Research On The Relationship Between Deformation And Design Parameters For Ultra Deep Foundation Pit Retaining Structure In Wuhan Yangtze River I Terrace

Since the beginning of twenty-first Century, with the rapid development of the construction and municipal engineering in Wuhan area, more and more foundation engineering and underground engineering appeared, which is more and more complex, and the scale and the depth of excavation is also larger and larger. Wuhan is prospered of water, the focus of city construction on both sides of the Yangtze River and Hanjiang River, deep Foundation pit is mainly concentrated in I-grade terrace of Yangtze River, however, the contradiction between the trend of development and the complex engineering geological conditions、hydro-geological conditions and the requirements of environmental protection and other factors in Wuhan Yangtze River level-I terrace area have become increasingly prominent, which has put forward many challenging project in the research, design and construction of deep foundation pit engineering, deep foundation pit supporting technology has become the hot and difficult in the construction of high-rise building in Wuhan area.Reasonable selection of soil shear strength index is the assurance to obtain realistic earth pressure distribution patterns. And it is basis for determination of security and economic support program. However, due to the conventional geotechnical tests ignore the specific conditions of deep foundation pit engineering,resulting in the experimental parameters have some deviation from the reality, which may lead to security problems of deep foundation pitor economy unreasonable. Therefore,for Wuhan Yangtze River level-I terrace, studying soft soil sampling、 shear strength test method、selection of shear strength index and obtaining physical and mechanical properties of soft soil engineering statistical laws and summarizing envelope deformation and studying optimal soil pressure mode and analyzing deformation factors and proposing deformation control method have important practical significance. These studies help to improve the design theory of foundation engineering, provide an important scientific basis for the design and calculation of deep foundation pit engineering in Wuhan and similar area.Take ultra deep foundation pit of the Zhonghai International Building in Wuhan Yangtze River level-I terrace as example. And take static sounding tests、soil sampling、soil test and physical and mechanical parameters as the basic starting points. Make correlation analysis between physical and mechanical parameters to obtain the statistical laws of soft soil physical and mechanical properties. Study the selection of soil shear strength index by using elastic foundation beam method and 3D finite element. For value problem of level inner support stiffness, derive horizontal stiffness theory analytic of the ring support system, and verify the correctness of theory analytic by using finite element numerical simulation method. Discuss deformation law of supporting structure combined with the measured data. Study the factors and deformation control method to foundation pit by 3D finite element. And consider the impact of groundwater on pit deformation by using coupled seepage and stress analysis. The main conclusions of this article as follows:1、Analyze the deformation and distribution model of earth pressure in Wuhan Yangtze River level-I terrace according to measured data deeply. Horizontal displacement curves of underground continuous wall basically presented "drum" phenomenon with both ends small and middle large. Wuhan Yangtze River level-I terrace presented typical two elements sedimentary law with deep weakness sticky soil upper and sandy soil lower part. The deformation form of retaining wall with supporting structure mainly presented arcuate deformation curve. The location about maximum horizontal displacement of underground continuous wall in deep foundation pit of the International Building is at 0.56H～0.64H, the average value is 0.59H. For deep weakness sticky soil upper, measured earth pressure is much smaller than rankine active earth pressure.It shows that measured earth pressure is between cq and cu shear strength and the caculated rankine active earth pressure is too large under using qq shear strength from core barrel. it is used cu strength indicators for a significance level of excavation firstly; it can be used for second and the three levels of excavation. For sandy soil lower part, the water and earth pressure is more close to measured data by using soil-water divided calculation caculation than soil-water jointly calculation.2、Obtain physical and mechanical parameters indicators of target stratum by site sampling and soil test in soft soil of Wuhan Yangtze River level-I terrace deep foundation pit. Analyze the correlation of physical and mechanical parameters indicators.Get the statistical laws physical and mechanical engineering properties of soft soil.Meanwhile study the influence of sampling method to physical and mechanical parameters indicators. Pointed out problem of sampling in Wuhan Yangtze River level-I terrace. Basically void ratio increases with the increase of natural moisture content. There is a high degree of correlation between them. The natural moisture of Wuhan Yangtze River level-I terrace Zhonghai deep foundation pit most lie in 0.85 to 1.5 times the soil liquefaction limit. Basically plasticity index increases with the increase of liquid limit water content. There is a high degree of correlation between them. But there is a lower degree of correlation between plasticity index and void ratio.There is a high degree of correlation between shear strength index under different test methods and Ps value. Get shear strength index by Backcalculating Ps value which is having certain reference value. This article provides the corresponding shear strength index when Ps value is 0.4-0.8MPa. The shear strength index of core tube sampling is significantly lower than thin-walled borrow sampling, and natural moisture and liquid limit index higher than thin-walled borrow sampling. In a word, core tube sampling to the disturbance of soil is largrer than thin-walled borrow sampling.The shear strength index is low. Now, Commonly use core tube sampling in Wuhan Yangtze River level-I terrace. And use qq shear strength index in design.Overall, the shear strength index of core tube sampling significantly lower. It will eventually result in overly conservative in design and make poor economy.3、Firstly propose the level stiffness calculation analytic in the ring support