Mechanical Behavior and Sulfate Resistance of Alkali Activated Stabilized Clayey Soil

Clayey subgrade soil requires treatment in order to make the subgrade stable for pavement structures. Treatment of clayey soil i.e. stabilization of clayey soil by cement, lime, and fly ash are established techniques used in geotechnical and highway engineering. Stabilization by alkali activation of fly ash is reported recently but literatures are limited. Present study investigates the stress strain behavior, peak stress and ultimate strain of clayey soil stabilized by slag and slag-fly ash blending by alkali activation. The peak stress as high as 25.0 N/mm² may be obtained at 50% slags content when 12 molar sodium hydroxide solutions were used. Peak stress, ultimate strain and slope of stress–strain curve of stabilized clay are controlled by Na/Al and Si/Al ratios. Stress–strain response and peak stress of slag and fly ash blended specimen are not governed by Na/Al and Si/Al ratios; rather the behavior is dependent predominantly on slag content.     

基于GDS室温条件下盐渍土强度特性的试验研究

以新疆(北疆)地区S201公路沿线的盐渍土为研究对象,通过三轴剪切试验对盐渍土的强度特性进行了较系统的研究。试验结果表明,盐渍土与非盐渍土在力学性能上没有本质的区别,即应力-应变曲线大致都呈双曲线型,抗剪强度与破坏面上的法向应力成正比,且符合库伦定律;含水量和含盐量的变化导致试样发生的破坏形式不同。在含水量低和含盐量高时,试件具有脆性破坏的特征,在含水量高和含盐量低时逐渐显示出塑性特征;含水量和含盐量的变化对盐渍土的强度参数有重要影响,其中对粘聚力C的影响较大,表现较明显,而内摩擦角仅对含水量的变化表现敏感;极限强度与含水量和含盐量之间的关系符合二次抛物线变化规律,在不同应力作用下,抛物线形式不同。     

Carbon steel slag and stainless steel slag for removal of arsenic from stimulant and real groundwater

Arsenic in groundwater is a serious environmental problem. The contamination of groundwater with arsenic has been of utmost concern worldwide. Steel slag is a solid waste generated from steel production. Although steel slags have been used for arsenic removal from water, this process has not been systematically or integratively researched. In this study, the arsenic removal capacity and mechanism were investigated for carbon steel slag, stainless steel slag and Fe-modified stainless steel slag based on an in-depth study. The study also evaluated the potential utilization of different steel slag for regeneration. The maximum adsorption of arsenic on carbon steel slag, stainless steel slag and Fe-modified stainless steel slag was 12.20, 3.17 and 12.82 mg g^?1 at 25 °C, respectively. The modification of stainless steel slag by FeC1₃ can generate more pore structures and larger surface areas, and 300 °C treatment produces the best regeneration efficiency. The ΔG values were negative for all of the steel slags, indicating the spontaneous nature of the adsorption process. The solution pH was a critical parameter for the removal of arsenic for steel slags. Under highly alkaline solution conditions, the mechanism of arsenic removal by carbon steel slag and stainless steel slag can be attributed to chemisorption, including chemical precipitation and coordination reactions, and under weakly alkaline solution conditions, electrostatic interaction and specific adsorption are the arsenic removal mechanisms by Fe-modified stainless steel slag. Regeneration of the Fe-modified stainless steel slag was better achieved than that of the other steel slags in the application of high-temperature treatment.     

Effect of sodium sulphate on the shear strength of clayey soils stabilised with additives

The effect of sulphates on the soil stabilisation using mineral additives such as lime, cement and fly ash has been reported by several researchers. The effect of sodium sulphate (Na₂SO₄) (0–6% by dry weight of soil) on the behaviour of the grey clayey soil (GS) and red clayey soil (RS) stabilised with lime (L) (0–8%), natural pozzolana (NP) (0–20%) and with a combination of lime-natural pozzolana (L–NP) was investigated. The soil specimens were subjected to testing of direct shear strength after 7, 30, 60 and 120 days of curing period. In the absence of Na₂SO₄, the results show that both clayey soils can be successfully stabilised with L or with a combination of L–NP, which substantially increases their shear strength and produces high values of shear parameters. However, at short curing period and for any content of Na₂SO₄, a further increase in shear strength and shear parameters is observed. Moreover, after 30 days of curing, the RS specimens stabilised with L or with NP alone are altered when the Na₂SO₄ is greater than 2%, whereas the GS specimens are not altered. However, the alteration of RS specimens is little when the L and NP are combined on curing with a high content of Na₂SO₄. Generally, the effect of Na₂SO₄ on both stabilised clayey soils depends on the curing time, percentage of additives used and their type, mineralogical composition of stabilised soils and Na₂SO₄ content.     

