Electrochemical strengthening of clayey sandy soils

Most previous studies and applications of electrochemical stabilization of soils through electroosmosis have been made on clayey soils. The object of this investigation was to find out if relatively small amounts of clay (1.5%–3.5%, by weight) present in a sandy soil would be enough for stabilization and strengthening to be possible. The results indicate increases of cohesion of the order of 100–200 lb./sq.ft. X-ray analyses of treated soils indicate that sheet structures of clays are reduced and silicates destroyed upon treatment by electroosmosis. Newly-formed minerals also cement the soil. These neoformations include gibbsite, limonite, calcite, hydrohematite, hydrogoethite (hydrolepidocrocite), hisingerite, allophane, allophanoid, gypsum, hematite, magnetite, nontronite, trona and natron (Na₂ CO₃, 10H₂O). The process seems to be irreversible.     

Coupled theory of mixtures for clayey soils

In this work, elasto-plastic coupled equations are formulated in order to describe the time-dependent deformation of saturated cohesive soils (two-phase state). Formulation of these equations is based on the principle of virtual work and the theory of mixtures for inelastic porous media. The theory of mixtures for a linear elastic porous skeleton was first developed by Biot (Theory of elasticity and consolidation for a porous anisotropic solid, Journal of Applied Physics, 1955, 26, 188–185). An extension of Biot's theory into a nonlinear inelastic media was performed by Prevost (Mechanics of continuous porous media, International Journal of Engineering Science, 1980, 18, 787–800). The saturated soil is considered as a mixture of two deformable media, the solid grains and the water. Each medium is regarded as a continuum and follows its own motion. The flow of pore-water through the voids is assumed to follow Darcy's law. The coupled equations are developed for large deformations with finite strains in an updated Lagrangian reference frame. The coupled behavior of the two-phase materials (soil-water state) is implemented in a finite element program. A modified Cam-clay model is adopted and implemented in the finite element program in order to describe the plastic behavior of clayey soils. Penetration of a piezocone penetrometer in soil is numerically simulated and implemented into a finite element program. The piezocone penetrometer is assumed to be infinitely stiff. The continuous penetration of the cone is simulated by applying an incremental vertical movement of the cone tip boundary. Results of the finite element numerical simulation are compared with experimental measurements conducted at Louisiana State University using the calibration chamber. The numerical simulation is carried out for two cases. In the first case, the interface friction between the soil and the piezocone penetrometer is neglected. In the second case, interface friction is assumed between the soil and the piezocone. The results of the numerical simulations are compared with experimental laboratory measurements.     

Residual-state creep behavior of typical clayey soils

There are few places in the world to monitor aseismic creep. One of them is the Ismetpasa segment of the North Anatolian Fault. The observations in the Ismetpasa showed that the creep rate progressively decreased along the 40 years before the 1999 Kocaeli-Golcuk (Mw = 7.6) earthquake and then started increasing. This phenomenon might be a systematic of the creeping segments. If it is the case, this behavior can be utilized for early warning before the expected major earthquake in the Marmara Sea. In this study, the creep rate of the segment has been studied by GPS and InSAR technologies. The results showed that the rate has decreased to 1.3 cm a year. This result might be an indication of stress starting increase. If the segment retains the decreasing trend and it is ceased by a major earthquake, it would be a proof of the relationship between the creep process and the earthquakes. Then, the creep process might be utilized for early warning.     

