Influence of Salt-Lime Stabilization on Soil Strength for Construction on Soft Clay

Construction on soft soil is one of the most challenging situations faced by geotechnical engineers. The heterogeneous and complex nature of soil, especially those containing organic clay, often makes it impossible for the construction specification to be addressed properly. Generally, clay exhibits low strength, high compressibility, and strength reduction when subjected to mechanical disturbance. This means that construction on clay soil is vulnerable to bearing capacity failure induced by low inherent shear strength. All these properties can be improved by the effective stabilization of soil. This study analyzed the effectiveness of incorporating salt-lime mixtures at various dosages in improving the strength increment of the soil. The results indicate that among different combinations of salt and lime, the best performance in terms of strength increase was achieved by adding 10% NaCl with 3% lime in the soil. The outcome of this study focuses on enhancing the ultimate strength of soil and its implementation in the field of foundation engineering.

Soil Strength Characteristics Along an Arable Eroded Slope

Undisturbed soil cores were taken from different slope positions （upslope, backslope and footslope） and soil depths （0-15, 20-35 and 100-115 cm） in a soil catena derived from Quaternary red clay to determine the spatial changes in soil strength along the eroded slope and to ewluate an indicator to determine soil strength during compaction. Precompression stress, as an indicator of soil strength, significantly increased from topsoil layer to subsoil layer （P 〈0.05） and was affected by slope position. In the subsoil layer （20-35 cm）, the precompression stress at the footslope position was significantly greater than at the backslope and upslope positions （P 〈0.05）, while there were no significant differences at 0-15 and 100-115 cm. Precompression stress followed the spatial wriation of soil clay content with soil depth and had a significant linear relationship with soil porosity （r^2 = 0.40, P 〈 0.01）. Also, soil cohesion increased with increasing soil clay content. The precompression stress was significantly related to the applied stress corresponding to the highest change of pore water pressure （r^2 = 0.69, P 〈 0.01）. These results suggested that soil strength induced by soil erosion and soil management wried spatially along the slope and the maximum change in pore water pressure during compaction could be an easy indicator to describe soil strength.

Effects of water salinity and content on particle size distribution and soil strength

The effect of NaCl on soil strength was investigated in this project based on salinity concentrations of 0 g/L, 5 g/L, 20 g/L, and 50 g/L as well as varying water contents of 15%-20%. Laser particle size analyzer was also performed to explain possible effects. From particle size analysis and strength tests, it is hypothesized that the strength of the soil is increased with the addition of certain salinity concentrations until there are reversed effects, which is between 20 g/L and 50 g/L from our study. The increase of strength is suggested to be the affect of a greater variety of particle sizes. Since NaCI plays a role in the particle size distribution, it also plays a role in the strength of soils. The degree of the effect of the water content also differs from concentrations, and could be due to the variation of hydration film thickness on particles, which is affected by the ions introduced from water.

Novel Hybrid X GBoost Model to Forecast Soil Shear Strength Based on Some Soil Index Tests

When building geotechnical constructions like retaining walls and dams is of interest,one of the most important factors to consider is the soil’s shear strength parameters.This study makes an effort to propose a novel predictive model of shear strength.The study implements an extreme gradient boosting(XGBoost)technique coupled with a powerful optimization algorithm,the salp swarm algorithm(SSA),to predict the shear strength of various soils.To do this,a database consisting of 152 sets of data is prepared where the shear strength(τ)of the soil is considered as the model output and some soil index tests(e.g.,dry unit weight,water content,and plasticity index)are set as model inputs.Themodel is designed and tuned using both effective parameters of XGBoost and SSA,and themost accuratemodel is introduced in this study.Thepredictionperformanceof theSSA-XGBoostmodel is assessedbased on the coefficient of determination(R2)and variance account for(VAF).Overall,the obtained values of R^(2) and VAF(0.977 and 0.849)and(97.714%and 84.936%)for training and testing sets,respectively,confirm the workability of the developed model in forecasting the soil shear strength.To investigate the model generalization,the prediction performance of the model is tested for another 30 sets of data(validation data).The validation results(e.g.,R^(2) of 0.805)suggest the workability of the proposed model.Overall,findings suggest that when the shear strength of the soil cannot be determined directly,the proposed hybrid XGBoost-SSA model can be utilized to assess this parameter.

