Study On Foundation Pit Dewatering Technology In Muddy Soil Site With Silty Sand Interlayer In Yangzhong Area

Because of the special historical causes,the floodplain along the Yangtze River is rich in groundwater and has complex engineering geological conditions.The underground engineering construction of the site is faced with such problems as high cost of deep foundation pit support,easy silting of dewatering wells and poor dewatering effect.In this paper,according to the typical muddy soil site with deep silt interlayer under the Yangtze River floodplain landform in the Yangzhong area of Zhenjiang River,through the means of laboratory test,theoretical analysis and numerical analysis,the research on deep foundation pit dewatering technology is carried out.The main work and results are as follows:(1)This paper analyzes the historical causes and permeability characteristics of silt with silt interlayer in Yangzhong area.Based on the historical data and engineering data of Yangzhong area,it is found that the soil layer in Yangzhong area is mainly formed by the deposition of Quaternary loose deposits in Cenozoic era.Influenced by the tidal change of the Yangtze River and the periodic flood of river water,the muddy silty soil layer contains silt interlayer with different thickness and uneven distribution.The permeability characteristics of silty soil with silty sand interlayer were further studied through indoor seepage test.The test showed that the horizontal permeability coefficient of the soil layer was larger than that of the vertical permeability coefficient,and the permeability coefficient increased correspondingly with the increase of the thickness of silty sand interlayer.(2)Through the self-made foundation pit dewatering test device,a single well dewatering laboratory test was carried out to study the influence of the location of the filter section and the silt interlayer and the filter material around the well on the dewatering effect.The results show that when the filter section of the dewatering well is located in the muddy soil,there will be a rapid decrease of the pumping capacity caused by silting.At this time,adjusting the height of the well pipe to increase the contact range between the filter section and the silt interlayer will significantly improve the dewatering effect.It is suggested that the elevation of the dewatering well be properly adjusted to improve the dewatering effect in the actual project dewatering.Through comparative analysis of different gradations,the dewatering effect can be improved The amount of water in the well is lowered by filtration material.It is concluded that the use of 50% fine sand +50% medium sand ratio can slow down the clogging,and the precipitation effect is ideal.(3)In view of how to determine the effect of variable top elevation of dewatering pipe to improve the dewatering effect,it is proposed to increase the coverage of silt interlayer in the filter section by changing the top elevation of dewatering pipe.The equivalent simulation can be achieved by increasing the thickness of silt interlayer in the original filter section.Based on this method,the single well and multi well precipitation numerical models are established by using Visual MODFLOW software to simulate and analyze the dewatering effect.The results of single well numerical analysis are similar to that of precipitation test,which further verifies the rationality of the above-mentioned equivalent simulation method;The numerical simulation of group well precipitation shows that the dewatering rate can be increased by 16.94% ? 36.93% by changing the top elevation of dewatering well.(4)Relying on the deep foundation pit project of Yangzhong Zhongyang science and Technology Plaza,through visual MODFLOW software establishes a three-dimensional geological model and carries out numerical simulation analysis on the dewatering scheme.The calculation shows that: according to the traditional dewatering well arrangement,the maximum drawdown of groundwater level is only-3.7m;Adjust the elevation of the pipe top of the dewatering well,increase the coverage of the filter section by 1m,and increase the maximum drawdown of the groundwater level from-3.7m to-5.5m,which can basically meet the requirements for drawdown in the dewatering design scheme.