Study On Energy Transfer Law And Macro-micro Mechanism During Compaction Of Loessial Silt

With the rapid development of China's economy,the modernization process has been accelerated and penetrated into all fields of social development.Especially with the implementation of strategic policies,such as the great development of the west and the rise of the central and western regions,the expansion of urban and rural construction is accelerating,and the scale of modern infrastructure projects such as airports,expressways and urban expressways is expanding rapidly.However,under the influence of landform,the contradiction of insufficient urban development space is increasingly prominent.In order to expand the construction land,taking engineering technology,construction cost and other factors into comprehensive consideration,the filling engineering is imperative.Considering the problem of regional soil quality and construction economy,the central and western regions of China are located in the upper middle reaches of the Yellow River.Most of the filling engineering use loessial soil.However,due to the lack of land resources,loessial silt with extremely complex engineering properties is also used as filler,and its compaction characteristic is a key factor affecting the quality of the filling engineering.In practice,compaction degree is generally used to evaluate the quality of soil filling engineering.How to accurately control the maximum dry density of laboratory test standard is the key problem of compaction degree calculation.The laboratory maximum dry density is obtained by compaction test,so how to obtain the effective laboratory maximum dry density is the key technical problem of filling quality control.Exploring the transfer process of compaction energy in soil can not only judge the compaction quality of soil,but also directly judge the effective utilization rate of compaction energy to avoid energy waste.Exampled by a certain filling area of Shanxi as the research object,on the basis of laboratory standard compaction method,a series of compaction test and the soil stress test has been conducted based on two different confined conditions.And Combined with electrochemical impedance spectroscopy and discrete element numerical simulation,the transfer law of compaction energy in loessial silt is studied from macro and micro perspectives.The compaction energy transfer law of loessial silt from granular to stable soil skeleton is obtained.The main research contents and conclusions of this thesis are as follows:(1)The compaction tests results show that the confined condition and the change of compaction mode(single blow or multiple blow)have a great influence on compaction results.Under the same compaction energy,the maximum dry density of compaction loessial silt under the condition of single blow confined condition is less than that under the condition of multiple blow and unconfined,and the hoop effect will be formed under the confined condition.Without changing the confined conditions,the effects of water content,compaction energy and paving thickness on the compaction effect of loessial silt are consistent with those of fine grained soil.(2)By using the stress testing system independently developed by the research group,the stresses within the soil during compacting process are measured for different confined conditions,which reveals the compacting of the soil is the dynamic consolidation process.It can be approximately equivalent to the incompletely elastic collision between the hammer and the soil.This process is extremely short,only a few to ten milliseconds,compaction energy can be transmitted to the soil in the form of waves,compaction impact force and soil bottom stress are manifested as a single peak curve that first increases and then decreases with compaction action time.The peak value of compaction impact force and the peak value of soil bottom stress increase with the increase of the stability of soil skeleton.When the water content exceeds the optimal water content,the effective stress in the compaction impact transfer process decreases and the soil bottom stress decreases.Therefore,in order to ensure the full utilization of compaction energy,its water content should be as close as possible to its optimal water content,w=[w_op,w_op+2%).(3)The ultimate strength of compacted loessial silt shows different laws under different water content and confining pressure,and the type of the stress-strain relationship change generally changes from softening type to hardening type with the change of confining pressure and water content,and there are critical confining pressure and critical water content.Under the same compaction energy and water content,the critical confining pressure is about 200k Pa?300 k Pa.Under the same compaction energy and confining pressure,the critical water content is the optimal water content under the corresponding compaction energy.(4)Based on the conservation of energy,the concept of"stress diffusion-transfer ratio"in soil is put forward,then the compacting effect mechanism of compaction energy in soil is explained.The research shows that the"stress diffusion transfer ratio"of soil in compaction process increases with the accumulation of compaction energy.When the compaction energy reaches 2000k J/m³,the stress diffusion transfer ratio tends to be stable,reaching?0.9.The density of soil is no longer significantly changed,even the compaction energy is further increased,which indicates that the soil skeleton reaches a stable state.Therefore,it is suggested that in practical engineering,the maximum economic compaction energy of loessial silt can be about 2000 k J/m³.(5)Numerical simulation analysis of PFC^2D discrete element shows that in the compaction process,the strong force chain develops from the bottom of the hammer to the bottom of the soil.When the vertical strong chain is completely formed,the soil reaches the densest state,and then the compaction can reach the maximum economic compaction energy.With the decrease of confined condition(i.e.the increase of barrel diameter to hammer diameter ratio),the confined restraint reduces the lateral displacement of soil particles,the hoop effect is gradually eliminated,and the compaction effect is improved.However,when the ratio of barrel diameter to hammer diameter reaches 4,the lateral diffusion of compaction reaches its limit,and the overall compaction effect decreases.Therefore,the distance between the hammer diameter and space between blow points should be reasonably selected in the project.It is suggested that the distance between the hammers should be 2?3 times of the hammer diameter in order to achieve the best compaction effect.