Research On The Basic Mechanical Properties Of Engineering Gravelly Soil And Dynamic Compaction Strengthening Mechanism

With the rapid development of economy, Blasting and mechanical excavation are adopted for gravel soil in China. A large artificial field is obtained from the methods of filling offshore and excavating hill. It is due to the loose and backfill uneven thickness gravel soil are filled on ground, And the understanding of constitutive relation and the mechanism of dynamic compaction are not ery clear. Thus, the theoretical and practical significance researching on this aspect is necessary. The main contents include:In this paper, the basic mechanical properties of gravel soil was researched, therein, mesoscopic and macroscopic action mechanism of dynamic compaction are studied systematacially.(1) Trough the compaction test and the direct shear test, the optimal mixing ratio of rough and fine materials was summarized. The maximum dry density and direct shear strength index of gravel soil reached the peak value at this time. It is found that generally the optimal ratio of rough materials account for 70～80% and the water content of fine soil should be lower than that of 3-4%. Through the triaxial compression and tensile test, the differences of the strength of triaxial compression and tensile were cleared revealed. The particle breakage test showed that the corresponding fragmentation parameter increased regularly with the increase of confining pressure.(2) A double yield surface elastoplastic constitutive model which is named PMC-Clay model is rasied. According to the failure mechanism of stress space, the model can be divided into 3 zones:ensile failure, the shear failure and the volume yield zone. PMC strength theory is used to determine the shear yield boundary in this model, and the modified Cambridge model is used to the volume yield boundary using tensile strength in the tension region. different plastic flow rule is constructed for three damage regions:associated phase flow rule using in the tensile failure zone, non associated plastic flow rule using in shear failure zone, and associated plastic flow rule using in the Volume yield area. The hardening and softening criterion is based on the true strength theory using the assumption of plastic strain hardening, and use the list form setting. Two parameters fitting methods were presented according to the experimental data.(3) Under the action of dynamic compaction, the compacting mechanism of the gravel soil includes 4 kinds of action:the particle breakage, the change of particle arrangement and contact, the change of the pore and movement of particles. The particle breakage test reveals the positive correlation between relative particle breakage rate and confining pressure, and determines the reasonable fitting formula. The particle breakage can improve the gradation of gravel soil, making it easier to reach the dense state, which is an important micro mechanism of treatment by dynamic compaction of gravel soil. Throughing the secondary development of particle flow theory PFC2D, the model of granular flow model is established based on the contact model of particles hysteretic type damping. The simulation results show that the strengthening mechanism of the gravel soil compaction is actually an interactional result of the increasing number of particles in this area and directional arrangement of long axis of the particles.(4) The numerical simulation analysis of the dynamic compaction is carried out by accepting the PMC-Clay model into software FLAC3D. The macro mechanism of dynamic response, deformation and compaction of gravel foundation under the action of dynamic compaction are studied. Through the group dynamic numerical model, the strengthening effect of the spacing between different arrangement of tamping points were compared, the effective and economic arrangement of tamping points parameters were determined. By using the similar principles, indoor model test device of dynamic compaction method is designed and the development and distribution rule of soil displacement and stress in the process of compaction are studied. The effective reinforcement depth calculation formula is put forward, which considers the factors such as the energy of the dynamic compaction, the soil characteristics and the size of the ram. Through 7 case tests, the formula calculating value is in good agreement with the measured results, which proves the rationality and accuracy of the formula in the description of the effective reinforcement depth of the gravel soil.(5) Dynamic compaction engineering tests of coarse gravel soil in different content were carried out. The test results showed that the dense state, the average ray velocity of the foundation soil, the bearing capacity and deformation modulus in the test rammer area using optimal mixed proportion of coarse and fine materials are obvious bettrer comparing with the other test results.