Creep characteristics and constitutive model for frozen mixed soils

The uniform settle caused by the permafrost creep is still the primary problem of engineering construction in cold region.To investigate the creep characteristics of frozen soils mixed with silty clay and coarse-grained sand,several triaxial creep tests of frozen mixed soils under different conditions(temperature,confining pressure,coarse-grained particle content) were performed,and the effects of the temperature,confining pressure and coarse particle content on the creep characteristics of frozen mixed soils were also analyzed.The results of the experiments indicated that when the confining pressure was low,the specimen exhibited an attenuation creep under a low-stress level(0.4-0.7) and a non-attenuation creep under a high-stress level(0.7-0.9).In contrast,when the confining pressure was high,the specimen had both initial and stable creep stages,but no the accelerated creep stage.The higher the content of coarse grains,the shorter the stable creep stage,and the easier to enter the accelerated creep stage for the specimen.Further,the long-term strength of frozen soils decreased with an increase in the content of coarse grains.Finally,a newly improved Nishihara model was proposed to consider both the hardening effect and damage effect by introducing both the hardening and damage variables,which can model the entire creep process of frozen soils modeled relatively easily.It was found that with the increasing content of coarse particle,both the strengthening and damaged effects in the creep process are reduced。

A new procedure for determining dry density of mixed soil containing oversize gravel

This paper presents a novel computational procedure for the maximum dry density of mixed soils containing oversize particles.At first,the large-scale compaction test data for mixed soils are analyzed by an artificial neural network to determine the main factors affecting the compaction.These factors are then imposed on a genetic programming method and a new mathematical equation emerges.The new equation has more conformity with the experimental data in comparison with the previous correction methods.Besides,the mixed soil dry density is associated with most base soil and oversize fraction specifications.With regard to the sensitivity analyses,if the mixed soil contains high percentages of oversize fraction,the mixed soil composition is governed by the specification of oversized grains,such as specific gravity and the maximum grain size and by increasing these factors,the mixed soil dry density is increased.In mixed soil with a low content of oversize,the base soil specification mainly controls the compaction behavior of mixed soil.Furthermore,if the base soil is inherently compacted with greater dry density,adding the oversize slightly improves the mixed soil dry density.In contrast,adding oversized grains to the base soil with a lower dry density produces a mixed soil with greater dry density.By increasing the maximum grain size difference between the oversize fraction and base soil,the dry density of mixed soil is enhanced.

Shear behaviours of cohesionless mixed soils using the DEM:The influence of coarse particle shape

The coarse particles in mixed soils can be cobbles or gravels,with the main difference being their roundness(an indicator describing particle shape characteristics at an intermediate scale).The influence of coarse particle shape(i.e.,roundness)on the macroscopic and microscopic shear behaviours of cohesionless mixed soils with various fines contents(FCs)was investigated via the discrete element method in this study.The shapes of coarse particles were formed using the rotation-invariant spherical harmonic method proposed by previous investigators.An equation was proposed to predict the initial void ratios of samples in this study.A decrease in the roundness of coarse particles can increase the peak friction angle(FC≤40%)and critical friction angle(FC≤30%).As the roundness of coarse particles decreases,the peak dilatancy angle initially increases and then decreases(FC≤20%).Furthermore,it was found that the roundness of coarse particles hardly affects the classification of cohesionless mixed soils,as determined by probing the percentage contributions of coarse-coarse,coarse-fine,and fine-fine contacts.When cohesionless mixed soils change from an underfilled structure to an interactive-underfilled structure at the critical state,the main forms of coarse-coarse contacts were discovered.Additionally,the force-fabric anisotropy mechanisms of the influences of the roundness and rolling resistance coefficient of coarse particles on the shear strengths of cohesionless mixed soils were found to be different.

