The Mechanical Characteristics Of Rockfill Materials Under True Triaxial Stress Conditions

Rockfill is a material with a wide range of sources and abundant reserves has been widely used in engineering practice(e.g.,earth-rockfill dams,subgrades,weak foundation treatments,and high backfills)owing to its properties of high compaction density,high permeability,high sh ear strength,and strong anti-liquefaction capacity.To date,the static behaviors of rockfill materials have been extensively investigated using the conventional triaxial testing method.However,the rockfill usually undergoes complex true triaxial stress states in practical engineering applications,and its mechanical behaviors are quite different from those shown in the results of conventional triaxial tests.Therefore,in this dissertation,to investigate the mechanical Characteristics of rockfill materials under general stress conditions,a new large-scale static and dynamic true triaxial apparatus at Tsinghua University(THU-SDTTA)was thoroughly tested and further developed,and sets of conventional triaxial and true triaxial tests were conducted.The main work and contribution of this dissertation are as follows.(1)The core device and key testing method for the true triaxial apparatus were developed.The innovative design of jointly employing the oval caps and special-shaped latex membrane effectively solves series of technical problems during true triaxial tests,such as specimen sealing in high pressure,device assembling and disassembling,pore-water pressure measuring,back pressure applying,deformation uniforming,and sensor installation.To further improve the measurement accuracy of displacement and volume change of the specimen,water submersible linear variable differential transformers(LVDT)were added in the minor principal stress direction.Then,various types of static and dynamic true triaxial verification tests were carried out to verify the reliability,accuracy,and applicability of the whole mechanical system,the electronic system,the loading system,and the newly designed sealing device.(2)To study the mechanical characteristics of rockfill materials under complex stress conditions,especially for the condition of stress state changing of the rockfill materials during construction and impounding of earth-rockfill dams,sets of tests,including conventional triaxial and true triaxial isotropic compression tests,compression tests in different directions,complex stress path tests of loading-loading and loading-unloading,and ?-plane shear tests,were conducted separately.The stress and deformation properties of rockfill under true triaxial complex stress paths were systematically investigated.Moreover,comparing the results of the two different types of tests,the influence of the factors such as the intermediate principal stress and the inherent and stress-induced anisotropy on the mechanical characteristics of rockfill materials was analyzed.(3)The particle breakage mechanism of rockfill materials with different stress paths was studied based on the changing of grading curves during conventional triaxial tests.In addition,the concepts of the axial length and the inclination angle were adopted as parameters to statistically analyze the distribution characteristic of shape and direction of rockfill particles.Then,combined with the triaxial test results,the anisotropy mechanism of rockfill was discussed.(4)Three constitutive models(i.e.,Duncan-Chang E? model,Duncan-Chang EB model,and Nanshui double yield surfaces elastoplastic model)commonly used in the analyses of earth-rockfill dams in China were adopted to simulate the complex stress path tests under conventional triaxial and true triaxial conditions.The applicability of the three constitutive models for rockfill under complex stress conditions was probed.Moreover,combined with the test results,the inherent and stress-induced anisotropy,the stress-strain behavior,etc.,of rockfill under complex stress path conditions were studied.