Research On Groundwater Control Method For Deep Excavation Engineering Of Yangtze River ? Terrace In Wuhan Area

There are many rivers in Wuhan city,lakes and complex hydrogeological conditions.Wuhan urban area basically extends along both sides of the Yangtze River and Hanjiang River.Grade I terraces widely distributed on both sides of the Yangtze River and Hanjiang River are the main areas of Wuhan urban construction.In recent years,accidents caused by improper design of foundation pit projects in Wuhan also mainly occur in this area.The first terrace of the Yangtze River in Wuhan has a typical binary structure.Its upper part is filled with miscellaneous fill,and its middle part is interbedded with clay,silty clay and silt,which is a weak permeable layer with poor permeability.The lower part is a layer of silty fine sand,medium fine sand and gravel,belonging to a confined aquifer with good permeability.The confined water in its lower part has strong hydraulic connection with Yangtze River and Hanjiang River.In recent years,the foundation pit of engineering construction projects in Wuhan has been developing in the direction of " deep and large".The waterproof curtain(falling bottom and hanging)has also been applied more and more widely in foundation pit dewatering,and the risk of foundation pit confined water dewatering also has an increasing trend.At present,the theory of foundation pit confined water dewatering in Wuhan area lags behind the engineering practice.On the one hand,the lag of theoretical research leads to the over-experience and conservatism of confined water precipitation in foundation pit,which also causes great waste of engineering.It may also cause confined water precipitation accidents in foundation pit under special circumstances,resulting in huge social impact and economic losses.In addition,some scholars have analyzed that before and after the completion of the Wuhan subway,due to the existence of waterproof curtain,the area of groundwater cross-section is reduced,and the change of groundwater flow field causes the local groundwater level to rise.With the completion of several subway projects and the development of urban underground space,it will have an impact on the regional underground water flow field and will have an unpredictable impact on the regional geological environment in Wuhan.Therefore,it is of great practical significance to study the groundwater control method for deep foundation pit engineering in Wuhan area.In this paper,some research work has been carried out for the purpose of groundwater control in the deep foundation pit of the 1st terrace of the Yangtze River in Wuhan area.Based on a detailed analysis of the geological and environmental conditions in Wuhan area.It is pointed out that the high-rise buildings in Wuhan area are mainly related to the 1st terrace accumulation plain area and the 3rd terrace Longgang area,while the 1st terrace of the Yangtze River is mainly involved in groundwater control in deep foundation pit engineering.Its hydrogeological conditions are very complex and are the key areas for groundwater control in deep foundation pit.There are different strata combinations in different areas of the first terrace.According to engineering practice,different strata combinations have great influence on foundation pit engineering.Therefore,the paper carries out a sub-regional study on the strata structure of the first terrace of the Yangtze River.According to different strata combinations,the first terrace of the Yangtze River is divided into three sections(riverside section,middle section and edge section).Based on the representative engineering in each section,the hydrogeological model of each section of the first terrace of the Yangtze River is abstracted,which lays a foundation for the research on groundwater control method of the first terrace of the Yangtze River in Wuhan area.We should firstly proceed from the environmental constraints around the foundation pit,then study the hydrogeological conditions,engineering geology and foundation pit conditions of the foundation pit,and make full use of the foundation pit retaining structure(underground continuous wall,curtain pile,etc.).Based on the favorable conditions created by groundwater control,based on technical,economic and environmental comparisons,a reasonable,effective and reliable groundwater control scheme is selected to establish a groundwater control model suitable for different sites to determine the groundwater control design.The key factor affecting the groundwater control effect is the depth of the water curtain of the side wall of the foundation pit.Therefore,only by fully grasping the law of the effect of the depth of the water curtain on the foundation pit dewatering,can we formulate a scientific and reasonable groundwater control plan.In this way,the purpose of controlling groundwater dewatering groundwater can be achieved.According to the different characteristics of stratigraphic age and stratigraphic combination,this paper analyzes the groundwater control method of the Yangtze River I terrace in Wuhan area.Through analysis and calculation,it can be known that the deep foundation pit precipitation in the Yangtze River I terrace area in Wuhan uses the method of “separation + precipitation”.Suitable to effectively control groundwater.That is to say,the suspended vertical curtain is used,and the reasonable and effective deep well precipitation is used to prevent the side wall piping and the confined water from inrushing.The control is mainly based on “reducing dredging and auxiliary seepage”,and adopting“suspended curtain + deep well precipitation” The method is scientific and reasonable.In order to compare and analyze the ground subsidence of various foundation pits under different stratum combinations after precipitation,the paper analyzes the ground subsidence mechanism and flow field characteristics caused by the deep foundation pit precipitation in the Yangtze River I terraces,the middle section and the edge section.Using the