Experimental study on geosynthetic-reinforced sand fill over marine clay with or without deep cement mixed soil columns under different loadings

Geosynthetics and deep cement mixed(DCM)soil columns have been widely used to improve soft soil grounds in many countries and regions.This paper presents an experimental study on a geosynthetic-reinforced sand fill over marine clay with or without DCM columns under different loadings.Two tests were conducted on the sand fill reinforced with fixed-end and free-end geosynthetics over marine clay under three-stage local loading to investigate the effects of the boundary conditions of geosynthetic reinforcement on reducing settlements.It is observed that the fixed-end geosynthetic sheet is more effective in reducing settlements than the free-end condition under identical local loading.Another test was conducted on the fixed-end geosynthetic-reinforced sand fill over the marine clay improved by DCM columns under single-stage uniform loading.The vertical stresses on the marine clay and on the DCM columns,as well as the tensile strains of the geosynthetic sheet in the overlying sand fill,were measured.The results revealed that the stress concentration ratio increases with an increase in consolidation settlements,and the maximum tensile strain of the geosynthetic sheet occurs near the edge rather than at the center of the top surface of the DCM columns.

Soil-cement mixture properties and design considerations for reinforced excavation

soil-cement is a mixture produced by grouting or mixing cement with soils. This paper reviews and discusses the general classifications of grouting techniques and the suitability of their applications.The mechanical properties of soil-cement mixture and the influence of sodium silicate added are discussed. Design considerations for deep soil mixed wall(DSMW) for excavation support and vault arch for tunnelling stabilisation are presented. Parameters for the numerical analysis of soil-cement mixture are evaluated and recommended.

Mapping Soil Organic Carbon Stocks of Northeastern China Using Expert Knowledge and GIS-based Methods

The main aim of this paper was to calculate soil organic carbon stock(SOCS) with consideration of the pedogenetic horizons using expert knowledge and GIS-based methods in northeastern China.A novel prediction process was presented and was referred to as model-then-calculate with respect to the variable thicknesses of soil horizons(MCV).The model-then-calculate with fixed-thickness(MCF),soil profile statistics(SPS),pedological professional knowledge-based(PKB) and vegetation type-based(Veg) methods were carried out for comparison.With respect to the similar pedological information,nine common layers from topsoil to bedrock were grouped in the MCV.Validation results suggested that the MCV method generated better performance than the other methods considered.For the comparison of polygon based approaches,the Veg method generated better accuracy than both SPS and PKB,as limited soil data were incorporated.Additional prediction of the pedogenetic horizons within MCV benefitted the regional SOCS estimation and provided information for future soil classification and understanding of soil functions.The intermediate product,that is,horizon thickness maps were fluctuant enough and reflected many details in space.The linear mixed model indicated that mean annual air temperature(MAAT) was the most important predictor for the SOCS simulation.The minimal residual of the linear mixed models was achieved in the vegetation type-based model,whereas the maximal residual was fitted in the soil type-based model.About 95% of SOCS could be found in Argosols,Cambosols and Isohumosols.The largest SOCS was found in the croplands with vegetation of Triticum aestivum L.,Sorghum bicolor(L.) Moench,Glycine max(L.) Merr.,Zea mays L.and Setaria italica(L.) P.Beauv.

Bearing Capacity Assessment of Collapsible Soils Improved by Deep Soil Mixing Using Finite Element Method

Problematic soils usually cause considerable problems to engineering projects. As an example, soil structure collapse caused by moisture increment or rising underground water level results in huge settlements. This type of problematic soil, named collapsible soil, can cause dramatic problems and should be amended where exists. Today, the use of different techniques for soil reinforcement and soil improvement is widely used to treat soil properties. One of these methods is Deep Soil Mixing (DSM) method. This method becomes more important in the cases of studying and examining collapsible soils. In this research, the settlement of amended collapsible soils, applying deep soil mixing method, is examined. The experiments show that soil amendment using this method, well prevents the settlement of collapsible soils giving rise to bearing capacity.

An elasto-plastic constitutive model incorporating strain softening and dilatancy for interface thin-layer element and its verification

The mechanical behaviors of the interface between coarse-grained soil and concrete were investigated by simple shear tests under condition of mixed soil slurry (bentonite mixed with cement grout).For comparison,the interfaces both without slurry and with bentonite slurry were analyzed.The experimental results show that different slurries exert much influence on the strength and deformation of soil/structure interface.Under mixed soil slurry,strain softening and shear dilatation are observed,while shear dilatation appears under the small normal stress of the interface without slurry,and shear contraction is significant under the condition of the bentonite slurry.The thickness of the interface was determined by analyzing the disturbed height of the sample with both simple shear test and particle flow code (PFC).An elasto-plastic constitutive model incorporating strain softening and dilatancy for thin layer element of interface was formulated in the framework of generalized potential theory.The relation curves of shear stress and shear strain,as well as the relation curves of normal strain and shear strain,were fitted by a piecewise function composed by hyperbolic functions and resembling normal functions.The entire model parameters can be identified by tests.The new model is verified by comparing the measured data of indoor cut-off wall model tests with the predictions from finite element method (FEM).The FEM results indicate that the stress of wall calculated by using Goodman element is too large,and the maximum deviation between the test data and prediction is about 45%.While the prediction from the proposed model is close to the measured data,and the error is generally less than 10%.

