Modelling rapid non-destructive test using light weight deflectometer on granular soils across different degrees of saturation

This study introduces an advanced finite element model for the light weight deflectometer(LWD),which integrates contact mechanics with fully coupled models.By simulating LWD tests on granular soils at various saturation levels,the model accurately reflects the dependence of the LWD modulus on dry density,water content,and effective stress.This model addresses and overcomes the limitations of previous finite element models for this specific problem.Simultaneously,this research presents the first experimentally validated fully coupled contact impact model.Furthermore,the research provides a comparative assessment of elastoplastic and nonlinear elastic models and contrasts an enriched node-tosegment method(developed in this study)with the more precise mortar technique for contact mechanics.These comparisons reveal unique advantages and challenges for each method.Moreover,the study underscores the importance of careful application of the LWD modulus,emphasising the need for sophisticated tools to interpret soil behaviour accurately.

Groutability classification of granular soils with cement grouts

This research aims to develop a methodology for applying the geostatistical method to generate a groutability classification for granular soils.To ensure the precision of the suggested technique,a total of 103 data samples were used.Predicting the groutability of granular soils has always been difficult because of many soil characteristics.As a result,a new two-dimensional graph,the groutability classification of granular soil(GCS)chart,was developed.GCS establishment was based on data analysis of the grain size of soil and cement-based grouts(N1 and N2),relative density(Dr)and fines content of the soil(FC),water/cement ratio of grout mixture(w/c),and grouting pressure(P),all of which have a direct impact on the groutability of soil media.The geostatistical method was used to develop and compile the GCS graph based on the aforementioned parameters with the use of coefficient S,which is a coefficient of the scoring set of parameters including P,w/c,Dr,and FC.The validation process was carried out hierarchically,with an additional set of 30 data.The proposed method has a prediction accuracy of roughly 96.7%,demonstrating a helpful tool.The proposed approach can be easily implemented in practical engineering situations because it has a comparable syntax to commonly used formulae.It should be noted that the proposed formula was only tested using the data samples collected,and the applicability of the produced procedure to other situations requires more examination.

Experimental Analysis of Hydraulic Conductivity for Saturated Granular Soils

Hydraulic conductivity is the ability of a porous media to transfer water through its pore matrix. That is a key parameter for the design and analysis of soil fluid associated structures and issues. This paper presents the test results of the vertical hydraulic conductivity kv carried out on one poorly graded sand and three gap graded gravely sand. It was found that the vertical hydraulic conductivity of saturated soil depends on the grain size distribution curve, on the initial relative density of the soil. Compilation of these current test results and other test results published, shows that the common approaches predict well to some extent the vertical hydraulic conductivity kv for the poorly graded sand materials and underestimate the kv values for gap graded gravely sand materials. Therefore, new approaches are developed for the prediction of the vertical hydraulic conductivity in saturated poorly graded sand and gap graded gravely sand. The derived results from the new approaches lie in the range of the recommended values by (EAU 2012) and (NAVFAC DM 7 1974).

Dilation and breakage dissipation of granular soils subjected to monotonic loading

Dilation and breakage energy dissipation of four different granular soils are investigated by using an energy balance equation. Due to particle breakage, the dilation curve does not necessarily pass through the origin of coordinates. Breakage energy dissipation is found to increase significantly at the initial loading stage and then gradually become stabilised. The incremental dissipation ratio between breakage energy and plastic work exhibits almost independence of the confining pressure. Accordingly, a plastic flow rule considering the effect of particle breakage is suggested. The critical state friction angle is found to be a combination of the basic friction between particles and the friction contributed by particle breakage.