Laboratory tests on enhancing strength of cement stabilized organicsoil with addition of phosphor gypsum and calcium almninate cement

In order to improve the engineering properties oforganic soil, a new stabilization agent is developed by theaddition of phosphor gypsum and calcium aluminate cement.The artificial organic soil is applied in the study and a series oflaboratory tests were carried out to explore new stabilizationagents and determine the optimal dosage. Unconfinedcompressive strength （UCS） and the pH value of soil poresolution were measured. The influence of organic content,agent composition and curing time on the UCS of sampleswere also researched. The test results show that the UCS ofstabilized organic soils by a new agent achieves approximately800 and 1 200 kPa at 28 and 90 d curing time, respectively.The pH test results show that a high alkaline environment is anecessary and not a sufficient condition for high strength. Thestrength of stabilized soil is related to the hydration product ofstabilization agent. The mechanism of strength formation wasalso explored by X-ray diffraction （XRD）, mercury intrusionporosimetry （MIP） and scanning electron microscope （SEM）tests. A large amount of ettringite is produced to fill the largepores of organic soils, which contribute to the high UCS valueof stabilized organic soils. The new agent can solidify theorganic soil successfully as well as provide a new approach totreat the organic soil.

Effect of discrete fibre reinforcement on soil tensile strength

The tensile behaviour of soil plays a significantly important role in various engineering applications. Compacted soils used in geotechnical constructions such as dams and clayey liners in waste containment facilities can suffer from cracking due to tensile failure. In order to increase soil tensile strength, discrete fibre reinforcement technique was proposed. An innovative tensile apparatus was developed to deter- mine the tensile strength characteristics of fibre reinforced soil. The effects of fibre content, dry density and water content on the tensile strength were studied. The results indicate that the developed test apparatus was applicable in determining tensile strength of soils. Fibre inclusion can significantly in- crease soil tensile strength and soil tensile failure ductility. The tensile strength basically increases with increasing fibre content. As the fibre content increases from 0% to 0.2%, the tensile strength increases by 65.7%. The tensile strength of fibre reinforced soil increases with increasing dry density and decreases with decreasing water content. For instance, the tensile strength at a dry density of 1.7 Mg/m^3 is 2.8 times higher than that at 1.4 Mg/m^3. It decreases by 30% as the water content increases from 14.5% to 20.5%. Furthermore, it is observed that the tensile strength of fibre reinforced soil is dominated by fibre pull-out resistance, depending on the interracial mechanical interaction between fibre surface and soil matrix.

Utilization of soil nailing technique to increase shear strength of cohesive soil and reduce settlement

This article deals with the assessment of the soil nailing technique with a vertical inclusion to improve the geotechnical parameters of cohesive soil. A series of unconfined compression tests and direct shear tests were carried out to establish the stressestrain relationship and strength characteristics of the reinforced clay sample by vertical steel nails. The shear strength performance of the new composite material was tested by varying the number of vertical inclusions, the embedment depth and the alignment radius. The results confirmed that the vertical bars/inclusions shared the vertical applied load with clay. Increase in the number of vertical inclusions significantly increases the shear strength and the stiffness with a remarkable reduction in settlement. When the clay samples were reinforced with six inclusions along the perimeter, the shear strength was increased to 231% for the embedment depth ratio equal to 0.85. To obtain the optimum effect in eliminating shear failure, the vertical inclusions should be extended to a deeper zone with sufficient numbers. It has been found that the vertical inclusions significantly influence the shear strength, and the brittle or general shear failure of the unreinforced sample can be diverted to partial/plastic shear failure.

Effect of Friedel's salt on strength enhancement of stabilized chloride saline soil

In the field of soil stabilization, only calcium silicate hydrate(CSH) and ettringite(AFt) as hydration products have been reported to directly contribute to the strength enhancement of the soil. A chloride dredger fill, an artificial chloride saline soil, and a non-saline soil were stabilized by Portland cement(PC) and PC with Ca(OH)_2(CH) with different contents. A series of unconfined compressive strength(UCS) tests of stabilized soil specimen after curing for 7 d and 28 d were carried out, and the hydration products and microstructure of the specimens were observed by X-ray diffractometry(XRD), scanning electronic microscopy(SEM), and energy-dispersive X-ray analysis(EDXA). The results showed that the strengths of PC+CH-stabilized chloride saline soils were much higher than those of PC-stabilized soils. A new hydration product of calcium aluminate chloride hydrate, also known as Friedel's salt, appeared in the PC+CH-stabilized chloride saline soils. The solid-phase volume of Friedel's salt expanded during the formation of the hydrate; this volume filled the pores in the stabilized soil. This pore-filling effect was the most important contribution to the significantly enhanced strength of the PC+CH-stabilized chloride saline soils. On the basis of this understanding, a new optimized stabilizer was designed according to the concept that the chloride in saline soil could be utilized as a component of the stabilizer. The strength of the chloride saline soils stabilized by the optimized stabilizer was even further increased compared with that of the PC+CH-stabilized soils.

