Engineering Properties Of Excavated Soil And Pit Landfill Settlement

China's urbanization process has been continuously advanced,and large-scale engineering construction has been carried out,resulting in a lot of excavated soil.According to the survey results of construction waste output of some Chinese cities in 2016 provided by the Ministry of Construction of China,the total output of excavated soil accounted for 75.5%,of which the annual output in Hangzhou reached 126.16 million tons,accounting for the largest proportion.Due to the shortage of disposal sites,since 2010,excavated soil in Hangzhou has been mainly transported by engineering vehicles and ships to the abandoned pits and lowlands in Deqing and Fuyang for backfilling:This terminal disposal method of excavated soil,an emerging disposal method at present,can not only solve the problem of insufficient land resources of landfill,but also avoid the safety risk of landslides.And it is an emerging disposal method to manage abandoned mine pits and make land resources.However,the landfill process of this way was just a simple dumping and backfilling of excavated soil.Many problems existed such as slow consolidation,low capacity utilization rate of landfill site,large post-construction settlement and low bearing capacity of foundation,which reflect our insufficient understanding of engineering waste soil pit backfilling technology.Therefore,the key to solve the problem is to deeply study the factors affecting the settlement and capacity of the excavated soil landfill,put forward the method of increasing the capacity of the landfill and controlling post-construction settlement,and standardize the design and operation process of the landfill.Through field investigation,in-situ test,laboratory test,the engineering properties of excavated soil in Huashan mine landfill in Deqing were summarized to obtain the saturation gravity curve and the law of compactness,degree of consolidation and excess pore pressureand with depth.Using LANDFILL landfill settlement and capacity analysis software to analyze the effects of water accumulation conditions,groundwater level,initial dry density,initial porosity ratio on landfill settlement and capacity with 3D model.The following conclusions were obtained:(1)There was a large gap between the excavated soil particles,which contained large particles such as gravel and bricks.The distribution of excavated soil was uneven,which in this landfill was mainly divided into two types:low liquid limit clay(CL)and sand-containing fine-grained soil(CLS).In the process of landfill,the landfill was in the state of water,and there was no drainage measures,in which excavated soil was close to or in the state of saturation and had a high water content.30.6%of the excavated soil samples were in the fluid-plastic state,42.9%of them were in the soft plastic state,and 26.5%of them were in the plastic state.The shallow soil samples above 2m and some deep soil samples were in the fluid-plastic state,and the soil quality was soft.The foundation bearing capacity was low,which was consistent with the result of DPT and SPT.Due to the lack of compaction treatment after landfill,the compaction was not sufficient and the dry density of filling soil was small.The average compactness of 38 soil samples from holes B1,B2,B3 and B4 and 9 topsoil samples was 0.75,and the average compactness of 11 soil samples from holes B5 was 0.83.The soil of the landfill had low compactness and poor permeability.When the saturation was between 0.8 and 1,the cohesive force of the excavated soil decreased with the increase of the saturation,and the internal friction angle decreased with the increase of the saturation.The corresponding shear strength under the pressure of each level was decreased as saturation increased.The consolidation test results showed that the excavated soil has high compressibility,the consolidation degree of the whole landfill was low,and the average consolidation degree was only 44.9%in 5 years of accumulation.The current landfill method will lead to low compactness,soft soil,low degree of consolidation and slow consolidation.(2)The numerical simulation results of LANDFILL software showed that the calculated geometric storage capacity of the landfill was 9.03×105m3.Under the current landfill condition,the actual landfill capacity was calculated as 9.86x105m3,and the actual landfill quality was calculated as 1.625×106t,with a capacity expansion rate of only 9.2%.With the completion of the main consolidation settlement,the expansion rate of the landfill can reach 21.6%,and extra excavated soil of 1.88×105t can be filled.12.4%of the pit capacity was not utilized,which has a great impact on the capacity of the landfill site.Therefore,the landfill technology needs to be studied and improved.The theoretical calculation results of settlement were slightly smaller than the software calculation results,but the error was not big,of which the average was less than 5%.LANDFILL software has the characteristics of separation of time and space,so the calculation accuracy of LANDFILL software was higher than the theoretical calculation results and the calculated result was closer to the actual.(3)When the groundwater level is reduced from 24m to 14m,the capacity increase of the pit landfill will reach 90%of the increase that precipitation can produce,while the remaining 14m of precipitation will only contribute 10%to the capacity increase.Lowering the groundwater level can increase the expansion rate of landfill and increase the capacity of landfill.However,with the continuous decline of groundwater level,the influence of goundwater level height on the expansion rate was becoming smaller and smaller,and the growth rate of expansion rate slowed down.In the actual project,if the method of lowering groundwater level is adopted to increase the capacity of the landfill,it is necessary to get the best capacity-increase method by considering the site topography,physical and mechanical properties of the filler and precipitation cost of the project.(4)As the initial dry density of the soil increased,the main consolidation settlement of the pit landfill gradually decreased,and the expansion rate was reduced according to the calculation method of the expansion ratio.However,the soil was more compact and the voids of the particles were less,so actually increasing the initial dry density can increase the capacity of the pit landfill,and the compression settlement by by the self-heavy load and the external load will be smaller.Through LANDFILL software to calculate the LANDFILL settlement,layered rate had a great influence on subsidence calculation results.To ensure the accuracy of the results,landfill capacity analysis should be conducted according to the actual landfill rate.(5)The design process and control measures for maximizing the capacity of the excavated soil pit landfill were proposed.It is necessary to measure the geometric storage capacity and volume of the landfill,determine the type of excavated soil,carry out pretreatment and modification,adopt a targeted backfilling scheme,pay attention to compaction drainage and landfill rate during backfilling,and do a good job of closing the field.