traditional stratified summation method and effective stress principle,the ground subsidence value caused by various types of foundation pit precipitation is calculated according to the water level difference before and after precipitation,and the basic laws of land subsidence are summarized as follows:(1)Deep foundation pit precipitation causes little land subsidence,generally within 100mm;(2)Deep foundation pit precipitation causes the ground settlement difference to be less than 1‰.The method of “suspended curtain + deep well precipitation” can be used for extensive precipitation,but it should be noted that the building is The joints of the two foundations are cracked due to the poor interface settlement;(3)The compression layer caused by the precipitation of the deep foundation pit is mainly the interaction layer,and the formation above the interaction layer,and the sedimentation of the lower sand layer is extremely small;(4)There is a certain relationship between land subsidence and stratum combination.From the riverside to the edge,the surface settlement has a decreasing trend;(5)The method of“suspended curtain + deep well precipitation” can effectively control the ground subsidence outside the pit.Through the study of groundwater control method and ground subsidence law of the Yangtze River I terrace in Wuhan area,it provides a scientific and reasonable reference value for the design of foundation pit groundwater control scheme on the Yangtze River I terrace.The paper holds that for the deep foundation pit precipitation on the Yangtze River I terrace in Wuhan area,it should be controlled by “reducing dredging and auxiliary seepage as the auxiliary”,and adopting the method of “suspended curtain + deep well precipitation” for precipitation.For the Yangtze River I terraces in Wuhan,different sections,deep foundation pit precipitation caused little land subsidence,generally within100 mm,the settlement compression layer is mainly concentrated in the interaction layer and above,the lower silt layer,sand layer The settlement is small.At the same time,the basic law of ground subsidence caused by deep foundation pit dewatering in Wuhan area is analyzed by establishing an ideal model for numerical calculation of foundation pit dewatering.However,these conclusions are only obtained by establishing an ideal model,and there is no calculation and analysis of the amount of water in the foundation pit.Therefore,the conclusions obtained must be checked by actual engineering.Based on this,the paper takes the Wuhan Metro Line 5 and Heping Park Station on the Yangtze River Istage terrace in Wuhan as an example to discuss the calculation method of the foundation water inflow in the case of the suspended water curtain in the Yangtze River I terrace.The paper analyzes the calculation of the water inflow in deep foundation pits under four conditions: no curtain condition,small depth of curtain embedding aquifer,a certain depth of curtain entering aquifer,and a large depth of curtain entering aquifer.It is considered that:(1)For the deep foundation pit without curtain,the groundwater seepage is a steady flow,and the calculation of the water inflow from the foundation pit can be calculated by the large well method,and can also be solved by numerical calculation.(2)When the vertical suspension curtain is implanted into the aquifer with a small depth(the depth of the concavation depth h3 is less than 1/3 of the confined aquifer M depth),the Yangtze River I terrace has a sedimentary rhythm,and its permeability coefficient is up to Gradually increasing,The water stop effect of the curtain is not obvious,and the curtain has little effect on the amount of water in the foundation pit.At this time,the amount of water in the foundation pit can be approximated by the analytical solution of Darcy's law,and it can also be calculated by numerical method.(3)When the vertical hanging curtain is implanted at a certain depth of the aquifer,ie M/3<h3<2M/3(h3 is the depth of the curtain inserted into the confined aquifer of the sand layer,and M is the pitted confined aquifer thickness).At this time,the hanging curtain water stop effect begins to be sensitive,and the water stop effect is more obvious.The water inflow from the foundation pit can be solved analytically using the theory of groundwater dynamics,and numerical calculation can also be used.(4)When the vertical hanging curtain is implanted into the aquifer with a large depth(h3>2M/3),the groundwater level of the foundation pit is unstable three-dimensional flow,and the analytical solution is more difficult to solve the water inflow of the foundation pit,which can be numerically calculated.Solution.The key point of dewatering for deep foundation pit engineering in Wuhan area is to prevent seepage damage(sand flow,piping,sudden surge)of high-head and strong permeable confined aquifer.The dewatering method of " mainly reducing drainage,and auxiliary seepage separation" is adopted for the lower confined aquifer,i.e.suspended vertical curtain and reasonable and effective dewatering method for deep well.At present,there is no theoretical basis for the depth of the suspended waterproof curtain embedded in the confined aquifer or the proportion of the thickness of the embedded aquifer in Wuhan area.Based on the analysis of the factors affecting the depth of waterstop curtain implantation,and through theoretical calculation and numerical calculation analysis,the calculation theory of water-crossing gate is preliminarily deduced,and is applied to the actual project of deep foundation pit of sanyanqiao north road station on Wuhan rail transit line 6.it is believed that the method of " hanging curtain+deep well dewatering" is generally applicable to the groundwater control of deep foundation pit engineering on the 1st terrace of the Yangtze river,and can achieve the purpose of effectively controlling the groundwater of deep foundation pit engineering.the actual depth of water-crossing gate is basically consistent with the theoretical calculation.