Effects of water and fines contents on the resilient modulus of the interlayer soil of railway substructure

This paper deals with the resilient behavior of the interlayer soil which is created mainly by the interpenetration of ballast and subgrade soils. The interlayer soil studied was taken from a site in the southeast of France. Large-scale cyclic triaxial tests were carried out at three water contents (w = 4, 6 and 12 %) and three fines contents corresponding to 5, 10 % subgrade added to the natural interlayer soil and 10 % fine particles (<80 μm) removed from the natural interlayer soil. Soil specimens underwent various deviator stresses, and for each deviator stress, a large number of cycles was applied. The effects of deviator stress, number of cycles, water content and fines content on the resilient modulus (M _r) were analyzed. It appears that the effects of water content and fines content must be analyzed together because the two effects are closely linked. Under unsaturated conditions, the soil containing high fines content has higher resilient modulus due to the contribution of suction. When the soil approaches the saturated state, it loses its mechanical enhancement with a sharp decrease in resilient modulus.     

Collapse behaviour of a lateritic soil

Collapsible soils are problematic by nature. They undergo collapse or sudden settlement or subsidence under a given stress when their water content is increased. Collapse is characterised by collapse potential expressed as ΔH/H x 100, where ΔH is collapse compression. The amount of stress applied and the water content at the time of collapse govern the amount of collapse. In other words, collapse potential depends upon the amount of stress and the water content. Loess and other wind-blown silts are examples of collapsible soils. This paper presents a parametric study on the collapse behaviour of a lateritic soil. Remoulded specimens of a lateritic soil passing through a 425 µm sieve were compacted in 1-D consolidometer or oedometer at a density of 13 kN/m³ for studying collapse under varied initial moisture contents and initial surcharge pressures. Collapse compression of the samples was induced by saturating the specimens with water contents corresponding to 100% degree of saturation. After collapse occurred, the 1-D consolidation tests were continued up to an applied stress of 160 kPa. Collapse behaviour was studied for the applied initial pressures (σ_i) of 10, 20 and 40 kPa and for the initial water contents (w_i) of 5%, 10%, 15% and 20%. Collapse compression and collapse potential decreased with increasing w_i for all σ_i.     

Compressive and Tensile Behavior of Polymer Treated Sulfate Contaminated CL Soil

In this study, the compressive and tensile behavior of polymer treated sulfate contaminated CL soil was investigated. Based on the information in the literature, a field soil was contaminated with up to 4 % (40,000 ppm) of calcium sulfate in this study. In addition to characterizing the behavior of sulfate contaminated CL soil, the effect of treating the soil with a polymer solution was investigated and the performance was compared to 6 % lime treated soil. In treating the soil, acrylamide polymer solution (15 g of polymer dissolved in 85 g of water) content was varied up to 15 % (by dry soil weight). Addition of 4 % calcium sulfate to the soil decreased the compressive and tensile strengths of the compacted soils by 22 and 33 % respectively with the formation of calcium silicate sulfate [ternesite Ca₅(SiO₄)₂SO₄)], magnesium silicate sulfate (Mg₅(SiO₄)₂SO₄) and calcium-magnesium silicate (merwinite Ca₃Mg(SiO₄)₂). With the polymer treatment the strength properties of sulfate contaminated CL soil was substantially improved. Polymer treated sulfate soils had higher compressive and tensile strengths and enhanced compressive stress–strain relationships compared to the lime treated soils. Also polymer treated soils gained strength more rapidly than lime treated soil. With 10 % of polymer solution treatment, the maximum unconfined compressive and splitting tensile strengths for 4 % of calcium sulfate soil were 625 kPa (91 psi) and 131 kPa (19 psi) respectively in 1 day of curing. Similar improvement in the compressive modulus was observed with polymer treated sulfate contaminated CL soil. The variation of the compacted compressive strength and tensile strength with calcium sulfate concentrations for the treated soils were quantified and the parameters were related to calcium sulfate content in the soil and polymer content. Compressive stress–strain relationships of the sulfate soil, with and without lime and polymer treatment, have been quantified using two nonlinear constitutive models. The constitutive model parameters were sensitive to the calcium sulfate content and the type of treatment.     

