| The method of surcharge preloading combined with dynamic compaction is a joint soft ground treatment method in recent years.This method combines the two methods of surcharge preloading combined with dynamic compaction,and after the soft subsoil was treated by the method of surcharge preloading and achieves a certain consolidation degree,Dynamic compaction method was used for soft subsoil.This kind of ground treatment Not only has the advantages of low cost and simple construction,but also has a good reinforcement effect and has been applied in some domestic projects.In terms of the two methods alone,the reinforcement mechanism is relatively mature and the number of engineering applications is numerous.However,the design method and reinforcement mechanism for the joint treatment of soft subsoil are still in the research stage.The design theory is far behind the engineering practice,Especially the effect of the value of the design parameters of the joint processing method on the reinforcement effect is more complicated.Therefore,it is necessary to carry out systematic research on this method and give a more reasonable design scheme and parameter values.In this paper,based on the actual project of soft ground treatment by the method of surcharge preloading combined with dynamic compaction,the field tests and the laboratory soil test were carried out.Combined with numerical simulation and GPR model prediction methods,the reinforcement mechanism of soft subsoil,reasonable range of design and the evaluation of treatment effects were comprehensively analyzed.It will provide the basis for the theoretical study of the method and the optimal design of scheme.The main work of this article and its conclusions are as follows:1.According to the actual engineering condition and the ground treatment plan,the experimental scheme of surcharge preloading combined with dynamic compaction of soft subsoil was designed to lay the foundation for comprehensive testing of the physical and mechanical property indexes of soft subsoil ground after treatment in different stages.The details are as follows:(1)The surcharge preloading stage test plan was designed.Monitoring of ground settlement,layered settlement,and pore water pressure were conducted during the surcharge preloading stage.The physical and mechanical property indexes of the soil were measured experimentally in lab after the surcharge preloading andU_t of the muddy soil layer reached 0.8 and 0.9 respectively.Standard penetration test,vane shear test and static penetration cone penetration test were performed at the site to analyze the treatment effect.(2)Dynamic compaction stage test plan was designed.WhenU_t(28)8.0or U_t(28)9.0respectively,the test area is selected to carry out the dynamic compaction test.The parameters of dynamic compaction test are:whenU_t(28)8.0,the single point ramming energy can be 2000kN.m,2200 kN.m and 2400 kN.m.The number of the single point ramming was 5,the number of ramming times was 2.whenU_t(28)9.0,the single point ramming energy can include 2000kN.m,2200kN.m,2400kN.m,and the number of single point ramming was 5.In addition the single point ramming energy was 2400kN.m,the number of the single point ramming was 6 and 7 respectively,and the number of times was 2.During process of the dynamic compaction,the ramming settlement of single point and the pore water pressure in the muddy soil layer were monitored.After the completion of the dynamic compaction,the laboratory soil test was conducted according to the dissipation of the excess pore water pressure,including the density,water content,and the combined liquid-plastic limit test.In-situ tests included Standard penetration test,vane shear test and static penetration cone penetration test.2.According to the designed test plan,in-site test and laboratory soil tests were carried out at each stage of ground treatment.Through the analysis of the test data,the change law of the bearing properties of soft subsoil is given and the range of values of important design parameters for ground treatment is determined.The specific research conclusions are as follows:(1)During the surcharge preloading period,the insertion of the plastic drainage belt significantly increased the drainage and consolidation speed of the muddy soil layer.The maximum settlement at the monitoring point was 712.6mm,the minimum settlement was 482.6mm,the average settlement was 577.6mm,and the preloading treatment was effective.It can reduce post-construction settlement and improve the stability and bearing capacity of soft subsoil.The law of consolidation settlement over time has two types of curves:three-stage type and two-stage type.The main reason for the analysis to produce this change rule is due to different geological conditions on site.(2)The layered settlement values within the depth range of the muddy soil layer were larger,the settlement value of the uppermost part is 118.5mm,the lowest part is13.5mm,and the value of the layered settlement within the 4m upper part of the muddy soil layer is larger,from the monitoring results,the consolidation of drainage is more significant,and the improvement of physical and mechanical properties of the soft subsoil is most obvious in this range.