Study On Characteristics Of Confined Water In Riverside Area Of Yangtze River In Wuhan And Groundwater Control Of Deep Foundation Pit In This Area

The riverside area on both sides of the Yangtze River and the Hanjiang River in Wuhan is an important area for Wuhan’s urban construction.With the continuous growth of urban population and vigorous development of underground space,more and more foundation pits with complex environment in this riverside area are excavated,and the depth and scale of excavation are becoming larger and larger.Because the engineering geological conditions and hydrogeological conditions in Wuhan riverside area are very complex,especially the characteristics of its confined aquifer,such as high pressure,strong water abundance and high permeability,it has brought great difficulties to the design and construction of groundwater control for the foundation pits in this area,even resulting in frequent accidents.Therefore,it is of great practical significance to study the characteristics of confined water in riverside area of Wuhan for groundwater control and safety design and construction of deep foundation pit engineering in this area.At present,for deep and large foundation pits in riverside area of Wuhan with complex hydrogeology,engineering geology and surrounding environment,conservative bottom-type water-stop curtain is adopted based on experience.Although this kind of groundwater control method meets the safety requirements of foundation pit engineering,it is difficult and costly to construct,and there is suspicion of over-design,which results in a great waste of engineering.Underground water control method with suspended water-stop curtain can effectively reduce project cost and generate great economic benefit.However,there is a serious lack of relevant theories and specifications for the groundwater control method of suspended water-stop curtain,such as the calculation method of water inrush in foundation pit under the condition of suspended water-stop curtain and the calculation method of water level drop outside the pit,etc.,which may be difficult to meet the requirements of safety and environmental protection of deep and large foundation pit near the river.How to improve the groundwater control theory and optimize the groundwater control design of foundation pit is the key problem to be solved urgently on the basis of ensuring the effect of foundation pit dewatering and meeting the requirements of surrounding environmental protection.Therefore,this study focuses on the characteristics of confined water and groundwater control of deep foundation pits in riverside area of Wuhan.Firstly,based on the monitoring data of water level of the Yangtze River and the monitoring data of confined water in riverside area,the functional model of water level fluctuation of the Yangtze River is put forward,and the hydraulic connection between confined water in riverside area and the Yangtze River is analyzed.Besides,the spatio-temporal prediction model of confined water level is constructed to reveal the characteristics and change rules of confined water level in riverside area.Secondly,in view of the problems that existing foundation pit dewatering specifications,dewatering design software and theoretical calculation methods are difficult to accurately and quantitatively evaluate the water inflow of foundation pit under the condition of suspended water-stop curtain,this thesis,based on the theory of Dupuit steady confined well flow and large well method model,puts forward the calculation formula of water inflow of foundation pit under the condition of suspended curtain and the water level drop formula outside the pit.It provides a theoretical basis for the design and analysis of precipitation in foundation pit under the condition of suspended water-stop curtain and its influence on surrounding environment.Thirdly,based on the field connection test of foundation pit engineering and combined with the construction process of water-stop curtain,the recommended values of reasonable permeability coefficient of water-stop curtain are given,which provides basis for the determination of permeability index of water-stop curtain by relevant codes or regulations of foundation pit engineering.Finally,based on the above research results,taking the ultra-deep foundation pit group of Hankou Binjiang business area of Wuhan as an example,the design optimization of groundwater control for foundation pit group is carried out.The main conclusions and some new observations obtained in this study are as follows.（1）A fluctuation function model of the Yangtze River water level is proposed,and the hydraulic connection law between the confined water and the Yangtze River in Wuhan riverside area is revealed.Based on the monitoring results of the water level of the Yangtze River for many years,it is found that the fluctuation of the water level of the Yangtze River have significant periodicity.This periodicity can be captured by a trigonometric function model,which can well describe the fluctuation characteristics of the water level of the Yangtze River.On the base of long-term hydrological monitoring results in the riverside area of Wuhan,it is shown that the confined water level in the riverside area has a significant hydraulic connection with the water level of the Yangtze River.And the strength of this hydraulic connection decreases exponentially with the increase of distance from the Yangtze River.The farther away from the Yangtze River,the smaller the influence of the fluctuation of confined water level on the fluctuation of the Yangtze River,and the weaker the hydraulic connection.