Discrete element method study of parameter optimization and particle mixing behaviour in a soil mixer

Soil mixing is an emerging research in the field of construction resource recovery.In this study,the mixing behaviour of soil particles in a mixer is numerically simulated by the discrete element method(DEM).A four-factor,three-level orthogonal experiment is designed to optimize the mixer design by selecting the fly-cutter speed,spindle speed,number of blades and fly-cutter diameter,using Lacey mixing index and power consumption as evaluation indicators.Then,the impact of soil cohesion and type on the mixing behaviour is investigated.The results show that the optimal parameter combination of this experiment is 280 rpm fly-cutter speed,40 rpm spindle speed,4 blades and 250 mm fly-cutter diameter.This optimal combination reaches a comparatively uniform state mix in 5.9 s with an average power consumption of 704.11 W.In addition,the wear and tear of the mixer increases as soil cohesion increases,while the mixing quality of materials declines,resulting in a“shaft hugging”phenomenon.The mixing efficiency varies greatly among different soil types,but the radial and tangential velocities have a similar law.This work can provide some guidance for the optimization design of a mixer and study of soil mixing.

土壤修复中固液混合过程的研究及应用

以土壤修复中淋洗混合过程为研究目标,使用自有设备开展了液固混合过程实验,以混合效果和单位质量能耗为目标函数,以混合比例、搅拌桨形式、搅拌转速以及搅拌时间等操作参数为考察变量,通过设计均匀实验,并利用DPS软件对实验结果进行数据处理,并经过实验验证,得到最优固液混合操作条件为搅拌桨类型为铲式,液固混合质量比为0.5,搅拌速度为70 r/min,搅拌时间为10 min,得到的平均混合度为0.94,单位质量物系功耗为12.1 kg/kJ。

A multi-attribute decision making approach of mix design based on experimental soil characterization

The clay mineral composition is one of the major factors that governs the physical properties of silty sand subgrade. Therefore, a thorough knowledge of mineral composition is essential to predict the optimum engineering properties of the soil, which is generally characterized by different indices like maximum dry density （MDD）, California bearing ratio （CBR）, unconfined compressive strength （UCS） and free swelling index （FSI）. In this article, a novel multi- attribute decision making （MADM） based approach of mix design has been proposed for silty sand- artificial clay mix to improve the characteristic strength of a soil subgrade. Experimental investigation has been carried out in this study to illustrate the proposed approach of selecting appropriate proportion for the soil mix to optimize all the above mentioned engineering properties simultaneously. The results show that a mix proportion containing approximately 90% silty sand plus 10% bentonite soil is the optimal combination in context to the present study. The proposed methodology for optimal decision making to choose appropriate combination of bentonite and silty sand is general in nature and therefore, it can be extended to other problems of selecting mineral compositions.

Simulation of Excess Pore Water Pressure During Deep Soil Mixing Columns Installing

Most of current studies of deep soil mixing (DSM) methods are focused on the soil strength improvement and soil treatment effectiveness. But the DSM installation leads to excess pore water pressure and soil disturbance, which will bring great harm to adjacent structures, such as shell tunnels and historic buildings. The procedure of excess pore water pressure buildup while large number DSM columns are installed is complicated. In order to find methods to predict and simulate the excess pore water pressure during DSM column installation, the complicated dissipation and buildup of excess pore water pressure through in-situ test are studied in this paper. In-situ test was conducted in soft clay near the Huangpu River in Shanghai. The pore water pressure was investigated by an automatic monitoring system. Test results indicate that the excess pore water pressure induced by one DSM column installation is composed of the compaction pressure and the reversing pressure. The empirical equations of excess pore water pressure dissipation and buildup were built by mathematical fitting methods. A compound method is proposed to simulate the excess pore water pressure due to DSM installation. Using this method to predict the excess pore water pressure in the situ test, results show a well agreement between the prediction and the measurements.