(3)According to the analysis of the monitoring results of the 8 groups of dynamic tests,whenU_t(28)8.0?U_t(28)9.0,the increase of excess pore water pressure which located within 9m from the surface(within 4m in the upper part of the muddy soil layer)is obvious after the dynamic compaction,and The spatial distribution is roughly bubble-shaped;The single point ramming energy selected by the dynamic compaction is not the bigger the better,and when it reach a certain value,and then increase the single point ramming energy has little effect on the excess pore water pressure,and the early stage the smaller the degree of consolidation achieved by surcharge preloading is,the more obvious the law is;The test results show that when the consolidation degree reaches 0.8 or more,the influence depth of dynamic compaction treatment can meet the requirements of engineering reinforcement depth.The reasonable range of ramming point spacing is 6m-8m.(4)On the condition of E(28)2400k N?m,the excess pore water pressure in muddy soil layer increased with the number of ramming.It was shown by the curve of the ramming settlement with the number of ramming,when the number of the single point ramming exceeded 5,the soil was failure and it caused a sudden increase of ramming settlement,and the increase in the excess pore water pressure in the soft soil was no longer significant.So in practical projects,it is necessary to determine the reasonable number of single point ramming based on the on-site geological conditions to make the soil reinforcement effect optimal.(5)On the conditions of the practical project,when the number of the single point ramming is 5,the excess pore water pressure at different depths reached more than 50%after 11 days,especially in the upper 4m range of muddy soil.It can reach more than 70%.According to the situation of dissipation of excess pore water pressure,the second time of ramming can be reasonably arranged.(6)The indexes of the laboratory soil test and the field tests change regularly with the parameters of the dynamic compaction test.The test results of various indexes showed that the increase of the energy and the number of the single point ramming can significantly improve the treatment effect of the muddy soil overall,but at the same time,when the single point ramming energy is larger,excessive number of the single point ramming may cause failure of the soil.At this time,increasing the number of single point ramming not only had no significant effect,but also may worsen the treatment effect.(7)From the analysis of comprehensive test results,it can be seen that,for this project,whenU_t?8.0,the compressive modulus and the bearing capacity characteristic value was fully able to meet the engineering requirements after a reasonable ramming energy and the number of ramming were applied to the soft subsoil and the excess pore water pressure was completely dissipated.3.Considering the changes of physical and mechanical parameters of soft subsoil in different processing stages,a numerical analysis model of surcharge preloading combined with dynamic compaction of soft subsoil was established.The feasibility of the model was verified through comparative analysis of actual measurement and simulation results.Under the condition of different parameters,further analysis was made that the effect of soft subsoil treatment and the main physical and law of change of physical and mechanical property indexes with the change of design parameters,the specific research conclusions were as follows:(1)The results of numerical analysis shown that after the surcharge preloading stage,the drainage consolidation is mainly performed horizontally and vertically after the plastic drainage belt is set in the muddy soil layer.The setting of the plastic drainage belt significantly accelerated the drainage consolidation speed.As the drainage consolidation progressed,the more the soft soil layer was in the upper part,the higher the excess pore water pressure dissipated.Consolidation settlement of the soil occurred at the same time as the drainage consolidation.The error of the simulated settlement was 6.37%compared with the measured value.The layered settlement value of the muddy soil layer is basically the same as the measured value.The simulation results showed that the established numerical model had high calculation accuracy.(2)According to the curve of the depth of the excess pore water pressure in soft subsoil under different consolidation degree,it was shown that although the value of the excess pore water pressure was the largest in the muddy soil upper layer,the dissipating velocity was faster near the upper layer,so at the late stage of drainage consolidation,the excess pore water pressure increased with depth,but the value is much less than the initial state overall.(3)Using the improved formula based on structural dynamics to calculate the maximum contact stress between the rammer and the soil surface,according to the impulse form of the impact load,the load was simplified to a half sine wave applied to the surface of the soil to simulate the dynamic compaction during dynamic numerical analysis.The time history curve of the contact surface of the rammer under strong ramming load was calculated.The development law of the curve was the same as the actual measurement rule of the dynamic compaction,which proved that the dynamic load was reasonable.