A generalized dilatancy angle equation of granular soil

This paper presents a generalized dilatancy angle equation of granular soil to cover not only the drained tests but also the undrained tests by introducing a generalized structure of soil:soil skeleton formed by soil particles and the fluid in soil voids,under the assumptions of the incompressibility of soil particles and the compressibility of the fluid in soil voids.For the drained tests,the generalized dilatancy angle equation of granular soil would be degenerated to its current dilatancy angle equation.However,for the undrained tests,the generalized dilatancy angle equation of granular soil was derived with aλparameter that was related to the stress-strain state of soil and the nature of the fluid in soil voids.Theλparameter was determined by the initial dilatancy angles of granular soil at the onset of shearing on the same initial state of the soil in the drained and undrained tests.In addition,the generalized dilatancy angle equation of granular soil was verified for application in calculation of the dilatancy angles of sands in the drained and undrained tests.

Modeling of ratcheting accumulation of secondary deformation due to stress-controlled high-cyclic loading in granular soils

An objective of this work is to develop a validated computational model that can be used to estimate ratcheting accumulation behavior of granular soils due to high-cyclic loading. An accumulation model was proposed to describe only the envelope of the maximum plastic deformations generated during the cyclic loading process, which can calculate the accumulated deformation by means of relatively large load cycle increments. The concept of volumetric hardening was incorporated into the model and a so-called overstress formulation was employed to describe the evolution of the accumulated volumetric deformation as a state parameter. The model accounted for ratcheting shakedown and accumulation such as a pseudo-yield surface(a shakedown surface) associated with loading inside the current virgin yield surface which was implemented into the well-known modified Cam-clay model. Finally, the model was calibrated using data from the stress-controlled drained cyclic triaxial tests on homogeneous fine grained sands. It is seen that the model can successfully represent important features of the ratcheting accumulation of both volumetric and deviatoric deformation caused by repeated drained loading over a large number of cycles.

A New Type of Granular Soil Stabilizing Binder

A new type granular soil stabilizing binder was prepared. Itscomposition was designed in the system ofslag-clinker-gypsum-activating agent. Its properties were comparedwith those of 425~# Portland blastfurnace-slag cement.

Grain crushing and its effects on rheological behavior of weathered granular soil

To disclose the grain crushing effects on the weathered granular soil rheological behavior,a series of rheological tests (odometer compression and triaxial shearing) were carried out.At the same time,the sieving analysis tests of these specimens were also executed before and after tests,and the grain crushing degree,Br and n5,were collectively adopted to estimate the grain crushing.The grain crushing degree depends on the stress path,stress level,and load time,especially,the longer load time and more intensive gradient shearing path will increase the grain crushing quantity.The Hardin crushing degrees Br are 0.191,0.118 and 0.085 in the ordinary compression,rheological compression and triaxial rheological shearing,respectively;The grain crushing degrees n5 are 1.9,1.4 and 1.32,respectively.The strain softening phase indicates the grain crushing and diffusive collapse,and the strain hardening phase indicates the rearrangement of these crushed grains and formation of new bearing soil skeleton.The rheological deformation of granular soil can be attributed to the coarse grain crushing and the filling external porosity with crushed fragments.

Assessment of the Internal Instability for Granular Soils Subjected to Seepage

The knowledge of the internal stability of granular soils is a key factor for the design of granular and filter for the geotechnical infrastructures such as dykes, barrages, weirs and roads embankment. To evaluate the internal instability of granular soils different criteria are generally used in the practice. However, the results of these criteria on the same soil may lead to different evaluations of the internal instability. In this paper the common criteria used for the internal instability have been presented and compared as far as possible. It was found that the most internal instability criteria define a limit value for the secant slope of the grain size distribution curve of the granular soils. Based on this finding an own criterion for the evaluation of the internal instability of granular soil has been developed and compared to the common criteria. A very good agreement between some criteria was found. Furthermore, a site specific assessment for the evaluation of the internal instability of granular soil has been proposed in order to get more confidence in this evaluation.

