Study On Deformation Mechanism And Control Technology Of Embankment Of National Highway Extension Project In Thick Marine Silt Strata

With the sustained and rapid development of the national economy,the construction of expressway is very active,and the upsurge of reconstruction and expansion engineering has also started.Due to the particularity of coastal soft soil,however,now many completed projects of high grade highway have occurred some diseases,especially the problem of uneven settlement where the new and old roadbed joining together,which seriously limits the construction of high-grade highway in coastal area.In this paper,based on the reconstruction and expansion engineering of 204 national highway 1#section of Guanyun,according to the special geological environment of thick sea marine,complex construction technology of soft soil foundation treatment and design conditions of new and old embankment's junction,carrying out the study on deformation mechanism and control technology of embankment of national highway widening project in thick marine silt strata.The main contents and conclusions are as follows:1.The engineering characteristics of the silt stratum in Lianyungang were studied by laboratory experiments from two aspects:physical mechanics characteristics and microstructure characteristics.The results showed as follows:The thick sea marine have some special properties,such as high water content,poor permeability,high void ratio,low shear strength,high compressibility,and its natural condition is in plastic flow condition,but compared with the silt in several other regions,the compressibility of Lianyungang silt is lower.The engineering characteristics of the silt reflected by its Microstructure characteristics are accordant with which measured by macroscopical test,which means part of the engineering properties of soil could be speculated with reliable microstructure characteristics.2.Based on actual engineering construction and subsidence monitoring,using PLAXIS 3D simulated the soft soil foundation treatment schemes of unilateral broaden section,including no treatment,preloading and powder spraying piles,and respectively carried on the comparison and analysis of the vertical displacement and stability of these three kinds of foundation treatments.Finally,the vertical additional stress and the distribution law of excess pore water pressure were further studied and analyzed to further studying the subgrade's deformation mechanism.In particular,based on the businesic solution,the vertical additional stress under unilateral widening embankment was deduced and verified by MATLAB.At the same time,the impacts of the unilateral broaden construction on the formation of the vertical additional stress was analyzed.It turns out that(1)Preloading can better control the uplift caused by excavation unloading effect,but effects of powder injection pile are more significant in controlling post-construction settlement,excess pore water pressure and roadbed's stability.(2)The horizontal displacement is opposite in the strata below the right slope of the new and old subgrade,and there is a turning point along the depth at z=2.5m.The horizontal displacement of the strata below the slope foot of the new and old road is smaller than that of the other two foundation treatment schemes under the foundation treatment scheme of cement injection pile,and the maximum horizontal displacement below the right slope foot of the old road is only 11.5mm.(3)The results of stability analysis show that the safety factor of each foundation treatment scheme is greater than 1.25 at different stages,which meets the requirements of the code for safety reserve.But relatively speaking,the greatest potential damage risk is occurred on the juncture of new and old subgrade on the surface of the earth,followed by the right slope of the widening subgrade.The stability of roadbed is enhanced by powder injection pile,while the safety factor of equal load preloading is smaller than that of no foundation treatment scheme during the construction period and the 5-month stowage period.After the completion of construction,as consolidation settlement tends to be stable,the safety factor of roadbed will increase and the roadbed will be more stable.(4)In the actual construction,it is necessary to pay more attention to the increase of the vertical additional stress in the junction of new and old embankment,especially the thick sea silt layer,and taking proper treatment measures in time.3.The numerical simulation results fully showed the superiority of the powder spraying piles.Based on the deficiency of existing pile curing agent,this paper proposed a new design formulation,and contrastively analyzed the shear strength and microstructure features of the solidified soil respectively in design formula and pure cement from different ages.It turned out that(1)No matter the early strength or the late strength of the designed formula solidified soil,were obviously higher than that of cement stabilized soil.(2)The cement still plays a dominant role in the design formula,high calcium contained fly ash and modified ultrafine granulated blast furnace slag powder not only generate hydration gel in itself,but also add a large number of calcium ions,which strengthen the hydration reaction of cement,thus improve the strength of solidified soil.Compared with the cement solidified soil,with the increase of age,the microstructure of the solidified soil is denser and the later strength is improved.The addition of acrylonitrile fiber reinforced the structure integrity of the solidified soil,while improving the shear strength of the solidified soil,its crack resistance can be improved too.(3)The design curing agent have great economic benefits,when the water content is 60%,in the same ages of 7 and 28 days strength,the price of design curing agent of each linear meter is lower than the pure cement curing agent 4.66 yuan and 5.08 yuan respectively,which accounts for about 28.8%and 31.4%of the pure cement curing agent's cost.4.Considering the shortage of powder injection pile,this paper puts forward the combined load preloading and powder injection pile combination technology with thick marine silt stratum conditions,and the settlement monitoring results showed the scheme can greatly reduce the total settlement and post-construction settlement of subgrade in bridgehead,which combines the advantages of powder injection pile and preloading,the two kinds of soft soil foundation treatment measures.5.Currently,the deformation coordination control technology of new and old embankment in soft soil stratum is one of the main problems of the reconstruction project.The numerical simulation results show that there will be obvious difference settlement between the junction of old and new embankment in the extension project in thick marine silt strata.Corresponding to the special geological conditions of thick sea silt stratum,the integrated application of step method and geogrid is put forward and its concrete construction scheme and technology was discussed.Finally the improvement scheme of deformation coordination control construction of new and old embankment in bilateral broaden section was puts forward.