Water repellency in oil contaminated sandy and clayey soils

Two sites from a humid tropical environment were studied with respect to soil water repellency caused by hydrocarbon contamination. Samples were analyzed for water repellency (molarity ethanol droplet method), total petroleum hydrocarbons, acute toxicity (Microtox) and field capacity. At both sites, water absorption times were logarithmically related to the molarity ethanol drop value (R > 0.95). In a sandy soil collected from an old separation battery which had been bioremediated, field capacity was strongly related to hydrocarbon concentration (R = 0.998); and at 10,000 mg/kg the calculated field capacity was only 75 % of the baseline. Water repellency was related to hydrocarbon concentration asymptotically and plant growth limiting values (severity > 3.0) were observed at low concentrations (2,400 mg/kg), even though toxicity was at, or below background levels. Bioremediated soil at this site had hydrocarbon concentrations only 1,300 ppm above background, but had extreme water repellency (severity = 4.6–4.7). Soil water repellency was also measured in a clayey, organic rich floodable soil, in a multiple pipeline right-of-way colonized by water tolerant pasture and cattails. Water repellency was associated with total petroleum hydrocarbon concentration (R = 0.962), but was not related to field capacity or toxicity. In this low-lying site, the water repellency observed in the laboratory is probably not representative of field conditions: samples taken at the end of the ten week dry season (and only four days before the first rains) showed ample moisture (> 80 % field capacity).     

Crack patterns in clayey soils: Experiments and modeling

The paper presents an experimental and numerical study to investigate the behavior of desiccated clayey soils. The performed tests permit to evaluate the crack pattern as well as the tensile strength as a function of suction. A new model that relates the porosity evolution to the suction and to the tensile strength was developed and implemented in the finite element program CODE_BRIGHT. The proposed model captured the initiation and propagation of cracks in a thin layer of desiccated clay and predicted crack patterns in terms of the Minkowski densities (i.e. average crack length and crack intensity factor). The effect of the heterogeneity of the tested specimens, modeled by random clusters, was also quantified. Copyright © 2011 John Wiley & Sons, Ltd.     

不同类型黏土的强度特性及其预测

强度特性是非饱和土力学中基础性的研究内容。目前对广吸力范围内非饱和黏土强度的预测研究相对较少。本文首先基于文献中粉质黏土、Madrid黏土和南阳弱膨胀土的非饱和强度特性进行了对比与分析。将不同类型非饱和黏土的强度特性大致分为3种类型:（1）在某一吸力范围试样出现强度峰值,并随着吸力值的进一步增大而降低;（2）达到某一吸力值后其强度几乎维持不变,不受吸力值的影响;（3）其强度随着吸力值的增大而增大。此外,基于现有考虑吸附水膜和毛细水作用的方法拟合广吸力范围内不同类型土的土水特征曲线,并将土水特征曲线分离成吸附土水特征曲线和毛细土水特征曲线。在非饱和土的抗剪强度公式中,认为吸力引起的非饱和强度增强部分主要由毛细水作用决定的,故将非饱和抗剪强度公式中吸力引起非饱和增强项的有效应力系数（即饱和度或有效饱和度）用毛细水对应的饱和度替代。最后,利用修正后的非饱和抗剪强度公式对3种较广吸力范围内非饱和土的强度进行了预测。预测结果表明过渡区段内的强度预测效果较好,但高吸力段非饱和强度的预测还有待进一步研究。     

The liquefaction of clayey soils under cyclic loading

This paper seeks to investigate the liquefaction of clayey soils, a phenomenon that has been the trigger for many natural disasters in the last few decades, including landslides. Research was conducted on artificial clay-sand mixtures and natural clayey soils collected from the sliding surfaces of earthquake-induced landslides. The undrained response of normally consolidated clayey soils to cyclic loading was studied by means of a ring-shear apparatus. For the artificial clay-sand mixtures, it was found that the presence of a small amount of bentonite (≤ 7%) would cause rapid liquefaction, while a further increase in bentonite content (≥ 11%) produced the opposite effect of raising soil resistance to liquefaction by a significant degree. It was demonstrated that the bentonite-sand mixture was considerably more resistant to liquefaction than the kaolin-, and illite-mixtures, given the same clay content. The test results of plastic soils revealed the significant influence of plasticity on the liquefaction resistance of soil. The microfabric of clayey soil was investigated by means of a scanning electron microscope. The analysis showed that the liquefaction potential of soil was strongly related to certain particle arrangements. For example, soil vulnerable to liquefaction had an open microfabric in which clay aggregations generally gathered at the sand particle contact points, forming low-strength “clay bridges” that were destroyed easily during cyclic loading. On the other hand, the microfabric of soil that was resistant to liquefaction appeared to be more compact, with the clay producing a matrix that prevented sand grains from liquefying. In the case of the natural soils, the obtained results indicated that their cyclic behavior was similarly influenced by factors such as clay content, clay mineralogy and plasticity. The relation between the liquefaction potential of natural soil and its microfabric was thus also established. On the basis of the obtained results, the authors posited an explanation on the mechanism of liquefaction for clayey soil.     