Stabilization of Expansive Soil with Cement Kiln Dust and RBI Grade 81 at Subgrade Level

The present research work deals with an expansive high plastic clayey soil with cement kiln dust (CKD) and stabilizer (RBI Grade 81). The physical and engineering properties of soil are plasticity, compaction, unconfined compressive strength (UCS), consolidation and California bearing ratio (CBR) of the clayey soil and clay treated with CKD and stabilizer were determined. Soil chemistry was examined before and after treatment using scanning electron microscope (SEM) and elemental dispersive spectrometer. The clay mixed with CKD, CKD and RBI Grade 81 was found that optimum contents are 10 % (CKD), 15 % CKD with 4 % RBI Grade 81, respectively. The result indicates that CKD alone will decrease maximum dry density and increase optimum moisture content. CKD with RBI Grade 81 slightly increases maximum dry density and decreases optimum moisture content. UCS increased with CKD alone and CKD with RBI Grade 81 from 88.3 to 976 kN/m², respectively. CBR values were increased by the addition of CKD, CKD with RBI Grade 81 from 1.65 to 21.7 %. With the curing time of 3, 14 and 28 days, UCS and CBR values were increased due to pozzolanic reaction from cementations material. The treated soil has considerable reduction in compression index. SEM images clearly indicate the formation of CSH and CAH gel.     

基于超微型贯入试验的黏性土干燥过程中结构强度演化规律研究

研究土体结构强度对了解土体开裂破坏过程和预防相关工程地质问题有重要意义。以南京地区下蜀土为对象,采用超微型贯入试验方法,分析了试样在干燥过程中的结构强度演化特征。结果表明:所采用的超微型贯入试验方法为定量研究土体在干燥过程中结构强度的演化规律提供了可行的途径,该方法具有操作简单、精度高和含水率适应范围大等优点; 在干燥过程中,当试样处于高含水率阶段时(w24.27%),剖面上的结构强度基本保持一致,空间差异性较小; 在低含水率阶段(w24.27%),试样结构强度空间差异性显著增强,随深度的增加,结构强度呈递减趋势并逐渐趋于稳定; 总体上,土体的结构强度在干燥过程中随平均含水率的减小呈指数递增趋势,这主要是由于土体水分蒸发导致吸力增加,土颗粒之间的作用力增强。此外,由于土体发生收缩变形,土体孔隙比减小,土颗粒间联接点增加,也会对结构强度产生贡献。     

Sorption of surfactants and personal care products in Indian soils

Sorption of three surfactants and personal care products in four types of commonly occurring Indian soils was extensively studied. The soils used in the study were red soil, clay soil, compost soil and sandy soil as classified by American Society for Testing and Materials (ASTM). The three surfactants used in the study were representative of cationic, non-ionic and anionic surfactant groups. The sorption of surfactants followed the descending order: sodium dodecyl sulphate (SDS) > trimethyl amine (TMA) > propylene glycol (PG). The maximum adsorption capacity (Q_max) was obtained in compost soil (28.6 mg/g for SDS; 9.4 mg/g for TMA and 4 mg/g for PG). The rate of adsorption was the maximum in compost soil followed by clay and red soils, and minimum for sandy soils. It is found that the Freundlich model fits the isotherm data better than the Langmuir model. Freundlich coefficient (K _f) increased as the organic content of soils increased. Desorption of target pollutants in tap water was 20–50% whereas acid desorbs 40–90% of target pollutants from soil matrix. It was also found that the adsorption and desorption were significantly affected by the presence of clay and organic matter. The results also indicate that surfactants and personal care products, especially TMA and PG, are highly mobile in sandy soil followed by red soil. Therefore, immobilization of target pollutants is most economical and effective in compost and clayey soils whereas for other type of soils the combination of physiochemical and biological process will be effective option for remediation.     

