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RESEARCH ARTICLEExperimental study on compaction-induced anisotropicmechanical property of rockfill materialXiangtao ZHANG,Yizhao GAO,Yuan WANG,Yu-zhen YU*,Xun SUNState Key Laboratory of Hydroscience and Engineering,Department of Hydraulic Engineering,Tsinghua University,Beijing 100084,China*Corresponding author.E-mail: Higher Education Press 2021ABSTRACTThe anisotropy of rockfill materials has a significant influence on the performance of engineeringstructures.However,relevant research data are very limited,because of the difficulty with preparing specimens withdifferent inclinationangles using traditional methods.Furthermore,the anisotropy test of rockfill materials is complex andcomplicated,especially for triaxial tests,in which the major principal stress plane intersects with the compaction plane atdifferent angles.In this study,the geometric characteristics of a typical particle fraction consisting of a specific rockfillmaterial were statistically investigated,and the distribution characteristics of particle orientation in specimens preparedvia different compaction methods were examined.For high-density rockfill materials,a set of specimen preparationdevices for inclined compaction planes was developed,and a series of conventional triaxial compression tests withdifferent principal stress direction angles were conducted.The results reveal that the principal stress direction angle has asignificant effect on the modulus,shear strength,and dilatancy of the compacted rockfill materials.Analysis of therelationship between the principal stress direction angles,change in the stress state,and change in the correspondingdominant shear plane shows that the angle between the compacted surface and dominant shear plane is closely related tointerlocking resistance associated with the particle orientation.In addition,different principal stress direction angles canchange the extent of the particle interlocking effect,causing the specimen to exhibit different degrees of anisotropy.KEYWORDSrockfill,inclination of specimen preparation,anisotropy,mechanical property,mechanism1IntroductionChina is rich in rockfill materials,which have manyexcellent engineering properties,such as high compacteddensity,high shear strength,good permeability,andfavorable liquefaction resistance 1,and have thereforebeen widely used in high-fill,high-earth-rockfill dams andrailway foundations,among many other engineeringapplications.This paper focuses on research on compactedrockfill in earth-rockfill dams.Both the concrete-facerockfill dam and core-wall rockfill dam are two commondam types in China,occupying an increasingly largeproportion of high dams built or under construction in thecountry.As the main filling materials of core-wall rockfilldams,rockfill materials usually account for more than two-thirds of the entire dam body,whereas in concrete-facerockfill dams,the proportion of rockfill materials can reachapproximately 99%of the entire dam body.In actual damconstruction processes,strong compaction causes therockfill particles to form distribution features with longaxes favorable along the horizontal direction,which enablethe mechanical properties of the rockfill to be anisotropic.(An important point to note is that the anisotropicformation mechanism of rockfill differs from that ofrocks in the earths crust 2.)Therefore,a rockfill dam isusually characterized by high density and transverseisotropy as a result of strong horizontal-layered compac-tion during the construction process.A rockfill dam is characterized by a complex stress stateduring the construction and water impounding processes.The major principal stress plane is not necessarilyconsistent with the compaction plane.The major principalstress plane in a rockfill dam is almost identical to thehorizontal compaction plane during the filling period of theArticle history:Received Dec 20,2019;Accepted Mar 22,2020Front.Struct.Civ.Eng.https:/doi.org/10.1007/s11709-021-0693-0dam,but changes significantly during the impoundingperiod.For a concrete-face rockfill dam,the upstreamwater pressure acts vertically on the inclined concrete faceduring the impounding period,which enables the majorprincipal stress plane in the rockfill behind the face todeviate from the compaction plane.For a core-wall rockfilldam,the upstream rockfill is subjected to upward buoy-ancy during the impounding period,allowing morehorizontal displacement to occur because of the waterpressure acting on the core wall.Consequently,thedirection of the major principal stress in both the upstreamand downstream rockfills may vary significantly.Further-more,once the constraint effect of the river valley isconsidered,changes in the major principal stress directionbecome more complicated.Both theory and practice have demonstrated that theanisotropy of rockfill materials has a significant influenceon the mechanical behavior of rockfill dams 1.Forexample,when the mechanical behavior of a high rockfilldam