Mechanism and Optimal Application of Chemical Additives for Accelerating Early Strength of Lime-flyash Stabilized Soils

To accelerate the early strength of lime-flyush stabilized soil for extending its further uses in highway and shortening highway constraction time, five kinds of chemical odditives were chosen on the basis of mechanism analysis of accelerating early strength in highway as a semi-rigid base materhd, and a series of experiments about the effect of differeat kinds of additives and quantity on the early strength of the stabilized soll were tested. The results show that chemical additives can efftciently improve the early strength of lime-flyush stabilized soil both the 7 d and 28d, and the optimum quantity for above chemical additive is 1.5%-2.5% approximately. Some suggestions for the practical construction were also proposed.

Shear strength of an unsaturated weakly expansive soil

To study the weakly expansive clay obtained from a slope along Wuhan—Shiyan expressway in Hubei Province,soil-water property tests and some unsaturated triaxial tests with suction control were conducted,and the soil-water retention curve（SWRC） and unsaturated shear strength of this soil were obtained.Results show that the air-entry suction and the residual degree of saturation of the tested soil are 106 kPa and 8%,respectively.The boundary effect zone and the transition zone can be identified on the desorption curve,but the residual zone is not so obvious.The unsaturated shear strength increases as suction increases within the range of controlled suction in the test,and friction angle,b,in the triaxial shear test is 17.6°.Based on the results,constitutive models for predicting the unsaturated shear strength using the SWRC were evaluated,and comparisons between prediction and measurement were made.It is concluded that for engineering purpose,the constitutive model should be carefully selected based on soil properties when predicting the unsaturated shear strength using the SWRC.

Strength of undisturbed and reconstituted frozen soil at temperatures close to 0 ℃

The strength of warm frozen soils in permafrost is fundamentally significant to estimate and predict the ground settlements from construction activities. A study was therefore initiated to assess the strength and its behaviors of undisturbed and reconstituted frozen soils at temperatures close to 0 ℃. A series of triaxial compression tests(TCT) were performed by using a developed testing apparatus and a matching specimen-preparation method. The confinement was applied from air pressure, the temperature in the specimen was maintained using two-end refrigeration, and multi-stage loading on a single specimen was adopted to determine the strength. The test results showed that the strength, both for the undisturbed and reconstituted frozen-soil specimens, was significantly dependent on the temperatures and independent of the applied confining pressures. Additionally, the strength of undisturbed frozen soils was about 1.6 times more than that for reconstituted frozen soils. These observations were closely associated with the structures existing between pore-ice and gravels with large diameters.

Strength and stiffness variation of frozen soils according to confinement during freezing

When water between soil particles is frozen, the strength and stiffness behavior of soils significantly change. Thus, nu- merous experimental studies in the laboratory have been carried out to characterize the strength and stiffness of frozen soils. The goals of this study are to evaluate the strength characteristics of frozen soils, which underwent confinement in freezing and shearing stages, and to estimate the stiffness variation by shear wave velocity during shear phase. The specimens are prepared in a brass cell by mixing sand and silt with 10% degree of saturation at a relative density of 60%. The applied normal stresses as confining stresses are 5, 10, 25 and 50 kPa. When the temperature of the specimens is lowered up to -5 ~C, direct shear tests are carried out. Furthermore, shear waves are continuously measured through bender elements during shearing stage for the investigation of stiffness change. Test results show that shear strength and stiffness are significantly affected by the confining stress in freezing and shearing phases. This study suggests that the strength and stiffness of frozen soils may be dependent on the confining stresses applied during freezing and shearing.

