Study On The Principles And Technical Methods For Grid Composite Foundation In Silt

The continuously developing high building or ultra high building request for foundation soil that can undertake more and more high load and whose deformation is much less. The foundation soil in most cities of our country has the binary medium model, which composes with the topsoil and the underneath soil. Because of the weaker strength and higher deformation of topsoil, it can't directly be the foundation of the high building or ultra high building. The traditional design method usually neglects the beneficial factors that the topsoil have a strength at the lateral confinement and the mechanism for deformation of the topsoil, but and directly adopts the expensive pile foundation that cuts through the topsoil, it delivers load and controls deformation to pass the high strength pile and the high bearing capacity soil. Therefore, it doesn't adapt with development direction and request of the social economy and soil mechanics that the foundation depth of the high and higher buildings is more and more deep.Accompany with the extensive development of research about the foundation soil and the demand of economic development, the problems that the foundation depth of the high building or ultra high building is deeper and there is not suitable foundation type using of topsoil are more and more obvious and urgent. In this paper, it was been proposed that the design principle of composite foundation with the lateral confinement structure, and the above-mentioned problem had been resolved. That the research has realistic meaning and objectivity is emphasized, the silt at Zhengzhou that has the typical characteristic of binary medium model is made use of and verified by new foundation type. By many means, such as the large-scale destructive static load experiment, indoor and on-site testing, prototype pile experiment and theories analytical, the interaction mechanism of the grid structure and the soil have been studied thoroughly and systematically.Basing on these results, the design calculation method of grid structure composite foundation and the construction technology and quality control system of foundation had been established through theory of one dimension of solid, theory of compound, theory of average, system standpoint and the design data, In-situ testing data, a great deal of foundation engineering, the international experiment research results. Through author pays more than effort for few years, the main results are as follows:(1)Through engineering Investigation, the indoor and on-site experiment and test, case investigation, theories analysis etc, the thorough research on foundation at Zhengzhou had been conducted in the combinations types of rock and soil, structure types, strength of foundation soil, mechanism of deformation and failure etc. The conclusion had been drawn that the silt failure under the concentrated load carry is not along the suppose shear face of traditional theories to slide, but the silt take place compression firstly and expanded with the center bulge and the side, and swelled up for break soil as follow increasing load, and break soil is outer circle in the foundation, and the way of the silt to be breached is called "compression-expansion-swelled up" .(2)The instability of the silt in foundation is one accumulate deformation and failure that constituted from the different depth local failure. Its process includes the seepage and compression-expansion-swelled up. When there is not lateral confinement, the process of local failure doesn't have limitless factor that the seepage path of the groundwater is shorter and pressure of the groundwater is higher, therefore, the lateral deformation take place usually at the topsoil, and the partial failure at the topsoil takes place. The failure of compression-expansion-swelled up that is according to the certain direction and the order layering constitutes the deformation and failure in the topsoil foundation.(3)According to the lateral confinement effect of the artificial structure on the foundation soil in the composite foundation, the deformation failure mechanism of the foundation soil, and the result of the indoor and outdoor lateral confinement experiment, the foundation of high-rise building is divided to two types including lateral confinement structure foundation and non-lateral confinement structure foundation firstly. In the non-lateral confinement structure foundation, when the foundation soil deformation is in the condition of K₀ (stationary soil pressure), the lateral deformation failure with obscure artificial structure constraint will happen. In the lateral confinement structure foundation, because the effect of artificial structure on the foundation soil, the lateral confinement of the foundation soil is formed and the strength in the vertical direction is enhanced, imperatively makes the vertical deformation of the superficial foundation soil the majority and controls the lateral deformation of which to zero gradually. Based on the one-dimension consolidation theory and the lateral confinement experiment, the design principle of the lateral confinement structure composite foundation is established, and the study on the cement agitation pile grid structure composite foundation, one type of the lateral confinement structure composite foundation, is carried out. It's indicated that the lateral confinement structure composite foundation has the availability in the compression -expansion- swelled up and seepage failure of topsoil and practicability in the shallow bury of high-rise building foundation and the utility of the topsoil.(4)According to the research of the engineering—mechanics property of the cement-soil churning pile and grid structure, the concept was put forward firstly, such as that a single pile is weak strength and is change strength and the foundation is high strength in the composite foundation. After perfected design method and construction craft, based to the analysis of stress and strain dint of the pile in the foundation soil and the test data and the conclusion of stress and strain of the pile, the deduced calculation formula of change strength of cement-soil churning pile is: rily depth noodles strength q_u that is decreased because the difference of the undertaken load p and resistance q of the pile is reduced. The pile strength that satisfies this calculation formula can immediately carry out the affixture stress in the foundation to deliver effectively.(5) Through the research, pointing out the differences between the pile and soil, grid structure, modulus of deformation implication of composite foundation and so on, and which has the relevance between the deformation modulus of grid structure composite foundation and the pile size,pile model measures E₂, the pile week earthen model measures Es,grill structure module E_p and so on. Through the stress condition analysis, inferring the subgrade modulus formula by taking thetheory of elasticity as the foundation is: E = mKK₁E₂ + (1- m)E_s, K₁= p /(p - q),K =E_p/(E_p+E₂). Its relevance through pile rate of displacement m, reductioncoefficient K in pile nose soil modulus, load reduction coefficient K₁ between pile earth expressions and so on.(6)Inferring the grid structure composite foundation bearing capacity f_spk, the bearing capacity reduction coefficientβand so on. The target formulas is: f_spk=β(1-m)f_sk+mR_a/A_p ,β= 1/(1+e) , which eliminatedβhas taken theempirical value the uncertainty, and has established the foundation soil primitive void ratio relations.(7) Through some high building of grid structure composite foundation movement which conforms to each standard, that has confirmed its principle and the method accuracy, according to this, compiled a set to be suitable the Zhengzhou area high-rise construction grid structure composite foundation design, the construction, overseeing, the examination technical guide, and which provided the basis for this foundation type design, the computation.