Effects of stress conditions on rheological properties of granular soil in large triaxial rheology laboratory tests

In order to study the rheological properties of red stone granular soil,a series of rheological experiments were executed on large tri-axial rheological apparatus.Under 100,200 and 300 kPa confining stress conditions,the rheological tests were carried out.These experiment results showed that the stress conditions,especially the stress level were the critical influencing factors of the rheological deformation properties.Under the low stress level(S=0.1),the granular soil showed the elastic properties,and there was no obvious rheological deformation.Under the middle stress level(0.2<S≤0.6),creep curves showed the linear viscoelastic rheological properties.However,under the high stress level(S>0.8) creep curves showed the non-linear viscous plastic rheological properties.Especially,under the stress level of S=1.0,the accelerated rheological phase of creep curves occurred at early time with a trend of failure.The stress level had obvious effects on the final rheological deformation of the soil sample,and the final rheological deformation increments nonlinearly increased with stress level.The final rheological deformation increment and step was little under low stress level,while it became large under high stress level,which showed the nonlinearly rheological properties of the granular soil.The confining pressure also had direct effects on final rheological deformation,and the final rheological deformation linearly increased with confining pressure increments.

Mobility and dynamic erosion process of granular flow:insights from numerical investigation using material point method

In order to understand the dynamics of granular flow on an erodible base soil,in this paper,a series of material point method-based granular column collapse tests were conducted to investigate numerically the mobility and dynamic erosion process of granular flow subjected to the complex settings,i.e.,the aspect ratio,granular mass,friction and dilatancy resistance,gravity and presence of water.A set of power scaling laws were proposed to describe the final deposit characteristics of granular flow by the relations of the normalized run-out distance and the normalized final height of granular flow against the aspect ratio,being greatly affected by the complex geological settings,e.g.,granular mass,the friction and dilatancy resistance of granular soil,and presence of water in granular flow.An index of the coefficient of friction of granular soil was defined as a ratio of the target coefficient of friction over the initial coefficient of friction to quantify the scaling extent of friction change(i.e.,friction strengthening or weakening).There is a characteristic aspect ratio of granular column corresponding to the maximum mobility of granular flow with the minimum index of the apparent coefficient of friction.The index of the repose coefficient of friction of granular flow decreased gradually with the increase in aspect ratio because higher potential energy of granular column at a larger aspect ratio causes a larger kinetic energy of granular soil to weaken the friction of granular soil as a kind of velocity-related friction weakening.An increase in granular mass reduces gradually the indexes of the apparent and repose coefficients of friction of granular soil to enhance the mobility of granular flow.The mobility of granular flow increases gradually with the decrease in friction angle or increase in dilatancy angle of granular soil.However,the increase of gravity accelerates granular flow but showing the same final deposit profile without any dependence on gravity.The mobility of granular flow increases gradually by lowering the indexes of the apparent and repose coefficients of friction of granular flow while changing the surroundings,in turn,the dry soil,submerged soil and saturated soil,implying a gradually increased excessive mobility of granular flow with the friction weakening of granular soil.Presence of water in granular flow may be a potential catalyzer to yield a long run-out granular flow,as revealed in comparison of water-absent and water-present granular flows.In addition,the dynamic erosion and entrainment of based soil induced by granular flow subjected to the complex geological settings,i.e.,the aspect ratio,granular mass,gravity,friction and dilatancy resistance,and presence of water,were comprehensively investigated as well.

A numerical analysis of the shear behavior of granular soil with fines

Shear behavior of granular soil with fines is investigated using the discrete element method （DEM） and particle arrangements and inter-particle contacts during shear are examined. The DEM simulation reveals that fine particles play a vital role in the overall response of granular soil to shearing. The occurrence of liquefaction and temporary reduction of strength is ascribed mainly to the loss of support from the fine particle contacts （S-S） and fine particle-to-large particle contacts （S-L） as a consequence of the removal of fine particles from the load-carrying skeleton. The dilative strain-hardening response following the strain-softening response is associated with the migration of fine particles back into the load-carrying skeleton, which is thought to enhance the stiffness of the soil skeleton. During shear, the unit normal vector of the large particle-to-large particle （L-L） contact has the strongest fabric anisotropy, and the S-S contact unit normal vector possesses the weakest anisotropy, suggesting that the large particles play a dominant role in carrying the shear load. It is also found that, during shear, fine particles are prone to rolling at contacts while the large particles are prone to sliding, mainly at the S-L and L-L contacts.