Novel Approach in Sampling and Tensile Strength Evaluation of Roots to Enhance Soil for Preventing Erosion

This paper presents novel approaches to address the complex issues associated with preservation, transportation, and tensile testing of the vegetation root samples needed for the enhancement of soil and prevent erosion. Readily availability of no equipment for in-situ assessment of the roots’ contribution to soil strength forces the researchers to transport the root samples to the lab for testing and estimating the contribution to the soil shear strength. Moreover, the standard procedures and apparatuses available in the public domain are regrettably suitable for testing of relatively stiffer materials. Therefore, conducting the tensile test of roots using off-the-shelf equipment often causes premature failure of the soft tissues and produces an erratic result, which ultimately leads to unrealistic soil shear strength. The experimental work replaced the traditional jaw type grips by innovative 3D-printed mold or metal ring with silicone, epoxy, and hot-glue to ensure a minimal degree of damage to the roots. Other scopes of the study include a comparison between fresh and refrigerated samples, the effect of sample storage temperature, pH, and Optimum Effective Root Area (OERA) per unit area of soil. Initial study conducted on the Bermuda grass (Cynodon dactylon) roots involved comparison for different approaches based on the gripping technic to select the best method. Finally, the paper included the results of tensile strength test performed on Spartina alterniflora root samples following the suggested guidelines thus helping better evaluation of root embedded soil shear strength, enhancing the resistance against soil erosion, and conserving the ecosystem.

Revisiting the Bjerrum's correction factor:Use of the liquidity index for assessing the effect of soil plasticity on undrained shear strength

The undrained shear strength （su） of fine-grained soils that can be measured in situ and in laboratory isone of the key geotechnical parameters. The unconfined compression test （UCT） is widely used in laboratoryto measure this parameter due to its simplicity; however, it is severely affected by sampledisturbance. The vane shear test （VST） technique that is less sensitive to sample disturbance involves acorrection factor against the soil plasticity, commonly known as the Bjerrum＇s correction factor, m. Thisstudy aims to reevaluate the Bjerrum＇s correction factor in consideration of a different approach and arelatively new method of testing. Atterberg limits test, miniature VST, and reverse extrusion test （RET）were conducted on 120 remolded samples. The effect of soil plasticity on undrained shear strength wasexamined using the liquidity index instead of Bjerrum＇s correction factor. In comparison with the resultobatined using the Bjerrum＇s correction factor, the undrained shear strength was better representedwhen su values were correlated with the liquidity index. The results were validated by the RET, whichwas proven to take into account soil plasticity with a reliable degree of accuracy. This study also showsthat the RET has strong promise as a new tool for testing undrained shear strength of fine-grained soils.

Digital mapping of soil physical and mechanical properties using machine learning at the watershed scale

Knowledge about the spatial distribution of the soil physical and mechanical properties is crucial for soil management,water yield,and sustainability at the watershed scale;however,the lack of soil data hinders the application of this tool,thus urging the need to estimate soil properties and consequently,to perform the spatial distribution.This research attempted to examine the proficiency of three machine learning methods(RF:Random Forest;Cubist:Regression Tree;and SVM:Support Vector Machine)to predict soil physical and mechanical properties,saturated hydraulic conductivity(Ks),Cohesion measured by fall-cone at the saturated(Psat)and dry(Pdry)states,hardness index(HI)and dry shear strength(SS)by integrating environmental variables and soil features in the Zayandeh-Rood dam watershed,central Iran.To determine the best combination of input variables,three scenarios were examined as follows:scenarioⅠ,terrain attributes derivative from a digital elevation model(DEM)+remotely sensed data;scenarioⅡ,covariates of scenarioⅠ+selected climatic data and some thematic maps;scenarioⅢ,covariates in scenarioⅡ+intrinsic soil properties(Clay,Silt,Sand,bulk density(BD),soil organic matter(SOM),calcium carbonate equivalent(CCE),mean weight diameter(MWD)and geometric weight diameter(GWD)).The results showed that for Ks,Psat Pdry and SS,the best performance was found by the RF model in the third scenario,with R2=0.53,0.32,0.31 and 0.41,respectively,while for soil hardness index(HI),Cubist model in the third scenario with R2=0.25 showed the highest performance.For predicting Ks and Psat,soil characteristics(i.e.clay and soil SOM and BD),and land use were the most important variables.For predicting Pdry,HI,and SS,some topographical characteristics(Valley depth,catchment area,mltiresolution of ridge top flatness index),and some soil characteristics(i.e.clay,SOM and MWD)were the most important input variables.The results of this research present moderate accuracy,however,the methodology employed provides quick and costeffective information serving as the scientific basis for decision-making goals.