Role of degree of saturation in denitrification of unsaturated sand specimens

Nitrate contamination of groundwater arises from anthropogenic activities, such as, fertilizer and animal manure applications and infiltration of wastewater/leachates. During migration of wastewater and leachates, the vadose zone (zone residing above the groundwater table), is considered to facilitate microbial denitrification. Particle voids in vadose zone are deficient in dissolved oxygen as the voids are partially filled by water and the remainder by air. Discontinuities in liquid phase would also restrict oxygen diffusion and therefore facilitate denitrification in the vadose/unsaturated soil zone. The degree of saturation of soil specimen (S _r) quantifies the relative volume of voids filled with air and water. Unsaturated specimens have S _r values ranging between 0 and 100 %. Earlier studies from naturally occurring nitrate losses in groundwater aquifers in Mulbagal town, Kolar District, Karnataka, showed that the sub-surface soils composed of residually derived sandy soil; hence, natural sand was chosen in the laboratory denitrification experiments. With a view to understand the role of vadose zone in denitrification process, experiments are performed with unsaturated sand specimens (S _r = 73–90 %) whose pore water was spiked with nitrate and ethanol solutions. Experimental results revealed 73 % S _r specimen facilitates nitrate reduction to 45 mg/L in relatively short durations of 5.5–7.5 h using the available natural organic matter (0.41 % on mass basis of sand); consequently, ethanol addition did not impact rate of denitrification. However, at higher S _r values of 81 and 90 %, extraneous ethanol addition (C/N = 0.5–3) was needed to accelerate the denitrification rates.     

Shear strength and pore-water pressure characteristics of sandy soil mixed with plastic fine

Recent earthquake case histories have revealed the liquefaction of mixtures of sand and fine particles during earthquakes. Different from earlier studies which placed an emphasis on characterisation of liquefaction in terms of the induced shear stress required to cause liquefaction, this study adopted a strain approach because excess pore-water pressure generation is controlled mainly by the level of induced shear strains. The current study includes the results of a set of laboratory tests carried out on sand specimens with the same relative densities and variation in the plastic fines (kaolinite or bentonite) contents ranging from 0 to either 30 % and consolidated at mean confining pressure of 100, 200 and 300 kPa using static triaxial test apparatus, in order to study the influence of fine content and other parameters on the undrained shear strength and liquefaction potential of clayey sand specimens; also, pore-water pressures in the specimens are discussed. Results of tests show that the peak strength decreases as the fines (kaolinite or bentonite) content increases up to a threshold content of fines (FC_th) after which, increases in plastic fine content lead to improve the peak shear strength of specimens, and also the ultimate steady-state strength has been improved due to the increased in plastic fines content. Also, pore pressure build-up in clayey sands is generally slower than that observed in pure sand.     

Soil-hydrological properties response to grazing exclusion in a steppe grassland of the Loess Plateau

Soil–water characteristics are necessary for water quality monitoring, solute migration and plant growth. Soil–water characteristic curve (SWCC) is a relationship between suction and water content or degree of saturation. However, little information is available concerning the impacts of grazing exclusion management on soil–water characteristics. Here, the soil–water characteristics of grasslands, which were excluded grazing for 5 (GE5) and 15 years (GE15), were studied. The saturated hydraulic conductivity (K _s), SWCC, particle composition, field capacity and some other indexes were determined. Results showed that the clay content and K _s of grassland soil were higher for GE15 than GE5. For both treatments, in low suction condition (≤100 kPa), the water holding capacity of 0–10 cm soil was the best. Water holding capacity of topsoil decreased gradually with the increasing of suction, and it reached the strongest when the suction reached 600 kPa. In all soil water suction, the water holding capacity of subsoil was the weakest. The van Genuchten expression was applicable for most of the samples, except 20–30 cm of GE5 and 10–20 cm of GE15. Dual porosity equation was applicable for all the samples. The soil–water capability and soil structure of which was fenced for 15 years is superior to that of 5 years. This study suggests that the enclosure management improved the soil structure and soil–water capability.     

