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.

Study on the shear strength of deep reconstituted soils

Based on analytical methods of strength studies for deep soils, direct shear tests were carried out to investigate the shear strength of deep reconstituted soils at different initial dry densities and amounts of water.The results indicate that the shear strength of deep reconstituted soils for identical amounts of water below the plastic limit is enhanced with increasing dry density and but reduced sharply at the critical density, the point at which coarse particles break down.Moreover, the shear strength for identical dry density decreases with additional amounts of water and the rate of degradation is the greatest at the critical density.This is because the friction resistance between coarse particles reduces with increasing amounts of water higher than the plastic limit.In order to obtain reliable strength of deep reconstituted soils, suitable dry densities and amounts of water are necessary.

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.

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.

Soil-water characteristics and shear strength in constant water content triaxial tests on Yunnan red clay

The shear strength parameters for geotechnical designs are obtained mainly from consolidated drained (CD) or consolidated undrained (CU) triaxial tests. However, during construction, the excess pore-air pressure generally dissipates instantaneously while the excess pore-water pressure dissipates with time. This condition needs to be simulated in a constant water content (CW) triaxial test. The study on Yunnan red clay is carried out to investigate the soil-water characteristics and the shear strength characteristics under the constant water content condition. Osmotic technique is used to obtain the soil-water characteristic curve. A series of CW triaxial tests are conducted on statically compacted specimens. The experimental results show that the soil-water characteristic curve has a low air entry value of 7 kPa due to large pores in non-uniform pore size distribution, and a high residual value exceeding 10 MPa. In addition, the initial degree of saturation and net confining stress play an important role in affecting the shear characteristics under the constant water content condition. Finally, a new semi-empirical shear strength model in terms of degree of saturation is proposed and then applied to Yunnan red clay. Simulation result shows that the model is capable of capturing some key features of soils. The model can be used in whole engineering practice range, covering both unsaturated and saturated soils.

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.

Shear strength features of soils developed from purple clay rock and containing less than two-millimeter rock fragments

Soil shear strength is an important indicator of engineering design and an essential parameter of soil precision tillage and agricultural machinery and equipment design. Although numerous studies have investigated the characteristics of different soil shear strengths, only a few of these works have paid attention to soils containing considerable quantities of rock fragments. To date, most studies on the effects of rock fragments on the shear strength have paid attention to the role of rock fragments with sizes >2 mm. The effects of rock fragments <2 mm in soil are generally ignored. Similar to rock fragments >2 mm, the presence of rock fragments <2 mm could also change the mechanical properties of soils. Thus, in the present study we evaluated the potential influence of <2 mm rock fragments on soil shear strength via an unconsolidated undrained(UU) triaxial compression test. Our results were as follows:(1) A certain quantity of <2 mm rock fragments presented in purple soils developed from clay rocks; and an appropriate quantity of <2 mm rock fragments could improve the shear strength of soils.(2) The different PSDs of soils containing <2 mm rock fragments mainly caused variations in the internal friction angle of soils.(3) The shear strengths of the two mudstone-developed red-brown and gray-brown purple soils was more sensitive to water than that of the shale-developed coarse-dark purple soil. As the soil water content increased from 9% to 23%, the changes in the cohesion, internal friction angle, shear strength, and the maximum principal stress difference were smaller in the coarse dark purple soil than in the two other soils. We therefore concluded that <2 mm rock fragments in purple soils exerted important effects on soil shear strength. A better understanding of the differences among the shear strength features of purple soils could help improve the design of agricultural machinery and equipment.

Determination Method for Shear Strength Parameters of Rock-Soil Mixtures Using Close-Range Photogrammetry and 3-D Limit Equilibrium Theory

Using a combination of close-range photogrammetry and three-dimensional(3-D) limit equilibrium theory, a determination method for the shear strength parameters of rock-soil mixture is presented. A close-range photogrammetry method is used for measurement of the 3-D terrain of the experimental target. Auto CAD Lisp and EXCEL VBA are used to perform 3-D limit equilibrium analysis of the stability of sliding mass and perform backanalysis of shear strength parameters. The presented method was used to determine the shear strength parameters of rock-soil mixtures at the Liyuan Hydropower Station. The 3-D terrain of sliding surface could be measured notably well using of closerange photogrammetry. The computed results reveal that the cohesion and friction angle of rock-soil mixtures were 3.15 k Pa and 29.88o for test A, respectively, and 4.43 k Pa and 28.30o for test B, respectively, within the range of shear strength parameters, as determined by field and laboratory tests. The computation of shear strength parameters is influenced by the mesh grid number, especially the cohesion of the rock-soil mixture. The application of close-range photogrammetry can reduce the siteworks and improve the computational efficiency and accuracy.