A constitutive model for granular soils

A simple constitutive model is presented to describe the mechanical behaviors of granular soils in a large stress range. A novel normal compression line(NCL) is first expressed by introducing a limit void ratio(e_L) in the double logarithmic scale.Subsequently, a state parameter(ξ) is defined to quantify the current state of granular soils, and a unified hardening parameter(H)that is a function of the state parameter(ξ) is developed to govern the hardening process of the drop-shaped yield surface.Combining with flow rule, a constitutive model for granular soils is proposed. Finally, the comparison between the predictions and the test results of Cambria sand and Coarse-grained material indicates that the model is able to describe the mechanical behaviors of granular soils in a large stress range.

FRACTIONAL ORDER MODELLING OF THE CUMULATIVE DEFORMATION OF GRANULAR SOILS UNDER CYCLIC LOADING

To model the cumulative deformation of granular soils under cyclic loading, a mathematical model was proposed. The power law connection between the shear strain and loading cycle was represented by using fractional derivative approach. The volumetric strain was characterized by a modified cyclic flow rule which considered the effect of particle breakage. All model parameters were obtained by the cyclic and static triaxial tests. Predictions of the test results were provided to validate the proposed model. Comparison with an existing cumulative model was also made to show the advantage of the proposed model.

A miniature triaxial apparatus for investigating the micromechanics of granular soils with in situ X-ray micro-tomography scanning

The development of a miniature triaxial apparatus is presented.In conjunction with an X-ray microtomography(termed as X-ray fiCT hereafter)facility and advanced image processing techniques,this apparatus can be used for in situ investigation of the micro-scale mechanical behavior of granular soils under shear.The apparatus allows for triaxial testing of a miniature dry sample with a size of 8 mm x 16 mm(diameter x height).In situ triaxial testing of a 0.4-0.8 mm Leighton Buzzard sand(LBS)under a constant confining pressure of 500 kPa is presented.The evolutions of local porosities(i.e.,the porosities of regions associated with individual particles),particle kinematics(i.e.,particle translation and particle rotation)of the sample during the shear are quantitatively studied using image processing and analysis techniques.Meanwhile,a novel method is presented to quantify the volumetric strain distribution of the sample based on the results of local porosities and particle tracking.It is found that the sample,with nearly homogenous initial local porosities,starts to exhibit obvious inhomogeneity of local porosities and localization of particle kinematics and volumetric strain around the peak of deviatoric stress.In the post-peak shear stage,large local porosities and volumetric dilation mainly occur in a localized band.The developed triaxial apparatus,in its combined use of X-ray|iCT imaging techniques,is a powerful tool to investigate the micro-scale mechanical behavior of granular soils.

MICROMECHANICS ANALYSIS FOR UNSATURATED GRANULAR SOILS

This paper aims at establishing an anisotropic stress expression for unsaturated pendular-state granular soils. Using the second-order fabric tensor, we formulate a micromechanics scheme of soils with statistically averaging method, and reveal that the macroscopic average stress of unsaturated granular soils in pendular-state is not isotropic. Not only is the stress from contact forces anisotropic due to the fabric, but also the capillary stress is directional dependent, which is different from the common point that the capillary stress is isotropic. The capillary stress of unsaturated pendular-state granular soils is determined by the orientation distribution of con- tact normals, so it is closely related to the initial and induced anisotropy of soils. Finally, DEM numerical simulations of triaxial compression tests of pendular-state soils at different degrees of saturation are used to verify the existence of above anisotropy of stresses.

A discrete element model for simulating saturated granular soil

A numerical model is developed to simulate saturated granular soil, based on the discrete element method. Soil particles are represented by Lagrangian discrete elements, and pore fluid, by appropriate discrete elements which represent alternately Lagrangian mass of water and Eulerian volume of space. Macroscale behavior of the model is verified by simulating undrained biaxial compression tests. Micro-scale behavior is compared to previous literature through pore pressure pattern visualization during shear tests. It is demonstrated that dynamic pore pressure patterns are generated by superposed stress waves. These pore-pressure patterns travel much faster than average drainage rate of the pore fluid and may initiate soil fabric change, ultimately leading to liquefaction in loose sands. Thus, this work demonstrates a tool to roughly link dvnamic stress wave patterns to initiation of liQuefaction nhenomena.