Influence of Infiltration of Sodium Chloride Solutions on SWCC of Compacted Bentonite–Sand Specimens

The degree of saturation of compacted bentonite buffer in deep geological repositories is subject to alterations from infiltration of groundwater and heat emanated from the waste canisters. The matric suction (ψ)–degree of saturation (S _r ) relations of unsaturated clays is represented by soil–water characteristics curves (SWCC) that are influenced by soil structure, initial compaction condition and stress history. Infiltration of groundwater besides increasing the degree of saturation can also alter the pore water chemistry; the associated changes in cation hydration and diffuse double layer thickness could impact the micro-structure and matric suction values. This study examines the influence of infiltrating sodium chloride solutions (1,000–5,000 mg/L) on the transient ψ–S _r relations of compacted bentonite–sand specimens. Analysis of the ψ–S _r plots, and X-ray diffraction measurements indicated that infiltration of sodium chloride solutions has progressively less influence on the micro-structure and SWCC relations of bentonite–sand specimens compacted to increasingly higher dry densities. The micro-structure and SWCC relations of specimens compacted to 1.5 Mg/m³ were most affected, specimens compacted to 1.75 Mg/m³ were less affected, while specimens compacted to 2 Mg/m³ remained unaffected upon infiltration with sodium chloride solutions.     

Measuring daytime CO2 fluxes from the inter-tidal mangrove soils of Indian Sundarbans

The aim of this research was to measure the rate of carbon dioxide (CO₂) exchange between the soil and atmosphere in the inter-tidal forest floor of the Indian Sundarbans mangrove ecosystem and to study its response with soil temperature and soil water content. Soil CO₂ effluxes were monitored every month at two stations (between April, 2011 and March, 2012); one situated at the land–ocean boundary of the Bay of Bengal (outer part of the mangrove forest) and the other lying 55 km inshore from the coast line (inner part of the mangrove forest). The static closed chamber technique was implemented at three inter-tidal positions (landward, seaward and bare mudflats) in each station. Fluxes were measured in the daytime every half an hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path non-dispersive infrared gas analyser. The fluxes ranged between 0.15 and 2.34 μmol m^?2 s^?1 during the annual course of sampling. Effluxes of higher magnitude were measured during summer; however, it abruptly decreased during the monsoon. CO₂ flux from the forest floor was strongly related to soil temperature, with the highest correlation found with temperature at 2 cm depth. No such significant relationship between soil water content and CO₂ efflux could be properly ascertained; however, excessively high soil water content was found to be the only reason which hampered the rate of effluxes during the monsoon. On the whole, landward (LW) sites of the mangrove forest emitted more than the seaward (SW) sites. Q ₁₀ values (obtained from simple exponential model) which denote the multiplicative factor by which the efflux rate increases for a 10 °C rise in temperature ranged between 2.07 and 4.05.     

Use of Sewage Sludge Ash and Hydrated Lime to Improve the Engineering Properties of Clayey Soils

Using various additives has been considered as one of the most common stabilization methods for improvement of engineering properties of fine-grained soils. In this research the effect of sewage sludge ash (SSA) and hydrated lime (HL) on compressive strength of clayey soil was investigated. For this purpose, 16 kinds of mixtures or treatments were made by adding different amounts of SSA; 0, 5, 10 and 15% by weight and HL; 0, 1, 3 and 5% by weight of a clayey soil. First, compaction characteristics of the treatments were determined using Harvard compaction test apparatus. So that, 12 unconfined compressive strength test specimens were made using Harvard compaction mold from each treatments taking into account four different curing ages, including 7, 14, 28 and 90 days in three replications. Therefore, a total of 192 specimens were prepared and subjected to unconfined compressive strength tests. The results of this study showed that the maximum dry density of the treated soil samples decreases and their optimum water content increases by increasing the amount of SSA and hydrated lime in the mixtures. It is also found that the adding of HL and SSA individually would increase the compressive strength up to 3.8 and 1.5 times respectively. The application of HL and SSA with together could increases the compressive strength of a clayey soil more efficiently even up to 5 times.     

