Forming condition of transient saturated zone and its distribution in residual slope under rainfall conditions

Rainfall, as one of the most significant factors triggering the residual soil slope failure, leads to not only the reduction of soil shear strength, but also the increase of soil weight and the decrease of matric suction as well. All these modifications in soil properties have important influence on the slope stability. The water infiltration and redistribution inside the slope are the preconditions of the slope stability under rainfall conditions. Based on the numerical simulation via finite element method, the water infiltration process under rainfall conditions was studied in the present work. The emphases are the formation, distribution and dissipation of transient saturated zone. As for the calculation parameters, the SWCC and the saturated permeability have been determined by pressure plate test and variable head test respectively. The entire process(formation, development, dissipation) of the transient saturated zone was studied in detail. The variations of volumetric water content, matric suction and hydraulic gradient inside the slope, and the eventually raise of groundwater table were characterized and discussed, too. The results show that the major cause of the formation of transient saturated zone is ascribed to the fact that the exudation velocity of rainwater on the wetting front is less than the infiltration velocity of rainfall; as a result, the water content of the soil increases. On the other hand, the formation and extension of transient saturated zone have a close relationship with rainfall intensity and duration. The results can help the geotechnical engineers for the deeper understanding of the failure of residual slope under rainfall condition. It is also suggested that the proper drainage system in the slope may be the cost-effective slope failure mitigation method.

An iterative algorithm for solving ill-conditioned linear least squares problems

Linear Least Squares（LLS） problems are particularly difficult to solve because they are frequently ill-conditioned, and involve large quantities of data. Ill-conditioned LLS problems are commonly seen in mathematics and geosciences, where regularization algorithms are employed to seek optimal solutions. For many problems, even with the use of regularization algorithms it may be impossible to obtain an accurate solution. Riley and Golub suggested an iterative scheme for solving LLS problems. For the early iteration algorithm, it is difficult to improve the well-conditioned perturbed matrix and accelerate the convergence at the same time. Aiming at this problem, self-adaptive iteration algorithm（SAIA） is proposed in this paper for solving severe ill-conditioned LLS problems. The algorithm is different from other popular algorithms proposed in recent references. It avoids matrix inverse by using Cholesky decomposition, and tunes the perturbation parameter according to the rate of residual error decline in the iterative process. Example shows that the algorithm can greatly reduce iteration times, accelerate the convergence,and also greatly enhance the computation accuracy.

改良预崩解炭质泥岩的力学行为及微观机理研究

预崩解炭质泥岩在中国西南地区常用作路堤填料。为改善预崩解炭质泥岩力学特性,采用粉煤灰、水泥和红黏土对预崩解炭质泥岩进行处理,通过三轴试验分析改良土的应力-应变关系和抗剪强度指标,并利用扫描电镜和X射线衍射表征其微观结构和矿物成分。结果表明,经水泥改良后预崩解炭质泥岩的应力-应变关系曲线由应变硬化型转变为应变软化型,但粉煤灰和红黏土不改变其应力-应变关系类型。三种添加剂对预崩解炭质泥岩黏聚力影响的大小顺序为:水泥>红黏土>粉煤灰。未加水泥的预崩解炭质泥岩均表现为鼓胀破坏,而经水泥改良的预崩解炭质泥岩均表现为剪切破坏或鼓胀-剪切破坏。由于胶结物能将土颗粒紧密胶结成团,使微观结构更加致密、稳定,因此,改良后预崩解炭质泥岩的力学特性得到显著增强。研究结果可为炭质泥岩在路基工程中的应用提供参考。

Effect of carbon black on the dynamic moduli of asphalt mixtures and its master curves

Modulus is one of the main parameters during the design of asphalt pavement structure, the newest specifications of China gives the dynamic moduli ranges of commonly used asphalt mixtures with base asphalt (BA) or styrene-butadiene-styrene modified asphalt (SBS MA), while the moduli ranges of mixtures with carbon black modified asphalt (CB MA) are not recommended. To investigate the CB effect on the dynamic moduli of CB MA mixtures, one commonly used asphalt mixture (AC-20) was designed with BA, SBS MA, and CB MA, respectively. Then, the uniaxial compression dynamic modulus tests were conducted at different temperatures and loading frequencies, the master curves of asphalt mixtures were analyzed based on the time-temperature equivalence principle. The results show that with increasing loading frequency, the temperature dependence of dynamic moduli of all asphalt mixtures tend to be less obvious. Both SBS and CB can decrease the temperature sensitivity of asphalt mixture, the SBS MA mixture has the lowest temperature sensitivity, followed by CB MA and BA mixture. In addition, CB and SBS can obviously improve the dynamic modulus of the BA mixture, enhance the anti-deformation performance of pavement structure, and the improvement effect of CB is almost the same with SBS.

基于微观结构的压实二元粉质黏土渗透与持水特性

粉质黏土路堤的耐久性在一定程度上受水分迁移控制,而后者取决于土的水力特性。本文研究了压实二元粉质黏土的水力特性。首先,按不同混合比和干密度制备了的二元粉质黏土试样,并利用扫描电镜和压汞仪分析了粉质黏土的微观结构。然后,通过变水头渗透试验和持水性试验,研究了粉质黏土的渗透性和持水性。结果表明,当黏土含量为100%时,压实后黏土颗粒分散性良好,并呈现定向排列特征。随着黏土含量降低,黏土颗粒的定向排列逐渐受到粉粒存在的干扰,导致粉粒周围形成许多大孔隙。但粉粒周围孔隙的数量随着干密度的增加明显减小。并且,随着黏土含量和干密度的增加,粉质黏土的渗透性降低而持水性增强。黏土含量和干密度越大,粉质黏土中的孔隙越少,对水体流动的阻碍越强。粉质黏土的渗透与持水特性均可通过孔径分布特征参数进行预测。