Study On Vibration Propagation And Reinforcement Mechanism Of Dynamic Compaction

Dynamic compaction(DC),as a common method of foundation reinforcement,is widely used in various large-scale construction projects.But at present,the understanding of dynamic consolidation mechanism is not deep enough.At present,the understanding of dynamic consolidation mostly focuses on the changes of static parameters before and after dynamic compaction,and there is little research on dynamic process of DC.Moreover,the calculation method of the reinforcement range of dynamic compaction mostly stays in the one-dimensional state by calculating the reinforcement depth,and there is no more effective method for the whole reinforcement extent.Based on 973 project "Research on Key Foundation Problems of Deformation and Stability Control of High Fill in Mountain Regional Airport",through two airports under construction(Chengde Airport and Beijing New Airport),this paper studies the propagation of vibration caused by dynamic compaction in soil-rock mixture and silt foundation,and puts forward a new understanding and calculation of reinforcement mechanism.The main contents of this paper are as follows:(1)For three different dynamic compaction levels(1000,1500,2000 k N·m),large-scale field tests on silt foundation were carried out at Beijing New Airport.The time history curve of vibration acceleration of tamping hammer,the propagation process of internal vibration acceleration and velocity of two-dimensional(vertical and horizontal)soil and the settlement law of tamping crater were obtained and analyzed.Pressuremeter tests and standard penetration tests show that the overall properties of the soil in the reinforced area have been significantly improved;the variation of the internal vibration of the soil is consistent with the settlement of the crater,showing a state of first increasing and then stabilizing;when the internal vibration acceleration and velocity of the soil exceed a certain value,the reinforcement of soil properties will occur;(2)The strengthening mechanism of dynamic compaction can be explained as that the hammer first gives momentum to the soil inside the compaction pit,and then pushes it into the surrounding soil.In the process of momentum transformation between the particles in the surrounding soil,irrecoverable displacement can be called displacement diffusionprinciple.According to this principle,the shape of the compaction crater is used.Through the verification of four groups of laboratory tests,it can be seen that the mathematical model can effectively calculate the settlement in dynamic compaction.(3)based on the field test of Chengde airport,the deformation of tamping crater is modeled by 3D scanning technology,and the assumption of tamping crater in settlement calculation model is verified;the settlement of hammer is measured,and the model can be used in the field calculation;on the basis of determining the reinforcement scope,the deformation of tamping crater is verified.The parameters in the model are inverted and the critical displacement of 0.04 m is obtained.The values are compared with the data in the literature,and the results are consistent.(4)The consolidation process of dynamic compaction is simulated by using particle flow software.The regularity of porosity and dynamic development during dynamic compaction is analyzed from the micro point of view and the vibration rules between the combination modes of the weight of hammer and height of falling are compared.Under the effect of dynamic compaction,the porosity is becoming smaller with the increase of distance to the crater.The effect of reinforcement is mainly under the hammer and concentrates in the range of 3-4 m in the lateral zone.The decay of vibration caused by "big weight with low falling height" is slower than that caused by "small weight with high falling height",so it can spread farther and reinforce deeper.