| Granitoidis one of the most widely distributed rock on the mainland, is a fundamental part of the continental crust.Weathering along the joint enters into the internal deep rock mass, forms thick redweathering crustbecause of the granite joint development, this is an important characteristic of granite landform in southeast China.Residual soil comes from the rock at the top of weathering crust, its original rock structure largely disappeared and remain in situ by the intense physical, chemical, and biological weathering.As a type of red weathering crust, Granite weathering crust has been widely attention due to the strong heterogeneity of the weathering and engineering geological factors of zonalGranite residual soil is a kind of special soil, and has its unique composition and structure features of engineering characteristics.Because of granite weathering residual soil with special engineering properties, easy to cause the soil strength after the encounter water loss, softening and disintegration, lead to geological disasters, often brings huge risk to construction and economic losses.With the speeding up of urbanization in our country development, many cities embarked on large-scale orbit transportation construction to alleviate traffic.In the construction of the subway and inter-city rail transit engineering in the southern coastal city, in order to avoid large area occupation of land, most adopt the way of the underground buried and tunnel construction at the station, the construction of underground engineering in granite residual soil engineering geological problems caused by a large number of emerging, then cause engineering accident in growing in number.Granite residual soil softening, pile foundation failure, slope instability, the tunnel collapse, deformation and so on engineering accidents.Starting from the reality of the practical need of engineering, to carry out the engineering characteristics of granite residual soil and engineering studies, through the comprehensive and systematic study on the engineering properties of granite residual soil, insight into the cause of its engineering geological problems, especially how to solve the engineering problem of granite residual soil, preventing engineering accidents, is undoubtedly an important and urgent research subject, not only has important practical significance, also has great economic value.Based on engineering example of Guangzhou Metro construction in granite residual soil area, relying on Guangzhou Metro corporation scientific research funds, in the Guangzhou area of geotechnical engineering characteristics of granite residual analysis and the research topic of metro construction method, through in-depth study systematically the engineering properties of granite residual soil, underground digging foundation pit design, granite residual soil treatment measures, etc., effectively solve the above problems, for engineering construction in the granite residual soil areas to provide reference for design and constructionStudy on the cause, the particle size composition, the physical and mechanical properties and the engineering properties of softening of the granite residual soil in Guangzhou areathrough data collection, outdoor drilling sampling, laboratory test, statistical analysis methods; Through Guangzhou Metro foundation pit design, mechanical calculation, numerical simulation, field experiment, in Guangzhou Metro line 6 second phase Station Luogang foundation pit of granite weathering foundation pit as an example, in-depth studied systematically in the foundation pit precipitation technology solve the granite residual soil treatment technology, effectively solve the Guangzhou Metro granite residual soil of problems in the design and construction of foundation pit.It has an important guiding significance for Guangzhou Metro engineering construction, also provides the reference for other cities similar subway under the condition of foundation pit construction.Main research conclusions and achievements:(1) The granite residual soil is the main product in the chemical weathering stage. The soil fraction character reflects the non-thoroughness in the physical weathering stage. The property of disintegrating and softening in the granite residual soil is greater than that in the migmatitic granite residual soil. The granite tends to be stable, and of less chance to soften and disintegrate, when clay particle (d<0.005mm) content exceeds 30%.(2) The natural water content is closely related to the environment. The water content of the concealed granite residual soil is higher than that of the surficial granite residual soil. Naturally, the granite residual soil is under hard-plastic-to-plastic condition, characterized as low liquid limit soil. It is of medium compressibility, and of week water permeability. Since the free swell ratio is low, it is not characterized as swelling soil. The three main microstructures are framework structure, flocculent structure and charged structure.(3) There is a large difference between the mechanical properties of granite residual soil and migmatitic granite residual soil. The cohesion and the friction angle in natural and saturated state vary with the saturability. Shallow as the ground water buries in the southern areas, the granite residual soil lies under the ground water for long, tending to be saturated. Thus, the cohesion and the friction angle in natural and saturated state vary in a small range.(4) Influenced by the material composition, granularity and internal structure, the granite residual soil has the natural character of water-softening, disintegrating, and strength-losing. It is inevitable to water-soften and to disintegrate under water. The main factor to control the disintegration time is water content. Lower the water content is, sooner the soil disintegrate. As the compactness rises, the granite residual soil is less disintegrative. Based on the grey correlation degree calculation, the correlation between factors and the disintegrating velocity in the experiment is revealed as X4 (valid porosity)> X5 (valid void ratio)> X2 (compactness)> X, (water content)> X3 (temperature). As the valid porosity rises, the immerging degree gets high, as with the high disintegration velocity and low anti-disintegration ability.(5) The soil strength indexes like c, (p tend to increase, but with less amplification, as the rainfall deepens. Rainfall strengthens the soil and protects the excavation effectively. The vertical permeability coefficient of each soil layer rises with the excavation depth and rainfall depth. The horizontal permeability coefficient falls as the excavation depth and rainfall depth increase. The reasons are as follows. Unloading made by the foundation pit excavation decreases the soil pressure and the soil void ratio, therefore increasing vertical permeability coefficient. Meanwhile, rainfall decreases the soil void and permeability horizontal coefficient. Since the soil water content is of great influence on the granite residual soil property, groundwater level must be lowered below the bottom before excavation.(6) The underground diaphragm wall is of fine continuity, suitable for kinds of grounds and groundwater conditions, and of high water preventing property. Considering the disintegration of granite residual soil, underground diaphragm wall and inner support are adopted in subway pit. In the same condition, the horizontal displacement calculated by the Li-Zheng deep pit calculation program is greater that by the Qi-Ming-Xing calculation program. As a vital pit stability factor, the horizontal displacement in the granite residual soil region is recommended to be calculated with the Li-Zheng deep pit calculation program.(7) Theoretical calculation, numerical simulation and field experiment are used to study the reasonable value of diaphragm wall embedded depth, in case of the open cut excavation in Tieluo Station of Guangzhou metro. It is showed that the excavation will be safe and the structure will be stable as the diaphragm wall embedded depth value is changed from the original design of 8 m to 7m or 6m. With the increase of the embedded depth value, the outward deformation of excavation changes little, whereas the inward deformation appears to decrease, and the axial force of steel support shows the same trend. It indicates that the safety factor characterizing the excavation stability is positively associated with the embedded depth value.(8) The weathered granite treatment programs are compared and elected under the unified economic indicators, which indicates that treating the granite residual soil through excavation dewatering is the most economical and reasonable method. In order to prevent softening and disintegration of granite residual soil through excavation dewatering, dewatering 20 days earlier than excavation, designing precipitation wells carefully and constructing in strict accordance with drawings. It is showed by practical test data of Tieluo Station that water level is dropped below excavation bottom, reaching expecting target. In addition, softening of granite residual soil is controlled effectively, and excavation is in a stable and safe state.(9) Numerical simulation is used to analyze characteristics including horizontal and vertical displacement, principal stress, and moment of diaphragm wall, axial force of internal bracing, vertical displacement of the soil outside the excavation. The analysis considers four operating conditions including dewatering one time or by step, and permeable or waterproof diaphragm wall. On the occasion of dewatering and excavating step by step, the diaphragm wall gets sinking as dewatering, while uplifting as excavating. Wall cracks and loose of joints linking wall with internal bracing appear because of vertical displacement fluctuation of diaphragm wall. The settlement value caused by dewatering one time is twice at least more than dewatering step by step, and the minimum principal stress value of diaphragm wall is 1.3 times more. The numerical simulation results of horizontal displacement of diaphragm wall and axial force of internal bracing are similar to field tests, and it will be reference for practical applying. |
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