Stress induced time dependent behavior of clayey soils

It is shown that time compression curve obtained from one-dimensional consolidation curve in the laboratory may include six phases. These are initial compression, first primary compression, transition from first primary compression to second primary compression, second primary compression, and transition from second primary compression to creep and lastly creep. This paper attempts to identify the quantitative beginnings and characteristics of these phases. A mathematical characteristic of all the soils that follow primary consolidation as per Terzaghi’s one dimensional consolidation theory is derived. It is known as the constant of primary consolidation. It is used to study the beginning of secondary consolidation and its effects on primary consolidation. Another characteristic of soils for creep and total absence of primary compression is derived. Methods are suggested for the determination of coefficients of Primary and Secondary consolidations and the compression index.     

The residual shear strength of some Hellenic clayey soils

Summary A programme of residual shear strength testing has been undertaken on a number of Hellenic soil types: Marls, Clays and Flysch. The residual strength was determined using the Bromhead ring shear aparatus. For the applied normal stress range the residual strength envelope was straight and the resultant residual friction angle values are correlated with the index properties, such as Atterberg limits and grain-size distribution. The influence of mineralogy on the residual friction angle is also considered.     

Rheological properties of clayey soils originating from flow-like landslides

Flow-like landslides in clayey soils represent serious threats for populations and infrastructures and have been the subject of numerous studies in the past decade. However, despite the rising need for landslide mitigation with growing urbanization, the transient mechanisms involved in the solid-fluid transition are still poorly understood. One way of characterizing the solid-fluid transition is to carry out rheometrical tests on clayey soil samples to assess the evolution of viscosity with the shear stress. In this study, we carried out geotechnical and rheometrical tests on clayey samples collected from six flow-like landslides in order to assess if these clayey soils exhibit similar characteristics when they fluidize (solid-fluid transition). The results show that (1) all tested soils except one exhibit a yield-stress fluid behavior that can be associated with a bifurcation in viscosity (described by the critical shear rate \( \dot{\gamma_c} \)) and in shear modulus G; (2) the larger the amplitude of the viscosity bifurcation, the larger the associated drop in G; and (3) the water content (w) deviation from the Atterberg liquid limit (LL) seem a key parameter controlling a common mechanical behavior of these soils at the solid-fluid transition. We propose exponential laws describing the evolution of the critical shear stress τ_c, the critical shear rate \( \dot{\gamma_c} \), and the shear modulus G as a function of the deviation w-LL.     

Deformation zones under square footings supported by clayey soils

Footing settlements depend not only on physical and mechanical properties of base soils, but also on applied load intensities and their distributions with depth, as well as on footing rigidity, shape and dimensions. An analytical expression relating rigid bearing plate and/or footing settlements to thicknesses of deformation (active) zones, which form below footing bottoms, has been previously offered by the author. The results of tests performed with 0.5, 1.0 and 4.0 m²-area square footings, constructed on undisturbed clayey soils and containing data describing active zone development, were collected from literature and analyzed. This paper presents graphical relationships between square footing settlements, active zone thicknesses and footing dimensions, which are verified by published test results performed with experimental square footings, having areas different than the ones selected for statistical analyses.     

