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This paper conducts an experimental study on the variable modulus foundation for frame-tube structure. The base reactions are measured and its correlation with deformation characteristic is analyzed. The following conclusions are summarized: (1) soil cement (volume ratio is 1:4) is used to strengthen the foundation soil at core tube area. The ratio of moduli of foundation soils at core tube area and external frame column is 2.23. The vertical deflections of medial axis and diagonal are 0.08‰ and 0.1‰. (2) Under the working loads, the base reaction at core tube area is highest and distributed uniformly. The base reaction at the area between the core tube and the external frame column decreases gradually. The overall distribution of base reaction shows a basin shape. (3) The ratio of foundation modulus at core tube area and external frame column area is 2.35. The ratio of base reactions at these two parts is 3.31, which shows a good correlation with the foundation modulus.

Based on theoretical analysis and experimental result, the principle as well as the interaction between pile and soil, the design parameters of composite foundation with cement-fly ash-gravel piles (CFG piles for short) are optimized, which are pile length, pile diameter, pile spacing, pile strength and thickness of cushion layer, in order to put forward the reasonable design parameters for reaching economic and reasonable target. Through the optimized analysis, it is put forward that bearing stratum of pile tip should select a relatively better soil layer, the diameter of CFG pile is usually appropriately to select between 350mm and 500mm, it is usually appropriately to select larger pile spacing under the permissible range between 3d and 5d being ruled by standard, it is appropriately to enlarge the pile diameter to reduce the requirement of pile strength in high rise and super high rise building which requiring bearing capacity of single pile is high, and the thickness of cushion layer is appropriately to select between 0.4 and 0.6 times of the pile diameter.

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