Research On Theory And Technical Measures Of Composite Foundation Optimization

A series of new growth point appear recently in the engineering practice of composite foundation deign and construction from the angle of its optimization. To make them summarizing and improving in a higher level so that to form a new transformation and a new design theory of composite foundation is now an important work both in practical and in theoretical sense. This paper is just aimed at the direction pushing these growth points a further mature in application and establishing the theory and measure of composite foundation optimization.1. The general target, the fundamental principal and the technical measures of composite foundation optimization are given and summarized, which makes the scattered and isolated methods for optimizing the composite foundation systematical and theoretical and then establishes the fundamental frame system of composite foundation optimization. This frame system is based on the interaction theory of the whole system of composite foundation, including the foundation, pile, soil basement, cushion and also pile platform, on their regulations of variation in different stages of composite foundation construction and execution, and also on the series of effective techniques in soil base treatment, hitting the target at harmonizing the requirements both in basement bearing capacity and basement settlement of practical engineering object. It is faced to"the whole system of composite foundation", to"the principal sequences of composite foundation"and to"the manifold measures of soil base treatment"for finding the optimized scheme in view as a whole, which grasps the essential factors reflecting the composite foundation optimization.2. The method coupling the appropriate results of prototype measurement with that of numerical calculation is adopted as an effective method in researching the problems of composite foundation optimization. An elastic-plastic analyzing model in large displacements is developed based on the Biot's consolidation theory, taking the ADINA software as its calculating platform. This model is characterized by applying the Cambridge model to describe the base soil, the contact mechanics model to describe the contact property between soil and piles, the"cell life death decay"technique and the"Reset and dig element death decay"to simulate the excavation of pile holes and stone filling in them, the"Rebar element"to simulate the vain net strengthening the soil base. These give a possibility to model flexibly the bearing characteristics of composite foundation in types of pile, of stratum and of pile-stratum during different period and order of their construction. This appears an obvious advance in calculating technique of composite foundation.3. The action mechanism and characteristics of composite foundation under some typical conditions selected as examples of optimizing in types of base soil improvement, in construction order as well as in post-stabilization of base soil are analyzed by using the developed software and calculating model. The result of analysis under the condition optimized in type of basement improvement has revealed the differences of bearing characteristics between the composite foundations in type of pile and in type of stratum, has improved both the definition of"post-construction settlement"of composite foundation and the method of its determination, and has analyzed also the effect of stiffness of pile and net, the effect of permeability of pile and base soils, and the effect of loading action on the magnitude of"post-construction settlement"during the execution of composite foundation. The result of analysis under the condition optimized in construction order of a composite foundation built with small piles of concrete without sand shows that the composite foundation of different construction order has different bearing capacity, the better planed in construction order composite foundation can bring the supper-construction weight into play and speed up the process of soil compression and soil consolidation, that leads to improving the bearing capacity of soil base or composite foundation. Meanwhile, a large part of settlement occurs during the period of construction, which largely decreases the"post-construction settlement"of building. As for the composite foundation treated by the method of post-construction stabilization, such as strengthened by different method in shallow part of soil base (for example, the soil replacement) and strengthened by grouting the deeper part of soil around piles, the result of their analysis has proved the fact that even the shallow improvement technique can obviously increase the bearing capacity of composite foundation. The influence of strengthened depth on basement deformation, on bearing capacity and on plastic domain in soil between piles are also analyzed.4. The methods of determining the"pile-soil stress ratio"and determining the"ultimate bearing capacity"of composite foundation under the condition of numerical calculation are proposed, which makes their values calculated close to that in practical condition. It makes also a progress in the field of numerical calculation of composite foundation.5. The shallow strengthening technique is extended to form a new thinking about"adjustable-bearing capacity design"of composite foundation, which can be used not only to avoid the shortcomings of the test result in laboratory, but also to apply simply, conveniently and easily. This possibility comes from that the piles as a main part of composite foundation are usually established at first, which gives a convenient condition to conduct the field loading test and to obtain a loading-settlement curve of soil base. Based on the change of general modulus of deformation before and after soil improvement this curve can be used to put forward a non-elastic method for estimating the bearing capacity of composite foundation. Such a method has been recommended and compiled it into a calculating program in this paper. The validity of the thinking and method of"adjustable-bearing capacity design"of composite foundation has been proved by comparing the results from numerical calculation and prototype measurement.The abovementioned series of research work may be taken as an important step leading the theory of composite foundation from its fundamental to its optimized and promoting the later theory into practical application.