砾钢渣抗液化特性试验研究

陈化的钢渣作为土工回填材料是废弃钢渣循环利用的主要途径之一。按土的工程分类方法,将废弃钢渣划分为砾钢渣、粗钢渣和细钢渣。针对砾钢渣,考虑固结应力比、振动频率、围压和含砾量等影响因素开展动三轴试验研究。分析了砾钢渣的应力、应变和动孔隙水压力的特性,分析了砾钢渣试样的动强度与振动次数、动应变与振动次数、孔隙水压力与振动次数和动应力与动应变关系。采用Seed和Finn提出的饱和砂土动孔压计算模型分析砾钢渣的动孔压曲线类型,并与传统砂砾土的抗液化强度进行比较。得出砾钢渣的抗液化特性较好,工程中可以用砾钢渣替代传统的砂土、砂砾土、砂砾料和砂卵石作为回填料,解决砂砾资源日渐短缺的问题。     

Testing Study of Subcritical Crack Growth Mechanism During Water Rock Interaction

Subcritical crack growth plays an important role in evaluating the long-term stability of structures in rocks. By applying the constant-displacement-relax method, two groups of test specimens that one immersed in groundwater and the other in air were tested to get the parameters of subcritical crack growth in double torsion test. The relations of the stress intensity factor K _I versus the subcritical crack growth velocity V were obtained under the two different environments, and the behavior of subcritical crack growth was also analyzed. The results showed: the relations of lg K _I  ? lg V accorded with linear rules, which is in good agreement with Charles theory; Compared with specimens in nature state, the lg K _I  ? lg V curves of saturated water specimens locate top left comer of those of air specimens. The slope of curve is smaller, and the intercept is bigger, which shows that the water–rock interaction speeds up the subcritical crack growth. And A increases 2.9 × 10¹⁸ folds but n decreases from 85.12 to 40.83 because of the water–rock interaction. Meanwhile, the fracture toughness K _IC also decreases obviously from 2.55 in air to 2.26 in water due to water rock interaction. The testing results provide a basis for time-dependence of rock engineering stability.     

非饱和抗剪强度指标c、φ值与含水率w的关系

非饱和土力学已经确认了基质吸力与非饱和土的强度指标有关,而基质吸力与含水率有着密切的关系。通过理论和三轴剪切试验分析,探讨了非饱和残积土抗剪强度指标c、φ与含水率ω变化的一般规律;同时结合已经推出的土水特征曲线关系,探讨抗剪强度指标c、φ与基质吸力μ的变化的一般规律。结果表明:随着非饱和土含水率ω增加、基质吸力μ的减小,黏聚力c有减少的趋势,内摩擦角φ变化不大。通过线性回归分析,黏聚力与含水率成半对数线性关系,黏聚力与基质吸力成线性关系。现有的非饱和土实用公式精确性较差,非饱和土抗剪强度指标有待进一步的研究与完善。     

Holocene Deposits in Saga Plain: Leaching Mechanism and Soil Sensitivity

The Saga Plain in Japan contains a 10–30 m thick Holocene clayey soil deposit with a natural water content generally more than 100% and a liquidity index (I _L ) larger than 1.0. Most of this is a marine deposit known as the Ariake clay formation. Using salinity in the pore water of this deposit as an index, the mechanism of post-depositional salinity leaching from the Ariake clay formation has been investigated. This has been achieved using current measurements of the salinity distribution in the deposit and the groundwater flow velocity in an underlying Pleistocene gravelly sand layer, together with advection–diffusion analyses. It is suggested that diffusion together with possible rainfall percolation and/or upward seepage flow from the Pleistocene gravelly sand layer was the main mechanism causing salinity leaching. Detailed analysis of the test results from four boreholes indicates that for the locations where the activity of the clay minerals was less than 1.25, salinity leaching probably accounts for the observed low undrained shear strength (<0.5 kPa) of remoulded soil samples, high values of the sensitivity (S _t ), and the formation of a quick clay.