Research Of Dynamic Stress Propagation Law Of Dynamic Compaction Method To Cohesionless Soil Subgrade

Dynamic compaction method is let the hammer fall freely from a certain height, to give the vibration energy to the foundation, and make strong shock waves and dynamic stresses in the foundation soil. Dynamic compaction method is a method to improve the strength of foundation soil, reduce compressibility, improve the liquefaction resistance of sand soil, eliminate the collapsibility of the collapsible loess, and also can improve the uniformity of the soil layers and reduce the possible differential settlement. In recent years, dynamic compaction method has been widely used for its low cost, simple operation and a series of advantages.At present, there are a lot of problems in the dynamic compaction theory research. The most concerned problems in the engineering application of dynamic compaction method are how to estimate the effective reinforcement depth and the influencing region, so as to make the reasonable compaction site plan, determine the tamping points spaces, tamping times and other parameters. But at present there is still no effective engineering practical quantitative model and accurate effective reinforcement depth formula, and can only estimate by empirical and semi-empirical formulas. Due to the numerous factors that may affect the consolidation effect, there is no results provide the quantitative understanding of the influence degree of various factors. So it is necessary to analyze the sensitivity of the various factors of dynamic compaction reinforcement effect, thus to establish an accurate formula to calculate the effective reinforcement depth. This has great practical significance.In this paper, the dynamic compaction field test was designed combined with the9th contract section located in Weifang Shandong Provice of QingLin express way (from Qingzhou to LinShu). In the test, dynamic stress was tested by the dynamic strain gauge to research the propagation law of cohesionless soil. The following conclusions were obtained:dynamic stress increased with the increase of the tamping energy, decreased with the increase of the vertical depth, and the attenuation rate gradually decreased. Within the tamping energy of2000-2800kN-m, the effective reinforcement depth of cohesionless soil was about6m and the influence depth was more than8m. Dynamic stress decreased with the increase of radial distance, and the attenuation rate gradually decreased. Dynamic stress’s attenuation rate along the radial direction was much larger than the vertical direction. Within the tamping energy of2000-2800kN-m, the effective reinforcement width of cohesionless soil was about3m. The space distribution of the dynamic stress when dynamic compaction method used to cohesionless soil was almost a "calabash".FLAC3D finite difference software was used to do the numerical simulation, compared the results with the field test results, the accuracy of the numerical model was verified. The main factors influencing the effective reinforcement depth of dynamic compaction were analyzed based on the numerical simulation results of different working conditions, the following conclusions were obtained:the value of the effective reinforcement depth was respectively proportional to the hammer’s weight and drop distance, and was inversely proportional to the hammer’s bottom area. A heavy weight hammer dropped from a low distance had better reinforcement effect and larger effective reinforcement depth than a light weight hammer dropped from a high distance. The orthogonal design method was used to analyze the sensitivity of soil parameters (soil density, compression modulus, Poisson’s ratio and the internal friction angle etc.) to the effective reinforcement depth. Through the range analysis method, Poisson’s ratio and density were obtained to be the most sensitive influence factors of the effective reinforcement depth.Combined with the analysis of the numerical simulation and the traditional empirical formulae, a new dimensional uniform formula for the dynamic compaction effective reinforcement depth of cohesionless soil was established. After the contrast verification, the results show that the formula can almost accurately calculate the effective reinforcement depth of cohesionless soil, and it can provide certain reference for the future similar dynamic compaction engineering.