糯米浆灰土无侧限压缩和三轴剪切离散元分析

糯米浆灰土是一种常见的遗址保护建筑材料,为研究糯米浆灰土压缩加载过程中的糯米-石灰胶结破坏和接触组构演化情况,采用离散单元法开展了糯米浆灰土无侧限压缩和三轴剪切离散元模拟。首先基于软胶结模型考虑糯米浆-石灰的强度特征,制备了糯米浆灰土离散元试样;然后通过参数敏感性分析为试样赋予合理的接触模型等效模量和胶结强度参数,最后对试样开展无侧限和三轴压缩模拟。结果表明:离散元模拟能再现糯米浆灰土加载试验的主要特征;加载过程中胶结破坏数量先缓后快增加,最终趋于平缓,胶结破坏呈现一定的聚集效应;无侧限压缩下胶结接触主要发生拉伸破坏,随着围压增加,剪切破坏接触数量增加;试样偏组构快速增加段为胶结破坏诱发。

Analysis of granular assembly deformation using discrete element method

The discrete element method is used to simulate specimens under three different loading conditions（conventional triaxial compression,plane strain,and direct shear）with different initial conditions to explore the underlying mechanics of the specimen deformation from a microscale perspective.Deformations of specimens with different initial void ratios at different confining stresses under different loading conditions are studied.Results show that the discrete element models successfully capture the specimen deformation and the strain localization.Particle behaviors including particle rotation and displacement and the mesoscale void ratio distributions are used to explain the strain localization and specimen deformation.It is found that the loading condition is one of the most important factors controlling the specimen deformation mode.Microscale behavior of the granular soil is the driving mechanics of the macroscale deformation of the granular assembly.

长江三角洲北翼第一硬土层理化特征及其地质成因

在长江河口两翼广泛分布第一硬土层(FHSL),研究其形成机制及工程地质特性对工程建设具有很好的指导意义。根据调查资料(钻孔935个,累计进尺42128 m)和试验资料,首次精确确认了长江河口北翼第一硬土层分布界线,研究了第一硬土层的形成年代、粒度特征、地球化学特征、工程地质特性等。研究表明:第一硬土层形成年龄为20~11 ka B.P.(多个光释光和14C测年资料);硬土层含水率随深度的增加有增大的趋势,表明气候自下向上逐渐变凉和变干;第一硬土层的颗粒级配、粒度分布频率曲线、C-M沉积图等特征显示,第一硬土层主要由粉砂、极细砂和粘土粒级组成,样品的粒度频率曲线主要呈单峰分布,反映出物质沉积前所受搬运营力性质单一,土体颗粒沉积以均匀悬浮占绝对优势,沉积环境是一种相对稳定的低能环境。第一硬土层的发育受气候控制,大致可以分为3个阶段:第1阶段(20~15 ka B.P.)为沉积与成土交替作用时期,且以沉积作用为主,硬土层剖面厚度主要受该阶段控制,至末次盛冰期结束;第2阶段(15~11 ka B.P.)为暴露成土期,这时洪水不能形成越岸沉积,加积作用基本停止,硬土层厚度不再明显增加,已形成的第一硬土区域受到频繁变迁的分合河网的侵蚀切割,形成多条不规则古河道和台地,硬土层逐渐脱水成陆,经历了风化成壤的过程;第3阶段(11 ka B.P.至今)为淹埋期,随着全新世的到来,气候变暖,海平面不断上升,硬土层被其上覆的海相沉积层掩埋,成岩作用开始直到现今。土体易溶盐含量较高,为典型氯盐渍土类型,自下而上具有从低变高的趋势,为海相层覆盖硬土层以后成岩过程造成的。