is analyzed using a numerical method such as thefinite element method(FEM),the calculated horizontaldisplacement and its relative relationship with the verticaldisplacement are far from the measured results.One of theimportant reasons for this deviation is that the anisotropyof the rockfill is not sufficiently considered in theconstitutive model.Therefore,the anisotropy of rockfillmaterials due to construction compaction has attractedmuch attention from engineers and academic communitiesin recent years.In particular,with the construction ofincreasingly higher rockfill dams,it has become moreurgent to perform research on the anisotropic properties ofrockfill.Researchers have been studying this topic from a varietyof perspectives.For example,Yang et al.3 and Jia 4conducted a variety of complex stress path tests toinvestigate the mechanical properties of rockfill undercomplex stress conditions and their effects on rockfilldams.With regard to the application of constitutive modelsto complex stress paths,many test results have shown thatmost existing constitutive models can be used toeffectively simulate properties along stress paths underthe condition of strain control or in a conventional triaxialstate 58.However,existing constitutive models are verydifficult to use for accurately simulating mechanicalproperties under some of the more complex stress paths.Thus far,few experimental studies have been conductedon the anisotropic mechanical properties of rockfillmaterials.On the other hand,available research data onthe anisotropic properties of soil have been focused mainlyon clay and sand 5,6,911.For example,Duncan andSeed 12 determined that anisotropy and stress reorienta-tion may cause the undrained strength of clay to changewith the direction of the failure plane.To study theanisotropy of sand,a variety of tests have been conductedusing different types of equipment,such as direct shearapparatus 13,14,plane strain tests 15,true triaxialapparatus 1621,and hollow cylinder torsional shearapparatus 2229.For example,Oda et al.15 investi-gated the anisotropic properties of sand using plane straintests,in which dense samples of Toyoura sand wereprepared in a tilting mold to provide different directions ofsample deposition with respect to the principal stress axes.Chaudhary et al.30 used a hollow-cylinder torsionalshear device to study the influence of initial fabric andshear direction on the cyclic deformation characteristics,such as the stressstrain response,shear modulus,anddamping ratio,of medium-density Toyoura sand.Shi 6used a true triaxial apparatus to conduct single-directionloading tests on coarse-grained soil in different principalstress directions and studied the stress-induced anisotropyof coarse-grained soil.Yang 28 employed differentmethods with an image-analysis-based technique and anappropriate mathematical approach in the preparation ofsand specimens in the laboratory,and quantitativelymeasured and compared the fabrics of the sand specimensat a microscopic level.They observed that the specimenprepared via dry deposition had a more anisotropicmicrostructure than that of the specimen prepared viamoist tamping.Yang et al.31 conducted a series oftriaxial andtorsional shear tests onToyoura sandto explorethe relationship between soil response and fabric aniso-tropy.Based on their results,the differences in theundrained stressstrain response among the differentlyprepared specimens were attributed to fabric anisotropy.Meanwhile,Suwal and Kuwano 32 measured theYoungs modulus and Poissons ratio in all three directionsof coarse-grained soil specimens by applying elastic wavesusing a disc-type sensor,and compared and analyzed thevertical and lateral differences of coarse-grained soil.Computational simulation,which is the third primarymethodology for solving a wide range of scientific andengineering problems 3336,can applied to the study ofthe anisotropy behavior of coarse-grained soils.Severalnumerical experiments investigating the properties ofgranular materials have already been conducted.Forinstance,Zhang 37 used the discrete element method toconduct numerical simulations of true triaxial tests ongranular materials under different shear modes and stresspaths,and investigated the influence of intermediateprincipal stress on the anisotropy and anisotropy strengthcharacteristics of granular materials under complex stressstates.Chu et al.38 proposed an elastoplastic modelsuitable for coarse-grained soil based on initial results ofanisotropy research on fine-grained soil,which may reflectthe anisotropy state of coarse-grained soil.Zhang et al.39modified a double-yield-surface elastoplastic constitutivemodel via stress transformation and introduction of a newstress ratio parameter according to experimental data,suchthat the anisotropy of coarse-grained soil is accounted forin the study.With regard to rockfill materials,on the other hand,whereas the aforementioned computational simulation2Front.Struct.Civ.Eng.methods may be extended