Geotechnical Characterization And Beneficial Utilization Of Urban River Sediments

With the accelerating process of urbanization,urban land resources are increasingly scarce.The main used management technologies for dredged sediments,such as landfill,ocean dumping,which induced a series of environmental geotechnical problems,cannot meet the rapid development of the city.Research on the beneficial uses of dredged sediments becomes a hot topic in related fields.Most cities have urban river with landscape features for the requirements of flood protection and foul water emission.These rivers lie in downtown area of the city with comparatively large population density.Due to human activities,a lot of waste water discharged into rivers,resulting in serious siltation.To maintain the appropriate functions of urban rivers,management had to regularly organize dredging,producing a considerable amount of dredged materials.The properties of urban river sediments may be different from the general sediments dredging out of lake or marine,needing for targeted treatment.However,the current treatment for dredged materials focused on marine or lake sediments,few studies have performed on the beneficial uses of urban river sediments.Therefore,it is necessary to study the beneficial utilization of urban river sediments.This paper relies on the State Water Pollution Control special demonstration project,taking the middle area of Qinhuai River in Nanjing city for the research object.Based on the system studies of properties and deposit characteristics of urban river sediments,thermal treatment and stabilization/solidification method for the beneficial uses of urban river sediments were investigated respectively.The thermal properties of fired bricks and the durability of stabilized soil were also experimentally investigated.The obtained research finding and understanding are as follows.(1)The sediments thickness of study area was measured by using self-developed sludge sounding ruler.Combined with site investigation of outfalls and surrounding environment,the regular pattern of sedimentation was analyzed.The physical-chemical properties of urban river sediments and the mechanical properties of remodeling soil were investigated by geotechnical tests,X-ray fluorescence(XRF),X-ray diffraction(XRD),and thermo-gravimetry and differential scanning calorimetry analysis(TG/DSC),respectively.The obtained results indicated that,affected by human activities,along the flow direction,sediments thickness gradually increases,organic matter content,liquid and plastic limits,coarse content,and the average particle size of sediments increased in varying degrees,while specific gravity decreased.Compared with other lake or marine sediments,urban river sediments has higher organic matter content and coarse content,lower clay content,its untreated remodeling soil with poor mechanical properties is difficult to be used directly on the project.(2)The urban river sediments were chemically,mineralogically and thermally characterized with XRF,XRD,and TG/DSC,respectively.Native and clay mineral were dominated by quartz and illite,respectively.The results provide theoretical foundation for admixture ratio selection,preparation and sintering of highly insulating brick.(3)The feasibility of the use of urban river sediments as primary raw material,Xiashu clay as secondary raw material in production of eco-friendly fired brick was investigated.To evaluate the effect of urban river sediments contents on pore-forming,large amount of urban river sediments ranging from 50%to 70%by mass were blended with clay and fired at 1000?,1050? and 1100?.The physical-mechanical properties,including loss on ignition,linear shrinkage,bulk density,and compressive strength,as well as the microstructural properties of the fired bricks were investigated.Besides,the environmental impacts of the fired bricks were also identified by leaching tests.The obtained results show that considering the balance between thermal insulation capacity and acceptable compressive strength,the bricks with 50%urban river sediments firing at 1000-1050? were more beneficial,complying with the requirements of GB5101-2003 for common bricks.The results of leaching tests confirmed that the fired bricks with urban river sediments are environmentally benign products.Therefore,urban river sediments are suitable as primary raw material in production of highly insulation brick.(4)The thermal conductivities at different ambient temperature of fired bricks were measured by steady-state method of EKO-HC110 type thermal conductivity tester.The change of thermal conductivity of fired brick,related to firing temperature and urban river sediments content,was studied from three aspects of change characteristic of mineral composite,porosity,and microstructure properties.The obtained results show that with the increase of ambient temperature,thermal conductivity increased a little.As the firing temperature increased,the amount of molten liquid gradually increased,filling small voids of brick.The contraction behavior was detected and microstructure was modified,so thermal conductivity of fired brick slightly increased.With increasing sediments content,organic matter content of the raw material gradually increased,the porosity of fired brick gradually increased,resulting in a significant reduction of thermal conductivity.The thermal conductivities of bricks with 50%urban river sediments firing at 1050? decreased at least 40%(0.820 to 0.480 W/m?).Compared with other organic pore-forming agents,urban river sediments have considerable pore-forming effects.(5)The unconfined compressive strength test and compression test were performed to study the change characteristics of the properties of stabilized sediments related to cement content,curing time and initial moisture content.The microstructural properties of stabilized sediments were investigated by SEM and XRD,respectively.The new solidification treatment method for urban river sediments with high organic matter content was proposed,that adding a basic component of lime and an expansion component of gypsum.Mechanical properties and environmental impacts of this new stabilized soil were studied.The results indicate that the curing effect gradually increased with cement content increases,and there is a minimum cement content.Due to the component of urban river silt,the curing strength of solidified sediments increase was mainly during the first 28d of age.The strength of solidified sediments can be sighnificantly improved by reducing its initial moisture content.Adding admixture can significantly improve the curing effect,and heavy metal contaminants can be effectively secured in frame.This treatment method is low cost,which can provide a reference for similar solidification treatment of high organic soil.(6)The wet and dry test was conducted to measure the parameters of compressive strength,loss soil weight,and volume change,in order to evaluate the wet and dry durability of the two solidified urban river sediments with different organic matter content.Results show that the wet and dry durability is controlled by stabilization agents,curing effects,and the formation of cracks induced by contraction and expansion.The compressive strength of stabilized soil decreased with the increase of wet-dry cycles under conditions of low cement content,while under conditions of high cement content,the strength of stabilized soil increased with the increase of wet-dry cycles;subsequent to that,the strength declined with increasing wet-dry cycles.The addition of stabilization agents could sighnificantly improve the wet-dry durability of stabilized soil.The urban river sediments stabilized with 5%cement or more is durable against wet-dry cycles.