| Open caisson is widely used,such as in the development of underground spaces such as bridge pier and abutment foundations,starting and receiving wells for pipe jacking construction,underground parking lots,and storage tanks.With the improvement of construction requirements,the sinking well is developing towards a larger and deeper direction.For example,the sinking well of a certain Yangtze River diversion project in Zhenjiang,Jiangsu,has a depth of over 40 meters,posing a huge challenge to the sinking construction of the sinking well.The sinking of a sunken well is closely related to its resistance and sinking posture,and the resistance is mainly the end resistance at the blade foot and the frictional resistance on the side wall.Therefore,it is of great significance for the construction of ultra deep caisson under complex conditions to study the soil failure mode,the size and distribution of caisson resistance,and the attitude control control technology of ultra deep caisson sinking.Based on the open caisson project of a Yangtze River water diversion project in Jurong,Jiangsu Province,this paper studies the settlement resistance calculation and attitude control technology of ultra deep stepped open caisson near the river.The main research content and conclusions are as follows:1.Using OPTUM G2 finite element limit analysis software,upper and lower limit numerical simulation analysis is conducted on the resistance of the blade foot end with tread in the Linjiang rich water sand layer.The failure mechanism of the soil at the blade foot is studied,and the failure modes of the soil at the blade foot of the open caisson under different blade foot shapes,burial depths,open caisson radii,and basin excavation conditions are explored,as well as their impact on the ultimate resistance of the blade foot end,The relationship curve between various influencing factors and ultimate end resistance is drawn.The results show that:(1)for the ultimate end resistance of the blade foot,the smoothness of the sinking well blade foot is much smaller than the roughness of the blade foot;(2)The ratio of depth to wall thickness h/B≥ 15 on the outer side of the open caisson,and the ultimate end resistance of the blade foot tends to stabilize,no longer increasing with the depth of penetration;(3)The decrease in the radius of the sunken well and the increase in the width of the foot surface of the sunken well blade result in a rapid increase in the extreme end resistance of the blade foot;(4)The ultimate end resistance of the open caisson significantly decreases after the bucket excavation of the blade foot soil,which is 0.54-0.85 times that of the unexcavated one;(5)In the future,similar projects can refer to the curve in this article for resistance prediction.2.Considering the soil arching effect,the distribution of soil pressure and the magnitude of frictional resistance on stepped sidewalls are studied and discussed.The paper further conducted experimental research on the drag reduction effect of bentonite slurry,taking into account the influence of the sand content of the slurry caused by the collapse of the sidewall sand soil,and different standing times on the shear friction resistance of the slurry concrete slab interface.The experiment shows that:(1)the formula for calculating the frictional resistance between the wellbore and the sandy soil is f=0.546 P,and the friction coefficient is μ= 0.546;(2)Using bentonite: soda ash: CMC: water=9:0.5:0.2:100 slurry,the calculation formula for frictional resistance of wellbore bentonite slurry is f=0.05P+0.252,and the friction coefficient is f=0.05P+0.252 μ= 0.05,the frictional resistance of the pipe wall decreases by 85% after grouting,and the drag reduction effect is significant;(3)When the standing time exceeds 24 hours,the friction coefficient after standing no longer increases,and the friction coefficient is about twice that of not standing;(4)When the sand content in the mud is 40%,the drag reduction effect of bentonite mud reaches 79%;When the sand content in the mud is 60%,the drag reduction effect reaches 46%.It is recommended in the project that bentonite slurry should have both a certain fluidity and a certain supporting force,and the collapse amount of the supporting sand should be 40% lower than that of the slurry.3.Using PLAXIS 3D finite element software,numerical simulations are conducted to investigate the stability and posture of circular caisson structures with cross beams at the bottom and different excavation methods(symmetric and asymmetric excavation).The results indicate that:(1)when the sinking depth of the open caisson is shallow or when each sinking starts,the top surface settlement and settlement difference are large,which is prone to uneven settlement.At this time,it is important to strengthen observation;(2)The settlement difference of the top surface without a cross beam is relatively large.It can be seen that the cross beam can reduce the uneven settlement of the caisson during the sinking process;In response to uneven soil texture,the cross beam can play a role in regulating the posture of the caisson;(3)Introducing the sinking slope Δm,it is found that Δm with cross beams is smaller than Δm without cross beams,and the fluctuation rate of Δm with cross beams is smaller than that without cross beams.When the sinking depth of the caisson reached 22.5 meters(with a large sinking depth),Δm with cross beams tend to stabilize,further proving that the cross beams can regulate the attitude of the caisson during the sinking process.4.The influence of water head difference between inside and outside the well on the stability of the bottom of the well under different sinking depths is derived,and a graph is drawn showing the relationship between the maximum allowable precipitation height and the sinking depth when the well is not drained.The results indicate that:(1)the stability of seepage and pressure resistance during the sinking of the open caisson is mainly related to the lowering water level inside the well.The larger the lowering water level inside the open caisson,the lower its corresponding safety and stability coefficient;(2)There is a linear relationship between the sinking depth and the maximum allowable precipitation height;(3)When the sunken well sinks to the top surface of the sandy soil,the depth that can be lowered is less than3.4 meters.It is not recommended to increase the sinking coefficient through precipitation inside the sunken well.5.Based on the specific supported project,the feasibility of the sinking resistance calculation chart is demonstrated through calculation of engineering examples.The calculation results indicate that:(1)there are significant differences in the sinking coefficient of the open caisson under different working conditions,ranging from 0.38 to 1.65;(2)In practical engineering,to ensure the smooth sinking of the caisson,continuous pumping of mud is carried out to fill the sidewall of the caisson,while keeping the blade foot as smooth as possible;Control the sinking speed of the caisson by controlling the excavation depth and reserved soil width;(3)The ratio of end resistance and side friction resistance at the edge of the sinking well is influenced by factors such as the roughness of the edge,the state of the sidewall soil,the excavation of the edge soil,and the effectiveness of sidewall drag reduction;(4)The proportion of blade end resistance to total resistance is large,while the proportion of side wall friction resistance is small. |
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