Evaluation of Shear Strength and Cone Penetration Resistance Behavior of Tropical Silt Loam Soil under Uni-Axial Compression

Laboratory investigations were conducted to study strength characteristics of silt loam soil of Ilorin, Kwara State, Nigeria, under uni-axial compression tests. The main objective of this study was to evaluate the effects of applied pressure and moisture content on strength indices such as bulk density, penetration resistance and shear strength of the soil and to develop relationships between the strength indices for predictive purposes necessary in soil management. The compression was carried out at different moisture contents determined according to the consistency limits of the soil. The applied pressure ranged from 75 to 600 kPa. Values of bulk density, penetration resistance and shear strength increased with increase in moisture content up to peak values after which the values decreased with further increase in moisture content. Regression models were used to describe the trends in the results for the soil. Results also showed that bulk density and soil strength normally regarded as indicators of soil quality are affected by moisture content and applied pressure and that these properties can be predicted using the models generated from the study.

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.

Large-scale direct shear testing of geocell reinforced soil

The tests on the shear property of geocell reinforced soils were carried out by using large-scale direct shear equipment with shear-box-dimensions of 500 mm×500 mm×400 mm (length×width×height). Three types of specimens, silty gravel soil, geocell reinforced silty gravel soil and geocell reinforced cement stabilizing silty gravel soil were used to investigate the shear stress-displacement behavior, the shear strength and the strengthening mechanism of geocell reinforced soils. The comparisons of large-scale shear test with triaxial compression test for the same type of soil were conducted to evaluate the influences of testing method on the shear strength as well. The test results show that the unreinforced soil and geocell reinforced soil give similar nonlinear features on the behavior of shear stress and displacement. The geocell reinforced cement stabilizing soil has a quasi-elastic characteristic in the case of normal stress coming up to 1.0 GPa. The tests with the reinforcement of geocell result in an increase of 244% in cohesion, and the tests with the geocell and the cement stabilization result in an increase of 10 times in cohesion compared with the unreinforced soil. The friction angle does not change markedly. The geocell reinforcement develops a large amount of cohesion on the shear strength of soils.

Low secondary compressibility and shear strength of Shanghai Clay

In order to investigate the compressibility, particularly the secondary compression behaviour, soil structure and undrained shear strength of Shanghai Clay, a series of one-dimensional consolidation tests (some up to 70 d) and undrained triaxial tests on high-quality intact and reconstituted soil specimens were carried out. Shanghai Clay is a lightly overconsolidated soil (OCR=1.2-1.3) with true cohesion or bonding. Due to the influence of soil structures, the secondary compression index Cα varies significantly with consolidation stress and the maximum value of Cα occurs in the vicinity of preconsolidation stress. Measured coefficients of secondary compression generally fall in the range of 0.2%?0.8% based on which Shanghai Clay can be classified as a soil with low to medium secondary compressibility. The effect of soil structures on the compressibility of Shanghai Clay is found to reduce with an increase in depth. Soil structure has an important influence on initial soil stiffness, but does not appear to affect undrained shear strength significantly. Undrained shear strengths of intact Shanghai Clay from compression tests are approximately 20% higher than those from extension tests.

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.

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.

Slope Stability Analyses of Outang Landslide Based on the Peak and Residual Shear Strength Behavior

Most of the natural and compacted fine-grained soil slopes that are in saturated or unsaturated condition undergo a large deformation prior to reaching failure conditions.Such slopes should be designed taking account of their strain-softening behavior using the residual shear strength (RSS) parameters.In this paper,the slope stability of a recently reactivated Outang landslide near the Three Gorges Dam in China is analyzed based on the RSS parameters of unsaturated soils.In addition,comparisons are provided in the FOS values of slope using both the peak shear strength (PSS) and RSS parameters.Firstly,a series of site investigations of the hydrologic and geologic conditions,ground surface displacements and cracks were described.The PSS and RSS behaviors of the sliding soils derived from a series of direct shear test results performed on saturated and unsaturated soil specimens are summarized.Secondly,a series of slope stability analysis were conducted considering the precipitation and Yangtze River water level variation within a representative period of 7 months,based on the PSS and the RSS properties.In this study,three different scenarios were considered,which include: i) considering only the precipitation with a constant water level;ii) considering only the decrease in water level without rainfall;iii) considering the combination of precipitation and decrease in water level.In each scenario,four steps were included to calculate the values of factor of safety (FOS) at different times.1) A steady-state seepage analysis was conducted with a constant total head at 525 m on the left boundary and 175 m on the slope surface below the Yangtze River water level.The initial pore water pressures were simulated in the slope under no precipitation and variation of water level.2) A specific boundary condition was applied on the slope surface to model the precipitation and Yangtze River water level variation.A transient seepage analysis was conducted to calculate pore water pressures at different times based on the initial pore water pressures.3) The FOS values at different times were calculated by the Morgenstern-Price method taking account of the variation of pore water pressures at different times,using the peak shear strength (PSS) parameters.4) The last step was repeated replacing PSS parameters with RSS parameters.The RSS parameters were lower than the peak values from laboratory’s direct shear test results for the soils in the sliding zones.The reduction in shear strength from peak to residual state under unsaturated soil condition was greater than that for a saturated soil.The FOS decreased almost linearly with time for the scenario in which only the influence of rainfall infiltration was considered.However,the total reduction in the FOS was relatively small.The FOS decreased rapidly at a linear rate with respect to time with a decrease in water level for the scenario in which Yangtze River water level decrease was considered.The FOS reached to a relatively constant value after Yangtze River water level reached the lowest value.The decrease in Yangtze River water level was the dominant factor that contributed to a reduction in the FOS.The FOS was strongly dependent on the development of the phreatic line after the Yangtze River water level reached the lowest value.The FOS calculated by RSS (i.e.FOSR) is less than unity;they were approximately 16% lower in comparison to that calculated by PSS (FOSP).If PSS parameters were used,the slope would still be stable even under the combined influence of precipitation and Yangtze River water level decrease.These results are inconsistent with the field observations.For this reason,the RSS parameters should be taken into account to evaluate reliably the slope stability of the Outang landslide.