of foundation support system. Analyze the scope of theory analytic deeply and verify the rationality and applicability of the analytic style by finite element methods.Study the effects on level stiffness of the ring support system.Put the support into two categories which are force lever member and force transmission lever member.so that the force model of the ring support system is more clear. the level stiffness calculation analytic in the ring support which is proposed in this paper can provide guidance for the design of foundation with ring support system.4、Obtain shear strength index of target formations in different experimental conditions based on deep excavation in soft soil sampling and soil test site. The deformation calculation analysis of deep foundation pit was made by elastic resistance method and numerical simulation of three-dimensional finite element. Get the deformation of foundation pit envelope structure and earth pressure and compare with the measured data, explore soil shear strength index test method which is suitable for the design of deep foundation pit in Wuhan Yangtze River level-I terrace. Under elastic resistance method, we can get that using under cq test conditions strength index is the closest to measured data based on provision of "m" value in Hubei Province local standards of technique regulations of foundation pit engineering when carry out design calculations of excavation.The shear strength was very low by qq test with the soil sampled from the core barrel, lead to the max calculated displacement of diaphragm retaining walls, which was far from the measured data, therefore, core tube should be prohibited when sampling in Geotechnical Engineering. On 3D numerical simulation, soil strength parameters under the uu, qq, rock core qq experiment condition, the horizontal displacement of underground continuous wall analog value is too large, and far away from the measured values, which should not be used as soil strength parameters of a foundation pit design; but under cu experiment condition, the horizontal displacement simulation of underground continuous wall is closest to measured values, cq followed. Earth pressure calculated and measured values are gradually increased with increasing depth, which has basically consistency law of development, within the scope of excavation, the measured earth pressure value is less than calculated value of the rock qq,qq and uu experiment condition, but closer to calculated value of the rock cu experiment condition, cq followed, which is consistent with underground continuous wall horizontal displacement law. The cu strength indicators are recommended to use. But from an engineering point of view the application, view of the horizontal displacement simulated value of underground continuous wall has little difference under the cq,cu test conditions, the earth pressure value under the cu test condition is less than the cq test condition, compared to cu, it is safer to use cq indicators in pit design, and cq indicators are more easily available in indoor geotechnical experiments, therefore, excavation designed can be used the cq strength indicators. Author recommend:it is used cu strength indicators for a significance level of excavation firstly; it can be used for second and the three levels of excavation.5、Carry out researching on deformation of soft soil foundation pit based on Hydromechanical Coupling,analyzing the effect on deformation of deep foundation and distribution of earth pressure taken by Groundwater seepage, provide a theoretical basis for groundwater controlling. Groundwater seepage do a greater impact on the force and deformation of retaining structure, and it has the maximum flow rate in the sand layer which has better permeability, the results are more consistent by 2D and 3D Hydromechanical Coupling analysis. Compare with the measured data of diaphragm wall horizontal displacement and settlement outside the pit, it’s necessary to do research on foundation pit by Hydromechanical Coupling. Also the groundwater level changes with the season and changes of source water have some influence on groundwater seepage and foundation pit deformation. As for ZhongHai international building which use the Vertical Cut-off Wall Keyed into Impermeable Stratum, the higher water table outside the pit, the bigger deformation and moment of diaphragm wall, also the settlement outside the pit and earth pressure. It’s necessary to monitor the groundwater outside the pit which can provide timely monitoring information to guide construction. The small horizontal displacement of wall is small but the settlement of ground is big when the waterproof curtain is deep. If the Vertical Cut-off Wall haven’t be setted to the impermeable layer, the flow rate will increased by 8 times, the upheaval of the excavation is significantly increased. The permeability of waterproof curtain has some influence on the deformation of foundation pit, the big permeability of waterproof curtain, the small wall moment and ground settlement. However,the upheaval of the excavation is small. If the permeability of waterproof curtain is less than 10-5m/d, it’s reasonable to ignore the influence on force and deformation of the retaining structure taken by the permeability of waterproof curtain.6、By using the orthogonal experimental design principle, the sensitive degree of soil strength parameters,the underground continuous wall thickness, ring beam stiffness, emission beam stiffness and modulus of elasticity to the influence on deformation of foundation pit supporting system is analyzed, and the level trend of various factors is investigated.The range value of the factors of soil strength parameters is maximum of 141.0mm, and the level trend curve changes significantly, so it is the key factor. The range value of the factors of underground continuous wall thickness and modulus of elasticity are 101.7mm and 93.7mm, which are close to each other and the change of level trend curve is second to the factors of soil strength parameters, so it is the important factors. The range value of the factor of ring beam stiffness and emission beam stiffness are54.8mm and 38.3mm, and the level trend curve changes gently, so it is a general factor. The soil strength parameters directly influence the amount and distribution of soil pressure, and the soil pressure loads on the supporting structure, which plays a decisive role in the deformation of supporting structure. Therefore, the selection of soil strength index should be the focus of foundation pit design, which just confirms the importance of the research on the selection of