Study On Safe Anti-floating Water Level And Anti-floating Measures Of Precipitation For Projects Closed To The River

Fuzhou is a typical coastal soft soil area with abundant groundwater,and the project closed to the river has typical binary stratigraphic structure characteristics.However,there are few researches on the selection method of anti-floating water levels and the influence of anti-floating measures on the surrounding environment for the special hydrogeology of the area closed to the river in Fuzhou.This paper takes a project closed to the river in Fuzhou City as the research object,based on deep analysis of the hydrogeology,engineering geological conditions,and meteorological data of the project site,combined with the morphological characteristics of the underground structure of the project and the surrounding buildings.The calculation and analysis of the safe anti-floating water level and base buoyancy of the site,the selection of anti-floating measures,and the impact analysis of deformation characteristics of precipitation and anti-floating measures during the construction stage of underground structures are conducted.The specific analysis and results are as follows:(1)Comprehensive specification method,semi empirical formula method based on groundwater seepage theory,and numerical simulation method are used to predict the anti-floating water level.The recommended anti-floating water levels obtained by the three methods for this site are 10.00 m,9.78 m,and 9.08 m,which verifies that the direct use of ground elevation or river flood level for the anti-floating water level of projects closed to the river is insufficient and one-sided.Based on the inverse calculation of the buoyancy resistance during the construction phase of the underground structure and the completion of the superstructure,it is concluded that the temporary buoyancy resistance water level during the construction period is 8.80 m,and the permanent buoyancy resistance water level is 10.00 m.The three methods comprehensively consider the rise of water levels in multiple aquifers and adjacent rivers on the project site,the surrounding drainage conditions,the permeability of the soil layer under the foundation,and the backfill quality of the fertilizer tank,to avoid the accidental selection of the anti-floating water level due to the short survey time and insufficient monitoring data.(2)Based on the HS-Small constitutive model,the dewatering excavation process of a prototype deep foundation pit project is simulated and analyzed.Based on empirical formulas and back analysis of the displacement monitoring data of the envelope structure,the parameters of the typical soil layer HS-Small model in Fuzhou region were obtained,which provided a reference for the parameter values in the following HS-Small finite element simulation.In order to analyze the influence of dewatering anti-floating measures taken during the construction of underground structure on foundation pit engineering and surrounding environment,a numerical model of foundation pit dewatering based on HS-Small is established according to the actual situation of the project closed to the river.It is found that the influence of groundwater is the most obvious during the period from foundation pit excavation to the completion of bottom plate construction.With the progress of dewatering excavation,the settlement of the foundation pit slope and surrounding buildings gradually increases and the uplift deformation appears at the bottom of the foundation pit.The horizontal displacement of deep soil outside the pit is a "parabola" shape,and the influence of dewatering on the foundation pit deformation is more obvious than excavation.Combining with the single variable method,the influence degree of the depth,mode,rate of dewatering measures and the depth of the waterproof curtain on the foundation pit and the surrounding environment is simulated.The analysis shows that fractional dewatering can effectively reduce the influence of the foundation pit on the surrounding environment.In this project,it is suggested that the precipitation depth should be controlled within 0.5～2.0 m below the bottom plate;Precipitation rate has a relatively small impact on the total settlement amount of the surrounding environment,but it has a greater relationship with the settlement rate;The waterproof curtain can extend the groundwater seepage path which has a significant impact on the control of surface settlement.Building settlement decreases with the increase of the embedded depth of the waterproof curtain,but the effect of the control of building deformation is not obvious after reaching a depth of 30 m.