（2）A prediction model of confined water level in riverside area is proposed to reveal the characteristics and variation rules of confined water.Considering the fluctuation characteristics of the water level of the Yangtze River as well as the hydraulic connection between surface rivers and confined groundwater,a prediction model for the confined water level in the riverside area is proposed based on the Boussinesq equation of groundwater seepage movement process.The model error is less than 10%verified by the hydrological monitoring results.Moreover,this model reveals the temporal and spatial characteristics and variation rules of confined water level in the riverside area,and provides a basis for the anti-floating design of foundation works and the dewatering control of foundation pits in the riverside area.（3）The formulas for calculating water inflow and water level drop outside the pit under the condition of suspended curtain are put forward in this study.Based on the theory of Dupuit steady confined well flow and modified based on large well method,the calculation Formula of water inflow in foundation pit considering suspended curtain condition and the Formula of water level drop out of foundation pit are put forward.Combining with engineering case and numerical simulation,these theoretical formulas are verified with high accuracy and feasibility in practical engineering.When r₀/M≤0.4,the area of foundation pit has a greater influence on the water level drop S outside the pit.When the insertion ratio L/M≥0.6,the increase of curtain depth has a significant effect on the reduction of water inflow.This calculation formula can provide the empirical effective value of curtain depth for foundation pit projects under different confined aquifer conditions,and can also optimize the number and arrangement of precipitation wells.（4）Critical values of permeability coefficient for suspended and bottom-down water-stop curtains are proposed.Based on the theory of Dupuit confined well flow and considering the influence of water-stop curtain of confined aquifer on foundation pit dewatering,combined with the connection test of Henglong Square in Wuhan,the influence of the curtain permeability coefficient under different curtain depths on the seepage field of foundation pit is numerically calculated,and the curtain permeability coefficient after completion of construction is obtained by inverting the connection test data.The recommended values of reasonable permeability coefficient of water-stop curtain are given:the critical value of permeability coefficient of suspended curtain is1.0×10^-5cm/s and that of bottom curtain is 1.0×10^-6cm/s.This study provides basis for the determination of permeability index of water-stop curtain by relevant codes or regulations of foundation pit engineering.（5）The connection and difference between Biot consolidation theory and Terzaghi theory in calculating settlement induced by precipitation of foundation pit are clarified.Through numerical simulation,it is found that the change of the permeability coefficient of the aquifer has certain influence on the settlement difference of the Biot consolidation theory and Terzaghi theory.The larger the permeability coefficient of the aquifer,the more significant the influence is.The depth of water-stop curtain has significant influence on settlement difference,and the deeper the curtain is,the more significant the influence is.In general,the settlement calculation value of Terzaghi theory is relatively large,which is relatively safe for engineering practice.From the technical point of view,since the fluid solid interaction between water and soil is considered in Biot theory,the calculation result should be more accurate than the actual value.When the settlement is taken as the optimal control standard of the water stop curtain,the Biot settlement calculation method considering the fluid structure coupling may be more appropriate.（6）The optimization study on the design of water-stop curtain for super deep foundation pit group of Binjiang business district in riverside area in Wuhan was carried out.By using the relevant parameters determined in the previous study,such as the confined water level,the water inflow of the foundation pit under the condition of the suspended water-stop curtain and the permeability coefficient of the water-stop curtain,the water-stop curtain design of the four foundation pits in the Binjiang Business district was studied by using the numerical simulation method under four conditions:open site condition（without considering the outer ring bottom type water-stop curtain）,closed site condition（considering the outer ring bottom type water-stop curtain with good water stop effect）,semi-closed conditions（considering the outer ring bottom type water-stop curtain with poor water stop effect or serious leakage）,and closed condition combining with considering the mutual influence of adjacent foundation pit dewatering.The results showed that,compared with the existing relevant specifications that the deep foundation pit in the riverside area needs to adopt the bottom dropping water stop curtain,the water-stop curtain for all of the four deep foundation pits in the Binjiang business district have different degrees of optimization space.Especially under the closed condition combining with considering the mutual influence of adjacent foundation pit dewatering,the results showed that all of the four foundation pits do not require bottom type water-stop curtain,and only a few ones need to set the suspended water-stop curtain combining with certain dewatering measurements that can the requirements of groundwater control.It greatly reduces the cost of groundwater control for deep foundation pit groups.This study provides a new insight for the underground water control design of deep foundation pit groups in riverside area.