上海黏性土与水泥混合后强度增长特性试验研究

对于使用水泥与上海黏性土进行混合加固的土体,其强度增长特性与水泥含量、加固土的初期pH值、养护时间有关。以上海4类黏土为研究对象,对加固土的强度增长特性进行了试验研究;探讨了加固土的养护时间、水泥含量、初期pH等与加固土强度的关系。试验结果表明,当上海黏土中水泥含量或者加固土的初期pH值大于某一临界值时,水泥加固土的强度将迅速增加,对于上海黏土,该临界pH值为11.7,对应的水泥含量为17%～20%。但当水泥含量达到一定值后,它对土体的pH值的影响开始变小,而且水泥土的强度趋于稳定的时间变长。     

Influencing factors of geometrical structure of surface shrinkage cracks in clayey soils

In order to investigate the effects of temperature, thickness of soil layer, wetting and drying cycles and soil types on geometrical structure of surface shrinkage cracks in clayey soils, special software Crack Image Analysis System (CIAS) for analyzing shrinkage crack patterns was developed. Eight groups of soil samples were prepared and subjected to drying to crack in laboratory. The number of crack segments and intersections, average crack length, width and aggregate area, crack intensity factor (CIF), and the corresponding probability density functions (PDF) of these parameters were determined by analyzing several crack patterns derived from different experimental conditions. The results show that the soil cracking behavior and the geometrical structure of crack patterns are significantly influenced by these considered factors. There is a tendency of crack length, width, aggregate area and their most probable value (MPV) related to the PDF increases with temperature increase. With thicker soil layers, the average crack length, width, aggregate area and CIF are increased, and the main distribution ranges of crack length, width and aggregate area are increased also. When the soil is subjected to multiple wetting–drying cycles, the soil surface generates more irregular and coarse cracks. The number of short and narrow crack segments increases significantly, and the CIF decreases with an increase in wetting–drying cycles. It is also observed that the extent of cracking is directly related to the soil fines fraction and its plasticity index (I_P). The greatest CIF and crack width are observed in the soils with the largest fines fraction and highest I_P. In addition, the ratio of numbers of crack segments to intersections ranges from 1.5 to 2, and cracking mainly takes place in three stages: main-cracks initiation stage; sub-cracks initiation stage; terminal stable stage.     

Modelling the transverse behaviour of circular tunnels in structured clayey soils during earthquakes

Acta Geotechnica - The paper presents novel results from advanced numerical simulations of the transverse behaviour of shallow circular tunnels in natural clays accounting for soil structure...     

Shear strength behaviour of compacted clayey soils percolated with an alkaline solution

In the French model of deep nuclear wastes repositories, the galleries should be backfilled with excavated argillite after the site has been filled. After thousands of years, the degradation of the concrete lining of the galleries will generate an alkaline solute (pH > 12) that would circulate through the backfill. The goal of this paper is to describe the impact of such solute circulation on the properties of compacted argillite. Since additives (bentonite, sand or lime) are often introduced in the remoulded argillite for the backfill, such mixtures were also studied. Saturated-portlandite water was circulated through compacted samples for 3, 6 and 12 months at 60 °C. The shear strength behaviour of the samples was determined with triaxial tests. The microstructure of the samples was analysed via mercury intrusion porosimetry tests and scanning electron microscopy. The results showed that the influence of the alkaline fluid on the properties of the argillite is a function of the nature of the additive. In the case of the calcareous sand, no major changes were observed. The pure argillite underwent a slight decrease in its cohesion due to limited dissolution of its clayey particles. Conversely, intense alteration of the bentonite–argillite mixture was observed, and the shear strength behaviour was modified. Lime addition improved the mechanical characteristics of the argillite.     

Analytical solution for one-dimensional consolidation of clayey soils with a threshold gradient

General approximate analytical solutions are developed for one-dimensional consolidation with consideration of the threshold gradient under a time-dependent loading. A comparison is made between the present solution and some available numerical solutions for a particular case, and the results show that the approach employed in this article is reasonable. The influence of the threshold gradient and the loading period on consolidation behaviour is investigated, and the results show that the moving boundary of seepage moves downward gradually. The greater the threshold gradient is, the slower the boundary moves. The excess pore pressure will not be completely dissipated at the end of consolidation, and the larger the threshold gradient is, the greater the residual excess pore pressure is. The average degree of consolidation considering the threshold gradient defined by settlement is different from that defined by pore pressure. Moreover, the greater the threshold gradient is, the larger the average degree of consolidation in terms of strain, whereas the smaller the average degree of consolidation in terms of stress. It is also shown that the longer the loading period is, the longer the time moving boundary takes to reach the bottom of the layer, and the greater the average degree of consolidation is.     