(4)After the application of dynamic loading,the excess pore water pressure in the muddy soil layer increased sharply in the upper part and increased with the number of ramming.From the contour maps of excess pore water pressure in muddy soil layers and the distribution rules along the depth and horizontal direction,it can be concluded that the effective reinforcement depth of dynamic compaction was generally in the range of8m-9m.This range basically included the upper 4m range of the muddy soil layers.The results showed that the depth of reinforcement of the ramming energy used can meet the engineering requirements.In addition,under the condition of different number of ramming,the influence range of the excess pore water pressure in the horizontal direction was more than 3m,and accordingly,the construction spacing can be reasonably selected.(5)On the premise of verifying the correctness of the model,the models under different conditions were calculated and analyzed.The results showed that,with the increase of the consolidation degree achieved after preloading,the ramming settlement decreased with the same number of ramming and the same ramming energy.When the ramming energy was constant,the ramming settlement did not decrease with the number of ramming.When the number of ramming reached a certain value,the further ramming will cause the soil to be failure,and the ramming settlement will continue to increase.According to the simulation results,the muddy soil layer the lower the degree of consolidation,the less the number of ramming when the soil was failure.in addition,for the soft subsoil,because of its low shear strength,the ramming settlement may not be significantly converged under certain ramming energy.The value of the single point ramming settlement cannot be used as the control criterion of stopping ramming.The higher the degree of consolidation of the soft subsoil,the greater the depth of transmission of the ramming energy,according to the numerical simulation results,When the consolidation degree reached 0.7 or more,the strengthening effect of dynamic compaction can be obtained.The modulus of the upper layer had a significant effect on the reinforcement effect,when the modulus was larger,the covering layer had a diffusion effect on the ramming energy,resulting in an increase in the influence radius and reduce in the influence depth,so it needed to be appropriate increasing the single point ramming energy to increase the influence depth.When the muddy soil layer was reinforced by the dynamic compaction method,it was not that the larger the ramming energy,the larger the excess pore water pressure generated,it should select the appropriate ramming energy to achieve the ideal excess pore water pressure according the actual conditions.4.The Gaussian process regression model(GPR)was introduced into the prediction of the compressive modulus and the characteristic value of subsoil bearing capacity.According to the design parameters and experimental test results,data samples were established.The main parameters were input samples,the compressive modulus and the characteristic value bearing capacity after the soft subsoil was reinforced were output samples,the model was trained and started forecasting after the prediction results met the accuracy requirements,and compare the prediction results with methods of BP neural network and LS-SVM.The specific content and conclusions are as follows:(1)The accuracy of the compressive modulus and the characteristic value of subsoil bearing capacity of muddy soils predicted by GPR model were higher than that predicted by BP neural network and LS-SVM method.The results show that GPR model had obvious prediction under small sample data conditions.(2)The GPR model was used to predict the compressive modulus and the characteristic value of subsoil bearing capacity under different parameters,and the prediction results were combined with the measured results to analyze the influence of the parameters change on the ground treatment effect.The results showed that with the increase of consolidation degree in the surcharge preloading stage,especially when the consolidation degree was greater than 0.8,the compressive modulus and the characteristic value of subsoil bearing capacity of the muddy soil increased after the dynamic compaction,which showed that with the increase of the consolidation degree,the reinforcement effect of the dynamic compaction on the muddy soil layer gradually became better.The values of the compressive modulus and the characteristic value of subsoil bearing capacity gradually increased during the continuous increase of the ramming energy,and in the actual project it is necessary to select a reasonable ramming energy according to the ground treatment requirements.Excessive ramming energy not only causes waste but also may cause failure of the soil and it will be counterproductive.The values of the compressive modulus and the characteristic value of subsoil bearing capacity showed a tendency of gradual increase as the number of ramming.According to the curve of the characteristic value of subsoil bearing capacity with the number of ramming,the increase of the characteristic value after the number of ramming is small,so it is reasonable that the number of ramming was 5 in large-area construction. |
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