Although cone penetrometers are often used for soundings and the evaluation of bearing capacity of paddy fields, the theory of the cone penetration resistance has not been clearly known yet. In this report, the authors try to analyze theoretically the penetration mechanism of cones based on the bearing capacity theory of foundations by G. G. Meyerhof. First, the authors express the bearing capacity component (q_1 = cN_c + p_0N_q) at a certain depth (D_f) as the function of the depth ratio (D_f/B). Next, some penetration tests are made using circular plates of five different base areas (A=1.2, 3.2, 5.0, 6.4, 10.0 cm^2), which are penetrated into the air-dried soil (c=0, φ=38°) at a low speed (ν=0.0017 cm/sec) and the results of tests are analyzed and the following conclusions are obtained. (1) The cone index (I_c) has a linear relation to the depth/width of a cone (D/2r) at a certain penetration depth (D) (Table 3). (2) The inclination of the straight line (showing the relation between I_c and D/2r) decreases linearly with the increase of the value of D, and the group of straight lines pass through a certain coordinates (4.91, 3.06). (3) The expression for the cone index (I_c) would be proposed generally as the next equation. I_c = a･D + (b － c･D)D/2r = m+nD/2r where a, b and c are constants which depend on the value of φ and the penetration speed, m is the bearing capacity component of a shallow foundation and n is the inclination of the straight line at a certain value of D.
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