Ground Response And Effect To Dynamic Compaction Of Saturated Silty(Sandy) Soils

The main objective of this paper is to improve the theoretical system and design method of dynamic compaction(DC).Firstly,the synthetical analysis of relevant information about the present theory and application study on DC is conducted.Then,considering saturated silty(sandy)soils as research subject,its characteristic under DC is studied through numerical analysis and field data.A new constitutive model related to soil mechanical properties in DC has subsequently been built.Further,the effect of DC on drainage consolidation of saturated soils is investigated based on improved numerical model.Finally,the design method to ascertain DC parameters in construction stage is proposed.The main contributions of this thesis are described as follows:(1)The development of DC in china will turn into the application of high energy level DC technology and DC method combined with dewatering through a deep analysis on development trends and characteristics of DC.Meanwhile,a database of 182 case histories of DC in china is presented.Some relationships between the DC construction parameters are analyzed based on these field data.These include the correlation of depth of improvement as a function of square root of energy level,the relation between hammer weight and drop distance,the relation between hammer diameter and energy level,the plot of normalized crater depth versus number of drops,the relation between hammer diameter and grid spacing,and the effective depth of improvement in various engineering projects.Many figures obtained from these statistical analyses offer a practical approach for geotechnical engineers to select initial DC construction parameters.(2)A parameter estimation method using multiple sources of field data based on a coupled hydro-mechanical model of DC and the Pareto multi-objective optimization technique is proposed to analyze dynamic properties of soils under DC.The hydro-mechanical model of DC is established by integrating the extended Biot dynamic equations,the CAP model and the contact force algorithm.The multi-objective optimization algorithm,Non-dominated Sorted Genetic Algorithm(NSGA-IIa),is integrated with the numerical model to identify soil parameters using multiple sources of field data.The proposed parameter estimation method may provide a rational tool to determine in situ dynamic properties of soils during DC,and overcome some limitations of the laboratory test and field test used in investigating DC problems.In addition,the proposed numerical method with CAP model underestimate the stiffness of soil mass under the condition of high strain rate loading and cannot well reflect the characteristics of soils by continual impact.(3)A new constitutive model by modeling the phenomenon of soils under DC is presented.The proposed model was augmented with Perzyna's viscoplastic formulation and soil stiffness reassignment equation based on CAP model for considering the strain rate effect and the variation of material properties during the process of DC,respectively,and then it was used to develop user-subroutine material of the FEM program LS-DYNA.The improved constitutive model is subsequently calibrated by static soil test data,dynamic experimental data and field test data.(4)The growth and dissipation of pore water pressure in saturated soils at the impact-consolidation process is investigated using a coupled hydro-mechanical model of DC with the improved constitutive model.The whole process of DC in saturated soil is continuous which consists of two stages,i.e.,transient loading due to impact,consolidation after the impact.At the impact stage of DC,the pore water pressure rises instantly to a moderately high level and drops sharply to zero or even negative values for a very short period of time,then enters the fluctuation process until remains almost constant.Subsequently,the consolidation begins to occur and the residual pore water pressure will decreases gradually with the increase of time.Further,the influences of some parameters on consolidation effect of saturated soils were investigated in detail.Results show the more tamping energy have been conduct,the greater pore water pressure have been induced,which making slow down the dissipation of pore water pressure and the strength growth of saturated soils.The bigger hammer radius produce the effect of pore water pressure to extend wider,but also slightly less deep.The permeability does not play an important role in the generated pore water pressure at the impact stage,although it has a great influence on the pore pressure dissipation at the consolidation stage.Moreover,the drainage structure introduced in the DC technique can accelerate the dissipation of excess pore water and possess the optimum depth and distance from the impact point.(5)A numerical investigation into the influence factors on the compaction effect and construction efficiency of silty(sandy)soils due to DC,and some suggestions applicable to DC of silty(sandy)soils are presented.For DC of silty soils,the key lies in how to deal with the relationship between the dissipation of pore water pressure and the increment of effective improvement depth.The applicability of the proposed suggestions is illustrated by comparing its construction scheme with primary program of DC applied in a hydraulic reclamation project of Shanghai.Results reveal the practicality of the proposed suggestions.In addtion,for DC of sandy soils,a developed method of estimating ground deformation caused by DC is presented.Firstly,five deformation variables are defined to describe characteristics of ground surface deformation.An extensive parametric study is then conducted to investigate the effect of each parameter on the five deformation variables.These include soil type and properties,tamper base area,momentum and energy per blow,continuous tamping.Finally,based on the results obtained,a forecast model is produced to describe ground deformation under DC.The applicability of the proposed procedure is then illustrated by comparing its predictions with two cases of DC applied in the field.The results of these comparisons indicate that the predictions using the developed method are reasonably realistic.