strength parameters of foundation pit of Zhonghai International Building located in Wuhan Yangtze River I grade terraces.7、By using the finite element numerical simulation method, the influence of the thickness of the enclosure wall, the closed ring beam number and sectional dimension of inner support, dowel bar section size and vertical spacing and digging sequence on deformation of retaining structures are studied, which provides the basis for deformation control.The underground continuous wall thickness has greater effects on its horizontal displacement, and the reasonable underground continuous wall thickness should consider the surrounding environment comprehensively. The factors of foundation pit retaining structure deformation control standards are determined. For the circle inner supporting system in this paper, the closed ring beam section size has a bigger effect on deformation of foundation pit, while the emission beam section size has a smaller effect on deformation of foundation pit. Therefore, increasing the ring beam and the ring beam section stiffness can improve the level of the inner support stiffness to effectively control the deformation of foundation pit. Track number of inner support and the vertical layout of support make a larger influence on deformation of foundation pit.Compared with the two tracks inner support, the displacement of foundation pit retaining structure is minimal under the circumstance of three tracks inner support, while the vertical layout position of support plays an important influence on the deformation of foundation pit by using the two tracks inner support. The first track of inner support is general set in the crown beam elevation, and when the second track support is set in the range of 0.56H-0.64H, the deformation of foundation pit can get control extremely. Therefore, setting the vertical position of support reasonably is an important aspect to control deformation of foundation pit. Digging sequence has a certain influence on the underground continuous wall horizontal displacement. For example, symmetrical excavation can reduce the maximum horizontal displacement of the underground continuous wall. However, the maximum horizontal displacement is not occurred in the completed of a certain layer of soil excavation, but when the earthwork quantity of asymmetric excavation is the biggest. To inner support deep foundation pit, which has a prominent spatial effect, hierarchical symmetrical excavation can also effectively control the deformation of foundation pit.Innovations of this dissertation mainly reflected in the following three points:1、It is the first time to study systematically the soft soil shear strength parameters selection of Wuhan Yangtze River I grade terraces and obtain a test methods which is suitable for the Wuhan Yangtze River I terraces soft soil shear strength indicators selection. Having a discussion about how the geotechnical shear strength parameters (cohesion, angle of internal friction) affect deep excavation earth pressure and deformation of the supporting structure in the ultra-deep foundation engineering design of Wuhan Yangtze River I grade terraces, through taking undisturbed wild soil, laboratory tests and obtained the soil shear strength indicators of various test conditions(qq:quick shear experiment, cq:consolidated quick shear experiment, cu:triaxial consolidated undrained experiment, uu:triaxial unconsolidated undrained experiment), and had a comparative analysis combined with on-site monitoring earth pressure and support structure displacement, focus on solving the relationship problems between the geotechnical shear strength parameters optimization selection and supporting structure deformation in Wuhan Yangtze River I grade terraces ultra-deep foundation engineering design, and prospective research is made about the next edition of Hubei Province local standards "excavation Technical Specification" revision.2、Systematically derive horizontal stiffness theory analytic of the ring support system for the first time.Analyze the scope of theory analytic deeply and verify the correctness of theory analytic by using finite element numerical simulation method. Analyze the factors to horizontal stiffness of the ring support system. It can provide strong theoretical basis for ring support design of foundation pit. About ring support system, obtain horizontal stiffness theory analytic by theory analytic deriving. Take Zhonghai ultra deep foundation pit as a example, set up numerical analysis model for the ring support system. Then wen can get the horizontal stiffness of ring support. Compare with analytical solutions.It can verify rationality of theory analytic and provide basis for selecting value of ring support horizontal stiffness.3、Initially establish statistical relationship of physical and mechanical engineering properties indicators in Wuhan Yangtze River I grade terraces.And provide total stress and effective stress of shear strength index according to Ps value. But there is no effective stress index of silt and silty soil in Hubei Province local standards of technique regulations of foundation pit engineering. The paper provides the corresponding shear strength index when Ps value is 0.4-0.8MPa according to fit regression curve equation under qq and cu test. It can provide some basis for selecting shear strength index.4、Deeply analyze the deformation law of deep foundation pit in Wuhan Yangtze River I grade terraces. Get the key factor of Influencing deformation in foundation pit. And put forward the deformation control method of foundation pit. Wuhan Yangtze River level-I terrace presented typical two elements sedimentary lawwith deep weakness sticky soil upper and sandy soil lower part. The deformation form of retaining wall with supporting structure mainly presented arcuate deformation curve. The location about maximum horizontal displacement of underground continuous wall in deep foundation pit of the International Building is at 0.56H-0.64H, the average value is 0.59H. Explored the impact degree to deformation of foundation pit by using the orthogonal experimental design principle which include soil strength parameters, the underground continuous wall thickness,ring beam stiffness, emission beam stiffness and modulus of elasticity. The soil shear strength index is key factor. Analyzing the selecting of soil shear strength index is having great practical significance. It can control the deformation of foundation pit by increasing the ring beam section stiffness、seting a reasonable vertical position for support、carrying out layered symmetrical excavation.