basically in unsaturated state due to rainfall infiltration and reservoir water level fluctuation. Meanwhile, a large number of examples show that the deformation processes of landslides always take a long period of time, indicating that landslides exhibit a time-dependent property. Therefore, the deformation of unsaturated soils of landslide involves creep behaviors. In this paper, the Burgers creep model for unsaturated soils under triaxial stress state is considered based on the unsaturated soil mechanics. Then, by curve fitting using the least squares method, creep parameters in different matric suction states are obtained based on the creep test data of unsaturated soils in the sliding zones of Qianjiangping landslide. Results show that the predicted results are in good agreement with the experimental data. Finally, to further explore the creep characteristics of the unsaturated soils in sliding zones, the relationships between parameters of the model and matric suction are analyzed and a revised Burgers creep model is developed correspondingly. Simulations on another group of test data are performed by using the modified Burgers creep model and reasonable results are observed.     

芬顿试剂在土壤DNA提取中的脱腐殖酸效果

为了探索提取土壤DNA之前去除腐殖酸的方法,采用芬顿试剂对土壤样品进行预处理,结果表明芬顿试剂可以有效地去除土壤中的腐殖酸,未经处理和经芬顿试剂处理20～100 min的土壤,均可提取到接近23 kb大小的宏基因组DNA。16S rDNA PCR扩增表明,40～100 min处理土壤提取的DNA可以成功扩增到目的片段,而处理20 min的土壤不一定能扩增到目的片段,未经处理的土壤则不能扩增到目的片段。芬顿试剂可以用于土壤预处理以去除腐殖酸。     

Instability of a geogrid reinforced soil wall on thick soft Shanghai clay with prefabricated vertical drains: A case study

A 7.6 m high geogrid reinforced soil retaining wall (RSW) was constructed at the end of an embankment on very thick, soft Shanghai clay with 12 m deep prefabricated vertical drains (PVDs). The settlement of the ground, the wall movement and pore water pressure were monitored during the construction. From day 118, halfway through the construction, unexpected pore water pressure increment was recorded from the pore water pressure meters installed in the PVD drained zone indicating a possible malfunction of the PVDs due to large deformation in the ground. After the last loading stage, on day 190, a sudden horizontal movement at the toe was observed, followed by an arc shaped crack on the embankment surface at the end of the reinforced backfill zones. The wall was analyzed with a coupled mechanical and hydraulic finite element (FE) model. The analysis considered two scenarios: one with PVDs fully functional, and the second one with PVD failure after day 118 by manually deactivating the PVDs in the FE model. The comparison between the measured and simulated ground settlement, toe movement, and pore water pressure supported the assumption on the malfunction of the PVDs. It is believed that the general sliding failure in the wall was caused by the increase of pore water pressure in the foundation soil and soils in front of the toe. It is suggested that possible failure of PVDs should be considered in the design of such structures, and the discharge rate of the PVDs and the pore water pressure should be closely monitored during the construction of high soil walls on soft soils to update the stability of the structures, especially for grounds where large deformations are expected which may cause the failure of the PVDs.     

非饱和土与特殊土力学的基本理论研究

对非饱和土与特殊土开展了持久深入的研究,取得了系统的创造性成果：在国内率先研制成非饱和土固结仪、直剪仪、渗气仪、标准三轴仪、温控三轴仪、多功能三轴仪和土工CT-三轴仪等一系列仪器设备,揭示了非饱和土与特殊土的水气运动规律及变形、强度、屈服、水量变化、湿陷、湿胀、细观结构演化、温度效应等许多重要力学特性规律;构建了岩土力学的公理化理论体系与多种组合形式的非饱和土的应力状态变量;提出了各向异性多孔介质的有效应力理论公式与非饱和土的有效应力理论公式;建立了非饱和土、湿陷性黄土和膨胀土的本构模型谱系（包括非线性、弹塑性、结构性损伤与热力耦合模型）与分别考虑密度、净平均应力和偏应力影响的广义土-水特征曲线模型谱系;创立了非饱和土三维固结理论及其固结模型谱系;自主研发了分析固结问题的系列软件,求得一维固结问题的解析解和二维固结问题的数值解,形成了完整的理论体系。应用研究成果解决了多项工程的疑难问题,表明所建理论能够指导实践,为工程决策提供理论支持和科学依据。     

Simulation of Methane Production in Anaerobic Rice Soils by a Simple Two-Pool Model

Methane (CH₄) is produced in flooded rice fields by anaerobic decomposition of applied organic residues, root-derived materials and native soil organic matter (SOM). Since CH₄ is an important greenhouse gas it is important to understand, and to be able to model, the processes which produce it. Anoxic incubation of soils employed in the cultivation of irrigated rice, with and without the addition of various potentially-available organic substrates, provides information on potential CH₄ emissions which can be incorporated into process-based models. In this study, a simple two-pool model is employed to simulate the CH₄ production of a number of anaerobically-incubated rice soils, and their responses to amendment with a variety of organic substrates. The model differs from most accounts of SOM transformation in that kinetics are microbially-mediated rather than first-order. Simulation yields a reproduction of the general trends of CH₄ production in response to amendments of acetate, glucose and rice straw.     

Impact of Bituminous Subballast on Railroad Track Deformation Considering Atmospheric Actions

High-speed ballasted track design standards usually require the use of sand and gravel layers as subballast to fulfill an accurate protection of the formation layers, not only against traffic loads, but also against the effects of weather. Seasonal changes in soil water content, or suction changes, are responsible for cyclic volumetric strains on railroad trackbed layers, thus on the infrastructure. Being almost completely water-resistant when compared with granular-only layers, bituminous subballast offers a higher protection of the subgrade, consequently improving its behavior along the infrastructure life cycle. This question is investigated through the comparison of the performance of the track formation against atmospheric actions, taking into consideration the unsaturated state of the geomaterials. The method adopted consists of modeling the vertical displacements of both bituminous and granular subballast designs through a finite-element coupled thermo-hydro-mechanical (THM) analysis. The comparison of the two design solutions confirms that the adoption of a bituminous subballast layer might allow important reductions in seasonal vertical displacements.