On the variability of the sediment concentration in currents on steep slopes: a simplified approach

Based on the analysis of the results of flume experiments and on a simplified theoretical approach, a formula is proposed for the evaluation of the total solid load in a water–sediment current. The obtained results show that the sediment concentration of the water–sediment currents gradually varies from the typical values of bed load to those typical of debris flows, even when the clay concentration in the whole current is not negligible in comparison with the total solid load. The proposed simplified approach confirms the theoretical results obtained by the application of more complex rheological models. Besides the proposed theoretical interpretation of the laboratory experimental results seems to confirm a unified view of the different types of solid transport, that has been presented elsewhere.     

Experimental and theoretical investigation on the compression behavior of sand-marine clay mixtures within homogenization framework

In this paper, the compression behavior of sand-marine clay mixtures was investigated, both experimentally and theoretically. The test data reveal that the Normal Compression Line of a sand-clay mixture depends on both the sand fraction and the initial water content of the clay matrix. The local stress in the clay matrix σ′_c is approximately close to the overall stress of the sand-clay mixture σ′ for a sand mass fraction of 20%. The stress ratio, σ′_c/σ′, falls significantly with increasing overall stress for a sand fraction of 60%, which may be attributed to the formation of clay bridges between adjacent sand particles. A compression model was formulated within the homogenization framework. First, a homogenization equation was proposed, which gives a relationship between the overall stiffness E and that of the clay matrix Ec. Then, a model parameter ξ was incorporated considering the sensitivity of the structure parameter on the volume fraction of the clay matrix. Finally, a simple compression model with three model parameters was formulated using the tangent stiffness. Comparisons between the experimental data and simulations reveal that the proposed model can well represent the compression curves of the sand-marine clay mixtures observed in the laboratory.     

Tensile strength characteristics of contaminated and compacted sand-bentonite mixtures

A theoretical and experimental study of the unconfined penetration test (UP) method was conducted to determine the tensile strength of contaminated and compacted sand-bentonite mixtures, which are used widely in the landfill construction sites as a clay liner. To do this, the original UP device and experimental procedure were modified to reduce the measurement errors and named as the improved unconfined penetration (IUP) test. Experiments were carried out to examine the variation in tensile strength as a function of disk diameter, loading rate and pH level. The results of the experiments provided the following important findings. The tensile strength increases with an increase in the disk diameter. The tensile strength is not sensitive to the loading rate in the range of 0.1%/min–1.0%/min. A specimen compacted with a low pH value of water shows a high tensile strength, because a lower pH solution increases soil particle bonding stresses.     

PERMEABILITY AND CONSOLIDATION OF SEDIMENT MIXTURES AS FUNCTION OF SAND CONTENT AND CLAY MINERALOGY

The current study is the first step in a systematic experimental research on the erosion behaviour of sand-mud mixtures. It concerns the effect of a varying sand content and clay mineralogy on the porosity, structure, strength and permeability of artificially generated sediment mixtures. The permeability of a sediment mixture is an especially significant parameter concerning the type of erosion that occurs. It determines if the erosion of the bed is either a drained or an undrained process, respectively indicating surface erosion or mass erosion. Measurements on various mixtures concerning the consolidation coefficient and the permeability have been executed. Results show a distinct transition of behaviour between a sand-silt dominated network structure and a clay-water matrix. The occurrence of these two types of structures appears to depend on the porosity of the volume fraction of sand related to silt, which is, therefore, an important parameter concerning the type of erosion. Finally, the study provides a valuable data set that can be used as a reference for following stages of this research on the erosion behaviour of natural cohesive sediments.     

Mechanical properties and time-dependent behaviour of sand-granulated rubber mixtures

The use of granulated recycled rubber as a lightweight material in civil engineering applications has been widely growing over the past 20 years. Processed waste tires mixed with soils have been introduced as lightweight fills for slopes, retaining walls, and embankments. It has also been considered as a damping material under foundations in seismic zones. Understanding the properties of sand-rubber mixtures is essential to evaluate its performance in geotechnical applications. Isotopically consolidated drained (CD) triaxial tests were conducted to investigate the effect of rubber size, content and saturation condition on the mechanical properties of sand-rubber mixtures. Moreover, the compressibility of the sand-rubber mixtures under sustained loading was investigated through one dimensional consolidation tests. The unit weight, shear strength and stiffness of sand-rubber mixtures decreased whereas deformability increased at increased rubber content. A non-linear stress-strain response was observed, that changed from brittle to ductile behaviour at increased rubber content. Sand-rubber mixtures, under one dimensional loading, exhibited significant settlement that increased as rubber content increased.     

Support vector machines applied to uniaxial compressive strength prediction of jet grouting columns

Learning from data is a very attractive alternative to “manually” learning. Therefore, in the last decade the use of machine learning has spread rapidly throughout computer science and beyond. This approach, supported on advanced statistics analysis, is usually known as Data Mining (DM) and has been applied successfully in different knowledge domains. In the present study, we show that DM can make a great contribution in solving complex problems in civil engineering, namely in the field of geotechnical engineering. Particularly, the high learning capabilities of Support Vector Machines (SVMs) algorithm, characterized by it flexibility and non-linear capabilities, were applied in the prediction of the Uniaxial Compressive Strength (UCS) of Jet Grouting (JG) samples directly extracted from JG columns, usually known as soilcrete. JG technology is a soft-soil improvement method worldwide applied, extremely versatile and economically attractive when compared with other methods. However, even after many years of experience still lacks of accurate methods for JG columns design. Accordingly, in the present paper a novel approach (based on SVM algorithm) for UCS prediction of soilcrete mixtures is proposed supported on 472 results collected from different geotechnical works. Furthermore, a global sensitivity analysis is applied in order to explain and extract understandable knowledge from the proposed model. Such analysis allows one to identify the key variables in UCS prediction and to measure its effect. Finally, a tentative step toward a development of UCS prediction based on laboratory studies is presented and discussed.     

三维极限平衡法在原位水平推剪试验中的应用

原位水平推剪试验是一种常用的试验方法,但由于在对类似与土石混合体这类极度不均匀的岩土体进行试验时,其滑动面通常极为不规则.作者在三维极限平衡理论分析的基础上,推导出了野外大型原位水平推剪试验在考虑三维滑动面时的强度参数计算方法,并采用VC++.NET编制了相应的数据处理系统(HPT-3D),为原位水平推剪试样提供了新的处理技术.将该法应用于虎跳峡龙蟠右岸土石混合体的现场水平推剪试验中,得到了一些有益的结论.     

虎跳峡龙蟠右岸土石混合体粒度分形特征研究

应用分维理论对虎跳峡龙蟠右岸分布的土石混合体粒度分布的分维规律进行了研究分析,建立了平均粒径与相应的分维数之间的定量关系模型。通过研究表明,土石混合体具有良好的统计自相似性,由于其本身为级配不良土,在分维曲线上表现为双重分形分布,这种特殊的分维分布与土石混合体的成因及形成过程有关。