Study On The Time Effect Characteristics Of Turfy Soil In Dunhua Region Of Jilin Province

With the development of economic construction in Jilin Province, railways, highways andother line engineering has gradually increased. There are many line projects which werecontributed to promote the traffic outside the province are under construction in JilinProvince currently. Many engineering are across the wetland environment unavoidablebecause of the long distance and high standard feature of line engineering. Especially theroad of Jilin to Hunchun and Heda highway are through a typical peat soil wetlands-turfysoil. Turfy soil is a class of quagmire, which is formed at the adequate rainfall and restoreenvironment, with the characteristics of high organic matter content, large void ratio, highcompression and low degree of decomposition. It is very difficult to meet the engineeringneeds with the special engineering properties. The engineering properties of turfy soil havesignificant time effect because the decomposition and organic matter content of plantresidue will change over time, causing a large number of undecomposed plant roots in turfysoil. The normal operation of turfy soil road will be effect by the change of engineeringproperties of turfy soil foundation, causing the original material composition, physicalproperties, mechanical properties and water properties of turfy soil will change with timegoing. The characteristics study of turfy soil with time going has the great theoretical andpractical value in order to avoid the construction accidents which are caused by engineeringproperties of turfy soil evolution over time.For the above reasons and necessity of the study, this paper were studied on the time effectproperties of turfy soil under the support of the National Natural Science Foundation ofChina " study on the engineering geological characteristics of turfy soil considering timeeffect in seasonal frost region " and " The highway construction technology of turfy soilarea ", etc four transportation construction projects in Jilin Province.The study began with the field survey of systematic study the formation and thesedimentary characteristics of turfy soil. A lot of field sampling and laboratory tests arecarried out in order to analysis the physical properties, chemical characteristics andmechanical properties of turfy soil at the year of 2004 and 2009. And also it is that using advanced equipment and technology to research the microstructure, mineral compositionand formation age. The study of time effect will use the natural profiling and constructoperation testing method to study the engineering geological characteristics of turfy soil onthe basis of indoor test data with time going under the natural history and engineering life.And giving the best foundation treatment which is not only meet the requirement offoundation bearing capacity, but also adapt the permeability, water conditions, drainagepath, water holding capacity and other aspects in wetland by comparing the moisturecontent, permeability and other changes of natural turfy soil in different foundationtreatment sections before and after embankment construction.It gains the following results through the study on the time effect characteristics of turfysoil in Dunhua region:1. Study on the transformation mechanism, engineering properties, formation ages andother properties of turfy soil at Dunhua region:1) Jilin Dunhua is located in the Northeast Plain wetland environment. Turfy soil is atransitional soil that is not swamped completely.2) The turfy soil is a kind of peat which has small density, high water content, highpermeability, large pore, acidic, high organic matter, high compressibility and formed lessthan 1 million years according to the indoor test results in Dunhua region (includingHuangsongdian section, Largestone town section and Jiangyuan town section).2. The qualitative and quantitative characteristics research on physical, mechanical,chemical and micro-structural of turfy soil at Heda road show that:1) The study area which has the characteristics of loose structure, low decomposition andlarge permeability that the vertical greater than or equal to the horizontal permeability isHolocene sediment and deposit unevenly. With the degree of decomposition increasing, thestructure of turfy soil changes from aerial structure to flocculation structure gradually; andthe porosity decrease, the structure gradually become denser than before. The sensitivitytest results show that the sensitivity of turfy soil is small and belonging to the lowsensitivity soil, which does not have structural properties and structural strength is weak.2) Using IPP software to quantitative analysis the pore characteristics of peat soil atdifferent depth. The results show that the direction angle of porosity is greater than 75°. It is illustrate that the soil have certain direction, large pores, flat pore shape and poorcircularity. And different depths have different pore directions. The pore direction anglereduced and the pore shape is changed from flat to round with the embedded depthincreased.3) We can see from the stress path graph of turfy soil that pore water pressure exit lagphenomenon and the lag time increased with the confining pressure increased. The porewater pressure increased dramatically when the lag time ended. It will lead the effectivestress reduce rapid.3. The evolution of engineering geological characteristics of turfy soil under project life areas follows:1) The density of natural turfy soil become larger over time and the increasing multiple isdecreased with the embedded depth increased, while the soil particle density is contrast.2) The moisture contents variation of turfy soil over time are different at differentfoundation treatment sections. The permeability of deeper and deep turfy soil will impactlowly after filling with porous materials to treat foundation. It means that this method donot change the penetration path and destruct the wetland ecology. So this foundationtreatment can be used at turfy wetland areas. Using gravel weathered rock plus compositeearthwork cloth to treat foundation have less environmental impact and the method can beused in turfy wetland area also.But sand pile foundation treatment is not suitable to use in wetland areas from theperspective of protecting wetlands point of view. Because it can be speed up the drainageand consolidation; and it will increase the vertical permeability of the turfy soilsignificantly. This method can also change the original water channel that let a lot of wateralong the surface of turfy soil down into the sand pile. It will impact the wetlandenvironment of turfy soil.3) The mechanical properties of turfy soil over time are changed. Compression indexchanged over time show that the properties of secondary consolidate compression of turfysoil have greater impact over time.4. Using GDS stress control device to simulate the subgrade of turfy soil, the test results areas follows: 1) At the load (shear) stage, the pore water pressure dissipation level is affected directlyunder different drainage ways. Due to the pore water pressure can not be dissipated, theeffective stress and foundation strength reduced and the soil deformation increased with theincrease of load on the cover.2) Faster loading rate will cause a large amount of pore pressure changes and grounddeformation. The ground strength decreases rapidly and the stability of foundation isaffected with faster loading rate. And slow loading can reduce the deformation and increasethe strength of foundation, but it will increase loading time substantially.3) The pore pressure, deformation and stress path have little difference under different testconditions on the various samples of turfy soil in the initial load (shear) and theconsolidation phase. The study results show that the characteristics of turfy soil depend onthe initial consolidation state.4) The study on the foundation compression modulus variation of turfy soil shows that thestrength of turfy soil decrease with the loading carry on. The foundation strength is notaffected significant under different drainage test conditions; but different loading rate willimpact the foundation strength. The foundation strength will be decrease as the loadinggradually; and stop loading consolidation process will increase the soil strength, which isfully demonstrated that the safety and necessity of the need of grade filling embankment.