Dynamic Behaviors Of Composite Foundation With Applications

Composite foundation made of cement mixing column or stone column is widely used in civil engineering because of its low price and efficiency. Complicated properties of the composite foundation and limited construction technique declined the composite foundation to fulfill the engineering safety. Therefore, field test to detect this kind of foundation seems to be very important. Block vibration test appears better foreground of market and tendency as it's higher rate of sampling and lower cost conpared to static loading test. Numerical analyses and primary practices of block vibration test to composite foundation made of cement mixing or stone columns are intensive studied in this dissertation. The main research work herein consists of the following parts:(1) The formulations of axisymmetrically infinite elements for dynamic analysis of vertical vibration problems in saturated composite foundations are presented. The theoretical basis as well as the implementation of the elements is discussed, and the element decay functions are derived using the analytical solutions of axially symmetric configurations. Using the proposed finite-infinite element method, the surface vertical displacements of air-saturated soil'(dry'soil) and of water-saturated soil with extremely low permeability subjected to a surface point excitation (called as the Lamb's problem) are calculated and the results agree very well with the existing theoretical solutions of single-phase elastic media. As an application, the velocity admittances of a concrete block resting on cement mixing or stone column saturated composite foundations are calculated. The influence of soil permeability and column rigidity on the dynamic response of the composite foundations is investigated.(2) Three kinds of infinite elements coupled finite element method are proposed and developed to the theory of wave propagation in viscoelastic medium. The computation program is compiled with FORTUNE. Using the program, vertical vibration properties of block on homogeneous foundation, stratified foundation and composite foundation with single column are studied. Influencing factors on admittance of block on homogenous foundation, including propagation velocity of shear wave of soil, diameter and thickness of the block, the location of response sensor and so on, are systematically researched. As to stratified foundation, influences of the thickness and embedment depth of soft soil stratum on velocity admittance of block on dual and three layered ground are primarily studied. The dynamic properties of block on the composite foundation with single column are related to the column dimensions and stiffness of soil surrounding the column. The effects of the column length, diameter or sectional area, the velocity of shear wave in column or soil surrounding it on velocity admittances corresponding to low frequency or resonant frequency are studied. The critical length of integrate column is given. The position of column defects is analyzed. The results display that the composite foundation stiffness is relevant to velocity admittance amplitude to low frequency.(3) By using the axis-symmetrical finite-infinite element hybrid method based on the elastic half-space, the sensitivity of mechanical admittance to the Young's modulus of column shaft along the depth is numerically analyzed, including effects of the shear wave velocity of subsoil and column length as well as defects of column shaft. The difference of sensitivity between cement agitated column and stone column is also analyzed. The mechanical admittances at low and resonant frequencies are investigated to evaluate the properties of composite foundation with defects on the column shaft or subsoil.(4) A program is developed with Levenberg-Marquardt method belonging to the least square method. By using the velocity admittance of the block on single column composite foundation, the mechanical parameters are inversed, and the effects of the defect variation in the column shaft and initial value to the inversion are analyzed. Several numerical simulations primarily prove the potential applicability of this method.(5) The behavior of vertical rigidity of circle loaded raft on homogenous elastic layer affected by friction of raft bottom is analyzed with FEM. The rigidity and deformation modulus of unsaturated or saturated sandy soil foundation will be underestimated with present formula when Possion's ratio is low because of the hypothesis as the load raft is perfect smooth. The dynamic stiffness of foundation derived from admittance at low frequency in resonance test are relevant to inertia force and damp ratio of the vibrating system, the conventional formulae of dynamic stiffness will be revised.(6) Two practical examples are introduced. The static loading test, mechanical admittance and block vibration tests are performed. The results of tests are in conformity with the research work in this paper. Using the inverse program, the elasticity modulus of cement mixing column is carried out based on the block vibration test results.