to describe some anisotropicmechanical properties of rockfill materials,little directexperimental evidence exists to verify whether thisapproach is reasonable.Oda and Nakayama 40 listed the following three mainsources of inherent anisotropy in granular materials:1)anisotropic distribution of contact normals,representingthe relationship among particles;2)distribution directionof the long axis of pores;and 3)distribution direction ofthe long axis of non-spherical particles.Through a biaxialcompression test of a two-dimensional bar assembly,Odaet al.41 also observed that the inherent anisotropy causedby 1)and 2)tended to disappear completely at the earlystage of inelastic deformation,whereas the inherentanisotropy caused by 3)remained into the final stage ofdeformation.Therefore,the anisotropy,which significantlyinfluences the mechanical properties of compacted rockfill,is caused mainly by the orientation of the particles.Inaddition,in the construction process,the degrees oforientation of rockfill particles due to different compactionintensities are different,and thus the degrees of influenceof the anisotropy of the materials on their engineeringcharacteristics are also different.This relationship indicatesthat the anisotropy of rockfill materials can have asignificant influence on the engineering performance ofhigh earth-rockfill dams,although relevant research workwill be necessary to verify this hypothesis.However,thusfar,little has been achieved on the most basic research databecause of the difficulties with preparing specimens withdifferent inclination angles using traditional methods.Furthermore,anisotropy tests on rockfill materials,espe-cially triaxial tests with different angles between theprincipal stress plane and the compaction plane,tend to becomplex and complicated.In laboratory triaxial tests to investigate the anisotropicproperties of soil,the traditional method of specimenpreparation is to deposit or to compact soil in a largecontainer,and then to cut the required specimen at differentinclination angles.In the case of sand,the specimen shouldbe frozen or sprayed onto a tilting mold before cutting.Thedisadvantages of using these methods are as follows:thestructure of granular materials may be destroyed duringcutting,and a specimen containing large particles cannotbe cut in a way that reflects the material characteristics.These limitations imply that it is not feasible to usetraditional methods,such as freezing,cutting,or tiltingmold cutting,to prepare specimens of rockfill materials,which are characterized by large particle sizes and nocohesion,at different inclination angles.Moreover,whereas torsion shear tests using a hollow cylinder canalso be applied to study the mechanical properties of sandor clay specimens when the principal stress plane intersectswith thecompaction planeat different angles,nosuchlargehollow-cylinder apparatus exists thus far for large-particle-size materials such as rockfill materials.Based on the aforementioned considerations,the geo-metric characteristics of the typical particle fraction of acertain rockfill material were first analyzed in this study.The distribution features of the particle arrangementdirections in simulated specimens with different compac-tion intensities were then investigated.To examine thehigh-density rockfill material,a set of specimen prepara-tion devices,which can enable the compaction plane andbottom plane of the rockfill specimen to have differentinclination angles,was manufactured.A series of conven-tional triaxial compression tests,in which the majorprincipal stress plane intersects with the specimencompaction plane at different angles,were then conducted.Based on relevant test results,the influence of the majorprincipal stress direction angle,i.e.,the angle between themajor principal stress plane and the compaction plane,onthe mechanical properties of the compacted rockfillmaterial was investigated,and the formation mechanismof the anisotropic shear strength of the compacted rockfillmaterial was analyzed.2Statistical analysis of geometriccharacteristics of rockfill particles andsimulated specimenThe rockfill materials used in this research were obtainedfrom the main rockfill material ground of a planned 300-m-high earth-rockfill dam in south-west China,with amaximum particle size of 600 mm in the originalgradation.Scaling the original gradation was necessarybecause of the limitation of the test equipment,and thus amaximum particle size of 40 mm was imposed on the testspecimen.2.1Particle shape features of tested rockfill materialIn most projects,rockfill materials are obtained viamechanical crushing after the rock mass is blasted.Theresulting rockfill particles are irregular and angular inshape,with axial length ratios greater than 1 and withmany angles.Figure 1 shows photos of typical large-andmedium-sized particles in the specimen.To quantitatively analyze the shape features of therockfill particles,three axial lengths were measured.Asshown in Fig.1,