Enhanced strength in novel nanocomposites prepared by reinforcing graphene in red soil and fly ash bricks

Low-dimensional nanomaterials such as graphene can be used as a reinforcing agent in building materials to enhance the strength and durability. Common building materials burnt red soil bricks and fly ash bricks were reinforced with various amounts of graphene, and the effect of graphene on the strength of these newly developed nanocomposites was studied. The fly ash brick nanocomposite samples were cured as per their standard curing time, and the burnt red soil brick nanocomposite samples were merely dried in the sun instead of being subjected to the traditional heat treatment for days to achieve sufficient strength. The water absorption ability of the fly ash bricks was also discussed. The compressive strength of all of the graphene-reinforced nanocomposite samples was tested, along with that of some standard (without graphene) composite samples with the same dimensions, to evaluate the effects of the addition of various amounts of graphene on the compressive strength of the bricks.

Influencing factors of compressive strength of solidified inshore saline soil using SH lime-ash

Through unconfined compressive strength test,influencing factors on compressive strength of solidified inshore saline soil with SH lime-ash,ratio of lime-ash(1-K),quantity of lime-ash,age,degree of compression and salt content were studied.The results show that because inshore saline soil has special engineering characteristic,more influencing factors must be considered compared with ordinary soil for the perfect effect of solidifying.

A new strength criterion for structured soils

Existing strength criteria are mostly formulated to describe the mechanical properties of reconstituted soils. However, the engineering characteristics of structured soils are different from those of reconstituted soils in many aspects, especially in their strength properties, Thus, the influence of soil structure （bonding and fabric） on the mechanical properties of structured soils cannot be correctly described, By analyzing the breakage mechanism of natural soils, the structured soils can be conceptualized as binary medium materials consisting of bonded blocks and weakened bands. On this basis, a new strength criterion is pro- posed for structured soils, The expressions of the strength criterion on both meridian and deviator planes are given to describe the strength properties of structured soils on these planes. The proposed strength criterion is compared with available test data under conventional and true triaxial stress conditions in the literature. It is observed that the proposed strength criterion agrees well with the test data.

偏高岭土基地聚合物对水泥固化红黏土的改善机制

为了研究偏高岭土对水泥固化红黏土的改善效果,开展了三种组合(纯水泥、水泥+偏高岭土以及水泥+偏高岭土+水玻璃)的红黏土固化试验。基于固化剂化学组分和固化土的干密度、pH值以及物相成分等,研究了新型复合碱激发体系作用下偏高岭土对水泥固化红黏土的作用机理。研究表明:当水泥、偏高岭土和水玻璃掺入比分别为12%、5%和3%时,红黏土固化效果最佳,相比于纯水泥固化土其强度提高了2.82倍。在n(SiO_(2))/n(Al_(2)O_(3))从2.53增加至4.05过程中,固化土强度发展较快,随后逐渐趋于稳定。由于水泥水化生成的Ca^(2+)能够平衡固化体系中的部分负电荷,在n(Na_(2)O)/n(Al_(2)O_(3))较小的情况下固化土强度得到了显著提升。最后通过固化土微观形貌及主要物相组成发现,新型复合碱激发体系的试样中含有无定形地聚物凝胶且主要物相特征峰峰值有所降低,说明产生了更多的地聚合凝胶产物。

上覆软弱路基加固对既有地铁隧道沉降的影响规律

为研究道路施工引起下方既有地铁隧道的沉降问题,在地铁隧道上方斜穿施工道路的基础上,进行了现场隧道沉降变形实测研究,分析了道路在路基注浆加固、路床和路面结构层施工阶段中地铁隧道的沉降曲线。建立地铁隧道-土体-道路模型对道路施工的注浆加固过程及路床和路面结构层施工进行模拟,通过比较地铁隧道沉降计算结果与现场实测值,验证了该精细化模型的准确性。基于此,分析了路床和路面结构层总施工厚度、道路土体性质、隧道下卧土层、隧道衬砌强度等关键参数对地铁隧道的沉降影响规律。结果表明:地铁隧道的沉降值与施工厚度呈正相关关系;道路的存在对隧道的沉降影响越小,其弹性模量和泊松比对地铁隧道沉降几乎没有影响;卧土层的弹性模量越大,土层越不易变形,且地铁隧道沉降越小;衬砌弹性模量增大对地铁隧道沉降影响反而越小。