Behaviors Of Deep Excavation Considering Disturbance Effects Due To Construction

With the emergence of the high-rise structure and the underground engineering, the quantity of deep excavation project rapidly increases. In the course of construction, the soils around deep excavation are disturbed continuously, which makes the stress-strain relationship of disturbed soil be different from the undisturbed soil. The influencing factors of soil masses due to the construction disturbance of deep excavation, including various change of stress state, stress path, moisture content, void ratio, failure of soil structure, separation or mixture of soil composition, strength and deformation characters, were analyzed. On these basis, the disturbance for the soil surrounding deep excavation from the construction and the construction risk were researched systematically, which is important on optimizing the design of deep excavation, ensuring the safety and stability and controlling the deformation induced by deep excavation.First, the disturbance due to construction is analyzed, and the expression of generalized disturbance degree and the evaluation system of disturbance induced by construction are put forward. According to three phase diagram of soil, the quantitative expression for disturbance due to construction of deep excavation is derived out, and the effect of disturbance on strength indexes of geomaterials is deduced using Taylor series expansion. The effect of disturbance on earth pressure and the stability of deep excavation are analyzed respectively, and the results show it shouldn't be neglected during the design and construction. On the view of controlling cutting of soil and lowering water table, the measures to reduce the disturbance due to construction of deep excavation are expounded in detail.Second, the experimental study on the soils in Hangzhou area show the soil has strong structural property, which displays high sensitivity (the index of sensitivity is about 6.7), the curves of compressibility are distinctively different between undisturbed sample and remolded sample in simple compression test. The stress-strain curve is generally strain hardening type, which can be described with hyperbolic functions. Under identical stress path, the failure stress of test specimen is increased with the consolidation pressure. The stress path has the intense influence on the deformation of soil surrounding deep excavation. Under different stress way, there is diversity for soil behaviors.Third, the results of direct shear test show that the moisture content has a great effect on the undrained shear strength and parameters of soil, cohesive strength and internal friction angle of undisturbed sample is varied with the quadratic trinomial of moisture content and decreased with the increase of moisture content. In additional, compared to the internal friction angle the effect of moisture content on cohesive strength is larger, and tends to the uniform with the increase of moisture content, in the meantime they are depend on the structure strength and binding effect of soft soil.Forth, the in site test for soil disturbance due to construction indicate that the physical and mechanical properties of soils, such as moisture content, void ratio et al., varies with the construction disturbance. The cone penetration tests in active zone show that the resistance on cone head for soil is reduced slightly due to excavation and is related to the depth of soil, the distance to retaining wall.Fifth, the in site tests of anchors shows the distribution of the friction force is not uniform and the stress attenuates along the length of anchor, which is the maximal in the orifice of hole and related to the level of friction force. The prestress in anchor varies in time and is correlative to the grouting amount, location for itself and locking load. The pullout resistance of anchor is sensitive to the excavating for deep excavation, and the displacement of retaining wall is influenced in certain degree when applying prestress on the anchor.Sixth, the characters of stress and deformation during the construction of deep excavation are analyzes, beginning with the stability of deep excavation, the concept of strong disturbance zone from excavation is set up. A elastic wedge model is used for analysis of disturbed zone of step-sloped deep excavation. The disturbed zone in the process of cutting can be calculated according to elastic plane-strain theory, and it is interpreted by an example. According to the zoning of disturbance due to excavation, the relation between the deformation and the engineering behavior of soil will be able to study and the corresponding support and reinforcing measures will be set out.Lastly, on the basis of past studies on disturbed functions, the expression of equivalent disturbed function is put forward. Relative intact state soil is modeled by elastic model and fully adjusted state soil is modeled by modified Cam Clay model respectively. The constitutive model based on the disturbed state concept is established, whose calculated results are similar to ones of the test, and there is uniform tendency between them. Then constitutive model developed is introduced into the finite elements analysis, and the results come from numerical calculation for the deformation of retaining wall is uniform to the measured in site. With the development of excavation, the disturbed zone is developed gradually and the equivalent disturbance degree increased too. Among the four influencing factors including the rigidity of retaining wall, embedded length of retaining wall, elastic modulus of retaining wall and the rigidity of inside supports, the first have a large effects on the soil disturbance.