Research On The Stability Analysis Of Variable Deep Mixed Column Composite Foundation

The embankment filling of highway in soft areas must ensure the stability of whole embankment Therefore, the subsoil under the road embankment must be reinforced. It is known that the composite foundation of cement soil deep mixed columns is a commonly used reinforced method in soft foundations. However, embankment failure or instability accidents are usually met with in real engineering practices. There are mainly two reasons. First, the quality of column body is nonhomogeneous and column-soil synergetic effect is hard to be realized in common cement soil deep mixed columns. Second, the failure mode of composite foundation of cement soil deep mixed columns is unclear, and its calculation method is irrational.To solve the above problems, IGE of SEU combined the technologies of variable cross-section and bidirectional mixing to improve the column quality and enhance the overall performance. In addition to bearing capacity and settlement calculation, stability analysis should also be performed for variable diameter deep mixed column composite foundation. In the stability analysis of this composite foundation, the traditional limit equilibrium method has been applied despite that the inherent characteristic of the foundation was not considered. Hence, a systematical stability analysis was discussed in this thesis by laboratory and insitu tests, theoretical and numerical analysis. The main results were listed as bellowed:1. Mechanical behaviors of the cement soils were first investigated by unconfined compression test, tensile test and triaxial test. Thereafter, the effect of cement content, curing time and initial moisture content on the strength, the empirical formula between strength and curing time and the correlation between tensile strength and unconfined compressive strength of Lianyungang marine cemented soils were obtained. Additionally, the lateral load test of the deep mixing column showed that the main failure models of the column were shearing and bending. Note that the horizontal displacement at failure was significantly suppressed while no improvement of lateral capability was observed in the shearing failure model for the single-lap column reinforced by geogrid. On the contrary, the lateral capacity can be increased in bending failure model with single-lap column reinforced by the glass fiber geogrid.2. Based on the potential possible failure model of the columns, the coupled strength reduction method of corresponding tension and shear damage was proposed. Results showed that the tension failure occurred for columns far from the intermediate embankment and shear failure took place for the columns close to the centre of embankment Hence, the new proposed method could differentiate the failure models of improved groud at different zones.3. The factors affecting stability of the composite foundation were studied by the orthogonal theory and multivariate statistical analysis. The results showed that the column diameter and spacing were the most key factors and, the length and strength were the secondary factors. However, the column strength was the most important factor to depth of slip surface. Additionally, empirical expression for predicting the safety factor and depth of slip surface of composite foundation were built up by multivariate non-linear regression analysis, which can consider the factors, i.e., the external load, column spacing, column diameter, column length and column strength.4. The influence mechanism of embankment stability at design parameters of T-shaped deep mixed columns (such as column spacing, enlarged column cap height, enlarged column cap diameter, column strength), the development of plastic zone, safety factor, and horizontal displacement of typical point were detailed analyzed by 3D numerical analysis. Columns beyond the shoulder were bending failure first at the variable cross-section and the lower section and those located at the center of embankments were of less horizontal displacement and bending moment. Hence, bending failure of adjacent columns were caused by the failure of columns located at the toe, then bending or shear failure of columns located at shoulder occurred, and shear failure of columns located inside the shoulder came into being, with the sliding surface passing through the column cap.5. The instability of cross-shaped deep mixed column composite foundation was composed of the bending failure of columns from the toe to shoulder, and shearing failure of columns located inside the shoulder. Sliding surface outside the shoulder should pass through the upper variable cross-section. The equivalent plastic strain and incremental displacement can be used to descript the evolution of the instability process. Displacement mutation can occur at the same time with plastic zone connection, which may also occur before plastic zone connection. Additionally, safety factor increased and horizontal displacement decreased gradually with the increases of column diameter ratio respectively.6. A practical calculation method of safety factor on bending failure model was proposed, which considered the interaction force between slices, the effect of deep mixing column in active bearing zone to lateral earth pressure, the influence of column cap on anti-dumping moment and anti-bending moment and the differences of failure depth between columns. Finally, the stability of an embankment case in Lianyuangang marine soft clay zone by the traditional circular slice method, tension and shear strength coordination reduction method, and practical calculation method based on bending failure mode, was conducted to compare the stability of the ground improved by the conventional and variable diameter deep mixing columns. The results showed the validity and advantage of design and stability control.