Effects of deep soil mixing on existing shield tunnels in soft soil ground

To mitigate the impact of adjacent construction on existing shield tunnels,deep soil mixing(DSM)has been widely used to reinforce the soft soil ground around shield tunnels.However,the construction of DSM may cause the movement of existing shield tunnels under soft soil and sensitive ground conditions,and reasonable installation parameters will reduce the impact of DSM construction on the existing shield tunnels.Based on the field tests of DSM installation parameters and a program of field measurements of existing shield tunnels during the DSM construction in Suzhou,the reasonable installation parameters of DSM were selected,and the movement of soil behind the soil mixing walls(SMWs)during multirow DSM installation was investigated.The movement of the shield tunnels caused by DSM construction were discussed in detail.The field test results showed that the DSM columns installed at a higher speed and a lower water-cement ratio enlarged the movement of the surrounding soil.The DSM should be installed at a lower speed and a higher watercement ratio to reduce the movement of the shield tunnels.The field measurement results showed that the displacement of the tunnel lining was primarily caused by the construction of DSM zones beside the shield tunnels,which led to vertical compression and horizontal expansion of the tunnel lining.The construction of DSM immediately above the shield tunnels caused uplift to the tunnels.In addition,the deformed shapes of the two shield tunnels were asymmetric,and the displacement of the spring lining was larger than that of the crown.By taking the reasonable installation parameters of DSM and under the protection of the SMWs,the deformation of the shield tunnels caused by the construction of DSM was effectively controlled,and the maximum displacement was within the control value of the shield tunnels in this study.

Numerical investigation on arching effect surrounding deep cylindrical shaft during excavation process

Predicting the inner displacements of deep vertical shafts during the excavation process has been a difficult task considering the geological,structural,and constructional influences.In fact,the two-dimensional(2D)analytical solution based on the retaining wall model remains insufficient for understanding the actual behavior during an excavation.This is because the deformation of vertical shafts becomes complicated due to the unexpected arching effect brought about by the three-dimensional(3D)flexible displacements occurring in the excavation process.Previous analytical solutions only considered the limit equilibrium.Therefore,the present study deals with a 3D soil-structure simulation by considering the displacements of a cylindrical shaft and the mechanical behavior of the surrounding soil as well as the geometry of the cylindrical structure.Moreover,this mechanical behaviors of the surrounding soil and shaft are controlled by the shaft stiffness;hence,the relationships among the shaft stiffness,mechanical behavior of the surrounding soil(in terms of earth pressure coefficient),and shaft displacement were investigated.A cylindrical model,120 m in depth and 20 m in diameter,was positioned at the center of a sand domain,and each excavation step was performed at an interval depth of 20 m.A 3D finite difference method analysis was applied using the modified Cam-Clay(MCC)model to represent the soil behavior.As a result,the present study provides a new normalized lateral earth pressure theory for excavated shafts by considering the 3D arching effect obtained from parametric studies using various levels of shaft stiffness.From a comparison with the analytical solutions of previous studies(Terzaghi,1943a;Prater,1977;Cheng&Hu,2005),it is found that the previous studies underestimated the earth pressure acting on the cylindrical shaft because they did not consider the accurate arching effect.

Transportation infrastructure settlement and heave distress:challenges and solutions

Transportation agencies spend millions of dollars annually to repair civil transportation infrastructure including pavements,earth structures and approach slabs distressed by soft compressible soils and expansive soils.Several research studies performed at the University of Texas at Arlington(UTA) focused on stabilizing these problematic soils so that they will provide better and more stable support to the transportation infrastructure.This paper focuses on a summary of two major distresses and mechanisms,and remedial measures for addressing these distress problems.A combined lime-cement stabilization method is fully evaluated in providing better support of pavement infrastructure,and these results are described here.Another major transportation infrastructure problem involving bridge approach slabs requires different treatment methods,and these results are briefly described.As a part of the recently completed research study assessments,both shallow and deep soil treatment methods for stabilizing soils are fully evaluated for their effectiveness in arresting the distress posed to the pavements and bridge approach slabs.These results along with a few future research needs are presented in this paper.