Stabilization of clayey soils with Tunisian phosphogypsum: effect on geotechnical properties

Phosphogypsum and cement have been reported to improve the physicochemical properties of clayey soils. The present study aimed to investigate the behavior of various soils with different particle sizes and chemical and mineralogical compositions in the presence of phosphogypsum and cement mixed at various proportions. These hydraulic binders were assayed on three different soil samples, and their effects were examined using a battery of standardized tests, including the Atterberg limit, uniaxial compressive strength, Californian Bearing Rate (CBR) test, thermogravimetric analysis (TGA), microstructure observation (SEM), and X-ray diffraction tests. The results revealed a significant effect associated with the variation of phosphogypsum content in the soils. Keeping the cement content constant in the mixture, the continuous addition of phosphogypsum was noted to allow shifting the domain of plasticity to the highest water contents, which reduces the sensitivity of the soil to water and to increase the strength of soil. An increase of CBR index with the addition of phosphogypsum and cement is obtained. This treatment could have positively influenced the optimum moisture content and the maximal dry density. The mixture of soil-phosphogypsum and cement could give new forms such as ettringite and hydrate indicators of the improvement of the mechanical properties of the soil. This improvement varies from one soil to another, depending on its granularity and its mineralogy. The mineralogical composition of the soil, particularly kaolinite, amount, and size grading, have direct effects on the physical and mechanical properties of the soils under investigation.     

Response evaluation of axially loaded fixed‐head pile groups in clayey soils

The aim of this paper is to investigate the interaction between the piles in a group with a rigid head and correlate the response of a group of piles to that of a single pile. For this purpose, a computationally intensive study using 3‐D nonlinear numerical analysis was carried out for different pile group arrangements in clayey soils. The responses of the groups of piles were compared with that of a single pile and the variation of the settlement amplification factor R_a was then quantified. The influence of the number of piles, the spacing, and the settlement level on the group response is discussed. A previously proposed relationship for predicting the response of a pile group, based on its configuration and the response of a single pile, has been modified to extend its applicability for any pile spacing. The modified relationship provides a reasonable prediction for various group configurations in clayey soils. Copyright © 2009 John Wiley & Sons, Ltd.     

Desiccation shrinkage of non‐clayey soils: a numerical study

A mesoscale model of desiccation of soil based on the evolution of the pore system idealized as bimodal is numerically examined. A simplified evolution of the model reveals a series of characteristics that qualitatively agree with the observed macroscopic experimental findings. The principal mechanism is deemed to be driven by the surface evaporation and water outflow generating a pore pressure gradient resulting in the shrinkage mainly of the largest pores. The amount of shrinkage is a function of (negative) pore pressure and is controlled by the compressibility of the solid matrix. The numerical model includes also the ensuing partial saturation stage initiated by the air entry simulated as a scenario with a moving phase interface inside the pore. The proposed model can be extended beyond the two‐mode porosity soils, to include the multi‐modal porosity, or its statistical representation.Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of crude oil contamination on geotechnical properties of clayey and sandy soils

The southern coastal plain of Iran at the Persian Gulf encounters oil pollution due to the historical oil exploitation, related tanker navigations and accidents, and petrochemical industrial expansions in the recent years. Therefore, it is important to investigate the geochemical properties of oil-contaminated coastal soils and sediments for engineering and environmental purposes. Here, an extensive laboratory testing program was carried out to determine the effects of crude oil contamination on some of the geotechnical properties of clayey and sandy soils such as CL, SM and SP sampled from the coastal soils from this area. The testing included basic properties, Atterberg limits, compaction, direct shear, uniaxial compression and permeability tests on clean and contaminated soil samples at the same densities. The contaminated samples were prepared by mixing the soils with crude oil in the amount of 2%, 4%, 8%, 12%, and 16% by dry weight. The results indicated a decrease in strength, permeability, maximum dry density, optimum water content and Atterberg limits. Knowledge of these effects of oil contamination is important in coastal engineering and environmental remediation activities of the studied coastal plain.