Notwithstanding the statistical theory of mechanics of granular material proposed by the senior author, was established basing on experimental evidences, still it contained several uncertain assumptions. By the bi-axial compression tests on assemblage of equal balls, almost all of assumptions were ascertained. Observing the failure phenomena, some interesting and suggestive findings were added. When the assemblage is compressed, loosely packed assemblage grows into denser one until its deviation of void ratio s approaches to a certain value s₀ before failure. The value k which was proposed before as a parameter connecting void ratio, deviation of void ratio and shearing strain was found to be almost constant during failure independently of the initial arrangement of balls, whereas the above explained s₀ depends thereon. The expressions connecting void ratio and angle of internal friction obtained before are still valid even if non-zero constant s₀ was introduced so as to fit in with experimental evidences.

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The increase of the ultimate bearing capacity of piles driven into a losse sand was analysed by a theory that included the change in the angle of internal friction of sand, φ, caused by pile driving.The results of some laboratory and field investigations are analysed by the proposed theory and a method is developed to estimate the increase of the ultimate bearing capacity of a single pile in pile groups driven into a losse sand.

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The paper consists of two parts. In Part I the effect of pile driving on the surrounding soil is discussed on the basis of observation of the pore water pressure in the soil, and the earth pressure and pore water pressure on the pile surfaces. Part II discribes the result of load tests on a single pile, group pile, and spread footing foundations. The piles were steel piles 30 cm in diameter and 5.55 m in embedded length. With the group pile foundation the piles were placed in a square array spaced 90 cm center to center. Three of the group piles and the single pile were instrumented. In order to investigate group action, the measurement was made on the axial loads in the piles, earth pressure on the pile surface, the base reaction at the bottom of pile cap, and the movements of the subsoil. The bearing capacity of the group pile foundation is compared with the single pile and spread footing foundations.

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