A Study on the Shear Strength Characteristic of Unsaturated Red Clay

The red clay in Chenzhou, Hunan province is mostly in unsaturated state. Simply applying the mechanical properties that derived from classic saturated soil mechanics often leads to slope failures in this region. In order to study the shear strength characteristic of unsaturated red clay in Chenzhou and to explore a shear strength equation that can be easily applied in engineering practice, a series of triaxial tests of saturated and unsaturated red clay samples were performed using the regular triaxial testing apparatus. The testing results show that the peak strength of red clay drops slightly before the moisture content of 30% but decreases sharply after that. The friction angle of red clay under unsaturated state is basically equal to the effective friction angle under saturated state, while the cohesion of unsaturated red clay is far much bigger than that of saturated one, which indicates that the matric suction makes a great contribution to the cohesion. By fitting the testing results with appropriate curves, the relationships between total strength parameters and with moisture content were obtained. The total increases logarithmically before the moisture content of 35% then decreases linearly, while decreases cubically with increasing moisture content.

Using Piezocone Tests for Analysis of Phreatic Water Conditions and Prediction of Soil Behavior in Tailing Dams

In this work the possibility of identifying two important aspects in the process of adopting soil parameters for calculating stability analysis models in tailing dams is discussed. The use of commercial computer programs for stability calculations allows obtaining numerically exact results. Its representativeness, however, will be linked to the correct definition of the phreatic regime and to the prediction of volumetric soil behavior during shearing (contractile vs. dilating materials). The theoretical principles for the selection of soils parameters for different failure models are briefly presented. Also, how the incorrect assumptions regarding material behavior can significantly affect the estimation of tailing dams’ stability. The results of CPTu tests for the diagnosis of the phreatic and mechanical condition of the materials are discussed and two examples are presented to remark on the care that should be taken to avoid incorrect soils parameters adoption.

中国南北方植物对土壤加固机制的差异性

为深入分析植物对土壤加固的影响效益和机制,该研究选取重庆缙云山地区和陕西延安黄土丘陵区种植一年的乔木(火炬树、榆树)和灌木(荆条、酸枣),测定其根系形态、力学参数和土壤的抗剪强度,通过RBMw模型计算根系固土效益,综合评估不同植物的固土效能和贡献度。结果表明:2地种植的物种平均根直径的差异不显著(P>0.05),重庆种植的乔木(火炬树、榆树)的根长、分叉数和根尖数显著高于延安,灌木未出现显著差异(P<0.05)。根系的抗拉强度与直径都符合负幂函数关系,其中平均抗拉强度最大的为荆条。除荆条外,同种植物根系的抗拉强度并未因不同地区的种植产生显著差异。4个植物种根系的固土效率为0.65~4.12 kPa,各物种间存在显著差异(P<0.05)。重庆2种乔木种植下的根土复合体有效黏聚力高于裸地(约10%),灌木种植下则普遍略低于裸地。除酸枣外,种植于重庆的4种植物的根系固土作用和效率都显著的高于延安(P<0.05)。研究结果可为不同地区固土护坡的树种选择提供理论依据。

海陆交互相软土固结抗剪强度指标变化规律研究

以珠三角地区海陆交互相淤泥质软土为研究对象,通过室内直剪试验探究不同固结压力和固结度下抗剪强度及抗剪强度指标的变化规律,提出抗剪强度及对应指标-固结度-固结压力三维Logistic数学模型。研究结果表明,当固结压力P≥200 kPa、固结度U≥40%时,软土黏聚力(c)和内摩擦角(φ)增长较为明显;当U=100%时,各级固结压力作用下的软土抗剪强度(τ)相较于初始状态分别提高了7.89、12.73、13.50、18.20、22.38 kPa;所给出的三维数学模型可直接计算出某一固结压力和固结度下的抗剪强度指标c、φ、τ。研究成果能够更为准确地评价该地区软土地基逐级加载过程中的整体稳定性。